Design of IMC filter for PID control strategy of open-loop unstable processes with time delay

A modified internal model control (IMC) filter has been proposed for the open-loop unstable process with time delay and a further IMCproportional-integral-derivative (PID) tuning rule is derived on the basis of the proposed IMC filter. Investigation of the IMC filter structure clearly demonstrates that a critically damped filter does not always provide satisfactory response and that this is probably due to lack of sufficient integral action. The conventional, critically damped filter is therefore replaced with a more general, second-order IMC filter for improved integral action. The present investigation shows that for the most unstable processes, the underdamped IMC filter provides the desired integral action and improves the closed-loop performance of the system except for the unstable processes with a strong lead term. An exhaustive simulation studies has been performed to show the advantage of the proposed method in both nominal and robust cases. By incorporating Kharitonov's theorem, closed-loop time constant (lambda) guidelines have been provided for the first-order delayed unstable process (FODUP). Copyright (C) 2010 Curtin University of Technology and John Wiley & Sons, Ltd.