# Physics Simulation Forum

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 Posted: Mon Feb 22, 2010 8:35 pm

Joined: Tue Jun 30, 2009 6:56 am
Posts: 20
Hi, I'm reading the PHD Thesis "Stable, Robust, and Versatile Multibody Dynamics Animation" of Kenny Erleben in order to implement a rigid body framework.

There is something I don't understand. At the page 99 there is the complete LCP problem formulation (see expression 4.216). Inside this expression, there is some matrices called A_aux and a vector called b_aux.

At the page 100, it's written that A_aux and b_aux correspond to a permutation of the third row and column in 4.61. What does this mean ? I don't understand what are the exact expressions for the A_aux matrix and the b_aux vector.

Could anyone help me to understand what should be put in the matrix A_aux and in the vector b_aux ?

Daniel

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 Posted: Fri Feb 26, 2010 4:25 pm

Joined: Mon Oct 12, 2009 6:23 pm
Posts: 39
Since no-one who knows what they're talking about is helping, I'll try

Looks like Aaux,baux are the friction coupling terms (E = all ones, mu = friction scaling). I don't totally understand what he has done with the friction direction vectors....

I find erleben great for coming up with the constraint jacobians, but hard to use for the full shebam. I combined his stuff with:
http://www.gphysics.com/files/IterativeDynamics.pdf

In particular, I believe Erleben is using the nonsymmetric friction formulation which is also a bit harder to solve than that in the iterative dynamics paper....

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 Posted: Sun Feb 28, 2010 8:11 pm

Joined: Tue Jun 30, 2009 6:56 am
Posts: 20
Thanks for your answer but I still don't understand. Actually, I have a problem to understand the sentence :

"A_aux and b_aux correspond to a permutation of the third row and column in 4.61."

For me, the third row in 4.61 is the matrix:

(-E^t mu 0)

and the third column in 4.61 is the matrix:

(E
0
0)

therefore, for me we have:

A_aux = (-E^t mu 0)

and

b_aux =
(E
0
0)

But I don't really believe that it's true !

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 Posted: Thu Mar 04, 2010 8:04 pm

Joined: Tue Jun 30, 2009 6:56 am
Posts: 20
Really, no one can help me with my problem ? No one can give me some clue about the meaning of this sentence ? It will help me a lot !

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 Posted: Thu Mar 04, 2010 9:49 pm

Joined: Sun Jun 26, 2005 6:43 pm
Posts: 3744
Location: California, USA
Quote:
But I don't really believe that it's true !

Could you be more specific and explain why you don't believe it's true?

Have you tried asking Kenny Erleben himself at the Open Tissue forums? http://www.opentissue.org/opentissuebb/
Thanks,
Erwin

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 Posted: Thu Mar 04, 2010 10:57 pm

Joined: Tue Jun 30, 2009 6:56 am
Posts: 20
Erwin Coumans wrote:
Quote:
But I don't really believe that it's true !

Could you be more specific and explain why you don't believe it's true?

For instance in the thesis, the vector b_aux seems to be a vector but according to what I have written above, b_aux would be a matrix because E is a matrix of dimension (etha x K) x K.

Erwin Coumans wrote:
Have you tried asking Kenny Erleben himself at the Open Tissue forums? http://www.opentissue.org/opentissuebb/

I'm gonna try !

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 Posted: Wed Sep 28, 2011 9:31 pm

Joined: Sat Dec 05, 2009 1:05 pm
Posts: 2
just curios what's the ending of the story a year later

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