Method for determining the rotational position of the drive shaft of a commutated DC motor

A method for determining the rotational position of a drive shaft of a commutated direct current (DC) motor as a function of the number of current ripples contained in an armature current signal of the motor as the motor drives the drive shaft, the method comprising:

• digitally sampling at sampling time points the armature current signal of the motor as the motor drives a drive shaft;
• generating a digitally sampled armature current signal value at each sampling time point;
• storing each digitally sampled armature current signal value in memory as the digitally sampled armature current signal values are generated;
• comparing the magnitude of the digitally sampled armature current signal value corresponding to a current sampling time point and the magnitudes of selected ones of the digitally sampled armature current signal values corresponding to previous sampling time points in a time interval containing the current and previous sampling time points;
• generating a rising slope detection signal if the comparison is indicative of an increasing tendency in the magnitudes of the digitally sampled armature current signal values over the time interval, the rising slope detection signal being indicative of a rising current ripple slope;
• and
• generating a falling slope detection signal if the comparison is indicative of a decreasing tendency in the magnitudes of the digitally sampled armature current signal values over the time interval, the falling slope detection signal being indicative of a falling current ripple slope.