APPLICATION OF DIGITAL CELLULAR RADIO FOR MOBILE LOCATION ESTIMATION


IIUM Engineering Journal, Vol. 24, No. 2, 2023 Harun et al. 
https://doi.org/10.31436/iiumej.v24i2.2803 

3D COLLISION AVOIDANCE SYSTEM FOR UNMANNED 

AERIAL VEHICLE (UAV) WITH DECENTRALIZED 

APPROACH 

MOHAMAD HANIFF HARUN1,2,3*, SHAHRUM SHAH ABDULLAH1,

MOHD SHAHRIEEL MOHD ARAS3, MOHD BAZLI BAHAR1,3 
AND FARIZ ALI@IBRAHIM3 

1
Department of Electronic System Engineering,  

Malaysia-Japan International Institute of Technology, 

 UTM KL Campus, Jalan Sultan Yahya Petra, 54100 Kuala Lumpur, Malaysia 
2
Faculty of Electric and Electronic Engineering Technology, 

Universiti Teknikal Malaysia Melaka,  

Hang Tuah Jaya, 76100 Durian Tunggal, Melaka, Malaysia 
3
Underwater Technology Research Group (UTeRG), 

Center for Robotics and Industrial Automation (CERIA), Faculty of Electrical Engineering, 

Universiti Teknikal Malaysia Melaka, 76100 Durian Tunggal, Melaka, Malaysia. 

*
Corresponding author: haniff@utem.edu.my 

(Received: 23 March 2023; Accepted: 20 May 2023; Published on-line: 4 July 2023) 

ABSTRACT: Unmanned aerial vehicles UAVs have been developed and refined for 

decades. Using an integrated software system, autonomous unmanned aerial vehicles 

(UAVs) perform missions automatically and return to a pre-programmed point. Malaysia 

has a lot of unoccupied airspace, yet autonomous UAV applications and research are still 

rare. In critical conditions, autonomous UAVs must deal with a variety of environmental 

and flight issues. This project involves a decentralized 3D collision avoidance system for 

an autonomous UAV. Ultrasonic, infrared, and laser rangefinders were chosen for the 3D 

collision avoidance system. The UAV's obstacle recognition and collision avoidance 

performance are also tested in four experiments. In various flight conditions, the 3D 

collision avoidance system can identify several material types and opacities by integrating 

selected rangefinders. Finally, the 3D collision avoidance system quickly reacts to 

obstacles in the X, Y, and Z axes. 

ABSTRAK: Kenderaan udara tanpa pemandu (UAV) telah dibangunkan dan diperhalusi 

selama beberapa dekad. Menggunakan sistem perisian bersepadu, kenderaan udara tanpa 

pemandu (UAV) autonomi melaksanakan misi secara automatik dan kembali ke titik pra-

diprogramkan. Malaysia mempunyai banyak ruang udara yang tidak berpenghuni, namun 

aplikasi dan penyelidikan UAV autonomi masih jarang berlaku. Dalam keadaan kritikal, 

UAV autonomi mesti menangani pelbagai isu alam sekitar dan penerbangan. Projek ini 

melibatkan sistem pengelakan perlanggaran 3D terpencar untuk UAV autonomi. Pencari 

jarak ultrasonik, inframerah dan laser telah dipilih untuk sistem pengelakan perlanggaran 

3D. Prestasi pengecaman halangan dan pengelakan perlanggaran UAV juga diuji dalam 

empat eksperimen. Dalam pelbagai keadaan penerbangan, sistem pengelakan 

perlanggaran 3D boleh mengenal pasti beberapa jenis bahan dan kelegapan dengan 

menyepadukan pencari jarak terpilih. Akhir sekali, sistem pengelakan perlanggaran 3D 

bertindak balas dengan cepat terhadap halangan dalam paksi X, Y dan Z.  

KEYWORDS: decentralized 3D collision avoidance; unmanned aerial vehicle; rangefinder 

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1. INTRODUCTION  

The IR 4.0 is increasing interest in autonomous UAVs. A UAV is an aerial vehicle that 

may be piloted from a distance [1]. UAVs are of use in military and civilian/commercial 

applications due to their wide range, inexpensive maintenance, quick deployment, mobility, 

and ability to hover [2]. The military uses UAVs for border security surveillance, 

reconnaissance, and target removal. Unmanned aerial vehicles are also used for search and 

rescue, parcel delivery, precision horticulture, and pharmaceutical transport. Multi-rotor 

drones, fixed-wing drones, single-rotor helicopters, and fixed-wing Vertical Take-Off and 

Landing (VTOLs) are the four main drone types, as shown in Fig. 1. 

The fact remains that each key drone type has its own benefits and drawbacks. For 

example, multi-rotor drones may hover due to their vertical direction drive framework, 

while single-rotor helicopters can hover and rotate due to their vertical direction drive 

framework. To put it another way, this type of aerial vehicle has slow movement speeds and 

requires extra energy to fulfil tasks [3]. However, fixed-wing drones and fixed-wing hybrid 

VTOLs can travel long distances due to their propulsion system and aerodynamic surfaces. 

