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Proceedings of the ISRM 2009 1. IFToMM International Symposium on Robotics and Mechatronics

September 21 - 23, 2009, Hanoi, Vietnam

Design and Control of a Six Wheels Terrain Robot

Chu Anh Mya, Huynh Van Dongb, Ha Huy Hunga and Vu Minh Duca

a Department of Mechatronics, Faculty of Aerospace Engineering, Le Qui Don Technical University, Hanoi, Vietnam

E-mail: mychuanh@yahoo.com bDepartment of Computation Engineering,, Faculty of Automation Engineering, Le

Qui Don Technical University, Hanoi, Vietnam Abstract. A design solution of a six wheels mobile robot capable of moving on terrain surface is presented in this paper. The design considers a mobile platform structure of six wheels robot in which a four bars parallel mechanism mounted on the main body linking with side wheels and a normal four bars mechanism connecting with the front wheel are used to make the robot move on terrain surface easily. A simple control method implementation is proposed also, and the whole robot system is integrated for real testing. Keywords. Terrain Robot Design, Parallel Mechanism, Kinematics of Six Wheels Robot

1. Introduction The terrain robot (rover) is a kind of mobile robot (vehicle) capable of travelling on rough terrain (like natural surface ground). The robot is expected to perform various tasks in special environments such as nuclear power plants, large factories, welfare care facilities, hospitals, and army areas in particular. Moreover, surface mobility of the terrain robot is crucial for accomplishing many tasks ranging from site preparation, construction and local transportation to prolonged exploration sorties many kilometers from the primary base. The terrain robot is needed to assist the astronauts in the day to day operation and maintenance of the base and all related infrastructure. The astronauts need terrain robots to transport personnel and supplies to and from the landing sites, storage facilities, and habitat modules. They may also need the robots capable of moving and hauling the soil for landing and habitat site preparation, radiation shielding, and burying biological and possible radioactive wastes. Whether it is short day sorties with unpressurized rovers, or month long sorties in large pressurized vehicles, terrain robot systems are the key element in extending exploration activities well beyond the immediate confines of the base and landing area. Mars has a surface area of approximately 144 million square kilometers, about the same area as all the combined land mass of earth. Clearly the astronauts need to be mobile to explore this vast new world. Eight successful rovers have been deployed on the Moon and Mars over the last 35

years. These include crewed vehicles as well as teleoperated (remotely piloted), and autonomous robots. The United States Lunar Roving Vehicle has been used on the Apollo missions and the Russia’s two Lunokhod lunar rovers has been explored the Moon. The Mars Pathfinder rover performs beyond expectations, and the Mars Excursion Rovers Spirit and Opportunity continue to perform well. The Lunar Roving Vehicle (LRV) is an electric vehicle designed to traverse the lunar surface, allowing the Apollo astronauts to extend the range of their surface extravehicular activities. The LRV has a mass of 210 kg and is designed to hold a payload of an additional 490 kg on the lunar surface. The frame is 3.1 meters long with a wheelbase of 2.3 meters. Each wheel has its own electric drive, a DC series wound 190 w motor capable of 10,000 rpm, attached to the wheel via an 80:1 harmonic drive, and a mechanical brake unit. Lunokhod 1 and 2 are a pair of unmanned lunar rovers landed on the Moon by the Russia in 1970 and 1973, respectively. The Lunokhod missions are primarily designed to explore the surface and return pictures. Lunokhod 1 has a mass of 900 kg and is designed to operate for 90 days while guided by a 5-person team from earth. Lunokhod 1 explored the Mare Imbrium for 11 months, traveling 11 km while relaying television pictures and scientific data. Mars Pathfinder is originally designed as a technology demonstration of a way to deliver an instrumented lander and a free-ranging robotic rover to the surface of Mars. Pathfinder not only

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accomplishes this goal but also returns an unprecedented amount of data. In its four months of operation the rover, named Sojourner, traversed a total distance of about 100 m. Sojourner has a mass of 11 kg and is about the size of a child's small wagon. In recent years, an increasing number of researches around the world are interested in the topic of terrain robot development, and the publications focus on a wide spectrum of directions such as the conceptual design (Bares et al, 1989), the structure optimization for locomotion (Apostolopoulos, and Bares, 1995), the mechanical design (Volpe et al., 1996, Thainwinboon et al., 2001), the control system (Chakraborty et al., 2005, Pierre Lamon et al., 2006, Kazuya and Hiroshi, 2005, Karam and Albagu, 1998), the prototype implementation (Volpe et al., 1996), etc. Based on the feature of system configuration for locomotion, the terrain robots can be divided into two main groups:

• the group of legs robots, and • the group of wheels robots.

