Dual speed control scheme of servo drive system for a nonlinear friction compensation

Servo motor drive systems with ball-screw and timing-belt are widely used in a numerical control, robot, factory automation and industrial applications. Most of the servo motor and drive system transfer the motor torque to loads via mechanical connections. However, the nonlinear friction of coupled mechanical devices reduces the performance of a servo drive. Especially, in a low speed range, tracking errors are serious due to the break-away friction and Stribeck effects. In this paper, a new dual speed controller is proposed for the compensation of nonlinear friction torque. The proposed dual speed controller has an outer speed controller and an inner friction torque compensator. The friction torque compensator adds additional torque corresponding to a nonlinear friction of mechanical devices, so the actual speed quickly tracks the reference value. Since the proposed nonlinear friction torque compensator uses the actual speed information without any motor parameters and mathematical model, the proposed compensator has a very simple structure and high stability. The proposed control scheme is verified by computer simulation and experimental results.