Longfellow makes a series of linear pots for servo systems.
(sorry, don't have a URL but any good 'source book' will list them).
With a suitable A/D you can have whatever resolution and sample
rate your require. They have units up to 6" in length and use a
glass base with evaporated metal coating so you get very long life.
(We used them on a muscle puller used in characterizing MD).

Also look at some of the linear optical sensors made by Koyocera
and the like. 10=100mm range, analog or digital outputs.

And the cheap no-name digital calipers I have do have an 'unoffical'
serial output. Clk & data, 38 bits, may tens of msec to clock
out the data (after all, it's battery powered so the processor
runs slowly to conserve power).

Robert

John Gardner wrote:
>
> Patrick J wrote:
> >
> > Is it possible to take a 'cheap' digital sliding caliper apart and use it as a
> > digital linear sensor w a '877 ? How do they work?
> > I have a 80mm long movement which takes min 0.10 sec which I'd like
> > to measure with 0.3 mm resolution. I know there are expensive versions
> > avail w RS232, I dont think they can handle that fast movement though
>
> Machine tool supply firms sell milling machine quill DRO retrofits
> which are functionally identical to digital calipers and have con-
> venient form factors for many apps. These have a range of 150 mm.
> Larger versions available.
>
> Note that most cheap digital calipers DO NOT have outputs. All such
> devices are quite slow - Reading a Mitutoyo "SPC" type device takes
> about 34 ms. Resolution is .01 mm for the cheap stuff. Max speed is
> 1 meter/sec.
>
> Mitutoyo SPC protocol is 3-wire, CLK, DATA, REQ.
> CLK and DATA are open-collector outputs.
> Pulling REQ low produces 52 clocks, DATA valid on
> CLK low, each group of 4 bits represents a hex #,
> lsb to msb:
>
> d0-d3     0Fh
>
> d4        sign digit, 0 is positive, 1 negative
>
> d5-d10    measurement value, BCD, MSD to LSD,
>
> d11       decimal point position in the 6-digit
>           measurement value, counting right to left
>
> d12       units, 0=mm, 1=inches
>
> Its easy to use these things if you can live with the speed.
> Faster devices are optical, referred to as "linear encoders",
> "linear scales", or sometimes "glass scales". AFAIK all these
> devices have quadrature outputs.
>
> regards, Jack
>
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