Capture Analog Input Voltage

[gleekd]

Hi, I've been trying to acquire voltage input from a tachometer with a cRIO chassis and an NI 9234 device.  Earlier, I used a C Series USB Single Module Carrier (NI USB-9162) along with LabVIEW's built-in DAQ assistant, and I was getting really good data, i.e. periodic single with clear peaks and valleys on the waveform graph.  But when I switched to using cRIO (NI 9074) and monitored the data with a waveform chart, I could no longer see a steady periodic wave anymore.  Instead, the signal has a lot of random peaks, and it has become impossible for me to determine the RPMs!  All I did is directly connect the Analog input channel to the waveform chart in a timed loop (with a 1kHz clock and 1ms period).  What am I doing wrong?  Could someone help?  

 

[gleekd]

Additonal question: would NI 9233 be sufficient for this testing purpose?  Its datasheet does say that it is for sound and vibration purposes mainly.  

[Aix-La-Chapelle]

Hi gleekd,

Were you using the 9234 with the 9162?  What was the sample rate you were utilizing?  The 9234 is definitely a way you can measure in Analog voltage.  What are your property settings on the 9234 in your cRIO? Have you gotten a chance to look at any of the 9234 examples in NI Example Finder?  There's a few in there about getting started with 9234 with a 9074.  Specifically if you go to example finder (Help » Find Example) then go to the search tab and search for the 9234, then you'll be able to take a look at some of the examples.  Let me know if this helps.  Thanks!

[gleekd]

Hi, Matt,

First of all, thanks so much for responding! 

Yes, I have looked at the examples, but they are kind of irrelevant to my problem.  To be more specific, I captured a few screenshots to help demonstrate what exactly I am having trouble with. 

The image below is a segment of my program for the cRIO.  This segment of code is in a timed loop with a 1kHz clock and 1 ms period.  I left other values as default.  The AI0 variable you see refers to the 9234 channel.  All I am interested in for now is the output plotted on the waveform chart.  What property settings of 9234 should I have set?  I didn't think there was an option to change the property.  I'm assuming that the AI0 outputs the voltage value that it captures from the tachometer, but please correct me if I'm wrong. 

The first image below is a segment of code I wrote for 9162, and it is inside a normal while loop.  The second image below is my settings of the DAQ assistant.  As you can see, my sampling frequency and 1kHz.  Other settings are left as default. 

I am expecting the output waveforms to look at least simliar, since the device (9234) I used to capture analog voltage is the same.  The only difference is in the chasses I used to connect the device (9234) with.  Why are the output graphs different?  Should I have used different period on the time loop? 

I hope I made my question clearer this time.  I'm looking forward to any possible solution you may have to this problem! 

Thanks!

[gleekd]

Is it possible that I may need signal conditioning??  I'm getting good waveforms on the oscilloscope and USB chassis, just not cRIO... Any suggestions??

[Aix-La-Chapelle]

Hi gleekd,

I tested this device out in both a cDAQ chassis and a cRIO chassis and did see a difference in the measurement when we had the chart in the same loop as the I/O node on the cRIO.  In the example for the 9234 getting started in scan mode, there is a high priority loop that gets the information from the I/O.  A separate loop then will communicate that data to the host and to the UI via a single process shared variable.  This way we produce results in one loop and consume them in the other.  That allows the producer loop to run quicker to capture the samples.  Also, I would make sure that you have your module configured for IEPE AC Coupled in your cRIO project.  Let me know if this helps.  Thanks!