[ODE] A 'real' demo of ODE

[Gregor Veble]

Hi Russ,

I saw my mistake a few moments after I sent my post. The issue of
singularity brings on another set of questions altogether, but I will
focus on other things here :).

As far as I can see, we are basically on the same track. The equation
for the link constraint that I sent earlier is a scalar function which
removes one degree of freedom per link (which are now *not* treated as
bodies). In the case of the multilink suspension there are five
(nonsingular) links removing 5 DOFs for 2 bodies, which is exactly what
you mention to be the optimal case. Are there any other issues I might
have missed?

-Gregor

Russ Smith wrote:
> 
> > That's interesting to hear, that a single more complex link is faster
> > than the sum of its parts. I was under the impression that the
> > numerical demand is predominantly dependent upon the number of
> > degrees of freedom removed. To use an example: if you
> > model the same sort of a link by either a single hinge joint, or by
> > using two ball-socket type of links, could you please explain where
> > the difference in numerical complexity comes about?
> 
> your example (modelling a hinge as two B&S joints) wont work because
> they will remove 6 dofs (degrees of freedom) from the system but
> a hinge only removes 5. so there will be a singularity.
> a better example is modelling a suspension with, say, 3 bodies and
> 3 hinge joints. sure ... the resulting system has the same number of
> degrees of freedom as if you had used a special suspension joint, but
> the 3 hinges are removing 15 dofs from 3 bodies instead of 5 dofs from
> just one body. as such the system matrix is bigger, it takes longer to
> solve, etc. this is why special purpose constraints are really useful.
> 
> russ.
>