Utilisation FFT

Bonjour à tous,
Je suis actuellement sur un projet utiisant les spectre de puissances et j'ai un problème au niveau des valeurs de puissance max de mes harmoniques. En effet celles- ci fluctuent énormément (entre 0 et 50%) suivant le placement de mes curseurs entre lesquels je fais ma FFT.
J'ai chercher dans la base de donnée de National Instrument mais rien ne dit que ce problème arrive. J'ai fais des tests complémentaires et je finis à la conclusion qu'il faut des tableaux d'échantillons de taille 2^n pour que les valeurs correspondent.
Donc est-ce que cela vous parrait juste et qu'il faut prendre en compte le zéro-padding ?
Merci d'avance de vos réponses.

Bonjour,
Oui j'ai regarder toutes les explications sur les FFT de LabView et rien n'est décrit dedans pour ce problème spécifique. La seul chose décrite dedans est qu'ils utilisent un algorithme de Cooley-Tukey mais rien d'autres...
Cordialement,

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Spectre fréquence sous labview

bonjour,
je dois définir le spectre d'un signal, pour cela j'ai tenté d'utiliser "FFT Power Spectrum.vi" et "FFT spectrum (Mag-phase).vi".
Dans les 2 cas le résultat graphique que j'obtiens en sortie de ces vi n'est pas correct, la fréquence est en y et ne cesse d'augmenter, au lieu d'être en x et stable.
En entrée de ces vi j'ai créé une waveform avec comme paramètre : 0 à t0, 0,0005 à dt, et un tableau que je remplis en dynamique avec les valeurs acquises par mes voies. Où ai je commis une erreur?
Merci d'avance.

here are my data,
Attachments:
soufflerie_principale.vi ‏74 KB
spectre.vi ‏137 KB

Erreur de calcul amplitude FFT à f = 0Hz

Avec l'objet sousMesuresSpectrales [subSpectralMeasurements.vi] de la librairie LabView, sous LabVIEW7.1\vi.llb\express\express analysis\SpectralBlock.llb, je constate un problème de calcul sur la FFT pour fréquence = 0 Hz en amplitude.
Dans mon application, j'ai exporté ce VI express pour n'utiliser que certaines de ses fonctionnalités, en autres propriétés : "Paramètres du signal" pour afficher sa FFT.
Normalement, à la fréquence = 0Hz, l'amplitude max et efficace doivent être identiques et correspondre à la composante continue i.e DC value. Ce résultat n'est pas observé. J'observe le rapport racine de 2 habituel pour une valeur pic ou RMS à f = 0Hz.

A présent , je réussis à mesurer mon accélération, obtenir le déplacement et ma vitesse ( pour tenir compte du gabari ) et je réalise un singnal sinusoidale de fréquence variable ! Dois-je utiliser la fonction PID pour mon asservissement ?
Je ne sais également pas comment tenir compte des points de cassures de mon "gabari" présent en pièce jointe auparavant ? Help

Extraire des données d'une FFT

Bonjour,
J'ai un VI qui me sert à échantillonner une tension alternative à 25000 Hz (à l'aide d'un accéleromètre, en fait un relevé de vibration), et je souhaite effectuer une transformée de fourier de ce signal afin d'obtenir un spectre et récuperer des valeurs spécifiques à certaines fréquences afin d'alimenter un tableau excel... (Valeur efficace de différents pics)
une idée ?

Bonjour,
Pour effectuer une transformée de Fourier d'un signal, vous avez plusieurs possibilités, en fonction que votre signal soit représenté par un tableau de doubles, ou par une waveform.
Vous pouvez utiliser la fonction FFT.vi, que vous trouverez dans la palette de fonctions Traitement du signal>>Transformées.
Vous pouvez également utiliser la fonction Analyseur de Distorsion Harmonic.vi , que vous trouverez dans la palette Traitement du signal>>Mesures sur Waveform.
Cependant, ces deux fonctions ne sont disponibles qu'à partir de la version Full de LabVIEW. Si vous disposez d'une version de base, ces fonctions ne seront pas disponibles.
Cordialement,
Olivier L. | Certified LabVIEW Developer

FFT d'un signal

Bonjour
J'ai réalisé une application qui échantillonne des signaux à 5kHz (avec une carte NIUSB6210).
Je souhaite garder uniquement le signal à 50Hz de ces signaux (avec une FFT) et avoir la courbe à 50 Hz.
Est ce que ce traitement et possible?
Si oui comment?
Merci
Cordialement
Damien

