Robot URDF Visualizer

COM (Center of Mass)

• Mass
  • Read from the URDF <inertial> block: <mass value="..." />.
  • Only links that have an <inertial> block are considered; the value is used in the Links panel and for inertia visualization.
• COM origin (position)
  • It is the <origin xyz="..." /> inside <inertial>, expressed in the link frame.
  • We parse this xyz for each link that has <inertial> and place a small purple sphere at that (x,y,z) in the link’s local frame, so the marker moves with the link.

Link Axes

• The axes shown are the link frame (the link’s origin frame), not the visual or collision frame. In URDF the link frame is the child frame of the joint that connects the parent link to this link; <visual>, <collision>, and <inertial> each have their own <origin> relative to this link frame.

Joint Axes

• In URDF, a joint can have <axis xyz="..." />, given in the joint frame: for revolute and continuous it is the rotation axis, for prismatic the translation axis. We only show axes for these three types.

Inertia

• In URDF, inside <inertial>, <origin xyz="..." rpy="..." /> gives the center of mass and orientation of the inertial frame in the link frame; <mass value="..." /> gives the mass; <inertia ixx ixy ixz iyy iyz izz="..." /> gives the 3×3 inertia matrix in that inertial frame.
• An equivalent uniform-density box is used to visualize the inertia, which is computed as follows.
  • If the inertia matrix has off-diagonal terms, we do an eigenvalue decomposition (Jacobi) to get principal inertias and principal axes.
  • Box dimensions follow from the principal inertias and mass, e.g. \(w^2 = (6/m)(I_{yy}+I_{zz}-I_{xx})\) where \(w\) is the box width, and similarly for height and depth.
  • The box is placed at the COM from inertial/origin xyz and oriented by the inertial frame rpy combined with the principal-axes rotation, drawn as a semi-transparent blue box.