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The difference between measurement value in step (3) and target angle in step(1) is set as the control value of the joint anglewheel-ground contact point corresponding to the max can be foundthe value of joint angle with the minimum calculated is set as the target valuethe wheel leg joint at is controlled45 678andand Fig.5 Control process Fig.6 Control system architecture V. EXPERIMENT In this experiment, the validity of the posture control was tested and verified through the robot's moving on a slope with different tilt angles. The terrain was shown in Fig.7. The experiment environment was a slop which was a combination of two tilt angles, and the maximum tilt angle was 12 β =° . Let the robot's yaw angle of the forward direction be θ (the angle between the forward direction of the X axis of the robot and the maximum slope line of the slope). In order to compare the effects of the posture control, we chose a fixed posture when the robot moved: on the horizontal plane, all of MOBIT's arms were perpendicular to the ground ( 90 iα =± ° ), and it was the fixed posture. °= 20 θ Fig. 7 Experiment environment of robot's posture control Let the yaw angle 0 θ =° , the robot with fixed posture moved in the direction along the maximum gradient with a speed of 20 / cm s . At this point, as without the method of posture control, the robot's roll angle and pitch angle entirely depended on the terrain's change, and roll angle was always 0°, the pitch angle's change was shown in dotted line in Fig. 15. And the solid line represented changes in the terrain contour; the dotted line represented the changes of the robot's roll angle and pitch angle when using posture control. It could be seen from the figure, when using posture control, the robot's pitch angle decreased which kept the body horizontal. (a) Pitch angle Roll angleReal angle010203040 (s)051015(°) (b) Roll angle Fig. 8 Posture control on a slope with 0 θ =° Let the yaw angle 20 θ =° , the robot moved with a speed of 20 / cm s , therefore, both of roll angle φ and the pitch angle ψ should be controlled. The ideal posture of the posture control was that both of the pitch angle and roll angle were 0 and maintaining a constant heightg H . In Fig.8, the solid line represented the ground slope's changes in theory in the directions of pitch and roll, roll angle and the pitch angle could be calculated according to the following equations:
arcsin(sin( ) sin( ))arcsin(cos( ) sin( ))φ θ βψ θ β== (5) Dotted line in the figure represented the changes of the robot's pitch angle and roll angle when using the posture control. It could be seen from the figure that the robot's pitch angle and roll angle obviously reduced. There was a peak error at the slopes' highest point which caused by the speed of the response of the feedback of the control of robot legs. Experiment’s pictures are shown in Fig.9. (a) Pitch angle (b) Roll angle Fig. 8 Posture control on a slope with 20 θ =° (a) Posture control on a slope with 0 θ =° (b) Posture control on a slope with 20 θ =° Fig.9 Experiments pictures Ⅵ. DISCUSS AND OUTLOOK Aiming at high moving capability and good adaptability based on its moving security insured, the original coupled optimization function was introduced synthesizing the analysis of stability and driving power, and a suboptimal solution which improves both the global traction and stability performance was presented for the robot. Our research can supply a theoretic base of coupled posture control of stability and traction optimization of HWLV mobile robot, build the coupled optimization criteria, and realize the coupled optimization with the posture control. The next step in our work will be evaluated and validated the practical feasibility of control algorithm by experiments in uneven terrains with this robot ACKNOWLEDGMENT This work was supported by National Natural Science Foundation of China under No.61305119, the Middle-aged and Youth Scientist Incentive Foundation of Shandong Province (No. BS2013ZZ006),and Project of Shandong Province Higher Educational Science and Technology Program(No.J14LA53 and No.J13LB05),Authors gratefully acknowledge the support. REFERENCES