How to create hinge/slide joints via equality for mechanism?

[Justpenz233]

Hi!

I am a graduate student, I use MuJoCo for my research on mechanism simulation.

My question

There are closed-loops in mechanism, which may contain many type of joints, yet some of the joints have to be created as equality in MuJoCo.

Thus I would like to know how to create hinge(revolute joint) or slide joint or cylindrical joint (both revolute and slide) by equality? Is MuJoCo capable of this for now?

Confirmations

• I searched the latest documentation thoroughly before posting.
• I searched previous Issues and Discussions, I am certain this has not been raised before.

[yuvaltassa]

Here is an example
https://github.com/google-deepmind/mujoco_menagerie/tree/main/robotiq_2f85

[Justpenz233]

Ah I see. While this is a feature we could add, it does not yet exist. What you want is a like a connect, but only constrains two axes rather than 3, right?

Yes, exactly!
'Connect' constrains 3d position only, 'hinge' equality (if there is) would constrain 3d position and 2 euler. 'Slide' would constrain 2d position and 3 euler. I think the other joint types could be done by the combination of the 'hinge' and 'slide' equality.

Do you think there is a way i could implement these features for now?

[yuvaltassa]

The simple way to do this currently is to use a regular slide joint with a weld constraint. Does that make sense?

[Justpenz233]

Like create a virtual body right? I am not sure if the mass of the virtual body would affect the simulation result, but sounds like it is the only method for now.

[yuvaltassa]

That's a valid concern. If you're worried about added mass you can add small armature to the slide as well.

[Justpenz233]

I see. But i'd like to know do you have plan to add this feature in the moment? This feature is quit essential for mechanisms.