Collision between two bodies is not always being detected

[adlarkin]

Intro

Hi!

I am a graduate student using MuJoCo for my research in robotic manipulation.

My setup

MuJoCo version 3.2.6 (python - pip install mujoco)

MacOS 15.3.1, arm architecture (apple M3 chip)

My question

I am using the franka panda model from mujoco menagerie (link here). I have noticed that when the two fingers have a joint position of 0, a collision is detected between the fingers sometimes, but not all the time. I've attached a video of what I am seeing, with collisions enabled in the viewer.

What I'm wondering is, is this a bug or expected behavior?

Screen.Recording.2025-02-22.at.1.11.27.PM.mov

This is what the fingertip collision mesh looks like, which makes me wonder if the non-convex nature of the surface is a contributor to this behavior:

Minimal model and/or code that explain my question

Download this model and then open it in the viewer (I am using scene.xml): https://github.com/google-deepmind/mujoco_menagerie/tree/main/franka_emika_panda

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]

Looks like the 0 position of the fingers is at the closed position, exactly. So position sensors have no control authority, they cannot press fingers together at 0 distance. So yes, this WAI. If it is a bug or not depends on how the real thing works...