high mobility, tracked robot

8/6/2019 High Mobility, Tracked Robot

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AM24 Enhanced Mobility Robot

Submitted byLim Sheng Jun (U036181M)

Department of Mechanical Engineering

In partial fulfillment of the requirements for the Degree ofBachelor of Engineering

National University of Singapore

Session 2006/2007


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SUMMARY

The objective of this project is to explore the feasibility of designing a robot that uses

a novel track mobility system that is able to overcome obstacles that would stop a

normal robot of similar dimensions. The robot would have to be able to overcome an

obstacle identical to its storage height and cross a gap 60% of its length in storage.

This project is held in collaboration with DSTA with the additional intention of

providing soldiers a means of surveying possibly dangerous environments at a safe

distance with the use of an obstacle crossing capable, man portable robotic platform.

In order to design the robot, a short literature research had to be done on present

robots that were used by various military groups to survey possible hostile

environments. At the initiation of the project, a set of criteria was stated for the basis

of this project; in addition, some additional criteria had to be determined before the

design stage. Following this, types of major components that could possibly be used

in the design were compared and examined. An analysis was then carried out in order

to select the appropriate parts to be used. The robots motors had to have enough

torque to overcome the obstacles and its centre of gravity had to be optimized for it to

cross gaps. Finally, sketches of the design concept were done on Solidworks.

The final design met the criteria set before it. The design is also easy to fabricate,

using common materials that can be bought of the shelf in already pre-formed shapes.

The unique feature of this design is the use of swing arms on the front of the robot.


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When not in used to climb obstacles, the arms can be left limp and can help guide

the robot over terrain which would otherwise hamper a robot of similar ground

clearance. They also serve as dampers when going down steps and reduce the amount

of impact on the robots chassis.

After working out the details in the initial stages, fabrication then commenced. Only

general tools and machines were used in the manufacture of the robot. This allowed

money to be saved as it would be costly to machine the parts of a one-off robot.

Once fabrication was completed, tests were run to see if the robot passed the basic

criteria set in the initial stages. Other tests were also performed to determine the

robots performance such as the maximum incline it can climb as well as the

transmission range of the radio controller and the transmitter range of the on-board

camera.

In all, the robot passes the basic criteria set forth to it easily. Some of

recommendations for improvements could be to increase the ground clearance of the

robot to unable it to clear obstacles more easily. Another recommendation would be

to use lighter aluminum wheels to save total weight as the solid ones are rather heavy.

More studies and tests have to be carried out to better understand the performance of

the robot in a real world scenario.


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ACKNOWLEDGEMENTS

Firstly, the student would like to express his heartfelt gratitude to his supervisor,

A/Prof. Gerard S.B. Leng for his warm personality, advice, guidance and assistance

through out this project. Without his valuable insights and advice, life during the

project would definitely be harder.

Next, the student would like to thank the staff of the Dynamics and Vibration lab, Mr

Cheng, Mr Ahmad, Ms Priscilla and Miss Amy for their patience and assistance in

helping out with administrative matters. The student would especially like to thank

Mr Cheng and Mr Ahmad for their guidance, fabrication help and help in the use of

lab machinery as well as helpful insights that have definitely made the fabrication

easier.

The student is also grateful to his fellow students in the same lab. He will not forget

the generosity and kindness that Eugene, Ali, Chann and Clement have given him

during the course of the project. In the course of this project, they have also become

this students good friends.

Also, the student is grateful to be working in collaboration with DSTA and Singapore

Combat Engineers. Working with CPT Teng, LTA Yap and LTA Zhu has given new

insight on what it feels to be a working engineer. This experience shall prove to be

valuable in future.


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Lastly, the student would like to thank his parents for their faith and love that they

have given him and the financial support to put him through many years of education.


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TABLE OF CONTENTS

SUMMARY i

ACKNOWLEDGEMENTS iii

TABLE OF CONTENTS v

LIST OF FIGURES ix

LIST OF TABLES xi

LIST OF SYMBOLS xii

1. INTRODUCTION 1

2. LITERATURE SURVEY 3

3. DESIGN OF ROBOT 5

3.1 Fundamental Criteria Robot Must Meet 5

3.2 Design Conceptualization 6

3.3 The Final Design 9

3.3.1 Most Advantageous Feature of the Final Design 9

3.3.2 Specifications of Final Design 10

4. FABRICATION OF PLATFORM 11

5. EXPERIMENTAL PROCEDURES AND RESULTS 12

5.1 Actual Specifications of Final Prototype 12

5.1.1 Weight 12

5.1.2 Velocity 12


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5.1.3 Additional Payload 13

5.2 Performance in Gap Crossing 13

5.3 Performance in Obstacle Clearing 14

5.4 Performance in Incline Climbing 15

6. GENERAL COMPONENTS 17

6.1 Main Components 17

6.1.1 Type of Control System 17

6.1.2 Type of Motors 18

6.1.3 Wheels or Tank Tracks 19

6.1.4 Power Source 20

6.2 Sizing up the Driving Motors 20

6.2.1 Estimated Maximum Mass of Robot 20

6.2.2 Type of Tracks and Wheels 21

6.2.3 Torque 21

6.2.4 Velocity 22

6.2.5 Criteria for Driving Motors 22

6.3 Sizing up the length of the Swing-arms 23

6.4 Sizing up the Swing-arm Motors 23

6.4.1 Torque 23

7. DISCUSSION 25

7.1 Actual Specifications of Final Prototype 25

7.2 Performance in Gap Crossing 26


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7.3 Performance in Obstacle Clearing 26

