# [ODE] why low mass geoms crash the simulation.

Ignacio García Fernández ignacio.garcia at uv.es
Wed Jun 14 03:55:56 MST 2006

```El mié, 14-06-2006 a las 11:28 +0200, LR escribió:
>  Hi,
>
> Changing the time step does not resolves my mass problem, then i done a very
> little simulation:
> 2 boxes (designs a L) connected with a hinge and a plane for the ground:
>
> If the boxes mass = 0,050kg and torque applyed into the hinge is 1 -->
> bodies take over the world (BANG !)
>
> If the boxes mass = 1    kg and torque applyed into the hinge is 1 --> OK
> If the boxes mass = 0,050kg and torque applyed into the hinge is between 0.1
> and 0.5 --> OK
>

Ok, it may be what M. McGuire pointed out. But if ou apply small forces,
it keeps stable. So it seems to me that it is a problem of stiffness.
[ Morgan. If it is an underflow problem, shouldn't it also crash with
small forces? ]

What stepsize are you using? With the crashing configuration, 0.05kg and
1Nm torque, use an stepsize of 0.0001

Let it go a few thousand steps, and see what happens.

> It seems that if units are not in the same range, simulation cannot resolve
> ... But with those so low values ...
>
> Is there a way to resolve it ? Or ... Does i have a problem anyway
> ???????????
>
>
> Laurent.
>
>
> -----Message d'origine-----
> De : ode-bounces at q12.org [mailto:ode-bounces at q12.org] De la part de Ignacio
> García Fernández
> Envoyé : mardi 13 juin 2006 18:47
> À : ode at q12.org
> Objet : Re: [ODE] lib question
>
> What you are experiencing is called numerical unstability.
>
> Let's do a test. Instead of increasing masses, sibstantially reduce the
> stesize of your simulation. Say, divide it by 10 or by 20. And tell us if
> this also solves the problem.
>
> The reason is that the frequency of the movement of the low mass body is
> very high, and the stepsize is not small enough to describe the solution.
>
> Hope this helps
>
> Ignacio
>
> El mar, 13-06-2006 a las 14:46 +0200, LR escribió:
> > Hy,
> >
> > I'm always trying to understant why low mass geoms crash the
> > simulation.
> >
> > A question : I'm using a plugin system: core is running simulation and
> > plugins can act on the core and on ODE. i built ODE ReleaseLib then
> > ODE code is embedded in each plugin and in the core.
> > I know it's not a good solution but at this time it's easier for me.
> >
> > My question is :
> > Does ODE keep "global" simulation variables (in this case it's normal
> > my simulation crash : plugins variables does not contains good
> > values), or does ODE keep all its variables inside an world object ?
> > (in this case pfouuu i don't understant what happend).
> >
> > Changing the mass resolves all my problems (it seems ...) but it's not
> > normal i must enter something like 0,500 kg for a little robot wheel
> > ....
> >
> > A precision : when plugins are mounted, core gives pointers to created
> > objects to the plugin
> >
> >
> > Laurent.
> >
> > _______________________________________________
> > ODE mailing list
> > ODE at q12.org
> > http://q12.org/mailman/listinfo/ode
--
May the source be with you
---------------------------------------------------------------
Ignacio García Fernández                  Instituto de Robótica
ignacio.garcia_at_uv.es                 Universidad de Valencia
http://robotica.uv.es/~ignacio/                Tlf. 96 354 3671
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