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Thread: Homebrew precision capacitive displacement measuring system?

  1. #1
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    Default Homebrew precision capacitive displacement measuring system?

    Has anyone seen diy versions or have idea what's under the hood?

    Possibly quite cheap way to measure very small displacements like precision spindle runout down to nanometer resolution (yes you read correct, 0.00000004 inches)

  2. #2
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    I've played around at work with CDCs which are amazingly sensitive. It wasn't very special from the interface point of view so I wouldn't expect any problems with a homebrew version. It all depends though on your embedded system skills.

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    Seems kinda blue-sky for the home shop. Where exactly would you use it?

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    It is not a cheap way to accurately measure down to nanometer resolution. My company makes a custom CDS for a Fabry-Perot interferometer used in a hyperspectral imager for chemical warfare detection. We achieve sub-nanometer resolution but it is extremely difficult. There are a lot of electronic subtleties that need to be accounted for, especially in a demanding environment (e.g. on a machine tool with microphonics, changing temperatures, dust, grime, etc.).

    But a home brew CDS with coarser resolution might be pretty doable.

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    Quote Originally Posted by Fasttrack View Post
    .

    But a home brew CDS with coarser resolution might be pretty doable.
    I was actually thinking something like 50 nanometer would be more than good for most uses. 100 um measurement range would be enough and linearity requirements are not that high either, I'm thinking that 1% would be great for many uses.

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    Haven't take more than cursory look on the electronics side but AD7747 looks promising single chip solution for half of the problem.
    http://www.analog.com/en/products/an...rs/ad7747.html

    Someting like say 10mm2 sensor area with 0.1mm gap would give abouts 1 pf capacitance. AD7747 claims resolution down to 20 aF so that would be abouts 2 nanometer resolution.
    Damn, sounds like another project to tackle..

  7. #7
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    In macro photography 100X magnification in a camera lens has a depth of field of 0.19 microns. It would be nice to be able to measure .15 microns plus/minus .03 microns.

  8. #8
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    Lightbulb

    Might be worthwhile to look at the Microchip CTMU (Charge Time Measurement Unit). Here is an app note claiming all sorts of applications, and possibly being able to solve world hunger and bring about global peace!

    http://ww1.microchip.com/downloads/e...U%2001375a.pdf

    http://ww1.microchip.com/downloads/e...tes/01250a.pdf

    More info:

    https://www.bnl.gov/edm/papers/Sergio_Rescia_020118.pdf

    http://www.capacitance-sensors.com/

    https://en.wikipedia.org/wiki/Capaci...acement_sensor
    Last edited by PStechPaul; 09-13-2017 at 05:01 PM. Reason: links, correction of CTMU

  9. #9
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    Until I got an autocollimator I was planning to make an electronic level using a differential capacitive sensor and a Blumlein bridge. Bently Nevada inductive proximity probes have very good resolution as well, but they cost more than making something yourself. An AD7747 would have vastly simplified the design.
    Last edited by Mark Rand; 09-13-2017 at 04:55 PM.

  10. #10
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    Quote Originally Posted by MattiJ View Post
    Haven't take more than cursory look on the electronics side but AD7747 looks promising single chip solution for half of the problem.
    http://www.analog.com/en/products/an...rs/ad7747.html
    Yes, that was the stuff we were using. I was building a load sensing bearing for washing machines at the time and was using this to measure the bearing displacement. It was the first time ever I've seen stuff specified in atto-anything.

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