An Improved Relay Auto Tuning of PID Controllers for SOPTD Systems

Using a single symmetric relay feedback test, a method is proposed to identify all the three parameters of a stable second order plus time delay (SOPTD) model with equal time constants. The conventional analysis of relay auto-tune method gives 27% error in the calculation of ku,. In the present work, a method is proposed to explain the error in the ku calculation by incorporating the higher order harmonics. Three simulation examples are given. The estimated model parameters are compared with that of Li et al. [4] method and that of Thyagarajan and Yu [8] method. The open loop performance of the identified model is compared with that of the actual system. The proposed method gives performances close to that of the actual system. Simulation results are also given for a nonlinear bioreactor system. The open loop performance of the model identified by the proposed method gives a performance close to that of the actual system and that of the locally linearized model.