[ODE] asymmetry in simulation

[Bill Sellers]

Dear Kay,

Sounds about par of the course to me. You'll always get small  
numerical errors and there are order of calculation effects that mean  
you won't get exactly the same values (and I assume the limbs of your  
character contact the ground at some point and so will generate  
contact forces that will certainly be different and will cause small  
perturbations from symmetry). I'd say this was unavoidable and the  
best solution is to make sure that it doesn't matter. If your  
character is basically stable then these values should remain small  
(e.g. a walking biped with big feet). If your character is unstable  
(e.g. dynamic gait) then you will need some sort of control system to  
alter the gait to compensate otherwise they will rapidly become very  
large - quite a tricky problem actually although if you are not  
bothered too much by realism you can just use some sort of tether to  
an invisible sky hook or constrain trunk motion appropriately.

Cheers
Bill

On 2 Jan 2006, at 19:38, Kay wrote:

> Hi, All:
>
> I built a character with perfect symmetric limbs with amotors, but  
> somehow after only one step of simulation the velocities of the  
> left and right side of the bodies start to differ(around 1e-17 for  
> a simulation step about 0.001), with or without symmetric forces.  
> Is this normal due to the simulation errors? Is there anyway to  
> preserve symmetry? Or maybe there is something that I did wrong  
> which caused the asymmetric drifts?
>
> -Kay
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Dr. Bill Sellers, Editor Primate Eye   Email:  
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