Frequency based tuning of SISO controllers for two-by-two processes

A systematic tuning method of SISO controllers for two-by-two processes is proposed. The first step consists of selecting, for each loop, the desired tracking closed-loop dynamics. From this information, the open-loop characteristics are deduced, leading to the frequency responses (magnitude and phase) of the unknown SISO controllers. An optimisation method is used to get a good approximation of each corresponding transfer function. Constraints are implemented to ensure a stable closed-loop system. For processes having delays, a Smith predictor structure is proposed. This frequency based tuning method is tested on three different processes (minimum and nonminimum phase) and leads to performances corresponding closely to the closed-loop specifications.