8. COST ANALYSIS 28

9. CONCLUSION 30

10. RECOMMENDATIONS 31

11. LIST OF REFERENCES 33

APPENDICES A

A.Specifications of Literature Survey Robots A

A.1 iRobots Packbot A

A.2 Securitas B

A.3 Tracked Reconfigurable Modular Robot C

B.Estimated Maximum Mass of EMR Calculations D

C.Coefficient of Friction between concrete and E

polyurethane tracks

D.Required Torque and Velocity for Driving Motors G

Calculations

E.Sizing Up Swing Arm Motor Calculations J

F. Specifications of Individual Components used K

F.1 Driving Motors K

F.2 Swing Arm Servos M

F.3 Radio Control System N


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F.4 Electronic Speed Controller P

F.5 Batteries Q

G.Experimental Data on the EMRs Velocity R


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LIST OF FIGURES

1. Example of Flipper Design used in Packbot

2. The Vertical Obstacle Problem

3. The Gap Crossing Problem

4. Variable Track Geometry System

5. CAD drawing of the Final Chassis design (without timing belts)

6. Picture of the completed prototype

7. Photos of Gap Crossing Test

8. Photos of Obstacle Clearing

9. The Process and Results of the Incline Climbing Test

A.1 iRobots Packbot

A.2 The Securitas Robot

A.3 Different Configurations of Tracked Reconfigurable Modular Robot

C Forces Acting on a Block on an incline

D Factors of the EMRs Power Requirements

E Factors of the EMRs servo requirements

F.1A Picture of Motor attached to Reduction Gearbox

F.1B Motor Performance Simulation from Manufacturers Website

F.1C Motor Dimensions from Manufacturers Website

F.2 Picture of Swing Arm Servo attached to Swing Arm

F.3 Pictures of Optic 6 and Electron 6 Transmitter and Receiver

F.4 Picture of Tamiya TEU-101BK Speed Controller


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F.5 Specifications of Battery from Manufacturers Website


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LIST OF TABLES

1. Specification of the EMR

2. Selection of type of Control System

3. Selection of Types of Motors

4. Wheels or Tank Tracks

5. Actual Specifications versus Estimated Specifications

C Results of Coefficient of Friction Test

G Experimental Data on EMRs Velocity


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LIST OF SYMBOLS

F: Force

Ff: Frictional force

g: Gravitational acceleration

M: Mass

R: Reaction Force

v: Velocity

: Angle of incline

: Static coefficient of Friction


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1. INTRODUCTION

In this day and age a greater emphasis has been placed on the safety and well-being

of soldiers. No longer are soldiers being put to unnecessary risks. Trainings and

exercises are carried out in controlled environments where by the risks or injury and

death are greatly minimized. This day and age is also one where weapons are getting

increasingly powerful and harder to detect. Examples of this include plastic anti-

personnel mines which cannot be detected using standard mine detecting equipment.

Hence with the advancement in technology, man has been starting to develop

machines to explore places that would be otherwise dangerous for him.

Presently, there are many robots on the market that do surveillance and monitoring

for soldiers. However, these robots tend to be heavy and bulky, making it difficult for

a single man to carry. Likewise, there are small-sized robots that exist, but due to

their small size; they are unable to overcome obstacles that a larger robot would have

no problem crossing. From these two points, we come to the main aim of this project

that is to design a small, light and relatively inexpensive robot that has mobility

superior to that of a typical similarly sized robot.

The benefits of such a robot are unquestionable. With the robot being easily man-

portable, rapid deployment of the robot can be carried out. Also the low relative costs

of it compared to robots of similar function would allow it to be mass deployed on

many fronts. The concept sounds easy enough but along the way, unanticipated


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problems could arise. More tests and studies would also have to be carried out on the

prototype to enhance it to make it more efficient in doing its tasks.

This report is written detailing the design, fabrication and testing procedures of the

prototype. Problems that arose would be addressed with an accompanying possible

solution. The report will then conclude with recommendations and a conclusion.


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2. LITERATURE SURVEY

The Enhanced Mobility Robot is a brand new final year project that is being offered

for the first time. As a result, there is absolutely no background material on it. As

such a robot is usually used for military service, public information on existing robots

are hard to find unless declassified, hence it is difficult to find detailed information on

this subject. A literature survey was conducted on various robots that would have a

similar function to the Enhanced Mobility Robot. The robots surveyed were iRobots

Packbot, the Securitas robot and the Tracked Reconfigurable Modular Robot. All

three robots utilize unique track mobility systems.

The Enhanced Mobility Robot is intended to be carried and deployed readily by a

single soldier. The closest robot that could be found having similar function and

capabilities is iRobots Packbot which is currently in service with the United States

military. The Packbot has seen active duty in Afghanistan and Iraq to great success as

an ordnance disposal robot as well as a scout robot.

