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Labjack U3-LV with button load cell - not getting full load range


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5 replies to this topic

#1 rdcragg

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Posted 19 April 2012 - 04:48 AM

Hello,

I am using Labjack U3-LV with a FC22 Compression Load Cell

http://www.meas-spec...ct.aspx?id=2438

The load cell is a high level Amplified Output so has 3 wires.

I am using Flowstone to monitor the Analogue input, the offset at zero load is approx 0.5v

As I increase the load there is a point where the voltage stops increasing (approx 2.7v I think) and stays fixed, this happens on two load cells so it is not a faulty load cell.

As I am new to load cells and DAQ's, what do I need to do to ensure that I can get the full load range of the load cell?

Maybe we should have bought the 4 wire version, I don't know.

Thank you

MC

#2 LabJack Support

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Posted 19 April 2012 - 08:18 AM

The U3's analog input range is 0-2.4V (0-2.6 can usually be achieved). You can force the U3 into a special 3.6V range by setting the negative channel to 32. This will cost you some resolution however. You will be operating in the differential specs: http://labjack.com/s...uide/appendix-a

#3 rdcragg

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Posted 20 April 2012 - 02:13 AM

The U3's analog input range is 0-2.4V (0-2.6 can usually be achieved). You can force the U3 into a special 3.3V range by setting the negative channel to 32. This will cost you some resolution however. You will operating in the differential specs: http://labjack.com/s...uide/appendix-a



Thank you.

So how do I get the load cell to give me full range in the 0 - 2.4V?

rdcragg

#4 LabJack Support

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Posted 20 April 2012 - 08:08 AM

You have a couple options. You could reduce the gain of the load cell's amplifier. Or you could use a resistive divider on the output. The divider is probably the easiest method. If you are not familiar with circuits the wiki page does a good job of describing the divider: http://en.wikipedia....Voltage_divider

#5 rdcragg

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Posted 13 June 2012 - 01:36 AM

Hello again, A resistive divider worked a treat, thank you. Now we have two issues with the FC22 loadcell with resistive divider on the Labjack U3 low voltage. 1.) Noise - we are seeing noise on the signal. What should I do to minimise noise? Do I need signal conditioning? 2.) Load changes with respect to temperature - I am seeing a change in the load when the temperature changes. I think this is a change in resistance due to temperature, particulary the resistors in the resistive divider on the output. What do I need to do with the circuit to compensate for temperature changes? Regards rdcragg

#6 LabJack Support

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Posted 13 June 2012 - 09:43 AM

1. How much noise are you seeing, in terms of volts? You expect +/-600uV from the U3 itself on a low-voltage channel using the 0-2.4 range. You can confirm this with a quiet source in that range such as a 1.5V battery connecting to FIOx and GND.

http://labjack.com/s...-and-resolution

2. It could be that the load is actually changing with temperature for some reason, or the excitation voltage is changing, or the load cell is changing, or the scaling resistance is changing, or the U3 analog input is changing. I would do different tests to try to isolate if one in particular is the main culprit.

If you suspect the scaling resistors, just set the system at a constant load such that the load cell outputs about 2.0-2.3 volts and then connect the output directly to the U3 without the scaling resistors. You can then compare temperature effect with and without the resistors, and can also compare noise while you are at it. If you want better resistors, check out the Y1453 series available at Digikey. Note from Section 2.6.3.6 of the U3 User's Guide that you should keep your resistors to 10k or less (or 200k or less if you use LongSettling).

Excitation voltage is a common problem. Note that even with your amplified load cell, the data sheet says the output is ratiometric with the excitation, so you should be measuring the excitation every time you measure the load cell, and use the measured excitation in your formulas rather than some constant. Note that this means excitation could cause tempco problems and noise problems. What are you using for excitation?

Note that your load cell is specified to give 4.0V output if the excitation is 5.0V. Thus rated max output is 80% of Vexc. You are allowed to use an excitation as low as 3.3V. If you use 3.3V, the max output from your load cell would be 2.6V and thus you can almost read to max load using a direct connection to a 0-2.4V input. Or you could use an excitation of 4.4V, which would give you a max output of 3.52V, which you could measure directly using the special 0-3.6V range. You can use DAC0 to provide these excitation voltages. In addition to being variable, DAC0 is more stable and has less noise than VS, but you should still measure the actual value in real-time rather than using a constant in your formulas.


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