SimplifiedIdealInvertedDecoupling.pdf - 1 SIMPLIFIED IDEAL OR INVERTED DECOUPLING Gagnon E Pomerleau A and Desbiens A GRAIIM(Groupe de recherche sur les

SimplifiedIdealInvertedDecoupling.pdf - 1 SIMPLIFIED IDEAL...

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1 SIMPLIFIED, IDEAL OR INVERTED DECOUPLING ? Gagnon E., Pomerleau A. and Desbiens A. GRAIIM (Groupe de recherche sur les applications de l’informatique à l’industrie minérale), Department of electrical and computer engineering, Laval University, Québec, Canada, G1K 7P4 ABSTRACT This paper presents a comparative study of simplified, ideal and inverted decoupling. The stability, robustness and implementation of the three decoupling methods are studied. The structured singular value (SSV) is used to carry out some comparisons. It is demonstrated that robust performance and robust stability of a nominally stable control system are equivalent for the three decoupling methods when the controllers are tuned to obtain identical nominal performance. A relation is derived between the presence of right-half plane (RHP) zeros of a process in series with its simplified decoupler and the instability of the ideal and inverted decouplers for the same process. This paper also describes a potential implementation problem related to the particular structure of the inverted decoupling. Finally, a recapitulative table of the main advantages and limitations of each decoupling method is presented. Keywords: Decoupling; Robustness; Stability; Implementation
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2 1. INTRODUCTION The choice of a decoupling method is a relatively complex task since all techniques have their advantages and limitations. Simplified decoupling is by far the most popular method. Its main advantage is the simplicity of its elements. Ideal decoupling, which is rarely used in practice, greatly facilitates the tuning of the controller transfer matrix. Inverted decoupling, which is also rarely implemented, presents at the same time the main advantage of both the simplified and ideal decoupling methods. Some authors have already compared simplified, ideal and inverted decoupling. Luyben [1] and Weischedel and McAvoy [2] have compared ideal and simplified decoupling methods using distillation column simulators. They concluded that simplified decoupling is more robust than ideal decoupling. According to Waller [3], stability problems encountered by Luyben [1] with ideal decoupling are explained by the fact that he used the same controller tuning for both decoupling methods. Weischedel and McAvoy [2] also kept the same controller tuning for both decoupling techniques, therefore leading to the same conclusion about robustness. Following these studies, McAvoy [4] concluded that ideal decoupling is very sensitive to modeling errors. To evaluate control systems robustness, Arkun and al. [5] proposed a general analysis procedure based on the singular values. To illustrate their methodology, they studied decoupling control systems applied to several distillation columns. The distillation columns used came from the literature [2,6,7]. They compared ideal decoupling and simplified decoupling. However, to be able to carry out direct analysis and comparisons with results already presented, they used the
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3 same decouplers and controllers parameters as found in the literature. As in the preceding studies,
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