The Packbot weighs in at 18kg, is less that 20cm in height and uses a patented flipper

design. Its capabilities include but not limited to, being 2m submersible in water,

having a maximum climbing slope of 60, ascending and descending staircases and

also wireless digital communications. The Packbot is an impressive robot however it

is rather large and heavy to be man-portable and it is also costly and reported to have

a cost of over SGD$10000 for each unit.


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The Securitas robot is a robot meant for the handling of hazardous materials. It is

large in sized but uses a unique 4 tracked system to overcome obstacles whereas the

Tracked Reconfigurable Modular Robot uses many instances, 2 or 3 links, of itself to

overcome obstacles. See Appendix A for more details on the various robots.

iRobots Packbot has size and function, compared to the other two robots, similar to

that as of the Enhanced Mobility Robot, hence Packbot will be used as a close

reference in the development of the Enhanced Mobility Robot.


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3. DESIGN OF ROBOT

3.1 Fundamental Criteria Robot Must Meet

The Enhanced Mobility Robot (EMR) will be operated in a fairly civilized area where

small steps and small drains are a common feature. The fundamental criteria of the

robot have been laid out right from the beginning. Below are the objectives the robot

is supposed to fulfill:

1) The robots footprint must be no bigger than 40cm x 30cm x 10cm in

storage.

2) The robot shall come with a wireless remote controller and optionally a

wireless video and remote display system.

3) The robot must overcome a gap of 24cm. i.e. 60% of its length.

4) The robot must overcome a vertical obstacle of 10cm i.e. 100% of its

height.

5) The use of a flipper design is prohibited. See Figure 1 for an example of

the flipper design.


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Figure 1: Example of Flipper Design used in Packbot

The robot will be designed and fabricated according to these criterions. General

performance will also be measured and tested using these criterions as the minimal

guideline.

3.2 Design Conceptualization

Once the criteria have been laid out, a design concept can be decided on to tackle the

problems of crossing the 24cm gap and 10 cm height while keeping the storage size

constraint in mind.

Flipper Track

System


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Figure 2: The Vertical Obstacle Problem

Figure 3: The Gap Crossing Problem

As seen from Figure 2 and 3, a robot with its centre of gravity at the middle of its

body will not be able to cross a gap more than half the length of its total length. And

also, it will not be able to climb up a vertical surface as high as its height as there

would not be enough traction.

Direction ofmotion Height

of robot= y cm

Height ofobstacle~ x cm

Length of

robot = 2x cm

C.G.

Crossing Failed


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Figure 4: Variable Track Geometry System

In order to overcome these two obstacles the robot design shall incorporate a variable

track geometry system to allow the tracks to be longer when crossing gaps, higher

when climbing vertical obstacles and more compact when in storage. The shape

manipulation of the track will be possible with the use of two swing arms either side

of the robot. This design could also be used with just wheels alone and without tracks.

The robot design shall be about 50cm long with its swing arms fully extended and

about 15cm high when it is climbing mode. This design is similar to the flipper

design used in Packbot but instead of using a separate track attached to the main track

like in Packbot, the shape of the main track system in this design changes.

With the basic design concept in place, it is now possible to evaluate the various

components that will be used to build the prototype. Off-the-shelf components will be

used to a large extent as they are relatively inexpensive compared to custom made

components. Purchasing from online retailers was not considered as it would be time

consuming to wait for delivery and not to mention more expensive and logistically

tedious.


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3.3 The Final Design

After sizing up the motors for the driving wheels, servos for the swing arms and also

selected the suitable components which will be discussed later, the whole concept can

be conceptualized in the final design as shown in Figure 5.

Figure 5: A CAD drawing of the Final Chassis design (without timing belts)

3.3.1 Most Advantageous Feature of the Final Design

The feature that sets the EMR apart from other robots is its variable track geometry

system with its swing arm design. Its advantages are as listed.

1) Allows the EMR to cross both gaps and vertical obstacles easier.


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2) The velocity at which the swing arms can swing allows the EMR to throw

itself over vertical obstacles, enhancing its vertical obstacle clearing

performance.

3) The swing arms when not in operation help the EMR negotiate uneven terrain.

4) The swing arms can swing down to lift the body of the EMR up. This can help

free the EMR from an obstacle if its body gets stuck on it.

3.3.2 Specifications of Final Design

The following specifications are what are expected of the final design. They are

estimated and based on information from catalogues. The information on how these

specifications were obtained can be found in the later sections.

Table 1: Specification of the EMR

General Specifications Remarks

Dimensions 400mm(L) x 300mm(B) x 100mm(H) Size constraintcriteria

Mass 6kg fully loaded Estimatedmaximum mass isused here

Speed 3 km/h

Type ofControl

Radio ControlRange: 1km maximum

Type of

steering

Differential One motor to

power each sidePower Source 7.2V R/C battery packs

For detailed information on the specific components, please go through Appendix F.


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4. FABRICATION OF PLATFORM

After the design has been thought out, fabrication can then be initiated. The design is

made out of mainly aluminum plates, bars and L-plates as they are readily available,

relatively cheap, have adequate strength and are easy to machine and bend for the

EMRs purpose. Only general machine tools such as the bench saw, bench drill and

turning machine for manufacturing the chassis were used. However, due to a lack of

work shop experience, the fabrication process of the chassis was time consuming.

Besides the self manufacture of the chassis, the aluminum timing pulleys and

polyurethane belts were subcontracted out. All the manufacturing activities were

conducted in the dynamics and vibrations labs workshop.

