Alexander Rice wrote:
> an interesting point was that i also used a stepper motor an a rather
> unusual way, if you get a really small bipolar stepper motor with the
> biggest step angle you can find (about 12 degrees is common) and then
> drive one of the coils with DC and apply your signal to the other coil,
> this results in a rotational movement between two step positions that is
> actually pretty proportional to the applied signal, if you want you can

As long as you're in the middle 50% or so of a step. The endpoints
will be quite rounded.

> even implement a feedback system based on the back EMF induced in the DC

Not quite, because the feedback is also none linear, in the same way.
Better to have an external position sensor (differential capacitor
works well and is common in galvo scanners).

> winding - this works far better than the more common speaker-mirror trick.

And a lot easier to construct. In theory you could get
nearly 24 degree beam deflection. With tiny mirrors placed
very close together you could make a decent
but slow X-Y scanner.

You could set the DC coil drive to something less than spec to
lower the magnetic spring constant, making the unit more
sensitive. However this is a trade off between moving mass
and applied power (force). You also need to make sure it's
a DC (PM permanent magnet) stepper, not a variable reluctance unit.
(Easy to tell. Does is 'cog' when you turn the shaft without
applied power. If it does, it's PM).
I have tons of old Decwriter paper feed motors. I'll have
to give this one a try. The coil resistance is low enough that
it could be driven directly by an audio power amp.

Robert

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