Robust control

Feedback control is used to compensate for unpredictable disturbances to a plant or process and for inaccuracies in predicting plant response because of errors or approximations in plant models. There have always been several techniques for designing compensators: feedforward control based upon measurements of disturbances and placing margins in the design specifications of feedback controllers are among the basic ones, while adaptive and self-tuning controllers and sliding mode control are among the more complicated ways of improving plant models. Robustness for many of these was built in through the allowance of safety margins, particularly gain and phase margins. An alternative, philosophically related to gain and phase margins, is the still-developing field of robust control theory, which entails mathematical design of control laws to meet defined uncertainty levels. In this chapter we briefly and superficially introduce this subject.