Need Help With Load Cell Selections!!

Discussion in 'The Projects Forum' started by TimThang, Nov 19, 2010.

  1. TimThang

    Thread Starter New Member

    Nov 19, 2010
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    Hey Everybody!

    I am currently doing a project whereby I have to detect a weight loss of 0.1g. However, my dead weight for my load cell is in the range of 3kg. So I'm looking for a load cell with a rated output of 3-5kg, but is yet still capable of detecting a 0.1g change in weight. Does anybody know any load cells that is capable of doing it? Or any other method I could adopt to solve this issue?

    Thanks
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    This will easily give a reasonable response - http://www.omega.com/ppt/pptsc.asp?ref=LCL&Nav=pref07

    Of course, the resolution of .1 gram is ultimately up to the electronics. You can cruise the Omega site to get some information about the use of instrumentation amplifiers. The resolution of .1 gm in 3 Kg is just barely possible with a 16 bit A to D converter.

    What is your electronic experience?

    Note: You can buy some strain gages (Omega's term) and make your own load cell. The challenge is to manage the fixture so the strain is large enough to show the change, but not so much as to destroy the gages.
     
    Last edited: Nov 19, 2010
  3. TimThang

    Thread Starter New Member

    Nov 19, 2010
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    0
    Hey,

    Thanks so much for the reply. I am currently a student studying mechanical engineering so I have little experience in dealing with electronics. To be safe, the load cell needs to have a output range of 5kg. Is a resolution of 0.1g not possible with such an output range? Or will i have to use an output range of 3kg?

    Thanks once again for your help
     
  4. TimThang

    Thread Starter New Member

    Nov 19, 2010
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  5. GetDeviceInfo

    Senior Member

    Jun 7, 2009
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    The strain gauge and it's associated circuit is analog in nature. Using quality op amps, you can magnify the resolution to very large proportions. In other words, you could realize changes in the .0001 g.

    However, there is always a trade off. In magnifying resolution, you also magnify errors. You now must treat your 'wiegh scale' as a system, and use the electrical circuit to best absorb the physical anomalies that come into play. Even then, errors will negate resolution at some degree. So now the question is thrown back to you, are you capable of building a scale that resolve .1g?
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
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    Since the load cell will be able to accept the load applied, it will register a .1 g change. Your problem is to make the electronics that will make the change resolvable. Did you read any material at the Omega site?

    The change is one part in 50,000. an A to D converter that can resolve that change has to be greater than 16 bits (32678 states). Here is a link to some 18 bit converters (131,072 states)- http://www.analog.com/dynamic/parametric/scResultsDisplay.asp?SearchType=PSS&ProductLine=ADC&la=en

    You might spend time researching noise control in the amplifier and techniques that may let you get all the performance possible out of the converter.
     
  7. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    Since you're in MechE, is there any way you can mechanically "tare" out the initial 3-5Kg in some sort of balance system, and just have a sensitive load cell indicates 0.1g changes? Maybe you can describe yor overall project...sometimes that lends itself to other approaches.

    Ken
     
  8. GetDeviceInfo

    Senior Member

    Jun 7, 2009
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    you could go to a high bit count converter, but you could also 'zoom' in on a scale range utilizing op amps.

    Regardless, the physical properties of the scale begin to introduce variables into the measurement, be it temp drift, mechanical resonance,ambient vibration,etc, etc.

    You'll find in such a project, that the resolution is in the mechanics. The electronics only report the current state of the sensor.
     
  9. John P

    AAC Fanatic!

    Oct 14, 2008
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    My experience of working with sensors is that if you're using simple techniques, a resolution of 1 part in a thousand is about the best you can get before your signal gets lost in noise. I'm sure it's possible to do better, but my guess is that you'll need good advice to do it as a beginner. And you're likely to need to use the best noise-eliminating techniques throughout your system.

    1 part in 1000 with a 5Kg range would only give you 5g resolution. That's not very good if what you really want is 0.1g.
     
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