how do i get the timestep of odesolver

Hello,

i do need the timestep in my equation. Is there a build in way to do that or do i have to monitor it somehow by storing it in a variable between iteration ?

Kind regards

Moritz

Answer 1

[Nathan Crosty]

The ODE solvers actually return time... You don't need to do anything extra.

[T,Y] = solver(odefun,tspan,y0)

Where T is the time, and solver is your choice of an ODE function.

Answer 2

[Moritz]

Yeah, that is obvious but i do need the timestep in my ode. With a fixed time step it would be easy but with a variable timestep solver i am not sure how to do that.

[Nathan Crosty]

The time that gets returned contains the actual "step time" that you are looking for. For instance, you can tell how long certain points of your simulation took with a variable step solver.

[Jan]

@Moritz: What exactly is a time-step in an ODE solver? Should steps rejected by the step-size control be considered or not?

Perhaps your problem would be more clear, if you show us the details of the function to be integrated.

[Richard Brown]

I suggest reformulating your problem so you don't need the time step. You can use the actual time t, but the step length is the business of the ODE solver, not your equation ...

[Moritz]

Richard, you are probably right. I am currently working on it.

If you are interested:

The Formulas: www.ing-mat.udec.cl/~rburger/papers/bck.pdf

It was formula (66) which confused me, but i think i can replace it with equation (56).

The numerical scheme is described on page 4545. I think the formulas are written for an implementation in e.g C++. Using loops is far to slow in matlab.

I solved the same model for centrifugation with the method of lines and ode45. But it was much easier for that case.

So i am rewriting the equations in differential form. Probably it would be possible to code a matrix. But i never did that and i am faster with the MOL.