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

We remove, for semiglobal output feedback control of uncertain nonlinear systems, a previous requirement that the high-frequency-gain sign 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. The underlying idea is that with the help of the high gain observer, the high-frequency-gain sign can be identified online, provided the designed control satisfies some domination condition.