ROBUST POSITION AND VIBRATION CONTROL OF AN ELECTROHYDRAULIC SERIES ELASTIC MANIPULATOR AGAINST DISTURBANCE GENERATED BY A VARIABLE STIFFNESS ACTUATOR

Abstract

This thesis discusses a hybrid robust control for trajectory tracking and vibration suppression of an electrohydraulic series elastic manipulator system (ESEMs) against nonlinear disturbances of a novel variable stiffness actuator (VSA). A combination of series elastic manipulator (SEM) and adjustable springs mechanism (ASM) provides the ESEMs a suitable dynamic in unknown environments as well as low energy consumption. Besides, the ESEMs can take advantages of a hydraulic actuator, such as high-power density, high torque-to-weight ratio by using electrohydraulic servo system (EHSs) as primary torque generator. However, the ESEMs is a complicated system with nonlinear elements, such as friction, leakage, coupling torque. The control performance is mainly affected by these factors. The control strategy in this study includes a positioning strategy for the EHS system and residual vibration elimination strategy for the end effector. The trajectory tracking controller used backstepping approach to create two loops, based on system states, the position control loop and the torque control loop. An adaptive fuzzy sliding mode control (AFSMC) was applied for each loop to ensure the stability and the robustness in the presence of the known uncertainty of both mechanical and hydraulic subsystem. Lyapunov’s theory proved the stability of the overall closed-loop system. In addition, a fuzzy input shaping (FIS) scheme was proposed to maintain the effectiveness of the vibration suppression to deal with the stiffness regulation over the expected operating range of the VSA system. By a number of real-time experiments, the evaluation of the hybrid controller in position and vibration control is carried out.