Friction pre-sliding control and sliding impulse compensation

Abstract

Precision position control in the order of micro metres demands that friction is either compensated for or included in the control system loop. The dynamic friction models including LuGre, Elasto-plastic and Generalized Maxwell-Slip (GMS) have been shown to model friction. Each of these dynamic models have the state variables of bristle displacement (z) and bristle displacement velocity dz/dt. They each have an equation that relates the bristle displacement velocity to the sliding velocity (v). This principle is disputed. It is shown that the presliding friction force can be analytically calculated from the applied electrical force, without having to have any relationship of the sliding velocity to the bristle displacement velocity. This leads to a feedback control system for presliding bristle displacement, without actual measurement of it.The dynamic friction models are currently used in real time control to provide feed forward compensation. Feed forward compensation is pre-emptive in that it provides anticipated friction force based on the planned velocity. A new control system strategy and method of overcoming the sliding friction forces called “impulse control” is developed which can be applied pre-emptively. Impulse control means the control of the time integral of a state variable. The term is not used to refer to control by short force pulses in this paper. The impulse control strategy is more generally applicable than just for the control of systems with friction. This friction control methodology is still awaiting a full practical implementation.