PID CONTROL : PERFORMANCE AND ROBUSTNESS

Abstract

PI and PID controllers are widely used in the industries and it is foreseeable that they will continue to be so. This thesis gives new insights and results on the performance and robustness of PI and PID controllers. Robust control design is an area of intensive research. Robustness is important as the process model used is often an approximation of the industrial process. Gain margin and phase margin are traditional measures of robustness. Designing a PID controller to meet gain and phase margin specification is a well-known design technique. If the gain and phase margin are not specified carefully then the design may not be optimum in the sense that there could be greater robustness and better performance. In this thesis, we study the change in ISE performance index when different gain and phase margin specifications are ll8ed in the design of PI and PID controllers and show how one can specify the gain and phase margin wisely to give the best of performance and robustness. PID controller can give improved performance over the PI controller when used appropriately. For controlling processes with dead-time, if the estimate of the dead-time is accurate, the Smith Predictor can give a faster set-point response than the PI or PID controller. The quantitative knowledge of the potential improvement that can be obtained from PI, PID and SP controllers is given in this thesis. This information will be useful when we need to choose among PI, PID and SP controllers for implementation. It may also be helpful in the design of auto-tuners and knowledge based feedback controllers.