[ODE] Wrong solutions for wheeled mechanism (slippage settings)

[Thomas Thüer]

Hi all,
I'm developing a simulator for mars rover type mechanisms. Building and 
setting up these rovers with ODE works fine, but I recently found out 
that setting the slippage on the wheel-ground contact joints can cause 
serious problems regarding the results of ODE's calculations. When I set 
the slippage to a very low value, it happens that the rover behavior 
doesn't correspond to real behavior when negotiating an obstacle. Take a 
rockerbogie type rover (6 wheeled) for example, it happens that the 
middle wheels are lifted at the same time the front wheels touch the 
obstacle and start climbing up, instead of staying on the ground and 
rotating around the revolute joint. Obviously, ODE doesn't find the 
appropriate solution due to the low slippage value. But on the other 
hand, when I allow more slip, the rover cannot negotiate the obstacle 
because it is slipping 100%. I cannot find the appropriate values for 
the contact joint settings that allow the rover to perform the real 
behavior on some obstacle, while other rovers do not cause any problems 
on the same obstacle.

Has anyone already encountered similar problems with wheeled mechanisms 
(vehicles) when negotiating obstacles?

Your help is highly appreciated, THANKS A LOT!
C'ya,
Thomas

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Thomas Thüer  
Autonomous Systems Lab, Swiss Federal Institute of Technology, Lausanne