Re: [CAD_CAM_EDM_DRO] Re: Poor Mans DRO

jethrobodine wrote:

>I had the same questions. How do commercial DRO encoders like Sargon work?
>I searched Ebay hoping to find a used commercial DRO to put on my low dollar mill/drill and was surprised that there were just a few listings. I found Bridgeport type mills with DROs for not much more than the price of a DRO system alone, interesting.
>
>

The standard optical scale used on DROs uses a glass plate about 1/4" thick,
with a heavy chrome film vacuum deposited onto it. it is then coated with a
photoresist, and exposed through a photomask to UV light. Etchants then
remove some of the chrome, leaving a pattern of clear and metal bars on the
surface of the glass. The clear and metal parts are about a 50:50
ratio, ie. the
slots equal the bars. There is often a stripe with a different pattern
that allows the
DRO box to realign the reference point after it has been turned off. Cheap
DROs don't have this feature.

There is a read head which wraps around the glass on one side of the strip.
It has a light source on one side, and light sensors on the other. the
sensor
side has two "analyzer" gratings with stripes of the same pitch as the
scale,
but it has them aligned in some manner so that when one analyzer grating
exactly lines up with the stripes in the scale, the other grating is 90
degrees
out of phase, ie. it is HALF covering the stripes of the scale. This
allows the
two sensors to see a quadrature variation in light as the head moves.
The quadrature method allows the DRO box to figure out both direction of
movement and amount of movement. It also allows 4 points to be detected
as the head moves one grating pitch along the scale. So, for a DRO readout
of .0005", the grating pitch of the scale needs to only be .002", or 500
lines
per inch.

There are many variations on this scheme, such as using 4 sensors and taking
the light ratio between them to compensate for light source degradation
over time. One of the things to watch is that cheap units have expendable
teflon or other sliders to keep the analyzer gratings from crashing into
the scale. When the sliders are worn down, the scale is destroyed by the
read head, guaranteeing that the unit must be replaced. The expensive ones
have tiny ball bearings riding on a hard track, and should give much longer
life.

Jon