Modeling a Winch Mechanism with a Tendon in MuJoCo

[caba0404]

Intro

Hello everyone,

I’m currently working on modeling a winch system in MuJoCo for my simulations.

My setup

As a first approach, I’ve defined the cable path using a spatial tendon as shown in the first image below:
<spatial name="Tendon" rgba="0 0 1 1" stiffness="K" springlength="0 L"> <site site="1"/> <site site="2"/> <geom geom="winch_geom" sidesite="4"/> <site site="3"/> </spatial>

In this setup:
• The tendon goes from the body (site 1) to the top of the winch housing (site 2),
• Passes over the winch (with the sidesite at point 4 on the geom),
• And connects to the rotating winch axis (site 3).

My question

This configuration works well up to a certain rotation angle (θ). However, beyond that angle, the tendon no longer wraps correctly around the winch. Instead, it takes a straight-line path from site 2 to site 4, skipping the intended curvature over the cylinder. This behavior is illustrated in the second image below.

Ideally, I’d like to simulate the cable wrapping continuously around the winch without having to model the cable explicitly as a composite body and rely on contact dynamics.

Does anyone have experience with:
• Forcing the tendon to follow a curved path over a rotating object?
• Better approximations for wrapping over cylindrical surfaces using MuJoCo’s tendon model?
• Tricks with additional sites, pulleys, or constraints that might help?

Any suggestions or insights would be greatly appreciated!

Thanks in advance!

Minimal model and/or code that explain my question

No response

Confirmations

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