There's no real magic to making your own encoders moderate resolution
encoders, except for one real caveat.

You need a pair of phototransistors spaced an appropriate distance apart,
that distance being ( n * line spacing ) + 1/2 line spacing where the black
parts and the white parts are both "lines". Rigidly mounting one of the
transistors and putting the other on a screw adjustable slide works just
fine, even for very fine resolutions.

Then you need a light source, one or more LEDs also working fine.

The problem is that the phototransistor cannot "see" light from more than one
space at a time for quadrature to work. This means either a phototransistor
with a very narrow acceptance angle or a light source with a very narrow beam.

How narrow is a function of the resolution as well as the spacing between the
LED and the transistor.

Suffice to say that at the level we're hoping for, .005", this is a bit of a
problem.

Recent experiments with telescope mirrors, and the making thereof, has me
more than little worried about the prospects of using an aperture of any
sort.

Perhaps cutting an existing encoder apart, then reassembling it with the
LEDs and transistors closer together may work.

The laser diode comment, though, is a very interesting one. Maybe with a
narrow enough laser diode, a person would use a wider acceptance
phototransistor and put the diode on the slide ...


Some interesting reading can be had at the USDigital site

http://www.usdigital.com/products/em1/

where they discuss their "homemade" encoder. Pretty tricky stuff !


Alan

--

Alan Rothenbush | The Spartans do not ask the number of the
Academic Computing Services | enemy, only where they are.
Simon Fraser University |
Burnaby, B.C., Canada | Agix of Sparta