Re: [simspark-devel] ODE Composite bodies

[Joschka B. <jos...@am...>]

Hi Sander,

thanks for keeping up the work on this!

On Feb 16, 2008, at 8:28 AM, Sander van Dijk wrote:

> On Sat, Feb 16, 2008 at 12:17 AM, Hedayat Vatankhah  
> <hed...@ai...> wrote:
> Yes, it crashes on my system too. :(
>
>
> :( What version + configure flags of ODE do you use? Weirdly enough,  
> the three systems we tested it on all have the same version (0.9 +  
> double precission) and only at one it doesn't work, so was hoping it  
> was a fluke. The solution I think would be to translate the body to  
> the center of mass (as also done in the  official ODE examples ),

Should be the other way round, but I guess that's what you meant ;-)  
Translate the masses so that the center of the composite mass  
coincides with the body's origin.

One thing I realize is that this seems to be quite a constraint when  
it comes to realism of the simulation. If you think of an example like  
a table made up of five bodies: a flat surface on the top, and four  
legs, e.g.

pos_top = (0,0,0)', pos_leg_1 = (-1,-1,-0.5)', pos_leg_2 =  
(1,-1,-0.5)', pos_leg_3 = (1,1,-0.5)', pos_leg_4 = (-1,1,-0.5)'

and

mass_top = 0.3, mass_leg_[1,2,3,4] = 0.1

and you assume point masses (which is the default in ODE, IIRC), you  
calculate the COM as:

COM = (pos_top * mass_top  + sum_{i=1}^{4} pos_leg_i * mass_leg_i) /  
(mass_top + sum_{i=1}^{4}  mass_leg_i)  %%% Latex like notation...

Which is the sum of all the positions weighted by their masses divided  
by the total mass. For this example, you'll get a COM at about  
(0,0,-0.286), so we need to subtract -0.286 from the z-coordinate of  
each of the leg masses in order to fulfill the ODE constraint.  
However, you probably don't want to change the position of the  
geometries, since then you get a different shape from what you  
intended. So the collision shape will be correct, but leg masses will  
be slightly higher than intended. Hopefully, it shouldn't be a big  
deal, but it's something to keep in mind when thinking about the  
realism of the robot models. (I hope the example is somewhat  
understandable... it's much easier to see with a drawing actually...)

> but I suspect this messes up joint placement..

It shouldn't be a problem, since you only move the masses, not the body.

> I'll have further look into it this weekend. Anyway thanks for  
> testing!

Thanks a lot!

Cheers,
Joschka