[ODE] discontinuities/accuracy of integration

[Marc Toussaint]

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/