How to create a closed-loop (ring) leaf structure with rigid “weld” in Articulation?

Omniverse Isaac Sim
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1

Hi all,

I’m trying to model a plant leaf with four connected rigid parts in Isaac Sim using USD/PhysX Articulation.
The structure is like a “ring” (closed-loop):

  • Leaf1 connects to Leaf2 by a hinge joint
  • Leaf2 connects to Leaf3 by a hinge joint
  • Leaf3 connects to Leaf4 by a hinge joint
  • Leaf4 should connect back to Leaf1 with a rigid “weld” (distance constraint, or fixed joint)

My current setup:

  • I use Articulation (chain of hinge joints) for all leaves
  • I tried to add a DistanceJoint (or FixedJoint) between Leaf4 and Leaf1 to form a closed loop

Issue:

  • PhysX reports: Articulation cannot contain a distance joint! (Please exclude it from articulation)
  • If I use only Articulation joints, I cannot create a “ring” because articulation must be tree-shaped

Questions:

  1. What is the recommended way to “weld” (rigidly connect) two non-parent/child links (e.g., Leaf1 and Leaf4) in the same Articulation, forming a closed-loop, in Isaac Sim/PhysX?
  2. Is there any official workaround to create a “closed kinematic loop” or “ring” structure in articulation?
  3. If not possible in Articulation, is there any other PhysX-supported mechanism in Isaac Sim to achieve this kind of loop with strict distance or rigid constraint?

Notes:

  • I do not want the “soft” constraint; it should be as rigid as possible, like a true weld.
  • Using PhysxSchema is not an option on my current setup.

Any advice, sample code, or official documentation reference would be appreciated!

Thanks!

3

Hi @zhaowentao,

Thank you for posting your issue. For the question on how to fix the joint, I think using a FixedJoint should weld the two links together. I think Distance Joints are not supported in Articulations.

In terms of closed kinematic chains, I think it is still possible to enable Articulation property on the prim tree. However, you cannot have nested Articulations. E.g. if you enable articulation for the base prim, you cannot have child prims that also have the Articulation enabled.

4

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