NI 9234 DC Offsets

[yewen]

Hello fellow professionals,

Background Info:

I am using the Sound and Vibration Assistant and a cDAQ with 5 NI 9234 cards for 5 triaxial DC accelerometers. Calibration sensitivity for each DC accelerometer have already been acquired and set to DC coupling.

Measurement Objective:

Determine vehicle baseline parameters in the low frequency spectrum (1 Hz to 30 Hz), hence the DC accelerometers for low frequency observations. Measurement accuracy and precision is of great importance.

Question:

Preliminary run/testing of 2 calibrated, triaxial (XYZ) DC accelerometers reveal offsets in graph. Are there methods within the Sound Vibration software to adjust the DC offsets without manually applying the offsets through post processing methods?

An applications engineer responded with the following solution, but does not seem logical when I specifically require DC coupling?

"Yes, you can remove DC offsets using the Sound and Vibration
Assistant.  When you are looking at the Configuration window where you set
the maximum/minimum inputs, sensitivity, etc., there is a tab entitled
'Device'.  Click on that tab and there is an option entited 'Coupling
Mode'.  Change the coupling from DC to AC.  Enabling AC coupling will
remove the DC signals from your signal, eliminating the need to perform
post-processing to remove DC offset values."

Attached is a figure of the 2 triaxial DC accelerometers with the DC offsets clearly shown.

   ai0 ~ X axis

   ai1 ~ Y axis

   ai2 ~ Z axis

The purple and green lines (Z axis) should be approx. 1g, but purple does not.

Thanks for your time, your technical support is greatly appreciated!

[GarPar]

Hi yewen-

     While AC coupling will remove the DC offset, it will also attenuate your signal in the low-frequency spectrum, so if measurement accuracy and precision are both of great importance, the 9234 (or any dynamic signal analyzer) may not be the best device for your application unless you are willing to remove the DC offset manually.  With AC coupling enabled, the -3dB cutoff is 0.5Hz and the -0.1dB point is 4.5Hz.

     So to answer your question, there is not a way using the Sound and Vibration Assistant to remove the DC offsets without manually applying the offsets through post processing methods, unless you want to enable AC coupling, which it sounds like you do not want to do.  I would recommend either sticking with the post-processing and maintain your DC coupling or look into a DAQ device, which has better absolute accuracy than a DSA card (which has better dynamic accuracy).

     I hope this helps.  Best of luck with your application!