Averaged RMS vs. averaged FFT peak

I have an application whereby I record several vibration signals of a spinning apparatus. The recording process is automated for a production stand, and two numbers are displayed for each channel - one is the averaged RMS value of the integrated waveform; the other is the discrete peak amplitude at the run speed frequency. Normally, the RMS value is larger than the discrete peak, as I expect. Every once in a while, though, the discrete peak is higher. Should this be possible?
Note that the waveforms are not recorded as part of the production process, and the observed results are not easily reproduced.

Hi AEI_JR,
Could you elaborate on any specific LabVIEW functions that you are using to get your RMS and FFT?
Also, how are you measuring this signal, what kind of numerical values
are you expecting, and how often are you seeing these suspected
outliers? This will provide a better understanding of what exactly is
expected behavior for your setup.
Developer Zone Tutorial: The Fundamentals of FFT-Based Signal Analysis and Measurement in LabVIEW a...
Thanks!

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What's the best program structure for multi-channel Averaged FFTs?

I have been successfully using a 8-channel FFT analyzer using a separate Averaged FFT Spectrum (Mag-Phase) VI for each channel, but I now need to expand my channel count from 8 to 16. All channels are simultaneoulsy read in to a buffer to maintain phase relatioships. Do I need to have 16 copies of the Averaged FFT Spectrum VI in my diagram to maintain the averages of each channel or can I utilize one FFT VI somehow within a For Loop? If a For Loop can be used, how can the averaged spectrums be maintained for each of the 16 channels?

You should be able to use a single FFT vi in a for loop. If your data is in a 2D array representation with each row representing a channel and you leave the indexing enabled you can pass your data array into the loop and it will strip off each row automatically. You could then pass each row of data to the FFT and pass the results out to the boundary of the for loop. This will then build an array containing the FFT results for each row (channel).
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Minimum difference between average and peak bitrage in mpeg2?

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How do you find an average of peak values in a chart?

I would ideally like to display a value alongside a chart that represents the average value of every three peaks. Right now I am only able to display a live value for the peaks. Is this possible?

Are you using the peak detector VI linked below? Using the index of peak locations and their corresponding values, you could try using a loop to build an array of the peak locations then averaging every 3 indexes of the array.
Peak Detector VI http://zone.ni.com/reference/en-XX/help/371361H-01/lvanls/peak_detector/
Regards,
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Averaging FFT of mutiple time series to get averaged fft results

Hi
I have multiple time series for the same set of measurement done at different times. Though all of them are not of same length but sampling frequency is same for all of them. When I do FFT of individual time series I will get magnitude & phase.
Is it correct to do linear averaging of the magnitude & Phase of FFT OR its necessary to convert magnitude & phase in to real & imaginary terms & then to averaging of real & imaginary parts for particular frequency & transform them back in to magnitude & phase.
To me later appear correct. Just want to confirm if there is any more simpler way to achieve this.
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Isha

If the data sets are of different lengths, then the FFTs will have different frequency resolutions and cannot be averaged by any simple technique. I suggest that you determine the longest possible data set and then zero pad all data sets to that length before doing the FFTs. Or take subsets of the data where all the subsets are of the same length. After all the data sets are adjusted to the same lengths I think you can average the magnitudes and phases separately.
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Cycle average and RMS VI outputting unexpected value.

I have an application that performs FFT analysis on an AC voltage singnal. As part of the whole setup, I've also included the Cycle average and RMS vi. When I've run the whole program, I've found out that the RMS value indicated is less than the highest magnitude component from the FFT analysis!(Which is incorrect). In addition I've found that the sub-vi, ma_ptmFetch.vi, within the cycle average and RMS VI, is returning an error(-20312), which says input waveform size is zero.
Can anyone help, with this problem?

two general observations
1) the differences in the RMS value an the highest magnitude in the FFT may be due to scaling, 0 to peak vs RMS in the FFT
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What are you trying to learn from the AC voltage? Are you performing some sort of power quality test?
Hope this helps,
Preston Johnson
Preston Johnson
Principal Sales Engineer
Condition Monitoring Systems
Vibration Analyst III - www.vibinst.org, www.mobiusinstitute.com
National Instruments
[email protected]
www.ni.com/mcm
www.ni.com/soundandvibration
www.ni.com/biganalogdata
512-683-5444

FFT RMS or Peak?

