Resolution of Rotor Position Measurement: Modeling and Impact on Speed Estimation

The aim of this article is to provide an accurate modeling of the resolution of rotor position measurement and to evaluate its impact on speed estimation required for electric drive control. Starting from the space vector model of a spatially quantized rotor position measurement, the novel concept of instantaneous quantized speed is introduced, which is shown to constitute the effective input to any speed estimation algorithm. Time-harmonic formulations that are valid under periodic torque disturbances are also derived. These expressions, based on modulation theory, justify the presence of otherwise unexplained time harmonics in the measured rotor position and estimated speed, which appears when the shaft is subjected to torque disturbances. Analytical and numerical results are provided on a well-known observer structure to validate the filtering action of speed estimation algorithms on the harmonics of the instantaneous quantized speed. Finally, experimental verification on a 400 W permanent magnet (PM) servo drive is presented.