[ODE] discontinuities/accuracy of integration

[Erin Catto]

You might try this (commercial): http://www.autolev.com

Or Google: multibody dynamics software 

-----Original Message-----
From: ode-bounces at q12.org [mailto:ode-bounces at q12.org] On Behalf Of Marc
Toussaint
Sent: Saturday, September 17, 2005 4:09 AM
To: ode at q12.org
Subject: Re: [ODE] discontinuities/accuracy of integration

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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>



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