Auto-tuning of cascade control systems

In this paper, a novel auto-tuning method for cascade control system is proposed. By utilizing the fundamental characteristic of cascade control systems, a simple relay feedback test is applied to the outer loop to identify simultaneously both inner and outer loop model parameters. A model matching PID controller tuning method based on Pade coefficient and Markov parameter approximation is proposed for the overall system performance. Consequently, well-established model based PI tuning rules for inner loop combined with the model matching method for outer loop make the auto-tuning process simple and yet effective. It can be directly integrated into commercially available industrial Auto-tuning systems. Two examples are given to illustrate the effectiveness and robustness of the proposed method.