How do I specify parameters to send to my ODE function when calling it using ode45?

Question

Josh Halff 2018-4-18

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/395904-how-do-i-specify-parameters-to-send-to-my-ode-function-when-calling-it-using-ode45

回答： Star Strider 2018-4-18

I'm trying to write a general function to propagate an orbit from initial position and velocity vectors using ode45, but I want to be able to specify mu in the script from which I call the funtion (via ode45).

Here is the function I'm propagating:

function [xdot] = RVorbitPropogation(t, x)
r = sqrt((x(1))^2 + (x(2))^2 + (x(3))^2); %Define R
xdot = [x(4); x(5); x(6); -(mu/((r)^3))*x(1); -(mu/((r)^3))*x(2); -(mu/((r)^3))*x(3); mu;]; %Define equation
end

I want to specify mu in the script so I can use this function for different bodies. Here is where I call the function:

[R_converted,V_converted] = oe2rv(oe, mu);
y0 = [R_converted(1), R_converted(2), R_converted(3), V_converted(1), V_converted(2), V_converted(3)];
[T, Y] = ode45(@RVorbitPropogation,tspan, y0);

Is there a way to add another input to ODE45 for mu? I've tried using

function [xdot] = RVorbitPropogation(t, x, mu)
r = sqrt((x(1))^2 + (x(2))^2 + (x(3))^2); %Define R
xdot = [x(4); x(5); x(6); -(mu/((r)^3))*x(1); -(mu/((r)^3))*x(2); -(mu/((r)^3))*x(3); mu;]; %Define equation
end

and

[R_converted,V_converted] = oe2rv(oe, mu);
y0 = [R_converted(1), R_converted(2), R_converted(3), V_converted(1), V_converted(2), V_converted(3)];
[T, Y] = ode45(@RVorbitPropogation,tspan, y0);

but that doesn't work because of the way ode45 understands inputs. Thanks for the help!

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Answer 1

Star Strider 2018-4-18

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/395904-how-do-i-specify-parameters-to-send-to-my-ode-function-when-calling-it-using-ode45#answer_315920

在 MATLAB Online 中打开

This works:

function [xdot] = RVorbitPropogation(t, x, mu)
r = sqrt((x(1))^2 + (x(2))^2 + (x(3))^2); %Define R
xdot = [x(4); x(5); x(6); -(mu/((r)^3))*x(1); -(mu/((r)^3))*x(2); -(mu/((r)^3))*x(3); mu;]; %Define equation
end

mu = 42;                                                        % Create ‘mu’
tspan = [0 5];                                                  % Create ‘tspan’
y0 = [zeros(6,1); 1];                                           % Create ‘y0’
[T, Y] = ode45(@(t,x) RVorbitPropogation(t,x,mu),tspan, y0);

figure(1)
plot(T,Y)
grid

Note that in the ode45 call, ode45 only ‘sees’ the ‘(t,x)’ arguments, allowing the anonymous function call to ‘RVorbitPropogation’ to acquire ‘mu’ from the workspace.