[Robotics] Linked group and rigid body constraints - problem.

[Morten Lind]

Thanks for replying, Benoit. 

Yes, I did experiment with the compounding. One immediate issue with it is 
that of the center of mass. One of the arms of the L-shape must be the rigid 
body, and the center of mass will be at that rigid body's center. 

If the center of mass is placed inside one of the arms, the motion will be 
weird and un-realistic. This will be apparent if letting a bunch of Ls fall 
into a box.

If trying to place the center of the rigid body between the arms, where the 
real physical center of mass is, and then trying to stack the Ls side by side, 
there will be a collision of the center of mass with one of the arms of the L 
lying "inside" it.

However, it seems that, otherwise, the collision and such works ...

Best,
Morten Lind.

On Wednesday 10 March 2010 21:04:50 Benoit Bolsee wrote:
> Did you try compound shape instead of constraint?
> 
> Compound is easy to setup and allows to create a complex rigid body made
> of several simpler elements based on parent-child relationship.
> 
> Starting from the two objects that form the L-shape, you make one parent
> of the other, select rigid body for the parent, static for the child,
> choose an appropriate collision shape for each and enable compound on
> both. The parent will combine the shape of child and you should get the
> same effect as with the constraint. As far as I know, this setup should
> work with linked library.
> 
> /Benoit
> 
> > -----Original Message-----
> > From: robotics-bounces at blender.org
> > [mailto:robotics-bounces at blender.org] On Behalf Of Morten Lind
> > Sent: vendredi 26 février 2010 8:48
> > To: Blender and Robotics
> > Subject: [Robotics] Linked group and rigid body constraints - problem.
> >
> >
> > Hi all
> >
> >  Background: In a manufacturing simulation I use a lot of
> > work pieces which
> > are handled by robots an transported by AGVs, as well as hung
> > on conveyor
> > carriers for painting. A very simple workpiece is an
> > L-shaped, square cross
> > section aluminium tube; a part of an office chair.
> >
> >  Having many such workpieces, like 100s, it is practical to
> > make it in a group
> > and then link all others, possibly from an external
> > .blend-"library". Due to
> > the non-convex nature of the L-shape, and the fact that this
> > non-convexity is
> > exploited in stacking the workpieces, I wish to model it as
> > two box-shaped
> > rigid bodies, each bounded by "box". These rigid bodies are
> > invisible, and one
> > should be doubly hinged to the other. To one of these, all
> > other geometric
> > parts are attached, notably a non-colliding object
> > representing the actual
> > geometric shape of the workpiece, and a small invisible cube
> > for a gripper
> > tool to sense contact with.
> >
> >  The method and the structure of the L-shaped compound works
> > splendid, so that
> > I get a very fine and realistic scenario if the work piece is
> > dropped into a
> > box with other parts or dropped on top of an AGV. This can be
> > verified when
> > douplicating the group directly, creating a couple of workpieces.
> >
> >  However, when linking the group to make linked duplicates,
> > it seems that the
> > constraints disappear, and the two rigid bodies just
> > separates. Trying out
> > another method where everything is parented to the "master"
> > rigid body does
> > not work, since the other rigid body will then have a parent
> > and not move;
> > dynamics for a rigid body with a parent seems to totally
> > disable dynamics for
> > that body. Trying to mend this parenting-problem by
> > un-parenting and replacing
> > by a couple of hinge-constraints on start-up does not alter
> > the immobility of
> > the rigid body that was born with a parent.
> >
> >  I have been trying various ways to solve the problem for
> > some days now. I
> > have searched a lot on Blender Artists, as well as generally
> > with Google, and
> > I have read a lot of docs and examples. Nothing seems to even
> > hint on the
> > existence of this problem. If anyone has any suggestions on
> > how to solve it,
> > it would be received warm-heartedly by me! Alternatively, but
> > not preferably,
> > if anyone could inform me that this is un-acheivable with
> > Blender 2.49b and
> > it's game engine, I would be grateful for that information,
> > and settle with
> > what I can get then.
> >
> >  Best regards,
> >  Morten Lind.
> > _______________________________________________
> > Robotics mailing list
> > Robotics at blender.org
> > http://lists.blender.org/mailman/listinfo/robo> tics
> 
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