Figure 6: Picture of the completed prototype


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5. EXPERIMENTAL PROCEDURES AND RESULTS

Once fabrication has been completed, several tests have to be carried out to see if the

prototype EMR meets the criteria that were initially set. These tests will be used to

gauge the following:

1) The actual specifications compared to the estimated specifications

2) Does the EMR manage to cross a gap 24cm wide?

3) Does the EMR climb a vertical obstacle 10cm high?

4) How well does the EMR perform?

5.1 Actual Specifications of Final Prototype

5.1.1 Weight

The weight of the EMR was measured using a digital weighing scale with accuracy of

1g. The weight of the EMR is 4.830kg including the batteries.

5.1.2 Velocity

The velocity of the EMR was measured by measuring the time it took to travel a

distance of 6m with a running start. A stopwatch was started when the EMR crossed

the start line and stopped when it crossed the finish line. This was repeated several


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times and the average velocity calculated. The EMRs maximum velocity was

0.71m/s or 2.5 km/h which almost the same speed as a humans slow walking speed.

However, the actual value falls short of the predicted one but is still acceptable as it

does not differ from the value very significantly. Furthermore, the speed of the EMR

was not a very crucial factor in design. See appendix G for experimental data.

5.1.3 Additional Payload

The EMRs capability of carrying extra loads was carried out by slowly adding

additional weights onto its chassis. The EMR moved easily with an additional weight

of 5kg on it on a horizontal flat surface. However, more weights and the addition of

weights on inclines were not tested for fear of possibly damaging the EMR.

5.2 Performance in Gap Crossing

The EMRs capability of crossing gaps was tested by simulating a gap of 24cm. This

was done by using a trolley and table of similar height and leaving the trolley 24cm

from the table. The length of the gap was measured using a ruler. Weights were used

to secure the trolleys legs so as to restrict its motion during the test. The test was

carried out 5 times. The process and result of the test can be seen from Figure 7. With

a short run up, and locking the swing arm servos in place, the EMR manages to cross

the 24cm gap without falling into it 4 out of 5 times. The only time when the EMR

fell into the gap was when the operator forgot to lock the swing arms in the horizontal


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position. Using the time markers on the video while taping the results of the

experiment, it was found that the EMR took about 0.7s to cross the gap, from the time

the front of the EMR left the surface of the trolley till when the rear of the EMR

crossed the ledge of the table.

Figure 7: Photos of Gap Crossing Test

5.3 Performance in Obstacle Clearing

The obstacle clearing test was carried out by using a horizontal aluminum bar of a

table that was whose top surface was 11cm above the ground as the obstacle. A ruler

was used to measure the distance of the top of the bar to the floor. Figure 8 shows the

process and result of the test. In order to pass this test, the EMR needed to make full


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ramp. Rubber matting was place over the ramp the increase the traction of the EMR

as the test was to test if the EMR had enough torque at the wheels to allow it to climb

the slope. Figure 9 shows the process and results of the test conducted at a 30, 40

and 45. The 45 slope was the steepest inclined tested as the EMR did not have

enough traction with the rubber matting at steeper inclines. As shown from Figure 9,

the EMR manages to climb up the 45 slope easily. The EMR took 5seconds to cover

a distance of 1m on the ramp; however, this was not a true gauge of the EMRs

climbing ability as full throttle was not utilized for fear of losing traction and hence

control on the slope and possibly damaging the EMR.

Figure 9: The Process and Results of the Incline Climbing Test


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6. GENERAL COMPONENTS

6.1 Main Components

6.1.1 Type of Control System

Table 2: Selection of type of Control System

Type of Control

System

Advantages Disadvantages Remarks Selected

Wired Reliable control Limited rangelimited by lengthof control wire

Adds weight tooverall system

PossibleEntanglement

Radio Control(R/C)

Long range

Easy toimplement

Entanglement

free

PC Control Same as R/CControl

Videoacquisition andcontrols can beintegrated

Harder and moretedious toimplement.

Additionalprocessor needsto be installed

Can becontrolledwirelessly bycomputer orusing a add-on R/Cmodule

Compared to a PC control system which can manage both controls and the

acquisition of video at the same time, the radio control system seems inferior.

However, the PC control system requires the additional installation of a


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microprocessor as well as the need to create a custom program for the robot.

Therefore the radio control system is chosen for its ease of installation and usage.

6.1.2 Type of Motors

Table 3: Selection of Types of Motors

Type of Motors Advantages Disadvantages Remarks Selected

PermanentMagnet Direct

Current

(PMDC)

High number ofrevolutions perminute (4000-

15000rpms)

Require Gearboxes toreduce angularvelocity

Electronic SpeedController Required

Cylindrical shapemakes mountingdifficult

Servo Motor Good Torque andeasy of control

Plugs directly into

radio receiver

Rectangularcasing withmounts makesmounting easy.

Compact andlight package

Rotational speed toolow to drive wheels

Fixed range of

rotation (60-180)

Modificationrequired toallowcontinuous

motion

For this project, both servo motors and PMDC motors will be used. DC motors will

be used for driving the main wheels as they provide adequate speed and torque which

can be tailored to match specifications using a reduction gearbox. Servo motors are

not chosen for this purpose as they spin too slowly although they have various


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advantages over PMDC motors. The servos on the other hand, will be use for the

control of the swing arm in the robot as they spin slower and delicate control is easier.

