I am trying to analysis a system by Fourier Transform. my system has 3 dominant modes. (1,0.05) (0.51,0.01),(0.46,0.01) the pairs are defined as (freq,damping_ratio)
I've got those result with eigenvalue analysis.the system contains 3 signal.so these frequency[1,0.51,0.46] must be find in these three signals too. so I've run FFT but I didn't see these frequency in the plot. what is the problem in your mind? why these frequencies do not exist in FFT plots?
Here you can get the signal this txt file contain 3 signal in it. I put last signal FFT(PSD) here. Tnx
as you see those frequencies don't exist in the 3rd-signal. its same for other 2 signals too. Also I will put my codes here.
t=linspace(0,30,3012); fS=ceil(inv(t(2)-t(1))); x=data(:,3); Nfft = 10 * 2^nextpow2(length(t)); psd = 20.*log10(fftshift(abs(fft(x,Nfft)))); freqs=[0:Nfft - 1].*(fS/Nfft); freqs(freqs >= fS/2) = freqs(freqs >= fS/2) - fS; freqs=fftshift(freqs); figure(1); plot(freqs, psd); xlim([-1.5 1.5]); xlabel('Frequency / Hz'); title (sprintf('PSD'));