I'm trying to build a model of a robot using a Kiwi drive. The intent was to approximate the behaviour of the wheels by having 0 friction in the axial direction, and some appropriate value in the tangential direction. However, it seems impossible to set direction-dependent tangential friction for contact pairs in MuJoCo. Are there any ways to implement this behaviour without directly modelling the wheels, each of which comprise at least 9 bodies?
You can specify axis-specific friction using the pair element, see documentation here and note that "friction" has 5 parameters: http://www.mujoco.org/book/modeling.html#pair The this case the pair would involve the wheel geom and the ground geom. But I don't see why you would want to do that for wheels. They should have friction in all directions, and be able to roll. Axis-specific friction makes sense for things like skating on ice for example.
Thanks for the reply, and I think that will work, but I'm curious as to why you think it's a bad idea. Omni wheels DO have "friction in all directions", and they obviously roll, but the effective tangential friction is vastly lower in the direction of the axis of rotation due to the rollers around the circumference of the wheel.
I am not actually familiar with the mechanism, but I was imagining that you would model the rolling part. If you are trying to simplify it and achieve the same overall effect, then anisotropic friction is indeed the way to go.