On-Line Tuning of Controllers for Unstable Processes

In this paper, a relay based on-line controller tuning method for open-loop unstable processes is presented. In the proposed method, prior to controller tuning a relay in tandem with the controller and plant induces limit cycle oscillations. Based on the limit cycle measurements, a ¿rst order transfer function model of the process dynamics is obtained. Fourier series based curve ¿tting with the options of nonlinear least squares method and trust-region algorithm has been used to detect the peaks and zero crossings of the noisy limit cycle output signals. Using a control tuning method based on the explicit process model the controller settings are re-tuned non-iteratively to achieve enhanced performance without disrupting closed-loop control. The results obtained with this method are presented for typical design examples. A comparison of the performances with those given by earlier workers for unstable processes is presented.