> I've just taken apart an older and defective HDD and saw how the head
> carrying arm is built:
>
> a flat coil with a large diameter (although not circular) moving
> between 2 enormously powerfull magnets.

That's called a "voice coil" because the physics is just like a loud
speaker.

> I got the idea to try to use this on a model R/C ship and thought of
> translating the PWM signal from the receiver to whatever kind of signal
> this arm will require to function.

I seriously doubt it is driven by PWM.  The reason for using a voice coil
drive is that it can respond very quickly.  It really jerks those heads back
and forth to minimize seek time.  The track to track tolerances are very
tight, and even a small amount of vibration would cause trouble.  To drive
this thing by PWM would require a very high frequency.  That would cause so
many other problems that using a linear drive in a servo loop is an easier
and certainly cleaner solution.  Keep in mind that high currents are only
required when the coil is moved because there is very little radial force on
the heads when they are staying on a track.


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