Calibration technique to remove series resistance errors in the sensed

Abstract

A motor controller controls a motor's rotational speed using sensed back electromagnetic force (back EMF) of the motor. The sensed back EMF is presumed to contain an error component caused by series resistance in the motor coil. A velocity sense signal is continually generated representing the sum of the measured back EMF and an error-cancelling quantity equal to the current in the coil scaled according to a gain-control signal stored in the controller. During the first part of a calibration operation, the coil current is set to zero until a time T.sub.1 at which the coil current is substantially zero and the motor velocity is substantially constant. The value of the velocity sense signal is stored at the time T.sub.1, this value representing the zero-current back EMF of the coil. A current pulse of predetermined magnitude and duration is then provided to the coil such that at a predetermined equal-speed time during the pulse the coil current is non-zero and the motor speed is substantially equal to the motor speed at time T.sub.1. At the equal-speed time, the gain-control signal is set to a calibrated value which causes the velocity sense signal to be equal to the zero-current back EMF value, so that the error-cancelling component is sufficient to remove the series resistance error. The calibrated gain-control signal is used during subsequent normal operation of the motor.