bb87928bce
Upstream: https://github.com/Rhoban/onshape-to-robot Upstream-Commit: 80e710700aac9573a2230f74f7ce9e094833a0bc Upstream-Branch: master
88 lines
3.1 KiB
ReStructuredText
88 lines
3.1 KiB
ReStructuredText
.. _kinematic-loops:
|
|
|
|
Handling kinematic loops
|
|
========================
|
|
|
|
Some robots have *kinematic loops*, meaning that the kinematic chain is not a tree but a graph.
|
|
|
|
Introduction
|
|
------------
|
|
|
|
Here is a 2D planar robot with kinematic loop, we assume the two first joints to be actuated and the others to
|
|
be passive:
|
|
|
|
.. raw:: html
|
|
|
|
<center>
|
|
<video width="70%" controls>
|
|
<source src="https://github.com/Rhoban/placo-examples/raw/master/kinematics/videos/planar_2dof_trajectory.mp4" type="video/mp4">
|
|
</video>
|
|
</center>
|
|
<br/>
|
|
|
|
|
|
However, robot description are usually **trees**. To model this type of robot, we break it down to a tree. We attach **frames** to this tree, and need to enforce **run-time constraints**.
|
|
|
|
.. image:: _static/img/opened_chain.png
|
|
:width: 300px
|
|
:align: center
|
|
|
|
|
|
Specifying closing constraints
|
|
------------------------------
|
|
|
|
While you could manually add :ref:`frames <custom-frames>`, ``onshape-to-robot`` provides a more convenient way to handle kinematic loops: **mate connectors**.
|
|
|
|
To achieve that, add a **mate** with the name ``closing_something``:
|
|
|
|
.. image:: _static/img/loop.png
|
|
:align: center
|
|
:class: padding
|
|
|
|
|
|
Support for ``<equality>`` in MuJoCo
|
|
------------------------------------
|
|
|
|
When using the :ref:`MuJoCo <exporter-mujoco>` format, ``onshape-to-robot`` will add ``<equality>`` constraints to enforce the kinematic loop.
|
|
|
|
For example, the above robot can be exported using the following ``config.json``:
|
|
|
|
.. code-block:: javascript
|
|
|
|
{
|
|
// Document URL, MuJoCo output
|
|
"url": "https://cad.onshape.com/documents/04b05c47de7576f35c0e99b3/w/68041f3f5c827a258b40039c/e/db543f501b01adf8144064e3",
|
|
"output_format": "mujoco",
|
|
|
|
// Disable the freejoint to fix the robot
|
|
"freejoint": false,
|
|
|
|
// Don't create actuators for passive joints
|
|
"joint_properties": {
|
|
"passive1": {"actuated": false},
|
|
"passive2": {"actuated": false}
|
|
}
|
|
}
|
|
|
|
Here is the result of the export:
|
|
|
|
.. raw:: html
|
|
|
|
<center>
|
|
<video width="70%" controls>
|
|
<source src="https://github.com/Rhoban/onshape-to-robot-examples/raw/refs/heads/master/videos/planar_2dof_mujoco.mp4" type="video/mp4">
|
|
</video>
|
|
</center>
|
|
<br/>
|
|
|
|
|
|
Ressources
|
|
----------
|
|
|
|
Here are some ressources on how to handle kinematic loops in software:
|
|
|
|
* `Onshape assembly <https://cad.onshape.com/documents/04b05c47de7576f35c0e99b3/w/68041f3f5c827a258b40039c/e/db543f501b01adf8144064e3?renderMode=0&uiState=665d8e3a6de6705d2f788204>`_ for the above example robot.
|
|
* MuJoCo `equality <https://mujoco.readthedocs.io/en/stable/computation/index.html#coequality>`_ constraints.
|
|
* In `pyBullet <https://pybullet.org/wordpress/>`_, you can use `createConstraint` method to add the relevant constraint.
|
|
* In the `PlaCo <https://placo.readthedocs.io/>`_ solver, you can create a `RelativePositionTask`. See the `kinematics loop documentation section <https://placo.readthedocs.io/en/latest/kinematics/loop_closures.html>`_ for more details. Some examples created with onshape-to-robot can be found in the `example gallery <https://placo.readthedocs.io/en/latest/kinematics/examples_gallery.html>`_.
|