Relay Feedback Auto-tuning of Process Controllers – A Tutorial Review

Auto-tuning of multivariable PID controllers from decentralized relay feedback

A tutorial review on process identification from step or relay feedback test

A set of general expressions is derived from a single asymmetrical relay feedback test for online plant identification. The expressions also remain valid for an odd symmetrical limit cycle test method. Using the expressions, the exact parameters of open-loop stable and unstable first-order plus time delay (FOPDT) and second-order plus time delay (SOPDT) transfer function models may be obtained from simple measurements made on the limit cycle. The approach can also be used to identify transfer functions of integrating processes. Conditions for the existence of limit cycles in unstable FOPDT and SOPDT processes are derived. The design of controllers for these processes is then considered and a simple, but very effective, approach using standard forms with a variable zero is presented. The advantages of using PI-PD control, compared with conventional PID or PI-D control, particularly for unstable and integrating processes is clearly shown. Examples are given to illustrate the value of the proposed general identification method and the improved system performance provided by the proposed controllers.

Autotuning and controller design for processes with small time delays

Obtaining Controller Parameters for a New PI-PD Smith Predictor Using Autotuning

The purpose of this work is to devise a relay feedback experiment that can identify two points on the Nyquist curve from a single test at the frequencies ω=0 and ω=ω u . An identification procedure is devised accordingly for this parameter identification. The theory of equivalent gain is described, the biased relay feedback system is analyzed, and the potential problems are explored. Parametric system identification is discussed followed by the conclusion

Use of biased-relay feedback for system identification

Frequent and large load changes are often encountered in chemical processes. This can be lead to erroneous results in system identification especially for the fully automated identification procedure in autotuners. Relatively speaking, the relay feedback test is less sensitive to load disturbance (e.g., as compared to the step test). However, as the magnitude of load increases, the estimates of ultimate gain and ultimate frequency deteriorate exponentially. In this work, the causes of errors are analyzed using a describing function and a remedial action is devised. Since load disturbance leads to asymmetric output responses, an output-biased relay is employed to restore symmetric output response. Furthermore, instead of a trial-and-error procedure, the bias value can be calculated immediately following system responses. Subsequently, an identification procedure is proposed to maintain the quality of the model in the face of load changes. Linear and nonlinear systems are used to test the effectiveness of t...

Cheng-Ching Yu

Papers

Use of biased-relay feedback for system identification

Autotune identification under load disturbance