[ODE] discontinuities/accuracy of integration
Jon Watte (ODE)
hplus-ode at mindcontrol.org
Sun Sep 18 19:07:18 MST 2005
Could you describe what you mean by "discontinuities" ?
ODE is time stepped. This means that it's discontinuous at every time
step; it's a stairstep function, not a smooth matematical function.
Regarding the solver, if you're using dWorldStep, the solver uses a
traditional full matrix solver; you could probably take out the part
where it starts solving the matrix (after creating it), and put in
whatever you want. There are already three solvers to choose from:
dWorldStep, dWorldStepFast, and dWorldQuickStep, so there should be
ample samples for a fourth implementation :-)
Cheers,
/ h+
Marc Toussaint wrote:
> Hi there!
>
> I'm using ODE in a scientific context, simulating robots to test
> machine learning techniques. Actually the accuracy of the physical
> integration is not critical for this application -- as long as the data
> collected from the simulation is smooth.
>
> However, ODE quite often produces discontinuities in joint velocities,
> especially if there are substantial torques applying on the joints.
> Consequently, the accelerations that I monitor for the system become
> unrealistic.
>
> I figured that the origin of such discontinuities might be how ODE
> realizes joint error reduction. (There are NO collisions/contacts in my
> scenario.) I tried playing around with ERP and CFM setting for
> different joints, but it seemed to me that (1) also for an optimal
> setting I couldn't get rid of the discontinuities completely, (2)
> finding an optimal setting (which is different for each joint,
> depending on the attached loads) is itself a hard problem.
>
> Q1: Can you confirm that the joint error correction mechanisms is most
> likely the origin of such discontinuities?
>
> Q2: Is there a way to access the ``internal error-reducing forces'' (or
> whathever other mechanisms there is) in an accurate quantitive way?
>
> Q3: Is there a principled way to get rid of the discontinuities (other
> than playing around with ERP and CFM parameters)?
>
> Q4: Is there a chance that the core of ODE (the physical integration
> engine) can be replaced/modified such that hard constraints are
> fulfilled exactly? E.g., could one easily replace the engine by a DAS
> solver like DASPK?
>
> Thanks!
> Marc.
>
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