MATLAB: How to plot an inflection point from a temperature profile data source

inflection pointMATLAB and Simulink Student Suite

Hello everyone,
I have been trying to plot a tangent to a curve. My data source is a temperature profile from a TCL (temperature control lab). It has two heaters and the data from the arduino Leonardo is collected every second. I have a set of 2000 data points from the curve. While plotting the inflection point, I am not able to get the tangent to the curve. I tried to smooth the curve but no proper tangent could be found. I have the file attached. Can anyone please help me?

Best Answer

hello
here you are
enjoy it ! 
% sliding avg method
temp_out = myslidingavg(temp, 50);
figure(1),
semilogx(time,temp,'b',time,temp_out,'r','linewidth',2);
% first derivative
dx = mean(diff(time));
dy = [0; diff(temp_out)./dx];
dy_out = myslidingavg(dy, 25);
figure(2),
plot(time,dy,'b',time,dy_out,'r','linewidth',2);
% point of inflection = fin peak of first derivative (but not the first
% sample transient)
ind = find(time>time(1)+5 & time<time(1)+200);
[peak,loc] = max(dy_out(ind));
time_inflection = time(ind(loc));
y_inflection = temp_out(ind(loc));
dy_inflection = dy_out(ind(loc));
% tangent equation
temp_tang = y_inflection+dy_inflection*(time-time_inflection);
figure(3),
plot(time,temp,'b',time,temp_out,'r',time(ind),temp_tang(ind),'--k','linewidth',2);
legend('raw temp','smoothed','tangent at inlection point');
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