[ODE] Hinge Forces

[Russ Smith]

> If I apply force F to body m2, and it has one hinge joint to m1, will
> there be a much larger error than if I make another joint like in the
> picture?

the answer is that the configuration on the left is the correct one. one
hinge joint is all you need to resist *any* amount of force that tries
to push the hinge out of alignment. ODE does not model the breakable
hinges that you buy from the hardware store, it models ones that are
infinitely strong. having said this, from time to time you may see
joints come slightly out of alignment, but this error usually has
nothing to do with the forces that are applied to them. reasons for
error might be:

  * the rotation of the joined bodies per timestep is large.
  * CFM is too high (this will give you "spongy" joints).
  * there is numerical error caused by redundant constraints.

the configuration on the right side of your picture falls into this last
category - you have two joints trying to do the same job (i.e. trying to
limit the same degree of freedom) so this can cause numerical error
(especially if CFM is small) and totally ruin the simulation.

> Is it better to represent the dynamics as simple as I can, or with...

as in most things, keeping it simple is best. but, as in most things,
figuring out what the simple solution is can by tricky.

russ.

-- 
Russell Smith
http://www.q12.org