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In my first answer, I didn't understand your proposal exactly. Now I see that you're suggesting to model the entire wheel as a single cylinder and set directional friction properties to simulate the mecanum wheel rollers.

ODE supports two friction coefficients (mu and mu2 in sdformat in this tutorial) with a pyramid friction model. The friction directions corresponding to each coefficient are computed to be orthogonal to each other and the contact normal unit vector. If the fdir1 parameter is not set, then the first friction direction is computed from the component of the global X axis that is orthogonal to the unit normal. The second friction direction is then computed to be mutually orthogonal with a vector cross product. In the case where objects are contacting a ground plane normal to the Z axis, then the X and Y axes are the first and second friction directions respectively.

If the fdir1 parameter is set to a non-zero vector, then that vector is interpreted as the desired first friction direction in the body-fixed Link frame of the object. This makes it challenging to model mecanum wheels, since the link frame will be rotating as the wheel spins. It would be better to define the friction direction relative to the wheel carrier (spindle). This would require a modification to gazebo, but it sounds like it would be useful.

Did that make sense?