Data acquisition duration does not seem to match sampling information

[Suleiman_Atif]

Hi,

I am trying to use LabVIEW 2017 to collect accelerometer time histories to be used in a vibration transmissibility investigation, however, the data acquisition seems to end prematurely in relation to the sampling information. 

When the DAQ Assist was configured using a sampling rate of 1k Hz for 10k samples, the acquisition ended at around 6 seconds. When this was changed to 500 Hz for 5000 samples, the acquisition ended at around 3 seconds. I am not sure how to explain this observation and I would appreciate any help with this. Please refer to the attached images of the instrument and sample data. 

With kind regards,

Suleiman

[GerdW · Accepted solution]

Hi Suleiman,

a sampling rate of 1k Hz for 10k samples, the acquisition ended at around 6 seconds. When this was changed to 500 Hz for 5000 samples, the acquisition ended at around 3 seconds. I am not sure how to explain this observation

Read the datasheet of your NI9234 module to "explain this observation"!

It always helps to read the manuals…

Spoiler

Hint: you observed a sample rate of ~1.6kHz instead of the expected 1kHz.

[cbutcher · Accepted solution]

With a simulated NI-9234, it's possible to test the available sample rates.

The reason for non-continuous availability is described in the datasheet already mentioned, but you can apparently supply a different master timebase if needed.

(This is in general true, but often modules have a timebase like 100MHz, allowing more "normal" data rates to be easily achieved).

Here's the DAQmx code to check the actual sample rate. Note that to use the NI-9234 with DAQmx you'll need either a cDAQ or one of the newer cRIO-904x (but I'm guessing this is already true for you since you're using the DAQ Assist node...)

Using DAQmx rather than the DAQ Assist node can often provide you with more information, or at least, the availability to find more information.

Edit: With the DAQ Assist node, if you wire "False" to the "Stop (T)" input, and then connect the "task out" to the same DAQmx Timing and Stop/Clear Task, you can get the same information...

[Suleiman_Atif]

Hi GerdW,

Thank you for your reply and for referring me to the device manual, I realise the sampling rate was too low previously. 

With kind regards,

Suleiman 

[Suleiman_Atif]

Hi Cbutcher,

Thank you for your reply and insight into this topic, it is much appreciated.

With kind regards,

Suleiman