MATLAB: How to set threshold to trigger for start and stop finding max value

MATLAB and Simulink Student Suitemax value envelope signal threshold pulsethreshold

Hi all,

I am measuring flowing particles with a system. The particle will start to induce a pulse signal once it just enters the sensing zone, and the max amplitude can be obtained around the center of the zone.

What I have considered method by used for and if function is :

Set a threshold for switch on and switch off record the data
Once the amplitude (y>threshold) which means the cell just enter the sensing zone, start the search
When the amplitude (y<threshold) which means the cell almost leave the zone, stop searching
Find the max value from this recording time interval
Also get the time interval

However, I am quite fresh in MATLAB so don't know how to write down them…

Thanks if someone can help me design this tool!

Best Answer

Can you post a diagram, plot, image, data or anything to help illustrate your situation?

http://www.mathworks.com/matlabcentral/answers/6200-tutorial-how-to-ask-a-question-on-answers-and-get-a-fast-answer

Otherwise, just try find:

indexEnters = find(y > threshold, 1, 'first'); % Index when particle enters and signa becomes big enough.
indexLeaves = find(y > threshold, 1, 'last'); % Index when particle leaves and signa becomes small again.

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MATLAB: Detecting transition in a noisy square wave

Madjer, I think simple thresholding with removal of short pulses should work. Just remove pulses that are shorter than the known width - than the minimum width you believe the rectangular pulse should be. Here's a complete demo:

% Initialization steps.  Brute force cleanup of everything currently existing to start with a clean slate.
% Just for the demo.  If you put this into your own function you'll want to get rid of the close and clear commands.
clc;    % Clear the command window.
fprintf('Beginning to run %s.m ...\n', mfilename);
close all;  % Close all figures (except those of imtool.)
clear;  % Erase all existing variables. Or clearvars if you want.
workspace;  % Make sure the workspace panel is showing.
format long g;
format compact;
fontSize = 20;
y = readmatrix('a.txt');
thresholdValue = 0.6;
% writematrix(a(:), 'a.txt');
subplot(2, 3, 1)
plot(y, 'b.');
yline(thresholdValue, 'Color', 'r', 'LineWidth', 2);
grid on;
title('Original Noisy Signal', 'FontSize', fontSize);
xlabel('x', 'FontSize', fontSize);
ylabel('y', 'FontSize', fontSize);
% Get a histogram, just for curiosity.
subplot(2, 3, 2)
histogram(y);
grid on;
title('Histogram of Original Signal', 'FontSize', fontSize);
xlabel('y (signal) value', 'FontSize', fontSize);
ylabel('Count', 'FontSize', fontSize);
% Find out where the signal is above the threshold.
% Threshold at 0.6
aboveThreshold = y > 0.9;
subplot(2, 3, 3)
bar(aboveThreshold);
grid on;
title('Above Threshold (Logical Index)', 'FontSize', fontSize);
xlabel('x', 'FontSize', fontSize);
ylabel('True or False', 'FontSize', fontSize);
% Get width of spikes above thresholdValue
props = regionprops(aboveThreshold, 'Area');
allAreas = [props.Area];
subplot(2, 3, 4)
histogram(allAreas);
grid on;
title('Histogram of Spike Widths', 'FontSize', fontSize);
xlabel('Pulse Width', 'FontSize', fontSize);
ylabel('Count', 'FontSize', fontSize);
% Get rid of spikes less than 5 in width
aboveThreshold2 = bwareafilt(aboveThreshold, [5, inf]);
subplot(2, 3, 5)
bar(aboveThreshold2);
grid on;
title('Filtered, Above Threshold', 'FontSize', fontSize);
% Get rid of gaps of value 0 that are less than 500 in width
aboveThreshold3 = ~bwareafilt(~aboveThreshold2, [500, inf]);
subplot(2, 3, 5)
plot(aboveThreshold3, 'LineWidth', 2);
grid on;
ylim([0, 1.2]);
title('Filtered, Above Threshold (Logical Index)', 'FontSize', fontSize);
xlabel('x', 'FontSize', fontSize);
ylabel('True or False', 'FontSize', fontSize);
% Now find the mean value in the high and low regions.
meanHighValue = mean(y(aboveThreshold3))
meanLowValue = mean(y(~aboveThreshold3))
% Create final signal
finalSignal = meanLowValue * ones(1, length(y)); % First create the low value.
finalSignal(aboveThreshold3) = meanHighValue; % Then create the high value.
% Plot the final signal.
subplot(2, 3, 6)
plot(finalSignal, 'LineWidth', 2);
grid on;
ylim([0, 1.2]);
title('Final, Denoised Signal', 'FontSize', fontSize);
xlabel('x', 'FontSize', fontSize);
ylabel('y', 'FontSize', fontSize);
g = gcf;
g.WindowState = 'maximized'

You can adjust the threshold, shortest pulse width, final denoised signal levels, or other parameters if you want.

MATLAB: How to obtain each each maximum value from each signal set

You have the Signal Processing Toolbox, so use the findpeaks function to get the peak amplitudes. It has a number of options so you can be certain to get only the values you want. If you want to get the amplitudes of the ‘valleys’ use findpeaks on the negative value of the signal vector.

Best Answer

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MATLAB: Detecting transition in a noisy square wave

MATLAB: How to obtain each each maximum value from each signal set

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