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Weighing Scales — Parallax Forums

Weighing Scales

Joe WalshJoe Walsh Posts: 2
edited 2014-07-07 02:49 in BASIC Stamp
Has anyone done a project to build an electronic weighing scales ?

I want to be able to weigh up to 600kg approx.

Is this possible ?

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Comments

  • remmi870remmi870 Posts: 79
    edited 2009-02-03 21:31
    the heart of the scale is a load cell, normaly this is a piece of metal with sensors on it that is cut in a particular way so when the block is deformed a certain way it will stress the sensors to give you a difference. if you can find a load cell that will match your weight requirements and also get the information on the wireing, normaly you could just use the RCTIME to determine the resistance of the load cell and find the weight.
  • Joe WalshJoe Walsh Posts: 2
    edited 2009-02-03 21:52
    I will do that.
    I should have perhaps put some things in perstective.
    I have an existing setup that connects three load cells to a common box.
    The electeonics in the box look complicated.
    You can flick a switch to read the data from each load cell.
    Having select a load cell you can also push a button to zeroise the 'no load' on the cell.
    The equipment is difficult to operate (sensitive to moisture etc.)

    I will look at the load cell(s) and try to determine exactly what they are.

    My thinking is that I might be able to replace electronic circuitry with a simpler configuration.

    What further information can I supply ?

    Am I on the right track ?
  • Tex4uTex4u Posts: 30
    edited 2009-02-03 21:54
    I've threatened to steal the better half's kitchen scale. Gut it and I'm off to the races. Of course, I have to sleep at some point so.... The scale she has goes up to 5Kg and the local kitchen-gadget store has them for about $30 or Amazon for about $17 with 2Kg range.
    Amazon.com/Digital Scale

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    Of all the things I have lost...

    I miss my mind the most.


    Post Edited (Tex4u) : 2/3/2009 10:07:01 PM GMT
  • Craig EidCraig Eid Posts: 106
    edited 2009-02-03 23:08
    Joe,

    The typical load cell system uses a Wheatstone bridge - see attached picture - to calculate the resistance of the strain gauge. The first three resistors (R1-R3) are high precision resistors, usually 1% or less with a low temperature coefficient.
    The fourth resistor (R4) represents the·strain gauge.

    You can measure the voltage with an analog to digital converter and calculate the resistance of the strain gauge, and the resultant weight, with some simple math. Parallax has an application note using a serial interface to the Linear Technology 12 bit, dual channel·LT1298.

    I can post more information if desired.

    Craig
    Bridge.JPG
    1280 x 768 - 27K
  • MSDTechMSDTech Posts: 342
    edited 2009-02-04 12:21
    If you already have the load cells, you should be able to easily duplicate the system using a basic stamp. The hard part is finding the characteristics of the cells you already have.

    I used a similar kitchen·scale as the link posted by Tex4u for the rain gage on my weather station. Depending on the characteristics of the particular load cell, you may find the output is too low for rctime or the LT1289 to give an accurate or precise reading. I ended up using an AD7705 to read the output of the cell. It has a built in amplifier as well as the ADC. This chip has some built in calibration capabilities - in my programming, I do an automatic calibration and·zero adjustment during startup.

    Do you have any information on the three cells you already have?
    Load Cell for Rain Gage.JPG
    2636 x 1250 - 727K
  • Tracy AllenTracy Allen Posts: 6,663
    edited 2009-02-04 18:53
    Joe,

    Search the forum for messages from Mohamed Refky, who has posted quite a bit on his weigh scale design. For example,
    http://forums.parallax.com/showthread.php?p=540915
    or in the egroups archive:
    forums.parallax.com/forums/default.aspx?f=19

    Generally you need a 16 bit or better ADC for load cells.

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    Tracy Allen
    www.emesystems.com
  • dredre Posts: 106
    edited 2009-02-06 20:21
    Perhaps this link might help.

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

    cheers, David
  • Mohamed RefkyMohamed Refky Posts: 47
    edited 2009-02-08 11:49
    Also see:
    http://forums.parallax.com/showthread.php?p=616474
    http://forums.parallax.com/showthread.php?p=718875

    Three load cells are connected in parallel directly to the AD7730
  • WatchingEyeWatchingEye Posts: 1
    edited 2014-07-07 02:49
    To build an electronic weighing scales, It is very important to collect some useful tools and sources from where you easily can get helping stuff. Load cells can be used a series scale by connecting them all. Pallet Truck Scale is also very useful. You must have the sources to get all assisting metrical and scales.
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