Design of state observers from a drift-observability property

In this paper, the problem of state observation with exponential error deca y for nonlinear systems affine in the input is considered and an observer i s proposed. For such an observer, the drift-observability property of the s ystem (i.e,, observability for zero input), together with an assumption on the input amplitude or on the observation relative degree, are sufficient c onditions for exponential convergence of the observation error. The existen ce of an exponential observer is correlated to the existence of a solution for an H-infinity Riccati-like inequality. Global and semiglobal convergenc e results are presented.