Labjack U3-LV with button load cell - not getting full load range
Posted 19 April 2012 - 04:48 AM
I am using Labjack U3-LV with a FC22 Compression Load Cell
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.
Posted 19 April 2012 - 08:18 AM
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
So how do I get the load cell to give me full range in the 0 - 2.4V?
Posted 20 April 2012 - 08:08 AM
Posted 13 June 2012 - 01:36 AM
Posted 13 June 2012 - 09:43 AM
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 220.127.116.11 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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