Accessing Spatial Tendon Data in Isaac Lab and Discrepancy with MuJoCo Menagerie Import

[vreithmei-droid]

Hi IsaacLab Team,

I am currently working on integrating Spatial Tendons into the Unitree H1 humanoid. While the visual integration and basic physics seem to work in Isaac Sim, I am encountering issues regarding data access via the API and consistency during sanity checks.

1. Accessing Tendon Data in Isaac Lab

I modified the h1.xml (from MuJoCo Menagerie), imported it into Isaac Sim, added a Spatial Tendon (stiffness = 10000), and saved it as h1_tendon.usd. In a simple drop test within Isaac Sim, the tendon behaves as expected. However, when training in Isaac Lab, I cannot find the tendon variables within the robot.data object.

• Question: Where is the spatial tendon length stored in the Isaac Lab API? Or where can I find the position of the root/ leaf and attachment points to calculate the length myself.

• Goal: I need to read these values for the observation space or reward calculation.

2. Discrepancy during Sanity Check (Default H1 vs. Imported XML)

I performed a sanity check: I compared the default/ minimal Isaac Lab h1.usd with my imported version where I set the tendon stiffness to 0. Theoretically, the training results should be identical. However, the behavior differs significantly.

• Observation: It appears that the default h1.usd in Isaac Lab/Isaac Sim differs from the h1.xml provided by the DeepMind MuJoCo Menagerie.

• Question: Can you clarify the origin of the official Isaac Lab H1 USD file? Are there specific damping, friction, or armature values applied that differ from the standard MuJoCo Menagerie XML?

Thanks in advance