>You need a servo control loop.  Somewhere the disk drive has positional
>feedback on the voice coil and a bunch of analog electronics to null out
>the position error.

There is no position feedback system in any modern PC class drives. It
would be too expensive. The feedback is obtained from the heads reading
the tracks directly. If the heads are over the wrong track the controller
sends a pulse to the voice coil servo to move in the right direction. The
heads are kept on a track by reading the track proper (there is more than
one head on at least one platter, and there are several ways to 'ride' a
track in lock).

Peter

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