A nonlinear state observer based on indefinite Riccati design

In this article we propose an observer for continuous-time nonlinear systems. The observer gain is computed by an indefinite Riccati differential equation. We prove that under certain conditions the proposed observer is an exponential observer by choosing an appropriate Lyapunov function. To examine the practical usefulness of the proposed observer we apply it to an induction motor for the estimation of the rotor flux and the angular velocity.