Fixed-wing VTOLs (vertical takeoff and landing) can launch and land vertically without a 

runway [3]. However, fixed-wing drones have many drawbacks. Fixed-wing aircraft require 

a lot of airspace to maneuver and direct themselves. Other than that, the aerodynamic wing 

surfaces produce lifting power through air impacts. 

UAVs have both military and commercial uses. They must be built to work in difficult 

settings, such as huge open spaces, dense tree clusters, or rocky mountain slopes [4]. As a 

result, UAVs will struggle to achieve their missions in these hostile environments. Outdoor 

operation of UAVs is even more problematic due to GPS errors, poor communication, and 

bad weather [5]. Sensors enable UAVs to scan their surroundings and avoid collisions. 

Collecting accurate environmental data is important to avoiding crashes and completing the 

mission. Due to the increasing utilization of multi-UAV cooperative operations, 3D 

collision avoidance is crucial. When functioning centrally, an unmanned aerial vehicle 

assesses all colleagues' inputs and outputs to avoid collisions. While a central 

communication hub is beneficial, it is difficult to expand [6-8]. Decentralized systems can 

also handle more UAV teams in congested situations. These are more stable and robust. 

Malaysia is believed to be falling behind in aerial vehicle research and development 

compared to countries like the US, China, and Iran. Limitations include lack of aeronautical 

knowledge, high sensor and communication system costs, and limited airspace. To boost 

Malaysia's aeronautics industry, aerial vehicle research and development must continue. 

Currently, only a few Malaysian aerospace companies are researching and developing 

autonomous UAVs to solve industrial and military concerns. 

In many situations, rapid deployment of aerial vehicles can change a crisis situation 

into a non-critical situation since they can gather information faster than humans. Wildfires, 

medication delivery, crime fighting, and search and rescue are examples [9]. Wildfires, like 

the 2019 Amazon rainforest fires, can erupt at any time throughout the dry season [10]. 

Also, the Amazon rainforest covers 5.5 million km2, making human patrolling and scouting 

impractical. Using flying vehicles to monitor the Amazon rainforest could help limit the 

damage and mortality caused by wildfires. 

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Fig. 1: Types of drones. 

2. THEORETICAL BACKGROUND 

Numerous theories and principles must be explored and incorporated into the design 

and development of the unmanned aerial vehicle with decentralized 3D collision avoidance 

system. Aerodynamic forces, center of gravity (CG), and time of flight (ToF) are all 

significant theories and principles of flight. 

2.1   Aerodynamic Force 

All aircraft in flight are subjected to the forces of thrust, drag, lift, and weight. 

Understanding how these forces interact and utilizing power and flight controls to manage 

them is critical for flight. Thrust, drag, lift, and weight are the four forces that act on an 

airplane in level, unaccelerated flight [11]. These are the terms that are used to describe 

them: 

a) Thrust—the force applied forward by the engine/propeller or rotor. It opposes or 
overcomes drag. By and large, it acts perpendicular to the longitudinal axis. 

b) Drag—a rearward, retarding force created by the wing, rotor, fuselage, and other 
projecting items disrupting airflow. Drag, on the other hand, opposes thrust and 

acts in the opposite direction of the relative wind. 

c) Lift—is a force generated by the dynamic action of the air on the airfoil that acts 
perpendicular to the flight path through the center of lift (CL) and perpendicular 

to the lateral axis. Lift opposes the downward force of weight in level flight. 

d) Weight—the total weight of the aircraft, crew, fuel, and cargo or baggage. Weight 
is a force that acts as a drag on the aircraft due to the force of gravity. It acts 

vertically downward through the aircraft's center of gravity (CG) and opposes lift. 

The sum of these opposing pressures is always zero in steady flight. According to 

Newton's Third Law, there can be no unbalanced forces in steady, straight flight, because 

every action or force has an equal but opposing response or force. This is true regardless of 

whether you are flying level or ascending or descending. 

This is not to say that the four forces are equal. This signifies that the opposing forces 

are equivalent to one another and hence cancel out their effects. In Fig. 2(a), the thrust, drag, 

lift, and weight force vectors appear to be identical in magnitude. The conventional 

explanation claims (without specifying) that thrust equals drag and weight equals lift. While 

this statement is true, it might be misleading. It should be known that the opposing 

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lift/weight forces are equal in straight, level, unaccelerated flight. Additionally, they are 

greater than the opposing thrust/drag forces, which are equal only to one another.  

This revision of the conventional "thrust equals drag; lift equals weight" formula 

explains that during climbs and slow flight, a component of force is directed upward and 

functions as lift, while a piece of weight is directed backward in the opposite direction of 

flight and acts as drag. In sluggish flight, thrust is directed upward. However, because the 

aircraft is in level flight, weight has no effect on drag, as illustrated in Fig. 2(b).  

