Change frequency level after filter

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vu ngothanh 2015-5-14

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https://ww2.mathworks.cn/matlabcentral/answers/216650-change-frequency-level-after-filter

评论： Star Strider 2015-5-15

First grahp í the source data. After I filter it, I got the grahp 2. But as you can see, in the second grahp, the yelow line(I don't know how to call it) is not the same with the 0-0 line(blue). So I want to change the frequency level from 0,5 to 0. How can I do it.

Thanks in advance

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Answer 1

Star Strider 2015-5-14

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You seem to be using a low-pass filter. Use a bandpass filter instead, with a low-frequency cutoff that will eliminate the D-C offset (value at 0 Hz), and with the high-frequency cutoff you used in your original low-pass filter.

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vu ngothanh 2015-5-14

编辑：vu ngothanh 2015-5-14

在 MATLAB Online 中打开

here is a part of my input data

000000,6.200694E-6,-1.875651E-6
906250E-5,1.195134E-5,3.528527E-6
812500E-5,6.992699E-6,-1.703543E-6
000117,9.885240E-6,8.637580E-8
000156,7.302614E-6,2.633568E-6
000195,1.484388E-5,9.827665E-6
000234,9.781935E-6,1.359972E-6
000273,9.954110E-6,2.633568E-6
000312,1.470614E-5,2.977783E-6
000352,9.024364E-6,3.617479E-7
000391,8.852189E-6,-3.611039E-7
000430,8.886624E-6,-2.922609E-7
000469,1.050507E-5,7.418159E-6
000508,1.056979E-7,-1.302822E-5
000547,5.133209E-6,-9.689335E-6
000586,1.109046E-5,4.216958E-6
000625,2.688323E-6,-5.248959E-6
000664,6.786089E-6,-1.152799E-6
000703,4.926599E-6,-8.243631E-6
000742,6.889394E-6,-2.922609E-7
000781,5.064339E-6,-4.987900E-7
000820,7.130439E-6,-6.143919E-6
000859,2.102928E-6,-3.699991E-6

This is my code:

 y=load('vibration_019.txt');
  a=y(:,1);
  c=y(:,3);
  b=y(:,2);
  figure(1);
  subplot(2,1,1);
  plot(a,b);    
  subplot(2,1,2);
  [B,A]=butter(2,0.01);
  b1=filter(B,A,b);
  plot(a,b1);

Now, when I fix the code as you said. the result like this

But, I want like this:

Star Strider 2015-5-15

在 MATLAB Online 中打开

There is not enough data to test the filter, but there is enough available to design it. I designed a bandpass filter with a lower frequency cutoff of 500 Hz and an upper frequency cutoff of 10 KHz. That should do what you want it to.

The code:

a = y(:,1);
b = y(:,2);
c = y(:,3);
% 
Ts = mean(diff(a));                 % Analyse Signal Spectrum
Fs = 1/Ts;
Fn = Fs/2;
fa = fft(a)/length(a);
fb = fft(b)/length(b);
Fv = linspace(0, 1, fix(length(fa)/2)+1)*Fn;
Iv = 1:length(Fv);
% 
figure(1)                           % Plot Fourier Transformed Signals
subplot(2,1,1)
plot(Fv, abs(fa(Iv)))
grid
subplot(2,1,2)
plot(Fv, abs(fb(Iv)))
grid
% 
Wp = [500 10000]/Fn;                 % Design Bandpass Filter
Ws = Wp.*[0.8  1/0.8];
Rp =  1;
Rs = 10;
[n,Wn] = buttord(Wp, Ws, Rp, Rs);
[b,a] = butter(n,Wn);
[sos,g] = tf2sos(b,a);
% 
figure(2)
freqz(sos, 128, Fs)                 % Check Filter Performance
% 
bfilt = filtfilt(sos, g, b);        % Filter Signals
cfilt = filtfilt(sos, g, c);
% 
figure(3)                           % Plot Filtered Signals
subplot(2,1,1)
plot(a, bfilt)
title('Velocity 1 Filtered')
grid
subplot(2,1,2)
plot(a, cfilt)
title('Velocity 2 Filtered')
grid

The documentation for the filter design is in the Signal Processing Toolbox documentation. It is relatively straightforward.

vu ngothanh 2015-5-15

Thank you very much. I has just fix it as you say. it work. Thank you one more time

Star Strider 2015-5-15

My pleasure!

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Change frequency level after filter

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