RELAY FEEDBACK EXPERIMENTS FOR PROCESS IDENTIFICATION

The relay feedback experiment was made popular in the field of process control by Astrom and Hagglund (1984). This experiment was suggested as a means to automate the Ziegler-Nichols scheme for determining ultimate gain and frequency information about a process. Their approach followed directly from a describing function approximation (DFA) to the nonlinear relay element. The objective was to use the obtained process information for automatic tuning of PID controllers. 

Astrom and Hagglund s work (1984) has prompted research in several different directions. One of these directions, and the focus of Chapter 8, is in the area of process identification, where the objective is to obtain a more complete and accurate model of the process firom data generated under relay feedback. Fitting a more complete process model (i.e. a transfer function model) normally requires knowledge of several points on the process Nyquist curve. Given that the standard relay experiment combined with the DFA identification technique is able to identify only a single point, fitting such a model either requires the availability of some prior process information (e.g. Luyben, 1987) or requires the user to conduct a series of relay experiments in 

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