Dynamic analysis and decoupled control of a heavy-duty walking robot with flexible feet based on super twisting algorithm

Huanhuan Ren,Chengzhi Su,Lizhong Zhang

doi:10.1177/0020294020974059

Abstract

In this paper, the dynamics and control strategies of a biped robot with 6-DOF parallel leg mechanism are studied. Firstly, the multi-body kinematic model and dynamic model of the robot are established. Secondly, the insufficient stiffness of robot’s feet and the coupling effect between the kinematic chains are considered in dynamics modeling, and the rigid-flexible coupling model is established by using ADAMS and finite element method. Finally, aiming at the position deviation and system vibration caused by the flexible foot, a strategy based on the combination of a computed torque controller (CTC) and a second-order sliding-mode super twisting algorithm (STA) is designed. At the same time, the control strategy based on CTC and PID and the control strategy based on CTC and sliding mode control (SMC) are designed to compare with CTC-STA. The results show that CTC-STA has faster regulation ability and stronger robustness than CTC-SMC and CTC-PID.

Highlights

The legs of most mobile robots are connected in series by a group of open-loop kinematic chains, which have the characteristics of large-scale workspace, poor stiffness, and simple dynamics.[3,4,5]
In order to improve the dynamic performance and the robustness of the control system, a strategy based on the combination of computed torque controller (CTC) and super twisting algorithm (STA) is designed to adjust the system uncertainty caused by the flexible foot
The contribution of this paper is to provide a dynamic analysis method of the flexible parts of the robot system, and take the flexible deformation into account in the dynamic control, and provide a feasible control strategy

Summary

Introduction

Mobile robots are more and more widely used in many fields and the analysis of function and stability is essential in robot research and application.[1,2] At present, the legs of most mobile robots are connected in series by a group of open-loop kinematic chains, which have the characteristics of large-scale workspace, poor stiffness, and simple dynamics.[3,4,5] In order to enhance the loadcarrying capacity and stiffness, more and more robots with parallel leg mechanism are being developed and studied.[6,7,8] In theory, since the load is supported by multiple chains, parallel leg mechanism tends to have a large load-carrying capacity and high structural stiffness.[9]. Van et al.[16] proposed a new method based on the combination of CTC, high-order SMC and fuzzy compensator to control uncertain robot manipulators by using only position measurements. In order to improve the dynamic performance and the robustness of the control system, a strategy based on the combination of CTC and STA is designed to adjust the system uncertainty caused by the flexible foot. The deformation u of flexible body module in ADAMS is obtained by calculating mode superposition equation (17). In order to obtain reliable simulation results, the effect of foot A’s deformation is treated as position disturbance, as shown in equation 18(b): Àðt[1] ÀbÞ2. The traditional CTC combines the calculation of nonlinear force term Vðq, q_Þ and gravity term GðqÞ in equation (11) and PD control into the controller to realize the decoupling and linearization of the system. The closed-loop system’s stability can be proven by Lyapunov function:

À ðQÞlminðQÞkXk

XTQX ð32Þ

Conclusion