Regarding contact and friction behavior for flat foot humanoid in MuJoCo

[r3tro-t3ch]

Intro

I am Vishnu Joshi a researcher in StochLab, Indian Institute of Science, Bengaluru. Me and my team are currently working on a walking controller using MuJoCo simulator and had a few observations.

My setup

MuJoCo version 3.1.4, python, ubuntu 22.04

My question

When the floor is a simple box geom with friction coefficient (0.8), the robot walks properly and has no issue, but when the same floor is defined by a hfield with the same friction coefficient (0.8) the foot is unable to stay stable.
I realised that increasing the nrow and ncol variable did improve the situation but it is not similiar to the behaviour when the floor is simple box geom. I tried going through all the resources and found that the number of contacts between geom and hfield is restricted to 50 and changing it can break the simulation.

I have attached two videos, one with simple box geom and other with hfield with respective names.

walking_box_geom.mp4

walking_hfield_short.mp4

Minimal model and/or code that explain my question

No response

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]

Read these links:
https://mujoco.readthedocs.io/en/latest/XMLreference.html#option-flag-multiccd
https://mujoco.readthedocs.io/en/latest/modeling.html#preventing-slip