Bonjour,
Merci d'avoir posté votre question sur le forum de national instruments.
Ce traitement est possible, il vous fait consulter l'aide labVIEW et aller dans recherche d'exemples. Il vous faut utiliser un passe bande en choississant le bonne ordre etc...
J'ai trouvé un VI sur la FFT. Vous pouvez le consulter et prendre exemple.
Brice S.
National Instruments France
Pièces jointes :
Frequency Analysis of a Filter Design - DFD.vi ‏32 KB

Extraire une fréquence précise d'une FFT

Bonjours,
Voila ma question, J'ai effectué une FFT sur un signal temporel échantillonné à 1024 S/s , j'ai donc tout mon spectre de 0 à 500Hz, maintenant j'aimerais en sortir l'amplitude à 50Hz +-3Hz (ceci afin de voir la contribution du réseau électrique dans le spectre de mon signal mesuré) et je ne sais pas vraiment comment m'y prendre, iest-ce qu'il y a un vi déja existant qui fait cela?
J'ai essayé d'utilisé le vi "Multiple Tone Exctract", cela marche mais je le trouve un peu lourd pour mon besoin.
Cordialement
Sébastien Kuenlin
Résolu !
Accéder à la solution.

Bonjour et merci d'avoir posé votre question sur le forum NI.
Effectivement un VI Express tel le "Multiple tone extract" est gourmand, c'est pourquoi il est possible d'utiliser la fonction "Extract Single Tone Information", de plus bas niveau, comme dans l'exemple proposé ci-dessous et en pièce jointe.
En espérant avoir répondu à votre question.
Mathieu TUR
National Instruments France
Mathieu_T
Certified LabVIEW Developer
Certified TestStand Developer
National Instruments France
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LabVIEW Tour
Journées Techniques dans 10 villes en France, du 4 au 20 novembre 2014
Pièces jointes :
Mesure d'un ton.vi ‏28 KB

Utiliser la fonction Refft ou fftex?

Bonjour,
Voilà le questionnement:
Je mesure une signal et j'obtiens alors un tableau de valeur correspondant à 1 période du signal.
Je souhaite enfait comparer ce signal avec un signal de référence et donc utiliser leur transformée de fourier. Quelle est la fonction la plus adaptée, refft ou fftex?
Autrement, si j'utilise la fonction ReFFT je voudrais savoir si je peux écrire cela :
TF=ReFFT(mon_signal, nbre_point, partie_im);
ce qui impliquerait que TF soit un tableau lui-aussi avec par exemple : TF[2][5] pour avoir le 5 eme point du tableau de la partie imaginaire. Est-ce vrai?
Vu que dans l'idée j'aimerai faire : diff=TF_mon_signal - TF_signal_ref;
De plus, si c'est le cas, comment définir la variable TF ( type TF; mais quel type?)
Le but de tout ça étant d'éviter le plus possible l'utilisation de boucles for pour calculer la différence entre les fft.
Merci ,
_trent_
Résolu !
Accéder à la solution.

Bonjour,
Merci d'avoir posté votre question sur les forums de National Instruments.
A l'heure actuelle, la fonction ReFFT est classée comme obsolette.
National Instruments ne vous conseille pas de l'utiliser pour vos applications.
Notre recommandation est d'utiliser plutôt la fonction FFTEx pour réaliser votre programme.
Vous pouvez d'ailleurs utiliser la fonction CreateFFTTable en conjonction avec FFTEx pour utiliser plusieurs fois votre calcul de FFT.
J'espère que ces informations vous aideront dans votre développement.
Bien cordialement,
Guillaume H.
National Instruments France
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>> NIDays 2011, le mardi 8 février au CNIT de Paris La Défense

MyRIO FPGA FFT Express VI timing analysis for multiple input mode - trying to perform fft's on a 3-axis accelerometer

