design and simulation of telepresence robot

Course: Modelling and Simulations of Mechanical Systems Master Degree in Mechatronics Engineering

University of Trento Course Project Report

Design of a Mobile Telepresence Device

Report

<Solomon Genene Gudeta>

<Tesfaye Asmera Mengesha>

<Wendwossen Bellete Bedada>

<Mehari Kebede Tesfay>

<August 30, 2013>

Modelling and Simualtion of Mechanical Systems – Master Degree Course

Report:Design of a Telepresence Device.pdf 1

1 Introduction

The mobile telepresence device implements the

three wheel inverted pendulum mechanism. The

pendulum is anchored to a base platform that has a

wheel mounted on each side and one small wheel on

the rear end of the base. The motor drives each front

wheel independently, while the rear wheel is freely

movable. The system has a body that supports the

screen and camera at the desired height level. To

make the conversation smooth and real, the handler

(screen, camera) has been made to have some degree

of mov’t (rotation).

In this project we have designed a new device which

is different to the one available in the market.

Started by building a prototype, determining

components and their specifications, we determined

QFD which is the basis for the determination of

target parameters. The kinematic and dynamic

analyses are studied for the simplified version of the

device neglecting some forces. To make the device

robust to external disturbances we have carried out

the controller design.

The device is basically limited to indoor

environments with small slope surfaces.

2 Customer requirements

After discussing on the customer requirements

given, we have considered the followings:-

1. Handling and maneuverable: The ability of the

system to move in narrow space and change

direction.

2. Stable (robust): stable with respect to external

disturbances and must be safe for the user-

interface device.

3. Controllable: The ability of the robot to move in

the required direction with desired speed.

4. Easy to control at low speed: ability of robot to

move with constant speed or stand still to

record panoramas and watch the scene

5. Carry user interface at 1m or adjustable: It is

required that the height at which the user

interface mounted is adjustable and sufficiently

high for communicating.

6. Dynamic performance: It is expected that our

robot must have the same speed and stability to

walking man. An average walking speed for a

man is 1.15 m/sec.

7. Compactness: describes the amount of space

taken by the robot.

8. Autonomous: collision avoidance, path

detection and independent battery source.

Requirement Rank

Handling and maneuverable 7

Stable (robust) 5

Controllable 4

User interface at 1m high or adjustable 2

Dynamic performance 2

Easy to control at low speed 1

Compactness 1

Autonomous 2

Tab. 1 Customer requirements

3 Benchmarks

The double, the beam, AVA-500 and VGO robot are

devices used as bench mark for our design.

VG

O

Double

Bea

m

AV

A-5

00

Ourd

esig

n

Handling and manoeuvrable 3 5 3 2 4

controllable 5 4 4 3 5

Easy to control at low speed 5 4 5 3 5

Carry user interface at 1.2m 4 5 3 5 5

Stable(robust) 5 3 3 5 4

Dynamic performance 4 4 5 3 5

Compact 4 4 5 3 4

Autonomous 3 3 3 5 3

Note: Legend

1. Non-satisfaction 2. Slightly satisfy

3. Satisfy some how

4. Quite satisfy 5. Fully satisfy

Table 2: Benchmarks

4 Technical Specifications

Using the QFD the importance of each specification

is determined.

Technical specifications Unit Priority

dimension m 100

Speed m/s

87

Height m 64

Weight Kg 62

Battery life Hours 55

Table 3: Technical specifications

Large lateral and longitudinal dimension decreases

maneuverability and increases the stability of the



system. Those dimensions are the highest in terms of

satisfying customer requirement.

Speed of the robot measures how well the dynamic

requirement is achieved. The ability to achieve

desired speed also increases controllability.

The height of the bar improves the visibility of user

interface screen mounted on it whereas degrading

the compactness and maneuverability.

Weight of the system decreases compactness and

dynamic performance of the system.

Higher battery life is achieved at a cost of size of

system (compactness) and handling difficulties. On

the other hand it makes the robot autonomous with

the capability of operating on its own power without

direct power supply.

5 Targets/Goals and parameters

Basing on the technical priority of the technical

specification derived from QFD the range of

parameters is determined.

