- #1
sinisterstarr
- 3
- 0
Homework Statement
I'm trying to solve a system of two second order linear differential equations with the ode45 function. It is a two degree of freedom problem with 2nd order derivatives of both variables, u and theta. I believe that's referred to as a "stiff matrix").
I'm very confident in the A and B matrices themselves, but not on the running of ode45. One of the two problems I'm having is the values in my y matrix. I'm just not sure what should be there. 1's have the same result as zeros. I know y is the dependent variable, I just don't understand what the function would want me to type there. What does its being zeros mean? What does its being ones mean?
My second problem is that I only know how to give a step input of force (f). As far as I can tell, the force input will be 628 Newtons the entire time. I tried to say f=628 when t<=0.015, but there is no "t" in the ode45 function. I'd like to give it an impulse if I can.
Homework Equations
My code so far is:
u0 = [0 ; 0];
theta0 = [0 ; 0];
FUN = @FUNA;
T = [0 10];
y0 = [u0 ; theta0];
[TT,YY] = ode45(FUN,T,y0);
plot(TT,YY(:,3));
function dy = FUNA(t,y)
%Constants of system
M = 70-5.876;
m = 5.876;
a = (((0.05)^2)+((0.13)^2))^0.5;
IG= 0.0233;
k = 500;
g = 9.81;
N = 2; %number of degrees of freedom
%Define sub matrices
MM=[(M+m) m*a ; m*a (IG+(m*a^2))];
K =[ 0 0 ; 0 (k-m*g*a)];
C =[0 0 ; 0 d*a*a];
F =[1; 0];
MI= inv(MM);
Z = zeros(N,N);
I = eye(N);
%Define matrices
A = [ Z I ; -MI*K -MI*C];
B = [ zeros(N,1); (MI*F) ];
f = 628;
y = [0; 0; 0; 0];
dy= A*y + B*f;
The Attempt at a Solution
I'd like to split this into four single order differential equations, but the second order derivative of both variables seems to stop me. I've run this code and I actually get plots, but they are either flat lines or ramp functions, depending on which of the four outputs I choose. I'm sure it's my use of the y matrix that's beating me.