Velocity controlled disk drive head retraction with offset removal

A control apparatus for controlling retraction of a transducing head from the surface of a recordable medium, the transducing head carried on a movable member actuated by an electric motor, the control apparatus comprising:

• a measuring circuit having a sensed back electromotive force (back EMF) as its output during a measuring phase, the sensed back EMF including an actual back EMF from the electric motor plus a measuring circuit offset voltage, wherein the actual back EMF from the electric motor is related to a velocity of the moveable member;
• an offset removing circuit connected to the output of the measuring circuit for determining and removing the measuring circuit offset voltage and providing the actual back EMF from the electric motor at its output;
• a driver circuit connected to the electric motor for providing the electric motor with a drive current during a driving phase having a magnitude based on the actual back EMF from the electric motor;
• and
• a controller connected between the offset removing circuit and the driver circuit for alternately activating the measuring circuit during the measuring phase and the driver circuit during the driving phase,
• wherein the offset removing circuit comprises an offset sampling circuit for sampling and storing the measuring circuit offset voltage during the driving phase, and a subtracter for subtracting the measuring circuit offset voltage stored in the offset sampling circuit from the sensed back EMF during the measuring phase.