What is a URDF inertia tensor?

A link's inertia tensor is the six numbers in its <inertial> block that tell the physics simulator how its mass is spread out — and therefore how hard it is to spin.

Glossary · ~2 min read

<inertial>
  <origin xyz="0 0 0.05" rpy="0 0 0"/>
  <mass value="1.2"/>
  <inertia ixx="0.004" ixy="0" ixz="0"
           iyy="0.004" iyz="0" izz="0.002"/>
</inertial>

ixx, iyy, izz are the moments of inertia about each axis; ixy, ixz, iyz are the products of inertia (zero for a body symmetric about those axes). Together with mass and the CoM origin, they fully describe how the link resists linear and angular acceleration.

Placeholder values (ixx=iyy=izz=1, or all near-zero) are common in hand-written URDFs and are almost never physically correct for the link's actual mass and shape — they're a frequent cause of jittery or unstable simulation. The full walkthrough, including collision-mesh mistakes that compound the problem, is in fixing URDF inertia and collision meshes.

Jointly computes the tensor directly from CAD geometry using Mirtich's exact polyhedral-mass algorithm, instead of leaving it as a placeholder.