The Nyquist stability criterion is a fundamental principle in control theory used to determine the stability of a linear time-invariant (LTI) system based on its frequency response. Specifically, it relates the open-loop frequency response of a system to the stability of the closed-loop system.
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Let be the open-loop transfer function. The winding of its Nyquist plot around , together with the number of unstable open-loop poles, determines the number of unstable zeros of , which are the closed-loop poles.