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Member
Frizzit
Posts: 19
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frequency response function modal analysis

After reviewing the signal analysis functions in DIAdem I have realized them to be a bit limited for modal analysis.  I have a couple hammer impact tests that I need to process a frequency response function for, and since this is brand new to me I'm not seeing anything in the embedded function list that is going to help me.  I was wondering if anyone out there has a couple of pointers on generating a FRF plot for modal hammer impact tests.  I did notice that the ChnFFT2 command allows me to generate a transfer function, coherance, and FFT Cross Spectrum channels for analysis.  Though I might be confused and this may be everything I need.  My FFT2 settings are below.

 

 

[code]

FFTIndexChn      = 0
FFTIntervUser    = "NumberStartOverl"
FFTIntervPara(1) = 1
FFTIntervPara(2) = 2500
FFTIntervPara(3) = 1
FFTIntervOverl   = 0
FFTNoV           = 0
FFTWndFct        = "Rectangle"
FFTWndPara       = 10
FFTWndChn        = "[1]/Time axis"
FFTWndCorrectTyp = "No"
FFTAverageType   = "No"
FFTAmplFirst     = "Amplitude"
FFTAmpl          = 1
FFTAmplType      = "PSD"
FFTCrossSpectr   = 1
FFTCoherence     = 1
FFTTransFctType  = "Spectrum H0"
FFTCrossPhase    = 0
FFTTransPhase    = 0
Call ChnFFT2("[1]/Time axis","'[1]/H_1' - '[1]/H_4'","'[1]/A_1' - '[1]/A_4'") '... XW,ChnNoStr1,ChnNoStr2

[/code]

Member
Rohama_K
Posts: 164
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Re: frequency response function modal analysis

Hi,

 

If you are looking for the three things you mentioned (transfer function, coherence, and FFT Cross Spectrum) then you are right about what ChnFFT2 generates. Are you looking for something else?

Rohama K.
Member
Frizzit
Posts: 19
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Re: frequency response function modal analysis

Standard modal analysis has something denoted as FRF.  I have a labview application note "The Fundamentals of FFT-Based Signal Analysis..."

 

Frequency Response Function


The frequency response function (FRF) gives the gain and phase versus frequency of a network and is typically computed as

where A is the stimulus signal and B is the response signal.

The frequency response function is in two-sided complex form. To convert to the frequency response gain (magnitude) and the frequency response phase, use the Rectangular-To-Polar conversion function. To convert to single-sided form, simply discard the second half of the array.

You may want to take several frequency response function readings and then average them. To do so, average the cross power spectrum, SAB(f), by summing it in the complex form then dividing by the number of averages, before converting it to magnitude and phase, and so forth. The power spectrum, SAA(f), is already in real form and is averaged normally.

Refer to the Frequency Response and Network Analysis topic in the LabVIEW Help (linked below) for the most updated information about the frequency response function.

 

http://zone.ni.com/devzone/cda/tut/p/id/4278

 

So the options for FFT2 are

 

No DIAdem does not calculate a transfer frequency response.
Spectrum H0 DIAdem calculates the transfer frequency response by dividing the FFT of the output signal (A) by the input signal (E): FFT(A)/FFT(E). DIAdem averages the amplitudes of the individual transfer functions.
Spectrum H1 DIAdem specifies the cross spectrum and the auto spectrum for each signal pair. DIAdem calculates the transfer frequency response by dividing the averaged spectra: Middle(cross(A,E))/middle(auto(E)). DIAdem does not average phases, because phases can delete each other.
Spectrum H2 DIAdem specifies the cross spectrum and the auto spectrum for each signal pair. DIAdem calculates the transfer frequency response by dividing the averaged spectra: Middle(auto(A))/middle(cross(E,A))

If you assign the values Spectrum H1 or Spectrum H2 to the variable FFTTransFctType, DIAdem averages and divides the cross spectra and the auto spectra and calculates the amplitudes last.

 

Which state auto spectrum when FRF is power spectrum. 

