# Locating point on x-axis where the slope is decreasing

3 views (last 30 days)
Tessa Kol on 10 Mar 2021
Commented: Star Strider on 10 Mar 2021
Dear all,
I used the following code to plot the angle over time:
figure
hold on
grid on
plot(Time, theta,'linewidth',1.5)
xlabel('Time [s]','FontWeight','bold')
ylabel('\theta [deg]','FontWeight','bold')
xlim([0 round(Time(end))])
legend('Opening angle','Location','northeast')
title('Opening angle')
The resulting graph is shown below. I want to index at what time the slope is decreasing. How can I do that?
The variables Time and theta to plot this graph can be found in the data.mat file. Star Strider on 10 Mar 2021
A reasonably robust way is to use the ischange function:
Time = D.Time;
theta = D.theta;
[TF,m,b] = ischange(theta, 'linear', 'Threshold',5);
chgpts = [find(TF); m(TF); b(TF)].';
chgidx = find(chgpts(:,2)<0,1);
figure
plot(Time, theta,'linewidth',1.5)
hold on
plot(Time(chgpts(chgidx(:,1))), theta(chgpts(chgidx(:,1))), 'xr')
hold off
grid on
xlabel('Time [s]','FontWeight','bold')
ylabel('\theta [deg]','FontWeight','bold')
xlim([0 round(Time(end))])
legend('Opening angle','Location','northeast')
title('Opening angle')
producing: The ‘Time’ value is then:
ChangePointTime = Time(chgpts(chgidx(:,1)));
that here is 6.01 seconds.
Star Strider on 10 Mar 2021
As always, my pleasure!

Sargondjani on 10 Mar 2021
Bascially you want to get the index when the change in theta becomes negative:
dtheta = diff(theta);
index = find(dtheta<0,1);%the 1 refers to the first entry where the statement is true

R2020a

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