An Improved Autotune Identification Method

K. Srinivasan and M. Chidambaram

Abstract
A method is proposed to estimate accurately the model parameters of a first order plus time delay (FOPTD) transfer function model using the conventional relay autotune method. Usually, the value of the delay is assumed or noted from the initial portion of the response of the system. Whenever identifying a higher order dynamics system by a FOPTD model, this method identifies wrongly the time constant as negative (Li et. al., 1991) due to the error in identifying the time delay which is due to error in the model structures. In the present work, an additional equation is formulated to calculate accurately the parameters of the FOPTD model. Even when the actual system is FOPTD and the time delay to time constant ratio is larger, then higher order harmonics can not be neglected in the output response. Also in the present work, a method is proposed to consider the higher order harmonics of the relay oscillations. Improved accurate values for the controller ultimate gain are obtained. The estimated model parameters of a FOPTD model are compared with that obtained by Li et al. (1991) and that with the exact model parameters of FOPTD systems. The performance of the controller designed on identified model is compared with that identified by Li et al. (1991). The proposed method gives a better performance.

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Keywords
Relay tuning, higher order harmonics, FOPTD, single test