Re: [CAD_CAM_EDM_DRO] Optical DRO

"Arne Chr. Jorgensen" wrote:

> From: "Arne Chr. Jorgensen" <instel@...>
>
> Hi,
>
> Here is something more I don't understand:
>
> Jon wrote: ( some time ago - digest 408 - somehow I have lost it )
>
> Well, you proposed something, using some old Hilger&Watts gratings,
> - I have
> no clue what these are.

Hilger & Watts (of England) made high precision measuring gratings for
many years,
and I believe still do make similar tools. These are quite similar to
the glass scale
in a DRO, but are often used by themselves, and placed over an object to
be
measured. One counts grating lines through a microscope.

> Further, I didn't understand what you meant by
> tilting them and how you would read them. You also mentioned glass
> scales.

If you place two of these scales with the grating side facing each
other, but at a slight angle,
you get a Moire pattern. If one scale is slid along its long axis
(perpendicular to the direction
of the grating lines) dark and light bands appear to move at right
angles to the motion.
These bands are much further apart than the grating lines. The spacing
of these bands can
be changed by changing the angle between the two gratings.

By placing two photocells (or better, 4) strategically over a small
piece of the grating which
is tilted with respect to the long one, you could make a quadrature
linear encoder with a
resolution of 4 x the line spacing of the grating.

I have set this up on the bench, and it works WELL! I also rebuilt a
micro indicator
that used this principle, so it is not my invention.

Jon