Hello everybody! The problem is the following. I am generating the robot from xacro to urdf and it works fine in Gazebo, RVIZ, etc. URDF format assumes that I can put only one object in it. Hence, our robot is fully mobile according to a scene. When I convert my urdf to mjcf (the kinematic tree is attached), one of the bodies becomes fixed. Can I manage it on urdf level somehow? It's significant for me because I have to generate a family of robots and modify mjcf file each time is very time-consuming. URDF, MJCF files, and kinematic tree are attached.