How to Add a Deformable Object Underneath a Humanoid Foot to Simulate Foot Padding?

[shubhankarKulk]

Intro

I am a Masters Student using MuJoCo for academic purposes

My setup

MuJoCo version 3.2.7. Using Python architecture. OS Ubuntu (WSL). Version 24.04

My question

Hi, I am currently working on a humanoid simulation project, and I am facing a challenge related to simulating foot padding. Specifically, I am trying to add a deformable object underneath the foot of the humanoid to improve the accuracy of the foot's motion, especially the rolling motion as it interacts with the ground.

Could anyone advise on how to approach this in a way that allows the deformable object to behave correctly in the simulation, especially in terms of interacting with the foot and the ground? Any suggestions or insights on how to implement this would be greatly appreciated!

Thank you!

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.

[Balint-H]

Hello!

It depends a bit on the scale of your simulation. Are you also simulating the rest of the body in e.g., a locomotion setting? Or just the foot itself as it is being loaded?

If it is on the body scale:

A simple approach you could do is to segment the foot into a handful of segments (even if you don't add articulation between them) and cover each segment with a collision primitive shape. The simplest case would be adding three small spheres in the plane of the foot, but you can scale up the complexity as needed. Then you can adjust the collision parameters with these geoms, making them softer or more rigid as needed. Check out the solver section of the documentation for more info on how to configure this in your model: https://mujoco.readthedocs.io/en/stable/modeling.html#csolver

Here's an example from the skeletal frame of MyoSuite's leg model:

The collision primitives covering it:

Labelling the joints of the 2 DoF ankle and the MTP joint:

If you need even finer grain I suppose you could try it using the elasticity plugin for higher resolution, deformable finite element-style model.

[shubhankarKulk]

I am trying to see if I can pair an flexcomp component for the underside of the foot of the current humanoid.xml model. I don't know if that is the best approach to go with my goal, as I want to try to find deformable foots for the entire body to see if reinforcement learning helps effective rolling motion of the foot.

[yuvaltassa]

Look at the video at the top of the latest changelog, (as of today, this will obviously change soon), I suspect this is what you want? (fast deformables with limited dofs)

[shubhankarKulk]

Yes, and I was wondering if I can pair it to another geometry, so that they move together. If there is a contact of the foot on the ground, it would first hit the deformable object. Wanted to try if this works for attaching it under the foot of the humanoid.xml model to start with.

[yuvaltassa]

Yes of course, just like those finger pads are pinned to the fingers.