[ODE] lib question

LR logreg at free.fr
Wed Jun 14 02:28:06 MST 2006


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

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


-----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


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.
> _______________________________________________
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> ODE at q12.org
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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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