Quicktune: A reliable automatic strategy for determining PI and PPI controller parameters using a FOPDT model

Abstract

It is well known that many complex processes are often satisfactorily controlled by means of simple two-term (PI) and three-term (PID) controllers. However, for those processes which use PI and PID loops, the task of ‘tuning’ can be very time-consuming and expensive operation. Consequently, any help offered to the process operator to ease this tedious task and at the same time produce consistent behaviour is very desirable. This paper presents a CAD software package that automatically determines the necessary PI parameters for two-term operation, identifies those situations where PI is not appropriate, and in these cases designs an alternative pPI strategy. The original software used the Ziegler-Nichols empirical settings, but this paper considers a range of options which (in theory) should realise a less oscillatory response. All of the strategies considered are based on a first-order-plus-deadtime (FOPDT) plant model. The paper also introduces a new set of tuning relationships which, despite their simplicity, appear to realise a consistent sensibly damped step response, much superior to the Ziegler-Nichols values. The paper concludes by considering two ‘real applications’, namely, flow and pH control, which illustrate the quality of the performance achievable using the CAD software