[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?


> 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
> Thanks!
> Marc.
> --
> http://homepages.inf.ed.ac.uk/mtoussai/
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