INOM EXAMENSARBETE TECHNOLOGY,GRUNDNIVÅ, 15 HP

, STOCKHOLM SVERIGE 2016

Omni wheel robotOmnihjulrobot

ANGELICA LEJDEBY

KARL HERNEBRANT

KTHSCHOOL OF INDUSTRIAL ENGINEERING AND MANAGEMENT

Omni Wheel Robot

ANGELICA LEJDEBYKARL HERNEBRANT

Bacherlor’s Thesis in Mechatronics

Supervisor: Baha Alhaj HasanExaminer: Martin Edin GrimhedenApproved: 2016-06-07

TRITA MMKB 2016:39 MDAB100

AbstractThis project is about building a three wheeled robot car with Omni wheels.Omni wheels can make a robot drive sideways without rotating first. They canmake a robot rotate at the same time as it goes straight ahead. An Omni wheelrobot can for example be good choice for a tracking robot, though it can drivemore effective than a robot car with regular wheels. The thing that speaksagainst Omni wheels is that it has more friction and it takes more power torotate the wheels.

This robot car is an obstacles avoiding robot that should with help ofUltrasonic sensors and IR-sensors be able to drive around in a room withoutcrashing in to objects or walls. With the help of Omni wheels the robot shoulddrive without rotating much which makes it more effective than a robot carwith regular wheels.

iii

SammanfattningOmnihjulrobot

Det här projektet handlar om att bygga en trehjulig robotbil med Omnihjul.Omnihjul kan göra det möjligt för en robot att köra i sidled utan att förstrotera. De kan också möjliggöra för en robot att rotera samtidigt som den körrakt fram i en rak linje. En Omnihjulrobot kan till exempel vara ett bra valsom spårningsrobot. För att den kan köra mer effektivt än en robotbil medvanliga hjul. Det som talar mot Omnihjul är att de har mer friktion och detkrävs mer kraft för att rotera hjulen.

Den här robotbilen är en hinderundvikande robot som med hjälp av Ult-raljudssensorer och IR-sensorer ska kunna köra runt i ett rum utan att krashain i objekt eller väggar. Med hjälp av Omnihjul ska roboten kunna köra utanatt rotera mycket, vilket gör den mer effektiv än en robotbil med vanliga hjul.

v

Preface

We would like to thank our supervisor Baha Alhaj Hasan for all help and supportthrough the whole project.

Angelica Lejdeby Karl HernebrantStockholm, May, 2016

vii

Contents

Abstract iii

Sammanfattning v

Preface vii

Contents ix

Nomenclature xi

1 Introduction 11.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Purpose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.3 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.4 Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2 Literature study 32.1 Motor control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.2 Omni wheels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.3 Ultrasonic sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.4 IR-sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.5 Microcontroller . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.6 H-bridge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3 Demonstrator 73.1 Problem Formulation . . . . . . . . . . . . . . . . . . . . . . . . . . . 73.2 Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83.3 Electronics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

3.3.1 H-bridge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103.3.2 Ultrasonic sensor . . . . . . . . . . . . . . . . . . . . . . . . . 103.3.3 IR-senor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113.3.4 Microcontroller . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3.4 Hardware . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123.5 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

ix

3.5.1 Ultrasonic sensor test . . . . . . . . . . . . . . . . . . . . . . 143.5.2 IR-sensor test . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

4 Discussion and conclusions 174.1 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174.2 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

5 Recommendations and future work 195.1 Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

5.1.1 Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195.2 Future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Bibliography 21

x

Nomenclature

SymbolsSymbols Descriptiond Distance (cm)t Time (s)v Speed of sound (m/s)γ Specific constantG Gas constant (J mol−1K−1)T Temperature (K)M Molar mass (kg/mol)x Distance (cm)

xi

Chapter 1

Introduction

A robot that is able to move in any direction immediately is very useful. It can beused in tight spaces or to more effectively follow something.

