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signal processing of vibration signal

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GOURANG UPADHYAY
GOURANG UPADHYAY on 4 Feb 2023
Commented: Paul on 4 Feb 2023
I have one vibration signal with sampling frequency of 1000 Hz i want to do signal processing of that when i did it my amplitude of signal after FFT is not same.
please help
clc;
clear;
close all;
%%Time Domain Plot
vib1=readtimetable("ks.csv","SampleRate",1000);
tiledlayout(3,1)
nexttile
plot(vib1.Time,vib1.Var2)
L=length(vib1.Var2);
N=2^nextpow2(L);
xlabel("Time(s)")
ylabel("Ampl(mm/s^2)")
title("Time domain analysis")
% % FFt
t=vib1.Time;
fs=1000;
delt=seconds(1/fs); %time step
Totaltime=t(end,end);
my_fft=2/N*(fft(vib1.Var2));%FFT of Signal
abs_fft=abs(my_fft); %Absolute value of fft
delf=fs/N; %Frequency Resolution
n2=0:1:N/2-1; %fft results are plotted for N/2 data points
fk=delf*n2;% frequency values
% abs_fft(1:N/2);
nexttile
plot(fk,abs_fft(1:N/2))
[v,p]=findpeaks(abs_fft(1:N/2),fk,'MinPeakHeight',1);
findpeaks(abs_fft(1:N/2),fk,'MinPeakHeight',1)
xlabel("Frequency (Hz)")
ylabel("Ampl(mm/s^2)")
title('Frequency domain analysis')
% % Spectral density
% nexttile
% pspectrum(vib1.Var2,vib1.Time,"spectrogram")

Answers (1)

dpb
dpb on 4 Feb 2023
Have gone through this innumerable times -- and the fft doc gives an example of normalizing the one-sided PSD to match the time series amplitude. See the following links and discussion/examples therein --
Somewhere there's a very long conversation in which I give a myriad of examples of the effects of sampling rate and time in output resolution of both frequency and amplitude. Real world signals rarely match the sampling rate identically so that the even with a pure tone without noise the total amplitude of a single frequency bin in the FFT/PSD may not match -- the energy is spread across the nearest bins if the bin center doesn't match precisely; hence to get the total energy estimate one must integrate the peak, not just pick the peak magnitude.
  1 Comment
Paul
Paul on 4 Feb 2023
Why should the PSD match the time-series amplitude? I can't find anything on the fft doc regarding power spectrum. It does talk about amplitude spectrum, but that's not the same thing.

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