System and method for determining position or speed of a commutated DC motor with error correction

A method for determining rotational position or speed of a mechanically commutated DC motor from motor current ripple due to commutation for controlling a motor driven power device, said method comprising the steps of:

• monitoring motor current of a mechanically commutated DC motor with a current sensor;
• detecting ripples in the motor current with a processor;
• calculating a ripple frequency based on time between two successive ripples with the processor;
• determining a ripple count between two successive frequencies with the processor;
• comparing the ripple count to a threshold value with the processor;
• determining an estimated ripple frequency from a motor model stored in a memory when the ripple count exceeds the threshold value;
• calculating a corrected ripple count from a ratio of the calculated ripple frequency and the estimated ripple frequency with the processor;
• and
• determining, with the processor, at least one of the motor position and the motor speed based on the corrected ripple count.