Nonlinear variable damping integral sliding mode control for electro-hydrostatic actuator

For electro-hydrostatic actuator (EHA), the traditional sliding mode controller has many defects, such as control signal chattering, difficulty in acceleration information acquirement and controller parameters tuning, which make the controller difficult to be applied in practice. To solve the above problems, the reduced order mathematical model of EHA is obtained reasonably by using singular perturbation theory, which result in avoiding the use of acceleration information. On this basis, a novel nonlinear variable damping integral sliding mode controller (NSMC) is synthesized by employing the reduced order model. NSMC can adaptively adjust the system damping ratio from underdamping to overdamping according to the position control error, and can effectively improve the position step performance. Besides, filter-based uncertainty estimator is designed to estimate and compensate the parameter uncertainties and external disturbances of EHA in real time. Due to the introduction of sliding mode surface integral term and uncertainty estimator, switching function is unnecessary, on the one hand, the free-chattering sliding control can be achieved. On the other hand, the whole dynamic process is determined directly by the dynamic of sliding mode surface, so that sliding mode surface parameters can be tuned directly according to EHA’s control performance index, which greatly simplifies the parameters tuning process. Meanwhile, the stability of the whole closed-loop system and sliding mode surface is proved via Lyapunov stability theory. Finally, by comparing respectively with PI, conventional sliding mode (SMC) and damping variable sliding mode controller (DVSMC), the simulation results indicated that NSMC can effectively improve EHA position tracking performance and enhance the robustness of parameter uncertainty and external disturbance.