Accuracy of Autotuning Identification Methods and Achievable Closed Loop Performance

ABSTRACT The paper compares two identification methods used to build models for control purposes to be used for autotuning, namely relay with additional delays and sinusoidal inputs. The two techniques allows to obtain more (2 or 3) points in the Nyquist plane and from them a model can be obtained by a least square method. The accuracy in identification which is obtainable by the two methods is evaluated and possible improvement in closed loop performance are illustrated. As expected, the identification by means of sinusoids is able to give higher accuracy, with respect to the inherently approximate relay techniques, when at least 3 tests can be performed; such tests however are more time consuming and more sensitive to measurements perturbations. Full advantages of the improved identification can be taken by adopting a model based controller.