Failure importing clearpath warthog urdf

[jcsantamaria]

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Isaac Sim Version

4.5.0
4.2.0
4.1.0
4.0.0
4.5.0
2023.1.1
2023.1.0-hotfix.1
Other (please specify):

Operating System

Ubuntu 22.04
Ubuntu 20.04
Windows 11
Windows 10
Other (please specify):

GPU Information

• Model: NVIDIA GeForce RTX 3070
• Driver Version: 560.94

Topic Description

Detailed Description

Importing the Clearpath Warthog URDF using the import function. Isaacsim reports errors and hangs. It is difficult to assess at which import step the import failed.

Steps to Reproduce

1. Launch Isaacsim with ROS2 extension enabled
2. Open a new scene
3. Try File->Import: navigate to warthog_description/urdf and select warthog.urdf (use the attachment compressed file: warthog_description.zip)
4. Import fails, application hangs.

Error Messages

(If applicable, copy and paste any error messages you received)

2025-04-21 18:52:51 [45,924ms] [Warning] [omni.usd] Warning: in AppendChild at line 775 of C:\g\122538378\USD\pxr\usd\sdf\path.cpp – Invalid prim name ‘e-stop’

2025-04-21 18:52:51 [45,924ms] [Warning] [omni.usd] Warning: in AppendChild at line 775 of C:\g\122538378\USD\pxr\usd\sdf\path.cpp – Invalid prim name ‘e-stop’

2025-04-21 18:52:51 [45,925ms] [Warning] [omni.usd] Coding Error: in _IsValidPathForCreatingPrim at line 3340 of C:\g\122538378\USD\pxr\usd\usd\stage.cpp – Path must be an absolute path: <>

2025-04-21 18:52:51 [46,016ms] [Error] [omni.kit.commands.command] Failed to execute a command: URDFImportRobot.
File “d:/isaacsim/extscache/isaacsim.asset.importer.urdf-2.3.10+106.4.0.wx64.r.cp310/isaacsim/asset/importer/urdf/scripts/extension.py”, line 459, in overwrite_callback
self._load_robot(dest_path, path)
File “d:/isaacsim/extscache/isaacsim.asset.importer.urdf-2.3.10+106.4.0.wx64.r.cp310/isaacsim/asset/importer/urdf/scripts/extension.py”, line 586, in _load_robot
omni.kit.commands.execute(
File “d:/isaacsim/extscache/omni.kit.commands-1.4.9+d02c707b.wx64.r.cp310/omni/kit/commands/command.py”, line 463, in execute
result = omni.kit.undo.execute(command, name, kwargs)
[…skipped…]
File “d:/isaacsim/extscache/omni.kit.commands-1.4.9+d02c707b.wx64.r.cp310/omni/kit/undo/undo.py”, line 78, in execute
result = _execute(command, name, level, history_key)
File “d:/isaacsim/extscache/omni.kit.commands-1.4.9+d02c707b.wx64.r.cp310/omni/kit/undo/undo.py”, line 459, in _execute
raise error
File “d:/isaacsim/extscache/omni.kit.commands-1.4.9+d02c707b.wx64.r.cp310/omni/kit/undo/undo.py”, line 420, in _execute
result = command.do()
File “d:/isaacsim/extscache/isaacsim.asset.importer.urdf-2.3.10+106.4.0.wx64.r.cp310/isaacsim/asset/importer/urdf/scripts/commands.py”, line 145, in do
return self._urdf_interface.import_robot(

<class ‘RuntimeError’> Used null prim
2025-04-21 18:52:51 [46,018ms] [Error] [omni.ui.python] RuntimeError: Accessed invalid null prim

At:
d:/isaacsim/extscache/isaacsim.asset.importer.urdf-2.3.10+106.4.0.wx64.r.cp310/isaacsim/asset/importer/urdf/scripts/extension.py(594): _load_robot
d:/isaacsim/extscache/isaacsim.asset.importer.urdf-2.3.10+106.4.0.wx64.r.cp310/isaacsim/asset/importer/urdf/scripts/extension.py(459): overwrite_callback

Screenshots or Videos

(If applicable, add screenshots or links to videos that demonstrate the issue)

Additional Information

What I’ve Tried

Import of turtlebot3_burger.urdf works fine.

