Disabling actuators during run-time from Python #2684

lewiships · 2025-06-17T08:52:24Z

lewiships
Jun 17, 2025

Intro

Hi!
I am a PhD student at Imperial College London and I use MuJoCo for my research on reconfigurable and adaptable robotics in the field of BioEngineering.

My setup

MuJoCo version: 3.3.1
Python API
Windows 11

My question

I am struggling with the actuatorgroupdisable functionality. I can get this to work well in the viewer, but is there a way to do this during run-time from Python?

I've tried digging through the simulate code to see how it's done for the viewer but I hit a dead end. Seems the Python bindings expose mjModel as read-only, but since editing the 'disableactuator' field in the model struct is possible for the viewer I'm wondering if there is a way to make it work for me also.

The documentation seems only to show how this works in the simulate window but I can find no evidence of anyone doing this or something similar from Python.

Any help is greatly appreciated.

Minimal model and/or code that explain my question

Model:

<worldbody>
    <light diffuse=".5 .5 .5" pos="0 0 10" dir="0 0 -1"/>
    <geom name="floor" type="plane" size="1 1 0.1" rgba="0.5 0.5 0.5 1"/>

    <body name="thigh" pos="0 0 1.4" euler="0 180 0">
        <!-- Thigh geometry using parameters -->
        <geom name="thigh_geom" type="cylinder" size="0.100 0.300"
              rgba="0.1 0 0.8 1" contype="0" conaffinity="0"/>

        <!-- Parameterized sites -->
        <site name="thigh_insertion" size="0.01" pos="-0.105 0 0"/>
        <!-- <site name="thigh_distal" size="0.01" pos="-0.105 0 0.200"/> -->

        <body name="knee" pos="0 0 0.400" euler="90 0 90">
            <!-- Joint position relative to shin length -->
            <joint name="knee_joint" type="hinge" axis="0 0 -1"
                   pos="0 0 -0.400" range="-75 90" damping="8" stiffness="10"/>
            <geom name="knee_wrap_geom" type="cylinder" size="0.100 0.100"
                  pos="0 0 0" rgba="0.6 0.6 0.6 0.3" contype="0" conaffinity="0"/>

            <body name="shin" pos="0 0.4 0" euler="-90 0 0">  <!-- Original offset maintained -->
                <!-- Shin geometry using parameters -->
                <geom name="shin_geom" type="cylinder" size="0.070 0.400" rgba="0.8 0 0.1 0.4"
                      solimp="0.9 0.95 0.001" solref="0.01 1"/>

                <body name="insertion_slide" pos="-0.07 0 0.300">
                    <joint name="insertion_slide_joint" type="slide" axis="0 0 1" range="-0.560 0" damping="5"/>
                    <geom name="insertion_slide_geom" type="box" size="0.02 0.02 0.035" rgba="0.5 0.5 0.5 1"/>
                    <site name="shin_insertion" size="0.01" pos="-0.02 0 0"/>
                </body>

                <!-- Parameterized shin sites -->
                <!--<site name="shin_prox" size="0.01" pos="-0.075 0 -0.330"/>-->
                <site name="end_effector" size="0.09" pos="0 0 0.390"/>
            </body>
        </body>
    </body>

    <!-- Velocity Test -->

</worldbody>

<tendon>
    <spatial name="knee_tendon">
        <site site="thigh_insertion"/>
        <site site="shin_insertion"/>
    </spatial>
</tendon>

<actuator>
    <muscle name="knee_actuator" tendon="knee_tendon" ctrllimited="true" ctrlrange="0 1" force="500"
    lmax="1.01" lmin="0.7" fpmax="10"/>
    <position name="insertion_lock" group="1" joint="insertion_slide_joint" kp="100000" inheritrange="1"/>
    <position name="insertion_move" group="2" joint="insertion_slide_joint" kp="50" forcelimited="true" forcerange="-15 15" inheritrange="1"/>
</actuator>

<sensor>

    <actuatorfrc name="knee_actuator_force" actuator="knee_actuator"/>


    <jointpos name="knee_joint_position" joint="knee_joint"/>
    <jointvel name="knee_joint_velocity" joint="knee_joint"/>


    <tendonpos name="knee_tendon_length" tendon="knee_tendon"/>
    <tendonvel name="knee_tendon_velocity" tendon="knee_tendon"/>


    <touch name="wall_contact_force" site="end_effector"/>
    <framepos name="end_effector_pos" objtype="site" objname="end_effector"/>
</sensor>

<!--<keyframe>
    <key time="0" qpos="-1.57081 0.3" qvel="0 0"/>
</keyframe>-->

Code:
The codebase is split across many files, so posting it all here wouldn't be practical. The core idea is this:

I’m using two actuators to control a sliding joint on the shin:

One actuator moves the joint to a desired location

The other simulates locking it in place

I want to disable or enable these actuators at runtime, but I’m struggling to find a working approach through the Python API.

Ideally, I want to do something like this:

import mujoco as mj

# Load model
model = mj.MjModel.from_xml_path(xml_path)

# Attempt to disable or enable actuators dynamically
model.disableactuator = 0  # Intended: no actuators disabled (i.e. enable lock)

If there's a clean workaround or alternative solution to toggle actuator control on and off at runtime, please let me know.

Confirmations

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

lewiships · 2025-06-17T10:36:51Z

lewiships
Jun 17, 2025
Author

Solved now, I was trying to access MjModel.opt directly instead of using MjOptions

0 replies

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lewiships Jun 17, 2025

Intro

My setup

My question

Minimal model and/or code that explain my question

Confirmations

Replies: 1 comment

Uh oh!

lewiships Jun 17, 2025 Author

lewiships
Jun 17, 2025

lewiships
Jun 17, 2025
Author