Re: Position Readout Question and Introduction

> From: George Potter <gpotter@...>
> Is anyone using the output from a DRO as the position indicator in a
> servo system. I've been contemplating this for quite some time, and
> am fascinated (specially because I have the DRO installed on the Mill).

It's done all the time on large cnc machine tools. What I mean is that the
output from a linear scale system (such as Sony's Digiruler) is translated
into a quadrature format that mimics the standard output of rotary
encoders. This is then used as the source of position feedback to the cnc
control. I once worked on a machine that had a Mitsubishi cnc control which
had also been retrofitted with Sony scales. You could select from either
feedback source by changing a parameter in software. The problem you will
have with using a regular DRO this way is that the scales themselves don't
present a useable output at their connectors. Additional circuitry inside
the readout box amplifies and otherwise conditions the raw scale signals
into essentially step and direction signals that it can count and display.
Unfortunately, I don't know of any DRO maker that provides an output jack
for auxiliary connections. It may be possible, with a little reverse
engineering, to tap into the digital circuitry in the readout box to obtain
a useable signal (abandon all warranty hope, ye who enter here!). You may
get the idea to build your own scale to quadrature box, but AFAIK all the
different scale makers have different, non-compatible scale electronics. It
would also depend upon whether the scales are optical, capacitive,
inductive, etc. The above mentioned Digiruler system can be had with
quadrature output, but the cost makes further discussion inappropriate for
the audience to which this mailing list is directed ;). There remains the
linear encoder/mylar scale system available from http://www.usdigital.com
which, despite its peculiar native resolution, is being successfully used
by some other list members to achieve a do-it-yourself DRO setup. This is
maybe the best, easiest method of achieving what you have in mind since the
encoders used have quadrature outputs.

Let us know more of your plans and you won't want for opinionated comments
on them!