Adaptive nonlinear output-feedback control with unknown high-frequency gain sign

A global adaptive output-feedback control scheme, which does not require a priori knowledge of high-frequency gain sign, is proposed for general nonlinear systems in output-feedback form. Unlike in Ding (1998), the removal of a priori knowledge of the high-frequency gain sign is not at the price of making growth restrictions on system nonlinearities, and furthermore, only the minimal number of parameters needs to be updated.