Member
Rohama_K
Posts: 164
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Re: frequency response function modal analysis

ChnFFT2 is the right function to start with analyzing hammer impact tests.

 

It is important that several impacts are measured as identical as possible. Using a trigger on the hammer signal gives results with the same phase on both channels on every impact. With a large number of measurements like this, it is possible to get an averaging in the ChnFFT2 and a coherence result.

 

PSD is usually used to compare noise levels (a spectrum without a few peaks and low noise). A hammer-impact is usually used to analyze mechanical structures with resonance-peaks (modes). In this case, it is better to calculate RMS or power spectrum. The difference is just a factor if all measurements and FFTs are calculated with the same number of values.

 

Modal analysis can go far beyond ChnFFT2. In this case, the response of a mechanical structure is tested on a large number of points on the structure. Hundreds of transfer functions are calculated and modes are isolated with special fit algorithms. The 3D behavior of the structure at each mode is displayed. This is not part of the DIAdem-functionality.

 

We need more information about how far you want to go with modal analysis and we need some sample data.

Rohama K.
Member
Frizzit
Posts: 19
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Re: frequency response function modal analysis

As far as I can tell the proper spectrum for ~ FRF is the H1 option.

 

We are not evaluating 3d behavior, though we are looking for resonance values.  I have some sample data sets, 1 accel response per strike, that can be provided, though eventually we want to evaluate multiple response locations per impact point.  Meaning we will have up to 14 accelerometers responses per hammer strike.  Can fft2 be indexed properly to evaluate multiple responses or do the impacts and responses have to be fed into the function at a 1 to1 ratio. 

 

Please email me for data sets if you require them.

Member
Rohama_K
Posts: 164
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Re: frequency response function modal analysis

You can have multiple impacts and response when using ChnFFT2 and that will produce multiple channels for the transfer function and FFT Cross Spectrum. You will however only have one coherence channel. 

Rohama K.
Member
Frizzit
Posts: 19
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Re: frequency response function modal analysis

I have determined my requirements regarding output, but I still don't know weather or not DIAdem can do it.  Do you know what it takes to evaluate structural mobility/impedance?

Trusted Enthusiast
Brad_Turpin
Posts: 2,875
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Re: frequency response function modal analysis

Hi Frizzit,

 

The short answer is that DIAdem is not designed to do full modal analysis.  We provide the atomic FFT and transfer function and curve fitting capabilities, but if you want to identify vibration modes and infer physical/material properties from the modal analysis, DIAdem does not help you with that.  Note that DIAdem can efficiently call any LabVIEW VIs, so if you have any of this working in LabVIEW, you could use DIAdem to call those VIs and display the results.  I would be happy to show you how to customize DIAdem with user menus or user icons to launch your LabVIEW functionality.  Note that DIAdem always installs the previous year's LabVIEW Runtime Engine, so you can use DIAdem to call a VI or source distribution without asking the DIAdem user to install the LabVIEW Runtime Engine.

 

Brad Turpin

DIAdem Product Support Engineer

National Instruments

Member
UBhatt
Posts: 1
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Re: frequency response function modal analysis

Hello guys,

 

 We are having some measured data for modal analysis of a particular structure and we have response and impact signal acquired by NI c_DAQ system.

 

Now, In order to find out FRF, I used FFT [Two time signals] function of ANALYSIS. But, one very important feature of our analysis is also to get coherence. Now, when we provide input and outout time signal to this function, we also need to configure it with some basic FFT functions like transfer function, coherence, amplitude etc. But coherence is not being calculated if we do not select more than one time intervals. Also, by selecting different number of time intervals and different overlapping , coherence is completely different. I am not much sure about this particular parameter. How to decide number of time intervals to be sure that coherence is correct in that.  And how much influence it will have in normal FFT calculations also with this number of intervals.

Trusted Enthusiast
Brad_Turpin
Posts: 2,875
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Re: frequency response function modal analysis

Hi UBhatt,

 

I am asking R&D about this.  Stay tuned.

 

Brad Turpin

DIAdem Product Support Engineer

National Instruments