[ODE] Collision constraint

[Joakim Eriksson]

Isn't there anyone that knows if this is an issue with ODE? 

/Joakim E.

> -----Original Message-----
> From: Bob Dowland [mailto:Bob.Dowland@blue52.co.uk]
> Sent: den 19 juni 2003 15:24
> To: ode@q12.org
> Subject: RE: [ODE] Collision constraint
> 
> 
> I think you've hit the nail on the head here - this is the 
> main snag with "penalty methods" (penetration correction 
> force). I don't know how ODE's "correction" works, perhaps 
> someone can explain but using a simple spring model (force 
> proportional to error) can give rise to these additional 
> "bouncing" effects.
> 
> If you think of the penetration as an error call it e and 
> note that it is a function of time e = e(t) then a simple 
> spring correction also called "proportional controller" 
> generates a force proportional to that error
> 
> F = F (e(t)) = kp.e(t)
> 
> this is the same ode as for a spring (giving "shm"). It's 
> nice and simple but does produce, well, a certain 
> springyness. An improvement on this is "proportional, 
> derivative control":
> 
> F = kp.e(t) + kd.d/dt{e(t)}
> 
> a spring with a damper. A little more complicated but gives 
> you a fighting chance at least. The trick, then, is to find 
> "good values" for kp and kd, (ie ones that make e go to zero 
> as quick as poss, not overshoot, and not oscillate about zero 
> - aka "critically damped") unfortunately this often comes 
> down to manual tweaking - depending on the situ.
> 
> I don't know if this helps in any way - I guess it would be 
> more useful to hear what some of the ODE vets have to say on this.
> 
> 
> > -----Original Message-----
> > From: Joakim Eriksson [mailto:jme@snowcode.com]
> > Sent: 18 June 2003 09:42
> > To: ode@q12.org
> > Subject: [ODE] Collision constraint
> > 
> > 
> > I have been looking a bit at the constraint system and there is one
> > thing I wonder.
> > 
> > As far as I can tell so does the constraint solver only 
> > return a force.
> > That force
> > is then applied to the bodies. So the force turns into a 
> velocity that
> > changes the body.
> > Now what happens if a body is far into for example a plane. So the
> > penetration
> > depth is large. That would force the 'c' part of the 
> > constraint equation
> > to have a
> > large value (To fix the penetration). If we would set the ERP to 1.0
> > then he
> > would need to fix the error as quickly as possible. Now 
> > because we calc
> > a force
> > wont that mean that the body will get a velocity that forces 
> > it not only
> > to quickly
> > move out of plane but also to continue away from the plane? Or am I
> > missing
> > something?
> > 
> > If the problem actually does exists it would show in the 
> > following way.
> > A fast moving object with a coeffiecent of resitition that 
> equals 0.0f
> > that collides
> > with for example a plane would still get a bounce effect. 
> How high the
> > bounce would be would depend on the ERP and the penetration depth.
> > 
> > /Joakim E.
> > 
> > 
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