SUMMARYThis paper presents design and experimental evaluation of a robust contact task controller for an electrohydraulic actuator that operates under significant system uncertainties and nonlinearities. The designed controller allows the actuator to follow a free space trajectory, and upon contact with an uncertain environment exert a desired force, by stably passing through the transition phase from free space to constrained space. The scheme is essentially the combination of two distinct control laws that are individually designed for position regulation in free space and force regulation during sustained contact. Both controllers are designed, using a nonlinear approach within the framework of quantitative feedback theory (QFT), to satisfy a priori specified stability, tracking and disturbance rejection specifications. They are then combined with a simple switching law to form a contact task controller. Due to the existence of a switching, the resulting control system is non-smooth. The stability of the controller is then analysed using an extended version of Lyapunov's second method under the condition of existence and uniqueness of Filippov's solution. Experiments, performed on a typical industrial hydraulic actuator, include motion through free space, contact with the environment and the transition between the two. The proposed QFT contact task control scheme enjoys the simplicity of fixed-gain controllers, is easy to implement, requires very little computational effort, is robust to the variation of hydraulic functions as well as environmental stiffness, and results in responses with good performance in both transient and steady-state periods. Additionally, the controller only requires measured contact force and actuator position as feedback; this makes the controller attractive for industrial implementation.
