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Abstract: Using a single symmetric relay feedback test, a method is proposed to identify all the three parameters of a first order plus time delay (FOPTD) model. On identifying a higher order dynamics system by a FOPTD model, the conventional method identifies a negative time constant [Li, W., Eskinat, E., & Luyben, W. L. (1991). An improved auto tune identification method. Industrial and Engineering Chemistry Research, 30, 1530–1541] due to the error in neglecting higher order dynamics in the system output. In the present work, all the parameters of a FOPTD model are estimated with adequate accuracy. Three simulation examples are given. The estimated model parameters of a FOPTD model are compared with that obtained by Li et al. [Li, W., Eskinat, E., & Luyben, W. L. (1991). An improved auto tune identification method. Industrial and Engineering Chemistry Research, 30, 1530–1541] and also that with exact model parameters of the system. The performance of the controller designed on the identified model is compared with that identified by Li et al. [Li, W., Eskinat, E., & Luyben, W. L. (1991). An improved auto tune identification method. Industrial and Engineering Chemistry Research, 30, 1530–1541] and with that of the actual process. The proposed method gives performance close to that of the actual system. Simulation results for stable and unstable systems are given. [Copyright &y& Elsevier]

A simple method is proposed to design PI/PID controllers for stable first-order plus time delay (FOPTD) systems. The method is based on matching the coefficient of corresponding powers of s in the numerator and that in the denominator of the closed loop transfer function for a servo problem. This method gives simple equations for the controller settings in terms of the FOPTD model parameters. Simulation results show that the method gives a similar response as that of Ziegler–Nichols (Z–N) method and better response than that of IMC method. Controllers are also designed by using two tuning parameters and the performance is best when compared to that of Z–N [ASME Trans. 64 (1942) 759] and Abbas [ISA Trans. 36 (1997) 183]. The controller settings give a robust performance for uncertainty in the process model parameters. The method is also extended to design PI/PID controllers for an unstable FOPTD system. Simulation results show that the present method gives improved performances: (i) for PID controllers over that of the controllers designed by Huang and Chen [J. Chem. Eng. Jpn. 32 (1999) 579], IMC method and that proposed by Visioli [IEE Proc. CTA 148 (2001) 180] and (ii) for PI controllers over the method of Jung et al. [J. Process Contr. 9 (1999) 265]. Theoretical analysis of stability and robustness of the proposed controller are also provided. [Copyright &y& Elsevier]

First order plus time delay (FOPTD) models obtained from the cyclic steady state of the relay feedback oscillations can suffer from poor performances for some processes such as first and second order processes with fast parasitic dynamics. Poor FOPTD models are due to high frequency oscillations that activate parasitic dynamics. Dynamic elements such as hysteresis and integrator added in the feedback loop can reduce the oscillation frequencies but they increase the experimental times. Here, to relieve these problems without increasing the experimental time and computational complexity, relay feedback autotuning methods that apply the method of weighted moments to relay feedback transients are proposed. The proposed methods use the same relay feedback tests as the conventional relay feedback methods and are simple to use computationally, providing conservative PI controllers for all the test batch processes and the above first and second order processes with fast parasitic dynamics. [ABSTRACT FROM AUTHOR]