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Hot!Load cell and PIC

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nidal
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2008/08/01 05:16:40 (permalink)
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Load cell and PIC

Hi all
 
     It is the  first time I use load cell , I still have not decided which load cell I will use yet  , but it seems that they are  all similar , 350 ohm output resistor and 2mv/v sensitivity analog output 0-10v . I could  not understand some terms and how deos that effect my readings and results , could anyone explain please ?
 
  • I will use a voltage divider to convert from 0-10v to 0-5v then I use opamp with unity gain , is that ok, Do I need filters  ?
  • I might  use a 300kg load cell , so what is the role of 2mv/v ? knowing I might use 10bit A/D or 12bit A/D ?
  • If anyone has experience in load cells , could you  suggest other methods to read them , please ?

 
  Sorry for disturb , and thank you all
 
 
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    DarioG
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    RE: Load cell and PIC 2008/09/01 02:20:42 (permalink)
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    Hello Nidal, I've no experience with Load Cell, but I presume you'll be getting that 0-10V output which should fit quite well a PIC's input - divided by 2 or 3 depending on its VDD, possibly with 2 simple resistors (if the output impedance of the cell is low enough).

    Sure an OpAmp is a safer measure, but IMO it's not needed.

    GENOVA :D :D ! GODO
    #2
    ste5442
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    RE: Load cell and PIC 2008/09/01 05:04:26 (permalink)
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    Hi,
    Can you post datasheet links to the load-cells you are thinking of using?

    Pretending to be able to write firmware since last week!
    #3
    microlab
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    RE: Load cell and PIC 2008/09/01 05:27:36 (permalink)
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    Loadcell specification: 2mV/V/300kg means that if you use 10V excitation, you get (10V x 2mV/V) = 20mV full scale output for 300kg.
    If you have a loadcell 2mV/V/5kg & you use 5V excitation, the linear equation will be 10mV for 5kg.
     
    Hope this is clear..
    Jayant
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    isa.guru
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    RE: Load cell and PIC 2008/09/01 08:17:40 (permalink)
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    ORIGINAL: nidal

    Hi all

      It is the  first time I use load cell , I still have not decided which load cell I will use yet  , but it seems that they are  all similar , 350 ohm output resistor and 2mv/v sensitivity analog output 0-10v . I could  not understand some terms and how deos that effect my readings and results , could anyone explain please ?
    • I will use a voltage divider to convert from 0-10v to 0-5v then I use opamp with unity gain , is that ok, Do I need filters  ?
    • I might  use a 300kg load cell , so what is the role of 2mv/v ? knowing I might use 10bit A/D or 12bit A/D ?
    • If anyone has experience in load cells , could you  suggest other methods to read them , please ?





    Sorry for disturb , and thank you all




    A load cell output is differential and is not 0-10V. The output signal is in the range of tens of millivolts.
    What you need is:
    =a good power to the cell - stable, regulated, within the mfg recommended range, the higher power, the higher signal
    =differential amplifier
    =either trim pots to adjust for the initial offset and span (+/-2-5 %) or do it in software at the calibration stage
    =The amplifier has be able to work with the power supply for the cell or can work on a separate power supply. But it has to work well at the common mode voltage from the cell.
    =With a proper gain of the amplifier you can scale the voltage to the PIC ADC allowable voltage
    =Use good resistors - 25 ppm for an instance, 0.5 or 0.1 %

    ADDED: and os course use a very good op amp :)
    post edited by isa.guru - 2008/09/01 08:31:27

    7th dan instrumentation and controls
    #5
    seyyah
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    RE: Load cell and PIC 2008/09/04 02:52:32 (permalink)
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    Since you will measure very small signals, a very good design and good parts are needed for reliable and stable operation. I don't know what your requirements are but check mcp3551 and related documents from microchip. Also complete front-end solutions and higher resolution, lower noise and more function ADCs are available from other manufacturers. Generally a suitable delta sigma adc is used for this job. Check the products and choose one of them that fits your requirements.
    post edited by seyyah - 2008/09/04 02:54:07
    #6
    debrajdeb
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    RE: Load cell and PIC 2008/09/05 06:51:26 (permalink)
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    I had designed a load cell indicator using 18F4520. But the built in ADC was not good enough, as the signal level is less. So I had used AD7718 -- Thats a sigma delta ADC with PGA. It gives good readings (resolutions) -- 24bit.
    The communication is thro SPI. Hope, this will be suitable for you too.
     
    Debraj.
    #7
    loneranger
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    RE: Load cell and PIC 2008/09/28 21:48:08 (permalink)
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    It seems this load cell has an amplifier built in and will give a output of 1 V for 2mv ouptut of just the 'mechanical loadcell'. I dont think you will need any filter but noise suppressing cap ~0.01 uF will be good near the pin
    #8
    RobotWhisperer
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    Re: Load cell and PIC 2019/11/19 11:45:56 (permalink)
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    Hi, is it possible to detect weight with load cell?
    Without measuring or displaying any value like a weighing machine?
    I mean i just want to detect a load on footboard. PIC will generate an output signal when a human stepped onto the footboard.
     
    if(there is a human on board)
    output_high(pin);
     
    how can i write this code in PIC C Compiler?
    Thanks...
    post edited by RobotWhisperer - 2019/11/20 02:58:26

    the man who is gonna conquer the world with his robot army.
    #9
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