Patent ID: 6845337

Claim:
A method for determining the rotational position of the drive shaft of a direct current motor, the method comprising: detecting current ripples contained in an armature current signal of the motor as the drive shaft of the motor rotates in response to being driven by the motor; counting the detected current ripples; determining whether a current ripple expected to be contained in the armature current signal at a probable time point is absent from within a tolerance time band containing the probable time point of the expected current ripple; if the expected current ripple is absent from within the tolerance time band containing the probable time point of the expected current ripple, then determining whether a current ripple is detected after the tolerance time band of the expected current ripple; if a current ripple is detected after the tolerance time band of the expected current ripple and the expected current ripple is absent from within the tolerance time band, then counting the expected current ripple as a detected current ripple; determining the rotational position of the drive shaft based on the counted current ripples; and dynamically changing the length of the tolerance time band as a function of an operating state of the motor as the motor drives the drive shaft.