 

(a) (b) 

Fig. 2: (a) Relationship of forces acting on an aircraft (b) Force vectors during a stabilized climb. 

2.2  Centre of Gravity (CG) 

A body in a gravitational field has an unlimited number of particles of varying sizes, 

each with its own weight. The total weight of the body is the resultant parallel force system 

created by those weights [12]. Moreover, when the gravitational field is uniform over a 

body, the center of gravity is also the center of mass. The UAV's center of gravity influences 

the UAV's balance while in flight. To improve overall flying performance and minimize any 

instability issues, the UAV's center of gravity must be on the same horizontal plane as the 

propellers.  

2.3   Vibration 

Vibration is the oscillating motion of a system or body of attached bodies [13]. Free 

vibration and forced vibration are the two main types of vibration. Forced vibration is 

created by an external periodic or intermittent force, whereas free vibration is caused by 

gravitational or elastic restoring forces. A UAV's vibrations come from its aerodynamic, 

mechanical, and flying motions. Due to electronic components like flight controllers, 

inertial measurement units (IMUs), and sensors are susceptible to vibrations, multirotor 

UAVs have motor-propeller produced vibrations. It is also vital to consider vibrations 

because a built UAV vibrating at its normal frequency would be disastrous. So, it is critical 

to isolate electrical components from vibrations and limit UAV vibrations as much as 

feasible. 

2.4   Time of Flight (ToF) 

The Time-of-Flight concept measures the distance between a sensor and an item by the 

time it takes for a signal to travel from the sensor to the object and back. The Time-of-Flight 

principle works with several signals (carriers), the most prevalent being sound and light 

[14]. Light is the carrier for TeraRanger sensors because it combines speed, range, weight, 

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and eye-safety. As a result, infrared sensors perform better than other distance sensors of 

similar size and weight. 

  

(a) (b) 

Fig. 3: (a) Direct ToF (b) Indirect ToF. 

Time-of-Flight (ToF) sensors detect distances by measuring the time it takes photons 

to travel from the sensor's emitter to a target and back. As illustrated in Fig. 3, both indirect 

and direct ToF have particular advantages. Both can measure a pixel's intensity and distance. 

Direct ToF sensors give out nanosecond-long light pulses and time how long it takes for 

some of the light to return. Indirect ToF sensors use continuous modulated light to calculate 

the distance to objects. The object distance from the sensor can be calculated using Eq. 1, 

where d is the object distance and c is the sound or light speed. 

𝑑 =
𝑐 × 𝑇𝑜𝐹

2
 (1) 

3. METHODOLOGY 

This section contains a mechanical description and the concerns related to the design 

of the fuselage, landing gear, and propulsion system are described in detail. The electrical 

and electronic description includes in-depth discussions of the thought processes behind and 

justifications for the choice of components including the power supply, ESC, flight 

controller, RC transmission, and range finder. Finally, the experimental procedures are 

presented, including the setup of all four experiments. 

3.1 Mechanical Part - Fuselage 

SolidWorks is used to design the multirotor fuselage. The fuselage design defines the 

type of multirotor aircraft, such as the hexa-copter depicted in Fig. 4(a). Thus, multirotor 

aircraft have 6 motors and 6 propellers for propulsion. The extra two motors in the Hexa X 

rotor arrangement provide more lift force. A hexa-copter has higher stability and can 

continue to fly even if one of its motors fails. Also, the bare fuselage design is more 

convenient and versatile for research and development. These motor mounts are colored 

differently from the other motor mounts to show the multirotor aircraft's heading. The 

fuselage is also constructed such that the motor arms may be stored and transported easily. 

The multirotor aircraft also features a cargo mechanism at the bottom for extra electronics 

or sensors. 

To build the multirotor aircraft, the SolidWorks file of the bottom plate is exported as 

a drawing exchange format (DXF) file. This ensures the bottom plate is accurate and closely 

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matches the drawing. The carbon fiber tube is cut to the desired length using a grinder, 

where the intended motor-to-motor length is 685 mm. 

3.2 Mechanical Part - Landing Gear 

Similarly, SolidWorks is used to design the multirotor aircraft landing gear, which is 

shown in Fig. 4(b). This style of landing gear also allows for extra cargo area. The landing 

gear must be designed to support the overall weight of the multirotor aircraft plus any 

additional payloads. The landing gear can also be retracted horizontally for storage and 

transit. 

Similar to the fuselage, the landing gear is made from carbon fiber plates and tubes, as 

seen in Fig. 4(b). The desired payload height clearance from the ground is 180 mm. 

 

(a)                                                (b) 

               Fig. 4: (a) Fuselage design (b) Landing gear. 

