USD Articulation — Multi-Arm Robots

Purpose

Assemble and validate multi-link robot articulations with ArticulationRootAPI, fixed joints, and Isaac robot schema overlays before deployment.

Prerequisites

• For standalone schema authoring: matching OpenUSD and isaacsim-robot-schema library artifacts.
• For live articulation interaction, extension commands, or rendering: built Isaac Sim and the environment from isaac-sim-orchestrator.

Limitations

• Targets Isaac Sim 6 / Kit 110 unless a section states otherwise.
• Does not replace official NVIDIA documentation for unsupported edge cases.

Troubleshooting

Schema authoring versus runtime interaction

The robot schema is generated and packaged from source/libraries/isaacsim_usd_schemas/robot/; use that distribution for standalone schema application and validation with OpenUSD. For robot-motion schema metadata, use the separate isaacsim.robot_motion.schema module. These schema packages do not require a Kit extension registry merely to import.

Kit commands, live timeline interaction, UI, and rendering stay on the Kit route.

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.

Available Scripts

Running scripts

From agent runtimes that expose skill execution helpers, invoke helpers with run_script():

run_script("scripts/apply_robot_schema.py", args=["--help"])

From a built Isaac Sim tree, run the same file with ./python.sh (Linux) or python.bat (Windows) from _build/*/release, or execute shell helpers directly when they do not require the simulator.

Multi-arm assembly (correct pattern)

assemble_multi_arm_robot(stage, chassis_usd, arm_usd, robot_root, arm_offset, chassis_body_path, arm_base_path) — spawn chassis + arm, create a FixedJoint, remove ArticulationRootAPI from the arm.

See scripts/multi_arm_assembly.py.

What does not work

• Referencing the same arm USD twice via sublayer composition.
• Using over "Geometry" to add arms in a parts layer.
• Assuming visual presence equals physical attachment.

Robot Schema overlay (Kit 110)

Apply the modern Isaac Robot Schema on top of the physics layer. The URDF/MJCF importers do this automatically; for hand-authored or retrofitted USDs apply it manually.

apply_robot_schema(stage, robot_prim, link_prims, joint_prims, site_prims, robot_type) — apply IsaacRobotAPI on root, IsaacLinkAPI on each link, IsaacJointAPI on each joint, IsaacSiteAPI on grasp/mount frames, then call PopulateRobotSchemaFromArticulation to fill ROBOT_LINKS/ROBOT_JOINTS. Valid robot_type tokens: Default, End Effector, Manipulator, Humanoid, Wheeled, Holonomic, Quadruped, Mobile Manipulators, Aerial.

See scripts/apply_robot_schema.py.

Validation checklist

Before trusting any multi-arm robot:

1. Exactly 1 UsdPhysics.ArticulationRootAPI on the chassis, nowhere else.
2. At least one chassis/root-branch FixedJoint anchors each top-level attached subsystem before flatten-before-deploy.
3. Every joint body is reachable from the single articulation root through one connected joint hierarchy.
4. IsaacRobotAPI present on the root; robot_type set to a valid token.
5. ROBOT_LINKS / ROBOT_JOINTS relations populated (call PopulateRobotSchemaFromArticulation if not).
6. Each grasp frame carries IsaacSiteAPI (not deprecated IsaacReferencePointAPI).
7. Render from four angles (front, side, 3/4, top).
8. Send to a vision LLM with a binary question ("Arms attached? PASS/FAIL").

Validation snippet (self-contained)

Run via $ISAAC_SIM_DIR/python.sh. Enforces exactly one ArticulationRootAPI, verifies the entire joint graph is connected to that root, checks for at least one root-branch FixedJoint attachment before flatten-before-deploy, and reports the Robot Schema state.

validate_articulation(usd_path) — assert exactly 1 ArticulationRootAPI, verify hierarchy connectivity, and print root-attached children plus Robot Schema state.

See scripts/validate_articulation.py.

Extend with whatever subsystem path patterns your asset uses. Binary goal: exactly one root, one connected articulated hierarchy, at least one root-branch FixedJoint anchor before flattening, and the Robot Schema overlay present.

Common failures