Technical specifications Unit Min Value

Max Value

Longitudinal length m 0.2 0.5

Lateral length m 0.15 0.25

Speed m/s 0.8 3

Mass Kg 5 10

Height m 1 1.5

Battery life Hours 5 -

Table 4.Target values

The desired value is extracted based on the table

above and the benchmark table. Attention is given to

the overall performance of the system.

Technical specifications Unit Desired value

Lateral length m 0.35

Longitudinal length m 0.17

Speed m/s

1.15

Height m 1.5

Mass Kg 5

Battery life Hours 6

Table 5.Desired values

6 System models and solutions

The 3D modelling of the system is used to study the

important behaviours that could influence the

customer requirements. The model has the following

physical bodies:

Fig1. The designed device

Chassis: The body of the robot that carries

the battery, the motor and the embedded

electronic system.

Wheels: two identical front wheels with the

same mass and radius and one rear wheel of

smaller radius to increase stability and

maneuverability of the device.

Bar: it is a vertical rod which supports the

screen. It is adjustable to vary the position of

screen in order to facilitate face to face

conversation. The bar has only vertical

mov’t and it can’t be tilted in any direction.

Screen handler: it is a body which is

deployed to carry the screen. It has

adjustable angular position to a limited

degree to mimic human like conversation

i.e. the robot can rotate its head to make the

conversion smooth and fluid.

To study the kinematics and dynamics of the system

we used MBsymba package and maple. The body

has six degree of freedom. The degrees of

movements are forward motion, lateral motion,

vertical motion of the bar, screen rotation, chassis

rotation, wheel rotation. Screen handler and the bar

body are connected by revolute joint, while

prismatic joint is used to direct the bar only in the

vertical direction. The chassis and wheels are

connected by revolute joint. We have considered the

forward rolling without slipping. Due to this

constraint the forward motion and rolling is related

and can be represented by one coordinate. So our

system has five DOF. The relative coordinates

approach is used to study the kinematic and dynamic

analysis of the model. The reference frames are

defined as indicated in the figure 2 below.



Fig .2 Device reference frames

Fig.3 Chassis and wheel reference frames, coordinates

RF1:= translate(x(t),y(t),r).rotate('Z',psi(t)): RF_w1:= RF1.translate (L1,-W/2, 0).rotate ('Y',

phi1(t)); RF_w2:= RF1.translate (L1, +W/2, 0).rotate ('Y',

phi1(t)); RF3:= RF1.translate (-(L-L1), 0, 0): RF4:= RF3.translate (0, 0,-l).rotate ('Y', beta(t)): RF2:= RF1.translate (0, 0, H+s(t).rotate('Z',

'theta(t)'))

Assuming that the body moves with constant

forward velocity, we didn’t consider the longitudinal

dynamics of the system. To achieve greater stability

and maneuverability the lateral and yaw dynamics of

the system are studied. The forward wheels have the

same side slip angle. It is assumed that the slip

angles are very small (0-0.5 ) It is determined from

tire contact point velocity. Sideslip angle is the angle

between a rolling wheel's actual direction of travel

and the direction towards which it is pointing. For a

free-rolling wheel this slip angle results in a force

parallel to the axle and the component of the force

perpendicular to the wheel's direction of travel. The

slip angle of rear tire [equation [1]] is greater than

the front tire [equation [2]] which implies over

steering.

( ) ( )

( ) ( ) ( )

( ) ( )

( ) ( )

( ) ( )

Fig.4 Wheel and body velocities

Being a 3D model, the dynamics of the system is

studied using the Newton-Euler equations.

Definition of bodies:-

Chassis: = make BODY (P, mc, 0, 0, izc): Bar : = make BODY (G, mb): Screen: =make BODY (Ps, ms, 0, 0, izs): Left wheel: = make BODY (RF_w2, mw, 0, iyw2, 0): Right wheel: = make BODY (RF_w1, mw, 0, iyw1, 0): Rear wheel: = make BODY (RF4, mr, 0, iywr, 0): Note: symbols are described in appendix [D]

We have considered only tyre contact point forces

ignoring internal reaction forces between bodies.

These forces are:

Reaction force between tire and floor: the

tire force that arises from friction in contact

patch and elastic deformation of the tires.

