analog input voltage measurement using PXI-6143

[Ahedayat]

I am using  NI PXI-6143 card in order to record the voltage from LVDT. I am using the VDT signal conditioner and want to hook up the Dc output of the signal conditioner to the Daq card. I connected the +output to the AI+ and - Output to the AI -.  The problem is that the measured amplitude using both PXI test panel and DAQMX functions are different than the actual DC voltage. I used the differential terminal configuration because it was the only option. I was expecting to get a constant DC value from the Daq card but it gives me some kind of square signal. (pic attached). If I use the basic Dc/RMS function, the averaged Dc value from the measured signal is different than the actual one. Interestingly enough, the maximum value shown in the graph is the real voltage. How can I measure the Dc voltage using the NI PXI-6143 card?

Thanks,

 Ahmad

[EEsRULEtheWORLD]

Hi Ahmad,

Have you used an oscilloscope to verify that the LVDT is in fact producing the correct DC voltage?  It's possible that this is the true signal being generated from the LVDT.

It could also be that your PXI-6143 is not functioning properly.  Does this behavior occur on different channels or when the module is in a different slot?  If you were unable to verify that the LVDT is producing the correct DC voltage with an oscilloscope, you may want to apply a known DC signal from a battery or signal source and connect it to the 6143.  Using test panels, determine if the signal is being acquired correctly.

[Ahedayat]

Thanks Sara, 

I followed the PXi-6143 manual about the wiring instruction. I found that I was missing the resistor between the - output of signal conditioner/indicator and AI GND of the terminal block. Putting the 10K resistor, the problem has been solved. 

Right now, I have a problem with the warning "Waiting for the real time to respond". After running the Vi, This warning appears and data acquisition process is stopped. I appreciate if you can take a look at the VI I am running on the RT.  

Thanks,

[Smootastic]

Hi Ahmad,

With the shift registers and whatnot, it looks like you are dynamically growing your array (using Insert into Array), which in most LabVIEW applications especially Real-Time is a big no-no.  As the array grows, you are writing all of the data into file.  This is probably starving a couple of threads, such as the Front Panel communication thread.  What you can do is try using a Producer Consumer Architecture, such as this example:

http://decibel.ni.com/content/docs/DOC-4610

You can send the Acquired data via RT FIFO to the lower loop, which can write the data to file.  It eliminates the need to use Insert into array, but you can replace the DAQ Assistants with your DAQ code.