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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 the 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 autotuning systems. Two examples are given to illustrate the effectiveness and robustness of the proposed method.