I've gotten familiar with the standalone gazebo project using .model and .world files to describe robots. Currently trying to figure out ROS and URDFs. It looks like the available joint types in URDF are limited to 1 axis rotations. Is there any way to implement a hinge2 type of joint (such as a steerable wheel) in URDF? It looks like the only way is to use two separate joints to do such a function. Am I missing something? Thanks.