Performance limitations in Field-Oriented Control for asynchronous machines with low resolution position sensing

The traditional control architecture adopted for asynchronous motors consists of a cascaded control with an inner current loop, based on field-oriented control and an external velocity loop. In the majority of industrial applications this architecture makes use of velocity and flux observers, both fed by a position measurement that is often characterized by low resolution. This paper investigates the effects of the velocity/flux estimation error on the performance of the current/velocity regulation. A methodology, based on a linearized model of the motion control system, is introduced to quantify the perturbations caused by these estimators. Large discrepancies between the stator currents and their estimates, as well as distortion of the mechanical transfer function are explained in the light of these simplified models. Guidelines to improve the design of the current/velocity regulators are presented too. An experimental case study demonstrates how these tools can be exploited to analyze the motion control system of a commercial washing machine.