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Thread: Need Help with Strain gauge use.

  1. #1

    Post Need Help with Strain gauge use.

    I am fairly new to all of this. What i would like to do is use a strain gage with a basic stamp logic controller. The strain gage will provide a variable resistance depending on the deflection of the object it is attached to. My question is how do I use this variable resistance to control other actions when it is in certain ranges, or is this not possible?

  2. #2

    Default Re: Need Help with Strain gauge use.

    Welcome aboard.

    The chief thing about strain gauges is that they change by only a small amount as they are put under strain.

    In practical terms, that means you'll probably be using a bridge connection of several gauges plus an amplifier to get the signal boosted to a level you can use with your Stamp. The amplified signal will feed into an A/D converter the Stamp can read.

    If you go to the Omega Engineering website, you will find several articles on using strain measurements. http://www.omega.ca/techref/default_eng.asp

    Cheers,
    Tom Sisk
    Sisk Consulting Inc.
    www.siskconsult.com

  3. #3

    Default Re: Need Help with Strain gauge use.

    Hi,

    One of the secrets to successfully using strain gages is to mount them to get the
    maximum strain, as their output is very low. If you read the specs on a strain gage based
    load cell for instance you will see that the full scale output is on the order of 3mV/V. And
    if the recommended [stable] bridge supply voltage is, say, 10 VDC then the output at full
    scale load is only 30mV! Thus the need for amplification; or a high resolution ADC. [I
    like the use of an instrumentation amplifier like the INA125.]

    In order to use the gages to sense a quantity other than just strain, for example, load, they
    must be mounted on an element which is under great strain due to the applied load. An
    excellent example would be to cement gages to the upper and lower surface of a diving
    board [a cantilever beam] at the diving board's supported end - the point of maximum
    bending moment, hense maximum strain.

    Historically gages have been used to measure the strain, load, deflection, pressure, etc. of
    a myriad of structures or structural elements. Digital bathroom scales are one of the most
    common examples. Take one apart and follow the tiny color-coded wires back from the
    signal conditioning/display module to an encapsulated mound on a piece of metal.
    Generally these wires have the colors of red [+ bridge power], black [(-) power] and
    either white or green [the strain gage bridge output]. Under the encapsulation you would
    find probably at least two gages [the other two resistors completing the wheatstone
    bridge are probably on the conditioning module].

    If you would like further discussion please let us know.

    cheers, David

  4. #4

    Default Re: Need Help with Strain gauge use.

    I already worked with strain gage in a electronic scale project.
    If you want I can send you a schematic of the circuit that was used in that project.
    Basically it has some analog passive filters, a instrumentation amplifier, a active analog filter, a diferential A/D and a simetrical power suply.
    All this components have to be inside a metallic shield.

    Quote Originally Posted by jst97610 View Post
    I am fairly new to all of this. What i would like to do is use a strain gage with a basic stamp logic controller. The strain gage will provide a variable resistance depending on the deflection of the object it is attached to. My question is how do I use this variable resistance to control other actions when it is in certain ranges, or is this not possible?

  5. #5

    Default Re: Need Help with Strain gauge use.

    Yes I would love for you to send me this schematic. I am technically trained in the mechanical arena so any help I can get with this would be greatly appreciated.

  6. #6

    Default Re: Need Help with Strain gauge use.

    David,
    Yes, I understand the operation of the strain gage, and the importance of a wheat stone bridge. What I need help with is how does an instrumentation amplifier work, and how do I use one. Also after I have an amplified signal how do I get the stamp to interpret this not as high or low but as a discrete value. As I have said lower in the Forum I am a Mechanical Engineering student, so my understanding of Micro controlers is very limited, I have had a few basic classes that involved the use of Micro contorlers and PLC's.
    I really appreciate the help
    Justin

  7. #7

    Default Re: Need Help with Strain gauge use.

    Justin,

    Here are several links which may offer answers to your questions:

    1] This shows calculations for determining the gain needed for a strain gage bridge:

    http://www.nakka-rocketry.net/strainlc.html

    2] This link is the datasheet for the instrumentation amplifier I use. There is a figure
    which shows the use of a TIP transistor when drawing power from the voltage reference.
    I would suggest including this for any bridge using less than 1,000 ohm gages.

    http://www.ti.com/lit/ds/symlink/ina125.pdf


    3] Here is an excellent resource for using an A-to-D converter; including the Stamp code:

    http://emesystems.com/OL2tlc2543.htm

    Let us know if this is the type of information you need.

    cheers, David

  8. #8

    Default Re: Need Help with Strain gauge use.

    Ok, IŽll prepare a text explaining the circuit as well as how to make it work.


    Quote Originally Posted by jst97610 View Post
    Yes I would love for you to send me this schematic. I am technically trained in the mechanical arena so any help I can get with this would be greatly appreciated.

  9. #9

    Default Re: Need Help with Strain gauge use.

    IŽm not having much time to dedicate to this subject.
    There is a long holiday next week here in Brazil and IŽll try to prepare the text explaining how the circuit works (and the circuit of course)

  10. #10

    Default Re: Need Help with Strain gauge use.

    Hi Fransisco,

    You mentioned an electronic scale project. Does this project use a strain gauge to measure weight?

    If so, then I'd be interested in seeing the circuit and the accompanying text also.

    Thank you,

    rmcb

  11. #11

    Default Re: Need Help with Strain gauge use.

    Sorry for delay.
    This is the circuit that was used in a electronic scale project.
    Electronic scales use load cells which are bases on 4 straing gauge arranged in a configuration known as wheatstone bridge.
    With such arrangement there is a strain gauge that is streched and opposite one is compressed. This way the resultant signal is higher.
    I simplified the circuit because it was intended to use a load cell that was very far from the amplifier.The original circuit used a ADS1242IPW which is a very accurate A/D converter with differential inputs and is able to compensated the flutuations in the power line used to power the load cell.
    The first stage has passive filters and there is a instrumentation amplifier followed by a active filter. The capacitors used in this filter must be made of polyester. All other capacitors can be ceramic or electrolytic ones. All resistors are 1%.
    I think that this circuit can be adapted to your needs. The layout must be designed with careful and a metal shield must be used to prevent interference.

    This circuit will produce approximately 5V from a strain gauge. However the signal is added to Vref (2.0V).
    So this feature has to be taken into account in the sofware that will process the signal.



    Quote Originally Posted by FranciscoTavares View Post
    I already worked with strain gage in a electronic scale project.
    If you want I can send you a schematic of the circuit that was used in that project.
    Basically it has some analog passive filters, a instrumentation amplifier, a active analog filter, a diferential A/D and a simetrical power suply.
    All this components have to be inside a metallic shield.
    Attached Thumbnails Attached Thumbnails Click image for larger version

Name:	wheatstone bridge.png‎
Views:	51
Size:	3.8 KB
ID:	92432  
    Attached Files Attached Files
    Last edited by FranciscoTavares; 05-16-2012 at 01:26 PM.

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