Limiting in feedback control systems

Limiting effects exist in feedback control systems because of the restricted range of operation of components or because of purposeful limiting introduced for the protection of equipment. Such limiting effects increase the difficulties associated with the analysis and synthesis of systems because of the nonlinear mathematical relations which result. As such effects can markedly modify system performance, there is a practical need for a simplified method for taking them into account. The effect of limiting on system performance is studied here in terms of the frequency response of the system. It is found that limiting of the most elementary form “ simple limiting ” improves system stability, except in those systems which are conditionally stable or have a conditional degree of stability. In such instances, precautions are suggested to prevent the limiting action from adversely affecting system performance. It is found that the velocity limiting of a servomotor introduces the effect of phase lead and hence can be used to improve system stability. On the other hand, the limiting of servomotor acceleration directly or indirectly (as by means of current or pressure limiting) is found to introduce a phase-lag effect which can be detrimental to system performance. Electronic analogue computer techniques were employed to obtain much of the data presented here.