Joint not moving with body. #2411

AKG47-Auto · 2025-02-08T23:01:46Z

AKG47-Auto
Feb 8, 2025

Intro

Hi!

I am a graduate student at UMich, I use MuJoCo for my research on Cargo Balancing Controller for Trucks.

My setup

I am using MuJoCo on Ubuntu 20

My question

I have created a simple plate carrying a box around the map. The steering motor/joint is supposed to stay at the center of the plate at all times but when the plate moves, the steering joint does not move with it. It stays fixed in its global position. It would be great if someone could help me with this. Same thing occurs for the plate slide joint.

Minimal model and/or code that explain my question

<!-- Default properties -->
<default>
    <joint armature="1" damping="1" limited="true" />
    <geom conaffinity="1" condim="1" contype="1" margin="0.001" rgba="0.8 0.6 .4 1" solimp=".8 .8 .01" solref=".02 1" />
</default>

<!-- World body (the environment) -->
<worldbody>
    <light cutoff="100" diffuse="1 1 1" dir="-0 0 -1.3" directional="true" exponent="1" pos="0 0 1.3" specular=".1 .1 .1" />
    <geom conaffinity="1" condim="3" name="floor" pos="0 0 0" rgba="0.8 0.9 0.8 1" size="0 0 0.125" type="plane"/>
    
    <!-- Plate body -->
    <body name="plate" pos="0 0 1">
        <geom name="plate_geom" type="box" size="2 2 0.1" friction="0.9" mass="5" rgba="0.6 0.6 0.6 1" condim="3"/>
        <inertial mass="5" pos="0 0 0" diaginertia="1.67 1.67 3.33" />
        
        <!-- Steering joint attached to plate -->
        <joint name="steering_joint" type="hinge" axis="0 0 1" pos="0 0 0" range="-30 30" damping="0.5"/>
        
        <!-- Plate joint for movement -->
        <joint name="plate_joint" type="slide" axis="1 0 0" pos="-2 0 0" damping="0.1" range="-5 5"/>
        
        <!-- Suspension joints -->
        <joint name="left_suspension" type="hinge" axis="1 0 0" pos="0 2 0" range="-1 1" damping="0.5" />
        <joint name="right_suspension" type="hinge" axis="1 0 0" pos="0 -2 0" range="-1 1" damping="0.5" />
    </body>

    <!-- Box body with free motion -->
    <body name="box" pos="0 0 3">
        <geom name="box_geom" type="box" size="0.5 0.5 0.5" mass="1" friction="1.9" rgba="0.8 0.2 0.2 1" condim="3"/>
        <inertial mass="1" pos="0.25 0.25 0.25" diaginertia="0.0833 0.0833 0.0833"/>
        <joint name="box_free" type="free"/> <!-- Free motion -->
    </body>
</worldbody>

<!-- Actuators for controlling joints -->
<actuator>
    <general name="plate_motor" joint="plate_joint" ctrlrange="-100 100" ctrllimited="true" />
    <motor ctrllimited="true" name="steering_motor" joint="steering_joint" ctrlrange="-10 10" gear="10"/>
    <motor ctrllimited="true" name="suspension_motor_l" joint="left_suspension" ctrlrange="-10 10" gear="10"/>
    <motor ctrllimited="true" name="suspension_motor_r" joint="right_suspension" ctrlrange="-10 10" gear="10"/>
</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.

yuvaltassa · 2025-02-09T11:15:46Z

yuvaltassa
Feb 9, 2025
Maintainer

This works as intended. The joint is relative to the parent.

2 replies

AKG47-Auto Feb 9, 2025
Author

What should I do to have the joint move with the body? I want to steer my plate as it moves along a given path.
I'm new to MuJoCo so any guidance would be highly appreciated.
Thanks

yuvaltassa Feb 12, 2025
Maintainer

Just ignore the joint visualization, it doesn't mean anything, especially for sliders. That said why is your slider after the hinge? That's a strange design choice. Generally speaking for free motion you want translation before rotation. For horizontal movement you generally have slider-slider-hinge on x-y-z, respectively. Other formulations are not wrong, but will lead to strange behavior far away from the origin...

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Joint not moving with body. #2411

AKG47-Auto Feb 8, 2025

Intro

My setup

My question

Minimal model and/or code that explain my question

Confirmations

Replies: 1 comment · 2 replies

yuvaltassa Feb 9, 2025 Maintainer

AKG47-Auto Feb 9, 2025 Author

yuvaltassa Feb 12, 2025 Maintainer

AKG47-Auto
Feb 8, 2025

Replies: 1 comment 2 replies

yuvaltassa
Feb 9, 2025
Maintainer

AKG47-Auto Feb 9, 2025
Author

yuvaltassa Feb 12, 2025
Maintainer