Stability criteria for nonlinear multivariable systems

In the paper, multivariable input/output stability criteria of a graphical nature for nonlinear multivariable feedback systems are developed. The derived stability criteria, which are of the circle type, use the frequency plots of the eigenvalues of a normal transfer-function matrix which approximates the linear part of the nonlinear feedback system. An iterative design procedure is introduced which reduces the error of approximation and the feedback systems sensitivity to disturbances, while ensuring the stability of the closed-loop system.