Kinematic analysis of leg mechanism of six-legged walking robot

Abstract: In order to increase the automation level of agricultural operations, broaden the application scope of agricultural robot, and improve the ability of adapting to the different working environment and flexible work, a novel three-degree-of-freedom leg mechanism used in the six-legged walking robot is introduced. This leg mechanism comprised a drive mechanism based on 2RUS+RU parallel manipulator and a traveling mechanism based on parallelogram mechanism. The motor of drive mechanism is fixed on body frame. This leg mechanism has not only the advantage of parallel mechanism, but also a good protectiveness. In this paper, kinematic analysis and simulation of leg mechanism of six-legged walking robot is accomplished. Firstly, based on the intrinsic relation between the angular velocity and the angular velocity of Euler angles of the dynamic platform, the relationship matrix between linear velocity and angular velocity of driving mechanism is established. Based on that, the entire Jacbian matrix in the 3×3 form of the leg mechanism is deduced by using the relationship matrix derivative method, and the explicit Hessian matrix in the 3×3×3 form of the parallel drive mechanism and the leg walking mechanism is obtained, which also adopts the method of derivative matrix. Secondly, with the rationed rotation angle of the revolute joint ranging in -45°, 45°, a distribution diagram of condition number of the integral Jacobian matrix is drawn. The condition number of integral Jacobian matrix is changed slowly and smaller in the central region of the workspace in this diagram, so that the mechanism flexibility is good in this area and can meet the requirements of the robot movement. Lastly, under the conditions that were step increment of 300 mm and crossing obstacle height of 200 mm, the trajectory planning of the foot end is accomplished and the track function of the foot end is presented based on the method of combined polynomial, which can make the robot stable and free from impact and have a good landing performance in the process of motion. Under the condition of the geometrical parameters of the leg mechanism, the simulation model of the robot's leg is established and the track function of the foot end is presented when the six-legged walking robot walks straight. The function of the trajectory is as input. The velocity and acceleration curves of driver deputy based on the analytical solutions and virtual prototype are described. Through the data analysis in curves, the accuracy of angular velocity and angular acceleration based on the theory and simulation results both can reach 10-3 mm, which indicates that the theoretic analysis is correct and feasible. In addition, the simulation results show that the maximum of angular velocity of drive joint is less than 0.33π rad/s and the maximum of angular acceleration of drive joint is less than 1.22π rad/s. Therefore, the driving speed and torque of leg mechanism are both smaller in the course of the planned trajectory movement, which is favorable to drive. Accordingly, the rationality of the trajectory planning is confirmed. The results can provide the theoretical reference for the development and control of the hexapod robot.