Internal model control with improved disturbance rejection

The two-degrees-of-freedom Internal Model Control structure allows a good combination of set-point control and load disturbance rejection for processes with time delay. A design principle for improved disturbance control with this structure is presented. It provides a response to load disturbances that is independent of the dynamics of the disturbance. The controller is suitable for slow load disturbances, deteriorating the control performance of time delay compensation controllers like the Smith predictor and the Dahlin controller. An earlier design approach, proposed in order to improve the disturbance rejection characteristics of the Smith predictor, is briefly discussed. The new design principle is compared with a simplified design method related to the original Dahlin algorithm. The comparison is made for equal robust stability. The comparison shows to what extent taking into account the disturbance dynamics is worthwhile, when designing a controller.