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Load Cell Signal Output

  1. Jul 18, 2010 #1
    Howdy,

    I am a mechanical engineering student with limited circuitry knowledge to start...

    Anyhow I have a [-500,+500]lbf load cell that is essentially a wheatstone bridge strain gauge. I want to give it either 9Vdc or 12Vdc excitation voltage since I have a regulated power supply that can output those voltages. The load cell is rated for 10V excitation with a maximum of 15V though.

    Anyhow here is my problem. I am using a NI USB 6009 DAQ to record the voltage output from the load cell. At zero load I tested the voltage outputs for both 9V and 12V input excitation voltages.
    9V input -> 4.19V output
    12V input -> 5.65V output

    The Full Scale range of operation I hope to use this load cell in would result in a maximum/minimum increase in output voltage to
    9V input -> 4.19V +/- 0.027V output
    12V input -> 5.65V +/- 0.036V output

    The other device I have is the USB 6009 DAQ that can ready +/- 10Vdc but it has limited resolution, if I hook the system up to the DAQ without conditioning the output signal I only get +/-5lbf output accuracy and that is when sampling at 1hz. I would like to maximize how fast I can sample and greatly improve the output accuracy. So I figure I need to "zero" the output voltage with a circuit and I suppose scale the output up to +/- 10V.

    I hope I am not asking too complex of a question. I just don't really know where to begin. All I have come up with so far is:
    -Low Noise Op-Amp to apply a GAIN to the output signal and power it with the DC voltage source
    -Use a voltage divider circuit in parallel with the excitation source voltage to "zero" the output voltage.

    I must not be doing any of it right though since the circuits I have thrown together from guides on the net have not done anything that I am wanting them to do :(.

    Any help on where I need to look now?

    Here is some other info on the load cell
    Nominal Input Resistance: 394ohms
    Nominal Output Resistance: 350ohms
     
  2. jcsd
  3. Jul 18, 2010 #2
    Would an instrument amplifier with a setup such as the one posted in another PF topic work?
    diffa2.gif

    I suppose it is called a voltage difference amplifier. I suppose I could have one input voltage be my load cell output and have a 2nd power source of 5V which would give me an output of Vo = 0.65*A where A = R3/R1 and I could amplify the signal with a gain of about 14.5 which would give me Vo=9.425V nominal and Vo=9.86V FS (500lbf load).

    Although I hear that I shouldn't be using an op-amp for a load cell amplifier. Instead I should be using an instrumentation amplifier. Can I perform a voltage difference amplifier with an Instrumentation amplifier as well??
     

    Attached Files:

  4. Jul 18, 2010 #3

    dlgoff

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    Science Advisor
    Gold Member

    You could use op-amps or instrument amps. Depends on your cell and what you want to do. e.g. how many varying elements, temperature compensation, linearizing.
    I found this http://www.analog.com/static/imported-files/seminars_webcasts/49470200sscsect2.pdf" [Broken] that might help in you circuit design.
     
    Last edited by a moderator: May 4, 2017
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