strain gauge interface

Discussion in 'The Projects Forum' started by growerdick, Oct 8, 2012.

  1. growerdick

    Thread Starter New Member

    Oct 8, 2012
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    I want to interface strain gauges from digital bathroom scales to A to D inputs in a micro.

    The strain gauges have three wires which represent one side of a wheatstone bridge, with a center common wire, a wire at the other end of the strain gauge that provides temperature compensation, and the other from the end of the strain gauge that reflects the amount of strain. The at rest readings are around 700 ohms.

    I have experience at programming micros, but not a lot in electronics. I need advice on the best op amp and a suggested op amp circuit, I think I need to incorporate a precision trimmer as part of the wheatstone bridge, and maybe in the op amp feedback, and would like to use a four op amp DIP chip if possible. The TI LMC6484 looks good but I say that without much actual knowledge, it may be expensive overkill.

    I have 5 volts on board.

    Thanks in advance for a schematic that I can wire up and test.
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    744
    Use an Instrument Amplifier for the detection , here is a n example http://www.national.com/ms/LB/LB-1.pdf

    using your IC, or if you can get the same output from one of the terminals of the Wheatstone, then just use a single op amp buffer.
     
    Last edited: Oct 8, 2012
  3. growerdick

    Thread Starter New Member

    Oct 8, 2012
    13
    1
    The schematic shown is not quite what I am looking for. I was hoping to use just one op amp section and was looking for advice on the resistor feedback values and the value range of the precision trimmers.

    I am not an experienced op amp person. I need some advice on whether a simple approach will do the job, and what it might look like with just one feed back resister which would include or just be a precision trimmer and what the values might be.

    I am playing with another strain gauge that has both sides at 1000 ohms, but the one with the 700 ohm outputs is the most available.

    What should the other side resistors of the wheatstone bridge be?

    Thanks.
     
  4. MrChips

    Moderator

    Oct 2, 2009
    12,416
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    You need to provide some more details:

    1) What is the part number of your sensor? Can you provide a data sheet or schematic?
    2) What is your intended application?
    3) What are you going to do with the output of the amplifier?
     
  5. growerdick

    Thread Starter New Member

    Oct 8, 2012
    13
    1
    The strain gauge is one of four out of the corners of a digital bathroom scale. It is a standard setup for a strain gauge where two identical sensors are placed on different parts of the steel piece that feels the strain. One sensor senses changes due to temperature and the other the actual very very tiny stretching of the steel piece.

    I want to determine the weight of a process to control it, and this seems like a good way to do it. A special bench will have the strain gauges at each corner. The micro I am using has several A to D and I plan to use one for each corner.

    I hope I can use the amplified output of a wheatstone bridge, of which the two resistors making up the strain gauge are one half, in an A to D conversion that will equate to weight. Perfect accuracy is not important, only the change as the process runs.

    As I have tried to find out how digital bathroom scales handle the output, I have seen some exotic methods described, but hope I can use the amplified output directly into an A to D and get what I need.

    I have never done an A to D amplification. I have spent a lifetime programming micros, but always for special purpose digital control and interfaces. My electronic background is very basic and limited.

    What I am looking for I described in the original post, a method to amplify the output into the mid 5v range with maybe two precision screw pots to trim the output to a standard level.

    I have devised a press that lets me see how much weight the strain gauge should be feeling so I can calibrate the data.
     
  6. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    744
    Try this design, assuming output 1 is positive compared to output 2, R1 to R4 is the Wheatstone sensor,

    the combined output will be the same as reading between output1 and output2, but referenced to ground.
    Have you got a picture of the sensor?
     
  7. growerdick

    Thread Starter New Member

    Oct 8, 2012
    13
    1
    Thanks very much.

    It will be a few days before I can test this since I have to wait for a Digikey delivery.

    I hope the attached image is adequate. The steel piece is square and very sturdy looking.
     
  8. growerdick

    Thread Starter New Member

    Oct 8, 2012
    13
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    What should be the values for R3 and R4.

    R1 and R2 are the elements in the strain gauge and are both 748 ohms unstrained.
     
  9. MrChips

    Moderator

    Oct 2, 2009
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    Ideally, R3 and R4 would be identical to R1 and R2 except experiencing no load.
     
  10. growerdick

    Thread Starter New Member

    Oct 8, 2012
    13
    1
    I am getting nice amplification, but would like to kick it up one more decimal, it is still under a volt and under 300 ma.

    A question occurred to me on how much load the wheatstone bridge is creating. It looks like it directly connects +5 and ground with very little resistance in between. Since the sensing of the load is only planned for once each minute, should I put a small relay to shut off the power when they are not being read?

    A lot of this system will use solar panel battery power and I want to use as little as possible. Cloudy weeks can causes the generator to start and burn expensive gasoline.

    Thanks again. It was great to see usable voltage as I pressed on the strain gage.
     
  11. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    ok if your using the circuit i gave, then increase the 100k resistor from the output to the inverting pin, to 1Meg it should increase your output.
     
  12. growerdick

    Thread Starter New Member

    Oct 8, 2012
    13
    1
    Thanks Dodgydave, I will.

    Do you have any thoughts on the load the wheatstone bridge is representing? Should I put a small relay and only turn it on when a measurement is taken?

    There will be four of these bridges and that might add up to a significant load. Am I correct in my assumption that they represent a voltage divider with parallel resistors as far as a resistive load is concerned?

    Thanks again.
     
  13. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Yes its just a Dual voltage divider, with one sense element per bridge, to increase the output use the modified cct below.

    Measure the current with an ammeter, or DVM to get the total load from the psu, then put a weight on say 10kgs /22Lbs and see what difference it makes.
     
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    Last edited: Oct 14, 2012
  14. growerdick

    Thread Starter New Member

    Oct 8, 2012
    13
    1
    Would it be wise to put maybe a .1uf ceramic parallel to the 1M resistor?
     
  15. growerdick

    Thread Starter New Member

    Oct 8, 2012
    13
    1
    I don't understand your latest sketch.

    There will be four strain gages using four op amp elements into four A to D converters. I was thinking of powering them all up via a small relay when measurements are taken to reduce load.
     
  16. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,969
    744

    its a mod to make the gain variable by replacing the 100K resistor with a 2.2Meg potentiometer and yes put a 100nF capacitor across the potentiometer, gain should give you upto x11 from the strain-gage.

    how you power them is up to you.
     
    Last edited: Oct 14, 2012
  17. growerdick

    Thread Starter New Member

    Oct 8, 2012
    13
    1
    Thanks. Hope you are having a pleasant evening in Lancashire.
     
    Dodgydave likes this.
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