Non-Isotropic Ball Joint?

[MishaTikh]

Intro

Hi!

I am a working on modeling a flexible catheter using a chain of bodies.

My setup

I am using the latest MuJoCo version with Python. I am currently modeling the joints between the bodies as virtual gimbals, with two bending joints at 90 degrees and one rotation joint connected by zero length bodies. This mostly works but is computationally expensive and is occasionally unstable.

My question

I would love to be able to move from the gimbal setup to using a single ball joint, but need non-isotropic tuning for simulation accuracy. I need to be able to set the torsional stiffness and damping independently from the bending. Is this possible with a standard ball joint?

Thank you!

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.

[MishaTikh]

I am still testing it, but I ended up writing an anisotropic joint plugin: https://github.com/MishaTikh/MuJoCo-Anisotropic-Joint

[MishaTikh]

I seemed to run into a potential bug within Mujoco. It crashes silently when trying to load the plugin via MJSpec, even though it works correctly when using basic XML. I opened up an issue here: #2757