Introduction to ROS2

[!IMPORTANT] Before you start: we will be using the Linux command line during this module. If you are not familiar with the command line, you can find resources at the bottom of this tutorial to help you. When you encounter a command you don't recognise, look it up and try to understand what it does.

COMP4034 uses ROS2 Humble Hawksbill (usually just called ROS 2 Humble) as the middleware for communicating with and controlling our robots. Humble is available on all B85 lab machines through a Docker container, and you can set up the same environment on your own machine. You do not need to install ROS 2 directly on your computer to complete the module. We provide a Docker-based environment for working with ROS 2. See ROS2 on your own machine.

This tutorial is designed to be completed in B85. In this tutorial, we will cover:

1. Some initial setup for your B85 workstation
2. How to run the TurtleBot simulation
3. The 3 key ROS2 concepts - nodes, topics, and messages
4. How to teleoperate (drive around) the TurtleBot in simulation

Initial setup

ROS2 is already installed on the lab machines and runs inside a Docker container. You will need to start the container every time you sign-in to the lab machines.

1. Open a terminal, and cd ~/COMP4034/src
2. In your terminal, open VSCode with code . (note the full-stop! This allows VSCode to run and you to continue using the terminal)
3. In your terminal, type xhost +local:docker. You will get a message that local network connections have been added to the access control list if this is successful
4. Inside VSCode, press ctrl + shift + p on your keyboard and then search and select "Dev Containers: Rebuild and Reopen in Container"

If this is successful, you will see a new terminal prompt open inside VSCode that's pointing to /home/ros2_ws. This is the terminal where you will run your ROS2 commands. Start by running source /opt/ros/humble/setup.bash.

You will also need to build your packages by running colcon build, then source install/setup.bash.

You can check your setup so far by typing ros2 topic list into this terminal and checking it returns some topics (if nothing else is running, you should see /rosout and /parameter_events).

Running the TurtleBot simulation

You can now run the TurtleBot simulation.

1. Run the following command inside the VSCode terminal: ros2 launch turtlebot3_gazebo turtlebot3_world.launch.py

[!NOTE] Gazebo (the TurtleBot simulation software) might take a few minutes to launch the first time as it loads in the world file. You may also find it doesn't open successfully the first time, due to the "spawn service" being unavailable. This is a strangeness with Gazebo itself trying to use a service before Gazebo has started it properly (!) - if this happens, ctrl+c in the terminal running Gazebo and try launching it again.

Once it's successfully launched, it should open the world below:

We will leave this open for the rest of this tutorial.

[!CAUTION] Don't close the terminal running Gazebo, else the simulation will close and you'll need to restart it. You can also ignore the warning that Gazebo displays about reaching EoL.

Exploring Nodes, Topics and Messages

ROS2 predominantly uses a publisher/subscriber architecture for communication. We will go into more detail in the lectures, but for now it's easy to understand "nodes" as small programs that each do one job. Nodes communicate with one another using topics, which carry messages of a defined type. A simple example of how these concepts work is below, using a node that runs the robot's LIDAR and a node that runs the robot's movement:

Go back to VSCode and open a new terminal - click the "+" sign at the right of the bar where "PROBLEMS | OUTPUT | DEBUG CONSOLE | ..." are displayed.

In this new terminal tab:

1. Source the terminal source install/setup.bash You need to do this to every new terminal!
2. run the command rqt.

RQT is a tool that allows us to visualise different elements of our ROS system. In this instance, we will set it up to show us the nodes and topics actively publishing. Press Plugins > Introspection > Node Graph in the top bar of RQT. Select your check-boxes to match the image below and press the refresh button.

In RQT, the circles are nodes and the rectangles are topics. You will need to use the terminal to find out what message type is used by each topic. Open a new terminal tab in VSCode (so that you keep RQT running in the background) run the command ros2 topic list -t.

You can use RQT to learn about each message type. In the top bar of RQT, select Plugins > Topics > Message Type Browser. You can use the drop-down boxes in the new view to select the message package, then the message type. You need to press the "+" button to add the message to your view. There is an example below for the Twist message:

You should use the useful links at the bottom of this page to learn more about nodes, topics, and messages.

Teleoperating the TurtleBot

Teleoperation lets us drive the robot around, like a remote-controlled car. Close the terminal tab running RQT, and open a new terminal tab. Then run ros2 run turtlebot3_teleop teleop_keyboard. You need to keep this window focused as you press the buttons on your keyboard, and you will need to be able to see Gazebo to see the robot move. This command runs a node (a simple program) that publishes Twist commands to drive the robot based on which key you press. Each press of the keyboard key increases the velocity of the robot. As you can see here, a Twist message is a data type which contains two geometry_msgs/msg/Vector3 objects named linear and angular, which are respectively the linear and angular velocity commands you send to the motors. At each key press, you are basically incrementing the velocity values.

While teleoperating, use another terminal to inspect the velocity commands:

ros2 topic echo /cmd_vel

Try pressing a movement key and observe how the Twist message changes.

When you've finished driving your robot around, make sure you press 's' to stop your robot and close all the terminal tabs.

Congratulations, you've now finished this tutorial! You should now be able to:

• start the COMP4034 ROS 2 environment;
• launch the TurtleBot simulation;
• explain the difference between a node, topic and message;
• inspect ROS 2 communication using ros2 commands and RQT;
• identify a topic's message type;
• explain what a Twist message represents; and
• teleoperate the TurtleBot using ROS 2.

You should now move onto the page Using real robots.

After you practice with the robots under the guidance of your demonstrators, you will be ready to start Practical 1 - Motion and Control.

Useful links

Using the Ubuntu terminal

Linux commands cheat sheet

ROS2 Humble tutorials

Understanding nodes

Understanding topics