Custom robot imported from MJCF cannot be loaded into Isaac Lab: Failed to find a single articulation when resolving

[Yasu31]

I'm encountering an error with having multiple articulations in the prim path when adding my custom MJCF robot model to IsaacSim. I'm probably missing something straightforward, and I'd greatly appreciate any guidance. I've prepared a minimal reproducible example using a sample MJCF gripper model available on my branch here: https://github.com/Yasu31/IsaacLab/tree/simple-mjcf-gripper.

After converting my sample MJCF gripper model to USD with:

python scripts/tools/convert_mjcf.py simple_gripper.xml simple_gripper.usd --fix-base --import-sites --make-instanceable

I saved it to source/isaaclab_assets/data and adapted hands.py and the Isaac-Repose-Cube-Shadow-Direct-v0 environment to load my simple_gripper. You can review my exact changes here: main...Yasu31:IsaacLab:simple-mjcf-gripper

However, when I run hands.py or the training script, the following error occurs:

Traceback (most recent call last):
  File "/home/yasu/IsaacLab/source/isaaclab/isaaclab/assets/asset_base.py", line 94, in <lambda>
    lambda event, obj=weakref.proxy(self): obj._initialize_callback(event),
  File "/home/yasu/IsaacLab/source/isaaclab/isaaclab/assets/asset_base.py", line 265, in _initialize_callback
    self._initialize_impl()
  File "/home/yasu/IsaacLab/source/isaaclab/isaaclab/assets/articulation/articulation.py", line 1166, in _initialize_impl
    raise RuntimeError(
RuntimeError: Failed to find a single articulation when resolving '/World/Origin1/Robot'. Found multiple '[Usd.Prim(</World/Origin1/Robot/base>), Usd.Prim(</World/Origin1/Robot/worldBody>)]' under '/World/Origin1/Robot'. Please ensure that there is only one articulation in the prim path tree.
2025-03-26 20:11:34 [10,075ms] [Warning] [isaaclab.actuators.actuator_pd] The <ImplicitActuatorCfg> object has a value for 'effort_limit'. This parameter will be removed in the future. To set the effort limit, please use 'effort_limit_sim' instead.
2025-03-26 20:11:35 [11,961ms] [Warning] [omni.hydra.scene_delegate.plugin] Calling getBypassRenderSkelMeshProcessing for prim /World/Origin1/Robot/base/finger_left/collisions.proto_finger_left_geom_id0 that has not been populated
[INFO]: Setup complete...
Traceback (most recent call last):
  File "/home/yasu/IsaacLab/scripts/demos/hands.py", line 166, in <module>
    main()
  File "/home/yasu/IsaacLab/scripts/demos/hands.py", line 161, in main
    run_simulator(sim, scene_entities, scene_origins)
  File "/home/yasu/IsaacLab/scripts/demos/hands.py", line 115, in run_simulator
    root_state = robot.data.default_root_state.clone()
  File "/home/yasu/IsaacLab/source/isaaclab/isaaclab/assets/articulation/articulation.py", line 107, in data
    return self._data
AttributeError: 'Articulation' object has no attribute '_data'. Did you mean: 'data'?

I found out that if in the Isaac Sim window, I do:

1. "File" -> "Save Flattened As..."
2. delete the worldBody

it can be loaded, but then it's not an instanceable asset anymore and the same error as #2113 happens where only the first robot in all of the environments is articulated, so this is not a solution:

Could you please advise me on what I'm missing? Removing the top level body from the MJCF didn't work.

[Yasu31]

update: by a suggestion from my colleague @riccardofeingold , I have changed the top-level body name in MuJoCo to worldBody (as in this commit Yasu31@9c907e5 ), which gets rid of the Failed to find a single articulation problem, as there is no more separate worldBody in the tree.

However, the problem reported in #2113 in which only the first instance is actuated still happens... I will close this thread as my original problem was solved.

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