Tracking error-based static friction compensation for a bi-axial CNC machine

Contour errors usually result from unmatched dynamics, limited axial bandwidth and undesirable nonlinearities. For circular contours, friction, especially static friction, deteriorates the circular contouring accuracy at quadrant positions where there is reversal of velocities or motion from standstill. A two-stage tracking error-based static friction compensation scheme is proposed to compensate for static friction during the transition from the presliding regime to the sliding regime. The compensation consists of two phases, which are implemented sequentially in time. The first is a function of tracking error and the second is a function of tracking error and velocity. The compensation signals are then imposed at the input of the velocity control loop. Experimental results show that the two-stage tracking error-based static friction compensation scheme can effectively improve the circular contouring accuracy at quadrant positions without complicated modeling.