Simple Proportional Integral Controller Tuning Rules for FOPTD and HOPTD Models

Significance 

Currently, proportional integral controllers are the most widely utilized controllers in a majority of process industries despite having only two tuning parameters. Several proportional integral controllers tuning techniques have already been developed, such as the Ziegler−Nichols method and the Cohen−Coon method, but both possess drawbacks as they are seen to being too oscillatory. Recent advances in this field have pointed out that the direct synthesis method and the internal model control method can provide very simple tuning rules for the first order plus time delay (FOPTD) model. Moreover, studies by Skogestad et al removed the drawback of the internal model control method to tune proportional integral controllers simply by limiting the integral time. Therefore, in order to overcome the shortcomings of the FOPTD model-based method, the half-order plus time delay (HOPTD) model-based method has to be assessed.

To this note, Professor Jietae Lee from Kyungpook National University in collaboration with Dr. Yongjeh Lee and Dr. Dae Ryook Yang from Korea University and Professor Thomas Edgar from University of Texas at Austin proposed a study whose main objective was to utilize two asymptotes of proportional integral controllers to evaluate existing tuning rules and to propose new tuning rules for FOPTD and half-order plus time delay (HOPTD) models. In order to achieve their goal, they compared the tuning rules for the two extreme cases of small and large time delays to the time constants. Their work is now published in the research journal, Industrial & Engineering Chemistry Research.

The research method employed by the authors commenced by computing the two asymptotes of the proportional integral controller tuning rules. The results of the prior process were then used to evaluate the performances and application ranges of the proportional integral (PI) controller tuning rules, providing guidelines for the selection of PI controller tuning rules. Next, the researchers selected the design parameters appropriately, where the iterative continuous cycling method, the guaranteed gain and phase margin method, and the Skogestad internal model control method were found to be applicable for the whole range of time delays.

The research team observed that by matching two asymptotes for small and large time delays over the time constant, a simple analytic tuning rule for the FOPTD model was arrived at. It was also noted that for some of the overdamped processes whose transfer functions had large zero terms, the half-order plus time delay (HOPTD) models were found to be better than the FOPTD models. Applying the technique of matching two asymptotes, a simple analytic PI controller tuning rule was developed for the HOPTD models.

Professor Jietae Lee and colleagues successfully presented novel FOPTD and HOPTD tuning rules that accommodate high order processes. More so, they modified the Skogestad-Internal-Model-Control model reduction rule and removed some ambiguities in practical applications. Altogether, the half-order plus time delay model-based method for tuning PI controllers seems to be applicable to almost all overdamped processes robustly..

Reference

Jietae Lee, Yongjeh Lee, Dae Ryook Yang, Thomas F. Edgar. Simple Proportional Integral Controller Tuning Rules for FOPTD and HOPTD Models Based on Matching Two Asymptotes.. Industrial & Engineering Chemistry Research. 2018, volume 57, page 2905−2916

Go To Industrial & Engineering Chemistry Research

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