Speed Sensorless Control of Induction Motors based on a Reduced-Order Adaptive Observer

A novel speed-sensorless indirect field-oriented control for the full-order model of the induction motor is presented. It provides local exponential tracking of smooth speed and flux amplitude reference signals together with local exponential field orientation, on the basis of stator current measurements only and under assumption of unknown constant load torque. Speed estimation is performed through a reduced-order adaptive observer based on the torque current dynamics, while no flux estimate is required for both observation and control purposes. The absence of the flux model in the proposed algorithm allows for simple and effective time-scale separation between the speed- flux tracking error dynamics (slow subsystem) and the estimation error dynamics (fast subsystem). This property is exploited to obtain a high performance sensorless controller, with features similar to those of standard field-oriented induction motor drives. Moreover, time-scale separation and physically-based decomposition into speed and flux subsystems allow for simple and constructive tuning procedure. The theoretical analysis based on Singular Perturbation method enlightens that a Persistency of Excitation condition is necessary for the asymptotic stability. From a practical viewpoint, it is related to the well-known observability and instability issues due to lack of back-emf signal at zero-frequency excitation. A flux reference selection strategy has been developed to guarantee Persistency of Excitation in every operating conditions. Extensive simulation and experimental tests confirm the effectiveness of the proposed approach.