Allow tendon springlength attribute to take two values between which the spring produces no force (a deadband).

If given two non-decreasing values, `springlength` specifies a deadband for spring stiffness, inside of which the force is 0 and outside of which force behaves like a regular spring, with the setpoint corresponding to the nearest value. This can be used to create tendons whose limits are enforced by springs rather than constraints, which are cheaper and easier to analyse. See test/engine/testdata/tendon_springlength.xml example model.

Spring-limited spatial tendons whose lower range value is 0 are rendered as catenaries.

Fixes #520.

PiperOrigin-RevId: 484509706
Change-Id: I6698e94ee36168f52d501e83057559805e05172d

doc/changelog.rst

@@ -9,8 +9,20 @@ Upcoming version (not yet released)
 General
 ^^^^^^^
 
-- Added :ref:`mju_fill`, :ref:`mju_symmetrize` and :ref:`mju_eye` utility functions.
+- The tendon :at:`springlength` attribute can now take two values. Given two non-decreasing values, `springlength`
+  specifies a `deadband  <https://en.wikipedia.org/wiki/Deadband>`_ range for spring stiffness. If the tendon length is
+  between the two values, the force is 0. If length is outside this range, the force behaves like a regular spring, with
+  the spring resting length corresponding to the nearest :at:`springlength` value. This can be used to create tendons
+  whose limits are enforced by springs rather than constraints, which are cheaper and easier to analyse. See
+  `tendon_springlength.xml <https://github.com/deepmind/mujoco/tree/main/test/engine/testdata/tendon_springlength.xml>`_
+  example model.
+
+  .. attention::
+    This is a minor breaking API change. ``mjModel.tendon_lengthspring`` now has size ``ntendon x 2`` rather than
+    ``ntendon x 1``.
+
 - Removed the requirement that stateless actuators come before stateful actuators.
+- Added :ref:`mju_fill`, :ref:`mju_symmetrize` and :ref:`mju_eye` utility functions.
 
 Version 2.3.0 (October 18, 2022)
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