Circle criterion

In nonlinear control and stability theory, the circle criterion is a stability criterion for nonlinear time-varying systems. It can be viewed as a generalization of the Nyquist stability criterion for linear time-invariant (LTI) systems.

Consider a linear system subject to non-linear feedback, i.e. a non linear element ${\displaystyle \varphi (v,t)}$ is present in the feedback loop. Assume that the element satisfies a sector condition ${\displaystyle [\mu _{1},\mu _{2}]}$, and (to keep things simple) that the open loop system is stable. Then the closed loop system is globally asymptotically stable if the Nyquist locus does not penetrate the circle having as diameter the segment ${\displaystyle [-1/\mu _{1},-1/\mu _{2}]}$ located on the x-axis.

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