Hi Everyone!
Project Background:
I've been working with the myRIO FPGA in an attempt to generate an application capable of sampling a tri-axis accelerometer and performing an fft on each axis. I've successfully developed an application for a single axis, but attempting to duplicate the code to sample the second and third axes in parallel results in an estimated 150% resource utilization for the tiny FPGA's LUT's. Additionally, I'm looking to avoid sequentially processing each accelerometer input using triggers and a single fft block because that reduces my fft update frequency significantly (e.g. I can't calculate another fft for input 1 until I calculate an fft for inputs 2 and 3).
After reading up on the fft vi, I'm thinking that I can use the M-interval input indexes / Continuous output indexes Input/Output Index Pattern mode. My thought is that I can edit the vi to remove any math that "recombines" these three vectors into a single fft, resulting in 3 separate fft's. I'm also hoping that this process requires less time than using the sequential method described above.
The Questions:
1. Has anyone done an fft on three inputs using the myRIO at sampling rates > 20kHz and fft sizes of 1024 or larger? If so, I may just be lacking some proper resource management.
2. Does anyone know where to find timing information on the M-interval input indexes / Continuous output indexes Input/Output Index Pattern mode? The manual only provides timing diagrams for singel channel / single input modes. I don't want to waste my time modifying the vi if it will still take 3x as long (assuming modifying the vi is even a possibility).
Further Information:
I already have an application written that samples the accelerometers at >20kHz and then performs the fft on the main processor, but now I'm looking to see if it is possible to perform all signal processing on the FPGA side. The processor performs decently enough, but the timing is not as consistent as I would like it to be. Lastly, I am aware that the myRIO itself has a built in accelerometer, but I need to mount the accelerometer in an environment where the myRIO would probably be damaged and definitely cannot fit.
Any thoughts are much appreciated! The excessive FPGA compile times for this thing make the old guess and check method less appealing.
-Chris

Hi Chris,
Thanks for posting and the detailed background on the project! To answer some of your questions:
1. The FFT Express VI does use a significant amount of space. The FPGA on the myRIO is somewhat limited space-wise. Your best option may to implement the FFT for 1 channel on the FPGA and the other two on the RT side.
2. I converted the FFT Express VI to a subVI and I am not sure if you can trim too much code from it. The subVI is also very complex so re-working it would be a significant amount of work. I could not find much documentation on M-interval input indexes / Continuous output indexes Input/Output Index Pattern mode timing.
I hope that this helps!
Thanks,
Frank
Application Engineer
National Instruments

How do I perform an FFT or ReFFT in VB6 using measurement studio

I am trying to make a filter by taking a 1D array of voltages and doing a ReFFT() and padding the high frequencies in the array with a value of zero and then going back to time domain with ReInvFFT(). Here is my code which does not seem to be filtering any high frequency content.
lngReturn = frmMain.CWAI1.AcquireData(varData, varBinaryCodes)
frmMain.CWDSP1.ReFFT varData, realdata, imgdata
For i = (UBound(realdata) / 100) To UBound(realdata)
realdata(i) = 0
imgdata(i) = 0
Next
frmMain.CWDSP1.ReInvFFT realdata, imgdata, varData
After the function is done varData has a 1D array of filtered real voltage values. It does not seem to be filtering and I get back exactly what I pass in. I know it must be something simple that I am missing perhaps the variable cannot be used to pass in and then back out the values??
Please shed some light as there are NO examples of this use of FFT.
Thanks

Arthur,
Thank you for your reply. But at this point, one of my points of confusion is what software I have available to me. I wish I had a useful central help desk to call up and just say, "Hey, load me up with all of the professional version(s) of Visual
Studio. But I do not know what we have license to. And I kind of need to know what the options are, so that I can ask a specific question of our vast buracracy.
For now, I have downloaded only free versions of Visual Studio client software. Since my company does have servers running Microsoft SQL Server, I am going to assume that we have proper licensing for those servers. But does that server license
allow me to get any professional versions of the PC client software for the various Visual Studio(s) 2012?
And if I get a professional version of Visual Studio 2012, does that do away with the three different flavors that I have right now? I have 2012 versions of SQL Server Management Studio, Visual Studio Express, and Visual Studio Shell (Integrated).
Your feedback is much appreciated.
Thanks!

How can i display the signal out of a FFT block to a graph?

What conversion is required in order to convert a the output of a FFT block to a waveform display? The signal input to the FFT block is a binary pattern signal!

Hello St Augustine,
Unfortunately, I cannot open these files that you have attached so I cannot tell you for certain where your problem lies. However, if you are using one of the FFT VIs from the Analzye>Waveform Measurements sub-palette, then you should be able to wire directly from the VI to a Waveform Graph. Make sure you are trying to wire to a Wavform Graph not a Waveform Chart. If you are using the FFT VIs on the Analyze>Signa Processing>Frequency Domain sub-palette, you should also be able to directly wire into a Waveform Graph, but please refer to the KnowledgeBase document linked below for more information about plotting complex numbers.
http://digital.ni.com/public.nsf/websearch/C010A823CEA80D5386256938005A066E?OpenDocument
Rega
rds,
Jyoti F
National Instruments

How can I use the Diadem FFT Analysis GUI to give me multiple outputs given multiple inputs?

