Contact frame alignment for cylinder-sphere collisions #2749

david-wb · 2025-08-07T13:58:23Z

david-wb
Jul 15, 2025

Intro

Hi, I'm a research engineer working on continuous control problems using RL and MuJoCo.

My setup

Mujoco 3.3.3
Ubuntu 22.04

My question

I'd like to use anisotropic friction to emulate omni-wheels rolling on the surface of a sphere. Please see the screenshot below. My plan was to set the friction of the wheel cylinder to be zero along the cylinder's z-axis. The problem is the contact frames are not aligned with the z-axis (rotational axis) of the wheel cylinders. Is there a way to make is so that the contact frame is always aligned with the cylinders frame? Thanks in advance!

Minimal model and/or code that explain my question

<mujoco model="robot_with_wheelbase">
    <compiler coordinate="local" angle="degree"/>
    <option timestep="0.001" integrator="RK4" gravity="0 0 -9.81"/>
    <default>
        <geom rgba="1 1 1 1" />
        <joint damping="0.2"/>
    </default>
    <visual>
        <global offwidth="1920" offheight="1080"/>
    </visual>
    <asset>
        <texture name="checker" type="2d" builtin="checker" rgb1="0 0 0" rgb2="1 1 1" width="100" height="100" mark="none"/>
        <material name="checker_mat" texture="checker" specular="0.5" shininess="0.6" texrepeat="10 10"/>
        <material name="robot_mat" rgba="0.5 0.5 0.5 1"/>
        <!-- <material name="wheel_mat" rgba="0.3 0.3 0.3 1"/> -->
    </asset>
    <worldbody>
        <!-- <light pos="0 0 10" dir="0 0 -1" diffuse="1 1 1" specular="0 0 0"/> -->
        <geom name="floor" type="plane" material="checker_mat" size="10 10 0.1"/>
        <body name="base" pos="0 0 0.15">
            <freejoint name="base_free"/>
            <geom name="base_sphere" type="sphere" size="0.15" mass="10" rgba="0.4 0.5 0.6 1"/>
        </body>
        <body name="wheel_base" pos="0 0 0.35">
            <freejoint name="wheel_base_free"/>
            <!-- Robot Chassis -->
            <geom name="chassis_geom" type="cylinder" size="0.15 0.01" material="robot_mat" mass="1.0" rgba="1 0.5 0 1"/>
            <!-- Vertical Rod -->
            <body name="rod" pos="0 0 0.3">
                <geom name="rod_geom" type="cylinder" size="0.01 0.3" rgba="1 0.2 0 1" mass="0.2"/>
                <!-- Ball Tip -->
                <body name="rod_tip" pos="0 0 0.3">                    <!-- Positioned at rod top (0.1 + 0.04 sphere radius) -->
                    <geom name="rod_tip_geom" type="sphere" size="0.04" rgba="1 0.0 0 1" mass="0.05"/>
                    <!-- Ball: radius 0.04m -->
                </body>
            </body>
            <!-- Wheel 1: Positioned at 0 degrees, tilted -45 degrees toward ball center -->
            <body name="wheel1" pos="0.12727922061357855 0 -0.07" euler="0 -45 0">
                <joint name="wheel1_joint" type="hinge" axis="0 0 1" damping="0.1"/>
                <geom name="wheel1_geom" type="cylinder" size="0.03 0.02" rgba="0.5 0.5 0.5 1" mass="0.5"/>
            </body>
            <body name="wheel2" pos="-0.06363961030678925 0.110227038425243 -0.07" euler="40.89339465 20.70481105 -112.2076543">
                <joint name="wheel2_joint" type="hinge" axis="0 0 1" damping="0.1"/>
                <geom name="wheel2_geom" type="cylinder" size="0.03 0.02" rgba="0.5 0.5 0.5 1" mass="0.5"/>
            </body>
            <body name="wheel3" pos="-0.06363961030678925 -0.11022703842524297 -0.07" euler="-40.89339465 20.70481105 112.2076543">
                <joint name="wheel3_joint" type="hinge" axis="0 0 1" damping="0.1"/>
                <geom name="wheel3_geom" type="cylinder" size="0.03 0.02" rgba="0.5 0.5 0.5 1" mass="0.5"/>
            </body>
        </body>

        <light name="diffuse_light" pos="0 0 5" dir="0 0 -1" directional="false" diffuse="0.8 0.8 0.8" specular="0 0 0" castshadow="false" />
    </worldbody>
    <sensor>
        <framequat name="angle" objtype="body" objname="wheel_base"/>
        <frameangvel name="velocity" objtype="body" objname="wheel_base"/>
        <framepos name="base_pos" objtype="body" objname="base"/>
        <framexaxis name="base_xaxis" objtype="body" objname="base"/>
        <frameyaxis name="base_yaxis" objtype="body" objname="base"/>
        <framezaxis name="base_zaxis" objtype="body" objname="base"/>
    </sensor>
    <actuator>
        <motor name="motor1" joint="wheel1_joint" gear="1" ctrlrange="-10 10"/>
        <motor name="motor2" joint="wheel2_joint" gear="1" ctrlrange="-10 10"/>
        <motor name="motor3" joint="wheel3_joint" gear="1" ctrlrange="-10 10"/>
    </actuator>
    <contact>
        <!-- Anisotropic friction: zero along wheel z-axis, non-zero for theta -->
        <pair geom1="wheel1_geom" geom2="base_sphere" friction="1 1 0"/>
        <pair geom1="wheel2_geom" geom2="base_sphere" friction="1 1 0"/>
        <pair geom1="wheel3_geom" geom2="base_sphere" friction="1 1 0"/>
    </contact>
</mujoco>

Confirmations

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

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Contact frame alignment for cylinder-sphere collisions #2749

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david-wb Jul 15, 2025

Intro

My setup

My question

Minimal model and/or code that explain my question

Confirmations

Replies: 0 comments

david-wb
Jul 15, 2025