INTERNAL MODEL CONTROL WITH IMPROVED DISTURBANCE REJECTION

The two-degrees-of-freedom Internal Model Control structure allows a g ood combination of set-point control and load disturbance rejection fo r processes with time delay. A design principle for improved disturban ce control with this structure is presented. It provides a response to load disturbances that is independent of the dynamics of the disturba nce. The controller is suitable for slow load disturbances, deteriorat ing the control performance of time delay compensation controllers lik e the Smith predictor and the Dahlin controller. An earlier design app roach, proposed in order to improve the disturbance rejection characte ristics of the Smith predictor, is briefly discussed. The new design p rinciple is compared with a simplified design method related to the or iginal Dahlin algorithm. The comparison is made for equal robust stabi lity. The comparison shows to what extent taking into account the dist urbance dynamics is worthwhile, when designing a controller.