Re: [CAD_CAM_EDM_DRO] Re: Optical film for use with DROs

Alan Rothenbush wrote:

> >You don't need a HEDS unit if you can do what you have done. All you
> >have to do is make 2 fixed gratings with the same line pitch as the
> >scale but one offset from the other by 1/4 line pitch. Place two
> >narrow angle LEDs above and two phototransistors underneath. The
> >fixed gratings will act as a shutter to the movable scale grating as
> >long as the distance between them is very small or the LED light beam
> >is collimated (narrow angle output). The phototransistor outputs will
> >be in quadrature.
>
> I tried this when I worked for a company that wanted to produce joysticks for
> use with Macs, using optics instead of pots.
>
> We had ENORMOUS problems with scratching when placed emulsion to emulsion.
> It didn't work at all when placed the other way 'round. Apparently, that
> small difference in spacing was enough to mess up something optical.
>
> This with much larger lines and spaces than we're talking about here.
>
> We eventually ended up sending each roll of film to Hollywood ! where the
> film was coated with something magic. It wasn't a forever solution, but
> did last long enough for the average user.
>
> I think the fact that USDigital is having so much trouble with their
> 500 line linear encoders and that no other manufacturer of encoders ever
> came out with a product this fine is interesting, and should be noted.
>
> Funny things start happening to light right around spacings like this ...

No, this is not a light wavelength problem, you have to go down farther
than that. I think it is a Litho film problem, or possibly just due to the
spacings they need to keep from scratching the film. Commercial DRO
scales have ~600 lines/inch (actually .01 mm resolution scales have
stripes at .04 mm, which is .001575", or 635.00127 stripes/inch.)

These are the generic DRO scales, they go about 4 times higher with
plain digital LED scales before they need to add interpolation.

Now, the scale grating and the analyzer gratings are EXTREMELY thin
metal layers evaporated on the glass. It is either Nichrome or Tungsten,
for hardness, corrosion resistance, etc. The emulsion layer of litho
film is REALLY thick, to get the very opaque black. This will be
counterproductive as the scale resolution goes up. Two thick
emulsions with adequate space to prevent them from wearing against
each other is going to make this pretty tricky!

Jon