Robust closed-loop process identification from step test

In this paper, a new identification technique for single-input single-output closed-loop processes from step test in the frequency domain is proposed. The method can identify multiple points on process frequency response over the important frequency range of interest. Consequently, an accurate transfer function can be obtained directly by matching the model in the frequency domain. Such a method is applicable to closed-loop with pre-tuned controller and can be used for on-line tuning of PID controllers. The applications on various typical processes have been employed to show both effectiveness and practicality of the method.