Detecting strain in cloth

Discussion in 'General Electronics Chat' started by MrSoftware, Jun 28, 2016.

  1. MrSoftware

    Thread Starter Active Member

    Oct 29, 2013
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    Can anyone suggest a strain gauge that works with cloth?

    A guy at work had an idea that requires some sort of strain gauge to determine if a piece of clothing is under strain, and I thought for sure there would be some off the shelf sensors available, but so far I'm not successful in finding one. We don't need an exact measurement, just to know if there is say 2# or more weight on a cloth strap, such as a pair of suspenders. Is there anything off the shelf for this?
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    There are load cells on ebay for $3.00.
    You may be able to adapt for the application.
    What about a fish weight spring scale?
    Max.
     
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  3. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Perhaps the attached PDF might give you an idea.

    Bertus
     
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  4. jpanhalt

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    Jan 18, 2008
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    If you assume the strain is not exactly uniform across the cloth, a colored thread woven into the cloth would provide a visual indication.
    John
     
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  5. JohnInTX

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    Jun 26, 2012
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    You guys are amazing.
     
  6. atferrari

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    Hola John,

    Could you point me to where I could read about it?l
     
  7. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    Very interesting.... are you planning on developing an underwear wedgie alarm?
    Seriously now, in the case of suspenders, I'd probably consider using a load cell attached to their clips.
     
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  8. atferrari

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    What could be a realistic range?
     
  9. Techno Tronix

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  10. GopherT

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    If viewed in polarized light, strain on clear polymers can be seen easily. Use fabric made of natural color (un dyed) synthetic fibers. Use bright unpolarized light from the rear and polarize the light that transmits through the fabric (usually through a clear acrylic beam. As the polymers align under strain, the chains align and polarization occurs and appears as a dark line when the transmitted light is filtered with a second polarizing filter.

    It will be much more difficult with fibers because they are already mostly aligned as the fiber is spun (if spun under tension). Use a fiber with huge elongation (an indicator of fiber that is not "drawn" when spun, it is simply wound out of the spinnerets.

    It would be an interesting experiment - I've never seen it done with five but works great with acrylic sheet.

    Rotating polarizer will allow one to see which direction the polymer alignment makes and, therefore, the strain vector.

    There are lots of youtube video demonstrations. If it is not immediately visible, the fiber could be placed in a fluid with refractive index matching the fiber to limit all the stray air-fiber refractive index mismatch that will interfere with your view. The fabric weave will literally dissappear if the fluid matches perfectly. Mixture of water and calcium chloride can be used to match most materials.

     
    Last edited: Jun 29, 2016
  11. AnalogKid

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    Bertus scares me.

    ak
     
  12. bertus

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  13. MrSoftware

    Thread Starter Active Member

    Oct 29, 2013
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    Thanks guys for the info. I'm hoping for something off-the-shelf that just gets inserted into the strap, like clipped-in, as opposed to woven in. At least for the prototype.

    To elaborate a little; the goal is to determine, electrically, if a garment is being worn. The garment will have straps that will be under tension when it is being worn. It's not actually suspenders but that's the closest thing I can think of without giving it away (NDA). A strain gauge (or load cell) that can be inserted in-line with the strap was the best thing I could think of, but I'm definitely open to additional suggestions.
     
  14. cmartinez

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    If that's the case, then a much simpler solution would be the use of a pre-loaded mechanical switch.

    Why complicate things with a load cell, which will give you a proportional analog signal that will later need post processing, when a simple digital output (yes/no) will do the trick?
     
  15. MrSoftware

    Thread Starter Active Member

    Oct 29, 2013
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    A mechanical switch would be fine too. There will be a micro processor on-board for other things so processing analog signals won't be an issue, but a mechanical switch that can be integrated into a strap would be equally OK.
     
  16. cmartinez

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    How many units do you need?
    Reason I ask is, maybe you can find a ready made model out there, or you could design (or integrate) your own.
     
  17. jpanhalt

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    If the straps are anywhere near the torso, you might listen for heart or breath sounds, or even just rubbing sounds from normal motion.

    John
     
  18. MrSoftware

    Thread Starter Active Member

    Oct 29, 2013
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    For now we just need a prototype. If there is interest then we would need volume, but not at this time. My goal is definitely to find something off-the-shelf that can be easily adapted to work, but so far my googling skills have come up with nothing. :(

    Listening for sounds is interesting, but it might be in a gear bag, trunk of the car, etc.. so we would need to differentiate between biological and environmental sounds, which is more processing than I would like to have to do.
     
  19. cmartinez

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    Another technique that could be implemented in parallel with a switch, is a motion sensor. Under what conditions will the garment be worn?
     
  20. hexreader

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    Apr 16, 2011
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    Just spotted this strange device:

    http://www.mikroe.com/click/stretch/

    (play the video for a better understanding)

    I cannot imagine that this is of any use in your application, but it might trigger an idea.
     
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