An Auto-Tuning Method Based On The Modified Relay Feedback Identification

Astrom proposed a closed loop identification technique based on relay feedback tests in 1984. First of all, the relay feedback tests serve as an alternative to the conventional techniques on trial and error. Second, the on-line relay technique saves much time on the identification process. Hence it is possible to realize auto-tuning based on the relay feedback technique and the auto-tuning controller became a research focus from then on.This paper summarizes the developments and the applications of the relay feedback in those years. The methods for relay identification can be classified into the time domain and frequency domain approaches. The time domain approaches can obtain the parameters with great accuracy. However, the complex calculations limited its further extension. Increasing practitioners took their efforts on the frequency analysis to get the approximate solutions. Although it has been proven adequate for a large number of control loops, there are significantly approximation deviations especially for processes with large dead time, or even nonlinear processes.This paper provides the new concept of the phase derivation to compensate the span between the approximation value and the true value. Hence the critical information can be analytically obtained according to the observed information in one relay test. Meanwhile, the phase derivation is applied in the identification, which is available from the single test and avoiding additional tests for prior information of the state gain and dead time. The closed loop identification allowed the system running around the working point during the process of the identification. There is less influence on the normal work. Additionally, this paper presents the analysis of the existence of the sustained limit cycle and the robust of the whole system.Finally, based on the study of the modified algorithm, an auto-tuning control software package is developed in the environment of Matlab 7.0. This software package adopted the modified relay feedback approaches to perform the system identification in frequency domain to get the critical information to realize the auto-tuning controller. This software package provides an effective tool for the control engineers to improve the performance of their controller or to solve practical problems.