Hands-On ROS for Robotics Programming

Collisions and physical properties

Collision tags specify the volumes that need to be taken into account by the physics engine to detect interference/clearance between bodies. The visual tag from URDF is ignored for the purpose of this computation since it's only used for visualization purposes. This means that, in general, you can decouple visible aspects of the robot (more detailed) from the envelope shapes that are used for computing interference (simpler shapes).

The inertia tag of a part specifies its mass and tensor of inertia (3 x 3), along with all of its components (only six components are needed since the matrix is symmetrical).

In the gopigo.urdf file under the urdf folder, you can find the blocks of XML for the base_link and caster ...