Also, the torque produced falls into the required range for the swing arms and they

are easy to mount.

6.1.3 Wheels or Tank Tracks

Table 4: Wheels or Tank Tracks

Wheels or

Tank Tracks

Advantages Disadvantages Remarks Selected

Wheels Easy installation

Lower motortorque needed todrive

Less traction Easilyavailable

Tank Tracks More traction

Proven on evensurfaces

More powerfulmotors neededto drive

Track

dislodging fromwheels could bea problem

Difficult tofind andlimitedvarietylocally

The nature of the robot will be climbing and crossing obstacles, hence the choice of

tank tracks. Tank tracks allow a much large contact surface with the ground

compared to wheels, hence a robot with tracks would have better maneuverability on

uneven terrain.


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6.1.4 Power Source

Since a radio control system has been decided to be used, standard 7.2V battery packs

frequently used in radio control cars will be used. Parts like the radio receiver that

require a 6V voltage will be powered by the same battery packs using a voltage

regulator. A 12V battery will be used to power the camera and video transmitter

6.2 Sizing up the Driving Motors

After the fundamental components of the robot have been decided, what is left will be

to choose the correct component from catalogs or from searching at stores. The

characteristics required of the motor being used have to be determined before a

choice and purchase can be made.

The main criteria for choosing motors are its torque. In order to calculate the required

torque for the motors, the maximum mass of the robot must be estimated.

6.2.1 Estimated Maximum Mass of Robot

For the estimated maximum mass of the robot, the size and mass of iRobots Packbot

have been taken as a guideline. The rational behind this being that the Enhanced

Mobility Robot (EMR) and Packbot have similar purpose with the sole main

difference being size. Packbot has the dimensions 87.9 cm (L) x 51.4cm x (B) 17.8cm

(H) and EMR has maximum dimensions of 40cm (L) x 30cm (B) x 10cm (H). The


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volume ratio between Packbot and EMR is roughly 6.7:1. Since Packbots maximum

mass with a full payload is 18kg by proportionality or volume, EMR should have a

mass of 2.7kg. However some safety factor should be added to this number, hence a

safety factor of 2 is employed and this number rounded up. Hence the estimated

maximum mass of the EMR is 6kg which is about a third of Packbots weight. The

robots mass of 6kg is also a reasonable amount of load that a single soldier could

carry. See appendix B for calculations.

6.2.2 Type of Tracks and Wheels

For the tracks and wheels of the robot, aluminum timing pulleys and double sided

polyurethane timing belts will be used. The rationale behind this being that

polyurethane tracks provide good traction and are wear resistant. The aluminum

pulleys are used to mate with the belts. The size of the drive wheels is decided to be

5cm in diameter. This size is suitable for the EMRs swing arm design. As the EMR

will be traversing mainly on concrete areas, the coefficient of the tracks on concrete

is tested. See appendix C for calculations on coefficient of the polyurethane tracks on

concrete.

6.2.3 Torque

The torque of the driving motors must be estimated in order to make sure that the

robot is able to cross an obstacle of 10cm in height. Total torque required for robot to


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move up obstacle =1.617Nm = 16.40 kg cm. Since there are two driving motors, each

motor needs to produce 8.24 kg cm of torque. However a safety factor needs to be

added due to internal friction as well as the friction in the timing belt system. A safety

factor of 1.5 shall be multiplied resulting in a required torque of 12.36 kg cm of

torque for each motor.See appendix D for calculations

6.2.4 Velocity

There has been no speed criteria set for the EMR. However, the EMR must be fast

enough to be of good mobility and yet slow enough to allow precise positioning and

control by a human operator. Hence the EMRs velocity should be comparable to that

of a walking human which is estimated to be around 4 to 5 km/h. Assuming the EMR

could move at a maximum of 3km/h, which is a slow walking velocity, the estimated

velocity at which the EMRs motors need to spin at was 10000rpm. See appendix D

for calculations.

6.2.5 Criteria for Driving Motors

From the above, we have a motor torque requirement of 12.36kg cm for each motor

and also estimate that the motor will give the EMR a maximum velocity of 3km/h.


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6.3 Sizing up the length of the Swing-arms

The swing arm of the EMR needs to long enough to allow it to extend its height

adequately and allow it to engage obstacles of 10cm high easily but short enough to

allow the EMR to be kept within its storage dimensions. The length of the swing arms

from its point of pivoting to the extreme end that would engage the obstacle is

0.175m. This length of the swing arms was obtained from creating various

configurations in Solidworks and seeing if the size of the swing arms would fit the

size criteria as well as being long enough.

6.4 Sizing up the Swing-arm Motors

The main criterion for the swing-arm motors is the torque. With the maximum

estimated mass of the robot already identified, we can go on to calculating the torque

needed for the motors to raise the robot on to a vertical obstacle.

6.4.1 Torque

The torque required to straighten the swing arms is calculated to be 5.04N m or 51.4

kg cm. Two servos, one for each side of the robot will be used to power the swing

arms. Hence the amount of torque required for each servo is 25.7kg cm. As servo

torques are already rated by their manufacturer and are reliable figures, there will not


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be a need to add a safety factor to the amount of torque required.See Appendix E for

calculations.


