Errors using ode45

I'm getting the following errors when trying to use ode45, and I'm not sure how to fix them.

function:

function xdot = eom(t, x, Ix, Iy, Iz)
%define constants
%Assume no external moments
Mx = 0;
My = 0;
Mz = 0;

xdot(1) = (Mx./Ix) - (Iz-Iy)*x(2)*x(3)/Ix;
xdot(2) = (My./Iy) - (Ix-Iz)*x(1)*x(3)/Iy;
xdot(3) = (Mz./Iz) - (Iy-Ix)*x(1)*x(2)/Iz;
xdot = xdot';


return

ode:

%Define PMOIs
Ix = 2887; %kg-m^2
Iy = 2887; %kg-m^2
Iz = 5106; %km-m^2

%Define initial conditions
x1_0 = 0.6; % OmegaX rad/s
x2_0 = 0; % OmegaY rad/s
x3_0 = 1.1; % OmegaZ rad/s
IC = [x1_0; x2_0; x3_0]; %initial conditions matrix

%define timestep
t0 = 0; %initial time
tf = 10; 
interval = [t0 tf]; %timestep

%Numerically solve ODE
[t,x] = ode45('eom', interval, IC,Ix,Iy,Iz);
omegaX = x(:,1);
omegaY = x(:,2);
omegaZ = x(:,3);

Errors:

Error using  / 
Matrix dimensions must agree.

Error in eom (line 8)
xdot(1) = (Mx./Ix) - (Iz-Iy)*x(2)*x(3)/Ix;

Error in odearguments (line 90)
f0 = feval(ode,t0,y0,args{:});   % ODE15I sets args{1} to yp0.

Error in ode45 (line 115)
  odearguments(FcnHandlesUsed, solver_name, ode, tspan, y0, options, varargin);

Error in HW11 (line 18)
[t,x] = ode45('eom', interval, IC,Ix,Iy,Iz);

I can't figure out what the errors mean or how to get rid of them. If somebody could please help me properly use ode45 to find the solution to this function that would be great!

Answer 1

[Stephan]

%Define PMOIs
Ix = 2887; %kg-m^2
Iy = 2887; %kg-m^2
Iz = 5106; %km-m^2

%Define initial conditions
x1_0 = 0.6; % OmegaX rad/s
x2_0 = 0; % OmegaY rad/s
x3_0 = 1.1; % OmegaZ rad/s
IC = [x1_0; x2_0; x3_0]; %initial conditions matrix

%define timestep
t0 = 0; %initial time
tf = 10; 
interval = [t0 tf]; %timestep

%Numerically solve ODE
[t,x] = ode45(@(t,x)eom(t, x, Ix, Iy, Iz), interval,IC);
omegaX = x(:,1);
omegaY = x(:,2);
omegaZ = x(:,3);
plot(t,x)


function xdot = eom(~, x, Ix, Iy, Iz)
%define constants
%Assume no external moments
Mx = 0;
My = 0;
Mz = 0;

xdot(1) = (Mx./Ix) - (Iz-Iy)*x(2)*x(3)/Ix;
xdot(2) = (My./Iy) - (Ix-Iz)*x(1)*x(3)/Iy;
xdot(3) = (Mz./Iz) - (Iy-Ix)*x(1)*x(2)/Iz;
xdot = xdot';

end

[Vishank Battar]

Thank you so much! That worked.

Could you please explain what "~" does?

And could you explain what this means as well?

@(t,x)eom(t, x, Ix, Iy, Iz)

I've never called a function in this manner. I understand the eom(t, x, Ix, Iy, Iz) part, but I don't understand what putting @(t,x) in front of it does.

Thank you again for your help!

[Walter Roberson]

function xdot = eom(~, x, Ix, Iy, Iz)

means that the function will be called with a first parameter but that value of the parameter will be ignored inside the function. You could also have coded

function xdot = eom(t, x, Ix, Iy, Iz)

but then MATLAB would helpfully warn you that you did not use t inside the function -- and a fair bit of the time, a warning like that is useful to point out that you might have accidentally forgotten some code, or that you might have accidentally mismatched between the names passed in and the names you use in the code.

[Walter Roberson]

@(t,x) eom(t, x, Ix, Iy, Iz)

means that you are creating an anonymous function that will accept two positional parameter. At the time the anonymous function is called, it will accept the two parameters and will call the function eom. Whatever was passed in as the first parameter to the anonymous function will be passed as the first parameter to eom -- which is indicated by matching the name of the first parameter to the use of that same variable. Likewise, the second parameter passed to the anonymous function will be passed as the second parameter to eom, indicated by matching the name of the dummy second parameter to that same variable name in the body of the code.

At the time the anonymous function is built, the then-current values of Ix, Iy, and Iz will be recorded and will be copied into the datastructure representing the anonymous function. When the anonymous function is run, the values will be pulled out of the data structure and passed in to appropriate positions to the function eom.

[Stephan]

Thank you Walter.

off topic BTW today I reported 14 or 15 accounts to the TMW contact you provided. Hope this helps to get rid of this problem.