Understanding Contact Forces and Friction #2375

BY571 · 2025-01-20T09:13:40Z

BY571
Jan 20, 2025

Intro

Hi!
I want to use MuJoCo to simulate a robot design however, I have difficulty understanding the contact forces and friction. I created a minimal and simplified example of a part of my robot.

My setup

mujoco: 3.2.2
python: 3.8

My question

My goal in this example is that the red ring guides and restricts the movement of the shaft on the robot. The spaces are a bit tighter, but it's generally the same setting. However, for the robot and this simplified example, the shaft just moves through the ring without restriction.

Can you help me identify the issue?

Minimal model and/or code that explain my question

<worldbody>
    <!-- Ground Plane -->
    <geom type="plane" size="5 5 0.1" rgba="0.8 0.9 0.8 1" />

    <!-- Base Structure -->
    
    <body name="base" pos="0 0 0.2">
        <joint name="base_joint" axis="0 -1 0" pos="0 0.2 0"/>
        <geom type="cylinder" size="0.1 0.05 0.1" rgba="0.7 0.7 0.7 1" euler="1.5708 0 0"/>
        <body name="shaft" pos="0 0 0.15">
            <!-- Shaft -->
            <joint name="shaft_joint" axis="0 1 0" pos="0 -0.2 -0.08"/>
            <geom type="cylinder" size="0.02 0.4 0" pos="0 -0.125 0.3" rgba="0.2 0.2 0.8 1" contype="1" conaffinity="1"/>
        </body>
    </body>
    <!-- Guiding Ring -->
    <body name="guiding_ring" pos="0 -0.125 0.5" euler="1.5708 0 0">
        <joint name="guiding_ring_joint" axis="0 0 1" pos="0 0 -0.2" frictionloss="0.002"/>
        <!-- Outer Ring -->
        <geom type="cylinder" size="0.06 0.01" rgba="0.8 0.2 0.2 1" euler="1.5708 0 0" contype="1" conaffinity="1"/>
        <!-- Hollow Center (Overlapping Negative Space) -->
        <geom type="cylinder" size="0.04 0.01" rgba="0.8 0.9 0.8 1" euler="1.5708 0 0" contype="0" conaffinity="0"/>
        
    </body>
</worldbody>

<actuator>
    <motor joint="base_joint" ctrlrange="-0.5 0.5" ctrllimited="true" gear="0.5"/>
</actuator>

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 · 2025-01-20T12:10:46Z

Balint-H
Jan 20, 2025
Collaborator

Hello,

The outer ring is not hollow, even if you add a non colliding second geom, the first one is filled/convex already and collides with the rod (and resolves the intersection by pushing it out).

To create a hollow ring you have a couple options:

Decompose the ring into e.g. box segments. You can potentially use composite objects for this for a shorter definition: https://mujoco.readthedocs.io/en/stable/modeling.html#composite-objects
Use a SDF torus: https://github.com/google-deepmind/mujoco/blob/main/model/plugin/sdf/torus.xml
Use a flexcomp: https://mujoco.readthedocs.io/en/stable/XMLreference.html#body-flexcomp

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Understanding Contact Forces and Friction #2375

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Understanding Contact Forces and Friction #2375

BY571 Jan 20, 2025

Intro

My setup

My question

Minimal model and/or code that explain my question

Confirmations

Replies: 1 comment

Balint-H Jan 20, 2025 Collaborator

BY571
Jan 20, 2025

Balint-H
Jan 20, 2025
Collaborator