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In article <3ucft3$kd8@osa.osa.com.au>, cjh@osa.com.au (Clifford Heath) wrote:

> I have been playing with a largish linear motor, basically a voice-coil
> from an old washing-machine sized HP disk drive, and its behaviour started
> me thinking.  This motor weighs about 22 Kg (50Lb) and the coil has a 2
> Ohm impedance.
> 
> I connected 1.2 Volts, expecting to see the coil *accelerate* along its
> 15 cm (6") throw, but instead it moved at a steady rate determined by
> the input voltage.  Ok, I understand that - the motion induces a reverse
> EMF that exactly counters the input voltage (inertial effects
> notwithstanding).
> 
> Consider this effect in a speaker - a rising voltage corresponds to
> acceleration and hence sound pressure.  AC voltage peak means peak
> acceleration and hence peak sound pressure (once again, inertial effects
> notwithstanding).  This is sensible in retrospect, but I had always
> considered that peak cone excursion (not peak acceleration) would occur
> at the voltage peaks so it was a surprise.  Obviously speakers will
> self-centre, so there must be some spring constant in play too.
> 
> Now consider the recording end.  A moving coil microphone with equivalent
> mechanical characteristics will respond in a matching fashion (meaning
> accurate reproduction is conceivable), but not an electret.  The electret
> measures diaphragm *displacement* (capacitively), not rate of displacement,
> doesn't it?  Won't this cause phase shifts and frequency response anomalies
> etc in the reproduction?  Or is it that the diaphragm displacement is a
> force measurement (sound pressure against an hopefully linear spring
> constant) that the effect is the same?  And in this case, the moving-coil
> microphone had better have a very small spring constant or else it measures
> the sum of two functions:  pressure against the spring constant and actual
> sonic displacement.  These two functions are 90 degrees out of phase as I
> have described.
> 
> It seems to me that the balance of the displacement and acceleration
> functions should match from the recording to the reproducing device.
> 
> Thoughts?
> 
> I had vague ideas of using the linear motor as a driver for a servo-
> controlled subwoofer... after all it was made to deflect a head assembly
> weighing two hundred grams up to 10 cm and stop to an accuracy of under
> .05mm within about 10 ms, and this it has done more or less continuously
> for the last ten years.  Its continuous power handling must be over 600
> watts, and its linearity and long throw make it an attractive idea.
> 
> Clifford Heath,   cjh@osa.com.au,         +61 3 871-1694    _ _
> Open Software Associates Limited,              fax -1711   / / \
> Box 401, Ringwood, Victoria, Australia 3134.               \_/_/

The motor assembly in the disk drive would probably make a reasonable SUB
SUB woofer motor but in the actual disk drive it is probably servo
controlled by feedback from the actual platter(s) it is reading. In the
case of old CDC drives the coil is driven hard forward and the tracks are
counted as it moves. When the heads are in the right posy the motor is
given a huge reverse (Braking) pulse to stop about the right spot. Its
average seek time was from memory around 12ms from anywhere to anywhere
(I,m Talking CDC drives here yours may be slightly different) giving it a
maximum frequency response over it's full travel of approx 83Hz with
feedback. What it's response would be without feedback would be
interesting to find out. Sounds like a promisisng thing. I hope to here
how it turned out. By the way the CDC used 400W Rms as I remember to do
it's thing.

Grajam Dinning 
TSUNAMI@Halibut.pnc.com.au

tsunami@pnc.com.au (Graham Dinning) writes:

>The motor assembly in the disk drive would probably make a reasonable SUB
>SUB woofer motor but in the actual disk drive it is probably servo
>controlled by feedback from the actual platter(s) it is reading. In the

I still repair these washing-machine drives (well, somebody has to :-)), 
and in most cases there are several feedback loops - the DEC RK07 had 3 (The
position loop from the servo track, the velocity feedback from a 
coil/magnet on the positioner, and a optical position/velocity feedback 
transducer used for load/unload). The second of these, the velocity feedback
would work without the rest of the drive being operation, and would probably
be useful for this application.

A basic position feedback system is going to be essential as well - 
otherwise any offset in the electronics is going to move the coil to one 
end stop and stick there. The optical load/unload transducer (if present) 
could be used here, or maybe you could design something to do this.


>case of old CDC drives the coil is driven hard forward and the tracks are

The CDC drives I've worked on have a velocity feeback coil inserted in 
the centre of the magnet assembly - it's the 3-4 wires coming out the 
back of the positioner.

>counted as it moves. When the heads are in the right posy the motor is
>given a huge reverse (Braking) pulse to stop about the right spot. Its

It's a little more complex than that, at least in the drives I've worked 
on. The positioner current is controller by a DAC that is driven by an 
encoded difference between the wanted cylinder and the actual cylinder. 
This is then combinded using analogue circuitry with the velocity signal 
(to provide damping), and a few other things. In some drives, 
particularly winchesters, the velocity information is provided by the 
positioner coil current.

>Grajam Dinning 
>TSUNAMI@Halibut.pnc.com.au

-tony