How to better control the composite model with light mass

[rodriguesvinay]

Intro

Hi!

I am a researcher, I use MuJoCo for my research on deformable object manipulation.

My setup

mujoco 3.1.6 (python version)
Ubuntu 20.04.6 LTS, 64-bit

My question

I see that when I do a linear pulling motion, the rope (cable) drifts laterally. I checked the friction is active which I kept as 0.3. But maybe it's due to it's light mass. I have 20 segments each of 0.0025 units. I say because when I increased the mass of the segments to 0.05 then it did not create the drift. The video below shows the drifting with the original mass.

I am curious what other parameter tweaking can help to prevent the drifting motion if I want to keep the same mass. Thank you!

2026-01-10_20-33-36.online-video-cutter.com.mp4

Minimal model and/or code that explain my question

If you encountered the issue in a complex model, please simplify it as much as possible (while still reproducing the issue).

Model:

minimal XML

<mujoco>
  <worldbody>
    <light pos="0 0 1"/>
    <geom type="sphere" size="1" rgba="1 0 0 1"/>
  </worldbody>
</mujoco>

Code:

import mujoco
import mediapy as media
model = mujoco.MjModel.from_xml_string(xml)
data = mujoco.MjData(model)

with mujoco.Renderer(model) as renderer:
  mujoco.mj_forward(model, data)
  renderer.update_scene(data)

  media.show_image(renderer.render())

Confirmations

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

[rueyday]

Hi! Thanks for the detailed question.

I think I had a few potential solutions for the lateral drift you're seeing with your lightweight rope. Since increasing the mass eliminated the drift, it's related to numerical stability and to the contact solver's convergence with lightweight bodies.

Maybe you can try adjusting the following parameters:

• Increasing solver iterations
• Improving contact stiffness and friction
• Reduce timestep for better numerical accuracy

Hope that helps (just stumbled on this question when I'm using mujoco myself)
Good luck with your research!