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Thread: Homebrew precision capacitive displacement measuring system?

  1. #11
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    Quote Originally Posted by ikdor View Post
    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.
    Analog wizardy goes probably to some ridiculous sub-atto-something.
    For example I thought that 1ppm accuracy was pretty good on resistance measurements until I stumbled upon ASL F900 resistance bridge. 20 ppb accuracy and 0.5 ppb resolution.

  2. #12
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    That sort of resolution is, of course, utterly ridiculous to even consider..... At least for absolute measurement, and long term stability.

    If you want RELATIVE measurements, and need stability only over a fairly ßhort time span, then you can definitely do it, although possibly not quite to those limits.
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  3. #13
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    Quote Originally Posted by J Tiers View Post
    That sort of resolution is, of course, utterly ridiculous to even consider..... At least for absolute measurement, and long term stability.

    If you want RELATIVE measurements, and need stability only over a fairly ßhort time span, then you can definitely do it, although possibly not quite to those limits.
    Not sure was this comment on ASL resistance bridges or capacitive sensors?
    But like I said 1% linearity or accuracy would be great for lots of uses. If you measure something like precision spindle runout it shouldn't matter that much if the measurement result is 1.00 um runout or 1.01um runout.

  4. #14
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    Pretty much a "whichever".

    We get into the "measuring microns" deal here from time to time. It is a far different thing to measure differences than to measure as an absolute. Even have had folks who should know better talk about interpolating to tenths or better on dial calipers with a magnifying glass.

    Home-brew systems to measure microns and better is getting toward that sort of talk.

    Just trying to get back to reality.
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  5. #15
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    Yes, and no.
    A digital micrometer will pretty well measure to 1 micron ..
    since they mic gage pins, accurately, 10 times out of 11/12, to 1 micron.
    One or two measurements were out +1 micron, on 3 different pins, repeated 12 times.
    Always +1, never more than +1um, never less.

    Of course, "good" import mikes.
    I import and occasionally sell ones made by Shahe.

    I tried 4 different ones that were all junk. Difference in cost was about 10-20€ per mike.
    The shahe mikes start about 60€ with 22% vat.

    The analog gages like Federal seem to repeat well, indicate well, and show tir readings down to .1 um or so.
    From what I have seen online and at EMO germany trade fair.
    I don´t own one, but will get one someday..

    I suspect it is perfectly possible to make a 0.1-1 um gage, that repeats to less than 0.1 um error.
    If you can design electronic circuits- which I cannot.

    The difference vs absolute is pretty true - as You said.
    At 1 micron in size, say 25.000 mm, 1 um TIR or less..
    Say 15 mm long (7205 AC/p4/dup).

    what about surface finish variations ?
    lobing ?
    Length of area measured ?
    Or Is a go/no go gage sufficient at 1 um difference ?
    Corner rounding ?

    I quite accept that there is a +/-1 um error band, in homebrew stuff.
    But I also insist that the error band tends to 0, and is not more than 1 um - if done well, with attention, and proper workmanship is used.

    Quote Originally Posted by J Tiers View Post
    Pretty much a "whichever".

    We get into the "measuring microns" deal here from time to time. It is a far different thing to measure differences than to measure as an absolute. Even have had folks who should know better talk about interpolating to tenths or better on dial calipers with a magnifying glass.

    Home-brew systems to measure microns and better is getting toward that sort of talk.

    Just trying to get back to reality.

  6. #16
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    Quote Originally Posted by J Tiers View Post
    Home-brew systems to measure microns and better is getting toward that sort of talk.
    Just trying to get back to reality.
    My homebrew DRO measures in half microns, how close it is to reality is more dependent on the process capability of the magnet tape manufacturer.... I expect it to be better than 5um though for short distances.

  7. #17
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    Quote Originally Posted by J Tiers View Post
    Pretty much a "whichever".

    We get into the "measuring microns" deal here from time to time. It is a far different thing to measure differences than to measure as an absolute. Even have had folks who should know better talk about interpolating to tenths or better on dial calipers with a magnifying glass.

    Home-brew systems to measure microns and better is getting toward that sort of talk.

    Just trying to get back to reality.
    Capacitive displacement sensing is not good for absolute measurements but killer for very small differences. Commercial ones go down to 0.00004 microns but I would do with less

  8. #18
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    Good with that......
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    Hashim Khan

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