When converting a URDF via
gz sdf -p robot.urdf > robot.sdf to SDF, links with no visual element get removed / are ignored.
Converting the attached URDF (source: robotiq 3finger xacro) via
gz sdf -p robotiq-3f-gripper_articulated.urdf > robotiq-3f-gripper_articulated.sdf results in the attached SDF.
While the URDF contains the link tool0 (
<link name="tool0"/>), the exported SDF is missing this link. This also effects the spawning of URDF models via the ROS plugin.
I guess this also breaks the kinematic tree in general. E.g. Gazebo cannot spawn models that have an "invisible" link in their kinematic chain.
Since this link is not part of the kinematic chain, it cannot be used as reference for spawning other models. This is however important if models are to be spawn relative to "invisible" links, like grasp links that are commonly used for grippers.