[ODE] discontinuities/accuracy of integration

[Bill Sellers]

If you want an open source simulator that calculates joints exactly  
then you might want to have a look at Dynamechs. It's hosted at  
sourceforge and hasn't seen much development for a while but it's  
pretty solid. I've been using it for a few years (you can see a few  
examples of what it does on my web page http://homepage.mac.com/wis/ 
ASL/). The limitation is that it only remains exact for tree  
structures and its contact system is rather primitive. On the plus  
side it can also do simple underwater simulation at the same time.  
I've just moved over to ODE because I wanted better contacts and I'm  
hoping that the amount of damping in my systems and using a  
relatively integration small step size will help smooth out any  
instability. So far I've been able to get away with quite a large  
integration step size with ODE and my models don't fall over as much  
which I think is primarily to do with having less bounce at the foot  
ground contacts. The numbers seem more or less the same so far too.

The thing I really miss is that Dynamechs makes you specify all the  
links relative to their upstream link which makes them much easier to  
set by hand. The downside is that it insists on using MDH coordinates  
for hinges that I find very non-intuitive (and as far as I can see  
they make you use local axes that may not be all that convenient).

Cheers
Bill

--
Dr. Bill Sellers, Editor Primate Eye   Email: wis at mac.com
Faculty of Life Sciences               Mob: 0775 107 8997
3.614 Stopford Building
The University of Manchester           http://homepage.mac.com/wis/ASL/
Oxford Road, Manchester, M13 9PT, UK

Primate Society of Great Britain WWW site http://www.psgb.org/

On 17 Sep 2005, at 12:09, Marc Toussaint wrote:

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