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.

Overview

Consider a linear system subject to non-linear feedback, i.e. a non linear element $\varphi(v, t)$ is present in the feedback loop. Assume that the element satisfies a sector condition mu_1,\mu_2/math>, 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 1/\mu_1,-1/\mu_2/math> located on the ''x''-axis.

General description

Consider the nonlinear system

: $\dot = \mathbf + \mathbf,$
: $\mathbf = \mathbf,$
: $\mathbf = \varphi(v, t).$

Suppose that

# $\mu_1 v \le \varphi(v,t) \le \mu_2 v,\ \forall v,t$
# \det(i\omega I_n-A) \neq 0,\ \forall \omega \in R^\text\exists \mu_0 \in mu_1, \mu_2,:\, A+\mu_0 BC is stable
# \Re\left \mu_2 C(i\omega I_n-A)^B-1)(1-\mu_1C(i\omega I_n-A)^B)\right0 \ \forall \omega \in R^.

Then $\exists c>0,\delta>0$ such that for any solution of the system the following relation holds:

:: $, x(t), \le ce^, x(0), ,\ \forall t \ge 0.$

Condition 3 is also known as the ''frequency condition''. Condition 1 the ''sector condition''.

External links

Sufficient Conditions for Dynamical Output Feedback Stabilization via the Circle Criterion

Popov and Circle Criterion (Cam UK)

References

* {{cite book, title=Nonlinear Dynamical Systems and Control: a Lyapunov-Based Approach., last1=Haddad, first1=Wassim M., last2=Chellaboina, first2=VijaySekhar, date=2011, publisher=Princeton University Press, isbn=9781400841042

Nonlinear control
Stability theory