Quarter Bridge Strain Gauge w/ Arduino

Discussion in 'General Electronics Chat' started by straits, May 31, 2012.

  1. straits

    Thread Starter New Member

    May 31, 2012

    I've been using an arduino to get a reading from a FULL bridge of strain gauges bonded to a steel bar with no trouble. My circuit on the breadboard includes the ina125P amplifier. The schematic is identical to this one: http://cerulean.dk/words/wp-content/handsketch.jpg - which I've been using at the start of this project. It worked fine and I could get the desired signal by changing the gain resistance as necessary. The signal was displayed on a Labview virtual instrument graph.

    This time I'm trying to do the exact same thing but with one 120 Ω strain gauge on a rod, in a bridge with three 120 Ω resistors of accuracy 1%. The schematic is the same again - 5V excitation across two terminals, and wired to the amplifier across the other two. However, regardless of how much I amplify the signal, I get absolutely no reading, other than something close to 0V and around 4.67V if wired slightly differently. I consulted a technician and the circuit I have is apparently fine - and the strain gauge itself is functional because I picked up readings from it with a strain gauge card.

    Any suggestions as to what I should try?
  2. crutschow


    Mar 14, 2008
    What was the output voltage of the first configuration?

    How much does the strain gauge change resistance in your second setup?
  3. straits

    Thread Starter New Member

    May 31, 2012
    The output voltage of the first configuration, when using resistance gain of about 300 Ω, was just under 2 Volts (when I bent the bar as hard as I could). In order to make it more sensitive I dropped the resistance to a drastically low level of 10 Ω and observed great sensitivity (very easy to get an output of 4.67). The ideal gain for that configuration was obtained by using 100 Ω.

    The strain gauge changes resistance by 50 - 100 μΩ in the positive or in the negative direction when twisted (used for torque measurement). That is the resistance change I read when I use a strain gauge card in an unrelated arrangement (the point of this is to make it work on an arduino).

    The way I am doing it in the picture, I get absolutely no signal even when I place 10 Ω across the gain resistance terminals (4+60,000/10 ...huge gain)
  4. Anders Bertelsen

    New Member

    Oct 10, 2012
    Hi Straits :)
    Will you share your information about how you make a full bridge strain gage, and put it into labview ?
    I am interested in making a axial rod with a full bridge strain gage attached.
    - And I have no info about the subject.

    If you want to share your discoveries I would be very very gratefull.
    Kind regards - Anders/Denmark.