# 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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### Answers (2)

Star Strider
on 16 Jan 2023

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')

x = M1(:,1);

y = M1(:,2);

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

Fs = 1/mean(diff(x))

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')

.

##### 8 Comments

Star Strider
on 27 Jan 2023

My pleasure!

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

.

Steven Lord
on 16 Jan 2023

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