RE: [Algorithms] Articulated multibody physics

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We implemented the Featherstone method (iirc) as seen in Mirtich's thesis
for Stuntman. Last I knew, it was a bit slow, and you needed to add bogus
extra degrees of freedom to stop the answers exploding numerically.

Jamie

-----Original Message-----
From: gda...@li...
[mailto:gda...@li...]On Behalf Of
mark_me
Sent: 21 March 2002 04:52
To: Algorithms List
Subject: RE: [Algorithms] Articulated multibody physics

I'm not sure if I understood your method right, It seems to me that you will
end up with broken joints immediately.
To do this correctly you need to consider the joints' forces , and if you
have more than one force applied on the multi body simultaneously ( on
different bones ) then you need to solve a linear system of equations ,
which is here much easier than solving for resting contacts , but you will
still have the problem of accuracy and you might need a velocity or position
stabilizer to make sure your bones are always connected. I haven't tried
that myself but Maybe one of the guys who tried it can tell us if it's fast
enough for real time systems.

There are some other ways like the relaxation method that Jacobsen described
, and I think some people even talked about using the penalty method here ,
but I can't see how it could be suitable for this purpose.

-----Original Message-----
From: gda...@li...
[mailto:gda...@li...]On Behalf Of Ron
Hay
Sent: March 20, 2002 7:25 AM
To: Algorithms List
Subject: [Algorithms] Articulated multibody physics

I'm currently working on a skeletal animation system for our software,
and have decided it would be extremely useful to be able to use the
skeleton to be used not just for animation, but also for vehicle (and
otherwise) physics.  So a motorcycle can be described as a single
main "bone" (the body), with a 1-dof rotator on the front connected
to a translational bone with a spring attached, and a similar setup
without the rotation on the back, represented the front and rear
wheels, respectively.

So, I did a web search for "articulated chains" and "multibody dynamics"
and "articulated dynamics" and so on.  I found many, many papers
and sites, which showed the difficulties of such a problem.  And here
I thought inverse kinematics was complicated.

Anyway, my coworker and I thought of a simplification that we think
would work decently, if not entirely 100% correct.  Which is fine by us.
I wanted to run it past the resident list physics gurus to get their
comments.

Basically, given a force on a bone, we first apply it directly to the
center of mass for the entire linkage.  Then, we apply a torque to
the entire linkage based on the force and its distance from the center
of mass.  This is pretty standard for a single body, from what I understand.

Seperately, for the bone the force is applied to, we determine the torque
on the bone by finding the component of the original force that is
perpendicular to the bone.  Then, we propagate the component of the force
that is parallel to the bone to the bones it is connected to, and continue
on figuring out the torques and "propagation forces" recursively through
the linkage.

I'm going to be coding this up today and tomorrow, and I'll let people that
are interested know how it turns out, but if anyone has any comments or
experiences with such a problem, please send them!

  Ron Hay
  Developer
  Cybernet Systems.

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