VCM head position detection and control with back EMF

A method of determining a speed and position of a read/write head in a disc drive, the disc drive comprising a voice coil motor to which the read/write head is attached and a current-sensor resistor electrically coupled in series with the voice coil motor, the voice coil motor being characterized by a coil resistance and a coil inductance, the coil resistance being characterized by a coil resistance voltage, the voice coil motor possessing a back electromotive force (back EMF) when the read/write head is in motion, the voice coil motor having a voltage across itself, the current-sensor resistor being characterized by a current-sensor resistor resistance, the current-sensor resistor conducting an electrical current, the current-sensor resistor having a voltage across itself when conducting the electrical current, the method comprising:

• (a) estimating the voltage across the voice coil motor;
• (b) estimating the voltage across the current-sensor resistor;
• (c) estimating the coil resistance voltage;
• (d) estimating the back EMF of the voice coil motor, based upon the voltage across the current-sensor resistor, the coil resistance voltage, and the voltage across the voice coil motor;
• and
• (e) estimating the speed and position of the read/write head, based upon the back EMF of the voice coil motor.