Dynamic Simulation Analyses of a Six-leg-wheel Hybrid Mobile Robot under uneven terrains

Yanjie Li School of Mechanical Engineering

Shenyang Ligong University Shenyang, China

e-mail:lyjsyit@sina.com

Abstract—A six-leg-wheel hybrid mobile robot was

designed to move under uneven terrains. The dynamic simulaition model of the six-leg-wheel hybrid mobile robot was built using ADAMS software. The kinematic and dynamic characteristic simulation analysis was presented. These characteristics includes the displacement, velocity and acceleration of each part of the robot and driving torque of joints, contact force and torque between the wheels with ground and the ability of obstacle negotiation. The simulation analyses provide the theory basis for the design of the robot control system based on kinematics and dynamics.

Keywords- Six-leg-wheel Hybrid Mobile Robot, Dynamic modelling ， simulation

I. INTRODUCTION Leg-wheel hybrid robot has been a hot topic in the

research field of mobile robot recently. Compared with the other kind of mobile robots, such as wheel-based mobile robot[1], biped robot[2], the leg-wheel hybrid robot[3] has the reconfiguration ability, effective maneuverability and good application prospect in the hazard environment. The dynamic modeling of the mobile robot is the basement of realizing the complex control algrithm.

The mobile mechanism of the six-leg-wheeled robot in this paper consists of 6 wheels and 6 swing legs. The legs and wheels are driven individually. The robot has the characteristic of multi-joints, multi-configuration and adapting to multi-kinds of environment . The kinematic and dynamic model is relatively complex.

The dynamic modeling of the six-leg-wheel hybrid robot was built using the software ADAMS[4] in this paper. The simulation can fullfilled the kinematic and dynamic simulation analyses of the robot under several kinds of road condition, and the obstacle-avoiding ability simulation.

II. THE ADAMS MODELING OF THE SIX-LEG-WHEEL HYBRID ROBOT

A. The structure of the leg-wheel hybrid robot The mechanical structure of the six-leg-wheel hybrid

robot consists of shaft, 6 swing legs and 6 wheels. The structure principle is shown in Figure 1. The mobile structure is lateral symmetry. The three groups of leg-wheel

mechanism are suited in lateral. The wheels are connected with shaft by swing legs. The legs and wheels are driven individually. Adjusting the configuration of the robot using swing legs, the robot can improve its maneuverability and its terrain traversability.

1——right front wheel；2——right middle wheel ；3——right rear

wheel；4——left front wheel；5——left middle wheel；6——left rear wheel；7——right front swing leg；8——right middle swing leg；9——

right rear swing leg；10——left front swing leg；11——left middle swing leg；12——left rear swing leg；13——shaft

Figure 1 The structure pricinple of the six-leg-wheel hybrid robot

B. The 3D Modeling of the leg-wheel hybrid robot based on ADAMS The 3D simulation modelling of the six-leg-wheel hybrid

robot was built using ADAMS software. The parameters of the modelling are shown as follows：the net mass of the robot is 60kg；the dimension is 1610 mm×660 mm×700 mm；the radius of the wheel is 230mm；the length of the legs is 260mm；the distance between the joints of the swing legs is 550mm；the width of the wheels is 80mm；the lateral distance between the wheels is 500mm；the height of the shaft is 300mm。The dimension of the model and the mass of the parts is set acoording to the parameters of the principle prototype.The 3D model of the leg-wheel hybrid robot is shown in Figure 2.

2010 Third International Conference on Intelligent Networks and Intelligent Systems

978-0-7695-4249-2/10 $26.00 © 2010 IEEE

DOI 10.1109/ICINIS.2010.176

308

Figure 2 The 3D Simulation Model of the six-leg-wheel hybrid robot

In the proceeding of modelling, the motion function is applied on each rotary joint to replace the real motor mototype and then the model can be simplified under the premise of achieving all the function of the robot. To simulate and analyze the climbing obstacle capability and motor torques of the robot, the step surface attached to the ground was built and the frictions between the wheels and the terrain were taken in count. The frictions on the rotary axes of the swing legs were relatively small and were neglected.