I have also tried removing parts from the warthog.urdf, but the import failure still occurs

Related Issues

(If you’re aware of any related issues or forum posts, please link them here)

Additional Context

(Add any other context about the problem here)

warthog_description.zip (580.1 KB)

[VickNV]

As noted in URDF Importer Extension — Isaac Sim Documentation, USD requires prim names to use only alphanumerics and underscores. You may need to follow the following note to make the changes.

Warning

To comply with USD prim name conventions, special characters in link or joint names are not supported and will be replaced with an underscore. In the event that the name starts with an underscore due to the replacement, an a is pre-pended. It is recommended to make these name changes in the URDF directly.

[jcsantamaria]

Hi VickNV, thanks for the reply.
I don’t see any violations in the naming of joints or links. Perhaps some of them are too long?
Here it is a grep of joint and links

  <link name="front_left_wheel_link">
  <joint name="front_left_wheel_joint" type="continuous">
    <joint name="front_left_wheel_joint">
  <link name="front_right_wheel_link">
  <joint name="front_right_wheel_joint" type="continuous">
    <joint name="front_right_wheel_joint">
  <link name="rear_left_wheel_link">
  <joint name="rear_left_wheel_joint" type="continuous">
    <joint name="rear_left_wheel_joint">
  <link name="rear_right_wheel_link">
  <joint name="rear_right_wheel_joint" type="continuous">
    <joint name="rear_right_wheel_joint">
  <link name="left_diff_unit_link">
  <joint name="left_diff_unit_joint" type="revolute">
  <link name="left_diff_unit_buttons_link">
  <joint name="left_diff_unit_buttons" type="fixed">
  <link name="left_diff_unit_antenna_link">
  <joint name="left_diff_unit_antenna_joint" type="fixed">
  <link name="left_diff_unit_headlight_link">
  <joint name="left_diff_unit_headlight_joint" type="fixed">
  <link name="left_diff_unit_taillight_link">
  <joint name="left_diff_unit_taillight_joint" type="fixed">
  <link name="right_diff_unit_link">
  <joint name="right_diff_unit_joint" type="revolute">
  <link name="right_diff_unit_buttons_link">
  <joint name="right_diff_unit_buttons" type="fixed">
  <link name="right_diff_unit_antenna_link">
  <joint name="right_diff_unit_antenna_joint" type="fixed">
  <link name="right_diff_unit_estop_antenna_link">
  <joint name="right_diff_unit_estop_antenna_joint" type="fixed">
  <link name="right_diff_unit_headlight_link">
  <joint name="right_diff_unit_headlight_joint" type="fixed">
  <link name="right_diff_unit_taillight_link">
  <joint name="right_diff_unit_taillight_joint" type="fixed">
  <link name="base_link"/>
  <joint name="base_link_joint" type="fixed">
  <link name="chassis_link">
  <link name="top_chassis_link"/>
  <joint name="top_link_joint" type="fixed">
  <link name="diff_link">
  <joint name="diff_link_joint" type="fixed">
  <link name="imu_link"/>
  <joint name="imu_link_joint" type="fixed">

[jcsantamaria]

Ok, I try loading the warthog.urdf in Robot URDF viewer (http://urdf.robotsfan.com/) and it all seems fine.

[VickNV]

I was able to resolve the import issue by renaming all .stl mesh filenames that contained dashes (- ) to use underscores (_ ) instead, and then updating all references to those filenames throughout the URDF, Xacro, and related files accordingly. After making these changes, the Warthog URDF was imported successfully into Isaac Sim.

[jcsantamaria]

Hi VickNV,
That did the trick!
Thank you!