Acceleration magnitude from accelerometers does not seem to match calibration device

[Suleiman_Atif]

Hi, 

I am trying to configure a LabVIEW program with a NI 9234 device with two accelerometers to output acceleration in m/s^2. The conversion factors from voltage to m/s^2 for each accelerometer were used after acquiring the raw voltage from the DAQ Assist, however, when connected to a calibration device which vibrates with an RMS acceleration of 10 m/s^2, the RMS from the collected data is much lower. 

The accelerometers also seem to not be centred around zero which I do not know how to explain. I would appreciate any help to understand these observations. The accelerometers are directly connected to the NI 9234 device into AI0 and AI1. Please see images of the experimental arrangement and the VI. 

With kind regards,

Suleiman 

[Ben]

Guessing...

You have not set-up and Acceleration task that include the excitation required by the accelerometer?

Go to MAX...

Create  task

Let the Wizard guide you and get it to work in MAX first.

When you are happy with it in MAX save the task.

Open a new VI

Drop Task constant on the diagram

Select the acceleration task

Right-click the Task constant and select "Generate config and example

Ben 

[Bob_Schor]

I'm sorry to say that several years ago, I worked with some Masters-level Engineering Students who wanted to measure accelerations using an inexpensive triaxial acclerometer.  The published specs for the three axes stated the offset and gain of each channel as values ± 10%.  They didn't understand why their readings didn't make sense.  At least you've heard about "calibration", so now you have to ask yourself, "who do you trust"?

Here's a simple test.  Take your accelerometer and put it flat on the table.  Measure the acceleration when you are not moving it.  Now turn it upside down, and repeat the measurement.  In one orientation, X and Y should be 0, and Z should be 9.8 m/s/s, and in the other you should get 0, 0, -1g.  If you get anything else, you aren't calibrated (or your table isn't level) (or your accelerometer axes aren't mutually orthogonal) (or something else is wrong ...).

Bob Schor

[Ben]

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@Bob_Schor wrote:

... (or something else is wrong ...).

Bob Schor

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As in "no power".

Ben

[PsyenceFact]

As I think Ben is implying, the output data look like the accelerometers are not receiving any power.  PCB refer to their version of IEPE as "ICP" and the 333Bxx accelerometers can operate on a current of between 2 and 20 mA.

In the DAQ Assistant, ensure that your channel type is set to Analog Input:Acceleration:IEPE, the Iex Source is set to "Internal" and the Iex Value to 2 mA.

Just as an aside, when I run the DAQ Assistant as you seem to have it configured I get error -200284.  I think this is a result of filling the acquisition buffer when running in N Samples mode.  Do you get this?

Andy

[db-vibration-balancing]

Yes I got same problem when taking calibration two accelerometers with NI USB 9234. The calibrator generated acceleration 10 RMS m/s^2 but I always get ~9.7 and ~9.4 results. Need to support from experienced guys.

[Henrik_Volkers]

Your vi with the calculations would be helpful...

accelerator type

Another important point is accelerometer mounting... clean surface, some grease (or oil) between the surfaces, known torque according to spec.  How do you fix it?

and the calibration report of the field calibrator 🙂

If you post your vi, have the raw voltages displayed in a graph and with some data shown, rigth click on the graph and select 'Make current values default' ..save the vi and post it.