Re: [CAD_CAM_EDM_DRO] U.S. Digital encoders

Hi Alan R.,

I'm currently trying to repair an old Vernitech (M15-127) panel shaft
encoder, so I'm curious U.S. Digital's "homemade" encoder? I didn't
find it, can you be more specific?

My current thinking is that the LED is out. I don't know if it is IR or
not. I've replaced two 2N3906 transistors, still no luck. It looks like
about 256 cpi, and by rotating the encoder wheel, I can see about 4
"slots" alternately aligning over each of the photocells in the flat
pancake sensor.

Alan KM6VV

Alan Rothenbush 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