Making an amplifier for 5 strain gauges

Discussion in 'The Projects Forum' started by Nevalite, Feb 20, 2012.

  1. Nevalite

    Thread Starter Member

    Feb 20, 2012
    45
    0
    Hello everyone,

    I want to make an amplifier for 5 full bridge strain gauges I have, the model of the strain gauge is the Omega SGT-2/350-FB11. The strain gauges are going onto the suspension members in a formula SAE car and will ultimately be attached to the data acquisition unit of the car (0-5V) needed.

    I did some reading and I understand the strain gauges output mV that need to be amplified with an operational amplifier to the 5V region.

    I need a wiring diagram for 1 strain gauge (wheatstone bridge) going to 1 amplifier (not sure which model to use, there are so many...) and I need to know if it would be possible to have 5 wheatstone bridges going into 1 bigger amplifier? Or maybe just 5 bridges into 5 small amps and I could put all of that in a box or something.

    I know this is a little vague but if someone could point me in the right direction here that would be awesome.

    Thanks a lot
     
  2. Felo

    Member

    Feb 20, 2012
    91
    13
    Hi,

    Look for the INA131 datasheet, that will give you some way to follow, I have used it in the past, is a bit expensive but is totally worth it.

    Good Luck
     
  3. Nevalite

    Thread Starter Member

    Feb 20, 2012
    45
    0
    Thanks for the reply.

    I looked up the instrumentation amplifier you suggested and it looks like a good choice. From what I can tell there aren't any op amps or instrumentation amps that take 5 inputs so I'm thinking I will run 5x INA131 all on one board and I will put that into a box and make my own 5 in 5 out setup.

    I just want to make sure I understand how these work, here are the wiring diagrams:

    [​IMG]

    Let me know if I have this straight:
    In this diagram 2 and 3 come from the strain gauge, 4 and 7 come from the power supply (battery), 5 is a ground and 6 is the output. 1 and 8 being a place to attach a resistor if I want to change the gain on the amp, 10k resistor here would make it 1000x the input voltage (set to 100x without a resistor.)

    And this is how it looks in a simplified form:
    [​IMG]

    My questions are as follow:
    1. Did I understand the diagram correctly?
    2. If my source is 12v from the car battery, will I need a resistor on the top and bottom of the wheatstone bridge? Will I need some for the amp as well (I think it needs 6.5V?)
    3. Any good way hook 5 of these up on one pcb?

    Thanks a lot.
     
  4. mcasale

    Member

    Jul 18, 2011
    210
    12
    Really, the first thing you need to do is answer about how "good" or "accurate" these signals need to be. That will determine how many bits you need in the ADC (very important), and how good the amplifier should be, and how good the voltage driving the bridge needs to be.

    The offset voltage and voltage/current noise of the amplifier will limit the performance at the low end. That should let you figure out what gain you need for the overall system.
     
  5. Felo

    Member

    Feb 20, 2012
    91
    13
    Hi, mcasale is absolutely right, if you want accuracy, you have make that you have a nice stable power source, look at the pic, this was not designed by me but I have used succesfully, it is for contioning of a load cell wich outout goes to a 12 bit adc, you could make 5 of this and analog multiplex as needed.

    Use it at your own risk!!
     
  6. Nevalite

    Thread Starter Member

    Feb 20, 2012
    45
    0
    Thank you for the replies.

    That circuit you posted Felo seems a lot more complicated than I want this to be. Could I not just attach the 5 amplifiers in parallel so they all get the same voltage then add a capacitor at the top to maintain constant voltage? As for how good I need the signal to be I'm not really sure what to answer, it needs to be good enough to be used by our data acquisition unit...
     
  7. Felo

    Member

    Feb 20, 2012
    91
    13
    Sure, you can use the same power, for the 5 of them, I don't see any problem with that
     
  8. Nevalite

    Thread Starter Member

    Feb 20, 2012
    45
    0
    I was thinking something simple like this:

    [​IMG]


    Do I need a capacitor to hold the voltage constant? Also do I need any resistors?
     
  9. crutschow

    Expert

    Mar 14, 2008
    13,055
    3,245
    Since you only have a single supply voltage it may be easier to use an instrumentation amp designed for a single supply such as one of these (click to check the single supply box in the second column and the 36V box in the fifth column).

    You may need a resistor across the gain adjust pins to set the gain of the amp.

    Edit: You should always place a large electrolytic cap (100μF or more) across the power line and 0.1uF ceramic caps across each amplifier power and common pins. In an auto environment you might also want a small resistor in series with the power from the battery (say 10 ohms) before the electrolytic cap for additional noise reduction.
     
    Last edited: Feb 22, 2012
  10. Nevalite

    Thread Starter Member

    Feb 20, 2012
    45
    0
    Those appear to be more or less the same as the INA131 except the INA131 has the gain adjustment resistor built in setting the gain to 100 by default.

    The 131 says it is dual supply while the 118 says single supply but they both have the same input/output configuration on their data sheets so I'm a bit confused by that.
     
  11. mcasale

    Member

    Jul 18, 2011
    210
    12
    The single-supply should not be a problem. I did not look up your strain gauge specs, but I am guessing the "zero" output voltage on each leg is +5V? What is the maximum voltage you expect on any leg of the bridge? If it's +1v (my guess), then the maximum gain of the amplifier can only be 5 (to give a maximum of +5V to your ADC.

    Remember that the amplifier will also amplify its own input offset voltage, which is spec'ed at 50uV. If you use a gain of 100, the ADC may see a zero offset of 5mV, and it can be positive or negative.
     
  12. Nevalite

    Thread Starter Member

    Feb 20, 2012
    45
    0
    Would this work:

    [​IMG]

    Would that work with a dual supply INA131? What would the difference be?

    Thanks
     
  13. bertus

    Administrator

    Apr 5, 2008
    15,649
    2,348
    Hello,

    Where will the gound connections of the capacitors and the 7805 go?

    Here is a PDF with a lot of single supply Op-Amp schematics.

    Bertus
     
  14. Nevalite

    Thread Starter Member

    Feb 20, 2012
    45
    0
    Grounded to the frame of the car I guess, the battery is also grounded there.
     
  15. Nevalite

    Thread Starter Member

    Feb 20, 2012
    45
    0
    Where should they go?
     
Loading...