In one hand, the robots of the second group employ the friction principle between wheels and the ground to move the system; in the other hand, the robots of the first group use leg locomotion gaits for walking on the surface (like human and animal). Since the same principle of friction is used for locomotion, the robot employing chain mechanism for moving belongs to the second group as well. In general, the leg robot moves quite flexibly and climbs steps easily. The body of the robot in moving is very stable, and the ground traction ability is very good. However, the control system requires complex with higher autonomous level, since the robot must decide how to walk, and where to place foots properly. Moreover, to reach to a given maximum velocity is not very easy because of the leg locomotion is not continuous. In the contrast, the wheel robot type is familiar in designing, not so complex, and easier to control. In practice, a number of robots of this type has been launched for many real applications such as Lunarkhov (Russia), Mars Rover, Sojourner (NASA - US), etc. To meet the increasing demand of using robot for particular purposes, many research programs with big amount of budged in over the world have been setting up. The research proposed in this paper is one of them in the context. In this research, a robot with structure of six wheels locomotion configuration equipped with a wireless camera has been designed, simulated and tested at Mechatronics Lab, Le Qui Don Technical University, Hanoi, Vietnam (see Fig. 1). The structure of six wheels like Shrimp prototype (Lamon and Siegwart, 2003) is analyzed and selected for the prototype implemntation. The mission of the robot is to move on rough surface of the ground, capture camera signal of the real world and send the

image back to the control place. The robot designed includes two modules: the mobile frame module and the manipulation module carrying the camera. The design process covers the functional requirements analysis, the selection of structure for locomotion , the general configuration design, the mechanical design and the electric-electronics design, etc. A prototype of the robot is implemented and tested successfully.

Fig. 1. The image of the 6 wheels robot 2. Mechanical Design To select a suitable structure for the robot design, several requirements and functions of the system illustrated in the Tab. 1 are considered and analyzed. Tab. 1. Requirements of the robot design

Requirements Description

The ground on which the robot moving

The terrain surface like natural ground

The obstacle height < 200 mm

The inclination angle of the surface

< 300

The depth of hole on the surface

< 200 mm

The radius of hole on the surface

> 1500 mm

Maximum velocity 1 m/s

Average velocity 0,3 m/s

- Obstacle on the ground: obstacle of 100 mm in high on slope of 150

Mixed terrain

- Mixed slopes: slope of 50 on slope of 100

Loading < 15 Kg In practice, meeting such the requirements there are a number of mechanical structures for locomotion. Two types of structure are taken into consideration in this case: the legs structure and the wheels structure.

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After comparing advantages and disadvantages of each type following common criteria, the wheels configuration is preferred, and the six wheels mobile platform is selected (the most suitable option) since it composes several advantages as

• more familiar with designers, • lower level of automation, • flexible to accelerate and change the moving

direction, and • most of spare parts and equipments for making

the robot are standardized and marketable. The structure includes 6 wheels, as can be seen in the simulation picture below (Fig. 2), of which one in front (head), one in back (tail), and two for each robot-side.

Fig. 2. Simulation of the configuration

The front wheel is mounted on four links mechanism (front mechanism) connecting to the robot body. The side wheels are mounted on four parallel bars each side.

With the four bars mechanism in front of the robot, the traction force F is generated when the front wheel approaches to obstacle under the applying of moment T (moment of electric motor) . The force F then produces a moment rolling up all the front subassembly of the robot. This makes the robot easy to overcome the obstacle even the radius of the wheel is less than the obstacle height (Fig. 3). To overcome obstacles (steps) of 200 mm in height as required, the diameter of wheels is selected as 350 mm. By simulating with ADAM software (Fig. 4), for the wheel diameter selected and other choices of applied moments, friction coefficients, dimensions of mechanisms, etc, the robot can overcome steps of 200 mm and satisfies all requirements of locomotion presented in Tab. 1.

Fig. 3. Four bars mechanism mounted in front of the robot

Fig. 4. Robot simulation in ADAM View

Four bars mechanism

Front wheel

Parallel mechanism

Side wheels

Back wheel

Robot body

Obstacle

4

x

y

yr

xr

v1

I

c

v

v6

v6xv6y

v5yv5

v5x

v2x

v2v2y

v3

v3x

v3y

v4

d1 d2

d6

d d5 d3 d4

v1y

v1x

v4y

v4x

vy vx

θs

θs

θ

ξ

ω

w

b1a1

a2

b2

Fig. 5. The kinematics model in horizontal plan

3. Kinematics analysis and motion control In general, the kinematics of robot plays important roles in modeling, design and control. The kinematics modelling of the terrain robot is very complex since the locomotion configuration is complex and the robot moves on not the flat surface, but the rough terrain. To reduce the complexity of the model, in this section a 2D kinematics modelling is considered. In Fig. 5, we denote Oxy: the reference frame, Cxryr: the robot coordinate system fixed to the robot body, c: the center of the robot,

