Assessment of Interaction in Process Control Systems

Interaction between control loops has long been recognised as an area for concern within the field of process control. The first formalised method for assessing the level of interaction present in a system of loops was Bristol’s relative gain array. Being based solely on steady state data, construction of the RGA is pleasingly simple and application of the technique has attracted significant researchers. Where strong interaction effects arise as a problem in practice, their elimination can be approached at a progression of levels each carrying a progressively increased cost implication. The decision as to which of these levels of approach is warranted in overcoming any specific case of interaction involves gaining insight into the mechanics of the system itself. In the future, development in the use of detailed dynamic modelling and simulation may lead to the possibilities of evaluating the degree and form of interaction present and of developing compensating strategies. In this paper we aim at drawing attention to the fact that in considering interaction between feedback control loops an important distinction should be drawn between cases of true (two-way) interaction and cases of what can be termed as “one-way interaction” or “disturbance propagation” from one loop to another.