Hi,
I've got a sine wave signal that oscillates about the zero axis in the negitave and positive. It is measuring wave height about the static water level in an water wave tank. I am calculating the FFT but the amplitude is lower than the wave height in the time domain. Does this mean that I'm calculating it wrong? I don't know wheather I should be using the FFT Peak or RMS? I assumed that the FFT amplitude should give me the same amplitude as the time domain.
Any advice would be greatly appreciated.
Thank you,
Donners
Solved!
Go to Solution.

Donners,
It can get complicated but I suspect that what you are seeing is a result of the frequency of the waves not matching exactly the frequency bins of the FFT. For example suppose that df = 1 Hz. Then each element of the FFT array represents the energy in a "bin" one hertz wide centered on integer frequencies. For a wave with most of its energy at 7.3 Hz the FFT will likely have non-zero values for both the 7 and 8 Hz bins. In real systems with noise and slight frequency variations it is not uncomon for the data to be spread over 10 or 20 bins. To get the fundamental amplitude of the signal you would need to combine the energy in all those bins. Since the FFT produces a complex output, the phase component also needs to be considered. Non-sinusoidal waveshapes mean that some of the energy is in harmonics. A narrow square pulse can have a quite large amplitude yet still have no large spectral components.
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Frequency Response Function & FFT & Inverse FFT (problem of unit Volts-RMS)

Hello everyone,
I am currently working on a VI in order to compare two analog signals : the first one corresponds to the output signal (my reference) which is sent by my data acquisition card to a shaker and the second one corresponds to the input signal recorded by an accelerometer fixed on the same shaker. The final goal of the VI is to correct the analog output signal by using the analog input recorded signal in order to have the vibrations on the shaker which corresponds to what we really want.
To summary, I have a problem of unit with the Volts-RMS...
So this is my method for the VI :
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Then, I divide the FFT just calculated with the Frequency Response Function calculated just before.
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I don't want to use the Volts-RMS unit because I absolutly want to use the blocs " FFT.vi " and " Inverse FFT.vi ".
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Thank you for your help,
Best regards,
Sebastien

Hello Preston,
No, I have not use the Sound and Vibration toolkit. I have only used the signal processing toolkit with the two toolboxes " Waveform measurement " and " Transforms ".
But I think that what I have done for the moment in my VI is correct (I have finished the complete VI). But I am not sure of the units (Volts, Volts-RMS...) and I would like to understand.
I have tried with the Sound and Vibration toolkit for the frequency response function (because you say me that it deals with all the unit conversion) and I can obtain the same results than with the " Frequency Response Function.vi " of the toolbox " Waveform measurement ".
But I would like to understand the units (see my previous post please). For example, for the FFT (the result is a complex), why sometimes it is in Volts, sometimes it is in Volts-RMS ? Is it possible to convert it ? How ?
If you want, I can attach on the forum my VI and that will maybe help you to explain me. Maybe it will help other people interested.
And if someone else can give me other precisions or advices about it, do not hesitate.
Thank you for your help,
Sebastien

Calculating FFT using the FFTAverageType variable in DIadem

Hi,
I have some doubts about how should I use the averagingType variable. I want to define this variable as "Arithmetic" and get the combined FFT of two trials of the same process, i.e I have two signals which represent the same process.
What I do is just calculate the FFT of each signal separately. Then I calculate the amplitude of each FFT signal and finally i combine these two signals by doing the mean for each amplitude. I.E if i have signal1FFT = (1 , 2 ......23 ) and signal2FFT = (2,4, ...43 ) the resulting signal should be ( 3/2 , 6/2 ...... 66/2 )
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There are several ways to approach averaging in the FFT environment.
FFTAverageType is should be "Arithmetic" which means that all FFTs are weighted equal.
The Exponential averaging is kind of a running average for special calculations which are typically done during a running measurement.
Much more important is the parameter in "Calculate average from:" (Amplitude or Autospectrum)
Amplitude is giving a linear result and Autospectrum is giving the result based on squared values. The squared result is the correct result from an energetic point of view.
Example:
If you have a sine with a peak of 2 for one second and a peak of 4 for another second, you might want to get and averaged peak of 3.This might be wrong.
The RMS value of a sine with a peak of 3 would be 2.121.
The RMS which is representing the energy in a signal is the root of the average of the squared values. The RMS of a signal with half the time 2 and half the time 4 is 2.236. This means, that a sine with the peak of 3.162 for two seconds has the same RMS as the signal with a peak of 2 and 4 for one second each.
If you average FFTs in DIAdem you need to decide between the linear average (based on the peak Amplitude) and the energetic correct squared average (based on the Autospectrum). Right or wrong depends on your circumstances.
Usually, other mathematic programs are giving you the choice to decide or they might just use one or the other method.

