MATLAB: Filter data that is not sampled uniformly

filternon-uniform samplingsavitzky-golay filtering

Hi, does anyone know how to filter data that's not sampled at a constant frequency? The data is based on pressure-crank angle, so I have a pressure sample at every 0.1 crank angle degree.

I'm trying to the use butter() function, but it requires a sampling frequency…

Regards, TheNovice

Best Answer

I have no mechanical engineering background, so I’m having a difficult time imaging what your data actually are. If it is constantly sampled, you could filter them with respect to the crank angle, or interpolate (resample) them at a constant sampling frequency in time. I have no idea how valid this may be, since I don’t have your data.

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MATLAB: How to to filter non-uniformly sampled data

Your approach is correct. Rather than using interp1, use the Signal Processing Toolbox resample (link) function. It incorporates an anti-aliasing filter, and is preferred for signal processing purposes.

A useful lowpass filter design prototype is:

Fs = 360;                                                   % Sampling Frequency (Hz)
Fn = Fs/2;                                                  % Nyquist Frequency (Hz)
Wp = 100/Fn;                                                % Passband Frequency (Normalised)
Ws = 101/Fn;                                                % Stopband Frequency (Normalised)
Rp =   1;                                                   % Passband Ripple (dB)
Rs = 150;                                                   % Stopband Ripple (dB)
[n,Ws] = cheb2ord(Wp,Ws,Rp,Rs);                             % Filter Order
[z,p,k] = cheby2(n,Rs,Ws);                                  % Filter Design
[soslp,glp] = zp2sos(z,p,k);                                % Convert To Second-Order-Section For Stability
figure(3)
freqz(soslp, 2^16, Fs)                                      % Filter Bode Plot
filtered_signal = filtfilt(soslp, glp, original_signal);    % Filter Signal

Make appropriate changes for your signal.

MATLAB: How to convert engine cylinder pressure data (bar) to frequency (Hz) using FFT or STFT

Save the pressure values (calculated or measured) as a vector, with a vector of the matching sampling times. (A higher sampling frequency is better.) Calculate the sampling frequency from the sampling times, the Nyquist frequency from the sampling frequency, then see the documentation for the fft (link) function to calculate and plot the fft.

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Related Solutions

MATLAB: How to to filter non-uniformly sampled data

MATLAB: How to convert engine cylinder pressure data (bar) to frequency (Hz) using FFT or STFT

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