[ODE] Implementing genuine Coulomb friction

[Martin C. Martin]

Thomas Harte wrote:
> 
> As I have mentioned in the past, ODE's 'pyramid' friction approximation simply isn't good enough for my simulation. ... Therefore, I would like to see the option of using a genuine friction cone incorporated into ODE.

Coulomb friction is only an approximation to real world friction, and it's
pretty bad at times.  In fact, there are some situations in which the
Coulomb constraints have no solutions, in other words, all possible future
motions are inconsistent with Coulomb friction.  There are other cases
where there's more than one solution, i.e. there are two or more futures
which are completely consistent with Coulomb friction.  I'm not sure how
hard it is to enumerate the set of possible futures with Coulomb
friction.  For all I know it's uncomputable.  But I'm guessing it's a
nightmare to do, and even if you could do it, you're stuck with choosing
one of the many alternatives, or choosing the "best" non-Coulomb future
when none are consistent.  I'm sure people have looked at this, so maybe
there's some clever way of looking at it that simplifies everything.  But
I wouldn't be surprised if there wasn't.

- Martin