A Performance Comparison between a Smith Predictor and a PID Controller for Control Valve Hysteresis and Process Non-Self-Regulation

The performance of a Smith Predictor compared to a PI controller for setpoint and load disturbances in self-regulating processes has been studied and documented extensively. These studies have shown that the Smith Predictor´s response to load disturbances is noticeably better and the Smith Predictor´s response to setpoint changes is dramatically better than that of a PI controller except for extremely small ratios of deadtime to time constant. However the effect on the comparison of derivative and batch reset action, control valve hysteresis, and process non-self-regulation has not been sufficiently documented. The relative merits of different performance criteria are discussed. A dynamic simulation program is used to generate results to compare a PID controller to a Smith Predictor for load disturbances and a PID controller with rate before reset and batch reset to a Smith Predictor for setpoint disturbances. These results are used to develop general conclusions on the capabilities of the two methods for applications in the chemical industry for varying degrees of deadtime, hysteresis, and process self-regulation.