[SciPy-User] Zero crossing detection

[Lafras Uys]

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> On 29 March 2010 15:31, Moritz Maus <moritz.maus at uni-jena.de> wrote:
>> Is there a way for the ODE-integrators to
>> a.) stop the simulation when a specific event occurred or
>> b.) doing a MatLab (Simulink)-like "zero crossing detection", (when
>> these timepoints are not pre-defined?)
> 
> The short answer is no. (And we apologize for this.)
> 
> There's a much more powerful set of tools available for working with
> ODEs provided in the python package pydstool; they provide many
> features including this one. Alternatively, there are a number of
> wrappers for the ODEPACK integrator LSODAR (which is included but not
> wrapped in scipy) which provides exactly this capability. Finally, you
> can get some approximation of this behviour using scipy's integrators;
> they can be made to advance "one step" of the underlying integrator at
> a time, and while in this mode they can be used to "backtrack" as far
> as the beginning of the step. So you can just keep advancing until you
> meet your criterion then backtrack.
> 
> This brings me to a warning: no ODE solver I know of is able to deal
> with "here there be dragons"-type events, that is, situations where
> you not only want the integrator to stop, but where it is not allowed
> to evaluate anything past that point (perhaps because your RHS
> function yields nonsense). The standard approach is to write RHS
> functions so that they detect out-of-bounds conditions and return some
> not-too-offensive value, hoping that this won't send the integrator
> into a frenzy.

Not SciPy either, but PySUNDIALS (http://pysundials.sourceforge.net/) is
a set of python bindings for SUNDIALS. SUNDIALS can handle triggers,
events, delays and so forth.

Lafras
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