AUTOTUNE IDENTIFICATION UNDER LOAD DISTURBANCE

Frequent and large load changes are often encountered in chemical proc esses. This can be lead to erroneous results in system identification especially for the fully automated identification procedure in autotun ers. Relatively speaking, the relay feedback test-is less sensitive to load disturbance (e.g., as compared to the step test). However, as th e magnitude of load increases, the estimates of ultimate gain and ulti mate 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 response s, an output-biased relay is employed to restore symmetric output resp onse. Furthermore, instead of a trial-and-error procedure, the bias va lue can be calculated immediately following system responses. Subseque ntly, 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 the proposed procedure. The res ults show that the proposed approach maintains the quality of estimate s under step-like and non-step-like load changes.