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7. DISCUSSION

7.1 Actual Specifications of Final Prototype

Table5: Actual Specifications versus Estimated Specifications

Specifications

Predetermined Actual

Remarks

Dimensions 400mm(L) x300mm(B) x100mm(H)

392mm(L) x298mm(B) x100mm(H)

Mass 6kg 4.83kg 6kg is the valueused for motorrequirements.

Velocity 3 km/h 2.5 km/h

MaximumClimbing

Incline

Not Applicable 45 Additional test tomeasure the

EMRs climbingability.

From the Table 5, the actual dimensions of the EMR prototype meet the requirements

of the size criteria.

Also the actual mass of the EMR is lighter than the maximum estimated mass of 6 kg.

A lighter body mass of the EMR will decrease the strain on the motors and lower the

consumption of battery power. This also allows the EMR to climb obstacles more

easily. Moreover, this allows the EMR to carry more payloads.

The actual velocity of the EMR is lower than that of the estimated maximum velocity

but not too far off. This could be due to the fact that 7.2V batteries are being used

when the motor has a higher voltage rating. The number of revolutions per min

increases with the applied voltage; hence if the estimated maximum velocity of the


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EMR is to be met, battery packs with a higher voltage have to be used. This figure

can also be met by using a gearbox with a lower gear reduction ratio; however, this

comes at the cost of lower torque at the drive wheels.

7.2 Performance in Gap Crossing

The EMR manages to cross the 24cm gap requirement. However, it is still possible

for the EMR to fall into the gap 24cm wide. This can happen if the operator of the

EMR is not careful. If the EMR approaches the gap at an angle other than completely

perpendicular to it, it might be possible for it to fall in. Also if the EMR operator does

not fully extend the swing arms and lock them in place, the same thing could happen.

Therefore the EMRs performance in gap crossing performance can be said to be

partially dependent on the skill of the EMR operator.

7.3 Performance in Obstacle Clearing

The EMR manages to meet the requirement of overcoming an obstacle of 10cm in

height. The EMR manages to clear the height obstacle rather easily most of the time;

however, there were some problems during some of the trial runs. During a few of the

trial runs, the EMR was left stuck on the horizontal bar. The part where the EMR was

stuck on was the length on its chassis where there was no track system. Even though

the EMR was perched on the horizontal bar, the EMR was still able to progress over

the bar by swinging its swing arms. The momentum created by the swing arms


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unsettled the EMR and allowed the tracks to grip onto the horizontal bar and

transverse successfully. This problem is partially due to the type of obstacle. Had the

obstacle been a normal concrete step, the EMR would be able to use its front tracks to

pull itself over the edge.


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8. COST ANALYSIS

Raw Material Components

Item (length) Unit Cost ($) Quantity Subtotal

($)

Remarks

Aluminum L-plate1.5mm thickness (6m)

16 1 16

Aluminum plate 3mmthickness (6m)

23 1 23

Bolts, Screws, Nuts 29 1 29

Steel bar 10mmdiameter (30cm)

10 1 10 Estimated asobtained fromthe workshop

Aluminum bar 10mmdiameter (60cm)

6 1 6

50mm diameteraluminum pulley35mm thicknesswithout flange

70 8 560

410mm x 30mm belt(double sided)

45 2 90

550 x 30mm belt(double sided)

60 2 120

25mm diameteraluminum pulley21.5mm thicknesswith flange

34 4 136

575mm x 16mm belt(single sided)

16.20 2 32.40

Needle roller bearings10 x 14 x 10mm

3.60 20 72

Hitec HSR 5995TGServo

166.50 2 333

Hitec Optic 6

Transmitter andreceiver combo

405.70 1 405.70

Sanyo 3600mAh 7.2VBattery Pack

90 2 180

Tamiya TEU-101BKelectronic speedcontroller

58.50 2 117


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Servo switches, servohorns and wires

73 1 73

Align 6V voltageregulator

28.90 1 28.90

RS- 540 motor 19 2 3830:1 ReductionGearbox

20.70 2 41.40

Sub Total: 2311.40

Labour Cost $5/hr 200hr 1000

Total: $3311.40

Total Cost of the EMR is $3311.40 without factoring the costs of the camera to be

mounted on it. Also, the price of the EMR can be decreased if it is mass produces as

parts like the aluminum timing pulleys and some of the timing pulleys had to be

custom fabricated. This added to a substantial part of the costs.


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9. CONCLUSION

1) The EMR has performed well on both the gap crossing and vertical obstacle

crossing tests. As it has performed well in these two tests well, it means that

the EMR has satisfied the performance criteria that were presented at the

initial point of the project. However it falls slightly short of the projected

maximum speed that it should move at.

2) The EMR is not only able to accomplish the performance objectives but also

remain within the size constraint criteria.

3) There were some problems that surfaced during testing such as the EMR

getting stuck on the horizontal bar during the vertical test and also the

possibility that the EMR operator forgets to lock the swing arms in place.

Another problem that could arise is that the EMR has a low ground clearance.

4) The cost of the EMR can be further reduced if items were purchased in bulk

and certain components replaced with other more practical ones.


