Abstract: The problems of observer design for nonlinear systems with linear output are discussed in this paper. Under the assumption that a differential Riccati equation is solvable, an exponential reduced-order observer is developed by means of coordinate transformation and the gain matrix of the proposed observer depends on the solution of the differential equation.The simulation results show that the performance of the proposed reducedorder observer is good.

Key words: nonlinear systems, differential Riccati equation, exponential convergence, reduced-order observer