3.3 Mechanical Part - Propulsion System 

The mechanical components of a multirotor aircraft propulsion system include BLDC 

motors and propellers. BLDC motors and propellers for multirotor aircraft must be chosen 

based on the above requirements and the aircraft's overall takeoff weight. The weight of the 

multirotor aircraft is the key element impacting the BLDC motor choices. To reach the 2:1 

thrust to weight ratio, the motors' total thrust must be at least twice the aircraft's entire 

takeoff weight [15]. The required motor thrust can also be determined using Eq. 2 from [16]. 

𝑇ℎ𝑟𝑢𝑠𝑡 =
(𝑇𝑜𝑡𝑎𝑙 𝑡𝑎𝑘𝑒𝑜𝑓𝑓 𝑤𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑎𝑖𝑟𝑐𝑟𝑎𝑓𝑡) × 2

𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑚𝑜𝑡𝑜𝑟𝑠
 (2) 

The datasheet for a BLDC motor frequently includes the motor's thrust. Important 

information such as Kv ratings (RPM per supplied volt) and current ratings (maximum 

current drawable safely) are also included in the datasheet [17]. Additionally, to maximize 

motor efficiency, most BLDC motors recommend a propeller size and design. The 

multirotor aircraft constructed for this project is expected to weigh 3 kg in total. Tarot 4108 

Brushless DC was finally selected. Six lithium-ion batteries (25.2 V), a 380 Kv motor with 

1 kg of torque, and 1355 carbon fiber propellers were also selected. The UAV is 13 inches 

in diameter and has a 5.5-degree pitch. Therefore, the multirotor should include axial 

propulsion, puller configuration, and two-blade propellers for better performance. 

 

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3.4 Electrical and Electronic Part – Power Supply 

A multirotor aircraft's power supply is chosen to meet the propulsion system's voltage 

and current needs before being stepped down to meet the electronics components' voltage 

and current requirements. After settling on the motors and propellers, the series and parallel 

configuration of battery cells can be estimated. This is because one 18650 Li-ion cell 

delivers 4.2 V when completely charged and may safely draw up to 20 A (depending on the 

model). For example, to provide 24 V for the motors, at least 6 cells must be connected in 

series. Batteries are rated in mAh and C, where mAh is the maximum power stored and C 

is the rate of discharge relative to maximum capacity. According to Eqs. 3 and 4 from [18], 

a 2000 mAh Li-ion cell at 10 C may discharge at 20 A for 6 minutes. 

𝐼 = 𝑄 × 𝐶 (3) 

𝐷𝑖𝑠𝑐ℎ𝑎𝑟𝑔𝑒 𝑡𝑖𝑚𝑒 (𝑚𝑖𝑛𝑢𝑡𝑒𝑠) =
𝐼

𝐶
× 60 

(4) 

Hence, the multirotor aircraft power supply will be LG HG2 3000 mAh 20 C 18650 Li-

ion batteries (6S4P) as shown in Fig. 5(a) with a total capacity of 12000 mAh and a 

maximum voltage of 25.2 V. With this Li-ion battery combination, the motors can run for 

12 minutes at maximum discharge current of 20 A. Because the motor can only draw 12.4 

A at full throttle, the multirotor aircraft can only fly for 20 minutes. Nickel strips are used 

to spot weld the Li-ion cells into 6S4P arrangement. 

3.5 Electrical and Electronic Part – Electronic Speed Control (ESC) 

The ESC controls the motor speed of the multirotor aircraft, affecting the thrust force 

generated. The key problem in selecting an ESC is that it can handle the voltage and current 

draw from the power supply to the BLDC motors at full throttle. As indicated in Fig. 5(b), 

the multirotor aircraft ESC chosen is the HobbyWing XRotor-40A. A safety factor larger 

than 2 means the ESC's continuous current rating is substantially higher than the motors' 

maximum current demand. 

3.6 Electrical and Electronic Part – Flight Controller 

The flight controller of a multirotor aircraft receives input signals and converts them to 

output signals for the motors. So, one way to find a good flight controller is to look at its 

specs. The Pixhawk 2/Cube flight controller was chosen for the multirotor aircraft since it 

is open source and popular among UAV developers. The Cube also has a double redundant 

microprocessor, triple redundant IMU, and can accommodate up to three GPS modules. The 

Cube's (I/O) ports are also attached to a carrier board that can integrate with the Intel® 

Edison single board computer. It also includes 14 PWM pins, 8 of which have a failsafe and 

manual override. 

Thus, redundant components in The Cube ensure that the flight controller does not fail. 

The carrier board also allows for easy attachment of sensors and other electronic 

components to the flight controller. Because Pixhawk 2 is an OSH, the dimensions and CAD 

model of the flying controller depicted in Fig. 5(c) are simply obtained. So, the flight 

controller may be exactly positioned in the multirotor center to ensure accurate IMU and 

accelerometer data. 

