I don't know about cheap, but General scanning make pen motors.

It seems from their website that they don't make the GM20, with which I am
familiar, any more, but they have a GM 100 here :
http://www.gsilumonics.com/c03oem_gal_frame/galvoframe.html there seems to
be a range of styles available including 2 degree types which may be the
most appropriate for this application.

They also do a series of closed loop scanners which include very accurate
integrated capacitive position sensors and associated electronics. While
this would be a more accurate sensor, it would also be WAY too expensive.

I have emailed them to see if they still do the GM20.

regards all
Mark

From: Al Allworth 


> My first sensor was made from a device called a "limited angle torquer"
> available from surplus. It has a magnet rotating in a stator winding
> supported by low friction ball bearings. I built an arm to mount a weight
of
> about 5 Lbs at a distance of about 7 or 8 inches. It was tilted a few
> degrees from vertical and had a period of about 4 seconds. I more than
> saturated my chart recorder on the Landers event and aftershocks around a
> 3.0 (several years ago) at a distance of about 150 miles. The garage could
> have fallen on it without breaking it. I also made a smaller version out
of
> a pen driver from a chart recorder that worked equally as well but you
could
> hold it in your hand. Using a heavy mass helps to overcome some of the
> friction problems with these kinds of devices.
>
> There are pen drivers built like a large version of a meter that could
> probably be made to work but I never found one when I was looking for
stuff
> like that.
>
> There must be a simple and cheap way to get something for strong motion.



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