The forces in the lateral direction which are

responsible for lateral motion are tire forces

that arise from tire slipping. These forces are

modeled using elastic deformation of tires

ignoring friction forces. The tire forces are

stiffness of the tire multiplied by slip angles.

( ) ( ) ( )

( ) ( )

( ) ( ( )

( ) –

( ) ( )

( )) ( )

Gravitational force



Control torque: the torque between chassis

and front wheels to move the device in

forward direction as well as to steer the

device in the intended direction.

Bar force: it is force that acts on a bar to

adjust height of the screen

Screen torque: it is the torque needed to

adjust the orientation of the screen.

Friction forces and air resistance forces are not

modeled therefore do not appear in the model. Since

there is no motion of wheels in vertical direction,

vertical forces do not appear in equation of motion.

The forces in the forward direction are determined

by employing the idea of rolling without slipping

using wheel equations from internal degree of

freedom. Equations are reported in appendix [A].

Fig.5Wheel contact point forces and joint torques

The bar and screen handler part of the system is

considered as subsystem. Equation of motion is

derived separately so that the force needed to adjust

height of the bar and the torque needed to rotate the

screen is computed as follows.

( ) (

( )) (

( ))

( )

( )

( ) ( )

Fig.6 Bar joint forces, screen torques and coordinates

From the above hypothesis two ODE equations of

the system are obtained. The equations are reported

on appendix [A]

7 Model Predictive Yaw Controller design

The state space representation of the linearized

system is derived using mbsymba package. The

system is a two input two output system.

The state variables are lateral velocity and yaw

rate: ( ( ) ( ))

The input vectors are right and left wheel

torque: ( )

The output vectors are lateral velocity and yaw

rate: ( ( ) ( ))

The following are the realization generated by using

from maple worksheet.

*

+

*

+

C=*

+ *

+

The device is inherently stable at the desired value

of design parameters. Since it is dynamical system

its stability is influenced by forward velocity due to

the fact that the slip angles are the affected by

forward velocity.

The target is to make the robots lateral motion to

behave as commanded by user action. This can be

done by generating yaw moment. The designed

MPC controller should minimize the error between

the desired yaw rate and robot yaw rate to make the

lateral response of the system stable. The trajectory

of the user is combination of linear and curved one,

the yaw rate on linear trajectory is assumed to track

the desired value of zero since the rotation of both

front wheels are equal. In the mean while when the

robot enters a curved trajectory, the yaw rate has to

track the desired value of 0.416rad/sec computed by

equation [7]. So the desired yaw rate of the whole

trajectory can be modeled by using a pulse of

specified width.

( )

Where is forward velocity, is slip angle, is

wheel width, is acceleration due to gravity.

Control sequence is obtained by minimizing the cost

function. Using MPCtool GUI embedded in



MATLAB with the following parameters the

response of the system is reported in the figure 8.

*

+ *

+

Prediction horizon P = 65, Control horizon M=5,

Model horizon N=TΔt, T=60, Δt=0.01ms

Fig. 7 Lateral velocity response of the system

8 Result discussions

The main outcome of this project is the stability of

the product at the desired speed of 1.15 m/s. Two

motor with a rating of 270mNm torque, gear ratio of

35 and 170RPM rated speed are deployed on the

front wheels to drive the system to the intended

place. With this rated speed of the motor and wheel

radius of 65mm, the device can move with the speed

of 1.15m/s. At this speed the system is found to be

stable with the dimension of the chassis is accurately

met and the weight of the whole body to be 4.88Kg.

The height of the device can be adjusted at any

desired height between 1m and 1.5 m with the

possible rotation of screen to make the conversation

fluid. The bar height and the screen rotation are

independent part of the system that are derived by

their dedicated motors. So, in this project the bar

vertical movement and screen rotation are studied

separately as a subsystem. The power consumption

of the motors is indicated below.

