[ODE] Weird Dynamics

[whitt013]

Forwarding this to the list....

==========================================================

Hey, I think I have a good idea of what your problem might be.  Per ODE docs:

"Currently the fixed joint does not support a non-identity relative rotation
between two bodies, it only supports a relative offset."

I can't see your videos from here, so I'm not sure what kind of rotation is
involved, but I'm assuming that since they are spheres, they are rotating
before they run into each other.  If this is the case, when two spheres
contact that don't have identical rotations, the fixed joint tries to force
them to get to that orientation.  So, you might try setting both spheres'
rotation to identity before you attach the fixed joint and see what happens.

David

>===== Original Message From Alexander Klyubin <A.Kljubin@herts.ac.uk> =====
>Hello!
>
>I've been playing around with ODE for a couple of weeks now. The
>simulations I've been running tend to produce weird dynamics and even
>"explosions", although I don't believe I'm using any special features or
>too large time steps. So far I've spotted and fixed a couple of bugs in
>my code. Nevertheless, I'm still struggling with weird behavior of
>simulated bodies.
>
>
>I've created a rectangular arena consisting of four massive walls. Small
>spheres are placed into the arena. At each time step a small force is
>applied in random direction to each sphere. If two spheres collide, they
>stick to each other forever via a fixed joint.
>
>The behavior I expect is that that eventually all spheres form a big
>fixed articulated body. Moreover, the more spheres form the body, the
>less it should move, since random forces applied to each sphere should
>on the average cancel each other.
>
>What I observe when running the simulation is that aggregate articulated
>bodies indeed form. However, their behavior is really weird: they start
>rotating too fast, or gaining highly improbable impulses, or even
>exploding. You can take a look at a movie showing one such run of the
>simulation.
>   http://homepages.feis.herts.ac.uk/~ka2by/tmp/aggregation.avi (1.9 MB,
>DivX 5).
>   http://homepages.feis.herts.ac.uk/~ka2by/tmp/aggregation.mpg (3.7 MB,
>MPEG).
>
>
>As I'm pretty new to ODE I believe that bugs in my code may be
>responsible for the weird behavior. However, the code is pretty simple
>and I can't spot any more bugs in it.
>
>Any hints and suggestions appreciated.
>
>Best Regards,
>Alexander Klyubin
>
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