Alan,

That is not a problem if you use a fixed n-line grating. The moving
grating and the fixed one have a "shutter" effect that simultaneously
open and close multiple line pairs at a time to admit light over a
much larger total area than a single line.

Further, if you have two fixed n-line gratings one offset from the
other by 1/4 line pair pitch on the same piece of film, you
automatically get quadrature without having to precisely position the
phototransistors.

Put the heels of your hands together, splay your fingers and rotate
one hand at the heel. You will see the "shutter" effect.

Mariss

--- In CAD_CAM_EDM_DRO@y..., Alan Rothenbush <beer@s...> wrote:
> 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