Sliding mode control of a hydraulic servo system with large frictional torque

Precision tracking control for electro-hydraulic servo systems is a critical but difficult issue to be addressed, especially in the situation of large friction torques and low velocities. To address this problem, a novel control scheme is proposed in this paper based on a sliding-mode variable structure controller and a friction observer. The sliding-mode variable structure control law and the sliding-mode friction observer are detailed. The bristle dynamic friction model is adopted to estimate the friction torques and reduce the effects caused by the nonlinear frictions. The proposed control scheme is verified and validated through experimental results. The results show that good position tracking performance with high precision is attained in the presence of dynamic friction even with large friction torques and very low velocities.