Design of 6-DOF parallel mechanical leg of wheel-leg hybrid quadruped robot

Abstract: In the field of agricultural engineering, a lot of work is carried out on rough terrains, such as rugged mountains or hills. Mobile robots are very suitable to be used in these areas, and these kinds of robots can work in the rugged mountains or hills environment without caring about the safety of workers. The wheel-leg hybrid quadruped robot is very suitable to be used in agricultural engineering. It can move with the dynamic driving wheels in the road with high speed, and also can walk by stepping alternately in the ruins with its mechanical legs. The wheel-leg hybrid quadruped robot can also skate in a flat road by the wheels without dynamic driving, and in this state, the robot can obtain the maximum speed with the minimum energy consumption. In order to design a kind of wheel-leg hybrid agricultural quadruped robot, which can achieve walking, dynamic wheel moving, and roller-skating, a new mechanical leg based on the 3-UPS(U-universal joint, P-prismatic joint, S-spherical joint) parallel mechanism is proposed. The structure parameters of the mechanical leg are determined. The design schemes of the mechanical leg and the wheel-leg hybrid quadruped robot are given. First, using the vector chain method, the inverse position equation is presented, and the velocity transmission model is established. The workspace analysis of the leg mechanism is performed and the 3D (three-dimensional) map of the workspace is drawn by searching method. The relationship between the structural parameters of the mechanism and the workspace is revealed. The 3D map of condition number of Jacobian matrix is drawn in the workspace based on the velocity transmission model. Also, a set of kinematics performance evaluation indices are defined, and the kinematics dexterity of the leg mechanism is analyzed. And the relationship curves between structure parameters and the kinematics performance evaluation indices are drawn. These curves reveal the influence of structural parameters on the flexibility of the mechanism. Then, based on the indices of workspace and kinematics dexterity, the structure parameters are analyzed by Monte Carlo method. By establishing the probability space model of each structure parameter, an excellent set of structural parameters are found: The distance between two universal joint's rotation center points of the fixed platform is 230 mm, the distance between two spherical joint's rotation center points of the moving platform is 70 mm, the maximum diameter of the telescopic sleeve is 60 mm, and the lengths of each branch's sleeve and the telescopic rod are both 500 mm. Finally, based on the excellent set of structural parameters, the virtual prototypes of wheel-leg hybrid quadruped robot and its parallel mechanical leg are designed. The motion simulation analysis of the virtual prototype is performed, and the driving speed curves of parallel mechanical leg's driving joints are obtained. Simulation results show that: The driving parameters of the mechanical legs are very stable, and the peak values of the driving parameters are within the reasonable range. These prove that the design scheme and the structural parameters of the mechanical leg are very reasonable. The results provide the theoretical reference for further study of wheel-leg hybrid quadruped robot.