Acquiring multiple voltage and current sine waveforms, and log them along with the respective time stamp in a file."

Question

"I am doing a data acquisition project using a 16 channel N.I. series 6020E card where I want to capture respective current and voltage sinusoidal signals. I am planning on capturing four pairs of matching current and voltage signals. I planned on taking one reading across the channels every fifteen minutes. This reading will need to be 60 data points per period or 3600 readings per second per channel in order for me to recreate the sine signal to determine the phase angle difference between the voltage and current. I was able to get one channel to capture a signal but could not get more than one working at a time. I am also trying to sycronize the data collection so all samples are with respect to each other. Thanks for any help."
-posted by cappy on 2/11/2002
John N on 2/12/2002 answered:
"Hi Cappy,
Could you post this question to the Hardware > Counter/Timer or Hardware > Multifunction I/O forums? I think you will get more responses there. Here is a link.
The hardware is usually a better differentiation since software can support such a huge range of functionality.
The data rates you are talking about sound reasonable. Are you using the USB or the AT interface?
One other thing to be aware of is that the samples taken will not be simultaneous due to the multiplexed nature of the MIO board measurements. However if you know the channel clock rate(output from one of the VI's), then you can calculate the time difference between each measurement and correct your phase measurements by that amount.
If you have more questions, please post this to the other forum and copy all of our discussion to let everyone know what we have talked about already.
Best Regards,
John Nieri
Applications Engineer
National Instruments"
I am using the USB connection. If anyone can help with or has a similar VI for this, it owuld be appreciated so I can work on the larger aspect of the total project.

Answer 1

The easiest VI to use for this acquisition is AI Acquire Waveforms in the data acquisition palette in LabVIEW. This VI allows you to perform a finite acquisition from multiple channels on your DAQ board. As John mentioned the acquisition from individual channels will not be truly simultaneous. There will be a slight time shift from one channel to the next on the order of the maximum acquisition speed of the card, 100kHz or 10 microseconds in your case. By accounting for this 10 microsecond delay between consecutive channels you can properly calculate the phase shift between voltage and current.
Christian L
NI Consulting Services
Christian Loew, CLA
Principal Systems Engineer, National Instruments
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