IMC−PID Controller Design for Improved Disturbance Rejection of Time-Delayed Processes

The IMC−PID tuning rules demonstrate good set-point tracking but sluggish disturbance rejection, which becomes severe when a process has a small time-delay/time-constant ratio. In this study, an optimal internal model control (IMC) filter structure is proposed for several representative process models to design a proportional−integral−derivative (PID) controller that produces an improved disturbance rejection response. The simulation studies of several process models show that the proposed design method provides better disturbance rejection for lag-time dominant processes, when the various controllers are all tuned to have the same degree of robustness according to the measure of maximum sensitivity. The robustness analysis is conducted by inserting a perturbation in each of the process parameters simultaneously, with the results demonstrating the robustness of the proposed controller design with parameter uncertainty. A closed-loop time constant λ guideline is also proposed for several process models to cover a wide range of θ/τ ratios.