friction between cable and geometry (eg capstan)

[jongwoolee127]

The feature, motivation and pitch

Hi MuJoCo team,

I’d like to request support for capstan friction in spatial tendons, which is important for cable-driven robotic hands. A simple model (T_high = T_low · exp(mu · theta)) would already be very useful.

Right now it’s hard to implement this in user code because spatial tendon wrapping does not expose wrap geometry (e.g., theta or entry/exit tangent points), and the tendon model effectively exposes only a single scalar tendon force—so users end up splitting tendons and manually approximating theta.

Could you either (1) add an optional built-in capstan friction model for cylinder/sphere wrapping, or (2) expose wrap geometry outputs (theta + entry/exit points) via API/sensors to enable robust plugins?

Thanks for considering,

Alternatives

No response

Additional context

No response

[SagarJadhav30]

MuJoCo already models tendon wrapping geometry internally, but that information is currently not accessible at the API level. This makes physically correct cable friction models difficult or impossible to implement outside the engine, even though the underlying data already exists.

[SagarJadhav30]

Exposing wrap geometry would also benefit other effects such as wear modeling, tendon compliance, and learning-based controllers.