Closed kinematic chains in Gazebo Sim

This package shows how to get a parallel robot working in Gazebo Sim. The provided launch file requires simple_launch and slider_publisher.

sudo apt install ros-${ROS_DISTRO}-simple-launch ros-${ROS_DISTRO}-slider-publisher

Overall approach

Contrary to Gazebo classic, Gazebo Sim does not handle closed kinematic chains in the SDF file. Actually what is not authorized is a link having two parents (which is necessary for a closed chain, but may also be a non-closed chain).

In order to obtain a closed chain, the SDF must thus describe the robot as a tree, where all links are correctly placed. This can be done manually with a plugin, or automatically with the unroll_loops executable.

DetachableJoint

This plugin is already part of Gazebo and is designed to attach or detach a joint between two models. In our case we only attach it, and we use it on the same model:

<plugin filename="libgz-sim-detachable-joint-system.so" name="gz::sim::systems::DetachableJoint">
  <parent_link>link_CD</parent_link>
  <child_model>four_bar</child_model>  <!-- of course I know him, it's me -->
  <child_link>link_CD_closing</child_link>
  <attach_topic>close_loop</attach_topic>
</plugin>

AttachLinks

A custom plugin is also provided in this package, namely AttachLinks. This one only takes the two link names:

<!--plugin filename="libattach-links.so" name="gz::sim::systems::AttachLinks">
  <parent>link_CD</parent>
  <child>link_CD_closing</child>
</plugin-->

SDF pre-parsing

The unroll_loops executable takes in a SDF file, possibly with closed kinematic chains. It will cut them and add DetachableJoint plugins to close the loops at runtime. See robot_launch.py for an example:

ros2 launch gz_attach_links robot_launch.py robot:=biglide_closed

Arguments are:

• -f <file>: input sdf file (default: none)
• -n <name>: spawned model name (default: name in file)
• -o <output>: name of the output file. If loops are detected then a new SDF file will be written with plugins instead of closed chains
• -v <version>: SDFformat version, if not given will use gz sdf --versions to find one

Robot examples

Three examples are given, see the model folder and the launch file.

• a four bar from the Gazebo classic example

• and a biglide that we use for teaching