Ignition with ros2_control

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

• https://github.com/ignitionrobotics/ros_ign
• https://github.com/ignitionrobotics/ign_ros2_control