Positive feedback in servosystems

IN THE DESIGN of servosystems, two essentially opposing requirements have to be met: fast response and stability. Nonlinear and switching techniques provide new means to decrease the response time of a system without sacrificing stability. An accepted criteria for the evaluation of the response of a switching-type positioning servosystem is based on the time required by the system to settle to the steady state without overshoot, after a step input has been applied to the system. D. McDonald and J. B. Lewis described methods to improve the response time of servosystems by varying the damping coefficient of the system during the transient period. Another method, described by McDonald and A. M. Hopkin, is to reverse the forcing function of the system during the transient period. This method, although providing fast response, requires special measures to avoid erratic operation near the rest position.