3.7 Electrical and Electronic Part – DC Transmission 

The multirotor aircraft uses RC transmission to receive radio signals from the 

transmitter. The transmission range and frequency are important factors to consider when 

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choosing an RC transmission system. The TTSRC X9 Remote Control 2.4 G 9CH 

Transmitter with Receiver X9D is chosen for manual mode since it has a transmission range 

up to 1 km and runs at 2.4 GHz. In autonomous mode, the 3DR 100 mW Radio Telemetry 

is chosen due to its 1.5 km transmission range and 915 MHz transmission frequency, as seen 

in Fig. 5(d) and 5(e). Moreover, both transmitting frequencies are legal. 

 

 

Fig. 5: (a) Li-ion battery (b) ESC XRotor-40A (c) Pixhawk 2 CAD model  

(d) TTSRC X9 (e) 3DR radio telemetry 

3.8 Electrical and Electronic Part – Rangefinder 

The primary purpose of rangefinders is to determine the distance between an object and 

the rangefinder, but they are also utilized in this project to identify obstacles and avoid 

collisions. Table 1 compares several types of rangefinders. 

Table 1: Comparison between rangefinders 

Parameters Ultrasonic Sensor Infrared sensor LiDAR sensor 

Manufacturer WAVGAT SHARP Benewake 

Model GY-US42V2 GP2Y0A710K0F TFMini 

Medium Ultrasound Infrared Laser 

Communication 

interface 

Serial UART Analog Serial UART 

Distance measuring 

range 

20 cm ~ 720 cm 100 cm ~ 550 cm 30 cm ~ 1200 cm 

To construct the decentralized 3D collision avoidance system, all of the rangefinders 

listed in Table 1 will be employed for sensor fusion, allowing the advantages of each 

rangefinder to outweigh the shortcomings of the others. Additionally, several simulations 

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and experiments will be undertaken to determine each rangefinder's performance in a variety 

of different environments. 

3.9 Electrical and Electronic Circuit Diagram 

Figure 6(a) illustrates the relationships between electrical components that are 

necessary for the building of the multirotor aircraft circuit diagram depicted in Fig. 6(b). 

The circuit schematic is created using Fritzing software to aid in the rapid and exact 

production of the electrical and electronic system. 

 
Fig. 6: (a) Electronic components relations, (b) Multirotor aircraft circuit diagram. 

Multiple GPIO pins are available on microcontrollers and single-board computers to read 

analogue and digital inputs from sensors and to output analogue and digital voltage signals. 

This is to respond to the mobile robot's continually changing electrical requirements. The 

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microcontroller is used to read data from ultrasonic sensors that generate and receive digital 

signals. Additionally, through Universal Asynchronous Receiver and Transmitter (UART) 

connectivity, the single-board computer receives GPS coordinates from the GPS Module. 

Additionally, single-board computers are utilized to get photographs of railway track defects 

and video recordings of the railway track from cameras using serial connection protocol. 

C++ and Python will be used to program the microcontroller and single-board computer, 

respectively. 

3.10 Experimental Procedures 

Various experiments are conducted to obtain the required results to achieve the project's 

objectives. The results are based on the specific performance index to determine the success 

of the research. One of them is the percentage rate of obstacle detection based on the sensor 

fusion algorithm. This parameter is essential to show how quickly the system detects 

obstacles and the percentage of successful detection. The other parameter is the reaction 

time for the system to perform collision avoidance when an obstacle is detected. The 

reaction time is vital to ensure the system's robustness in all situations. Four experiments 

will be conducted to evaluate all the performance parameters for the system. 

A. Experiment 1: Obstacle Detection on Multiple Type and Opacity of Material  
                          Test 

Objective : To measure and determine the capabilities for obstacle detection 

on multiple type of material based on LiDAR, Sonar, and IR 

sensors 

Parameters : Manipulated Variable: Type of rangefinder and material 

Responding Variable: Detection of obstacle 

Apparatus : White polystyrene, cotton hoodie, clear plastic, translucent 

plastic, wooden plank, black box and rangefinders 

Procedure : 1. Ultrasonic, laser, and infrared rangefinders are positioned at a 
fixed distance from the wooden plank.  

2. The microcontroller controlling the rangefinders is 
programmed to obtain the distance between the rangefinders 

and obstacle. 

3. The distance obtained is recorded and compared with the 
fixed distance set between the rangefinders and obstacle.  

4. Step 1 to 3 is repeated by changing the type of material to 
wooden plank, cotton hoodie, clear plastic, translucent 

plastic, white polystyrene and black box.  

5. The results obtained are tabulated in table form.  

B. Experiment 2: Obstacle Detection Range in Different Environments Test 

Objective : To verify and evaluate the obstacle detection range for each type 

of rangefinder in various environments. 

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Parameters : Manipulated Variable: Flight environment. 

Responding Variable: Obstacle detection range of rangefinder 

Apparatus : Wooden plank, rangefinders, and multirotor aircraft 

Procedure : 1. Ultrasonic, infrared, and laser rangefinders are mounted at the 
same position on the multirotor aircraft.  

