having problem plotting ramp function

the line f(0001:3000)=((f0/3)*dt);

was supposed to maek the f(t) plot having a ramp function of f0/3 * time when time is at 0~3

I tried f(0001:3000)=((f0/3)*time) and it got an error which says "Unable to perform assignment because the left and right sides have a different number of elements."

please help

[Shivam]

Hi Yin,

Could you attach the data dt for me to debug the issue? You can use the paperclick icon to attach the data.

[Tien Yin]

here's the file i've been working on

[Tien Yin]

by the way here's how i want my f(t) to look like

[VBBV]

@Tien Yin You can modify the present code as below. A better way to do is to use piecewise function as mentioned by @Sam Chak

clc
clearvars
m=5;
k=18;
c=1.2;
f0=100;
wn=sqrt(k/m);
cc=2*m*wn;
Dr=c/cc;
wd=wn*sqrt(1-Dr^2);    
time=linspace(0,10,5001);
idx1 = find(time==3); % 
idx2 = find(time==5); % use find     
f=zeros(1,length(time));
for kx = 1:numel(time)-1
if kx <= idx1
    f(kx+1)=f(kx) + (f0/3)*(time(kx+1)-time(kx));
elseif kx>idx1 & kx<=idx2
    f(kx+1) = 100;
else
    f(kx+1) = 0;
end 
end    

figure;
plot(time,f);xlabel('t(s)'); ylabel('f(t)') 
xticks(1:1:10);grid
axis([0 10 0 110])   

[Tien Yin]

Thanks! So far I think using heaviside is a clearer way to present function too

Answer 1

[Shivam]

Hi Yin,

You can use the below script to get the desired figures.

m = 5;
k = 18;
c = 1.2;
f0 = 100;
wn = sqrt(k/m);
cc = 2*m*wn;
Dr = c/cc;
wd = wn*sqrt(1-Dr^2);

dt = 0.01; % assumed dt
time = 0:dt:10;

f = zeros(1, length(time));
f(1:round(3/dt)) = (f0/3) * (0:dt:3-dt); % Linear increase from 0 to 3 seconds
f(round(3/dt)+1:round(5/dt)) = f0; % Constant from 3 to 5 seconds
% f remains 0 after 5 seconds

g = (1/(m*wd))*exp(-Dr*wn*time).*sin(wd*time);
x = conv(f, g)*dt;
x = x(1:length(time)); % Ensure x is the same length as time

figure;
subplot(221); plot(time, f, 'r'); xlabel('t(s)'); ylabel('f(t)');
subplot(222); plot(time, g); xlabel('t(s)'); ylabel('g(t)');
subplot(223); plot(time, x); xlabel('t(s)'); ylabel('位移(m)');

Hope it helps.

[Tien Yin]

THANKS ALOT

Appreciate your hard work

Answer 2

[Sam Chak]

Hi @Tien Yin

The signal in the image is a piecewise function that consists of three sub-functions defined over different intervals: , , . You can apply the piecewise() function from the Symbolic Math Toolbox, or you can use the MATLAB indexing approach (though this method may not be suitable for publication in a journal).

Alternatively, you can use a direct math formula to combine the sub-functions into a one-line equation, as shown below. Since the second and third sub-functions can be described using a Heaviside function, this effectively reduces the representation to "two" sub-functions.

If you are interested, you can find another example here:

https://www.mathworks.com/matlabcentral/answers/2026819-how-to-find-the-transfer-function-of-a-simulink-output-plot#answer_1371299

t   = linspace(0, 10, 1001);
t1  = 3;
t2  = 5;

% Functions at subintervals
f1  = 100/3*t;                  % y1, Ramp function      at t < t1
f2  = 100*heaviside(t2 - t);    % y2, Heaviside function at t > t1

% Applying the Piecewise Function Put Together (PFPT) formula
f   = f1 + (f2 - f1).*heaviside(t - t1);

% Plot
plot(t, f, 'linewidth', 1.5), grid on, ylim([-50, 150])
title('Piecewise Function')
xlabel('t')
ylabel('f(t)')

[Tien Yin]

thanks alot!

i'm new to MATLAB now I learnt a new way to display piecewise function!