Plant parameter identification under relay control

A set of general expressions are derived from a single asymmetrical relay feedback test for online process identification. Using these 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 SOPDT process may also possess a 'stable' or 'unstable' zero. The expressions remain valid for an odd symmetrical limit cycle. Examples are given to illustrate the value of the proposed general identification method.