URDF / MJCF -> USD Conversion

Two conversion paths and one export path.

XACRO inputs

The URDFImporter core does not parse XACRO. Two supported paths:

Recommended — import directly from a running ROS 2 node

isaacsim.ros2.urdf adds a dedicated import path that queries the robot_description parameter on any node (typically robot_state_publisher) via the standard GetParameters service, resolves package:// URLs, writes the URDF to a temp file, and feeds it to URDFImporter. The node is responsible for XACRO expansion, so this also covers launch-file-only distributions that never ship a static URDF.

UI: File -> Import from ROS2 URDF Node (opens an import window with the same collider / robot-type / mesh options as the standard URDF importer).

Python (preferred over the deprecated URDFImportFromROS2Node Kit command):

from functools import partial

from isaacsim.asset.importer.urdf import URDFImporter, URDFImporterConfig
from isaacsim.ros2.urdf import RobotDefinitionReader

config = URDFImporterConfig(
    usd_path="/path/out",
    merge_fixed_joints=True,
    fix_base=False,
    robot_type="Manipulator",
)
importer = URDFImporter()

def _on_description(urdf_abs_path: str, package_found: bool) -> None:
    # `urdf_abs_path` is a temp file with package:// already resolved.
    config.urdf_path = urdf_abs_path
    importer.config = config
    importer.import_urdf()

reader = RobotDefinitionReader()        # singleton
reader.description_received_fn = partial(_on_description)
reader.start_get_robot_description("robot_state_publisher")

Requires the isaacsim.ros2.urdf extension (depends on isaacsim.ros2.bridge for the ROS 2 runtime), the ROS 2 environment sourced before launching Isaac Sim, and a reachable node publishing robot_description. The reader runs asynchronously — the callback fires once the GetParameters service replies.

Fallback — offline xacro CLI

Use when there is no live ROS graph. Requires only the xacro package (pip install xacro or apt install ros-$ROS_DISTRO-xacro):

xacro robot.xacro > robot.urdf
xacro robot.xacro arm_id:=fr3 hand:=true > robot.urdf

# Inside a sourced ROS 2 workspace so package:// / $(find-pkg-share) work:
source /opt/ros/$ROS_DISTRO/setup.bash
source install/setup.bash
xacro $(ros2 pkg prefix --share my_robot_description)/urdf/robot.xacro \
  > robot.urdf

Pass the resulting .urdf to Path 1 or Path 2.

Path 1 — Isaac Sim importer (recommended for RL/Lab)

Modern public API: isaacsim.asset.importer.urdf.URDFImporter + URDFImporterConfig (dataclass) and the matching isaacsim.asset.importer.mjcf pair. The post-import isaacsim.asset.transformer runs by default and restructures the USD output (collects dependencies, runs registered rules for physics conversion, materials routing, etc.).

from isaacsim.asset.importer.urdf import URDFImporter, URDFImporterConfig

config = URDFImporterConfig(
    urdf_path="/path/robot.urdf",
    usd_path="/path/out",
    merge_fixed_joints=True,
    fix_base=False,
    collision_from_visuals=True,
    collision_type="Convex Decomposition",
    joint_drive_type="force",
    joint_target_type="position",
    override_joint_stiffness=800.0,
    override_joint_damping=40.0,
    robot_type="Manipulator",          # robot-schema token
    run_asset_transformer=True,         # default True; applies transformer profile
    run_multi_physics_conversion=True,  # URDF -> PhysX/MuJoCo physics
)
output_usd = URDFImporter(config).import_urdf()

URDFImporterConfig fields (defaults)

CLI (Isaac Sim)

source/standalone_examples/api/isaacsim.asset.importer.urdf/urdf_import.py auto-enables omni.scene.optimizer.core and isaacsim.robot.schema, then applies the config.

"$ISAAC_SIM_DIR/python.sh" \
  "$ISAAC_SIM_DIR/source/standalone_examples/api/isaacsim.asset.importer.urdf/urdf_import.py" \
  --urdf      /path/robot.urdf \
  --usd-path  /path/out \
  --merge-fixed-joints \
  --fix-base \
  --joint-drive-type force \
  --joint-target-type position \
  --collision-from-visuals --collision-type "Convex Decomposition" \
  --robot-type Manipulator \
  --ros-package my_pkg:/abs/path/to/my_pkg
# --no-run-asset-transformer to skip the transformer profile.

--robot-type choices come from usd.schema.isaac.robot_schema.get_allowed_tokens(Attributes.ROBOT_TYPE): Default, End Effector, Manipulator, Humanoid, Wheeled, Holonomic, Quadruped, Mobile Manipulators, Aerial.

