Support for Capstan Friction in Spatial Tendons #3021
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Description
This PR implements the necessary data exposure to support capstan friction models (e.g., Euler-Eytelwein formula) for spatial tendons. It exposes the internally computed wrap geometry (wrap length, entry point, exit point) which was previously discarded.
Fixes #3005
Changes
mjdata.h
Added new fields to mjData to store per-wrap-segment geometry:
mjtNum* wrap_length: Arc length of the wrap on the geometry surface.
mjtNum* wrap_entry: 3D position of the tendon entry tangent point.
mjtNum* wrap_exit: 3D position of the tendon exit tangent point.
mjxmacro.h
Updated MJDATA_POINTERS to include the new fields, ensuring automatic memory allocation and pointer initialization in
mj_makeData
.
engine_core_smooth.c
Modified
mj_tendon
to populate the new wrap_length, wrap_entry, and wrap_exit fields using the values computed by
mju_wrap
.
Motivation
Currently, users cannot implement physically accurate capstan friction models (T_high = T_low * exp(mu * theta)) because the wrap angle theta (or arc length) is not accessible via the API, even though it is computed internally. This change exposes that data, enabling advanced cable-driven robot simulation.