Load Cell specifications

Discussion in 'The Projects Forum' started by davis103, Jul 17, 2008.

  1. davis103

    Thread Starter Member

    Jun 9, 2008
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  2. SgtWookie

    Expert

    Jul 17, 2007
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    For each volt applied across the load cell's Wheatstone bridge, with a full-scale deflection, you will get 20 millivolts, within +-5%. So, if you applied 10v across the bridge, you would get 200mV full scale deflection.

    When you're working with load cells like this, you really need to use instrumentation amplifiers. An INA128 would be a good choice. Attempting to build an instrumentation amplifier from individual op amps is quite a chore, and your CMRR goes in the toilet unless you're using 0.01% resistors, which are quite expensive.

    Please go to Intersil's website, and download Application Note 1298. (AN1298.0) It does a great job of explaining why and how to use instrumentation amplifiers.

    The INA128 was developed by Burr-Brown, now Texas Instruments. Go to TI's website and search for it. It's not very expensive, and very easy to use.
     
  3. davis103

    Thread Starter Member

    Jun 9, 2008
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    well the thing is that this load cell it already has a built in amplidied signal because its range is from 0.5 to 4.5 volts depending on the weight. I think they are just specifing the non amplified output. but i can not seem to make the connection between force and voltage
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    Ahh, I see. Well, with no force on the cell, and 1 volt applied to the load cell itself (sans amplifier) you would read a certain voltage drop across the cell's Wheatstone bridge. With maximum rated force applied, you would read 20mV more of a drop.

    Be careful to not exceed the maximum rating of the load cell, or it will be destroyed.
     
  5. davis103

    Thread Starter Member

    Jun 9, 2008
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    so you are saying that if i have this load cell connected at 5 volts, then the drop would be 100mv in the cell. How can i relate this to the force vs voltage. I tested it and connecting it at 5volts it gives me an output of 0.5 volts already amplified with o force applied
     
  6. Wendy

    Moderator

    Mar 24, 2008
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    Actually load cells are generally differential devices, think Wheatstone Bridge as the Sgt suggested. Perfect for Op Amp use, if the op amp is a good one. Voltage drop by itself is a bad way to use them, the amount of change is extremely small, and needs a really stable form of amplification (again, Wheatstone Bridge).


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    OK, reread the postings. We used to make them at my old company for custom robotics. Unless you buy a matching meter generally you need to calibrate them. Known weights would do this just fine. Even with a meter calibration is key, these suckers are not solid state devices, and are generally hand made, so there will be minor variations.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    There is normally a calibration sheet supplied with the load cell.

    Since 0.5v is your no-load zero, 4.5v should be your full deflection reading, or a delta of 4v.

    So, if you're getting a 100lb cell, every 0.04V above your zero reference (0.5v) is 1 lb, +/-5%.

    Keeping track of that zero reference voltage is the trick. It's difficult to get the zero repeatable.

    I suggest using twice the rating of your expected load; ie: if you might see up to 25lbs, go for the 50lb unit. Keep in mind that a shock load exceeding a certain percentage of your load cell's maximum rating may destroy it. Consult the manufacturer for details.
     
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