Support Parallel Robots?

Does Isaac sim support parallel manipulators like the Stewart Platform?

I am designing a variant of the Stewart Platform with cardan joints in Onshape - with the goal to simulate some simple routines with collision detection.

I can simulate close-chain mechanisms (like four-bar linkages), so I think you should be able to simulate parallel manipulators too.

The key difference is that you should exclude some joints from articulations since the API requires link relationships to be a tree structure. But the joints will still be simulated normally (for parallel robots there must be some joints that are not actuated by motors, these joints should be the ones excluded from the articulation API).

One problem I am facing now is that the excluded joints are not constrained very well. For example, for a revolute joint connecting two links, the links should only rotate around the joint, but in simulation, you will find the links are translating between each other with a small dislocation (depending on “position solver iteration”). Increasing position solver iteration will mitigate the problem but will instead slow down the simulation.

I guess it is because the PhysX engine needs more iteration to make the close chain more accurately simulated. I do not know if this will make the dynamics of close chains unreliable or unrealistic. I hope the experts in Nvidia can answer the question.

Thank you for the great reply - I can confirm that this is the correct approach. Model the robot as a tree as far as you can, and then add the loop-closing joints with the exclude from articulations flag set. The loop-closing joints will be simulated as external constraints on the articulation, so you may have to tune the positions iterations / lower the timestep to allow good solver convergence.

The official docs briefly address this too: Articulations — Omniverse Extensions documentation

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