Component Current

consumption

Motors 1.06A+0.8A=1.86A

Ipad 0.74A

Microcontroller 0.05mA

Motor controller 0.07A+0.1A=0.17A

Table 6 Power consumption

In order to have a battery life of about 6 hours which

is equal to the average battery life of Ipad, the

battery capacity of (1.86A+0.05A+0.17A)*6

hours=12.48Ah is needed. The power consumption

of Ipad is 4440mAh and it is autonomous. The

dynamic performance of the system is studied under

different turns and yaw controller which can handle

critical turns is designed using Model Predictive

Control. This is implemented by studying the yaw

and lateral dynamics of the system. The desired

value of yaw rate is calculated by equation [7].The

under steer coefficient is determined by calculating

the distribution of load. Accordingly, 70% of the

load is on the front wheels while 30 percent of the

load is on the rear wheel. The weight of the device:

W = 4.88*9.81 = 47.873N

Weight on front wheels (Wf) = 0.7*47.873 = 33.511

Weight on rear wheel (Wr) = 0.3*47.873 = 14.3619

Then the desired yaw rate will be 0.416 rad/sec.

The tire forces are modeled using linear behavior of

elastic deformation considering linear side slip.

Fig. 8 Lateral force vs. slip angle



Evaluating performance of the system versus the

target values derived from customer requirement the

device is easily controllable, compact, stable and

cost effective. The bench mark analysis is carried

out to compare the existing devices in the market

with our design and it is found that the designed

device could be more efficient and satisfies customer

requirements.

9 Conclusions

3D model is used to design telepresence robot. In the

design the analysis of lateral dynamics and yaw

dynamics is carried out to increase the dynamic

performance of the device. The case of forward

rolling without forward slipping and with constant

forward velocity is studied. Considering side slip,

yaw controller is designed based on the desired

value of yaw rate to control the lateral response of

the system while turning. Based on the hypothesis

discussed, the device can move at human walking

speed of 1.15m/s. It has an adjustable height of

30cm between 1.2m and 1.5 m to make the

conversion smooth. The main limitation in this

project is that the longitudinal dynamics and the

effect of slipping in the longitudinal direction are not

studied. In addition to this the centre of mass of the

device is not optimized and the friction forces are

not modelled and studied. The future

recommendation will be to consider these problems

to make the device efficient.

My role in the project was:

Component identification and specification

determination

Kinematic and dynamic analysis

Bench mark analysis

MPC controller design

References

[1]. Lecture slides and materials, Modelling and

Simulation of Mechanical System course,

Professor Francesco Biral, University of

Trento, Italy

[2]. http://www.doublerobotics.com/

[3]. http://www.vgocom.com/

[4]. http://www.robotshop.com/

[5]. www.thecartech.com

[6]. Lecture slides, Model predictive control

course, Dr. Solomon Kidane, Addis Ababa

University, Ethiopia.

[7]. Stability of three wheeled vehicles with and

without control system, M.A.Saeedi and

R.Kazeemi, K.N.Toosi University of

Technology, Iran.

[8]. Beam and AVA-500

http://www.doublerobotics.com/

http://www.vgocom.com/

http://www.robotshop.com/

http://www.thecartech.com/



Appendix

A. Kinematic and Dynamic equation of system and forces

{

( )

( )

( )

( )

( )

( )

}

Tire contact point forces in longitudinal direction

( ) ( ) ( )

( )

( ) ( ( )

( ) – ( ) ( )

( ))

Tire contact point forces in lateral direction

{ ( ) ( )

( )

( )

( ) ( ) ( ) ( )

( )

( )

}



B. Quality Function Deployment (QFD) using house of quality (HOQ) tool

C.

Han

dli

ng

and

man

euve

r

abil

ity

Contr

oll

able

Eas

y t

o c

ontr

ol

at l

ow

spee

d

Car

ry u

ser

inte

rfac

e at

1m

or

adju

stab

le

Robust

(st

able

)

