Nonlinear observers: a circle criterion design

We design observers for a class of systems with monotone sector nonlinearities. The resulting nonlinear estimation error system is represented as the feedback interconnection of a linear system and a time-varying multivariable sector nonlinearity. Using LMI software, an observer gain matrix is computed to satisfy the circle criterion and, hence, to drive the estimation error to zero. The new observer is incorporated in output feedback control, as illustrated by an example.