MJCF import (parallel API)

from isaacsim.asset.importer.mjcf import MJCFImporter, MJCFImporterConfig

config = MJCFImporterConfig(
    mjcf_path="/path/robot.xml",
    usd_path="/path/out",
    import_scene=True,            # include MJCF scene settings
    merge_mesh=True,
    robot_type="Quadruped",
    override_gain_type="fixed",   # MuJoCo actuator gain type
    override_bias_type="affine",  # MuJoCo actuator bias type
    override_gain_prm=[kp, 0, 0, 0, 0, 0, 0, 0, 0, 0],          # position control
    override_bias_prm=[0, -kp, -kd, 0, 0, 0, 0, 0, 0, 0],       # position control
)
output_usd = MJCFImporter(config).import_mjcf()

CLI: source/standalone_examples/api/isaacsim.asset.importer.mjcf/mjcf_import.py (mirrors URDF: --mjcf, --usd-path, --import-scene, --robot-type, --override-gain-type, --override-bias-type, etc.).

Legacy MJCF commands MJCFCreateAsset / MJCFCreateImportConfig are deprecated; use MJCFImporter directly.

Migration: for the broader omni.importer.mjcf / omni.importer.urdf → isaacsim.asset.importer.* rename map, see Renaming Extensions.

Asset Transformer (what run_asset_transformer=True runs)

isaacsim.asset.transformer executes ordered USD rule pipelines. Default post-import rules include UrdfToMjcPhysxConversionRule / MjcToPhysxConversionRule and material routing. To run manually on an existing USD:

from isaacsim.asset.transformer import AssetTransformerManager, RuleProfile

manager = AssetTransformerManager()
profile = RuleProfile.from_json("path/to/profile.json")
report  = manager.run("input.usd", profile, package_root="/tmp/out")

CLI: source/standalone_examples/api/isaacsim.asset.transformer/run_asset_transformer.py (--input, --profile, --output).

Robot Schema applied during import

The importer applies the modern usd.schema.isaac.robot_schema:

• IsaacRobotAPI on the robot root prim (stores robot_type, ordered link/joint relations, named-pose container).
• IsaacLinkAPI on rigid links.
• IsaacJointAPI on joints.
• IsaacSiteAPI on sites (replaces deprecated IsaacReferencePointAPI).
• IsaacNamedPose prims for named poses; manage via the isaacsim.robot.poser module (see manipulation-ik).

Validate after import:

from pxr import Usd
from usd.schema.isaac.robot_schema import Classes, get_allowed_tokens, Attributes

stage = Usd.Stage.Open("/path/out/robot.usd")
robot = next(p for p in stage.Traverse() if p.HasAPI(Classes.ROBOT_API))
print(robot.GetAttribute(Attributes.ROBOT_TYPE).Get())

Path 2 — Isaac Lab convert_urdf.py / convert_mjcf.py (config.yaml)

For Isaac Lab-native workflows you have a config.yaml per robot under assets/isaaclab/Robots/<Robot>/:

asset_path: /path/to/robot.urdf      # pre-expand XACRO first
usd_file_name: robot_name.usd

force_usd_conversion: true
make_instanceable: true              # critical for parallel envs
import_inertia_tensor: true          # use URDF inertia
merge_fixed_joints: true
self_collision: false

fix_base: false
default_drive_type: none             # "none" | "position" | "velocity"
default_drive_stiffness: 0.0
default_drive_damping: 0.0

link_density: 0.0
convex_decompose_mesh: false

Run:

cd "$ISAAC_LAB_DIR"
./isaaclab.sh -p scripts/tools/convert_urdf.py \
  --config /path/MyRobot/config.yaml --output /path/MyRobot/
# MJCF equivalent:
./isaaclab.sh -p scripts/tools/convert_mjcf.py \
  --config /path/MyRobot/config.yaml --output /path/MyRobot/

Drive presets

make_instanceable: true

GPU mesh instancing. Without it, 4096 envs * full mesh = VRAM blow-up. With it, parallel envs share one mesh in VRAM. Always set for RL.

fix_base by robot type

URDF export (USD -> URDF round-trip)

isaacsim.asset.exporter.urdf is the inverse path; useful for sharing imported USD assets back to ROS or other URDF-consumers.

from pxr import Usd
from isaacsim.asset.exporter.urdf import UsdToUrdfConverter

stage = Usd.Stage.Open("/path/robot.usd")
converter = UsdToUrdfConverter(
    stage,
    root_prim_path="/World/robot",
    mesh_dir_name="meshes",
    mesh_path_prefix="./",
    visualize_collision_meshes=False,
    variant_selections=None,
)
converter.export("/path/urdf_out")

CLI: source/standalone_examples/api/isaacsim.asset.exporter.urdf/urdf_export.py (--usd-path, --output-dir, --root-prim, --mesh-prefix, --variant SET=SELECTION).

Common pitfalls

Pip-wheel availability (standalone usage)

Each importer/exporter is also published as a standalone pip wheel for non-Kit consumers. See .cursor/rules/pip_packaging.mdc for the design.

Build via ./repo.sh build_standalone_wheels --ext <name>.