Re: plastic strips for encoders

On Sun, 30 May 1999, Elliot Burke wrote:

> Many people have written about using the US Digital plastic strips for
> encoders.
> Has anyone calculated the thermal effects of the plastic?
> The CTE of plastics is so much larger than other engineering materials that
> even a few degrees F over a foot will make a very inaccurate measurement.
> When I get to my office I'll look up the exact numbers, but they are
> freightening.
> If the strip was glued to a piece of glass it would work better, but might
> not fit in the reader.
> If the encoders use transmissive optic a metal backing wouldn't work.
>
> Elliot Burke
> HighTide Instruments
>

hi Elliot,

another tass* guy here too :-)

you are of course correct that a plastic scale would be suspect, but
the scale is probably a piece of litho film. if the film is clamped
or glued to the machine, then even if the film overhangs in the clear
for the reader, then it would be constrained by the machine structure.

i believe that there would be interest in an "open source" effort on
scales for machines, and the HP readers don't have the desired increments.
so we could design a reader with english increments of say .001 inch
and get the reader grill and scales produced on 8 mill litho film by
photoplotters with high accuracy which make tooling film for printed
circuit boards. these wouldn't have the accuracy of the chromium on
glass scales, but would be adequate for many of us on this list.

if your electronics are a computer then the HP readers and corresponding
scales are no problem to compute, but some people would like smaller
units and there are inexpensive quadrature counters with LCD screens,
for instance from Red Lion, that would work with such a project's scales.

-ron
rjw@...

*www.tass-survey.org