USD Articulation — Multi-Arm Robots

Two layers stack on a robot USD:

1. Physics: UsdPhysics.ArticulationRootAPI, UsdPhysics.Joint, UsdPhysics.RigidBodyAPI, PhysxSchema.PhysxArticulationAPI.
2. Robot Schema (usd.schema.isaac.robot_schema): semantic overlay used by isaacsim.robot.poser, the importers, manipulators examples, and any tool that walks "this robot's links / joints / named poses".

Rule: visual plausibility is not mechanical correctness. If it doesn't articulate, it doesn't exist.

Multi-arm assembly (correct pattern)

from pxr import Usd, UsdGeom, UsdPhysics, Sdf, Gf

# Spawn chassis
chassis = stage.DefinePrim("/World/Robot", "Xform")
chassis.GetReferences().AddReference(CHASSIS_USD)