Isaac Sim 4.5.0 URDF Importer: Links with defined properties (mass, inertia, collision) reported as "no body properties" and merged

Omniverse Isaac Sim
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1

Isaac Sim Version

[o] 4.5.0

Operating System

[o] Ubuntu 22.04

GPU Information

  • Model: RTX 4080
  • Driver Version: 550.144.03

Topic Description

Hello,

I’m encountering an issue with the URDF importer in Isaac Sim version 4.5.0. Several links in my turtlebot3_lime.urdf file, which have <inertial> and <collision> tags clearly defined, are being reported by the importer as having “no body properties (mass, inertia, or collisions)” and are subsequently merged into their parent links. This occurs even for critical links like base_link, camera_link, and link1.

The most critical issue is with my main chassis link, base_link. It has all necessary physical properties defined, but the log shows it’s being merged into base_footprint (which is a property-less link in my URDF). This effectively removes the primary physical body of the robot.

base_link URDF snippet:

<link name="base_footprint"/>

<joint name="base_joint" type="fixed">
  <parent link="base_footprint"/>
  <child link="base_link"/>
  <origin rpy="0 0 0" xyz="0 0 0.060"/>
</joint>

<link name="base_link">
  <visual>
    <origin rpy="0 0 0" xyz="0 0 0"/>
    <geometry>
      <mesh filename="meshes/turtlebot3_bigwheel_orin/base.stl" scale="0.001 0.001 0.001"/>
    </geometry>
    <material name="light_black"/>
  </visual>
  <collision>
    <origin rpy="0 0 0" xyz="-0.065 0 0.0225"/>
    <geometry>
      <box size="0.266 0.266 0.095"/>
    </geometry>
  </collision>
  <inertial>
    <origin rpy="0 0 0" xyz="0 0 0"/>
    <mass value="1.3729096e+00"/>
    <inertia ixx="8.7002718e-03" ixy="-4.7576583e-05" ixz="1.1160499e-04" iyy="8.6195418e-03" iyz="-3.5422299e-06" izz="1.4612727e-02"/>
  </inertial>
</link>

Similarly, other links like link1 (part of an attached manipulator), camera_link (the main camera body), base_scan (LIDAR), and caster_back_left_link/caster_back_right_link also have <inertial> and <collision> tags defined but are reported as merged.

Example: link1 URDF snippet (has visual, collision, and inertial):

<link name="link1">
  <visual>
    <origin rpy="0 0 0" xyz="0 0 0"/>
    <geometry>
      <mesh filename="meshes/open_manipulator_sara/link1.stl" scale="0.001 0.001 0.001"/>
    </geometry>
    <material name="grey"/>
  </visual>
  <collision>
    <origin rpy="0 0 0" xyz="0 0 0"/>
    <geometry>
      <mesh filename="meshes/open_manipulator_sara/link1.stl" scale="0.001 0.001 0.001"/>
    </geometry>
  </collision>
  <inertial>
    <origin xyz="3.0876154e-04 0.0000000e+00 -1.2176461e-04"/>
    <mass value="7.9119962e-02"/>
    <inertia ixx="1.2505234e-05" ixy="0.0" ixz="-1.7855208e-07" iyy="2.1898364e-05" iyz="0.0" izz="1.9267361e-05"/>
  </inertial>
</link>

Log

2025-05-31 09:32:43 [605,737ms] [Warning] [isaacsim.asset.importer.urdf] link base_link has no body properties (mass, inertia, or collisions) and is being merged into base_footprint
…
2025-05-31 09:32:44 [607,061ms] [Warning] [isaacsim.asset.importer.urdf] link link1 has no body properties (mass, inertia, or collisions) and is being merged into base_link
2025-05-31 09:32:44 [607,061ms] [Warning] [isaacsim.asset.importer.urdf] link battery_link has no body properties (mass, inertia, or collisions) and is being merged into base_link
2025-05-31 09:32:44 [607,061ms] [Warning] [isaacsim.asset.importer.urdf] link camera_link has no body properties (mass, inertia, or collisions) and is being merged into camera_bottom_screw_frame
2025-05-31 09:32:44 [607,061ms] [Warning] [isaacsim.asset.importer.urdf] link caster_back_left_link has no body properties (mass, inertia, or collisions) and is being merged into base_link
2025-05-31 09:32:44 [607,061ms] [Warning] [isaacsim.asset.importer.urdf] link caster_back_right_link has no body properties (mass, inertia, or collisions) and is being merged into base_link
2025-05-31 09:32:44 [607,061ms] [Warning] [isaacsim.asset.importer.urdf] link base_scan has no body properties (mass, inertia, or collisions) and is being merged into base_link
2025-05-31 09:32:44 [607,061ms] [Warning] [isaacsim.asset.importer.urdf] link base_link has no body properties (mass, inertia, or collisions) and is being merged into base_footprint

My questions are:

  1. Is this behavior a known issue with the URDF importer in Isaac Sim 4.5.0?
  2. Are there specific URDF formatting nuances or importer settings (e.g., related to fixed joints, articulation roots, or specific parsing requirements for <collision>/<inertial> tags) that I might be overlooking which could cause this, particularly for the base_link?
  3. What steps can I take to ensure that these critical links are correctly imported with their physical properties intact and not inappropriately merged?

I have also uploaded a zip file containing the full URDF and its associated mesh files with my forum post for complete reference.

Thank you for your time and assistance.
turtlebot3_lime_description.zip (10.0 MB)

2

@Misoka_An i think this is an unexpected behavior. there is a proposed workaround from one of the mod in another thread from a few months ago where another user experienced something similar:

hopefully they can resolve the issue in the near future.

5

Hello @Misoka_An,

This issue has been fixed in Isaac Sim 5.1.0. The release includes a specific fix: “Check for child link mass and avoid merging if it’s defined (only merge reference frames)” 1.

Your links with defined mass/inertia/collision properties should now import correctly without being incorrectly merged. Please upgrade to 5.1.0 and re-test your URDF.

Thanks for reporting this!