to measure displacement...

Discussion in 'General Electronics Chat' started by toffee_pie, Apr 12, 2013.

  1. toffee_pie

    Thread Starter Active Member

    Oct 31, 2009
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    hi guys,

    i need a design that measures distance, nothing too complex and distances will be not too great, 10/15 mm or so

    what is the best way to approach this?

    i am thinking strain gauge, proximity sensors, op amps, or some software even such a labview which i quite like but have not used it for years.

    just want to get some thoughts here, again i suppose the key work here is simplicity as i don't want something over kill.

    cheers.
     
  2. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    What accuracy?

    Ken
     
  3. BillB3857

    Senior Member

    Feb 28, 2009
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  4. toffee_pie

    Thread Starter Active Member

    Oct 31, 2009
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    i am going to do a simple wheatstone bridge and use a op amp to hook up to a scope on the output voltage.

    i am not sure what value strain gauge to get or what type, again i need something pretty simple and this works for me.

    i am measuring displacement >| --- - |<

    accuracy not paramount but will be in the order of mm.

    i need 2 strain gauges either side of my source object which will expand under test.

    any idea on what value resistance of strain gauge?

    thanks
     
  5. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    "Expand 15mm"? 15mm on a strain gauge doesn't sound logical sensor choice.
    Maybe you can explain exactly what you are trying to do. What is the overall project.

    Ken
     
  6. BillB3857

    Senior Member

    Feb 28, 2009
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    I agree that 15mm is a very wide stretch for a strain gauge. For those not versed in mm, that is a little over 1/2 inch. Again, the question would be..... How will the measuring device be physically connected to whatever it is you are trying to measure? What is moving, what is stationary?
     
  7. MrChips

    Moderator

    Oct 2, 2009
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    How about a ruler, measuring tape or vernier calipers?
     
  8. BillB3857

    Senior Member

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    I work on video cameras that measure the diameter of a stream of glass. Observe the video signal on a scope and measure the width of the pulse defined by the edges of whatever you are measuring.
     
  9. toffee_pie

    Thread Starter Active Member

    Oct 31, 2009
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    its actually a pouch cell, it will expand during discharge etc, i want to measure its displacement to see how much., 10mm would probably be the maximum, i think 5mm might be more realistic.

    i got a instrumentation amp i can use for the +/- inputs and measure the output voltage on the amp.

    i want to use 2 strain gauges so a half bridge or full bridge circuit, not sure what value resistors on the gauge though.?
     
  10. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    Call me dense, but I still can't visualize how you are going to arrange two strain gauges to measure the pouch expansion?

    Ken
     
    Last edited: Apr 12, 2013
  11. #12

    Expert

    Nov 30, 2010
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    Strain gauges don't move much. You would have to put a spring in the mechanism so the strain gauge is not fractured. Then measure the results as per the usual way, opamps, etc.
     
  12. toffee_pie

    Thread Starter Active Member

    Oct 31, 2009
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    the units involved will be relatively small (from the strain gauge) but i hope i can get eligible results out of the instrumentation amp.

    the pouch cell will expand and will cause restiveness changes in the Wheatstone bridge and thus on its output terminals, these will be fed into the amp

    well that's the plan anyway.

    any other methods are over kill at the moment, ie buying specific hard ware or the like.
     
  13. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    Yes, strain gauges put out a very small signal, and yes that can be amplified. Strain gauges output a voltage proportional to force, not distance. So I still can't see how you are going to mechanically implement them in your apparatus. Maybe you could post a sketch.

    Ken

    Got to correct myself. The output is proportional to distance, but on the microscopic scale.
     
    Last edited: Apr 13, 2013
  14. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Tiny magnet and linear hall effect sensor. That will work well from 0-15mm.

    The non-linearity should be easy enough to compensate in software, depending on accuracy and resolution needed.
     
    #12 likes this.
  15. joeyd999

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  16. MrChips

    Moderator

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    You could use a linear slider potentiometer.
     
  17. atferrari

    AAC Fanatic!

    Jan 6, 2004
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    As far as I know, LVDTs can translate displacement to voltage with good repeatability.

    /edit
    Didn't know that somebody had posted about them already!
    /edit
     
  18. joeyd999

    AAC Fanatic!

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    And, you can get at least 13 bits of accuracy using only a 555, a 4066, and a handful of discretes.
     
  19. toffee_pie

    Thread Starter Active Member

    Oct 31, 2009
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    thanks for the replys guys, appreciated.

    let me digest these last few posts.
     
  20. toffee_pie

    Thread Starter Active Member

    Oct 31, 2009
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    Linear variable transformers are overkill also, have you seen how much they cost..

    So, i will have to make do with a strain gauge and back to my initial question of a resistance that will give me maximum amplification factor for a op amp?. , INA337AIDGKT i think is ok to use.

    regards
     
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