Question about isaac sim 4.5 and isaac sim 5.0+

[iguhuifeide]

Outstanding performance！

I found that when I collected data and performed fitting using isaac sim 5.1, the fitting results were satisfactory. However, when I used isaac sim 4.5 to collect data and perform fitting, it was difficult for the process to converge.

I noticed that your readme.md specifically mentions, "These warnings are expected and indicate that certain physics properties (e.g. joint viscous friction) are not supported by the older simulator version. While the examples may still run, the physical fidelity and sim-to-real accuracy will be reduced."

I don't understand why using Isaac Sim 4.5 to collect data and simulate results in fitting failure, because even in older versions, the physics engine used for data collection should be consistent with the physics engine used for fitting.
I would like to ask you what caused this problem? Thank you！

[fbjelonic]

Thanks for the detailed question.
While Isaac Sim 4.5 and 5.1 both use PhysX, the effective physics interface exposed to the user is not identical. PACE was developed and validated against Isaac Sim 5.1, and in 4.5 some physics properties that PACE expects (e.g. certain joint or drive parameters) are either not supported, named differently, or ignored with warnings. As a result, the parameter set that is optimized during fitting may not actually be fully applied in the simulator, which can lead to poor conditioning and non-convergence.
We did not actively maintain backward compatibility with Isaac Sim 4.5, mainly due to limited capacity. Supporting 4.5 would require identifying and mapping the relevant physics parameters correctly for that version.
If you’re interested in contributing a fix for Isaac Sim 4.5, we’d be happy to review a PR and include it as a dedicated compatibility branch/tag.

What exactly do you mean by "I don't understand why using Isaac Sim 4.5 to collect data and simulate results in fitting failure"? Did you collect data on the real robot and used Isaac Sim 4.5 for the fitting or did you collect the data in Isaac Sim 4.5 and fitted in 4.5?

[fbjelonic]

Closing this due to inactivity.

I shared the explanation above regarding Isaac Sim 4.5 vs 5.1.
If you have further details about your setup (real robot vs sim data collection), feel free to reopen or comment.