T

i ix iyv v⎡ ⎤= ⎣ ⎦v : the velocity of the wheel i (the wheels are indexed from 1 to 6) expressed in Cxryr, θ: rotation angle of the robot, θs: steering angle of front and back wheels, I: the instantaneous center of velocity (

T

x yI I⎡ ⎤= ⎣ ⎦I ),

ω θ= , T

x yd d⎡ ⎤= ⎣ ⎦d : the distance from I to C,

T

i ix iyd d⎡ ⎤= ⎣ ⎦d : the distance from the center of wheel i to I, r: radius of wheels all,

. (for 2,3,5,6)ix iv r iω= = (see Fig. 6), . (for 1,4)j jv r jω= = , and

[ ]Tx y θ=q : the generalized coordinates vector where (x, y) is coordinate of point C,

Fig. 6. Velocity of a wheel The generalized velocity vector can be formulated as

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cos sin 0sin cos 0

0 0 1

x

y

vxy v

θ θθ θ

θ θ

⎡ ⎤⎡ ⎤ −⎡ ⎤⎢ ⎥⎢ ⎥ ⎢ ⎥= = ⎢ ⎥⎢ ⎥ ⎢ ⎥⎢ ⎥⎢ ⎥ ⎢ ⎥⎣ ⎦⎣ ⎦ ⎣ ⎦

q (1)

In Cxryr we can express the following relationship. iy yix x

iy y ix x

v vv vd d d d

ω θ= = = = =− −

Suppose that at every time on the given trajectory ξ , the following approximation can be acceptable.

5 6x xv v= , 2 3x xv v= , and 1 4x xv v= . This assumption can be understood that the angular velocities of two left wheels are equal, the angular velocities of two right wheels are equal, and the angular velocities of two steering wheels are equal as well. In fact, this means that we accept the slip of wheels when the robot mobiles on a curve trajectory. If we denote

, ,L R Sv v v as the velocities of left, right and steering wheels, respectively, the corresponding angular

velocities , ,L R Sω ω ω can be calculated as L

Lvr

ω =,

RR

vr

ω =, and .cos

SS

S

vr

ωθ

= . We suppose more that

the distance d approximates the curvature radius of ξ at point c . Thus angular velocity of robot θ can be approximately calculated through d and vx . Therefore, to move the robot on the trajectory ξ(t) with velocity ( ) ( ) ( )

T

x yt v t v t⎡ ⎤= ⎣ ⎦v at time tj, we need to control the wheels in such a way that

1 ( )Lj xj jv wr

ω ω= + ⋅ (2)

1 ( )Rj xj jv wr

ω ω= − ⋅ (3)

1 1 2 21 ( )tan

2j

Sjxj

a b a bv

ωθ −

⎛ ⎞+ + += ⎜ ⎟⎜ ⎟

⎝ ⎠ (4)

cos

xjSj

Sj

vr

ωθ

= (5)

4. Electrical design The general structure of electrical design depicted in Fig. 7 includes a central computation unit (microprocessor – 8515 AVR), signal control amplifier, actuators (motors), viewing element (camera), wireless telecommunication module (control signal transmitter TWS –HS-1 and receiver RWS-436), and PC user interface. The camera is used for viewing status of the robot when the robot moving; the image of camera is transferred into the PC interface via wireless communication (maximum distance of 100 m). To control the robot moving forward, backward, or turning left and right, the personnel needs to click on corresponding buttons of the interface designated on PC. The control signal is transferred with the radio signal transmitter; and then the signal is received by the radio signal receiver. The signal is finally processed by the microprocessor 8515 AVR to produce voltage levels for activating motors of wheels.

Fig. 7. Diagram of electrical design

Control Interface

(PC)