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10. RECOMMENDATIONS

The following are some recommendations that may be worth examining should the

project be continued.

1) The tracks system of the EMR is rather heavy. The aluminum timing pulleys

are solid and having so many of them on the EMR adds to quite a substantial

weight of the system. The pulleys should be replaced with spoked or hollow

timing pulley designs to reduce weight or they could be replaced with

alternative materials such as plastics. If the weight of the track system can be

reduced, the EMR will be able to carry more pay load and also reduce its

power consumption.

2) The ground clearance of the EMR should be increased as much as possible.

The EMR has no problems running on flat grounds when all the performance

tests were conducted, however, if the tests were conducted on uneven terrain

with a lot of sand and gravel, it would be possible that the EMR would get

stuck and not be able to complete the test.

3) Another point to improve on the EMR is to make its size constraint slightly

longer. If this is done, it can allow the EMR to have longer swing arms. The

consequence of this, being that the EMR will be able to climb higher obstacles


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such as normal stair steps which the present EMR design does not. The EMR

prototype can only traverse small steps for the time being.

4) If possible the track system should run along the entire length of the EMR,

providing it with a driving contact area with the ground at all times so that it

can traverse uneven terrain more easily.


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11. LIST OF REFERENCES

1. Chen, C. X., and Mohan M. T., Reactive Locomotion Control of Articulated-

Tracked Mobile Robots for Obstacle Negotiation, Proceedings of the 1993

IEEE/RSJ International Conference on Intelligent Robots and Systems,

Yokohama, 1993, pp. 1349- 1356.

2. Romiti, A., and Raparelli, T., Four track mobile robot for non structured

environments:, Advanced Robotics, 1991. 'Robots in Unstructured

Environments', 91 ICAR., Fifth International Conference on Intelligent

Robots and Systems, Yokohama, 1991, pp. 926- 930.

3. Liu, J., Wang, Y., Ma, S., and Bin, L., Analysis of Stairs-Climbing Ability

for a Tracked Reconfigurable Modular Robot:, Safety, Security and Rescue

Robotics, Workshop, 2005 IEEE International, Kobe, 2005, pp. 36-41

4. The Price of Freedom: Packbot,

http://americanhistory.si.edu/militaryhistory/collection/object.asp?ID=480

5. Shigley, J. E., Mischke C. R., and Budynas R. G., Mechanical Engineering

Design, McGraw-Hill, New York, 2004.

6. Beer, F. P., Johnston, E. R. Jr., and Clausen, W. E., Vector Mechanics for

Engineers-Dynamics., McGraw-Hill, New York, 2004.

7. Ang, K. K., and Wang C. M., Statics and Mechanics of Materials, McGraw-

Hill, Singapore, 2003.


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APPENDIX A: Specifications of Literature Survey Robots

A.1 iRobots Packbot

Figure A.1: iRobots Packbot

Uses flipper design on front end or on both front and back ends

Dimensions: 87.9 cm (L) x 51.4cm x (B) 17.8cm (H)

Available in various configurations for exploration and bomb disposal

Weighs 18kgs fully loaded

60 maximum incline climb ability

2m water submersible

Digital wireless communication

Proven usage under U.S. Army


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B

A.2 Securitas

Figure A.2: The Securitas Robot

Uses four sets of tracks powered by 2 motors

Tracks can pivot about mounting position allowing adaptation to various terrains

Weighs about 1000kgs

45 maximum incline climb ability

Climbs steps up to 230mm high

Maximum speed of 0.3m/s

For handling of hazardous materials


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A.3 Tracked Reconfigurable Modular Robot

Figure A.3 Different Configurations of Tracked Reconfigurable Modular Robot

2 or 3 or even more module robot

Uses many instances of itself to climb stairs and cross gaps

Maximum speed of 0.3m/s climbing stairs

Wired design at present, wireless control in development

Designed for steps up to 180mm high


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APPENDIX B: Estimated Maximum Mass of EMR Calculations

Maximum Volume of Packbot = 87.9 x 51.4 x 17.8

= 80 421 cm3

Maximum Volume of EMR = 40 x 30 x 10

= 12 000 cm3

Ratio Packbot to EMR Volume = 80 421 / 12 000

= 6.70

Maximum Mass of Packbot = 18 kg

Maximum Mass of EMR = Maximum Mass of Packbot / Ratio Packbot to

__ __EMR Volume

= 18 / 6.70

= 2.69 kg

Estimated Maximum Mass of EMR (S.F =2) = 2.69 x 2

= 5.37 kg

= 6 kg (rounded to nearest whole

______number)


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E

APPENDIX C: Coefficient of Friction between concrete and

polyurethane tracks

The static coefficient of friction between the poly urethane tracks and concrete is

obtained by the following methodology.

Figure C: Forces Acting on a Block on an incline

Let the static coefficient be .and the block be at rest

Mg sin = Ff = R -(1)

Mg cos = R -(2)

The static coefficient of friction can be obtained when the block starts to move

backwards down the slope when is increased to a certain value.

R

Ff

Mg cos

Mg sin


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F

Substituting (2) into (1)

Mg sin = Mg cos

= tan

From this result, we can obtain the value of coefficient of friction by finding the angle

at which the block starts to move down the slope.

