Hybrid Continuous-Discrete State Observer for Induction Machine in Presence of Magnetic Saturation
Résumé
This paper addresses the problem of state observation of induction motors facing large flux and speed variations. In these conditions, state observation (and control) cannot be properly performed without accounting for the nonlinear nature of the motor magnetic circuit. A key feature of this study is that the state observer design is based on an experimentally validated continuous-time nonlinear model taking into account the saturation nature of a commercial induction motor of 7.5 KW. A continuous-time nonlinear state observer is thus obtained, using the high gain design technique, and is shown to be convergent when applied to the (continuous-time) system model. The point is that, in modern control applications, continuous-time systems are only observed through sampled output signal measurements. Then, one classical solution consists in constructing a discrete-time approximation of the (continuous-time) observer. This solution is somewhat computationally burdensome and does not preserve the convergence properties of the original continuous observer. Here, a different solution is developed according to the hybrid continuous-discrete (HCD) estimation principle. The obtained HCD observer is formally shown to preserve a quite satisfactory estimation accuracy when applied to the (continuous-time) system model. This theoretical result is confirmed by simulation, considering an operational protocol involving large speed and flux variations making the motor operate both in the linear on nonlinear parts of its magnetic characteristic.