1.1 Background

A triangular platform with three Omni wheels such as this is generally called a KiwiDrive (Figure 1.1). It is because of that a Kiwi Drive can move in any directionimmediately [1]. If a robot can drive in any direction immediately it is calledholonomic. That means in other words that it has two degrees of freedom on thefloor (A car where all the wheels are lined up has only one degree of freedom andare therefor not holonomic). The Omni wheels can move in 90 degrees to their axis,that way you can mount the wheels facing different directions [2].

In this project the Omni Wheels will be used to build an obstacles avoidingrobot. It will avoid obstacles with the help of Ultrasonic sensors and IR-sensors.The difference between the two sensors will be researched.

1.2 Purpose

The main purpose of this robot car is that it should be able to drive by it self andavoid obstacles, with the help of Ultrasonic sensors and IR-sensors. The differencebetween the two sensors will be tested to research which sensor will give the bestviewing angles for this robot. The different cases will be tested with Ultrasonicsensors, IR-sensors and with both.

1

CHAPTER 1. INTRODUCTION

Figure 1.1. Three wheeled Omni Wheel robot (Kiwi Drive)

1.3 ScopeThe scope of the will be that only three of Ultrasonic sensors and three IR-sensorswill be used. The Arduino UNO does not have the processing power to do trigono-metric calculations so we will have to use a look-up table. The budget for theproject will be 1000 SEK.

1.4 MethodThe construction will be built in wood in two stories to make room for all thecomponents. The construction will be built like a triangle as there is three motorsand three wheels that are used. There will also be two different sensors used,Ultrasonic and IR-sensors, and there will be three of each. Ultrasonic and IR-sensors are used to make the robot able to avoid obstacles. The Ultrasonic sensorswill be placed on each side of the triangle and the IR-sensors will be placed in eachcorner. That way the robot will have the best range to see the obstacles.

2

Chapter 2

Literature study

2.1 Motor control

Since the Omni wheel robot only has three wheels it will be moving at angles. Forthe robot to translate at a particular angle, each wheel must rotate at a particularrotational velocity and direction. To determine the wheels speed trigonometriccalculations need to be done. Those calculations might not always be possible asthe robot must depend on the computational processing ability.

A microcontroller that does not do anything else can do about 3 trigonometricalcalculations per second. Unfortunately it is needed to do much more than trigono-metrical calculations. Therefor it might be useful to use a trigonometry look-uptable. Those are used to solve trigonometrical functions for very slow computersand were often used for a couple of years ago before the computers had gotten faster.For the trigonometry lookup table to work it is needed to do the trigonometricalcalculations before hand and save them in a memory. The program will then justreference them in a list. It wont be fully accurate, but it will work faster then tomake the microcontroller do the calculations [1].

2.2 Omni wheels

An Omni wheel is a wheel with several small rolls around the circumstance (Canbe seen in Figure 2.1). Omni wheels are made to be driven with full force and beable to slide laterally with ease. It is suppose to make a more effective drive bybeing able to drive sideways without turning the front of the platform. Thereforthe wheel can make a robot able to drive in any direction.

3

CHAPTER 2. LITERATURE STUDY

Figure 2.1. An omni wheel there the arrows shows the directions where the wheelcan move.

2.3 Ultrasonic sensors

Ultrasonic sensors use the speed of sound to calculate the distance to an object. Todo this they send out a high frequency signal and measure the time it takes for itto bounce on an object and come back (Figure 2.2). The frequency is higher than20 kHz, usually around 40 kHz, so the sound cannot be heard by humans [3]. Thedistance, d, can be calculated with

d = t · v2 (2.1)

where t is the time and v is the speed of sound. It is divided by two becausethe signal has traveled back and forth.

The speed of sound depends on the temperature and is given by

v =√γGT

M(2.2)

Here are γ a specific constant, in air around 1.4, T is the temperature in Kelvin.G is the gas constant which is 8.314J mol−1K−1 , M is the molar mass of the gas.The molar mass for dry air is about 0.02896 kg/mol [4].

4

2.4. IR-SENSORS

Figure 2.2. Ultrasonic sensor process. Here it can be seen how the Ultrasonic sensorsees an object, by transmitting an ultrasonic wave and receiving the wave after it hasbeen reflected to the microphone.

