MATLAB: Remove noise from strain gauge measurements

remove noisestrain gauge

Hi, I have been working on filtering strain gauge data but having problems with filtering the Strain gauge data. My data looks as shown below.

After using low-pass 5th order butter worth filter with a cut off frequency of 85Hzt, i am able to cut down the noise but i still could not able to get a smooth curve that i am expecting.Here is how data looks after filtering

As you can see in the image, the data still shows some noise. Can anyone help in filtering the data for a smoothed curve?

i tried using other cut off frequencies but nothing is working.

Any help will be a lot appreciated.

Thank you.

Best Answer

You probably need to look closely at your signal with the fft (link), specifically the code between the first (top) two plot figures, and use that information to determine your cutoff frequencies.

With respect to designing the filter, the Signal Processing Toolbox designfilt function is likely easiest to use. My IIR filter design procedure is here: How to design a lowpass filter for ocean wave data in Matlab?

Related Solutions

MATLAB: How to Butterworth Filter with Bandpass [10 500] with sampling rate 1000

You have to divide your passband and stopband frequencies by the Nyquist frequency to normalise them:

Fs = 1000;                                          % Sampling Frequency
Fn = Fs/2;                                          % Nyquist Frequency
Wp = [10  495]/Fn;                                  % Norrmalised Passband Frequencies
Ws = [5   499]/Fn;                                  % Normalised Stopband Frequencies
Rp =  1;                                            % Passband Ripple (dB)
Rs = 25;                                            % Stopband Ripple (dB)
[n,Wn] = buttord(Wp, Ws, Rp, Rs);                   % Optimal Filter Order
[b,a] = butter(n, Wn);                              % Calculate Filter Coefficients
[sos,g] = tf2sos(b,a);                              % Convert To Second-Order Sections For Stability
figure(1)
freqz(sos, 4096, Fs)                                % Filter Bode Plot

You cannot have any frequency exactly at zero or exactly at the Nyquist frequency, so I designed your passband to be close enough to them to be stable. In your filter, you can also use a high-pass design with a lower cutoff of 10 Hz, since your upper passband is at the Nyquist frequency. The passband and stopband ripple values are necessary to specify in the design but irrelevant to a Butterworth filter. These are acceptable values for most filter designs (for example Chebyshev) that require them.

You would use the ‘sos’ and ‘g’ variables in your actual filter:

filtered_signal = filtfilt(sos, g, signal);

(This design works well in R2016a and should also work in earlier releases.)

MATLAB: How to design IIR highpass filter with cutoff frequency of 20Hz and FIR bandpass filter with cutoff frequency of 10Hz and 15Hz

I can’t design a fir filter to those specifications with any of the fir design functions, regardless of the order of the filter. The normalised frequencies are simply too low and the passband too narrow.

The best I can do is a Chebyshev Type II filter:

Fs  = 8000;
Fn = Fs/2;
Wp = [10  15]/Fn;
Ws = [ 6  25]/Fn;
Rp = 10;
Rs = 40;
[n,Ws] = cheb2ord(Wp,Ws,Rp,Rs);
[b,a] = cheby2(n,Rs,Ws);
[sos,g] = tf2sos(b,a);
figure(2)
freqz(sos, 2^16, Fs)
set(subplot(2,1,1), 'Xlim', [0 25])
set(subplot(2,1,2), 'Xlim', [0 25])

I also had to increase the fft length 2^16 and narrow the frequency axis to [0 25] to see the details of the passband. Note that a Chebyshev Type II filter has a flat passband, so you can state the passband ripple, ‘Rp’, to be essentially anything you want.

If you want a fir bandpass filter with those characteristics, you would probably have to design separate highpass and lowpass filters and cascade them. The best way to do this would be with dfilt objects, using dfilt.cascade.

Best Answer

Related Solutions

MATLAB: How to Butterworth Filter with Bandpass [10 500] with sampling rate 1000

MATLAB: How to design IIR highpass filter with cutoff frequency of 20Hz and FIR bandpass filter with cutoff frequency of 10Hz and 15Hz

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