Accelerometer data with high skewness for USB cDAQ-9178 chassis with multiple NI-9234 modules

[EvenLund]

We are using different types of triaxis accelerometers from PCB connected to a National Instruments USB cDAQ-9178 chassis with multiple NI-9234 IEPE modules. We see that sometimes the acceleration data is not centered around zero, but have a high skewness (beard-like appearance). The center figure below is from a PCB 356A15 triaxis accelerometer for the Y axis, and you can see that the positive amplitude is about 150 m/s^2 whereas the negative amplitude is peaking to -400 m/s^2. We are calibrating the sensor with a handheld calibrator and none of the PCB accelerometers seem to have any error or damage. The mentioned accelerometer is not the only sensor that we can see this behaviour at times.

• Does anyone have any idea of why we are experiencing this? We try to ground the cDAQ chassis all the time, but we are discussing whether this could be due to voltage potential differences/grounding problems. Without hardware knowledge it is difficult to troubleshoot.
• Is there any way to mitigate this problem or fix the current data? Any tips will be welcomed as we really need to use previously collected data.

Regards,

Even Lund

[Henrik_Volkers]

First guess: IEPE sensors conditioners usually have a high pass filter.  So the center of the peak to peaks don't have to mach zero 😉

How does you wfrm look like ?  What is the mean value?

Do you reach any limitations ... mechanically, or in the range?

[EvenLund]

Sorry for not elaborating: The curves shown are the waveforms. The 9234 is set for AC so the signal should center around zero. You see this happening when the acquisition starts running.

There are no limitations mechanically and if you integrate this signal into for example displacement it is drifting to infinity, which is not the case. This is a pipeline with fluid flow being measured, by the way.

Best regards,

Even

[Henrik_Volkers]

What type of hand held 'calibrator' do you use?  

I assume the exitation should be a sine at fixed frequency (~160Hz?) and the RMS value is assumed to be constant..

What samplerate is used?

[Henrik_Volkers]

Can you post one good and one bad data example (raw data?)

[EvenLund]

We currently have a SV 110 with two frequencies (80/160 Hz) and variable amplitude (1-10 m/s^2) and previously used a RION VE-10 and a PCB 394C06. This should not matter, though, as data seems normal with sine wave distributed around zero. This is also independent of sample rate.

[EvenLund]

This is already posted with the data on the right (Z-axis) showing expected response and data in the middle (Y-axis) showing the bad response.

[Henrik_Volkers]

You posted some pictures... I asked for raw data (tdms,lvm,csv, or .... 😉

[Henrik_Volkers]

How do you read 200m/s² peaks with an exitation <= 10m/s²?

(OK, scaling seems wrong....  but there isn't any scaling in your pictures anyway 😄 )

I'm really interested in the cause of your readings, since I work for more than 10a in the field of accelerometer calibration.

[EvenLund]

The output that you see is from vibration measurement on a pipe with fluid flowing through. I just mentioned the calibrator to tell that the signal seems fine with that, so we do not think there are anything wrong with the accelerometer.