Further results on guaranteed dominant pole placement with PID controllers

In the literature (Journal of Process Control 19(2009):349-352), the problem of dominant pole placement with PID controller for a high order or delay system is considered. Two step-by-step systematic design procedures for guaranteed dominant pole placement with PID controller based on root locus and Nyquist plot are proposed, respectively. However, we find that, in some cases, the dominance on the transient response of the closed-loop system may degrade since the introduced two closed loop zeros are close to the pre-selected two dominant poles with the increase of K_P value. To handle this problem, the desired region of which all the closed-loop poles lie in except the pre-selected dominant poles is redefined and a special pre-filter is recommended to use at the input. Furthermore, two alternative dominant poles with critical damping ratio are selected and the corresponding design formulae of PID controllers are derived. The property of nonovershoot can usually be obtained by our improved method. Two same examples as those of [1] are used to demonstrate the effectiveness of the improved method.