Semiglobal output feedback control of uncertain nonlinear systems with unknown high frequency gain sign

In this paper, we remove, for semiglobal output feedback control of uncerta in nonlinear systems, a previous requirement that the high-frequency-gain s ign be known a priori. Our proposed control scheme is also of the high-gain observer-type, but it involves a switching between two forms of control, T he underlying idea is that with the help of the high-gain observer, the hig h-frequency-gain sign can be identified on-line, provided the designed cont rol satisfies some domination condition.