[ODE] Picking objects

[Bob Dowland]

Guys,

some time ago I had to simulate a robotic arm and control the pos/ori of the end effector using output from a 6d pos/ori tracker - lots of fun. What I did with that might be of interest and might not but there we are.

I used what you might call a "reference proxy" which responded perfectly (ie kinematically not dynamically) to the user input. The motion of this proxy was totally free and unsullied by the ravages of physics/collision response - it could go through walls, outreach the arm and so on.. The arm then just "did the best it could" to follow it. Where "doing the best it could" meant not violating it's various dynamic and geometric constraints. This worked very nicely.

As far as spring and damper mechanisms go the main problem seems to be the tuning issue. In particular in graphical sims the spring (and underlying metaphor :) ) will often get "stretched" far beyond any reasonable physical limits / dynamic operating range - so in a sense the model breaks down. One awful hack that works quite well is to treat the spring damper as a black box, tune stiffness and so forth to taste (so that things are good where you mostly need them) then maintaining a very clear idea of what velocity/acceleration ranges you want to allow just clamp the outputs.

Bob.

> -----Original Message-----
> From: Manohar B.S [mailto:sciphilog at yahoo.com]
> Sent: 30 June 2004 10:55
> To: Olivier Parisy
> Cc: ode at q12.org
> Subject: Re: [ODE] Picking objects
> 
> 
> Hello James,
> Hello Olivier,
> 
> > It's sad we have to get to such tricks as soon as
> > model control is needed... Perhaps someone who
> > developped a stable servo example could contribute
> > it in ODE distribution someday ? I know it would
> > solve most of my animation/interaction needs !
> 
> Well, I think there's a serious need for model control
> or Motion controller interface with the physics
> engines (which ever it be). Well, oscillations are
> part of the physics and the physics engine is happy
> becos it is honestly doing what it is supposed to do.
> We always expect some thing that we get convinced of
> due to our past experience with the nature
> (irrespective of right mathematical justification).
> 
>  At this time I am not very much informative about how
> to make a very "convincing" and "Accurate" motion
> control interface but I believe that Fully logic based
> contoller is the best option. But the requirements are
> like this:
> 
>  1. The Physics body/geom should follow the position
> and orientation of the Motion sensor as quickly as
> possible (with accuracy).
>  2. Oscillations should be minimum or atleast
> invisible.
>  3. Forces applied on the physics body/geom should be
> clean enough to be applied to the forcefeedback
> device.
>  3. The algorithm should be computationally lite.
> 
> ___________
> Fuzzy Spring-Damper will have variable spring konst
> and variable damping konst, depending on the rule
> table entries.
> 
>  I would be happy to dedicate time for research in
> this like.
> 
>  -- \
> Manohar BS
> 
> 
> 
> 		
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