NI-9161 & 9215 use in SE 2.5

[RJJE]

I can gather data using NI-DAQmx Base 2.2 using the above and the included NI-Data Logger but so far have been unsuccessful using SE 2.5.

Is this even possible?

Thanks

Randy

[RJJE]

After reading further posts I think the answer is no. No problem, I will use my NI CompactDAQ chassi with the 9215 analog input cards to capture the signals.

All I want to do is measure power use in my home but SE 2.5 is not co-operating with me (probably the other way around). I can get the voltage and current waveforms into SE and calculate the instaneous power wave form but from there I am lost. Anyone toss me a tidbit of info to point me to the right direction?

Randy

[Chris_W.]

Hi Randy,

It sounds like you're right there. If you're able to calculate the power over time in your house, what are you trying to do next? If you're just logging the data you should be able to save the data to an ascii file by adding a step and using the Save/Load Signals drop down and then Save to ASCII/LVM file. 

Chris W

[RJJE]

Hi Chris

I would like to get the 60 Hz current, voltage and the 120 Hz power waveforms into a more meaningful way. I believe I would convert the voltage and current to RMS but not sure if that is correct for the power.

Then perform an  average for each, say over 1 second as this would reduce the file size created, then do the final calcs in a spreadsheet for kWhrs. I don't understand the meaning of # of averages in the average time function.

Thanks

[Chris_W.]

Hi RJJE,

I'm not sure what you're talking about when you say 120Hz power waveform, but it sounds like you may know what you're doing there. "Number of avg." in the Time Averaging step, specifies the number of previous points in time to use to create the current "average" point. This doesn't actually decimate the data so that your end file will be smaller.

You can, instead, use the "Statistics" step found in Analysis>>Time Domain Measurements>>Statistics, and select "mean." At the bottom of that configuration window, if you select reset "Restart measurement on each iteration," it will average the number of points you specify as "Samples to Read" in the Timing Settings of your DAQmx acquire step and write that amount to your .txt file each iteration of the loop. It will repeat this process each iteration of your loop of tasks. This does not show the times that corrospond with the measurements when you write it to the .txt file, but when you are taking an average over a period of time that converts n values to one average value, you lose the meaning of the timing of those values anyway. I hope this helps.

Chris W

[RJJE]

Hi Chris

Instantaneous power is V * I and since I am generating the voltage & current sinewaves (easier to experiment with) I have them both in phase (power factor = 1). So power will be twice the frequency swinging from 0 to the upper peak. If you were to capture the V and I waveforms and multiply them and the power sinewave swings into negative that would tell you the V & I waveforms are out of phase.

I have been experimenting with the Statistic function, question would be, if I sample at 1k/sec and mean at 1k samples would the result to file would be at roughly 1 second or so which would include hardware overhead time?

I did find a NI example of a power monitor application using higher end hardware and LabView but I have to make do with SE for home use. Since I have been good this year maybe Santa will bring me a LabView Full version and hopefully upgrade my Multisim 2001 to Multisim 10.1.

Thanks for your help.

[Chris_W. · Accepted solution]

HI RJJE,

I'm not exactly sure what you're asking when you say  "would the result to file would be at roughly 1 second or so which would include hardware overhead time?," but what it sounds like you're asking is whether the file is being written to update approximately every 1 second. If this is what you mean, then the answer is approximately yes.

In your acquisition step you have to specify the Sample Rate and the number of samples. so if you use this as a continuous acquisition, then each iteration of your loop there will be 1000 samples that will be processed by the Mean step, then that one mean value will be written to your file. This process will then repeat until you stop the task. That means that from the time that Signal Express reads the 1000 samples from your PC RAM until it writes that mean value to your file, it would be 1 second plus the amount of time that it takes Signal Express to retrieve that data from PC RAM as well as the time it takes the Mean function to process those 1000 samples, plus the amount of time it actually takes to write that Mean value to the file.

If you're looking at the time between iterations to be about 1 second then these other values will be very small in comparison. They are really just dependent on the speed at which your processor can handle these other functions in your task.