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Strain Gauge - Sensor Application Help

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by CB900, Jan 11, 2017.

  1. CB900

    CB900 New Member

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    Hi,

    I'm currently working on a test bench which incorporates a small spring that needs to be evaluated. The test bench counts a certain component movement until the spring fails.
    The spring looks like a rounded capital A with long parallel side (below the cross bar if you like) the diameter if the springs wire is approx 0.25 - 0.5 mm. the distance between the ends of the legs is approximately 5mm

    Now, I originally intended for both protruding legs to activate 2 small micro switches but the additional load has influenced the data. The physical dimensions prohibit the use of an opto slot type sensor (even SMD)

    The only thing I can think of is a strain gauge on double sided sticky foam pads so that the spring would touch it (and deform it) at the end of movement - would this work and could I use one sensor for both legs (legs slghtly bends sensor in 2 places.

    Hope this makes sense
    Thanks
    Doc
     
  2. JonSea

    JonSea Well-Known Member

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    The foam tape is going to severely reduce the deflection/strain the strain gauge will experience.

    Are you trying to sense how many cycles the spring has experienced? Is something driving it during testing?
     
  3. tomizett

    tomizett Active Member

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    Could an inductive proximity switch help? I agree that trying to attach a strain gauge with foam tape sounds like it could be trouble, especially in terms of long-term repeatability.
     
  4. dave

    Dave New Member

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  5. OBW0549

    OBW0549 Member

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    That won't work. Strain gauges MUST be bonded firmly to the piece whose strain they're trying to measure; otherwise, the bonding medium takes up a portion of the strain, rendering the strain reading inaccurate. Using sticky foam tape, your strain readings would be completely meaningless-- if you get any strain reading at all.

    What would be the purpose of this strain measurement, anyway? What are you trying to detect (or calculate) with it?
     
  6. CB900

    CB900 New Member

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    Thanks all for the replies,

    To give all some background, the eye of the spring is purely for mounting. The springs parallel legs are inserted and protrude through a piece of moveable piece of metal via a small rectangular hole and an intermediate small worm gear (6mm dia).
    The current 'spring' breaks between the worm drive and metal, not at worm drive or metal. We need to get numbers for oscillations of the metal before failure for feedback (arse kick the manufacturer). Currently we find that one leg breaks first

    Because the loadings, spring dimensions are so small if any type of mechanical switching is used it could sway the data, searching around I cant find any proximity sensor that is SO small that would pick up the legs individually or a LED slot sensor that would fit in between or around the legs - hence I came back to my question

    I just need something that does not impact on the mechanics of the system but could detect 1 or 2 deformations that could ultimately be counted by a uP. Without drawing diagrams and uploading the best analogy I can think of is if you stick 2 fingers out and touched the back of your other hand this is what I want to measure. your hand being the gauge and your finger(s) being the spring leg - would a strain gauge be able to do this?

    Thanks
     
  7. alec_t

    alec_t Well-Known Member Most Helpful Member

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    Can you insert a piece of insulator-backed metal foil somewhere so that the spring leg contacts it to complete a circuit when moved?
     

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