2.4 IR-sensors

IR-sensors has an emitter that emits infrared (IR) rays. The rays are then detectedby a detector [5]. By looking at the intensity of the received light it is possible todetermine if an object is near the sensor. Light colored objects will reflect more IRlight than dark colored objects [6].

2.5 Microcontroller

A microcontroller is small computer on a single integrated circuit with a processor,memory and programmable input/output pins. Microcontrollers are used in a lotproducts that needs to be automatically controlled.

The microcontroller in this project is the Arduino Uno which is based on theATmega328P. The Uno has 14 digital input/output pins and 6 of them can be usedas PWM outputs. PWM stands for pulse-width modulation and it is used to get avariable continuous voltage from a source which can only be on and off. There arealso 6 analog pins and the clock speed is 16 MHz. The flash memory is 32 KB. [7]

2.6 H-bridge

An H-bridge is a circuit used to change the direction of the voltage trough a DC-motor [8]. The name H-bridge comes from the arrangement of the four switches, S1to S4 in figure 2.3.

5

CHAPTER 2. LITERATURE STUDY

Figure 2.3. The H-bridge is the part in red. The M is a dc-motor and S1-S4 is fourswitches.

The switches works in pair, S1 together with S4 and S3 with S2. When S1 andS4 are closed and S2 and S3 are open the current will flow through the motor asin the left image in figure 2.4. When open S1 and S4 and closing S2 and S3 thecurrent will flow in the opposite direction as in right image in figure 2.4. This makeit possible to drive the motor both forward and reverse. When all switches areclosed their will be a short circuit between the batteries plus and minus side. Thiswill most commonly make the switches melt [9].

Figure 2.4. The current direction trough the dc-motor depends on which of theswitches are open and closed. The left image shows the direction when switch S1 andS4 are closed and the right shows the direction when S2 and S3 are closed.

6

Chapter 3

Demonstrator

3.1 Problem Formulation

The research for this project is to determine which sensor will have best viewingangle. The questions that will be answered is

• What is the minimum width between two objects the robot can pass?

• What is the minimum height of objects which the omni wheel robot willdetect?

• How small object can the ultrasonic sensors detect?

• How small object can the ir-sensors detect?

7

CHAPTER 3. DEMONSTRATOR

3.2 Software

The code is written in Arduino 1.6.8. The flow of the program can be seen in figure3.1

Figure 3.1. Flowchart of the software

8

3.2. SOFTWARE

All of the sensors are named 1 to 3 as in figure 3.2. The sensors on each side ofthe robot are then grouped together (see table 3.1) with one ultrasonic sensor andtwo ir-sensors. The robot can drive in three directions without rotating and theyare numbered the same way as the ultrasonic sensors. The robot starts driving indirection 1 if it is clear. If it there is something in front of it the robot will try todrive in direction 2. If that does not work it tries to drive in direction 3 and thenit rotates. The ultrasonic sensors are set to react to object 40 cm in front of them.

Sensor 1: Ultra 1, IR 1, IR 3Sensor 2: Ultra 2, IR 1, IR 2Sensor 3: Ultra 3, IR 2, IR 3

Table 3.1. How the sensors are grouped together in the software.

Figure 3.2. The robot from above with the name of the sensors.

9

CHAPTER 3. DEMONSTRATOR

3.3 Electronics

The omni wheel robot consists of the following electronics:

Microcontroller: Arduino UNOH-bridge: Pololu DRV8833 Dual Motor Driver CarrierUltrasonic sensor: HC-SR04IR-sensors: IR Infrared Obstacle Avoidance Sensor

3.3.1 H-bridge

The omni wheel robot will use the Pololu DRV8833 h-bridge (figure 3.3). TheDRV8833 has two h-bridge drivers so it can drive two dc-motors. It can ouput 1.5A (2 A peak) at 5 V. The power supply voltage ranges between 2.7 V to 10.8 V[10].

The robot has two DRV883. One that is controlling two motors and one thatcontrols one motor.

