Ignition with ros2_control internals

continue to Let's try Ignition Fortress, just wanted to comprehend the architecture and design how it works for Ignition with ros2_control.

Build and Run ign_ros2_control

Install ros:galactic on ubuntu:20.04, see Installing ROS 2 via Debian Packages
Build ign_ros2_control

# source ros:galactic env
source /opt/ros/galactic/setup.bash

# build sample colcon workspace
export IGNITION_VERSION=fortress
mkdir -p ~/ros_ign/src
cd ~/ros_ign/src
git clone https://github.com/ignitionrobotics/ign_ros2_control
rosdep install -r --from-paths . --ignore-src --rosdistro $ROS_DISTRO -y
cd ~/ros2_ign
colcon build

Run ign_ros2_control

ros2 launch ign_ros2_control_demos cart_example_velocity.launch.py
...
ros2 run ign_ros2_control_demos example_velocity
...

Architecture / Design

this is ros2_control framework overview, as we can see user needs to develop Controller, Interface and Plugin such as Sensor, System and Actuator to integrate with ros2_control.

Implementation Detail

from ros2_control perspective, Ignition is one of System hardware object. IgnitionSystem possesses IgnitionSystemInterface inherited from hardware_interface::SystemInterface.
in other words, Ignition is just a hardware to control and plugged in to ros2_control.

we can refer to the implementation as below.

then sending velocity messages via /velocity_controller/commands topic to /velocity_controller to write the data IgnitionSystem joint slider_to_cart.

  <ros2_control name="IgnitionSystem" type="system">
    <hardware>
      <plugin>ign_ros2_control/IgnitionSystem</plugin>
    </hardware>
    <joint name="slider_to_cart">
      <command_interface name="velocity">
        <param name="min">-15</param>
        <param name="max">15</param>
      </command_interface>
      <state_interface name="position"/>
      <state_interface name="velocity"/>
      <state_interface name="effort"/>
    </joint>

IgnitionROS2ControlPlugin is one of the Ignition Gazebo System plugin library.
It provides callback methods such as Configure, PreUpdate and PostUpdate.

Configure will be called during initialization. in this plugin, it will read URDF from parameter server and instantiate ros2_control ControllerManager with parameters.
PreUpdate will be called before modify state before physics runs. It calls controller_manager_->write that leads to IgnitionSystem::write to update ignition::gazebo::components.
PreUpdate is mapped to IgnitionSystem::read.

Reference