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Improve armature documentation

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PiperOrigin-RevId: 742583729
Change-Id: I18d1c95a91f4d70b842c680d4a0f4d0b04476894
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Yuval Tassa
2025-04-01 01:16:06 -07:00
committed by Copybara-Service
parent 400a379b70
commit 05f3e914b0
4 changed files with 29 additions and 6 deletions
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doc/XMLreference.rst
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@@ -2151,14 +2151,28 @@ rotations as unit quaternions.
corresponding to mjModel.qpos_spring is also used to compute the spring reference lengths of all tendons, stored in
mjModel.tendon_lengthspring. This is because :ref:`tendons <tendon>` can also have springs.
.. image:: images/XMLreference/armature.gif
:width: 40%
:align: right
:class: only-light
:target: https://github.com/google-deepmind/mujoco/blob/main/test/engine/testdata/armature_equivalence.xml
.. image:: images/XMLreference/armature_dark.gif
:width: 40%
:align: right
:class: only-dark
:target: https://github.com/google-deepmind/mujoco/blob/main/test/engine/testdata/armature_equivalence.xml
.. _body-joint-armature:
:at:`armature`: :at-val:`real, "0"`
Additional inertia associated with movement of the joint that is not due to body mass. This added inertia is usually
due to a rotor (a.k.a `armature <https://en.wikipedia.org/wiki/Armature_(electrical)>`__) spinning faster than the
joint itself due to a geared transmission; in this case the added inertia is known as "reflected inertia" and its
value is the rotational inertia of the spinning element multiplied by the square of the gear ratio. The value applies
to all degrees of freedom created by this joint.
joint itself due to a geared transmission. In the illustration, we compare (*left*) a 2-dof system with an armature
body (purple box), coupled with a gear ratio of :math:`3` to the pendulum using a :ref:`joint
equality<equality-joint>` constraint, and (*right*) a simple 1-dof pendulum with an equivalent :at:`armature`.
Because the gear ratio appears twice, multiplying both forces and lengths, the effect is known as "reflected
inertia" and the equivalent value is the inertia of the spinning body multiplied by the *square of the gear ratio*,
in this case :math:`9=3^2`. The value applies to all degrees of freedom created by this joint.
Besides increasing the realism of joints with geared transmission, positive :at:`armature` significantly improves
simulation stability, even for small values, and is a recommended possible fix when encountering stability issues.