Tracking control of an electrohydraulic manipulator in the presence of friction

Analysis and estimation of friction and compensation for its effects in the control of an electrohydraulic manipulator is addressed in this paper. The specific hydraulic manipulator is an integral part of an automated fish processing machine which has been developed in our laboratory. The analysis reveals that considerable static and dynamic friction exists in the system. An available nonlinear observer for Coulomb friction, is modified to simultaneously estimate friction, velocity, and acceleration. A novel observer-based friction compensating control strategy is developed for improved tracking performance of the manipulator. The approach is based on acceleration feedback control. Experimental investigations show that this controller significantly outperforms a conventional, proportional-plus-derivative (PD) controller. The general approach presented in this paper, may be applied to compensate for friction in any servomechanism, particularly when the actuator dynamics is not negligible.