[ODE] discontinuities/accuracy of integration
erincatto at sbcglobal.net
Sat Sep 17 14:25:53 MST 2005
You might try this (commercial): http://www.autolev.com
Or Google: multibody dynamics software
From: ode-bounces at q12.org [mailto:ode-bounces at q12.org] On Behalf Of Marc
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?
> 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)
> If you want to get rid of velocity discontinuities and satisfy
> constraints exactly, you need a different engine.
> -----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
> I figured that the origin of such discontinuities might be how ODE
> realizes joint error reduction. (There are NO collisions/contacts in
> 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?
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