2. Multirotor aircraft is set up in a sunny environment. 

3. The wooden plank is positioned at a fixed distance of 10m 
from the multirotor aircraft.  

4. The distance between the wooden plank and multirotor aircraft 
is decreased to 9 m, 8 m, 7 m, 6 m, 5 m, 4 m, 3 m, 2 m, and 1 

m.  

5. The readings of the rangefinders are recorded. 

6. Step 2 to 6 is repeated by setting up the multirotor aircraft in a 
cloudy environment and a night environment. 

7. The results obtained are tabulated in table form.  

C. Experiment 3: Collision Avoidance Reaction Time Test 

Objective : To measure and study the reaction time of the multirotor aircraft 

on collision avoidance. 

Parameters : Manipulated Variable: Velocity of incoming multirotor aircraft 

Responding Variable: Reaction time of multirotor aircraft 

Apparatus : Rangefinders, radio controller, telemetry, laptop, and multirotor 

aircraft 

Procedure : 1. Multirotor aircraft with rangefinders mounted at forward 
facing position is set up at an empty and open field.  

2. Mission Planner is launched on laptop and the flight path 
of the multirotor aircraft is planned. 

3. The flight controller is configured to record the velocity of 
multirotor aircraft, rangefinder readings, and throttle level 

of each BLDC motor.  

4. The multirotor aircraft took off manually in position hold 
mode using a radio controller and was controlled to move 

at a direction parallel to the surface obstacle at a speed of 

2 m/s.  

5. The data logged in graph form is tabulated in table form. 
6. Step 4 to 6 is repeated by changing the speed of the 

multirotor aircraft to 4 m/s and 6 m/s. 

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D. Experiment 4: 3D Collision Avoidance Test 

Objective : To evaluate and demonstrate the capability of the multirotor 

aircraft on 3D collision avoidance. 

Parameters : Manipulated Variable: Position of obstacles. 

Responding Variable: Collision avoidance of multirotor aircraft 

Apparatus : Wooden plank, rangefinders, telemetry, laptop, and multirotor 

aircraft 

Procedure : 1. Multirotor aircraft with mounted rangefinders is set up at 
an empty and open field.  

2. Mission Planner is launched on laptop and the flight path 
of the multirotor aircraft is planned. 

3. The multirotor aircraft took off in autonomous mode and 
executed the flight path planned.  

4. Wooden plank is introduced along X-axis, Y-axis and Z-
axis of the multirotor aircraft.  

5. The reaction of the multirotor aircraft is observed and 
recorded. 

6. The observations obtained are tabulated in table form. 

4. RESULTS AND DISCUSSIONS 

Results are based on the design of the multirotor aircraft and the 4 real experiments are 

conducted using the proposed multirotor aircrafts throughout this research. 

4.1 Design of Multirotor Aircraft 

As seen in Fig. 7(a) and 7(b), the built multirotor aircraft includes the following 

characteristics: Hexa X, typical fixed skid landing gear, two blade propellers, puller 

arrangement, and axial propulsion. Due to the cylindrical form of the motor arm, motor 

mount, and landing gear, clamping is needed to secure them in place; drilling a hole through 

the carbon fiber tube will reduce its strength. Silicon rubber dampers are installed on the 

carbon fiber tube that will come into touch with the ground to absorb impact forces on the 

landing gear system. The 6S4P 18650 Li-ion battery pack is arranged in Fig. 7(c), and it is 

designed to be sealed using a non-conductive waterproof sealing tape.  

These dimensions are critical because they reflect the multirotor aircraft's virtual 

cylinder dimension for 3D collision avoidance. Thus, the width of the multirotor aircraft 

with propellers and the height of the multirotor aircraft with battery pack are utilized to 

establish the proportions of the virtual cylinder for 3D collision avoidance. As a result, the 

virtual cylinder has a diameter of 1014.7 mm and a height of 300.36 mm as shown in Table 

2, which is used to construct a 3D collision avoidance system. 

4.2 Obstacle Detection on Multiple Types and Opacity of Material Test 

The performance of the collision avoidance system for multirotor aircraft is directly 

affected by the performance of the several rangefinder types on identifying multiple types 

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and opacity of material. This experiment will also assess each rangefinder's ability to detect 

obstacles. Table 2 lists the findings. 

As shown in Table 3, ultrasonic sensors can successfully detect all materials used. Also, 

both laser and infrared rangefinders can detect all materials except transparent materials like 

clear plastic where incorrect readings occur. Contrary to expectations, ultrasonic sensors 

were able to detect soft materials like cotton hoodies with ease. However, humans typically 

wear cotton-based clothes, thus it is critical that the rangefinder utilized can detect it. 

 

Fig. 7: (a) Multirotor aircraft frame design, (b) Modelling of multirotor aircraft design,  

(c) Modelling of 18650 Li-ion battery pack. 

