Identification and control of an overshoot lead-lag plant

Whenever a process has two competing dynamic effects of two different time scales and the slower effect is less influential to the overall steadystate behavior, the resulting step response typically exhibits an overshoot leadlag behavior. In applying a conventional PI controller to this type of process will lead to very long time periods of offset under the closed-loop stability consideration. In order to overcome this problem while using existing industrial controller equipment, a simple modification is proposed which adds a filter to the PI controller structure to cancel the lead portion of the overall dynamics. Moreover, a systematic procedure is suggested to identify the model parameters and to give guidelines of selecting the filter time constant and the PI settings. The combined identification and control procedure for lead-lag processes is very robust and provides an excellent closed-loop performance.