I have a dynamic system. The equation is ddot = (M)\(F - K*d); Where M,K are (4,4) constant matrix and F and d are vector. I want to solve this equation using ODE45.

In my case The force vector F is variable. The values are changing with time. So to solve ode I've used a for loop. To update the solution of ODE45 I need to use something like this 'T(i,:)'. but it's showing an error 'Subscripted assignment dimension mismatch.' How can I solve an ode for variable dc input?

for i = 1:100
p1 = (power.dat)';
F = p1(:,i);
d0 = [333,333,333,333];
tspan = 0:0.1:1;
[t,T(i,:)] = ode45('Heat',tspan,d0);
end

Answer 1

[Walter Roberson]

When you use more than two entries in tspan (as you have done here), the second output of ode45 will be length(tspan) by length(d0) -- that is, it will have the integrals of each of the 4 elements, at each of the time steps. If you only need one of those 4 columns, then output to a variable and select the column you need.

[Md Shahidul Alam]

Thank you Mr. Roberson for your kind response. Actually my problem is the force vector F is a function of tspan's step size. The force vector is changing with the step size. The equation,ddot = (M)\(F(step size) - K*d);Normally in all tutorials,examples, I've found that the force vector is constant. In my case it's changing with step size. Is it possible to use ODE function in that case? Thanks in advance!