Design and Analysis of IMC based PID Controller for Unstable Systems for Enhanced Closed Loop Performance

In this paper, design of PID controller is analyzed for unstable second order processes with time delay based on IMC method and H2 minimization. A new desired closed loop transfer function is obtained based on which the PID controller is designed. Maclaurin series is used to approximate the controller expression as a PID controller. Improved closed loop performances are achieved with the proposed method when compared to the recently reported methods in the literature. Comparative analysis has also been carried out with modified Smith predictor schemes and showed that the proposed method is superior. Further, an analysis is carried out based on maximum sensitivity for arriving at systematic guidelines for selection of the closed loop tuning parameter which is essential for unstable systems. The bounds for this tuning parameter are analyzed using the maximum sensitivity.