How to import URDF and export the USD with flatten structure

[AndreaRossetto]

I'm trying to import my urdf model into isaaclab but everytime I get a not flat USD model

In this image you can see the Tree structure

I'm not able to obtain the flat model structure. I'm using the GUI but also using the python script I obtain the same result. How can I do it?

[vick-yu]

The tree structure you're seeing is expected -- this is the Isaac Sim Asset Structure,
which the URDF importer generates by default starting in Isaac Sim 6.0. It's a multi-file
USD package rather than a single flat file.

What the importer generates

By default, the importer runs the "Isaac Sim Structure" asset transformer, which produces:

robot_name/
  robot_name.usda         <- entry point (this is what you open/reference)
  payloads/
    base.usd              <- single-layer robot base
    robot.usda            <- robot schema
    geometries.usd        <- instanced meshes
    materials.usda        <- materials + textures
    Physics/
      physics.usda
      physx.usda
      mujoco.usda

The prim hierarchy inside the stage (links, joints, etc.) is still there -- it just
lives inside payloads/base.usd. This structure separates physics engines, enables
variant switching, and optimizes mesh instancing. It's described in the
Isaac Sim Asset Structure docs.

Using the output with IsaacLab

Point IsaacLab to robot_name/robot_name.usda (the interface file at the top of the
package). That composes everything correctly.

Getting a single flat .usda file instead

If you specifically need a single file without the multi-layer package, disable the
asset transformer:

Python API:

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

importer = URDFImporter(URDFImporterConfig(
    urdf_path="/path/to/robot.urdf",
    usd_path="/path/to/output",
    run_asset_transformer=False,
))
output_usd = importer.import_urdf()

Standalone script:

cd ~/isaacsim/isaacsim && ./python.sh \
  standalone_examples/api/isaacsim.asset.importer.urdf/urdf_import.py \
  --urdf /path/to/robot.urdf \
  --usd-path /path/to/output \
  --no-run-asset-transformer

This writes a single robot_name.usda directly to the output folder.

Note: the GUI does not currently expose this option -- you'll need to use the Python
API or the standalone script to get the flat output.

"The Isaac Sim Imported assets are organized in a specific structure to make it
easier to manage, reuse, and simulate them."
-- Asset Structure docs

Let us know if the multi-file package is causing a specific issue in your IsaacLab
workflow -- happy to help dig into it further.

[AndreaRossetto]

Thank you, In the old version of isaaclab I used the flat USD so for the sensors my path was "{ENV_REGEX_NS}/Robot/torso", so I'm facing some problems for example with a contact sensor for multiple rigid body

feet_contact_sensor: ContactSensorCfg = ContactSensorCfg(
        prim_path="{ENV_REGEX_NS}/Robot/UDroid/foot.*",
        history_length=4,
        update_period=0.02,
        track_air_time=True,
        track_pose=True,
        filter_prim_paths_expr = [ groundName ],
    )
    
contact_forces = ContactSensorCfg(
        prim_path="{ENV_REGEX_NS}/Robot/UDroid/.*", 
        history_length=4, 
        track_air_time=True, 
        track_pose=True
    )

but with the new structure I'm not able to reproduce the same result. Is there a way to do it?

[vick-yu]

Could you please provide the exact versions of Isaac Lab and Isaac Sim that you are using for both the working and non-working setups? Additionally, include a minimal example script and a USD file to help reproduce the issue, as well as the problematic Kit log file.

Have you also tried using the --no-run-asset-transformer flag that I mentioned earlier?

[vick-yu]

Hi @AndreaRossetto, we haven't heard back since our last message asking for version details, a minimal repro script, and whether the --no-run-asset-transformer flag helped. We'll close this for now to keep the queue tidy. If you're still hitting this, you can reopen it directly using the 'Reopen issue' button on GitHub, or open a new issue referencing this one so we have the context. Thanks for reaching out!