Considerations on the disturbance attenuation problem for PI/PID controllers for a generic load disturbance dynamics

Disturbance attenuation is often recognized as the primary concern of a control system. Regulation of the operating conditions is the usual task work to be pursued by a feedback controller. However, much of the academic works almost concentrate on set-point experiments for controller evaluation. Even those that explicitly concentrate on the disturbance attenuation problem, usually concentrate on input load disturbances. They exclude the specific, but usually not considered in the literature, case when the path from the disturbance to the controlled variable is different to the path from the controller output to the controlled variable. There are some process units such as heat exchangers or distillation columns where this situation may be encountered. However actual feedback controllers, usually of PI/PID type, do not take into account this situation. The point raised in this paper is that an increment of performance can be obtained if the information regarding the disturbance dynamics is included in the PI/PID design stage.