How can i find this minima?

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Peter Bu
Peter Bu on 7 Oct 2017
Commented: Cedric Wannaz on 8 Oct 2017
Hi everyone!
I have a question regarding finding peaks. I already used the "peakfinder"- function by Nathanael Yoder to find the 5 peaks (maximum), but I also have to find the marked values (minima). By doing it with his function I do not get them properly
. Does someone have an idea how to solve it? Many thanks in advance!

Accepted Answer

Star Strider
Star Strider on 7 Oct 2017
One approach is to use diff or gradient to calculate the approximate derivative of your data. Then ‘threshold’ the negative values of the derivative to be as large as necessary to detect the abrupt negative changes. Those will give you the points where the change is greatest. Calculating the points with the red arrows should be relatively straightforward.
The easiest way to find those using findpeaks is probably to first negate your signal (so the ‘dips’ become peaks), then use findpeaks to find the ‘peaks’ of the negated signal. Subtract as many indices as you need from those returned by findpeaks to get the points indicated by the arrows.

More Answers (1)

Cedric Wannaz
Cedric Wannaz on 7 Oct 2017
While it is not a very robust method, these points are the last points of the curve before the derivative becomes very negative. You could get them simply with:
pos = find( diff( signal ) < -4 ) ;
or use [diff(signal)<-4], false] for performing logical indexing, and you may want to subtract some offset to the positions if you want to get points a little before the signal starts going down.
Cedric Wannaz
Cedric Wannaz on 8 Oct 2017
My mistake, I updated variable names to simplify the notation and I forgot to change this one! See the updated comment.

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