Frequency Response Function and FFT for Modal Analysis

Question

R SW 2016-12-12

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/316502-frequency-response-function-and-fft-for-modal-analysis

评论： Lingyuan Kong 2021-1-22

BrassFRF.xls

I am experimentally testing a material to retrieve its natural frequencies through modal analysis. The type of testing is based around impulse response. My sampling frequency was 10kHz. My acccelerometer's sensisitivty was 100mV/g and my impact hammer's sensitivity was 2.25mV/N. My aim is to obtain an accurate FRF graph. My entire code:

%reading excel file

dataset = xlsread('BrassFRF.xlsx','Sheet1','A1:C50000');
%%Creating and filling data variables
a=dataset(:,2); 
n=dataset(:,3);
%%Converting acceleraltion voltages into acceleration (mm/sec^2)
a=a.*((1/0.1)*9.81);
%%Converting force voltages into Newton (N)
n=n.*(1/0.00225);
%%Creating FFT using acceleration
L=length(a);
NFFT=2.^nextpow2(L);
V=fft(a,NFFT)/L;
Fs=10000;
freq=Fs/2*linspace(0,1,NFFT/2+1);
subplot(3,1,1)
plot(freq,2*abs(V(1:NFFT/2+1)))
xlabel('Frequency (Hz)');
title('Velocity FFT ')
%%Creating FFT for using force
F=fft(n,NFFT)/L;
subplot(3,1,2)
plot(freq,2*abs(F(1:NFFT/2+1)))
xlabel('Frequency (Hz)');
title('Impulse FFT ')
%%FRF
FRF=V./F;
subplot(3,1,3)
plot(abs(FRF))
xlabel('Frequency (Hz)');
title('FRF')

Please tell me if you find any errors that can be the cause of an incorrect FRF. My suspect is the impulse FFT.

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Lingyuan Kong 2021-1-22

The last section is wrong, it should be:

FRF=V./F;

subplot(3,1,3)

plot(freq, abs(FRF(1:NFFT/2+1)))

xlabel('Frequency (Hz)');

title('FRF')

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Answer 1

durukan dilek 2017-11-1

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编辑：durukan dilek 2017-11-1

did you obtain frf of acceleration/force? why did you write V/F? why did you use velocity?

L=length(a); NFFT=2.^nextpow2(L); V=fft(a,NFFT)/L; Fs=10000; freq=Fs/2*linspace(0,1,NFFT/2+1); subplot(3,1,1) plot(freq,2*abs(V(1:NFFT/2+1))) xlabel('Frequency (Hz)'); title('Velocity FFT ') set(gca, 'YScale','log')

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Answer 2

Deniz Kny 2019-3-6

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how did you know your sampling frequency was 10khz? I have a accelerometer data and, trying to obtain fft like you. i need a Fs but i dont know how can i get this info.

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imthiyas manarikkal 2019-11-8

You can actually check your resonance frequency or natural frequency. The sampling frequency must be twice of your natural frequency (Nyquiste Theorem)

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Answer 3

Jose Ordaz 2019-12-19

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Hello,

i'm working in the same way, trying to obtain the natural frequency and damping of some materiales using the information from a hammer and a accelerometer. Can you please help me with this?, how you solve your problem with the FRF?.

thanks