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(.):  linear systems, PI-controller, controller fragility, stability margins, Nyquist criterion, D-decomposition
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(.):  Due to their simplicity and efficiency, PI controllers are widely used to solve a wide range of practical problems. Many modern controllers synthesis methods involve the numerical solution of optimization problems. Therefore, analyzing the fragility of the resulting PI controllers is particularly important. This article examines a control system with a second-order linear plant under a stabilizing PI controller. In this context, a new approach for analyzing the fragility of the PI controller is proposed. Fragility refers to the property of maintaining closed-loop system stability under static variations in controller parameters. The proposed approach is based on a well-known frequency-domain method for studying system robustness the "breaking by parameter" technique. The essence of the method lies in transforming the closed "plant-controller" system into a form where the studied parameter is extracted from the closed loop and treated as a separate scalar block (static controller). The remaining part of the system is treated as a fictitious control plant. Using the Nyquist criterion, the stability region (not necessarily connected) is found for the parameter under study, which is equivalent to constructing a one-dimensional D-decomposition. The resulting stability region is identical to that of the original "plant-controller" system. In the considered problem of PI controller fragility analysis, the parameters under study are its coefficients. A numerical example demonstrates the application of the proposed approach to PI controller fragility analysis for a widely used linearized model of a first-order plant with time delay.

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