Closed Loop Identification of Nonlinear Systems 1

In recent years, several new methods for the identiication of approximate models of an open loop plant on the basis of closed loop data have been presented. In this paper, we extend two of these methods to the nonlinear case: we consider that both the plant and the controller can be nonlinear. The rst method is a two-step procedure. The sensitivity function of the closed loop system is identiied through a high order non-linear model and it is used in the second step to simulate a noise free input signal for an open loop like identiica-tion of the plant. The second method identiies the right coprime factors of the plant through an open loop like identiication of the ltered sensitivity and complementary sensitivity functions. For both methods, we assume that the measurement noise enters the system under a high SNR assumption.