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Strain gauge Wheatstone bridge

Discussion in 'General Electronics Discussion' started by beedees, Mar 31, 2015.

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  1. beedees


    Mar 30, 2015
    Hi to everybody!
    I'm working in a project to amplify the signal of a strain gauge. In particular I use two strain gauges, one installed to measure the strain of a rod and the other used as a dummy gage for temperature compensation.
    So I need bridge completion module or more precisely a half-bridge to connect the two strain gauges and in which there are two completion resistors.
    I have found this product

    but I don't understand how it works. I'm interested in MR1-350-127. Is there the possibility to complete the bridge with two strain gauges or have I to use resistors too to complete the bridge? Is there a end to connect supply and one for ground? Is there the possibility to use a potentiometer to balance the bridge?
    I am lack of experience in this kind of application, so I ask you.
    Obviously if exist other products or products that satisfy these requirements I'm all ears! :)
    Thank you and sorry for my English!
  2. Gryd3


    Jun 25, 2014
    I think it would be best to do a little more research in how a wheatstone bridge operates.
    Also, if you have two strain gauges, don't condemn one to simply be a temperature compensation... configure it like this : [​IMG]
    Rg+ and Rg- are your strain gauges, which will both change in an equal manner with temperature so at rest there will always be 50% of Vex present on the right half of the bridge.
    Using two resistors of equal value on the other half will always keep 50% of Vex present on the left side of the bridge. Now, when the strain gauge flexes, the right half of the bridge will subtly drift away from 50% of Vex... you need to compare the voltage on the left side, to the voltage on the right. This can be done with an opamp.
    (Think about prodding this with a multi-meter... if you leave the black probe on the negative line, and measure both side of the bridge, the readings will be roughly the same... you may not even see a difference... but if you put the black probe on one side of the bridge, and a red probe on the other the meter will read 0 when it's at rest and will show a very small voltage. The opamp will amplify this to a much higher value so that it can more easily be seen.)

    Additionally, if you only want to use one strain gauge, there are alternative methods to compensate for temperature. (usually the 'wire' that is run to the sensor is compensated for... not the actual sensor).
    *note that RL1-3 are not actual resistors, but represent the resistance in the wire used.
    Using 2-wires to run to the sensor will cause the bridge to become unbalanced if the wire's resistance changes. Using a 3-wire technique will compensate for that... or you could just use option 1 up above... and use two sensors on opposing sides of the item you wish to monitor.
  3. beedees


    Mar 30, 2015
    You're right! But I have to measure tension and compression, so if I put two strain gauge one opposite to the other I compensate the temperature but also the effect of tension and compression. Sorry, it's my fault because I didn't mentioned this particular in post #1.
    Do you know if there are bridge completion module or better half-bridge with two resistors already mounted?
  4. Gryd3


    Jun 25, 2014
    Yea, you are right in that the compression or tension will be more apparent because the top and bottom half of the strain gauge side of that bridge will both change at the same time. (One goes +, other goes -)
    I don't see this as a down-side, but perhaps I am missing something. The result of using two gauges like this will still allow you to measure both tension and compression in one of two ways... When the object is neutral, tune the bridge to be equal. Tension will provide a positive voltage, compression will provide a negative voltage (or reversed..)
    The other option will require some fiddling... but you can tune the bridge to make the ADC output 50%. As it approaches 0% you are in compression, as it approaches 100% you are in tension.

    But... To answer the other question, I am unsure of any particular modules. When I was diggin around with strain gauges, I was looking at opamps and instrumentation amplifiers...
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