Table 2: Dimension of multirotor aircraft 

Dimension Length (mm) 

Motor to motor length 684.50 

Breadth (Without propellers) 669.77 

Width (Without propellers) 761.45 

Height (Without battery pack) 229.00 

Breadth (With propellers) 922.99 

Width (With propellers) 1014.70 

Height (With battery pack) 300.36 

 

Because laser and infrared rangefinders use light to measure distance between objects 

and the rangefinder, they have trouble identifying transparent materials. Light does not 

reflect from clear plastic since it is transparent and has low reflectivity. However, when 

using translucent materials such as translucent plastic, both the laser and infrared 

rangefinder may detect it due to the material's reflection. 

Thus, the advantages of the ultrasonic rangefinder over laser and infrared rangefinder 

can be exploited to overcome the disadvantages. However, ultrasonic rangefinders have 

drawbacks including loud readings and limited obstacle detection range that can be 

overcome by laser and infrared rangefinders. 

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Table 3: Obstacle Detection on Multiple Type and Opacity of Material Test 

Type of 

Material 

Material Used Type of Rangefinder (RF) 

Ultrasonic RF Infrared RF Laser RF 

Detection Observation Detection Observation Detection Observation 

Hard Wooden plank √ Good √ Good √ Good 

Soft Cotton hoodie √ Good √ Good √ Good 

Transparent Clear plastic √ Good X Bad X Bad 

Translucent Translucent plastic √ Good √ Good √ Good 

Bright White polystyrene √ Good √ Good √ Good 

Dark Black box √ Good √ Good √ Good 

Table 4: Obstacle Detection Range in different Flight Environments Test Using Ultrasonic, Infrared and Laser Rangefinder 

Flight 

Environment 
Detection Range (m) 

Ultrasonic RF Infrared RF Laser RF 

1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10 

Sunny √ √ √ √ √ √ √ X X X √ √ √ X X X X X X X √ √ √ √ √ √ X X X X 

Cloud √ √ √ √ √ √ √ X X X √ √ √ X X X X X X X √ √ √ √ √ √ √ √ X X 

Night √ √ √ √ √ √ √ X X X √ √ √ X X X X X X X √ √ √ √ √ √ √ √ √ √ 

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4.3 Obstacle Detection Range in Different Environments Test 

While an ultrasonic rangefinder is generally unaffected by its environment (such as 

illumination, brightness, and weather), it is tested to confirm its function in a variety of 

conditions. The infrared and laser rangefinders' functionality will be impacted by the 

illumination conditions in the environment. In bright flight situations, the average illuminance 

on the surface of the obstacle is between 32 Klux and 100 Klux, whereas in cloudy flight 

environments, the average illuminance on the surface of the obstacle is between 1 Klux and 20 

Klux. Needless to add, the average value of illuminance on the surface of obstacles during night 

flight conditions is less than 1 lux. Table 4 summarizes the findings. 

According to Table 3, the ultrasonic rangefinder can detect obstacles from 1m to 7m in 

various flight situations. Also, because ultrasonic rangefinders use air to measure distances 

between themselves and obstacles, the weather does not affect their effectiveness. The 

performance of the laser rangefinder varies with flight conditions since the surface of the barrier 

is illuminated differently in different flight situations. The laser rangefinder has a 6 m obstacle 

detection range in sunny conditions, 8 m in overcast conditions, and 10 m in night flight. The 

obstacle detection ranges of a laser rangefinder increase with decreasing surface illuminance. 

Nevertheless, the laser rangefinder's obstacle detection range in Table 1 matches its 

performance during night flight. 

The infrared rangefinder's performance does not match the criteria in Table 1, which only 

identifies obstacles 1 to 3 meters away. Surprisingly, the degree of illuminance does not impact 

the infrared rangefinder's effectiveness in sunny, overcast, or night flight conditions. This is 

true for this particular infrared rangefinder device. The chosen ultrasonic and laser rangefinders 

are suited for obstacle identification in the X- and Y-axis of the multirotor aircraft frame 

because they can identify obstacles at a distance. The ultrasonic rangefinder also offers a larger 

obstacle detecting area, increasing the success rate of the collision avoidance system. Due to 

the design of the multirotor aircraft's frame, the infrared rangefinder can be employed for 

obstacle detection in the Z-axis. The Z-axis multirotor aircraft outermost components are the 

battery and landing gear, whereas the X- and Y-axis outermost components are the propellers. 

4.4 Collision Avoidance Reaction Time Test 

The reaction time of a multirotor aircraft is the time it takes to avoid an obstacle. 

Preventing collisions requires quick response from multirotor aircraft when obstacles are 

detected. This experiment is designed to estimate the reaction time of a multirotor aircraft so 

that it can react quickly when an obstruction is detected. Table 5 shows the results. 

Table 5 shows that when travelling at 2 m/s, the multirotor aircraft reacts promptly when 

an obstruction is detected. However, at 4 m/s, the multirotor aircraft made evasive maneuvers 

30 milliseconds after recognizing the obstruction. The multirotor aircraft also experienced a 35 

milliseconds delay before performing evasive moves when going at 6 m/s. 

