Change frequency level after filter
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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
采纳的回答
Star Strider
2015-5-14
0 个投票
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.
8 个评论
vu ngothanh
2015-5-14
Star Strider
2015-5-14
Use the filter design functions in the Signal Processing Toolbox to design your filter. The filter you are using with that filter call is actually not doing any filtering.
For example, your filter has b=2, a=0.01 and freqz plots it as:
b = 2;
a = 0.01;
figure(1)
freqz(b, a, 512)
I do not have your signal, and I do not know what frequencies you want your filter to pass, so I cannot be specific about your bandpass filter design.
vu ngothanh
2015-5-14
编辑：vu ngothanh
2015-5-14
here is a part of my input data
0.000000,6.200694E-6,-1.875651E-6
3.906250E-5,1.195134E-5,3.528527E-6
7.812500E-5,6.992699E-6,-1.703543E-6
0.000117,9.885240E-6,8.637580E-8
0.000156,7.302614E-6,2.633568E-6
0.000195,1.484388E-5,9.827665E-6
0.000234,9.781935E-6,1.359972E-6
0.000273,9.954110E-6,2.633568E-6
0.000312,1.470614E-5,2.977783E-6
0.000352,9.024364E-6,3.617479E-7
0.000391,8.852189E-6,-3.611039E-7
0.000430,8.886624E-6,-2.922609E-7
0.000469,1.050507E-5,7.418159E-6
0.000508,1.056979E-7,-1.302822E-5
0.000547,5.133209E-6,-9.689335E-6
0.000586,1.109046E-5,4.216958E-6
0.000625,2.688323E-6,-5.248959E-6
0.000664,6.786089E-6,-1.152799E-6
0.000703,4.926599E-6,-8.243631E-6
0.000742,6.889394E-6,-2.922609E-7
0.000781,5.064339E-6,-4.987900E-7
0.000820,7.130439E-6,-6.143919E-6
0.000859,2.102928E-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
2015-5-14
What is the sampling frequency of your data?
Are:
a=y(:,1);
your sampling times?
vu ngothanh
2015-5-15
编辑：vu ngothanh
2015-5-15
Star Strider
2015-5-15
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
Star Strider
2015-5-15
My pleasure!
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