High Precision Control for Hydraulic Servo Systems with Large Frictional Torques and Experimental Research

A novel control scheme for hydraulic servo systems with large frictional torques is proposed based on a sliding-mode variable structure controller combined with a frictional state observer. The sliding-model variable structure control law and the sliding-model frictional state observer are detailed. The LuGre dynamic friction model is adopted to estimate the frictional torques and reduce the effects caused by the nonlinear frictions. Experimental research on hollow hydraulic motor systems is carried out to validate the proposed control scheme. The experimental results show that the application of the proposed control scheme is successful and the good position-tracking performance with high precision is attained in the presence of dynamic frictions even with large frictional torques and very low velocities.