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Cartesian end-effector control using site transmission.

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- Added `refsite` attribute to actuators with `site` transmission.
- Such actuators now have a well defined length and can be used for e.g., Cartesian end-effector control.
- Added example models and documentation describing the new feature.

PiperOrigin-RevId: 471561218
Change-Id: I538f09af9600d5c53992c4a9fed325648aab4fa3
This commit is contained in:
Yuval Tassa
2022-09-01 10:06:11 -07:00
committed by Copybara-Service
parent 834e8dd506
commit 46da1285af
10 changed files with 283 additions and 55 deletions
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doc/computation.rst
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@@ -255,14 +255,20 @@ These three components of an actuator - transmission, activation dynamics, and f
actuator works. The user can set them independently for maximum flexibility, or use :ref:`Actuator shortcuts
<CActuator>` which instantiate common actuator types.
Transmission
.. _geTransmission:
Each actuator has a scalar length :math:`l_i(q)` defined by the type of transmission and its parameters. The gradient
:math:`\nabla l_i` is an :math:`n_V`-dimensional vector of moment arms. It determines the mapping from scalar
actuator force to joint force. The transmission properties are determined by the MuJoCo object to which the actuator
is attached; the possible attachment object types are :at:`joint`, :at:`tendon`, :at:`jointinparent`,
:at:`slider-crank`, :at:`site`, and :at:`body`. The :at:`joint` and :at:`tendon` transmission types act as expected
mechanically and correspond to the actuator applying forces or torques to the target object.
Transmission
~~~~~~~~~~~~
Each actuator has a scalar length :math:`l_i(q)` defined by the type of transmission and its parameters. The gradient
:math:`\nabla l_i` is an :math:`n_V`-dimensional vector of moment arms. It determines the mapping from scalar
actuator force to joint force. The transmission properties are determined by the MuJoCo object to which the actuator
is attached; the possible attachment object types are :at:`joint`, :at:`tendon`, :at:`jointinparent`,
:at:`slider-crank`, :at:`site`, and :at:`body`.
The :at:`joint` and :at:`tendon` transmission types act as expected and correspond to the actuator applying forces or
torques to the target object. Ball joints are special, see the :at:`joint` documentation in :ref:`actuator<general>`
reference for more details.
The :at:`jointinparent` transmission is unique to ball and free joint and asserts that rotation should be measured
in the parent rather than child frame.
@@ -273,11 +279,18 @@ Transmission
also be modeled explicitly by creating MuJoCo bodies and coupling them with equality constraints to the rest of the
system, but that would be less efficient.
:at:`site` and :at:`body` are degenerate transmission targets, as their length :math:`l_i(q)` is always 0.
They can therefore not be used to maintain a desired length value, as with a position actuator. Site
transmissions correspond to applying a Cartsian force/torque at the site, while :el:`body` transmissions correspond
to applying forces at contact points belonging to a body. For more information about adhesion, see the
:ref:`adhesion<adhesion>` shorcut documentation.
:at:`site` transmission (without a :at:`refsite`, see below) and :at:`body` transmission targets have a fixed zero
length :math:`l_i(q) = 0`. They can therefore not be used to maintain a desired length, but can be used to apply
forces. Site transmissions correspond to applying a Cartsian force/torque at the site, and are useful for modeling
jets and propellors. :el:`body` transmissions correspond to applying forces at contact points belonging to a body, in
order to model vacuum grippers and biomechanical adhesive appendages. For more information about adhesion, see the
:ref:`adhesion<adhesion>` actuator documentation.
If a :at:`site` transmission target is defined with the optional :at:`refsite` attribute, forces and torques are
applied in the frame of the reference site rather than the the site's own frame. If a reference site is defined then
the length of the actuator is nonzero and corresponds to the pose difference of the two sites. This length can then
be controlled with a :el:`position` actuator, enabling Cartesian end-effector control. See the :at:`refsite`
documentation in :ref:`actuator<general>` reference for more details.
Activation dynamics
Some actuators such as pneumatic and hydraulic cylinders as well as biological muscles have an internal state called