How can I use the RMS.vi over more than one period?

[User_1979]

Hi everyone,

I am using the Cycle average and RMS.VI to get the RMS value of a alternative voltage (2kHz). To acquire this signal I am using a differentiel AI of a 6225 card (at 250kS/s) and the RMS value is correct, it works fine. However from what I have understood, this VI calculates the RMS value according to only one period no matter how many samples I have acquired (2000 samples in the VI I have attached). Is this correct? If so, is ithere a way to use it over all of the 2000 samples to get a more averaged value? Or does another VI exist?

I could repeat several time an acquisition of a short number of samples (a period) and make an average but it would take some time and I have to do this with 39 other Inputs on the same card one after the other...

Thank you for your help,

User

[Broken_Arrow]

If you would get your data without the Express VI, in a loop, you could use the RMS PtByPt.vi and watch the RMS value build to the desired sample. You could also convert that express VI data type to a plain integer array and use RMS.vi.

 

Message Edited by Broken Arrow on 10-20-2009 12:51 PM

[User_1979]

Hi Broken,

Thank you very much for your answer. I have replaced the previous rms.vi by your point to point one (but keeping the Daq assistant as it was) and it doesn't work I get a wrong and unstalble rms value. Why did you suggest not to use the Daq assistant? Doesn't this VI give a rms value of the 1000 (for instance) samples once the Assistant Daq sends has send them to him...? Moreover, could you tell me if this VI sends a rms value of X compleat periods (even if I have send X periods and a half for example)?

Thanks again,

User

[Broken_Arrow]

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User_1979 wrote:

....could you tell me if this VI sends a rms value of X compleat periods (even if I have send X periods and a half for example)?

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It gives the RMS value of only the newest set of data.

The RMS and RMS Pt. By Pt. work for me, but it depends on how you break out that Dynamic Data from the Express VI, which was why I suggested you didn't use it - mainly because you can't tell what happens to that data when you wire it to a function and it coerces. Actually, I should say, I can't tell what happens to it. You may know, but I don't.  

In the attached screenshot, I have "made" a 2k sine wave with an amplitude of 5. The indicators show RMS value.

[User_1979]

Hi Broken,

Thank you for making up your .VI

I am completly new to using point to point solutions... I don't understand why we have to use a for loop  while entering the number of samples in the VI. The last value of the "now" array is the final RMS value that must be considered? The first 1299 values are only "a bulding up of the final RMS value", is this correct?

What do you think about the "average and rms.vi" where I get 10 RMS values : using this VI, I am sure that the rms is calculated on a entire period each time. I would then just make an average of the 10 values. The values have always been very stable.

 User