Disturbance rejection in process control

This paper deals with the problem of disturbance rejection in process control. The disturbances could be external or generated by model uncertainties. Different types of disturbances are considered and also different approaches are outlined to reject or reduce their effects. After a general introduction about the relevance of the disturbances in process control, the controller design methodologies based on Disturbance Observers and Active Disturbance Rejection approaches are presented and their advantages and limitations when dealing with either non-minimum phase or unstable plants are discussed. Also, the use of iterative learning control techniques, mainly for batch processes, is outlined. The main issues involved in each approach are illustrated by different applications. Some conclusions and comparisons are finally drafted and some open problems are presented.