fix(urdf): rotate inertia tensors from the inertial frame into the link frame - #707
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…nk frame URDF gives each link's inertia tensor in its <inertial> frame, which <origin rpy> may rotate relative to the link frame. The loader kept only the origin translation and passed the tensor through unrotated, so links with rotated inertial frames got wrong inertia, rne, coriolis and gravload results. Apply I_link = R I R^T. Related to petercorke#686 and petercorke#688.
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| Metric | Results |
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| Complexity | 9 |
| Duplication | 0 |
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Summary
URDF gives each link's inertia tensor in its
<inertial>frame, whose pose relative to the link frame is<origin xyz rpy>.Link.Iexpects the tensor about the centre of mass with axes parallel to the link frame. The loader kept only the translation of that origin and passed the tensor through unrotated, so any link with a non-zero inertialrpygot a wrong inertia tensor, and therefore wrongrne,inertia,coriolisandgravload, with no warning.This rotates the tensor into the link frame,
I_link = R I Rᵀ, in a small helper inURDFRobot.py.ris unchanged, sincexyzis already the centre of mass in the link frame.Affected models include those with rotated inertial frames in the bundled data (the Trossen arms, and the Clearpath Ridgeback base used by
FrankieOmni) and robot_descriptions models such as the UR series.Related issue
Related to #686 and #688 (dynamics accuracy of URDF models).
Checklist
mainand all need the compiled extension or IPython.tests/test_urdf_inertial.py, covering an unrotated frame, a yaw that swaps the x/y moments, a general rotation againstR I Rᵀ, the centre of mass, and anrnecheck where a 90° roll movesiyy = 2onto the joint axis, so a unit joint acceleration needs torque 2 (3 before the fix). The three rotation tests fail without this change.