How can i found the infelction point from the data and remove the data before the first and last infelction points.

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JINU SUDHAKARAN Mr 2023-1-16

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https://ww2.mathworks.cn/matlabcentral/answers/1894820-how-can-i-found-the-infelction-point-from-the-data-and-remove-the-data-before-the-first-and-last-inf

评论： Star Strider 2023-1-27

data.xlsx

I need to find the inflection point from the experimental data using matlab. Kindly help me for that. Thanks in advance

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Answer 1

Star Strider 2023-1-16

在 MATLAB Online 中打开

The data are quite noisy. After filtering them, it is not obvious what sort of inflection point you want or how to define it.

M1 = readmatrix('https://www.mathworks.com/matlabcentral/answers/uploaded_files/1265285/data.xlsx')

M1 = 1430×2

0 32.2683 0.0050 32.2683 0.0100 32.2683 0.0150 32.2683 0.0200 32.2683 0.0250 32.2683 0.0300 32.2683 0.0350 32.2683 0.0400 32.2683 0.0450 32.2683

x = M1(:,1);

y = M1(:,2);

% yf = sgolayfilt(y, 3, 451);

Fs = 1/mean(diff(x))

Fs = 200

Fn = Fs/2;

L = numel(x);

NFFT = 2^nextpow2(L);

FTy = fft((y-mean(y)).*hann(L),NFFT)/L;

Fv = linspace(0, 1, NFFT/2+1)*Fn;

Iv = 1:numel(Fv);

figure

semilogy(Fv, abs(FTy(Iv))*2)

grid

xlim([0 10])

yf = lowpass(y, 0.5, Fs, 'ImpulseResponse','iir');

dyfdx = gradient(yf) ./ gradient(x);

d2yfdx2 = gradient(dyfdx) ./ gradient(x);

figure

yyaxis left

plot(x, y, 'DisplayName','Unfiltered Data')

hold on

plot(x, yf, '-r', 'DisplayName','Filtered Data')

hold off

ylabel('Data')

yyaxis right

plot(x, dyfdx, 'DisplayName','First Derivative Of Filtered Data')

hold on

plot(x, d2yfdx2, 'DisplayName','Second Derivative Of Filtered Data')

hold off

yline(0, '-g')

ylabel('Derivatives')

grid

legend('Location','best')

.

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Star Strider 2023-1-20

在 MATLAB Online 中打开

Finding the second derivative zero-crossings is straightforward.

Try something like this —

M1 = readmatrix('https://www.mathworks.com/matlabcentral/answers/uploaded_files/1265285/data.xlsx')

M1 = 1430×2

0 32.2683 0.0050 32.2683 0.0100 32.2683 0.0150 32.2683 0.0200 32.2683 0.0250 32.2683 0.0300 32.2683 0.0350 32.2683 0.0400 32.2683 0.0450 32.2683

x = M1(:,1);

y = M1(:,2);

% yf = sgolayfilt(y, 3, 451);

Fs = 1/mean(diff(x))

Fs = 200

Fn = Fs/2;

L = numel(x);

NFFT = 2^nextpow2(L);

FTy = fft((y-mean(y)).*hann(L),NFFT)/L;

Fv = linspace(0, 1, NFFT/2+1)*Fn;

Iv = 1:numel(Fv);

Fco = 0.5;

figure

plot(Fv, mag2db(abs(FTy(Iv))*2), 'DisplayName','Fourier Transform')

grid

xlim([0 10])

xlabel('Frequency')

ylabel('Magnitude (dB)')

xline(Fco, '-r', 'DisplayName',sprintf('Lowpass Filter Cutoff Frequency: %.2f',Fco))

legend('Location','best')

yf = lowpass(y, Fco, Fs, 'ImpulseResponse','iir');

dyfdx = gradient(yf) ./ gradient(x);

d2yfdx2 = gradient(dyfdx) ./ gradient(x);

d2zix = find(diff(sign(d2yfdx2)));

for k = 1:numel(d2zix) % Second Derivative Zero-Crossings

idxrng = max(1,d2zix(k)-1) : min(d2zix(k)+1, numel(x));

xz(k,:) = interp1(d2yfdx2(idxrng), x(idxrng), 0);

yfz(k,:) = interp1(x, yf, xz(k));

end

Function_Value_At_Inflection_Points = table(xz, yfz, 'VariableNames',{'x','y'})

Function_Value_At_Inflection_Points = 3×2 table

x y ______ ______ 1.4709 34.213 2.4326 36.47 3.188 38.038

figure

yyaxis left

plot(x, y, 'DisplayName','Unfiltered Data')

hold on

plot(x, yf, '-r', 'DisplayName','Filtered Data')

hold off

ylabel('Data')

yyaxis right

plot(x, dyfdx, 'DisplayName','First Derivative Of Filtered Data')

hold on

plot(x, d2yfdx2, 'DisplayName','Second Derivative Of Filtered Data')

hold off

yline(0, '-g', 'DisplayName','Derivatives 0 Line')

for k = 1:numel(xz)

xline(xz(k), '--m', sprintf('x = %.2f',xz(k)), 'DisplayName', sprintf('Inflection Points x = %.2f',xz(k)))

end

ylabel('Derivatives')

grid

legend('Location','best')

This entirely depends on how you want to define the inflection points and how you want to process the signal before calculating them.

.

JINU SUDHAKARAN Mr 2023-1-27

THANK YOU

Star Strider 2023-1-27

My pleasure!

If my Answer helped you solve your problem, please Accept it!

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Answer 2

Steven Lord 2023-1-16

The ischange, islocalmin, and/or islocalmax functions may be of use to you, as might the corresponding Live Editor Tasks Find Change Points and Find Local Extrema.

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How can i found the infelction point from the data and remove the data before the first and last infelction points.

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How can i found the infelction point from the data and remove the data before the first and last infelction points.

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