[ODE] discontinuities/accuracy of integration

Marc Toussaint mtoussai at inf.ed.ac.uk
Sat Sep 17 12:09:06 MST 2005


Thank you Erin for these details! Accessing the Lagrange multipliers might  
already help me out.

Still, do you know of alternative engines that would solve my problems?

Best,
Marc.

> ODE operates at the velocity level, not the acceleration level. So
> velocities can change quite a bit in one step.
>
> Since ODE uses a Baumgarte form of constraint stabilization, the
> stabilization forces are mixed in with the constraint forces. You should  
> be
> able to access the constraint forces (Lagrange multipliers) easily.
>
> If you want to get rid of velocity discontinuities and satisfy  
> constraints
> exactly, you need a different engine.
>
> Erin
>
> -----Original Message-----
> From: ode-bounces at q12.org [mailto:ode-bounces at q12.org] On Behalf Of Marc
> Toussaint
> Sent: Friday, September 16, 2005 1:40 PM
> To: ode at q12.org
> Subject: [ODE] discontinuities/accuracy of integration
>
> 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.
>
> --
> http://homepages.inf.ed.ac.uk/mtoussai/
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