PID controller tuning of nonstationary flow processes

A systematic procedure for the tuning of a time-variable PID controller is described in the paper. The complexity of the time-varying process model involving variable flow rates and considerable time delay is first reduced by introducing a modified time-scale, which leads in a model with constant parameters. The model is then identified with respect to the new time scale by a relay experiment. PID controller is tuned based on the model and transformed into time domain, which leads to a controller algorithm with coefficients, which vary as the flow rate changes. The performance of the controller is demonstrated by comparing it to the performance of a constant coefficient PID controller. Different model based tuning methods are also compared, and it is shown that the classical Ziegler-Nichols tuning can be improved considerably. Copyright (C) 1998 IFAC.