I have a few hundred channels I would like to perform FFT analysis on. They are mostly of different lengths, but I know I will have groups of 7 that will have the same number of data points. The default GUI will accept those 7 sets since they are the same size, but it will only give me one output as a combination of the sets. Is there a way to get Diadem to give me an FFT output for each set without having to select each data set individually?

The way it worked with the default channels, I had to make sure I wasn't going crazy because the way you demonstrated worked very well for me. That being said, for some reason what I am doing does not work. I've recently found that it is in changing the details in the GUI that makes my output different than yours. The differences are as follows:
1) I do not use a time channel. That is always left blank.
2) in the 'Window Functions' tab, I change the window type to Hanning
3) In the 'FFT Functions' tab, only amplitude is checked with the properties 'Peak amplitude', No Third/Octave, and Average amplitude is changed to arithmatic.
4) In the 'Time Intervals' tab 'Length/Start/Overlap' is checked, Interval length is changed to 1024, and overlap (%) is changed to 50.
I've also found that changing any number of these details from the default will cause my proplem to crop up. I am fairly sure that most of those options are just as feasibly done to one channel as they are to multiple. Is this a bug, or am I misunderstanding a property of FFT that has gone over my head? Please try changing a few of those properties since it seems like it should work just as well for all the channels as it does for just one, but that doesn't seem to be the case.
Another point of confusion is that while the window previewing the inputs in the GUI will show all the inputs I have selected, the window previewing outputs only appears to preview a single unknown output. That is just a side note though.

How many cores can Adobe Premiere Elements 12 utilise on a CPU?

I am looking into purchasing 1 of 3 different products right now and Elements is one of those with others being competitors. Due to Elements not utilising OpenCL or Cuda increased rendering speed, other software looks like a better solution. However, the ones I am currently looking at only use up to 4 cores. The processor I am looking at is the Intel 4930k 6 core processor, will Elements utilise all cores effectively? Also I have heard Elements runs poorly on Windows 8.0 and 8.1 as compared with 7.* . Is this the case?
-Best Wishes, A

ABane
This is not Adobe. Just a user to user forum where some of us do not have 6 core processor. to give you a first hand answer and and all of us do not have access to Adobe Engineering support documents that are not typically seen online. We visit here on a non scheduled basis, so a posted thread does not activate an instantaneous reply process.
If you want an Adobe answer to your question, you might click on the following link, and, when it opens, click on the statement
"Still need help? Contact us." in order to bring up Adobe Chat for the topic of "Downloading, Installing, and Setting Up".
Contact Customer Care
Interesting, it looks like a new format for the old link. But, let us see how effective it is with this new format.
Premiere Elements 12 should be able to use 4 cores. Probably 6 cores for encoding, but I cannot give you the definite answer.
Supported devices | Adobe Premiere Elements
I can tell you first hand, Premiere Elements 12/12.1 runs well on Windows 7 Professional SP1 64 bit, Windows 8 64 bit, and Windows 8.1 64 bit.
And these computer are using dual core processors.
ATR

Block Planning Display block options eg Percent of block utilised and Qtys?

Any hints how to display block quantities and percent utilised in PPDS area?
I have managed so far to get the following for blocks that cross between months.
1. Resource table. (/SAPAPO/RPT - Resource Planning Table )Can highlight both columns that the specific block is in, click "show operations", get the operations to show in the bottome screen, and the FILTER on the specfic dates to get the block quantities.
This works to get a block quantity but would need to do a slow manual process for every block!
2. Receipts View (/SAPAPO/RRP4). Can set up a visualisation profile which shows my characteristic in a column in the report, and then I can select a SINGLE resource from the front selection and get a listing of the quantities from that single resource, and then with filters on dates and/or characteristic, I can get the quantities.
But this is slow and manual per resource.
3. Detailed scheduling board shows VISUALLY whether blocks look full or not, but I cant find any way of getting any block quantities or percent complete from this view.
Ideally, I want a way I can download all resource quantities and block quantities and then determine percent utilized in the blocks and put alerts on this to show which blocks are under-utilised.
Any ideas????

