Disturbance rejection: A central issue in process control

This paper deals with the problem of disturbance treatment in process control. Disturbances can be external or generated by model uncertainties. Different approaches are outlined to reject or reduce their effect. Other than the simple feedforward, the most popular controller design methodologies to deal with disturbances are reviewed. Disturbance Observers, Uncertainty and Disturbance Estimator and Active Disturbance Rejection Control approaches are presented and their advantages and limitations highlighted. Also, the presence of time delays is considered. The main issues involved in each approach are illustrated by small examples.