Figure 3.3. Pololu DRV8833 H-bridge.

3.3.2 Ultrasonic sensor

The omni wheel robot will use the ultrasonic sensor HC-SR04. It has a minimumrange of 2 cm and maximum range of 400 cm with the accuracy of 3 mm. Thesensor need 5 V to operate functionally. The sensors detection angle is 15°[11]. Theright cylinder in figure 3.4 is the speaker which sends out a pulse that echoes backto microphone to the left if there is an object in front of it. The distance can becalculated from formula 2.1.

10

3.3. ELECTRONICS

Figure 3.4. Ultrasonic sensor (HC-SR04)

3.3.3 IR-senor

The IR-sensor (figure 3.5) can detect objects between 2 cm to 30 cm. The detectionangle is 35°. It can be powered with both 3.3 V and 5 V directly from the micro-controller [12]. The sensor is digital so it output low (0) when it detect somethingand high (1) when it is not.

Figure 3.5. The IR-sensor used in the project

3.3.4 Microcontroller

The microcontroller is the Arduino UNO (Figure 3.6). It is powered by a 9 Vbattery.

Figure 3.6. Arduino UNO

11

CHAPTER 3. DEMONSTRATOR

3.4 Hardware

The omni wheels in this project is the "48mm Omniwheel for LEGO NXT" madeby Nexus Robot (figure 3.7). They have a load capacity of 2 kg [13].

Figure 3.7. Omniwheel for LEGO NXT

The connection of the hardware can be seen in figure 3.8. All the componentsare powered by a 9 V battery. The battery is connected to an Arduino UNO whichis connected to the H-bridges and the sensors.

Figure 3.8. Shows how the hardware is connected. All of components is poweredfrom the 9 V battery which is connected to the UNO.

12

3.4. HARDWARE

The robots frame is made from plywood and it has two floors (figure 3.9). Thesize of the robot were kept down so the sensors would be able to cover 360°. Themotors are attached to the lower floor on the underside with cable ties. The batteryare also placed on the lower floor so the center of gravity will be held low. All ofthe electronics are on second floor.

Figure 3.9. The omni wheel robot

13

CHAPTER 3. DEMONSTRATOR

3.5 ResultsThe first test tested how small gaps the robot could drive trough (see figure 3.10).Both types of sensors were used in this test. The result is presented i table 3.2.

Figure 3.10. Test 1 tested the smallest gap the robot could drive through.

x (cm) d (cm) Result Comment40 20 Ok35 20 Ok30 20 No IR-sensor detects the walls

Table 3.2. Result from test 1. x is the distance between the walls. d is the shortestdistance the ultrasonic sensors will react to objects right in front of it.

None of the sensors could see anything but objects that are at the same heightas the sensor it self.

3.5.1 Ultrasonic sensor testWhen the robot only used the ultrasonic sensors it drove without crashing as longas the objects were wider than 3 mm and more than 5 cm away, see table 3.3 formore results. 360° viewing angle was not possible to get but every long side gotcovered when the viewing distance were set to 40 cm.

Object width (mm) Distance (cm)3 58 1112 1421 20

Table 3.3. Result from ultrasonic sensor test

14

3.5. RESULTS

3.5.2 IR-sensor testWith only the ir-sensors it was not possible to avoid obstacles. As seen in table3.4 the maximum distance were 15 cm but it changes a lot. Sometimes it did notdetect anything. The viewing angle were 0°.

Object width (mm) Distance (cm)3 712 1021 15Table 3.4. Result from the ir-sensor test

15

Chapter 4

Discussion and conclusions

4.1 Discussion

In the end the robot was almost working the way the thought was in the beginning,though it had some problems. The biggest problem was the frequency of the PWM-signals in two of the PWM ports and the blind spots, where the sensors could notsee. Besides of those problems the robot was working quite good.

