Patent ID: 6839653

Claim:
A method for determining the position of an element driven by the drive shaft of a commutated direct current (DC) motor, the method comprising: detecting current ripples contained in an armature current signal of the motor as the motor drives the drive shaft; counting the detected current ripples; performing a frequency spectral analysis of the armature current signal in order to determine frequency components of the armature current signal; analyzing the frequency components identified in the frequency spectral analysis of the armature current signal in order to determine which of the frequency components of the armature current signal is indicative of the frequency of the current ripples contained in the armature current signal; monitoring the current ripple frequency for changes over time while the motor drives the drive shaft; modifying the number of counted current ripples upon a change in the current ripple frequency in a given time interval if the change in the current ripple frequency deviates from the current ripple frequency more than a threshold deviation for the given time interval; and determining the rotational position of the drive shaft based on the modified number of counted current ripples.