This paper first reviews in brief, the different methods presented in the last ten years and then outlines the Nyquist stability criterion for linear control systems with time-delay, using dual-locus diagram technique. The variation of the loopgain and time-delay are investigated when the time-delay occurs in the forward loop or the feedback-loop or both. The criterion employs a delay chart prepared beforehand and only requires the plotting of a simple curve derived from the open-loop transfer function. The stability condition of a simple control system is used to illustrate the method.
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