Adding a 'minimum hold' option to fft spectrum vi

Hello
I wish to add a 'minimum hold' function to the averaging parameters in
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Hi ramsey,
I have been trying to find the VI's that you mentioneed but don't seem to be able tofind them in my pallettes. I believe they may be from the modulation toolkit in LV 7.1? Is this the setup that you have? Please let me know as there have been some significant changes with the analysis VI's since that revision.
Thanks
Rob L
NI Applications Engineer
UK & Ireland
It only takes a click to rate this message ;-)

Sound and Vibration kit - Peak Hold

I am evaluating a dynamic signal and would like to gather peak hold information from SVT 1/3 octave output. The SVT FFT tool has a "peak hold" function that allows for gathering spectral data over time, but it does not output an array of values. Is there a way I can capture peak hold from the SVT 1/3 octave output and create an indexed array that integrates a dynamic signal in a "peak hold" manner?
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Hello mstocker,
From your post this is what I understand:
You want to perform peak hold averaging with your Third-Octave
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Third-Octave Analysis.vi has a selection for the averaging type you can
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the results of several measurements into one final measurement. The
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If I have misunderstood your post, please reply with clarification. I would really appreciate it.
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Any way to add harmonic cursors to fft data?

I have a vi that plots vibration data from accels. I would like to place markers on the fft peaks that identify harmonics of the main cursor. Anyone out there that has done this?
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Ron Brook
Solved!
Go to Solution.

I assumed that the way to make it work flawlessly would be to use property nodes. I'll let you know after I get some time to implement that. I am also working right now on performing a peak search on my fft data. I have it working, but the peak frequencies don't match the data. I am still in trouble shooting mode on this one as well. My basic vi, which saves all six time waveforms and averaged spectra to the server, with another vi to recall all of it, including calculated overalls works very well.
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Applying of FFT and Octaves analysis to an analog input

Is it posible to aply, in real time, with LabVIEW 5.0 an FFT and Octaves analysis to an analog input obtained with a Labpc 1200? If yes could you please give me a hinch? Thank you very much.

There is basic FFT analysis built into the libraries that ship with LabVIEW Full Development System (FDS). However I would recommend considering the Sound & Vibration toolkit as an add on for LabVIEW. This contains more complete FFT analysis solutions (including averaged FFT analysers) and has a complete range of octave analysis (full and fractional analysis).
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This brings me onto the question of the board that you are using. Depending on the type of signals that you are working with, you may find that there are a number of limitations to using the Lab-PC-1200.
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How to see the peak values in a chart?

I have the following situation: I am applying the FFt to an acceleration signal. In the FFT-peak graph I see different peak frequencies. However I would like to know how much are those peak values and either display them in the chart on top of the peaks or extract them in numerical indicators. Is any of these solutions possible in LabView?
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GetMaxiumumExample.vi ‏81 KB

What is the fastest way to calculate the rms voltage from a large voltage measurement file?

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Hi Dave,
Just a thought, If you have a 60 Hz signal, then the 100 Khz is way overkill for the sampling rate, If you would resample the waveform at say 20 X 60 hz= 1200 Hz you should have the same details that would be in the 100 Khz sample signal. That should take substantially less time to do a RMS calculation on the resampled channel. The two commands are listed below.
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Averaged RMS vs. averaged FFT peak

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