tricky ode system from m file
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Starting from t=0 there is one equation system, then after some time, constraint is met and other system is necessary, then this 2nd system meets constrain and changes back to first and so on. I tried some ideas, but they didn't quite work.
Constraints are simple: once x(2) reaches X2 other system should turn in and once x(3) reaches X3 1st system should turn back in.
function xp=uzdevums1(t,x)
%parameters
r1 = 0.1; r2 = 1; r3 = 0.2;K1=100;K2 = 100; K3 = 100;X2=25;X3=10;speedx2 = 0.02; speedx3=0.02;
xp=zeros(3,1);
if (x(2)>X2)&&(xp(3)>=0)
xp(1)=r1*(1-x(1)/(x(2)+x(3)))*x(1);
xp(2)=r2*(1-x(2)/K2)*x(2)-x(2)*x(1)*speedx2;
xp(3)=r3*(1-x(3)/K3)*x(3);
elseif (x(3)>X3)&&(xp(2)>=0)
xp(1)=r1*(1-x(1)/(x(2)+x(3)))*x(1);
xp(2)=r2*(1-x(2)/K2)*x(2);
xp(3)=r3*(1-x(3)/K3)*x(3)-x(3)*x(1)*speedx3;
end
This is one way that doesn't quite work. I hope there is some easier way to do this, because i don't see how to do it in the hard way.
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Akzeptierte Antwort
Jan
am 7 Jan. 2013
Bearbeitet: Jan
am 7 Jan. 2013
And you want to integrate uzdevums1 by ODE45 or a similar function?
Do not do this. The ODE-integrators have a stepsize control, which is substantially confused by discontinuities. In consequence the number of steps and function evaluations can be increased very much, such that the accumulated rounding errors will influence the results.
Use event functions to get the switching points, stop the integrator and restart it with the former final values. See doc odeset.
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Ryan G
am 7 Jan. 2013
What do you mean doesn't work? It sounds like you need a new state to define what condition you are currently in. This way you will have a default operating point instead of using the if-else condition every time.
Technically I would use Simulink/Stateflow to accomplish this and it would be ideal. However, within the bounds of the ode solver you could do something like this:
if(myState == true)
%ode equations
myState = x(3)>X3)&&(xp(2)>=0;
else
%other ode equations
myState = (x(2)>X2)&&(xp(3)>=0);
end
This will keep you in the current state until the condition is met to go back to the previous (or next) state.
2 Kommentare
Jan
am 7 Jan. 2013
You could store the state persistenly, see "help persistent". But as explained in my answer, I recommend to avoid this.
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