Stable control of an electro-hydraulic actuator during contact tasks: Theory and experiments

This paper describes a systematic approach for controller design and stability analysis of hydraulic actuators during contact task. The goal is to design and experimentally evaluate a robust controller that: (i) is capable of following a reference position or a reference force in free or constraint space, respectively and (ii) guarantees the stability during the transition phase. A piecewise continuous controller is designed which regulates the actuator's position and upon contact with the environment switches to a force controller. Since the system is nonsmooth due to a discontinuous controller, a new extended version of Lyapunov's second method for nonsmooth systems is used for stability analysis of the developed control system. The stability is proven by using a smooth Lyapunov function candidate under the condition of existence and uniqueness of Filippov's solution (1964, 1979). Experimental results verify that the proposed controller can effectively control contact tasks; meanwhile it is realizable and therefore suitable for practical implementation.