Strain Measurement Error using PXI-4330

[Lynniz]

Hi All,

I have set up a lab experiment to compare the measured strain using PXI-4330, measured strain on another different DAQ system, and the theoretical strain.  I have set up a cantilever beam system, and I put different weights (2g to 90 g) on the free end of the beam to deflect the beam.  I used the DAQ Express in LabView to build a simple program to measure strain.  I balanced the strain gauge and shunt the circuit before I start the experiment.  I am seeing the measured values (both PXI 4330 and the other DAQ) were reading ~20 ustr higher than the theoretical strain @ 90 g of weight.  The error of the slope between the theoretical and the measured is ~ 20%.  I also have tried to use a different method to deflect the beam, and the error was very close.  Please let me know what I am doing wrong here. 

Thanks in advance,

Lynniz

[TimmTheEnchanter]

Hi Lynniz,

Could you clarify how your ~20 ustr offset correlates to an input voltage for your setup?  It should be specific to the change in resistance you expect and the excitation voltage you are providing.  This would be able to tell us if we're within the specifications of the PXI-4330.

[Lynniz]

Hi,

Both DAQ systems are supplying 10 V of excitation voltage to the strain gauges.  The gauge factor for the strain gauges are 2.11.  Are these what you are asking for?

Thank you,

Lynniz

[GarPar]

Hi Lynniz-

     What TimmTheEnchanter is talking about is what that ustr reading corresponds to in Volts.  If you look at page 3 of the PXIe-4330 specifications, there are some accuracy specs that you might look at.  20% would probably be out of spec, but you will need to calculate it based upon how for you are out of spec in Volts.

     So two separate DAQ systems are reading the same value?  Could it be that your theoretical is either incorrect or makes some assumptions that are unaccounted for in the real world?  What about the strain gauge - is it connected correctly?

     I hope this helps.  Have a great day!