On the compensation of friction forces in precision motion control

Friction is a primary disturbance in motion control systems. Low velocity friction, which includes stiction, causes the motion of an axis to have a nonsmooth start and to stop near zero velocity introducing positioning and tracking errors. Friction has been studied extensively in tribology. Mathematical models exist to characterize friction mechanisms. One definite information available from the models is that friction force primarily depends on the axis velocity. Based on this information, a practical approach for friction compensation is to use a feedforward controller, the output of which depends on the command velocity. Repetitive control can be applied to synthesize such feedforward control laws by repeated trials. The effectiveness of this approach is demonstrated by experiments on a machine tool feed drive. Also discussed are standard feedback control approaches for friction compensation and an open-loop pulse control strategy for situations where a motion axis must be moved over a minute distance