How does Labview determine the period of a periodic waveform for RMS calculation?

[chassan]

Hello,

Does Labview use "zero crossings" to determine the period of a waveform when calculating RMS, or is there another method?

Thanks,

Chris

[Hillary_C_E]

Hi Chris,

Can you explain a little more about your situation? Are you using the rms.vi or another vi? What are you calculating the RMS of? With a little more information we should be able to provide better help.

[billings11]

Unless you are using some sort of special function for periodic RMS it is not calculating period at all and is simply calculating root-mean-square by definition on the array of data in the waveform.

[chassan]

billings11,

Thanks for the information.  Does that mean that for a periodic waveform the programmer is responsible for ensuring that only full periods of data are fed into the RMS VIs?  Wouldn't data that was not an integer multiple of a full cycle of the waveform result in an incorrect RMS value for the waveform?

Thanks,

Chris 

[chassan]

Hillary,

Thanks for the reply.  I'm not using any particular VI at this point.

Do different VIs calculate RMS differently?  I am interested in calculating the RMS value of a periodic waveform.  So I am interested in the RMS value over 1 cycle of the waveform.  Therefore I want to ensure that only full cycles of data are being used in the RMS calculation otherwise the calculated value will not be valid (I think.).

Thanks,

Chris

[Hillary_C_E]

Chris,

Billings is correct that a RMS vi is calculating simply the root-mean-square of the array of data. Different vis don't do it differently but I was curious as to which vi you were using. If you only want the RMS for one period of data then you do need to make sure that only that data is being passed into the RMS vi.

[billings11]

Exactly.  If you are trying to measure an accurate RMS of a periodic waveform, you should use a waveform subset function (or whatever you want) to truncate the waveform to an integer number of periods.  If you are not sure of the exact period there are also some functions that can tell you the fundamental frequency, etc.

[chassan]

billings11 and Hillary,

Thank you both for the information and assistance.  It has been (and will be) very useful.

Best Regards,

Chris