Is diffsim supported in the MuJoCo backend?

[Siqi-MarvelLi]

I would like to ask whether the differentiable simulation (diffsim) feature is currently ready for the MuJoCo backend. I noticed in the documentation that MJWarp is not currently differentiable, and I’m wondering what the current status is.

Additionally, could you clarify which backend(s) currently support differentiable simulation?

[adenzler-nvidia]

Speaking for MuJoCo Warp the documentation is still accurate differentiability is neither supported nor actively being worked on. We are hoping to get there at some point, it's quite a bit of work though.

[momo-van]

@Siqi-MarvelLi please provide some information on how MJWarp differentially can help your use-case. Feel free to open a feature request in MJWarp and/or Newton repo.

[ytliu-inventec]

Hi @momo-van @adenzler-nvidia

I would like to ask if there is any progress on this?

Our team is targeting several server assembly tasks:

• dimm insertion (https://www.youtube.com/watch?v=CXQOEV0-UCU)
• rj45 insertion (https://www.youtube.com/watch?v=oSBQWlal91s) or (https://www.youtube.com/watch?v=1kNWTuYqLzI)
• cable routing (https://www.youtube.com/watch?v=ZGg7peSq_6I)
• bolt tightening (https://www.youtube.com/watch?v=ROCSHXWypvA)
• module assembly (https://www.youtube.com/watch?v=s0Do3UkRXEQ)
  The videos are only illustrations, as I do not possess real videos. These are all contact-heavy tasks and require realistic contact dynamics, and this is why we are building upon the Newton simulator.

We encounter multiple challenges regarding this approach, mainly:

• Newton's native solver stack is unstable, and object clipping through one another is still commonly seen. On the contrary, the mujoco_solver is relatively stable. But then it does not provide an analytic gradient like mujoco_mjx, which limits its capability to perform downstream tasks, like analytic gradient-based system identification or RL methods for contact-heavy tasks.
• Newton's has not ported Mujoco's cable model. This is critical for realistic RJ45 cable insertion and cable routing tasks.

I am happy to answer questions and provide more details.

[adenzler-nvidia]

Thanks for bumping this, it's probably a good idea to look at this: google-deepmind/mujoco_warp#1226 which is a community-contributed PR to mujoco warp, with a bit of discussion. The author does have a standalone fork that has a partial implementation, maybe that helps your case. could you try that out and give some feedback if you encounter any problems?