Radio Signal Transmit Module

Signal Receiver

Microprocessor

Amplifiers

Actuators

Control Person

Camera

6

C_Xta1

CompRx

10uF

U3

AT90S8515

9

1819

20

2930

31

40

2122232425262728

1011121314151617

12345678

3938373635343332

RST

XTAL2XTAL1

GN

D

OC1BALE

ICP

VCC

PC0/A8PC1/A9

PC2/A10PC3/A11PC4/A12PC5/A13PC6/A14PC7/A15

PD0/RXDPD1/TXD

PD2/INT0PD3/INT1

PD4PD5/OC1A

PD6/WRPD7/RD

PB0/T0PB1/T1PB2/AIN0PB3/AIN1PB4/SSPB5/MOSIPB6/MISOPB7/SCK

PA0/AD0PA1/AD1PA2/AD2PA3/AD3PA4/AD4PA5/AD5PA6/AD6PA7/AD7

Y1

P1

DB9 MALE

594837261

C6

5V

ThaoTac4

C7

ThaoTac8

DKPack9

R_Reset

DKPack7

DirPack4

10uF

DKPack2

C4

C_Reset

Xta2

ChipRx

CompRx

10uF

ThaoTac3

ThaoTac9

C3

DKPack10

DKPack8

DirPack5

DKPack3

10uF

0V

10uF

Xta1

Xta1

ChipTx

ThaoTac2

C_Xta2

DirPack10

U2

MAX232A

1

34

5

1615

26

129

1110

138

147

C1+

C1-C2+

C2-

VCC

GN

D

V+V-

R1OUTR2OUT

T1INT2IN

R1INR2IN

T1OUTT2OUT

DirPack1

ThaoTac10

ChipRx

DirPack6

DKPack4

SW

_Res

etCompTx

ThaoTac1

DirPack9

DirPack2

Xta2

ThaoTac6

ChipTx

DirPack7

DKPack5

5V

5V

DKPack1

0V

ThaoTac5

ThaoTac7

compTx

DirPack8

DKPack6

DirPack3

C5

Fig. 8. AVR 8515 microprocessor 5. Conclusion A prototype of six wheels terrain robot is designed, implemented and tested. The configuration of locomotion is suitable and meets the requirements of travelling on natural surface. During the test of the robot, several problems arises which need further studies: the optimization of the mechanical structure, the more efficient control algorithm that minimizes wheel slip, the dynamical control of actuator for travelling on a given path within a geometrical error limited, etc. 6. References J. Bares, M. Hebert, T. Kanade, E. Krotkov, T.

Mitchell, R. Simons, W. Whittaker. 1989. Ambler: An Autonomous Rover for Planetary Exploration. IEEE Computer , Vol. 22(6), pp. 18-26.

Apostolopoulos, D., Bares, J. 1995. Configuration of a Robust Rappelling Robot. Proceedings of the 1995 IEEE International conference on Intelligent Robots and Systems (IROS), Pittsburgh, PA, US.

Dimitrios S. Apostolopoulos. 2001. Analytical Configuration of Wheeled Robotic Locomotion. PhD thesis, The Robotics Institute, Carnegie Mellon University, USA.

Apostolopoulos, D. 1996. Systematic Configuration of Robotic Locomotion. Technical Report CMU-RI-TR-96-30, The Robotics Institute,

Carnegie Mellon University, Pittsburgh, PA, US.

R. Volpe, J. Balaram, T. Ohm and R. Ivlev. 1996. The Rocky 7 Mars Rover Prototype, Proceedings of IROS, pp. 1558-1564.

M. Thainwinboon, V. Sangveraphunsiri and R. Chancharoen. 2001. Rocker-Bogie Suspension Performance. Proceedings of the Eleventh International Conference in Automotive Engineering. Novermber 2001, Shnghai, China.

Yim, M. 1994. Locomotion with a unit-Modular reconfigurable Robot. PhD Thesis, Stanford University, USA.

Chakraborty, Nilanjan and Ghosal, Ashitava. 2005. Dynamic Modeling and Simulation of a Wheeled Mobile Robot for Traversing Uneven Terrain Without Slip. Journal of Mechanical Design. Vol. 127(5). pp. 901-909.

Pierre Lamon, Ambroise Krebs, Michel Lauria, Roland Sieg. 2006. Wheel torque control for a rough terrain rover.

Lapierre et al. 2007. Combined Path-following and Obstacle Avoidance Control of a Wheeled Robot. The International Journal of Robotics Research . Vol 26. pp. 361-375.

Lamon P., Siegwart R. 2003. 3D-Odometry for rough terrain – Towards real 3D navigation. Proceedings of IEEE International Conference on Robotics and Automation (ICRA2003), Taipei.

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Kazuya Yoshida and Hiroshi Hamano. 2005., Motion Dynamics and Control of a Planetary Rover With Slip-Based Traction Model, Tohoku University Press.

K.Z.Karam and A.Albagul. 1998. Dynamic Modelling and Control Issues for a Three Wheeled Vehicle. Proc. 5th Int. Con. on Control, Automation, Robotics and Vision, Singapore.

A. Albagul & Wahyudi. 2004. Dynamic Modelling and Adaptive Traction Control for Mobile Robots. International Journal of Advanced Robotic Systems. Vol 1 (3).

Karl Iagnemma, Hassan Shibly, Adam Rzepniewski, Steven Dubowsky. 2001. Planning and Control Algorithms for Enhanced Rough-Terrain Rover Mobility. Proceeding of the 6th International Symposium on Artificial Intelligence and Robotics & Automation in Space (i-SAIRAS 2001), Canadian Space Agency, St-Hubert, Quebec, Canada.