Lab 1: Intro to ROS 2

Learning Goals

• Getting familiar with ROS 2 workflow
• Understanding how to create nodes with publishers, subscribers
• Understanding ROS 2 package structure, files, dependencies
• Creating launch files

Before you start

It's highly recommended to install Ubuntu natively on your machine or in a virtual machine for development in simulation. However, if you can not install Ubuntu, you can still use the simulation inside Docker containers. For the following instructions, if you have Ubuntu installed natively, ignore the directions for using Docker.

We'll be using Classroom 50 throughout the semester to manage submissions for lab assignments. You can commit and push your work as often as you need, but a plain push does not count as a submission. When you're ready to submit, you need to push a tag named submission:

git push                            # your commits
git tag submission
git push origin submission          # this triggers the autograder

The autograder builds your package and checks the deliverables, then posts your score as a Release on your repo (check the Releases page or the commit's status check a few minutes after you tag). To resubmit, move the tag to a new commit:

git tag -f submission
git push --force origin submission

The best scored submission push is counted as your final submission and its grade will be your lab's grade.

The autograder finds your work by name. You should use the names the deliverables specify: package lab1_pkg, nodes talker and relay, topics drive and drive_relay, and launch file lab1_launch.py. Otherwise, the autograder will not be able to grade your work and your submission may get the wrong grade.

For a template of what your submission is expected to look like, please check out this template submission.

1. Overview

The goal of this lab is to get you familiar with the ROS 2 workflow. You can complete the coding segment of this assignment in either Python or C++. Pick one language and write the whole package in it. This is the easiest lab in which to try a new language, and the two workflows differ slightly in ROS 2, so if you want to learn both, redo the nodes in the other language in a separate scratch workspace. The autograder only grades the one lab1_pkg you submit.

In this lab, it'll be helpful to read these tutorials if you're stuck:

https://docs.ros.org/en/jazzy/Tutorials.html

https://roboticsbackend.com/category/ros2/

2 Getting ready (Native Ubuntu)

Install ROS 2 following the instructions here: https://docs.ros.org/en/jazzy/Installation.html.

Next, create a workspace:

mkdir -p ~/roboracer_ws/src
cd roboracer_ws
colcon build

Move on to Section 3 once you're done.

3: ROS 2 Basics

Now that we have access to a ROS 2 environment, let's test out the basic ROS 2 commands. In the terminal, run:

source /opt/ros/jazzy/setup.bash
ros2 topic list

You should see two topics listed:

/parameter_events
/rosout

If you need multiple terminals and you're inside a Docker container, use tmux.

4: Creating a Package

Deliverable 1: create a package named lab1_pkg in the workspace we created. The package needs to meet these criteria:

• The package is written in one language: either Python (an ament_python package) or C++ (an ament_cmake package). Don't mix the two in one package. The autograder accepts either.
• The package declares the ackermann_msgs dependency in package.xml, along with rclpy or rclcpp for the language you chose.
• If declared properly, the dependencies can be installed using rosdep.
• Your package folder should be neat. You shouldn't have multiple 'src' folders or unnecessary 'install' or 'build' folders.

5: Creating nodes with publishers and subscribers

Deliverable 2: Create two nodes in the package we just created. You can use either Python or C++ for these nodes.

The first node will be named talker.cpp or talker.py and needs to meet these criteria:

• talker listens to two ROS parameters v and d.
• talker publishes an AckermannDriveStamped message with the speed field equal to the v parameter and steering_angle field equal to the d parameter, and to a topic named drive.
• talker publishes as fast as possible.
• To test the node, set the two ROS parameters through the command line, a launch file, or a YAML file.

The second node will be named relay.cpp or relay.py and needs to meet these criteria:

• relay subscribes to the drive topic.
• In the subscriber callback, take the speed and steering angle from the incoming message, multiply both by 3, and publish the new values via another AckermannDriveStamped message to a topic named drive_relay.

6: Creating a launch file and a parameter file

Deliverable 3: Create a launch file lab1_launch.py that launches both of the nodes we've created. If you want, you could also set the parameter for the talker node in this launch file.

7: ROS 2 commands

After you've finished all the deliverables, launch the two nodes and test out these ROS 2 commands:

ros2 topic list
ros2 topic info /drive
ros2 topic echo /drive
ros2 node list
ros2 node info /talker
ros2 node info /relay

8: Deliverables and Submission

In addition to the three deliverables described in this document, fill in the answers to the questions listed in SUBMISSION.md.

9: Grading Rubric

• Correctly creating the package: 25 Points
• Correctly creating the nodes: 25 Points
• Correctly creating the launch file: 25 Points
• Written questions: 25 Points