Extract natural frequencies from acceleration data by FFT

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medo_sk
medo_sk 2011-7-29
Hi all,
I have problems when calculating frequencies from sample data by FFT in Matlab. The data file shows acceleration in cm/s^2. Sampling frequency is 200 Hz. The file can be downloaded here: http://ifile.it/pqdvhwx. Can you please share a Matlab code for this?

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回答（2 个）

Bjorn Gustavsson
Bjorn Gustavsson 2011-7-29
At the matlab command line prompt type: help spectrogram
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Bjorn Gustavsson
Bjorn Gustavsson 2011-7-29
Ok, but it would only take you about 30 s to test wouldn't it? I happen to have used that very function to find natural frequencies for sampled signals - as far as I understan this is the first stab-function to use for this purpose - especially if one expects that the dominating frequencies varies.

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medo_sk
medo_sk 2011-7-29
Here is the code done so far. I do not know how to proceed.
% Signal.mat is imported as variable CH1
g=981; %gravity acceleration [cm/s^2]
Fs=200; %Sampling frequency
dt=1/Fs; %Sample time
L=length(CH1); %Length of signal
t=(0:L-1)*dt; %Time vector
CORR=(CH1(:,1)-mean(CH1(:,1)))/g; %Shift of signal to zero value
%Plot Original signal
subplot(311), plot(CH1);
title('Original Signal');
xlabel('Samples');
ylabel('Acceleration [cm/s^2]');
%Plot Original signal in Time Domain
subplot(312), plot(t,CH1);
title('Original signal in Time Domain');
xlabel('Time [s]');
ylabel('Acceleration [cm/s^2]');
%Plot Corrected signal in Time Domain
subplot(313), plot(t,CORR);
title('Corrected signal in Time Domain');
xlabel('Time [s]');
ylabel('Acceleration [cm/s^2]');
n=get(gca,'ytick');
set(gca,'yticklabel',sprintf('%.7f |',n'));
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medo_sk
medo_sk 2011-7-29
Thanks Arnaud for your tips.
If I use the example file and adapt it to my data I do not get reasonable results. I am expecting something different from what is calculated by Y = fft(y,NFFT)/L.
Arnaud Miege
Arnaud Miege 2011-8-1
Thanks for formatting your answer, much appreciated!! Can you post the code you are using to compute the FFT of your data?

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