Performance evaluation of nonlinear surfaces for sliding mode control of a hydraulic valve

This paper discusses the problem of position control of a directional valve in absence of a plant model. Due to strong nonlinearities of the system, advanced control strategies are mandatory if ideal closed-loop performance is recommended. Oftentimes, strategies based on complex PID-control structures using nonlinear gains for different controller branches, containing many highly correlated parameters are introduced to reach the desired performance. In order to overcome the complexity, this paper discusses different sliding mode strategies and ways of defining the closed-loop performance by using nonlinear sliding surfaces. A new analytic approach based on the idea of dynamical pole placement to define an adaptive sliding surface is presented for increasing the closed-loop performance. The automated parameter design is carried out by using a hardware-in-the-loop optimization of the step responses. Test results confirm an improved closed-loop performance for the introduced approach compared to other linear and nonlinear sliding surfaces.