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Add <damper> actuator shortcut and related mjGAIN_AFFINE.

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**Rationale**

A general `mjGAIN_AFFINE` actuator adds the term `force = (c + kp * length + kv * velocity) * ctrl`. The damper shortcut restricts it to `kv * velocity * ctrl` and sets the `ctrllimited` attribute to true.

**API**

```xml
<actuator>
  <damper ctrlrange="0 1" kv="1"/>
</actuator>
```

* Required attributes: `ctrlrange` (>=0)
* Optional attributes: `kv` (>=0)

PiperOrigin-RevId: 455366866
Change-Id: I88773abbeccabb6442f215c9fc05e205df63f692
This commit is contained in:
Alessio Quaglino
2022-06-16 06:20:49 -07:00
committed by Copybara-Service
parent e0b6ba4a13
commit 307cf692e5
14 changed files with 283 additions and 34 deletions
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doc/modeling.rst
+2 -11
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@@ -545,18 +545,9 @@ Actuator shortcuts
~~~~~~~~~~~~~~~~~~
As explained in the :ref:`Actuation model <geActuation>` section of the Computation chapter, MuJoCo offers a flexible
actuator model with transmission, activation dynamics and force generation components that can be specified
independently. The full functionality can be accessed via the XML element :ref:`general <general>` which allows the user
actuator model with transmission, activation dynamics and force generation components that can be specified independently. The full functionality can be accessed via the XML element :ref:`general <general>` which allows the user
to create a variety of custom actuators. In addition, MJCF provides shortcuts for configuring common actuators. This is
done via the XML elements :ref:`motor <motor>`, :ref:`position <position>`, :ref:`velocity <velocity>`,
:ref:`intvelocity <intvelocity>`, :ref:`cylinder<cylinder>`, and :ref:`muscle <muscle>`. These are *not* separate model
elements. Internally MuJoCo supports only one actuator type - which is why when an MJCF model is saved all actuators are
written as :el:`general`. Shortcuts create general actuators implicitly, set their attributes to suitable values, and
expose a subset of attributes with possibly different names. For example, :el:`position` creates a position servo with
attribute :at:`kp` which is the servo gain. However :el:`general` does not have an attribute :at:`kp`. Instead the parser
adjusts the gain and bias parameters of the general actuator in a coordinated way so as to mimic a position servo. The
same effect could have been achieved by using :el:`general` directly, and setting its attributes to certain values as
described below.
done via the XML elements :ref:`motor <motor>`, :ref:`position <position>`, :ref:`velocity <velocity>`, :ref:`intvelocity <intvelocity>`, :ref:`damper<damper>`, :ref:`cylinder<cylinder>`, and :ref:`muscle <muscle>`. These are *not* separate model elements. Internally MuJoCo supports only one actuator type - which is why when an MJCF model is saved all actuators are written as :el:`general`. Shortcuts create general actuators implicitly, set their attributes to suitable values, and expose a subset of attributes with possibly different names. For example, :el:`position` creates a position servo with attribute :at:`kp` which is the servo gain. However :el:`general` does not have an attribute :at:`kp`. Instead the parser adjusts the gain and bias parameters of the general actuator in a coordinated way so as to mimic a position servo. The same effect could have been achieved by using :el:`general` directly, and setting its attributes to certain values as described below.
Actuator shortcuts also interact with defaults. Recall that the :ref:`default setting <CDefault>` mechanism involves
classes, each of which has a complete collection of dummy elements (one of each element type) used to initialize the