Tutorial: Using teleop_twist_keyboard ROS node to control a diff drive robot in Ignition

This tutorial intends to give a step-by-step guide into how to use ROS tools in conjunction with an Ignition based simulation. You should be able to achieve a result like the one in this video at the end of the tutorial.

We will be using Ignition Dome and ROS Melodic; however, please note that this tutorial is valid for Ignition releases that are compatible with ros_ign_bridge and ROS releases that are compatible with teleop_twist_keyboard and ros_ign.

We'll start with the steps to complete the tutorial, and later will be explained what's going on behind the scenes.

Pre-Requisites

• Install Ignition Dome

  • Instructions can be found here: Dome Installation

• Install ROS Melodic

  • Instructions can be found here: ROS Melodic Installation

• Install ros_ign. We'll be using ROS Melodic version

       # Add https://packages.osrfoundation.org
       sudo sh -c 'echo "deb http://packages.osrfoundation.org/gazebo/ubuntu-stable `lsb_release -cs` main" > /etc/apt/sources.list.d/gazebo-stable.list'
       wget https://packages.osrfoundation.org/gazebo.key -O - | sudo apt-key add -
       sudo apt-get update
       # Install `ros_ign`
       sudo apt install ros-melodic-ros-ign

• Install teleop_twist_keyboard

   sudo apt-get install ros-melodic-teleop-twist-keyboard

Steps to complete this tutorial

Install this demo package

mkdir -p ~/ign_teleop_tutorial_ws/src
git clone [email protected]:Blast545/ign_tutorials.git ~/ign_teleop_tutorial_ws/src
cd ~/ign_teleop_tutorial_ws
catkin_make

We will open two different terminals sharing the same environment. In each, we'll run the following commands:

Run launchfile with ROS, Ignition and ros_ign

# Shell 1
source ~/ign_teleop_tutorial_ws/devel/setup.bash
roslaunch roslaunch ign_tutorials diff_drive_demo.launch

Start teleop_twist_keyboard node

# Shell 2
rosrun teleop_twist_keyboard teleop_twist_keyboard.py

After running this, you should have an environment similar to this one:

And in the teleop_twist_keyboard terminal you can send commands to the system to move the vehicle robot:

Understanding the system architecture

For this tutorial, we've used the example world tunnel.sdf that includes a diff drive ground vehicle and shows various features available within the simulator. Including the ignition simulator, the whole system works like this:

• The teleop_twist_keyboard node transforms the user inputs into ROS Twist commands.
• The ros_ign_bridge subscribes to those ROS Twist commands and converts them to Ignition Transport msgs.
• The Ignition Transport msgs are used by diff drive plugin inside the tunnel.sdf that's running in the simulator.
• The simulator moves the robot accordingly, based on the diff drive plugin output.

We can see from the command used for the ros_bridge within the launchfile:

  <node
    pkg="ros_ign_bridge"
    type="parameter_bridge"
    name="$(anon ros_ign_bridge)"
    output="screen"
    args="/model/vehicle/odometry@nav_msgs/[email protected] /cmd_vel@geometry_msgs/[email protected] ">
  </node>

The msg input/output goes to the /cmd_vel topic (this topic applies to both the Ignition Transport and ROS), it takes geometry_msgs/Twist as input and uses ignition.msgs.Twist as output. Same principle applies for the odometry information displayed in rviz.

Also, taking a look to the SDF file from ign_ws/src/ign-gazebo/examples/worlds/tunnel.sdf we can see:

<plugin
  filename="ignition-gazebo-diff-drive-system"
  name="ignition::gazebo::systems::DiffDrive">
  <left_joint>left_rear_wheel</left_joint>
  <left_joint>left_front_wheel</left_joint>
  <right_joint>right_rear_wheel</right_joint>
  <right_joint>right_front_wheel</right_joint>
  <wheel_separation>1.25</wheel_separation>
  <wheel_radius>0.3</wheel_radius>
  <odom_publish_frequency>1</odom_publish_frequency>
  <topic>cmd_vel</topic>
</plugin>

This is the diff drive plugin configured to receive commands in the cmd_vel topic and to control the four joints: left_rear_wheel, left_front_wheel, right_rear_wheel and right_front_wheel that control the movement of the ground vehicle.

Further steps

As homework, can you tackle some challenges to continue the work provided here?

• Try sending commands to the robot using directly the Ignition Transport layer.
  • Check this ignition tutorial: Moving the robot.
  • Check the structure of the tunnel.sdf model.
• Can you change the launchile to use a different physics engine? Like TPE.
  • Check ign-physics tutorial: Switching physics engines.
• The diff_drive.sdf example world (ign_ws/src/ign-gazebo/examples/worlds/diff_drive.sdf) does not use the same cmd_vel topic to receive velocity commands, can you modify the commands to make it work in this scenario?
  • Check the diff_drive.sdf example commands and adjust the ros_ign bridges properly.

Want to dig deeper inside Ignition gazebo? Browse the following links:

• Browse the main Ignition portal.
• Browse the official tutorials for Ignition Dome.
• Check discussions and announcements in the Gazebo/Ignition community forum.