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Lite3 and X30 feet declare a singular inertia tensor on the links that take every ground contact

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Assessment

Difficulty
3/5
Estimated time
1-2 days
Newbie friendliness
72/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Active
Domain
robotics

Research direction

Start with the foot links at the cited locations in Lite3/urdf/Lite3.urdf and X30/urdf/X30.urdf, then compare their collision geometry and inertial declarations with Lite3.xml and X30.xml. Check the dont_collapse="true" ankle joints and the LIDAR_* links; done means the URDF and MJCF descriptions consistently use valid inertias and pass the referenced URDF preflight.

Written by the indexing model from the issue text.

Description

All four feet on both quadrupeds declare a mass and a collision sphere, and an
inertia tensor that no rigid body can have.

Lite3 (Lite3/urdf/Lite3.urdf:121, and FR/HL/HR at 224/326/428):

<link name="FL_FOOT">
  <collision><geometry><sphere radius="0.022"/></geometry></collision>
  <inertial><mass value="0.02"/>
    <inertia ixx="0" ixy="0" ixz="0" iyy="0" iyz="0" izz="0"/></inertial>
</link>

A solid sphere of that radius at that mass is

I = (2/5) m r^2 = 0.4 * 0.02 * 0.022^2 = 3.872e-06 kg m^2

The declared tensor is all zeros -- eigenvalues (0, 0, 0), not positive
definite.

X30 (X30/urdf/X30.urdf:129, +237/346/454): mass 0.06, sphere radius 0.036,
diaginertia = 1e-12. The solid-sphere value is

0.4 * 0.06 * 0.036^2 = 3.1104e-05 kg m^2

so the declared value is 3.11e7 times too small. The four LIDAR_* links
carry mass 1e-05 with an all-zero tensor as well.

The correct values are computable from the links' own collision geometry --
the two numbers above -- so no CAD lookup is needed if the feet really are
approximately solid spheres.

One caveat worth stating, because it decides whether this matters. A
zero-inertia body on a fixed joint is harmless if the consumer merges it into
its parent. But the joint immediately below carries dont_collapse="true":

<joint name="FL_Ankle" type="fixed" dont_collapse="true">

which is an explicit instruction to importers not to merge it. So the model
asks for the foot to be kept as an independent body and gives it a singular
inertia. Whichever half is intended, the two disagree, and consumers differ on
what they do about it -- MuJoCo's compiler and Drake reject a non-PD tensor,
while some importers silently substitute a bounding-volume inertia and others
integrate it as-is.

Also worth noting the MJCF siblings repeat the same values (Lite3.xml
diaginertia="0.0 0.0 0.0", X30.xml 1e-12), so this is not a URDF-only
transcription slip and fixing one file would leave the other wrong. I checked
that specifically because I had expected the MJCF to be the recovery source.

Found while running a URDF preflight over published quadruped descriptions;
the checks are open source, so the arithmetic above is reproducible on your side
without adopting anything of ours.

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