AGSCalabrese wrote:
> Dave wrote:
> > Seriously, anyone doing this kind of contactless distance measurement
> > is using optical interferometry. A laser diode, a beam splitter, a
> > corner reflector and a pair of phototransistors with a quadrature
> > decoder to count fringes.
> >
> > Now, the OP did say "resolution" and not "accuracy", so maybe there  
> > isn't a requirement for measuring the total span accurately, just
> > changes in distance.
> 
> Could you tell me more about practical optical interferometry ?  Have  
> you built such a device ?  Are there parts or kits to do this that  
> cost less than $100 / $200 / $500 / ??? to do this ?  Are there any  
> diagrams showing this setup ?

No, I've never done one. I just know they're used in systems like the
photoplotters used to produce PCBs. They have 0.0001" accuracy in 2D
over several feet in both directions.

The basic concept is illustrated here:

http://en.wikipedia.org/wiki/Michelson_interferometer

and further information here:

http://en.wikipedia.org/wiki/Optical_interferometry

It looks like Agilent might be a good place to start:

http://www.home.agilent.com/agilent/product.jspx?nid=-536900386.0.00&cc=US&lc=eng

Also, Zygo, although I know nothing about them:

http://www.zygo.com/?/met/markets/stageposition/zmi/&gclid=CN6boa_g0JwCFYdB5god9hYqKw

And no, this stuff isn't cheap -- but you've got a stringent set of
requirements, based on what you've told us so far. There's no free lunch!
Once you've got a working prototype set up, perhaps you can find ways to
reduce the cost by tailoring it more closely to your specific application.

-- Dave Tweed
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