Steady-state errors and the circle criterion

The authors address nonlinear control systems containing a memoryless nonlinear element and show that, when the circle condition for stability is met, corresponding propositions hold concerning the relationship between the nature of the error response and the system type. A convergent algorithm is given for computing the limit of the steady-state response whenever it exists. It appears that results presented can be obtained for the case in which N is preceded by a linear subsystem K(s) that takes into account integrators. In that case the key frequency-domain condition concerns the locus of the product K(iω)G(iω) and its relation to the critical circle, etc