Re: [CAD_CAM_EDM_DRO] Re: DRO linear scales discussion

a_w_abate wrote:

> >
> > What you do is take a piece of the scale and place it in a holder
> > that allows you to tilt it a few degrees from parallel to the main
> > scale. You put two LEDs on one side, and two photodiodes
> > on the other side at some separation. When the tilt is adjusted,
> > you will find a setting that causes bands of light and dark to sweep
> > across the sensors such that when one sensor is at 50% brightness,
> > the other is at either 0 or 100%. At that tilt, you have obtained
> > a quadrature (90 degrees) phase shift.
>
> I'm confused. You do not have to tilt the scale to get the signal
> phase shift. You have to adjust the width between the LED/sensor pair
> until you obtain the need shift. Then, lock it down. You could do
> this with a fine pitch screw.

Tilting the analyzer grating makes it possible to make one assembly,
with ONE section of grating and the sensors all fixed to each other.
Tilting this assembly is MUCH easier than positioning one grating
section with an accuracy of .0001" or better. I'd like to see you
accurately position the movable grating the way you suggest.
It would clearly be a trial and error technique, with no way to
move it just a little bit. Every trial would be a completely random
adjustment. Even the tilt trick is pretty tough, but you can SEE
the Moire pattern, and get quite close by eye, then use a scope
to refine the quadrature phase shift. I actually worked on a sensor
that was made this way to read +/- 5 Micro Inch, and it was fairly
easy to set it correctly.

Jon