Strain Gauge Amplifying problems

Discussion in 'The Projects Forum' started by Motaz Ayyad, Mar 10, 2014.

  1. Motaz Ayyad

    Thread Starter Member

    Mar 10, 2014
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    Hi,

    i am working on strain gauge training kit which will help the incoming students after me to apply the concepts of one of the force sensors

    but since i live in the 3rd world none of the instrumentation amplifiers are available such as INA122...etc

    the only Op/Amps available are :
    LM358
    LM386
    LM324
    uA741
    TO072

    and i know most of them are poor
    i used LM358 as first amplification which made a gain approx 100 then i used anther one but it ends up with complete unstable system

    and i am thinking of building instrumentation amplifier from one of the above Op/Amps

    but i have no idea about how? that's my problem or if anyone could have another solution

    thanks you and sorry for my bad English :/
     
  2. t06afre

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  3. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    One of the all-time great ***analog*** books.

    The amps you have available (324, 358, TL072) will work fine in a traditional 3-amp circuit if you keep their limitations in mind. Primarily, bandwidth and errors. What is the highest frequency you are conditioning?

    ak
     
  4. Motaz Ayyad

    Thread Starter Member

    Mar 10, 2014
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    Thanks for both of you

    the book was very helpful

    but still i didn't get it

    what is the mean of limitation?
    and the frequency condition?

    i tried to build the circuit in the attachment with LM358 instead of uA741
    and with another different thing which is i used +6 and 0 as power to the amps instead of +12 -12

    the output was constant to approx 4.42V

    thanks again
     
  5. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    We need the complete circuit, including the strain gauge stuff, to help. Can you post a schematic?

    4.4 V might be the output saturating against the positive rail, which means there probably is something about the input it doesn't like.

    ak
     
  6. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    4.4V does suggest output saturation. Try adding a small DC offset voltage to one or other input, to overcome any inherent input offset.
     
  7. Motaz Ayyad

    Thread Starter Member

    Mar 10, 2014
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    this is the full schematic that i build

    another question , i am also trying to get one of the INA but i don't know which one to buy -which could reach or not-
    any suggestion as plan b for an INA ???
     
  8. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    hi M,
    The lower two OPA's are incorrectly connected.
    Their +/-Vin are incorrect.

    E

    EDIT:

    Look at this edited drawing, also check the Offset pot for the bridge
     
    Last edited: Mar 17, 2014
  9. Motaz Ayyad

    Thread Starter Member

    Mar 10, 2014
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    thank you, i will test it today with these changes i hope it will work

    also could you tell me which one of the INA series is suitable for this application??
     
  10. ericgibbs

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    Jan 29, 2010
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    hi,
    I use the AD623, it will work on a single supply voltage.
    If you decide on a different type, ensure that its suitable for single supply operation, many IA require dual voltage supplies.
    E.
     
  11. Motaz Ayyad

    Thread Starter Member

    Mar 10, 2014
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    after the reconnecting as the edited circuit
    the bridge output voltage approx to 3mV
    the up OPA output voltage is 5.49V
    the down OPA output voltage 1.40V
    the last OPA output voltage .675 and not changing

    i guess maybe the resistors combinations is the problem?
     
  12. ericgibbs

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    hi,
    I will check out your resistor values, let you know what I find.
    E
     
  13. ericgibbs

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    Jan 29, 2010
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    hi,
    It looks OK to me.

    Run two simulations in LTS.
    First image is with the bridge in balance, second image bridge offset to give 11.1mV input signal.

    Gain set for approx 80.

    Note: I have grouped the 10R and 270R's into one 290R.

    Using LM358 OPA with a +5V supply will limit the maximum Vout to approx +3.5V..

    E
     
  14. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    hi,
    This sim shows a 0 thru 10mV input signal sweep and Vout, looks OK.

    Note the non linearity on Vout at near zero and at approx +4V due to using a LM358 at +5Vsupply.

    E
     
  15. Motaz Ayyad

    Thread Starter Member

    Mar 10, 2014
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    thank you very very much
    it works -with some sort of instability-
    a problem with DC offset in the bridge which i am working on it
    i am changing the gain with 1K Var. Res.

    and also i want to thank you for telling me about an application like LTSpice i will check it out, i see it very useful app.

    in addition, in a neighbor country(Egypt) i found these three instrumentation amplifiers
    which one you suggest?

    AD524 75L.E. approx 10.78$
    AD620AN 30L.E. approx 4.31$
    AD624 95L.E. approx 13.65$

    and i think the three are Dual voltage rail
     
  16. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    Hi,
    The AD620 is an older version, I would choose the AD624 , BUT it is specified to work with a dual supply voltage.

    I have attached the LTS asc file for the IA which I plotted earlier, it may help you get started.
    Join the free Yahoo LTSpice user group for models and examples.

    E

    EDIT:
    You will find its VERY difficult to adjust the bridge offset which such a high value pot.!!!
     
  17. Motaz Ayyad

    Thread Starter Member

    Mar 10, 2014
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    thank you for everything :)

    everything works fine , but still have the problem with the offset at no load
    for example i have 3V offset at no load with large gain that measure small things

    any idea to slove this problem? , i tried to make the wheatstone bridge more balance but with no luck
     
  18. t06afre

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    Do you use a full bridge or a half bridge setup? And also do you now use a commercial instrumentation amplifier or a home build one using opamps?
     
  19. Motaz Ayyad

    Thread Starter Member

    Mar 10, 2014
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    i am using full bridge, one strain gauge and 3 resisters
    and i am using home build instrumentation amplifiers (3 O/P) , first i was using lm358 then i replaced them with TL072
     
  20. t06afre

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    Have you tried inserting a variable resistor in the part of a bridge that are made by resistors. This way you can tune the bridge into balance much better. I would suggest using a multiturn pot since small values will give large swings in the output.
     
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