Aha. Found it.
in the DS Planning board.
click an order on block and then EDIT/SELECT/ALL FOR SAME BLOCK
then
EXTRAS / EVALUATION / EXTENDED OPERATIONS LIST - this gives all the quantities for selected block
then
EXTRAS / EVALUATION / RESOURCE UTILIZATION - select blocks and get percent utilisation for blocks

FFT Frequency Resolution vs. Graph Update Rate of a Vibration Signal

Hello,
First, what I know:
The frequency resolution (Hz) of an fft is 1/acquisitionTime (e.g. if 2 seconds of data are acquired, the frequency resolution is 1/2s = 0.5Hz).
What I'm Wondering:
Let's say I'm displaying an acquired signal on a graph and freq res = 0.5Hz. The graph update rate is therefore approx 2sec.  This may appear to be an application with a problem from a user's perspective.
Can I increase the update rate?
It seems that with vibration data (i.e. mostly non-periodic) I could create a buffer that is 2sec long.  The first data acquisition sequence will fil the buffer with 2sec of data. Then I could just acquire something shorter than 2sec of data (let's say 1/4sec). If I am acquiring data continuously and I push the 1/4sec of data into the front of my buffer then perform the fft again, I have effectively increased my graph update rate to 1/4sec and my users think I'm great.
Can anyone shed any light on whether or not (why/why not) this will work?
Thanks for your time,
Chris

The best tool NI provides for your application is the Zoom FFT. This allows you to specify the span (frequency range of the FFT) and the zoom (the delta f such as 0.5hz) in the FFT. The Zoom FFT has a built in buffer, called overlap and allows you to set the overlap so that it updates at the appropriate rate, event though you may need many seconds of data to get the delta-f in the FFT you are looking for. We package all this and manage units to display the correct units for power density, power, linear, magnitude, rms, 0 to peak, peak to peak, etc. We even provide a express VI for the zoom FFT and it is also part of the Sound and Vibration Assistant, requiring no programming at all.
http://digital.ni.com/public.nsf/allkb/F70DB3441DC5AB9986256E97005BA825
http://zone.ni.com/devzone/cda/tut/p/id/6107
http://zone.ni.com/devzone/cda/epd/p/id/5505
http://sine.ni.com/nips/cds/view/p/lang/en/nid/12152
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

How do I get my DAQ to have frequency resolution of 0.5 hz after FFT ?

Hi all,
I was just wondering how do I get a frequency resolution of 0.5hz after the FFT from this VI.
Or perhaps there can be some advice on where I must improve it.
Appreciate all advice, thanks!
-How81
as of now, the waveform graph i obtained, I wanted it to be of 0.5 hz, hence I used its property to set it at 0.5hz.
Message Edited by How81 on 10-13-2007 03:31 AM
Attachments:
Final DAQ.vi ‏107 KB

Hi How81,
   I won't comment on your code, I'm unable to see it (I have LV 71.1). So I'll discuss a little bit the DSP.
   If you want such a low frequency resolution, you have to sample a little longer. If your top-frequency is 25 Hz, I don't see the need for sampling at 1 kS/s, that gives a Nyquist freq. of 500 Hz!!! You'll only have a large amount of data, not of your interest!
   Another good advice, is to sample for a long time. To have a REAL resolution of 0.5 Hz, It is not sufficient to sample 2 MSample for one microsecond! So, you have to sample at least for 2 seconds, but I strongly suggest to sample for a longer time. In DSP,you almost always have to use some tolerances.
   Another matter is FFT resolution due to number of samples. If you sample 2 kS/s for 1 second, and you'll get a "2000 point FFT (...)", you'll see 1000 equally spaced frequency samples, from 0 to 1000 Hz (ie 1 Hz spacing), you can't have a resolution of more than 1 Hz. I would suggest something like sampling at (say for tolerance) 100 S/s, (top freq. you seee in FFT: 50 Hz), for 10 seconds: so you have 1000 points, but you have a resolution of 0.1 Hz.
   A part from this, always consider that FFT works in 2-powers, so it's better to do a 1024 points FFT.   Then you can use zero padding techniques to "adjust" fft to see samples at right frequencies, or sample 1024*10 samples in time at 1024 S/s and then decimate..... you have many choices.
   Hope this can help. Have a nice day!
graziano

Utilisation FFT

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