Saturated IEPE Accelerometer in First Measurement

Updated Aug 13, 2021

Reported In

Hardware

  • CompactDAQ Chassis

Software

  • LabVIEW

Issue Details

I am acquiring vibration from an IEPE(Integrated Electronics Piezo-Electric) accelerometer with a NI-9234 module, and I am observing the maximum value of saturation for the first 5 seconds of data acquisition:

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If I run the same acquisition for a second time, without changing any parameter, I can get the right results (right now, the accelerometer is mounted on a static component, so the signal is only noise):
 

image.png

If I run my application in LabVIEW FPGA and RT, through TDMS files, I get a similar result:

image.png

This behavior is not related to the NI-9234 module or any data acquisition device, but it is a specification and expected behavior of the accelerometer.

Solution

Every accelerometer has an electrical specification called "Settling Time", determined by the company which can be found in the product's datasheet. During the period of "Settling Time" the accelerometer should not be used for data acquisition, or else it will provide incorrect values.
Including a “delay”, equal to the "Settling Time" of your accelerometer, in your code will make the accelerometer settle and ready to be used. As a result, the issue will be fixed.

Additional Information

For the accelerometer PCB 356A44, the "Settling Time" is [=<5 seconds], as the datasheet of the product shows:

datasheet.png