How can I disregard part of the signal when calculating midline of my data signal?

I have a waveforem that starts regular oscillations after like 2 seconds of running the program. How can I get midline of the remaining signal after T=2, so basically disreagrading the first 2 seconds of the output wave. Also, is there an easy way to find frequency of this signal(Again while disregarding the first 2 seconds of wave)?I have attached a sample wave for reference.

Currently I am just using:

lnsig=length(y);

midline = mean(y)*ones(1,lnsig)

Also, if there's a way that I can keep changing the time after which I calculate for midline that would be great!

Answer 1

[Mathieu NOE]

hello

see code below for midline and signal oscillation frequency computation (cycle to cycle "zero" crossing detection)

a PID tuning problem ?

data = extract_data_from_figures('2.fig');
VN = data.names;
data = data.Y;
%     {'X'      }
%     {'midline'}
%     {'target' }
%     {'y vs x' }

x = data(:,1);
y_target = data(:,3);
y = data(:,4);
 
idx = (x>=1); % valid data for t > 1 s
midline = mean(y(idx));


%%find frequency of this signal
threshold = midline; % 
t0_pos1 = find_zc(x,y,threshold);
period = diff(t0_pos1); % delta time of crossing points
freq = 1./period; % signal frequency

figure(1)
subplot(2,1,1),plot(x,y,'b',x,y_target,'g',x(idx),midline*ones(size(x(idx))),'k--',t0_pos1,midline*ones(size(t0_pos1)),'dr','Linewidth',2);
legend('signal','target','midline','crossing points');
xlabel('Time(s)')
subplot(2,1,2),plot(t0_pos1(2:end),freq,'b.-','linewidth',2,'markersize',12);grid on
xlim([min(x) max(x)]);
legend('signal rate (frequency)');
xlabel('Time(s)')
ylabel('Freq(Hz)')

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
function [Zx] = find_zc(x,y,threshold)

% positive slope "zero" crossing detection, using linear interpolation
y = y - threshold;
zci = @(data) find(diff(sign(data))>0);  %define function: returns indices of +ZCs
ix=zci(y);                      %find indices of + zero crossings of x
ZeroX = @(x0,y0,x1,y1) x0 - (y0.*(x0 - x1))./(y0 - y1); % Interpolated x value for Zero-Crossing 
Zx = ZeroX(x(ix),y(ix),x(ix+1),y(ix+1));

end


function data = extract_data_from_figures(filename)
%%
% 
% Input  : File name <filename.fig> 
%          with multiple plots in a single file
% Output : struct data
%        : data.names  contains names of the display object Yvalues
%        : data.Y      contains the actual plot values withthe first column
%                      containing the x-values
%
% Written by Chetanya Puri, 2019
% Last Modified: Nov 6, 2019
%

fig = openfig(filename); % Open figure and assign it to fig object
dataObjs = findobj(fig,'-property','YData'); % Find all graphic objects with YData, in our case line values
xval        = dataObjs(1).XData; % Find the X-axis value
Ymat = [xval(:)]; % Create a matrix with first column of x values
for i=1:length(dataObjs)
    legend_name{i,1} = dataObjs(i).DisplayName;
    yval        = dataObjs(i).YData;
    Ymat = [Ymat yval(:)]; % Keep appending column vectors
end
close(fig); % close the figure
data.names = ['X';legend_name]; 
data.Y = Ymat;
end

[Smriti Mohan Shetty]

Thanks! I will not be using figures to extract data, I will be using a mat file like the attached here, how would the code change for that?

[Mathieu NOE]

hello again

see below

data = load('Newexp0.5Gain_1.6Target.mat');
x = data.theta(1,:);
y = data.theta(2,:);
 
idx = (x>=1); % valid data for t > 1 s
midline = mean(y(idx));
%%find frequency of this signal
threshold = midline; % 
t0_pos1 = find_zc(x,y,threshold);
period = diff(t0_pos1); % delta time of crossing points
freq = 1./period; % signal frequency
figure(1)
subplot(2,1,1),plot(x,y,'b',x(idx),midline*ones(size(x(idx))),'k--',t0_pos1,midline*ones(size(t0_pos1)),'dr','Linewidth',2);
legend('signal','midline','crossing points');
xlabel('Time(s)')
subplot(2,1,2),plot(t0_pos1(2:end),freq,'b.-','linewidth',2,'markersize',12);grid on
xlim([min(x) max(x)]);
legend('signal rate (frequency)');
xlabel('Time(s)')
ylabel('Freq(Hz)')
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
function [Zx] = find_zc(x,y,threshold)
% positive slope "zero" crossing detection, using linear interpolation
y = y - threshold;
zci = @(data) find(diff(sign(data))>0);  %define function: returns indices of +ZCs
ix=zci(y);                      %find indices of + zero crossings of x
ZeroX = @(x0,y0,x1,y1) x0 - (y0.*(x0 - x1))./(y0 - y1); % Interpolated x value for Zero-Crossing 
Zx = ZeroX(x(ix),y(ix),x(ix+1),y(ix+1));
end

[Smriti Mohan Shetty]

Can I also find time period of wave, after 1 second?

[Mathieu NOE]

yes this is done by the code above