How to find the zero crossing in x and time data sets?

156 views (last 30 days)
How can I find the zero crossing in the data sets?
figure()
plot(x,t)
  3 Comments
vimal kumar chawda
vimal kumar chawda on 10 Jun 2021
Yes we dont have 0.000000000000 so we have range is 10^-3 so all the value fall under 10^-3 are accepted. Now how can we do it as we have two sets x& t?

Sign in to comment.

Accepted Answer

Scott MacKenzie
Scott MacKenzie on 9 Jun 2021
Edited: Scott MacKenzie on 10 Jun 2021
Here's what I put together. The variable fCross is what you are looking for.
% data from posted matlab.mat files
f = readmatrix('testdata1.txt');
t = readmatrix('testdata2.txt');
tiledlayout(3,1);
nexttile;
plot(t,f);
hold on;
axis([1 10 -1.2 1.2]);
nexttile;
fAbove = f .* (f >= 0);
fBelow = f .* (f <= 0);
area(t, fAbove, 'FaceColor', 'r');
hold on;
area(t, fBelow, 'FaceColor', 'g');
axis([1 10 -1.2 1.2]);
nexttile;
fCrossRaw = find(diff(fAbove>0));
fCross = fCrossRaw ./ length(t)*10; % as per axes
plot(fCross, zeros(1,length(fCross)), '*r');
hold on;
axis([1 10 -1.2 1.2]);
xline(fCross, 'color', [.7 .7 .7]);
yline(0, 'color', [.7 .7 .7]);

More Answers (2)

Stefan Schuberth
Stefan Schuberth on 27 Jul 2022
Edited: Stefan Schuberth on 27 Jul 2022
If you have (x,y) data and want to do it without using loops try that:
i=find(y(1:end-1).*y(2:end)<0); % index of zero crossings
m=(y(i+1)-y(i))./(x(i+1)-x(i)); % slope
x0=-y(i)./m+x(i); % x coordinates of zero crossings linear interpolated

Joel Lynch
Joel Lynch on 9 Jun 2021
Edited: Joel Lynch on 9 Jun 2021
idx = find( f(2:end).*f(1:end-1)<0 )
Will return the left-hand indicies of cross-over points.
To get the exact X-values where the cross-over occurs, you would have to do some linear intepolation (inverted)
t_zero = zeros(size(idx));
for i=1:numel(idx)
j = idx(i); % Need both the index in the idx/t_zero vector, and the f/t vector
t_zero(i) = interp1( f(j:j+1), t(j:j+1), 0.0, 'linear' );
end
Note: this will fail if the cross-over happens on the last i value (i+1 would extend outside the range of the dataset)
  4 Comments
Scott MacKenzie
Scott MacKenzie on 11 Jun 2021
Edited: Scott MacKenzie on 29 Jun 2021
Yes, Joel's code gives the exact cross-over point. Bear in mind, however, that this is exact for the linearly interpolated data. The actual data are empirical, so it is not possible to know the exact cross-over point.
It probably doesn't matter much in this case, since the data appear to be gathered at a high sampling rate.

Sign in to comment.

Categories

Find more on Mathematics in Help Center and File Exchange

Products


Release

R2021a

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!