How to attach a body at the end of a composite plugin cable #3269

FilipeJInacio · 2026-05-12T09:45:52Z

FilipeJInacio
May 12, 2026

Intro

Hi!

I am a PhD student at IST University of Lisbon, I use MuJoCo for my research on tethered satellites.

My work focuses on generating simulation data for a Fault Detection system for a chaser satellite capable of docking with other satellites. The docking mechanism consists of launching a tether with a gecko-like adhesive material attached at the tip. The objective is to determine whether the docking attempt was successful.

I have been using MuJoCo for about a week.

My setup

Windows 11 Home 25H2
Python 3.12.10
Mujoco 3.7.0

My question

To model this system in MuJoCo, I created a simplified chaser satellite as a free-floating box with a cable attached using mujoco.elasticity.cable through a composite cable structure:

<body name="chaser" pos="0 0 0">
    <joint name="chaser_joint" type="free"/>
    <geom name="chaser_geom" type="box" size="0.1 0.1 0.1" material="material_chaser" mass="1"/>
    <body name="cable" pos="0.1 0 0">
        <composite type="cable" curve="s" count="81 1 1" size="1" initial="none">
            <plugin plugin="mujoco.elasticity.cable">
                <config key="twist" value="1e7"/>
                <config key="bend" value="3e6"/>
                <config key="vmax" value="0.05"/>
            </plugin>
            <joint kind="main" damping=".015"/>
            <geom type="capsule" size=".005" rgba=".1 .2 .8 1" condim="1"/>
        </composite>
    </body>
</body>

I would like to attach an additional body representing the gecko-inspired docking tip to the end of the cable.

So far, I have tried using a weld equality constraint between the tip body and the final cable segment. However, the weld tends to detach relatively easily during the simulation (even at reduced timesteps of 0.1ms) (maybe I'm doing something wrong?).

My idea was to expand the composite cable into explicit bodies/joints using:

model = mujoco.MjModel.from_xml_path("file.xml")
mujoco.mj_saveLastXML("expanded.xml", model)

and then manually attach the docking tip to the final generated body in the chain.

Unfortunately, the exported XML does not seem to reload correctly, likely due to the generated plugin/composite chain (Basically, the kernel dies while attempting to load the model. This might be a bug?).

My main question is:

What is the recommended way to attach an additional body to the end of a mujoco.elasticity.cable composite cable without relying on a weld constraint?

Minimal model and/or code that explain my question

Simplified version using the weld constraint.

<mujoco>
    <option timestep="0.001" impratio="10" gravity="0 0 0" magnetic="0 0 0" density="0" viscosity="0" integrator="implicitfast" solver="Newton">
        <flag energy="enable" contact="enable" sleep="disable"/>
    </option>

    <compiler angle="degree" inertiafromgeom="true" autolimits="true"/>

    <extension>
        <plugin plugin="mujoco.elasticity.cable"/>
    </extension>

    <asset>
        <texture type="skybox" builtin="gradient" rgb1="1 1 1" rgb2=".6 .8 1" width="256" height="256"/>
        <material name="material_chaser" rgba="0.8 0.6 0.4 1"/>
        <material name="material_target" rgba="0.4 0.6 0.8 1"/>
    </asset>

    <worldbody>
        <light name="sun" directional="true" pos="100 50 200" dir="-1 -0.5 -2" diffuse="1.0 0.95 0.85" specular="0.3 0.3 0.2" castshadow="false"/>

        <body name="chaser" pos="0 0 0">
            <joint name="chaser_joint" type="free"/>
            <geom name="chaser_geom" type="box" size="0.1 0.1 0.1" material="material_chaser" mass="1"/>

            <body name="rope" pos="0.1 0 0">
                <composite type="cable" curve="s" count="81 1 1" size="1" initial="none">
                    <plugin plugin="mujoco.elasticity.cable">
                        <config key="twist" value="1e7"/>
                        <config key="bend" value="3e6"/>
                        <config key="vmax" value="0.05"/>
                    </plugin>
                    <joint kind="main" damping=".015"/>
                    <geom type="capsule" size=".005" rgba=".1 .2 .8 1" condim="1"/>
                </composite>
            </body>
        </body>

        <body name="tether_tip" pos="1.1 0 0">
            <joint name="tether_tip_joint" type="free"/>
            <geom name="tether_tip_geom" type="cylinder" size="0.01" rgba="0 0 1 1" mass="0.5" fromto="0 0 0 0.05 0 0"/>
        </body>
    </worldbody>

    <equality>
        <weld body1="tether_tip" body2="B_last"/>
    </equality>

    <actuator>
        <motor name="chaser_x" joint="chaser_joint" gear="1 0 0 0 0 0" ctrlrange="-0.1 0.1" ctrllimited="true"/>
        <motor name="chaser_y" joint="chaser_joint" gear="0 1 0 0 0 0" ctrlrange="-0.1 0.1" ctrllimited="true"/>
        <motor name="chaser_z" joint="chaser_joint" gear="0 0 1 0 0 0" ctrlrange="-0.1 0.1" ctrllimited="true"/>
        <motor name="chaser_spin_x" joint="chaser_joint" gear="0 0 0 1 0 0" ctrlrange="-0.1 0.1" ctrllimited="true"/>
        <motor name="chaser_spin_y" joint="chaser_joint" gear="0 0 0 0 1 0" ctrlrange="-0.1 0.1" ctrllimited="true"/>
        <motor name="chaser_spin_z" joint="chaser_joint" gear="0 0 0 0 0 1" ctrlrange="-0.1 0.1" ctrllimited="true"/>
    </actuator>

</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.

FilipeJInacio · 2026-05-12T09:57:38Z

FilipeJInacio
May 12, 2026
Author

After review, I'm basically having the same issue as #1951.
Because it is unsolved and because I present the problem more cleanly, I'm asking in more detail here.

Update 1: I found that the cable and the tip were colliding. That was the reason for the gap. After removing the collision between the cable and the tip, the gap created by the weld is reduced by a lot.

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How to attach a body at the end of a composite plugin cable #3269

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FilipeJInacio May 12, 2026

Intro

My setup

My question

Minimal model and/or code that explain my question

Confirmations

Replies: 1 comment

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FilipeJInacio May 12, 2026 Author

FilipeJInacio
May 12, 2026

FilipeJInacio
May 12, 2026
Author