The robot had trouble with blind spots, there the sensors can not see. The firstidea was to only use three Ultrasonic sensors, but once tested the Ultrasonic sensorswas not enough. The robot crashed into walls and chairs with the corners of therobot, where the Ultrasonic sensors can not see. Then IR-sensors was used in thecorners of the robot to solve the blind spots. That made the robot crash a bit lessinto walls and chairs, but there was still some blind spots left which led to somecrashing still happening though it was less than with only Ultrasonic sensors. Thatmade both the Ultrasonic and IR-sensors to be the choice of sensors for this project.

Besides the blind spots the IR-sensors sometimes had problem with the light inthe room. If the sun beamed in through the window and the robot was in thosebeams the IR-sensors sometimes reacted to the light like there was an object in theway. Because of that the robot could mostly be driven in spaces where there onlywas light from lamps and not from the window.

Another problem with the robot is that one of the wheels has a lower speedthan the other two. That is because of the PWM-signals. The wheel that rotatesslower is connected to two PWM ports that has another frequency than the PWMports that the other two wheels are connected to. That can be solved by changingthe two H-bridges to one H-bridge that only need one PWM-signal instead of two.That way all the three wheels can be connected to a PWM port with the samefrequency and that way they will rotate with the same speed. This robot has twoH-bridges and two PWM-signals per motor. That is because of that the time wasto tight to be able to change the H-bridges and the coupling on the robot and at

17

CHAPTER 4. DISCUSSION AND CONCLUSIONS

the same time make sure to have a working robot at the end. That way the robotwas left with the two H-bridges.

4.2 ConclusionsWith just ultrasonic senors the robot will drive without crashing if objects appearstraight in front of the sensors. Driving with only IR-sensors did not work at all.The sensors did not detect object further than 15 cm away from the sensor and notat all if they were not straight in front of the sensor. With both of the sensor, seefigure 4.1, the result were similar to when only the ultrasonic sensors were used.Even if the size of the robot were kept down 360°viewing angle was impossible.

Figure 4.1. The green fields shows were the sensors reacted on objects. The redfields shows the blind spots.

18

Chapter 5

Recommendations and future work

5.1 Recommendations

5.1.1 SensorsThe combination of ultrasonic sensors and IR-sensors did not worked for 360° view-ing angle. For that two ultrasonic sensors attached to two servos that can rotate180° would be better. The IR-sensor could work if the robot would follow a darkline on the floor but not as an obstacle avoiding sensor. Using six ultrasonic sensorscould also work.

5.2 Future workTo improve the viewing angle the ultrasonic sensors would be placed on the corners,over the wheels, instead of between the wheels. Changing h-bridges to one that onlyneed one pwm signal per motor.

19

Bibliography

[1] Makezine. http://makezine.com/projects/make-40/kiwi/

[2] Society of Robots. http://www.societyofrobots.com/robot_omni_wheel.shtml 2016-03-10

[3] Nationalencyklopedin. http://www.ne.se/uppslagsverk/encyklopedi/lång/ultraljud 2016-03-09

[4] Tony Burden. Termodynamik med kompressibel strömmning. Stockholms Uni-versitet 2011

[5] maxEmbedded. http://maxembedded.com/2011/06/sensor-fundamentals/2016-05-06

[6] Carnige Mellon’s Robotic Academy. http://education.rec.ri.cmu.edu/content/electronics/boe/ir_sensor/1.html 2016-06-01

[7] Open Electronic. http://www.open-electronics.org/how-to-build-an-omni-wheels-robot/ 2013-05-08

[8] Hans Johansson. Elektroteknik. KTH 2013

[9] Chuck McManis. http://www.mcmanis.com/chuck/robotics/tutorial/h-bridge/ 2016-06-04

[10] Pololu. https://www.pololu.com/file/0J534/drv8833.pdf 2016-05-07

[11] Elec Freaks. http://www.micropik.com/PDF/HCSR04.pdf/ 2016-05-07

[12] Ebay seller czb6721960. http://www.ebay.com/itm/131356972026?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT 2016-05-07

[13] Robotshop. http://www.robotshop.com/en/48mm-omniwheel-lego-nxt-servo.html 2016-05-07

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TRITA MMKB 2016:39 MDAB100

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