Input signal and arguments in Ode45. Not enough imput arguments

Hi everybody!

I have an issue when using Ode45. I want to resolve a second order diferential equation where some parameters are fixed and one is a sinusoidal signal.

First of all I get the data from the main:

    a=1;
    b=1;
    c=1;
    x_in has the values of a sinusoidal signal
    [t,x_sp]=ode45(@Dinamica,[0 20],[0 0]);

The function:

    function dx_sp=Dinamica(t,x_sp,a,b,c,x_in)
        u=x_sp(1)
        v=x_sp(2)
        dx_sp=(x_in*a*b^2)-(2*c*b*v)-(u*b^2)
    end

¿How should I introuduce those values into my function?

Matlab says: not enough imput arguments.

Thanks!

Answer 1

[Bjorn Gustavsson]

The function dinamica expects 6 input arguments. However, the function, f that ode45 takes as input is handled as a function of two input arguments, so inside ode45 that function is called like this:

dydt = f(t,y);

Then inside your dinamica it crashes because it is expecting the 6 inputs. You have to change the call to something like this:

[t,x_sp]=ode45(@(t,x) Dinamica(t,x,a,b,c,x_in),[0 20],[0 0]);

However, inside dinamica you have a multiplication with the entire x_in array, that is every value of something you claim to be something varying harmonically. That will not work. You have to estimate the value of x_in at the time t. To do that you can do something like this:

function dx_sp=Dinamica(t,x_sp,a,b,c,t_in,x_in)
  u=x_sp(1);
  v=x_sp(2);
  x_in_at_t = interp1(t_in,x_in,t,'pchip'); 
  dx_sp=(x_in_at_t*a*b^2)-(2*c*b*v)-(u*b^2);
end

and change the call to:

[t,x_sp]=ode45(@(t,x) Dinamica(t,x,a,b,c,t_in,x_in),[0 20],[0 0]);

HTH

Answer 2

[Walter Roberson]

http://www.mathworks.com/help/matlab/math/parameterizing-functions.html

[Walter Roberson]

However:

x_in has the values of a sinusoidal signal

That implies that it is non-scalar.

dx_sp=(x_in*a*b^2)-(2*c*b*v)-(u*b^2)

All of x_in is used there, so dx_sp would be non-scalar.

    [t,x_sp]=ode45(@Dinamica,[0 20],[0 0]);

The boundary conditions are [0 0], a vector of length 2. Your function has to return a column vector the same length, so length 2. Therefore your code would fail unless x_in is a column vector of length 2.