Auto

nom

ous

Dynam

ic

per

form

ance

Com

pac

t

SC

OR

E

Handling and

maneuverability

- 1 1 1 1 1 1 1 7

Controllable 0 - 1 0 0 1 1 1 4

Easy to control at low speed 0 0 - 0 0 1 0 0 1

Carry user interface at 1m 0 1 0 - 0 1 0 0 2

Robust (stable) 0 1 1 0 - 1 1 1 5

Autonomous 0 0 0 1 0 - 0 1 2

Dynamic performance 0 0 1 0 0 1 - 0 2

Compact 0 0 0 1 0 0 0 - 1

Handling and maneuverability 7 9 1 1 1 3

Controllable 4 3 9 0 0 1

Easy to control at low speed 1 1 9 3 3 1

Carry user interface at 1m height

or adjustable

2 0 0 1 9 9

Robust (stable) 5 3 3 9 0 3

Autonomous 2 0 1 0 9 0

Dynamic performance 2 0 9 3 3 1

Compact 1 9 0 9 3 3

Technical priority 100 87 62 55 64

Technical difficulty HI HI LOW LOW HI



C. Components and specifications

Component specifications

i. Wheel 100: Quantity:2

Diameter:130mm

Width:26mm

Diameter of hub:5mm

Weight :500g

ii. Communication device: Type: ipad mini

Size:200mmx135mmx7.2mm

Weight:308g

Quantity:1

Protocol: 802.11 a/b/g/n wifi

Sensors: accelerometer, gyroscope

iii. Mounting bracket: Quantity:2

Type: EMG 30

Weight:27.5g

Thickness:2mm

iv. Chassis: Dimension: 350mmx170mm

Weight: 500g

Type: plastic + steel

v. Wheel Motor: Quantity:2

Type: EMG 30 gear motor with encoder

Rated voltage:12V

Rated speed: 170RPM

Rated current:530mA

Rated output: 4.22w

Encoder counts per turn: 360

Stall current: 2.5A

No load current: 150mA

Connector: 6 way JST

Weight: 242g

Size:30mm(28.5mm)x86.6mm

vi. Motor: for adjusting height vii. Quantity:2

Type: stepper motor, unipolar

Voltage:12V DC

Resolution: 1.8 degree/step

Length:39mm

Current:0.4A

Resistance:30ohm

Inductance:30mH

Max. radial force:28N

Max. axial force: 10 N

Weight: 280g

Torque: 36 oz/inch

viii. Wheel motor controller: Quantity: 1

Type: MD25 dual H-bridge

Voltage: 12V

Current: up to 2.8 A

Current consumption:0.1A

Size: 71mmx60mm

Interface: serial or I2C

ix. Stepper motor controller: Quantity:2

Power supply: 4.75 to 30V

Size: 69mmx61mmx1.6mm

Current consumption: 70mA

Output current:750mA

x. Microcontroller: Quantity:1

Type: Aurdino Leonardo(ATmega 32u4)

Input voltage: 7-12V

DC current per I/O pin: 40mA

Pin:20

Flash memory:32KB

Clock speed: 16 MHZ

Size: 69mmx53mm

xi. Body bar: Dimension:120cm

Weight:780

Type:780g

Adjustable height:30cm Handler:

Dimension: 210mmx140mmx8mm

Weight: 100g

Adjustable degree:60degree

Type: plastic



xii. Battery: Type: sealed lead acid battery

Quantity:1

Nominal voltage: 12V

Size:95mmx98mmx151mm

Capacity: 12Ah

Weight:1667g

xiii. Rear wheel: Type: caster wheel

Quantity:1

Diameter:40mm

Wheel width:15mm

Plate:25mmx25mm

Weight:200g

xiv. Charger: Type: MFA 12V 500mA SLA charger for 12Ah

Quantity:1

Input:120-240V AC

Output:12V

Change rate: 0.5A

Mass of the system:

Mass of wheel: 500g

Mass of caster: 200g

Mass of chassis: 3000g

Mass of bar: 780g

Mass of screen: 408g

Dimensions:

Width of chassis (W):170mm

Depth of chassis: 200mm

Length of chassis: 350mm

Radius of wheels: 65mm

Radius of caster: 40mm

Adjustable height max. : 30cm

Caster mounting road: 50mm

Height of bar max: 150cm

D. Symbols

Symbol Parameter name

r Radius of front wheel

r1 Rear wheel radius

W Distance between front wheels

H Bar length

L Length of chassis

l Caster length

mc Mass of chassis

mr Mass of rear wheel

mw Mass of front wheel(left and right has same mass)

mb Mass of bar

iyw1and iyw2 Front wheels moment of inertia

iwyr Rear wheel moment of inertia

Izc Chassis moment of inertia

izs Moment of inertia on screen holder

Documents

design and simulation of telepresence robot