[ODE] coulomb friction model?

[Erin Catto]

An interesting constraint that involves more than 2 bodies is a gear
constraint. For example, joint 1 rotates twice for every meter joint 2
translates. In a joint coordinate formulation this is trivial, but for a
body coordinate system like ODE, it is more difficult.

Erin

-----Original Message-----
From: ode-bounces at q12.org [mailto:ode-bounces at q12.org] On Behalf Of Nguyen
Binh
Sent: Wednesday, October 20, 2004 9:20 PM
To: Graham Fyffe
Cc: ode at q12.org
Subject: Re: [ODE] coulomb friction model?

On Wed, 20 Oct 2004 16:06:23 -0300, Graham Fyffe <gfyffe at gmail.com> wrote:
> Hi all!  I'm trying to use a coulomb friction model for my own
> simulation stuff.  I am in the middle of decyphering Baraff's paper
> and website, where he prescribes using a modified Dantzig method of
> sorts.  My question is, what algorithm does ODE use? 

    Hi-low Dantzig on Step (normal)
    a sort of iterative on StepFast (obsolete now, IMHO)
    Better Iterative (K. Murty) in QuickSep(recommended)

    All the meterials in Barraff paper seems to archaic now. I don't
know any modern physics lib use it now.

> I've tried
> parsing through the step.cpp file, but I can't extract the algorithm.

    I will be surprised if you could find the algorithm at first read. :)

> There seem to be optimizations based on the two-body joints that mask
> the true meaning to me.  I've read about the jacobians and matrices in
> the joints how-to document as well, but it is very light on what
> actually goes into the matrix in order to handle the "max force"
> properties of friction.  I've also red Russ's articles on the subject
> in the Games Programming Gems books, but again it is light on details
> about how to construct the matrix.  Can anybody help me out?

   The thing Russ wrote in GPG is just how to create a joint.So it's a
bit cryptic.

> BTW, I'm not using ODE for this project because the model is very
> simple; I only care about angular velocities of some parts and the
> rest of the system is totally fixed.  Also, ODE is unusable for it
> because I have linear constraints involving up to 5 of the bodies,
> whereas ODE only supports 2 :(

   Hmmm..... IMHO no physical simulation libary exists now use more
than 2 bodies constraints. All the 5 bodies constraints must be
converted to 2 bodies-constraints system.

-- 
Nguyen Binh
www.glassegg.com
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