[ODE] Sliding force + constraint force = horizontal/vertical trajectories?

[brel at arcor.de]

Hello!

I have been using an ODE simulation in my masters project, i modelled a two hinge joint arm that can move in the horizontal plane and that drags a "pen" along to draw lines. to control it I applied torques to the joints, rather than to set the velocity. What I noticed is that, if there was no or a very low torque applied to the "ellbow" joint, i.e. the forearm was dragged along with the upper arm, the trajectory the pen followed was EXACTLY horizontal or diagonal. I checked it by turning the world model by 45 degree, it is a thing in ode, not in the controller. The effect disappears without contact friction and gets weaker as  the torque applied to the ellbow gets larger, so I reckon it has something to do with the combination of the sliding friction model and the constraint force model. Can anybody help me with that?

Best

Marieke

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