How to set proper configuration to obtain peaks of my data

Question

Andriy Voshchenko 2022-6-23

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https://ww2.mathworks.cn/matlabcentral/answers/1746475-how-to-set-proper-configuration-to-obtain-peaks-of-my-data

评论： Star Strider 2022-6-27

Hi!

I want to extract peaks from my response spectrum for the future modal combination alalysis. My spectras have the following image if I use findpeaks command (this graphic is pretty dominant for one frequency, some others may have different form)

:

I´ve considered using the next settings:

findpeaks(F(:,2),F(:,1),"MinPeakHeight",5,"MinPeakDistance",2)

In this case the results are the following which is what I actually want

Nevertheless, if I change the previous line slightly changing the peak distance I no longer capture the main peak

findpeaks(F(:,2),F(:,1),"MinPeakHeight",5,"MinPeakDistance",1)

Instead it captures one of the first peaks of the data. My question is, how can I configure it so it only captures one peak for a certain X interval and this peak always being the highest value there? As I already mentiones, I have quite a few data sets to analyze and I´d like to make it less manual and avoid changing settings for each case.

Tnank you!

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Andriy Voshchenko 2022-6-23

Smoothing a data isn´t really an option since it reduces the peaks amplitude significally (thos peaks are response for the resonance and i don´t want to "smoothe" them), I just want it to capture the max value in each interval, is it possible?

Increasing peak distance is kinda lazy solution because it works in this case but my natural frequencies are 1.3, 2.6, 3.7. 3.9, etc......... Depending on load case I may have peaks as close as 1Hz, I need to tell the programm to somehow retain only the max one.

The other solution is to create a "for" loop and try to extract peaks in predefined intervals but I don´t really like this solution neither because of the probability of it not detecting/detecting extra peaks depending on my intervals separation.

Mathieu NOE 2022-6-23

编辑：Mathieu NOE 2022-6-23

I like to be challenged ... do you have some files with a few representative data ?

... I'd like to see what I can obtain.

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Answer 1

Star Strider 2022-6-23

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https://ww2.mathworks.cn/matlabcentral/answers/1746475-how-to-set-proper-configuration-to-obtain-peaks-of-my-data#answer_991935

It would help to have some representative data.

However, since the data are resonances, and the desired result seems to indicate that the peaks are possibly integer harmonics of the greatest peak, one way to approach this is first to take tha max of the data (returning the maximum and its index), and then use the x-value of the maximum (or the index of the maximum, depending on your findpeaks call) to set the 'MinPeakDistance' parameter, perhaps a 90% or so of the x-value or the index value. (Using 'MinPeakProminence' to define the peaks in noisy data might also be an option, however the data do not appear to have much noise. It would be necessary to experiment with that option.)

This involves finding the maximum and its index before calling findpeaks, however that might be the easiest way to get the desired result.

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Andriy Voshchenko 2022-6-23

在 MATLAB Online 中打开

Thank you for your answer! The solution you are talking about works pretty good in case the peak response is the first resonance. However, the results may vary drastically. Just to give some context, I´m getting the response spectras from Ansys as real + imaginary part of force/moment and amplitude+phase of the displacement. Those response spectras depend on the load case, damping ratio and the point where I observe them (there are a few supports in the structure, usually want to see the reaction forces there and also displacement of some unconstricted point).

For future analysis i want to extract the peak values, so i obtain the absolute value of the complex response

Fcomplex=Freaction(:,2)+i*Freaction(:,3); %complex reactions
Fvalue=abs(Fcomplex); % magnitude

And later on I try to obtain those peaks in order to apply some modal combination technique (CQC, SRSS, ABS). All that just to estimate the peak time response to the load spectra (kinda new in those things so if I´m missing something please let me know)

There are a few cases and each of them contains a few points of interest also. The response spectras may vary a lot, as example

Or also

I was trying to obtain those response peaks somehow and apply the combination methods, what do you think would be the best option?

Now that I think about it, i DO have the modes of the structure so i can just load them on X axis and somehow tell the program to find a peak near that value? How can I do that? Since the peak isnt necessary going to be on the loaded mode shape but +/-error so it has to detect the maximum value in some proximity...... Maybe define that error manually and do something like max(function) on interval [mode-error,mode+error]

I was thinking "findpeaks" could do the work but if there is any other method please let me know.

p.s. Attach the picture of modal analysis of the first 12 modes as example of what values will be included (obviously not all of them have response since the mass participation is very different)

Mathieu NOE 2022-6-27

hello again

@Andriy Voshchenko

the coding is not complicated but I'm worried that the list of modes is much longer than the number of peaks present in the response spectra as computed by @Star Strider

what do we do with the extra ones ? also the "matching" is not super good between spectra values and modes values

Star Strider 2022-6-27

在 MATLAB Online 中打开

That is actually straightforward using the interp1 function:

Freaction = readmatrix('https://www.mathworks.com/matlabcentral/answers/uploaded_files/1043710/Freaction.xlsx');

ModeFreqs = readmatrix('https://www.mathworks.com/matlabcentral/answers/uploaded_files/1046865/modes.xlsx');

%% Imaginary to real

Fcomplex=Freaction(:,2)+i*Freaction(:,3); %complex reactions

Fvalue=abs(Fcomplex); % magnitude

F=[Freaction(:,1) Fvalue];

[pks,locs] = findpeaks(F(:,2), 'MinPeakProminence', 5);

Freqs = F(locs,1);

Amplitudev = interp1(F(:,1), F(:,2), ModeFreqs);

Mode_Amplitude = table(ModeFreqs, Amplitudev)

Mode_Amplitude = 10×2 table

ModeFreqs Amplitudev _________ __________ 1.3421 11.299 3.6806 16.174 3.9112 5.9781 3.9429 7.1333 4.9696 55.494 5.1433 59.957 5.3496 74.118 6.2745 37.595 6.4024 28.312 6.8875 14.667

figure

plot(F(:,1), F(:,2))

hold on

plot(F(locs,1), pks, '^r')

hold off

grid

fpos = get(gcf, 'Position');

set(gcf, 'Position', fpos+[-50 -100 300 200]); % Stretch Limits To Inprove Readability

% text(F(locs,1), pks, compose('A = %6.1f\nF = %4.1f Hz',[pks, F(locs,1)]), 'Horiz','center', 'Vert','bottom')

text(ModeFreqs, Amplitudev, compose(' \\leftarrow F = %4.1f Hz, A = %6.1f',[ ModeFreqs, Amplitudev]), 'Horiz','left', 'Vert','middle', 'Rotation',90, 'FontSize',8)

The figure is a bit difficult to read because some of the mode frequencies are close together.

.

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Answer 2

Andriy Voshchenko 2022-6-27

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https://ww2.mathworks.cn/matlabcentral/answers/1746475-how-to-set-proper-configuration-to-obtain-peaks-of-my-data#answer_994415

That is the best solution so far for spedctras which excitate a lot of modes, Thank you @Star Strider!

Depending on the case I guess I can use a simple findpeaks or a more sophisticated solution you provided.

Also, @Mathieu NOE, answering your questions: you find all the peaks for all the modes and simply filter them (set some minimum frequency and don´t count the results below it) and yeah, I´m aware of the matching problem, there is nothing to do with it since its due to the structural damping and that is where I want the solution actually, to obtain the max value which is close to my freq and not just value for the response for that freq.

I guess with all the info you guys provided I can keep going without problem, thanks a lot!

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Star Strider 2022-6-27

As always, my pleasure!

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