MATLAB: How do i add a function that needs inputs to the main code? (Nonlinear pendulum code)

equation of motionfunctioninput valuesMATLABmultiple functionsnonlinearnonlinear pendulumode45pendulumplottingvariables

Hi guys,
I have the following two functions, pend_l and pend_n and the script file pend_solve.
the functions contain a variable "wsq" which needs to accept input values of L,M, b, so that the resulting value of wsq is substituted in the equations in
pend_l and pend_n.
I have been able to find the value of wsq as a seperate function, but i need to consolidate it with the rest of the code.
I do not know if i should add the expansion of wsq to the functions or to the script file, so that the output, with the plots and all, takes the value of wsq into consideration, after i input the values of L, M, and b.
Below is how i find the value of wsq seperately
function wsq = findwsq 
prompt1 = 'Enter value of L';
prompt2 = 'Enter value of M';
prompt3 = 'Enter value of b';
L = input(prompt1)
M = input(prompt2)
b = input(prompt3)
wsq = (12*9.81*L)/((4*(L.^2))+(b.^2)-(12*L*M)+(12*(M.^2))); 
disp(wsq)
end
My function files are:
pend_l
function xdot = pend_l(t1,x)
wsq=3.5;         %i need to replace the constant value (arbitrarily taken as 3.5) of wsq with the above function 
xdot = [x(2); -wsq*x(1)];
i need to replace the above wsq value with the function wsq such that pend_l computes xdot using the inputted values of L, M, b
similarly, i have
pend_n
function ydot = pend_n(t2,y)
wsq=3.5;         % same value of wsq in both cases is required
ydot = [y(2); -wsq*sin(y(1))];
and my script file,
pend_solve
clear all;
clc;
clf;
tic;
tspan = 0:0.01:10;
a=pi/2;
b=0;
x0 = [a; b];
[t1,x] = ode45(@pend_l,tspan,x0);
X1 = x(:,1);
X2 = x(:,2);
y0 = [a ; b];
[t2,y] = ode45(@pend_n,tspan,y0);
Y1 = y(:,1);
Y2 = y(:,2);
figure(1);
subplot(2,2,1);
plot(t1,X1); 
xlabel('Time (s)');
ylabel('Displacement (rad)');
hold on;
grid on;
plot(t2,Y1);
% legend('Linear','Non Linear');
subplot(2,2,2);
% figure(2);
plot(t1,X2);
xlabel('Time (s)');
ylabel('Velocity (rad/s)');
hold on;
grid on;
plot(t2,Y2);
subplot(2,2,3);
plot(X1,X2);
hold on;
plot(Y1,Y2);
xlabel('Displacement (rad)');
ylabel('Velocity (rad/s)');
grid on;
toc;
upon executing pend_solve with fixed value of wsq (without using the function wsq), i get three plots.
How do i make it so that the function wsq becomes a part of the main code such that the plots are plotted upon taking the computed value of wsq into consideration?

Best Answer

To integrate wsq into your main code, you can do the following:
  1. Update the definition of pend_l and pend_n to take the wsq parameter:
function xdot = pend_l(t1,x,wsq)
    xdot = [x(2); -wsq*x(1)];
end
function ydot = pend_n(t2,y,wsq)
    ydot = [y(2); -wsq*sin(y(1))];
end
2. Update pend_solve to pass in wsq. Since ode45 only take function handles that accept two inputs, we have to define an anonymous function that calls pend_l and pend_n and pass in wsq to them. Relevant MATLAB documentation can be found here. Below I'm reproducing only a portion of your code and modified it:
wsq = 3.5; %Replace this with a call to findwsq function
[t1,x] = ode45(@(t,y) pend_l(t,y,wsq),tspan,x0);
...
[t2,y] = ode45(@(t,y) pend_n(t,y,wsq),tspan,y0);
...
3. Now you can use findwsq function to set wsq according to user input
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