Role of secondary integral action in cascade control

Abstract

A comparative study of cascade control configurations involving PI-PI (PI mode for both the outer and the inner loop) versus PI-P (PI for the outer and P for the inner loop) has been made. The primary (outer) and secondary (inner) processes are represented by first order plus dead time models. Simulation results, obtained under both primary and secondary load disturbances, reveal the relative benefits that could be derived by employing a PI controller (instead of a P controller) for the inner loop. The added secondary integral action produces a 10 to 100 fold improvement in control performance to secondary load upsets under certain practically relevant loop conditions, namely, (i) an outer process with significant dead time (> 30% of the primary time constant) and (ii) an inner process with negligible dead time and a relatively small time constant (< 20% of the primary time constant).