- Added passive forces plugins

- Added new `cable` composite type:
  * The `initial` parameter specifies the joint at the starting boundary: `free`, `ball`, or `none`.
  * The boundary bodies are exposed with the names:`B_left` and `B_right`.
  * The vertex initial positions can be specified directly in the XML with the parameter `vertex`.
  * The orientation of the body frame **is** the orientation of the material frame of the curve.

- Added new `cable` passive force plugin:
  * Twist and bending stiffness can be set separately with the parameters `twist` and `bend`.
  * The stress-free configuration can be set to be the initial one or flat with the flag `flat`.
  * New cable example showing the formation of plectoneme.
  * New coil example.
  * New belt example showing interaction between twist and anisotropy.
  * Added test using cantilever exact solution.

PiperOrigin-RevId: 480033694
Change-Id: I491271bce8fccb185961477e903e5a72d172c8a3

doc/XMLreference.rst

@@ -2286,7 +2286,7 @@ coordinates results in compiler error. See :ref:`CComposite` in the modeling gui
    All automatically generated model elements have names indicating the element type and index. For example, the body at
    coordinates (2, 0) in a 2D grid is named "B2_0" by default. If prefix="C" is specified, the same body is named
    "CB2_0". The prefix is needed when multiple composite objects are used in the same model, to avoid name conflicts.
-:at:`type`: :at-val:`[particle, grid, rope, loop, cloth, box, cylinder, ellipsoid], required`
+:at:`type`: :at-val:`[particle, grid, cable, rope, loop, cloth, box, cylinder, ellipsoid], required`
    This attribute determines the type of composite object. The remaining attributes and sub-elements are then
    interpreted according to the type. Default settings are also adjusted depending on the type.
 
@@ -2316,6 +2316,12 @@ coordinates results in compiler error. See :ref:`CComposite` in the modeling gui
    elements are equality-constrained to remain connected (using the "connect" constraint type). The softness of this
    equality constraint is adjusted with the attributes solrefsmooth and solimpsmooth.
 
+   The **cable** type creates a 1D chain of bodies connected with ball joints, each having a geom with user-defined type
+   (cylinder, capsule or box). The geometry can either be defined with an array of 3D vertex coordinates :at:`vertex`
+   or with prescribed functions with the option :at:`curve`. Currently, only linear and trigonometric functions are
+   supported. For example, an helix can be obtained with curve="cos(s) sin(s) s". The size is set with the option
+   :at:`size`, resulting in :math:`f(s)=(size[1]*\cos(2*\pi*size[2]), size[1]*\sin(2*\pi*size[2]), size[0]*s)`.
+
    The **cloth** type is a different way to model cloth, beyond type="grid". Here the elements are connected with
    universal joints and form a kinematic spanning tree. The root of the tree is the parent body, and its coordinates in
    the grid are inferred from its name - similar to rope but here the naming format is "CB2_0". Neighboring bodies that
@@ -2370,6 +2376,16 @@ coordinates results in compiler error. See :ref:`CComposite` in the modeling gui
    smoothness-preserving equality constraint for box, cylinder and ellipsoid types. For all other types they have no
    effect. They obey the same rules as all other solref and solimp attributes in MJCF, except their defaults here are
    adjusted depending on the composite type. See :ref:`CSolver`.
+:at:`vertex`: :at-val:`real(3*nvert), optional`
+   Vertex 3D positions in global coordinates (cable only).
+:at:`initial`: :at-val:`[free, ball, none], "0"`
+   Behavior of the first point (cable only). Free: free joint. Ball: ball joint. None: no dof.
+:at:`curve`: :at-val:`string(3), optional`
+   Functions specifying the vertex positions (cable only). Available functions are `s`, `cos(s)`, and `sin(s)`, where
+   `s` is the arc length parameter.
+:at:`size`: :at-val:`int(3), optional`
+   Scaling of the curve functions (cable only). `size[0]` is the scaling of `s`, `size[1]` is the radius of `\cos(s)`
+   and `\sin(s)`, and `size[2]` is the speed of the argument (i.e. `\cos(2*\pi*size[2]*s)`).
 
 .. _composite-joint: