Posted 03 August 2008 - 01:24 PM
Posted 04 August 2008 - 09:22 AM
Here is the wikipedia page about a wheatstone bridge:
Posted 29 August 2008 - 03:42 PM
Posted 15 September 2009 - 12:25 PM
Posted 15 September 2009 - 12:37 PM
I would use the test panel in LJControlPanel for initial troubleshooting, rather than DAQFactory. It is a simpler program and eliminates more unknowns.
First identifying if there is a problem with the load cell.
Use your DMM to measure the voltage from FIO0 to FIO1 at the same time as you monitor it in LJCP. If you change the weight on the load cell by 3000 pounds, you should see the voltage output change by 15 mV.
You need an instrumentation amplifier such as the LJTick-InAmp. See above. Your full expected signal swing is only 0-15 mV, and the resolution of a differential input on the U3 is about 1.2 mV, so you will only have about 12 steps across your entire range.
Secondly, understanding if the load cell is hooked up to the LabJack correctly and if it is how I can get better numbers i.e. decide what the offset should be and what the gain should be in order to convert signal mV to force in lbs.
Posted 15 September 2009 - 03:13 PM
Posted 15 September 2009 - 03:17 PM
Posted 15 September 2009 - 03:57 PM
What load do you have during this measurement? If you have 0 load, then all these measurements sound correct. If you have 3000 pounds, you should see that one line changes to about 2.5105 compared to GND and the other changes to about 2.4955 compared to GND, and thus the difference between them is about 0.015 volts.
I have a DMM and I am measuring the voltage across the FIO0 and FIO1 terminals. The voltage that I measure across them is 0 VDC. I have taken the time to make certain that I have good connections between the terminals and the wires of the load cell. Between GND and FIO0 I have a value of 2.503 VDC and between GND and FIO1 I also have a value of 2.503 VDC. This makes me believe that I do not have the LabJack hooked up correctly, the Load Cell is broken, or I do not have the LabJack set up properly.
You need an instrumentation amp. You can buy an in-amp chip (or 2-3 op-amps) and build your own, but I recommend the LJTick-InAmp from LabJack:
I did have another question though about the amplifier. Could I, using and OpAmp circuit amplify the return signal in order to get more than 12 steps. I think that you suggested a LJ sponsored amplifier, but I am not sure where I should look for that.
The other option is the U6, which has a very nice instrumentation amplifier built-in.
Posted 06 March 2013 - 01:48 PM
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Posted 19 March 2013 - 12:13 PM
Posted 19 March 2013 - 12:26 PM
Posted 19 March 2013 - 02:21 PM
Posted 19 March 2013 - 02:58 PM
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