Questions about Creating and Using Tendons in Isaac Sim

Omniverse Isaac Sim
1

Topic Description

Questions about Creating and Using Tendons in Isaac Sim

Detailed Description

I am interested in understanding how tendons work in Isaac Sim and how to create/use them in my robotic models.

My specific questions are:

How do I create tendons in Isaac Sim?
    What is the recommended workflow or tool for generating tendons within Isaac Sim?
    Is there a graphical interface, or do I define them via USD or another scripting method?

Can I create tendons externally (e.g., in Blender) and import them into Isaac Sim?
    If so, what is the correct export/import process?
    Are there specific constraints or file formats required for tendons to be recognized by Isaac Sim?

Can tendons be created or manipulated programmatically (e.g., with Python scripts) in Isaac Sim?
    If yes, what API or modules should I use?
    Are there example scripts or documentation to help guide custom tendon creation and control?

Are there key things to consider or common issues when working with tendons in Isaac Sim?
    (e.g., limitations, compatibility, or best practices)

Where can I find example USD files or templates that demonstrate tendon usage in Isaac Sim?

3

Hi @trent.black, thank you for posting your questions.
Isaac Sim supports PhysX tendons for simulating coupled joint mechanisms like cable-driven systems. There are two types:

  • Fixed Tendons: Couple multiple joints with fixed gear ratios
  • Spatial Tendons: Model physical cables/ropes passing through attachment points

1. Creating Tendons in Isaac Sim
USD-Based Definition
The primary way to define tendons is through USD. This can be done programmatically as examples can be found here: Articulations — Omni Physics
or through Isaac Sim GUI. I think it is also possible to author USD with UsdPhysics API in blender, but I am not familiar with this and haven’t seen an example of import from Blender.

For Fixed Tendons, you apply PhysxTendonAxisRootAPI to the root joint of the tendon chain and PhysxTendonAxisAPI to other participating joints. Key attributes include gearing (coupling ratio), stiffness, damping, limitStiffness, and offset.
For Spatial Tendons, attachment points are defined on rigid bodies. Properties include stiffness, damping, limitStiffness, and offset (rest length).

Are there key things to consider or common issues when working with tendons in Isaac Sim?
(e.g., limitations, compatibility, or best practices)

Best Practices:

  • Use Fixed Tendons for mechanical coupling (gear systems, differential drives)
  • Use Spatial Tendons for cable/rope simulation (robotic hands, pulley systems)
  • Set appropriate stiffness values to avoid simulation instability
  • Use damping to control oscillation behavior

You can find examples using Fixed and Spatial Tendon in the physics examples prepackaged with Isaac Sim. Go to Windows>Examples>Physics Examples and search for Tendon.

4

Hi @michalin ,

I am trying to use one of the examples from articulations.html link you sent

I am getting an error when running the script: ImportError: cannot import name ‘PhysxSchema’ from ‘pxr’

I am using Isaac Sim 5.1.0.

Environment Notes:

  • Using ./python.sh wrapper.

  • SimulationApp is initialized before the import.

  • no docker

Has this extension been removed in 5.1.0? or are those scripts outdated?

Could you please advise on how to run these example scripts on Isaac Sim 5.1.0.

Thanks for the help