Design and experimental evaluation of robust variable structure control for hydraulic actuators

In this article we design and experimentally evaluate a variable structure force control law for a hydraulic system interacting with the environment. For this purpose we use a nonlinear mathematical model where the nonlinear dynamics are explicitly addressed in order to achieve robust performance over a wide range of operating conditions. The sliding mode procedure is carried out to design a robust controller that satisfies performance specifications for force regulation. The controller is derived using the equivalent control method complemented by a reaching law approach to further improve the performance. After deriving the dynamics of the equilibrium manifold, rigorous stability verification for the system is provided. The controller was implemented on a fully instrumented hydraulic actuator. A set of experiments were performed to study the effect of variation in force set-point, supply pressure, hydraulic parameters, and environmental stiffness. The experimental results show that the proposed controller is robust to the variation of hydraulic parameters as well as environmental stiffness.