URDF Importer doesn't import Joint Structure Correctly

[K-Wince]

Description

When using the URDF importer, the robot will import into the scene, however upon trying to test it, the robot is shown to have all of the links independent of the previous links in the chain.

I believe this is because all of the joints label the Body0 param as the root prim, rather than the previous prim in the chain. This can be fixed by going through the joints and changing the Body0 param, suggesting it is this.

An example of the URDF file is provided in the txt.

Robot URDF

Isaac Sim version

6.0.0-rc.22

Operating System (OS)

Ubuntu 24.04

GPU Name

RTX 4070

GPU Driver and CUDA versions

Driver 590.48.01, CUDA 13.1

Logs

No response

Additional information

No response

[vick-yu]

Hi @K-Wince,

Thanks for the detailed report and the URDF file.

I investigated this and found two things:

1. The provided URDF is missing <geometry> elements.

Every link in your URDF has <visual> and <collision> elements, but none
of them contain a <geometry> child. For example:

<visual>
  <origin rpy="0 0 0" xyz="0 0 0"/>
  <material name="">
    <color rgba="0.75294 0.75294 0.75294 1"/>
  </material>
</visual>

This is missing the required:

<geometry>
  <mesh filename="package://kinova_description/meshes/base_link.STL"/>
</geometry>

This usually happens when running xacro without the mesh package available,
or when mesh paths can't be resolved. Could you check your xacro expansion
and ensure the mesh files are referenced correctly?

2. The importer should handle this more gracefully

Even with missing geometry, the URDF joint chain is well-defined and the
importer should either reject the file with a clear error or import the
kinematic structure correctly without visuals. Setting all joints' Body0
to the root prim is incorrect.

On Isaac Sim 5.1, the importer aborts early when it encounters the missing
geometry. On 6.0 it continues but produces broken joint topology.

Workaround: Re-export your URDF with proper mesh references. The Kinova
Gen3 meshes are available in the ros2_kortex
package. Run your xacro with the mesh package paths properly sourced.

[K-Wince]

Hi @vick-yu,

Sorry, I should have been clear about that. I removed the geometry tags myself since they included hardcoded paths when using the xacro command. It imports with meshes into IsaacSim with those tags in there, but the Body0 issue is still present. I've also tried the way of using the ROS2 Node method and URDF file method with our full robot and the issue remains.

Thanks for helping!

[vick-yu]

Hi @K-Wince,

Thanks for the clarification. I was able to reproduce the Body0 bug on Isaac Sim 6.0.0-rc.22 using a Kinova-like URDF with valid geometry. The trigger is the world link (a link with no visual/collision/inertia) connected via a fixed joint — referred to as a "Ghost Link" in the underlying converter.

Root cause: Isaac Sim 6.0.0-rc.22 bundles urdf-usd-converter v0.1.0, which has a known bug where all joints' Body0 targets are set to the root prim when a Ghost Link is present at the root of the kinematic tree.

This is already fixed upstream. The fix landed in urdf-usd-converter v0.1.1 (released 2026-03-30):

• Issue: If the root is a GhostLink, the body0 of the joint is always the root newton-physics/urdf-usd-converter#98
• Fix: Fixed a bug where all the body0 components of joints would become root prims when a Ghost Link was present newton-physics/urdf-usd-converter#99

[K-Wince]

Hi @vick-yu,

Ahh, that makes sense. Thanks for the clarification, looking forward to it being fixed in the next release. Thanks again for all of your help!