Unusual dynamic behavior of the rope #2617

Longyouyuan · 2025-05-10T18:44:55Z

Longyouyuan
May 10, 2025

Intro

Hi!

I am a graduate student at IIT, I use MuJoCo for my research on rope manipulation.

My setup

Mujoco 3.2.2 / windows 11

My question

I built two identical rope models using ‘’composite‘’. One has its top end fixed, and the other has its bottom end fixed. Both ropes start in a horizontal position and then swing freely.
In theory, the motion of the ropes should be symmetric, but the two show noticeably different motion trajectories.
Could you please help me understand why this is happening?

Minimal model and/or code that explain my question

Model:

<mujoco model="Cable">
  <option viscosity="0.0002"/>
  <option timestep="0.002" />
  <include file="scene.xml"/>

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

  <statistic center="0 0 0.7" extent="2"/>  <!-- View define -->

  <visual>
    <global elevation="-30"/>
  </visual>

  <compiler autolimits="true"/>

  <size memory="20M"/>

  <worldbody>
    <composite type="cable" curve="s" count="41 1 1" size="1" offset="-1 0 1.2" initial="free">
      <plugin plugin="mujoco.elasticity.cable">
        <!--Units are in Pa (SI)-->
        <config key="twist" value="0e7"/>
        <config key="bend" value="0e5"/>
        <config key="vmax" value="0.00"/>
      </plugin>
      <joint kind="main" damping=".0015"/>
      <geom type="capsule" mass=".0025" size=".005" rgba=".8 .2 .1 1" condim="1"/>
    </composite>
    <body name="slider" pos="0.0 0 1.2">
      <joint name="ctr" type="slide" axis="1 0 0" damping="0.0"/>
      <geom size=".02" mass="0.5" rgba="1 1 1 0"/>
      <camera name="follow_slider" mode="track" target="slider" pos="0 -2 0.5" axisangle="1 0 0 60"/>
    </body>
    <site name="moving_goal" type="sphere" size="0.02" rgba="1 0 0 0" pos="0.0 0 1.2"/>
  </worldbody>


  <worldbody>
    <composite prefix="2" type="cable" curve="s" count="41 1 1" size="1" offset="0 0.05 1.2" initial="free">
      <plugin plugin="mujoco.elasticity.cable">
        <!--Units are in Pa (SI)-->
        <config key="twist" value="0e7"/>
        <config key="bend" value="0e5"/>
        <config key="vmax" value="0.00"/>
      </plugin>
      <joint kind="main" damping=".0015"/>
      <geom type="capsule" mass=".0025" size=".005" rgba=".8 .2 .1 1" condim="1"/>
    </composite>
    <body name="slider2" pos="0.0 0.05 1.2">
      <joint name="ctr2" type="slide" axis="1 0 0" damping="0.0"/>
      <geom size=".02" mass="0.5" rgba="1 1 1 0"/>
    </body>
  </worldbody>

  <equality>
    <connect name="right_boundary" body1="B_last" body2="slider" anchor=".025 0 0" solref="0.001 1" solimp="0.9 0.95 0.001"/>
    <connect name="right_boundary2" body1="2B_first" body2="slider2" anchor=".0 0 0" solref="0.001 1" solimp="0.9 0.95 0.001"/>
  </equality>

  <contact>
    <exclude body1="B_last" body2="slider"/>
    <exclude body1="2B_first" body2="slider2"/>
  </contact>

  <actuator>
    <position joint="ctr" kp="10000" kv="10" />
    <position joint="ctr2" kp="10000" kv="10" />
  </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.

v-r-a · 2025-05-11T21:31:16Z

v-r-a
May 11, 2025

Hi,

Just an observation---the motion symmetry deteriorates as the count increases.
count="5 1 1" and anchor=".25 0 0" (I guess the anchor location = 1 / (n-1)), n=5, works pretty good
count="11 1 1" and anchor=".1 0 0" works fine for some time
count="21 1 1" and anchor="0.05 0 0" not great
count="41 1 1" and anchor="0.025 0 0" pretty bad

You could test if changing the integrator or reducing the timestep has any further effect.

Reiterating, for one rope, the root of the kinematic tree is constrained, and for the second rope, the end. Someone with a better understanding of the forward dynamics computational pipeline will hopefully respond.

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Unusual dynamic behavior of the rope #2617

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Longyouyuan May 10, 2025

Intro

My setup

My question

Minimal model and/or code that explain my question

Confirmations

Replies: 1 comment

Uh oh!

v-r-a May 11, 2025

Longyouyuan
May 10, 2025

v-r-a
May 11, 2025