Precise measurement of angular position and speed of spindle motor during spin-up by computing differences in winding current rise time

Abstract

DC brushless motor with no extra sensor for motor position and speed is popularly used in industries, including disk drive industry. At very low speed the back electromotive force (EMF) voltage generated by the spindle motor does not provide a reliable estimation of velocity. Also at start-up stage, the rotor could be at any random initial position. To find a correct commutation state to drive motor forward, rotor position information is needed. This paper describes a new method to interpret the current rise time of each motor winding phase as an improved inductive sensing method to find motor position and speed at start-up stage. The measuring procedure is same as standard known inductive sensing method but it uses the differences of the rise time of each phase, rather than only looking for the fastest current rise phase. Based on the equations derived in this study, the motor continuous angular position in the 360° circle can be revealed and thus the speed can be derived precisely. It provides the opportunity for optimizing the acceleration commutation, and more important, it gives much precise speed measurement to allow the exit of inductive sensing phase with excellent speed consistency. This can minimize the overall spin up time and improve the power efficiency and acoustics.