Example package for ignition systems plugin

[ruffsl]

It would be nice to have an example of how to package a custom systems plugin for ignition. For example, the classic gazebo version of dolly uses the libgazebo_ros_diff_drive plugin for mapping command velocities to the joint control for a respective wheelbase of the robot:

dolly/dolly_gazebo/models/dolly/model.sdf

Line 330 in 349c352

<plugin name='diff_drive' filename='libgazebo_ros_diff_drive.so'>

The ignition gazebo version of dolly uses the libignition-gazebo-diff-drive-system instead:

dolly/dolly_ignition/models/dolly_ignition/model.sdf

Lines 330 to 332 in 349c352

	<plugin
	filename="libignition-gazebo-diff-drive-system.so"
	name="ignition::gazebo::systems::DiffDrive">

For instances where an existing plugin bundled in ignition may not suffice or not exist, it would be helpful to have a packaged example of something like the diff_drive plugin as a local package here.

https://github.com/ignitionrobotics/ign-gazebo/tree/5b21ec87306c35b3494adc25694fac8b8d1bb7de/src/systems/diff_drive

Perhaps a simple ignition plugin to control the tail_joint of dolly, that could be controlled via topic to active waggle, passively dampen, or hold still/break. It would also provide as a reference template for the cmake and ament exports necessary for ignition to find and load the externally compiled plugin.

dolly/dolly_ignition/models/dolly_ignition/model.sdf

Line 296 in 349c352

<joint name='tail_joint' type='revolute'>

Personally, I'd like to prototype an ackermann steering model, but want to avoid having to rebuild the entire ign-gazebo code base until the plugin is working and ready to upstream, keeping my development confined to a minimal dolly like colcon workspace/repo. As of yet, the current Ignition Robotics plugin and systems docs do not specify how to write your own, external from the upstream binaries.

• https://ignitionrobotics.org/api/plugin/1.1/tutorials.html
• https://ignitionrobotics.org/api/gazebo/3.3/index.html

[chapulina]

an example of how to package a custom systems plugin for ignition

There are a few examples here:

https://github.com/ignitionrobotics/ign-gazebo/tree/master/examples/plugin

This tutorial should also be helpful:

https://ignitionrobotics.org/api/gazebo/3.3/createsystemplugins.html

The recommendation is to write pure CMake packages for all functionality that isn't ROS-specific and use the ros-ign bridge to connect to ROS. This is meant to make the code more reusable. For example, on Gazebo-classic, we have a pure-cmake gazebo::DiffDrivePlugin, and also the ROS-enabled gazebo_plugins::GazeboRosDiffDrive - which has always been a pain to maintain.

I'd like to prototype an ackermann steering model, but want to avoid having to rebuild the entire ign-gazebo code base until the plugin is working

With the separation of concerns above in mind, I'd recommend you write a ROS package that contains:

• An Ackermann drive ign-gazebo system that's a pure CMake project and links against ign-gazebo, but not against ROS
• A launch file that spawns a model with the plugin above, and also starts the ros_ign_bridge for the cmd_vel twist message

Perhaps a simple ignition plugin to control the tail_joint of dolly, that could be controlled via topic to active waggle, passively dampen, or hold still/break. It would also provide as a reference template for the cmake and ament exports necessary for ignition to find and load the externally compiled plugin.

I agree that this would be a valuable example, I'll look into adding one

As of yet, the current Ignition Robotics plugin and systems docs do not specify how to write your own, external from the upstream binaries.

I think we could do a better job linking to the resources I pointed out above: the example plugins and the tutorials.

[cocodmdr]

Hello,

Are there news on this issue ?

I would like to build a modified version of diff_drive plugin in order to control a third directionnal wheel. Instead of the caster wheel there is a motorised fork with a motorised wheel.

I am trying to understand how to add a plugin but I am facing issues when I import custom plugins.

Regards.

[chapulina]

Hi @cocodmdr , did you look into the example system plugin?

https://github.com/ignitionrobotics/ign-gazebo/tree/master/examples/plugin/system_plugin

Also this tutorial:

https://ignitionrobotics.org/api/gazebo/4.0/createsystemplugins.html