Strain Gauge Sensitivity

Discussion in 'The Projects Forum' started by ntwhee0, Jun 5, 2014.

  1. ntwhee0

    Thread Starter New Member

    Jun 4, 2014
    3
    0
    I'm a prosthetist (no EE background)
    I have a Health O Meter bathroom scale, which has four strain gauges that have 3 wires each.
    I'm trying to hack it to send S+ and S- to AD623an for amplification.
    I'm told that S+,S- voltage needs to be around 2mV.
    I'm getting about 355 mV at S+,S- and I don't understand why.

    I've sending about 5 volts (4.99) into E+ ,E-

    Resistance between E+ and S+ is 1048 Ohms, between E+ and S- is 1048 Ohms, between E- and S+ is 1048 Ohms, and. Between E- and S- is 1045 Ohms.

    Here's my math to figure S+ and S-:
    4.99*1048/(1048+1048)=2.495 V,
    the voltage between S+ and E- should be
    4,99*1045/(1048+1045)=2.4914 V,
    the voltage between S+ and S- should be
    2.495-2.4914=0.0036 V=3.6 mV

    But i'm not getting 3.6 mV. I'm getting 355mV, and I don't know why. I want to get about 3.6 mV

    Where should I begin looking to figure this out??

    ps. (testing the voltage between each gauge's red and black wire (or red and white wire), I get about 640 ohms at each gauge.)
     
  2. ericgibbs

    Senior Member

    Jan 29, 2010
    2,499
    380
    hi,
    You should measure 3.583mV with those bridge resistance values.
    [see image]

    The value you measure 'I'm getting 355mV,' is numerically very close to 3.58mV expected, are you sure that you are reading and measuring the voltage correctly.?

    E
     
  3. celticlord

    New Member

    Jun 10, 2014
    1
    0
    Could someone please give me a pinout and and help to hook this up to a Basic stamp. I dont need to know how to connect it just how this pinsout. All I could gather is it uses a diff amp for the strain gauges.
     
  4. ntwhee0

    Thread Starter New Member

    Jun 4, 2014
    3
    0
    Hi ericgibbs,

    Thanks for checking the math.

    I finally got the correct sense voltage.
    I had not properly probed the OEM pcb to know the correct scheme for E+,E-,S-,S+.
    I figured it out by randomly trying different schemes.
     
  5. ntwhee0

    Thread Starter New Member

    Jun 4, 2014
    3
    0
    He Celticlord,

    As per your request, You'll notice the Health O Meter's circuit board has labels of S1, S2, S3, and S4 in very small text at the boarder of the board where each gauge's wires connect to the board. I soldered to the RED wire connection points only; Excite positive to S1, Sense positive to S2, Sense negative to S3, and Excite negative to S4. I used AD623an to amplify the signal. Then the signal goes to an STF4 discovery board.
     
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