When an obstruction is detected, the multirotor aircraft's reaction time is sufficient to avoid 

it. Rangefinders also read distances at 100 Hz, or 10 milliseconds each reading. As a result, the 

reaction time for the multirotor aircraft is 0–35 milliseconds. That's because the laser 

rangefinder's maximum obstacle detection range in diverse flight conditions is 6 meters, so the 

multirotor maximum speed is limited. It will also be unable to avoid a collision with an 

obstruction if it exceeds 6 m/s. 

4.5 3D Collision Avoidance Test 

The multirotor aircraft should be able to avoid obstacles on the X, Y, and Z axes. Figure 8 

(a) shows the intended autonomous mission flight path with an impediment noted in red. The 

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multirotor aircraft must also avoid an impediment at waypoint 3 on the map upon landing. The 

mission speed for the multirotor aircraft is set to 1 m/s and the altitude to 2.5 m. Table 6 contains 

the results seen in Fig. 8(b)–(d). 

Table 5: Collision avoidance reaction time test results 

Velocity of multirotor aircraft (m/s)  Reaction time of multirotor aircraft (1 x 10-3 s) 

2 0 

4 30 

6 35 

 

 

Fig. 8: (a) Autonomous mission flight path with obstacle position marker (b) Roll and pitch angle data 

logged of multirotor aircraft (c) Yaw angle data logged of multirotor aircraft (d) Rangefinders data 

logged of multirotor aircraft 

Table 6: 3D Collision Avoidance Test Results 

Position of obstacle Observation 

Along X-axis Successful avoidance 

Along Y-axis Successful avoidance 

Along Z-axis Successful avoidance 

Figure 8(b) and 8(c) show the multirotor aircraft's attitude and heading reference system 

data logged during the autonomous mission. Moreover, positive pitch indicates forward 

motion, while positive roll indicates rightward motion. Also, the multirotor yaw angle shows 

the multirotor heading. On the other hand, Fig. 8(d) shows the rangefinder data logged during 

the autonomous mission, with D0 being the forward rangefinder reading, D45 the northeast 

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rangefinder reading, D90 the east rangefinder reading, and D315 the northwestern rangefinder 

reading. Also, Dup is the rangefinder reading for upwards and CDIS is the rangefinder reading 

for downwards. The multirotor aircraft avoided collisions along the X, Y, and Z-axis in Table 

6.   

To get from waypoint 0 to 1, the multirotor aircraft first pitches forward. The program for 

this experiment makes the multirotor aircraft conduct evasive maneuvers when it detects a 

forward obstruction. The forward barrier is between waypoints 0 and 1. Also, when a multirotor 

aircraft detects a forward obstruction, it will roll to the right until it no longer detects the barrier 

and then proceed forward. The multirotor aircraft then yaws 90° and pitches forward to move 

from waypoint 1 to 2. The back obstacle is also parallel to the path from waypoint 1 to 2. When 

a multirotor aircraft detects a rear obstacle, it is programmed to roll away from it and pitch 

ahead. To proceed from waypoint 2 to waypoint 3, the multirotor aircraft must yaw for roughly 

120° and pitch forward. Also, when the multirotor aircraft reaches waypoint 3, it immediately 

enters LAND mode. An obstacle is positioned at waypoint 3 underneath the multirotor aircraft, 

which will undertake evasive maneuvers, such as pitching backward, until the object is not 

spotted, then land. Thus, the multirotor aircraft can avoid obstacles on the X, Y, and Z axes. 

5. CONCLUSIONS 

The 3D UAV collision avoidance system has been built and tested. The 3D collision 

avoidance system can also detect several material types and opacities in varied flight 

environments by combining selected rangefinders. Finally, the 3D collision avoidance 

algorithm can react quickly to obstructions in the X, Y, and Z axes. 

In the future, the hexa-rotor design can be improved with better obstacle detection systems 

such as 360° LIDAR and solid-state LIDAR, or vision-based obstacle detection systems such 

as stereo cameras and depth sensors. As obstacle detection coverage and performance improve, 

better collision avoidance algorithms can be built. Suitable filters should also be attached to 

the obstacle detection system to eliminate inconsistent readings owing to electrical disturbances 

and sensor interference. 

ACKNOWLEDGMENTS 

The authors would like to thank Ministry of Higher Education (MOHE) for sponsoring this 

work under the grant no. FRGS/1/2022/TK07/UTEM/02/19. We wish to express our gratitude 

to honorable University, Universiti Teknikal Malaysia Melaka (UTeM) and Ministry of Higher 

Education (MOHE). Special appreciation and gratitude especially for Universiti Teknologi 

Malaysia (UTM) KL Campus, Malaysia-Japan International Institute of Technology (MJIIT), 

Fakulti Teknologi Kejuruteraan Elektrik dan Elektronik (FTKEE) and Underwater Technology 

Research Group (UTeRG), Center for Robotics and Industrial Automation (CERIA) and 

Ministry of Higher Education (KPT) for supporting this research. 

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