A general class of linear time-invariant feedback control systems with a distributed delay in the feedback line is treated. Differently from present literature, both limits of the delay are taken as non-zero, which brings an interesting new perspective to the problem. The authors start with a theorem stating the equivalence of a general class of such distributed-delay systems to a system with multiple discrete and independent delays. Although this connection has been recognised earlier, what is interesting and novel in this study is the first deployment of a paradigm called the cluster treatment of characteristic roots (CTCR). CTCR declares the stability outlook of the system in the space of the delays non-conservatively and exhaustively. This capability forms a very important foundation over which the authors build further contributions. Firstly, the authors describe a systematic procedure to use distributed-delayed feedback logic in order to stabilise an unstable plant. Secondly, we present an intriguing but very simple control strategy which is called sign inverting control. This seemingly paradoxical proposition (i.e. the inversion of the control polarity) imparts considerably enhanced robustness of the control system against the variations in delay bounds. Example case studies are provided to validate these features.

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Stabilisation of open-loop unstable plants under feedback control with distributed delays

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