Experimental Results

Using the above methodology, the polyurethane tracks were tested on a concrete slab

and the angle of the slab measured when the polyurethane tracks start to slide down.

was obtained several times and the average obtained was used to calculate the

coefficient of static friction.

Table C: Results of Coefficient of Friction Test

Attempt 1 Attempt 2 Attempt 3 Average

37 35 36 36

= tan

= tan 36

= 0.7265


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G

APPENDIX D: Required Torque and Velocity for Driving Motors

Calculations

Figure D: Factors of the EMRs Power Requirements

Estimated maximum mass of EMR = 6kg

Diameter of drive wheels = 5cm

Coefficient of static friction of tracks = 0.7625

Mg sin = 6 x 9.81 x 10/25

= 23.54 N

10cm

50cm

Mg sin

R

Mg cos

Force required at wheels

Ff

25cm


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H

Ff = R

= 0.7625 x 6 x 9.81 x cos

= 41.13 N

Force required at wheels = 41.13 + 23.54

= 64.67 N

Torque at wheels = 64.67 x 5/2 /100

= 1.617 Nm

Torque required per motor = 1.617 / 2

= 0.8084 Nm

Torque required per motor (S.F. = 1.5) = 0.8084 x 1.5

= 1.213 Nm

= 12.36 kg cm


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J

APPENDIX E: Sizing Up Swing Arm Motor Calculations

Figure E: Factors of the EMRs servo requirements

Taking moments about Point A:

50 x RB = Mg x (502 - 102)0.5 /2

50 x RB = 6 x 9.81 x 24.49

RB = 28.83 N

Torque Required of Swing arm servos = RB x Length of Swingarm

= 28.83 x 17.5 / 100

=5.046 Nm

=51.44 kg cm

Torque Required per Swing arm servo =51.44 /2

=25.7 kg cm

10cm

RAy

Mg

RAx

RB

A

B

50cm

17.5cm

Torque Required of Swing arm servos

25cm


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K

APPENDIX F: Specifications of Individual Components used

Only the more important components are described in this section.

F.1 Driving Motors

The driving motors in used in the EMR are a pair of Mabuchi Motors RS-540 motors.

Figure EB shows the performance characteristics of the motors when running at a

load of 520 g-cm which is close to the assumed maximum load of 500 g-cm used for

calculations.

Figure F.1A: Picture of Motor attached to Reduction Gearbox


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L

Figure F.1B: Motor Performance Simulation from Manufacturers Website

Figure F.1C: Motor Dimensions from Manufacturers Website


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M

F.2 Swing Arm Servos

The servos used for operating the swing arms are a pair of Hitec HSR-5995TG digital

Servos. This servo is light and compact but develops a huge amount of torque for its

size, hence making it rather expensive.

Figure F.2: Picture of Swing Arm Servo attached to Swing Arm

Specifications:

Dimensions: 40mm x 20mm x 37mm

Weight: 62g

Operating Speed: 0.15sec/60 at 6.0V/

0.12sec/60 at 7.4V

Output Torque: 24.0kg cm at 6.0V

30.0kg cm at 7.4V


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N

F.3 Radio Control System

The radio control system used the EMR is the Hitec Optic 6 and Electron 6

transmitter and receiver respectively. This system allows the control of 6 actuators at

once. The Optic transmitter allows the mixing of channels. As a result, the forward

and backward motions of the EMR were controlled by the vertical movement of the

left hand stick while the direction of the EMR was controlled by the horizontal

motion of the right and stick. The swing arms are controlled by a switch on the

transmitter than activates a small servo to turn on power to the swing arm servos. The

controls of the swing arm servos are mapped to the vertical motion of the right hand

stick. A transmitter that does not allow the mixing of channels would otherwise make

controlling the EMR difficult and complicated.


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O

Figure F.3: Pictures of Optic 6 and Electron 6 Transmitter and Receiver

Specifications:

Electron 6 receiver:

Dimensions: 45.5mm x 22.5mm x 15.0mm

Weight: 17g


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P

F.4 Electronic Speed Controller

The speed controller being used to controller the two drive motors are a pair of

Tamiya TEU-101BK speed controllers, often used in radio control race cars. The

speed controllers are connected to both the batteries and receiver to allow precise

control of them. These Tamiya speed controllers also allow the reversing of the drive

motors.

Figure F.4: Picture of Tamiya TEU-101BK Speed Controller

Specifications:

Dimensions: 40.7mm x 36.8mm x 22.4mm

Weight: 54g

Maximum continuous current: 60A


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APPENDIX G: Experimental Data on the EMRs Velocity

The velocity of the EMR was measured by measuring the time it took to travel a

distance of 6m with a running start. A stopwatch was started when the EMR crossed

the start line and stopped when it crossed the finish line. This was repeated several

times and the average velocity calculated.

Table G: Experimental Data on EMRs Velocity

Distance Traveled Time Taken Velocity

6m 8.50s 0.7058 m/s6m 8.43s 0.7117 m/s

6m 8.40s 0.7143 m/s

6m 8.56s 0.7009 m/s

6m 8.56s 0.7009 m/s

Average Velocity: 0.706 m/s

As we can see from the table, the average velocity obtained by the EMR is 0.706 m/s

which is also roughly 2.5 km/h, which corresponds to a slow human walking speed.

high mobility, tracked robot

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