The angular velocity of each wheel and swing leg, the height of the step and the static and dyanmic friction factors between wheels and terrain were built as design variables. A dialog box was designed to input the data of the design variable easily. The dialog box names as “variable_box”is shown in Figure3. Using the dialog box, the data of the design variable of the model can be input. The upside of the dialog is the inputing area of the height of the step, static friction factor and dyanmic friction factor. The lower left of the dialog is the inputting area of the angular velocities of the wheels. The lower right of the dialog is the inputting area of the angular velocities of the swing legs. The angular velocities are negative while the wheels or the swing legs rotate clockwisely. The unit of the velocity is / so .

The setting of the height of step can be used to analyze the climbing obstacle capability of the robot; The setting of the static and dynamic factors can be used to simulate the different contact conditon between the wheel and ground. The seting of the motion velocity of the wheels and swing legs can be unsed to simulate the motion of the joint motors.

III. DYANMIC SIMULATION OF THE ROBOT USING ADAMS

Using the ADAMS simualtion model of the six-leg-wheel hybrid robot, the dynamic analyses under different motion condition can be achieved. The motion conditions in the simulation shown in Figure 4 were as follows: the height of the step is 30mm, the static friction factor between the wheels and the terrain is 0.3, the dynamic fricion factor is

0.1, the wheels rotate clockwisely, the angular velocity of all the wheels is 30 / so

，no motion was applied on each swing leg and the simulaiton time was 25s.

Figure 3 The dialog box of input the design variable

The figure 4(a) presents the curves of position, velocity and acceleration on the mass center of the robot shaft；The figure 4(b) presents the curves of the angular velocity and angular acceleration of the robot shaft. The curves represented the change of the robot posture while the robot climbing the 30mm-height step under no control algrithm.

Figure 4(c)~(e) show respectively the curves of the position, velocity and acceleration of the mass center of front-left wheel, middle-left wheel and rear-left wheel. Figure 4(f)~(g) shows the forces and torques between each wheel and ground, while contact1~contact6 represent the reaction respectively acting on the front-right wheel, middle-right wheel, rear-right wheel, front-left wheel, middle-left wheel and rear-left wheel.

Although no motions were acted on the joints of the swing legs, certain forces and torques act on the joints of the swing legs due to the coupling effect among the joints of the robot while the robot moving. Figure 4(h)~(i) present the foces and torques acting the joints of the left swing legs.

（a）The curves of the position, velocity and acceleration on the mass center of the robot shaft

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（b）The curves of the angular velocity and angular acceleration of the

robot shaft

（c） The curves of position, velocity and acceleration on the mass center

of the front-left wheel

（d）The curves of position, velocity and acceleration on the mass

center of the middel-left wheel

（e）The curves of position, velocity and acceleration on the mass

center of the rear-left wheel

（f）The contact forces between wheels and ground

（g）The contact torques between wheels and ground

(h) The forces acting on the front-left leg, middle-left leg and rear-left leg

(i) The torques acting on the on the front-left leg, middle-left leg and rear-

left leg Figure 4 The dynamic simulation example 1 of the six-leg-wheel hybrid

robot Change the parameters as follows, the height of the step

is 10mm, the static friction factor between the wheels and the terrain is 0.1, the dynamic fricion factor is 0.05. The simulation results were shown in figure5.

（a）The curves of the position, velocity and acceleration on the mass

center of the robot shaft

310

（b）The contact forces between wheels and ground

（c）The contact torques between wheels and ground

Figure 5 The dynamic simulation example 2 of the six-leg-wheel hybrid robot

IV. CONCLUSION The dynamic model of the six-leg-wheel hybrid robot

was built using ADAMS. A dialog box was designed to set the design variable such as the height of the step, the static and dynamic factors between wheel and ground and the motion of the wheels and swing legs. Using the dynamic simulation, the dynamic simulation curves can be achieved. The climbing obstacle capability of the robot can also be simulated using the dynamic model. The model will be used to analyze the dynamic characteristic of the robot under complex motion condition such as moving on the uneven terrain in the future works. The analyses can provide the theory basement for the design of the control system of the robot.

ACKNOWLEDGMENT This research is supported by the Fundation of State Key

Laboratoryof Robotics(No.:RLO200807)

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