Matlab Help:Stop ode45 with OutpuFcn

[stevenumber5]

Hi everybody,
I'm trying to solve this first-order ODEs system:and I'd like to stop the solution when y, i.e u(3), becomes negative:

%system
function rhs=particle(t,u)
global k
rhs=[u(2);
    -k*u(1)*sqrt(u(2)*u(2)+u(4)*u(4));
    u(4);
    -9.81-k*abs(u(4))*sqrt(u(2)*u(2)+u(4)*u(4))];

where u(1)=x,u(2)=dx/dt,u(3)=y,u(4)=dy/dt

I use ode45 as solver, and in order to stop it the option OutputFcn.
It stops after few values, when y is already positive.
Here is the script

%ode script

global k

t0=0;tend=10;

k=0.065;

a=20*pi/180;

u0=[0 20*cos(a) 1.5 20sin(a)];

options = odeset('OutputFcn',@output);

[time,result]=ode45('particle',[t0,tend],u0,options);
%output function

function status = output(t,y,status)

[val,pos]=min(y)

status=(val>=0)

if status==1

tend=t(pos);

end

Thank you all
Best Regards

 

[NateD]

It looks like your output function is not properly set up to return an appropriate status value. The output function should return either 1 or 0 depending on whether the integration should continue or not. You can use an if statement to check if y is negative and then return 0 if it is, otherwise return 1.

 

[oswaler]

,

I would like to suggest a few things to improve the ode45 solver and output function in this case. Firstly, it is important to properly define the variables and parameters used in the system and script. This will make it easier for others to understand and reproduce the results. Additionally, it would be helpful to provide some context or background information on the system being modeled and the significance of stopping the solution at a negative y value.

In terms of the ode45 solver, it may be beneficial to try different integration methods or adjust the tolerances to see if that changes the behavior of the solution. It is also worth considering if the system itself needs to be modified in order to accurately model the desired behavior.

As for the output function, it may be helpful to include more information in the status output, such as the time and corresponding values of the variables at which the solution was stopped. This will provide more insight into the behavior of the system and can aid in further analysis.

Overall, it is important to carefully consider the system and solver being used, and to continuously evaluate and adjust the approach in order to accurately model and understand the behavior of the system.