FFT of bearing signal

Question

Eris Prifti 2023-8-2

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2003517-fft-of-bearing-signal

评论： Jon 2023-8-15

vettore.mat

Hi everyone, i have a problem : when i do the fft of a signal i don't see the first harmonic , instead the fft show the first harmonic multiplied for 3. In fact my fundamental harmonic is 16,66 Hz e when i do the FFT it show 49,80 Hz. Is possible this? I insert the code and the signal on this. Also the sampling frequency is 10kHz.

S=size(N);
L2=S(1,1);
NFFT= 2^nextpow2(L2);
Y=fft(N,NFFT)/L2;
f1=f/2*linspace(0,1,NFFT/2+1);
f2=2*abs(Y(1:NFFT/2+1));

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Jon 2023-8-2

I'm not seeing anything in either your attached code or data file that indicates what the sampling frequency is. I don't know how you can scale the fft in Hz without knowing the sampling frequency.

Also in your attached code, you reference a variable called N, but the attached data file only has one column of data named x11. You also don't tell us the value of f in your code, is that the sampling frequency? If so what is it's value?

Eris Prifti 2023-8-2

在 MATLAB Online 中打开

Sorry,

for fs is 10Khz and N indicate the size of the column named x11. I hope this is sufficiente

S=size(x11);
Unrecognized function or variable 'x11'.
L2=S(1,1);
NFFT= 2^nextpow2(L2);
Y=fft(x11,NFFT)/L2;
f1=fs/2*linspace(0,1,NFFT/2+1);
f2=2*abs(Y(1:NFFT/2+1));

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Answer 1

Jon 2023-8-2

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2003517-fft-of-bearing-signal#answer_1282387

在 MATLAB Online 中打开

vettore.mat

In the attached code, I independently calculated the spectrum and compared to your calculation. It seems correct that your data has a very strong component at 50 Hz and higher harmonics of 50 Hz. Note also in the time domain plot, the 0.02 second period (50 Hz) oscillation is clearly visible. This gives added confidence that the scaling is correct. I also see a smaller peak at approx. 28 Hz. I agree that there is nothing significant at harmonics of the rotating frequency, which you report as 1000rev/min*1 min/60s = 16.667 Hz.

So either there is extremely little imbalance in the shaft, or perhaps the shaft frequency is not actually 1000 rpm. Regarding the 50Hz component, are you in a country where the electrical line frequency is 50 Hz. If so you are likely picking up "hum" from the power supply, or nearby power lines.

% % % S=size(N);

% % % L2=S(1,1);

% % % NFFT= 2^nextpow2(L2);

% % % Y=fft(N,NFFT)/L2;

% % % f1=f/2*linspace(0,1,NFFT/2+1);

% % % f2=2*abs(Y(1:NFFT/2+1));

% Assign parameters

fs = 10e3; % sampling frequency

% Load data from data file, data is in x11

load('vettore.mat')

% Find the length of the signal

L = numel(x11);

% Make sure that length of signal is even, if it isn't throw away last

% value to make it even

if rem(L,2)~=0

x11 = x11(1:end-1);

L = L-1;

end

% Compute the fft

Y = fft(x11);

% Compute the two sided spectrum P2

P2 = abs(Y/L);

% Compute the single sided spectrum P1

P1 = P2(1:L/2+1);

P1(2:end-1) = 2*P1(2:end-1);

% Define the frequency domain f and plot the single sided spectrum, P1

f = fs*(0:(L/2))/L;

% Plot results

% Plot the time domain signal

t = (0:L-1)*1/fs;

figure

plot(t,x11)

xlabel("time [s]")

ylabel("x11")

title("time domain signal x11")

xlim([0,0.1]); % zoom in

% Plot spectrum

figure

plot(f,P1)

title("Single-Sided Amplitude Spectrum of X(t)")

xlabel("f [Hz]")

ylabel("|P1(f)|")

xlim([0,300])

% Compare to OP's code

S=size(x11);

L2=S(1,1);

NFFT= 2^nextpow2(L2);

Y=fft(x11,NFFT)/L2;

f1=fs/2*linspace(0,1,NFFT/2+1);

f2=2*abs(Y(1:NFFT/2+1));

figure

plot(f1,f2)

title("OP's Spectrum")

xlabel("frequency [Hz]")

ylabel("Magnitude")

xlim([0,300])

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Jon 2023-8-4

在 MATLAB Online 中打开

vettore.mat

If you have the dsp toolbox you could use the function iircomb. Here's an example

% Assign parameters

fs = 10e3; % sampling frequency

bw = 0.005; % normalized bandwith of comb notch filter

lineFreq = 50; % line frequency in Hz

% Load data from data file, data is in x11

load('vettore.mat')

% Find the length of the signal

L = numel(x11);

% Make sure that length of signal is even, if it isn't throw away last

% value to make it even

if rem(L,2)~=0

x11 = x11(1:end-1);

L = L-1;

end

% Compute the fft

Y = fft(x11);

% Compute the two sided spectrum P2

P2 = abs(Y/L);

% Compute the single sided spectrum P1

P1 = P2(1:L/2+1);

P1(2:end-1) = 2*P1(2:end-1);

% Define the frequency domain f and plot the single sided spectrum, P1

f = fs*(0:(L/2))/L;

% Plot results

% Plot the time domain signal

t = (0:L-1)*1/fs;

figure

plot(t,x11)

xlim([0,0.1]); % zoom in

% Plot spectrum

figure

plot(f,P1)

title("Single-Sided Amplitude Spectrum of X(t)")

xlabel("f [Hz]")

ylabel("|P1(f)|")

xlim([0,300])

% Implement comb filter to filter 50Hz hum and harmonics

n = ceil((fs/2)/lineFreq)*2; % number of harmonics

[b,a] = iircomb(n,bw,'notch');

h = freqz(b,a,L/2+1,fs);

figure

plot(f,20*log10(abs(h)))

xlabel("f [Hz]")

ylabel("Magnitude [dB]")

title('Comb Filter Magnitude Response ')

% Filter the signal

x11f = filter(b,a,x11);

% FFT the filtered signal

% Compute the fft

Yf = fft(x11f);

% Compute the two sided spectrum P2

P2f = abs(Yf/L);

% Compute the single sided spectrum P1

P1f = P2f(1:L/2+1);

P1f(2:end-1) = 2*P1f(2:end-1);

% Define the frequency domain f and plot the single sided spectrum, P1

f = fs*(0:(L/2))/L;

% Plot spectrum

% % % figure

% % % plot(f,P1f)

% % % title("Single-Sided Amplitude Spectrum of notch filtered X(t)")

% % % xlabel("f [Hz]")

% % % ylabel("|P1(f)|");

% Plot it again, zoomed in

figure

plot(f,P1f)

title("Single-Sided Amplitude Spectrum of notch filtered X(t)")

xlabel("f [Hz]")

ylabel("|P1(f)|");

xlim([0,300])

Eris Prifti 2023-8-11

Hi jon,

i have another question : is possible to impement a Homomorphic filtering demodulation for signal like bearing in matlab and can you write the code if possible pls?

Jon 2023-8-15

Sorry, I'm not familiar with that technique. In general, I can offer you advice on particular problems you are having, but don't have time to write actual applications for people. If you start to work on your Homomorphic demodulation, and have specific MATLAB problems please open up a new question so others can see it and help you.

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