Force Displacement

[Arthuruuuuuu]

Dear all,

I have to aquire force and displacement and i am using 2 kinds of modules: NI 9220 and NI 9237 respectively. The problem is: both signals are not in phase.  They start to measure at the same time but on the run of the test they get different times for picks (maximums) when suppose to be at the same instant. 

Does any one can help me please ?????

[dsb@NI]

the 9237 and the 9220 have different delays based on the different converter technologies they use.

The delta-sigma converter on the 9237 results in the published signal delay: Input delay (40 + 5/512)/ƒs + 4.5 µs.

You can correct for this delay using DAQmx property nodes:

http://www.ni.com/white-paper/11369/en/#toc5

You could also correct for this delay in post processing as recommended here:

http://venus.ni.com/intsearch/app/main/p/pg/1/ps/50/sn/n2:en/q/synchronization%20with%20delta%20sigm...

Even after the signals are aligned based on the published delay, it is still possible (maybe even expected) to measure phase delay between force input and vibration response due to system dynamics.

[dsb@NI]

This may also be a good resource:

http://www.ni.com/tutorial/5376/en/

[Arthuruuuuuu]

hey,

thanks for your answer but actualy i am new using national instruments and labview. 

So, i didnt understand the tutorial above posted because i am using a VI with DAQ assistent and not understanding why to create a new phisical chenal. 

Thanks again,

Arthur

[dsb@NI]

Assuming that you are sampling at 50k Hz, alignment to the nearest integer sample means that 40 samples must be removed from the beginning of the delayed signals, and 40 samples must be removed from the end of the other signals.

This corrects for the integer-sample delay in the 9237.

Due to system dynamics, you should still expect to measure a phase delay in the displacement response.

[Arthuruuuuuu]

Hey Doug,

thanks for your answer. That is other doubt I still have:

When you say aquiring in 50 Ks/s, this is the maximum rate NI9237 suports.

Althoug NI 9220 suports a rate twice higher. 

Should I introduce two icons of daq assist on my VI ? And also how should I set the samples to read and rate ?

thanks again. 

[dsb@NI · Accepted solution]

given current level of experience, recommend using single instance of DAQ Assistant to acquire from both hardware devices using a single task. Yes, that means that the sample rate is limited by the 9237, but that also means that you get hardware synchronization with no additional programming. 

Set the sample rate to 50k Hz. You tell us why it should be different.

Set samples to read to capture interesting data (10k samples if you need 0.2 s of data for analysis, 50k samples if 1 s data is required to allow the responses to dampen out).

[Arthuruuuuuu]

Hey Doug, still some doubts:

What you mean when you say "same task"?

Also, this VI you shown me is able in labview or shoul I build this onde ? Thanks again,

Arthur.

[dsb@NI]

From http://www.ni.com/product-documentation/2835/en/

... An NI-DAQmx task is a collection of virtual channels, timing and triggering information, and other properties regarding the acquisition or generation. ...

The VI snippet in my previous post can be dragged from the discussion forum post into a Block Diagram in LabVIEW 2017 or later. Insert the code you need from that copied code into your VI. Or, if you want to, you can also use palette VIs to reconstruct the code. Here is a list of the VIs used:

DAQ Assistant (you already have a configured DAQ Assistant)

Convert from Dynamic Data (configure Resulting data type = 1D array of waveform)

Split 1D Array

Round to Nearest

For Loop (x2)

Get Waveform Subset.vi (x2)

Build Array

Simple Error Handler

Here is a screenshot with VI names shown:

[Arthuruuuuuu]

Hey Doung, thansk again, brow.

trying to build this structure you sugested me in your last poust i have got 2 erros.

I could not conect last simples erro handle and also aligned array. I tryied to substitute the aligned array by a waveform chart or graph but wasn't sucessfully.

How can i do to see the signals during my test is ruing ? 

Thanks a lot,

Arthur.