[ODE] Contact Joints

[Nguyen Binh]

Hi Tota!

contact force : fc  = trans(J)*lamda
acceleration along contact normal : ac = J*a + k

With inequality constraints we have, ac = J*a + k >= 0.

But we actually don't model like that. We just need a maths model that
fits the nature of contact:
1. contact force push, not pull => fc >= 0
2. contact force * acceleration = 0

So our model :
 a) if fc = 0 -> no contact -> don't care about ac -> we model J*a + k = 0
 b) if ac = 0 -> resting contact -> fc >= 0 and J*a + k = 0

That's maths model does fit contact's nature.
So the difference between contact and joints constraint is the
complementarity of fc and ac not the constraints equation.

-- 
Nguyen Binh
www.glassegg.com
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...currently searching for a graduate school in physical simulation...
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----- Original Message -----
From: Cristian Tota <cristian.tota at vion-software.ro>
Date: Thu, 2 Sep 2004 00:43:30 -0700
Subject: [ODE] Contact Joints
To: "[ODE]" <ode at q12.org>

 
Hi, 
  
I have a theoretical question about contacts, maybe someone can
explain this. The ODE 0.5 documentation states that the joint equation
is J*v - c = 0.
I understand this for bilateral constraints, but how is this used in
contact constraints, where there are inequations instead of equations,
like J*v - c >= 0 ?
  
Thank you in advance, 
Cristian 
  

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