The fundamental tradeoff between performance and robustness - A new perspective on loop shaping - Classic control revisited part II

Control systems must fundamentally trade off performance with robustness to plant uncertainty. For example, increasing the controller gain is desirable for improving tracking and disturbance rejection, but only up to a point at which uncertainty in the plant gain can potentially render the closed-loop system inevitable discrepancy between the true plant and its model. Hence, controller design aims at achieving an acceptable tradeoff between the conflicting goals of tracking or regulation performance versus robustness to plant uncertainty. Gain and phase margins are used in loop-shaping controller and design as a measure of robustness. In this article, only stable plants are considered, and plant uncertainty model is used.