How to actuate a joint with MjSpec? #2636

tonitonitonitoni · 2025-05-16T17:20:52Z

tonitonitonitoni
May 16, 2025

Intro

Hi!

I am a graduate student at TMU, I use MuJoCo for my research on space robotics.

My setup

Python Mujoco version 3.3.0 in Windows 10 WSL2 Ubuntu 22.04 coding in VSCode Jupyter notebooks

My question

I am trying to actuate the free joint in my model using the MjSpec model editing API.
I can add the free joint and add the actuator but I cannot add the actuator to the free joint.

I am trying to replicate this XML:

<mujoco>
    <compiler angle="radian"/>
    <option gravity="0 0 0"/>
  <worldbody>
    <body pos="0 0 1" name="chaser_base">
        <freejoint name="free_joint"/>
        <geom type="box" size=".25 .25 .25" rgba="0 .9 0 1" mass="100"/>
    </body>
    </worldbody>
    <actuator>
        
        <position name="motor_tx" joint="free_joint" gear="1 0 0 0 0 0"/>
        <position name="motor_ty" joint="free_joint" gear="0 1 0 0 0 0"/>
        <position name="motor_tz" joint="free_joint" gear="0 0 1 0 0 0"/>
        
        <position name="motor_rx" joint="free_joint" gear="0 0 0 1 0 0" />
        <position name="motor_ry" joint="free_joint" gear="0 0 0 0 1 0" />
        <position name="motor_rz" joint="free_joint" gear="0 0 0 0 0 1" />
    
    </actuator>
</mujoco>

I tried to add it via mujoco.MjSpec.from_file but got ValueError: Error: free joint can only be used on top level.

When I omit the <worldbody> tag the model compiles, but no actuators show up when I input spec.actuators.

Minimal model and/or code that explain my question

import mujoco
spec = mujoco.MjSpec()

chaser_base = spec.worldbody.add_body(name="chaser_base")
width, height, depth = 0.2, 0.2, 0.2
chaser_base.add_geom(
    name="chaser_base_box", 
    type=mujoco.mjtGeom.mjGEOM_BOX, 
    size=[width, height, depth],
    mass=30,
    rgba=[1, 1, 0, 1]
    )
free_test = chaser_base.add_freejoint(name="free_joint")
spec.add_actuator(name="free1")

Confirmations

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

quagla · 2025-05-19T08:13:32Z

quagla
May 19, 2025
Collaborator

Hi, we don't have actuator shortcuts (positions) in MjSpec yet. I would recommend having a look at https://github.com/google-deepmind/mujoco/blob/main/src/xml/xml_native_reader.cc#L2094 which is how we parse actuators in XML. You basically have to do the same in MjSpec - note that the code I linked already uses mjSpec, but it's the C interface rather then Python one.

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tonitonitonitoni May 16, 2025

Intro

My setup

My question

Minimal model and/or code that explain my question

Confirmations

Replies: 1 comment

Uh oh!

quagla May 19, 2025 Collaborator

tonitonitonitoni
May 16, 2025

quagla
May 19, 2025
Collaborator