Experimental comparison between PID control and friction compensation control for a class of nonlinear system with friction

A point-to-point positioning control of systems with highly nonlinear friction was studied. In view of variable friction caused by the change of load torque, an experimental comparison was made between the valve-controlled hydraulic motor servo system with PID control and that with friction compensation control. The experimental results show that the gross steady errors are caused by friction when the system is controlled by the conventional proportional control algorithm. Although the errors can be reduced by introducing the integral control, the limit cycle oscillation and the long setting time are caused. The positioning error for a constant load torque can be eliminated by using fixed friction compensation, but poor positioning accuracy is caused by the same fixed friction compensation when the load torque varies greatly. The dynamic friction compensation based on the error and change-in-error measurements can significantly improve the position precision in a broad range of change of load torque.