Stability criteria for instrument servomechanisms with coulomb friction and stiction

IT IS WELL KNOWN that relative motion between rubbing surfaces is opposed by friction forces. In general, a force directly proportional to velocity is produced by a component known as ¿viscous friction.¿ Normally, another component, the coulomb friction force C, produces a force that is essentially constant regardless of velocity. In addition, if the surfaces are initially stationary, the force required to start motion is greater than the coulomb friction force. Since the surfaces seem to adhere, the effect is called ¿stiction¿ and the magnitude of the required starting force is called the stiction force S. These effects are illustrated graphically in Fig. 1. Note that the ordinate may be force or torque, and the dimensions of C or S obviously must be adjusted as needed.