# [ODE] lib question

Bill Sellers wis at mac.com
Wed Jun 14 03:13:14 MST 2006

```Sometimes you really do need to use very low values for your
integration step. My horse simulation sometimes falls apart with an
integration step of 1e-3 but works quite happily at 1e-4 and when
I've been playing around with stiff springs I've had to drop to 1e-5.
This is double precision and dWorldStep using ERP="0.2" and CFM="1e-10".

Cheers

Bill
On 14 Jun 2006, at 10:28, LR wrote:

>  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
>
> 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
> http://robotica.uv.es/~ignacio/                Tlf. 96 354 3671
>
>
>
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> http://q12.org/mailman/listinfo/ode

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