What does DAQmx Create Virtual Channel (TEDS AI Accelerometer) actually do?

[testingHotAir]

Hi all,

I have a basic vibration logging application for which I'm using TEDS enabled accelerometers.  I'm configuring a task using the Create Virtual Channel (TEDS AI Accelerometer) to add the correct channels, but none of the data from the TEDS table appears to be getting pulled into my channels. 

Does this VI pull scaling information from the TEDS such that the values I read out are scaled to the appropriate unit?  Is there a way to force it to drag all the data from the TEDS through so that when I save a waveform to a tdms file they end up as channel properties?

I've included a screenshot of the VI that creates the task for reference.  Thanks!

[testingHotAir]

Definitely no scaling information carried through to the datalog...

[JaredRo]

Hi testingHotAir,

Can you provide me information about the hardware you're working with?  This will help me to troubleshoot your issue. 

[testingHotAir]

Hi Jared,

I'm using a cDAQ-9181 with a NI 9234 4ch accelerometer module.

[JaredRo]

The TEDS scaling data should be carried over to your TDMS write.

The NI-9234 only supports TEDS Class 1.  Do you know if the TEDS on your accelerometer is Class 1 or Class 2?  Just for your reference, here is a resource describing the difference:

What is the Difference Between Class 1 and Class 2 TEDS Sensors?
http://digital.ni.com/public.nsf/allkb/B0F3B23FE37C17FA86256E7400796684

If it is supposed to be compatible I can take a quick look at your code to see if I can determine anything that should be changed.

[testingHotAir]

They are TEDS 1 accelerometers.  I can actually see the entire TEDS table both in MAX and in my code (I read out the bitstream for my own amusement). 

To collect the data I'm using a DAQmx Read N Chan N Samp - 1D Waveform.  The output waveform array is wired directly to a TDMS write VI.  The properties on the channel end up as shown in the attached image; note that no scaling information is carried thorugh.

[JaredRo]

testingHotAir,

If you can get the TEDS table into LabVIEW, you should be able to pick out the pieces of information you want in your TDMS file properties and insert them manually using the TDMS Set Properties VI.  Here is the help file for this VI:

TDMS Set Properties Function
http://zone.ni.com/reference/en-XX/help/371361H-01/glang/tdms_set_properties/

[testingHotAir]

Yes, I did that.  Unfortunately that doesn't tell me how the DAQmx VI scaled the incoming data.  Did it use the scaling information found in the TEDS table?  Are the values I'm writing to file therefore m/s^2?  How can I be sure they are not raw voltage readings coming in?

[JaredRo]

Hi testingHotAir,

Accelerometers are typically rated with a Zero-g Voltage range.  This value can be found on the accelerometer’s specifications document.  You can determine if the output value is scaled by reading in voltage with the accelerometer stationary and checking if the value falls within the Zero-g range or if it does not.  If the voltage falls outside of the Zero-g range and you know the scaling factor to convert from Volts to m/s^2 you can further verify that the values that you’re reading are being scaled by the amount you would expect them to be.  Good luck!

[testingHotAir]

Yes, I could hack around and figure out if this DAQmx VI is doing what I think it should be doing, but in reality I'm just not going to use it if I have to go to the trouble of reading the TEDS and verifying the scaling every time I set up a task.  With no documentation about what it does to scale the voltage signal, it's a useless VI.