Lab 1 – Logger Exercise

Autonomous Systems Course · CRTA Lab, University of Zagreb

This repository contains a template for the first laboratory exercise. The goal is to implement a ROS 2 node that subscribes to one or more topics and logs the received data to the terminal and/or a file using ROS 2 logging utilities.

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Prerequisites

Before you begin, make sure the following are installed on your system:

• Ubuntu 22.04 (recommended) or a compatible Linux distribution
• ROS 2 Humble (or the distro specified by your instructor)
• Python 3.10+
• colcon build tool

If ROS 2 is not yet installed, follow the official guide: https://docs.ros.org/en/humble/Installation.html

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Creating a ROS 2 Workspace

If you do not already have a workspace, create one with the following commands:

mkdir -p ~/ros2_ws/src
cd ~/ros2_ws

Source your ROS 2 installation (add this to ~/.bashrc to avoid repeating it):

source /opt/ros/humble/setup.bash

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Cloning the Repository

Clone the package into the src directory of your workspace:

cd ~/ros2_ws/src
git clone https://github.com/CRTA-Lab/lab1_logger_exercise.git

Your workspace structure should now look like this:

ros2_ws/
└── src/
    └── lab1_logger_exercise/
        ├── lab1_logger_exercise/
        ├── resource/
        ├── test/
        ├── package.xml
        ├── setup.cfg
        └── setup.py

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Installing Dependencies

From the root of your workspace, install any declared dependencies:

cd ~/ros2_ws
rosdep install --from-paths src --ignore-src -r -y

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Building the Package

Build the workspace using colcon:

cd ~/ros2_ws
colcon build --symlink-install

After a successful build, source the local workspace overlay:

source install/setup.bash

Tip: Add source ~/ros2_ws/install/setup.bash to your ~/.bashrc so you don't have to source it manually every time.

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Running the Node

Once built, run the logger node with:

ros2 run lab1_logger_exercise <node_name>

Replace <node_name> with the entry point defined in setup.py (e.g. logger_node).

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Task Description

The exercise is split into two parts. Both nodes are provided as templates — your job is to fill in the missing logic marked with # Fill.

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Part 1 — Logger Node

Objective

Implement a ROS 2 node that subscribes to the /odom topic and saves the robot's position and orientation to a CSV file.

What to implement

• Subscribe to /odom (nav_msgs/msg/Odometry)
• On every received message, extract:
  • x — position along the X axis (pose.pose.position.x)
  • y — position along the Y axis (pose.pose.position.y)
  • yaw — heading angle, converted from quaternion to Euler angles
• Append each row to a CSV file in the format:

x,y,yaw
1.234,0.567,0.312
...

• On node shutdown (Ctrl+C), close the CSV file cleanly and display a trajectory plot of the recorded path (x vs y).

How to run

Terminal 1 — start the logger node:

ros2 run lab1_logger_exercise logger_node

Terminal 2 — play back the provided ROS bag file:

ros2 bag play <path_to_bag_file>

Once the bag file finishes playing, press Ctrl+C in Terminal 1 to stop the logger node. A plot of the recorded trajectory will appear automatically.

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Part 2 — Path Publisher Node

Objective

Implement a ROS 2 node that reads the CSV file saved in Part 1 and publishes the recorded trajectory as a nav_msgs/msg/Path message in the odom frame.

What to implement

• Read the CSV file generated by the logger node
• For each row, create a geometry_msgs/msg/PoseStamped with:
  • header.frame_id = "odom"
  • pose.position.x and pose.position.y from the CSV
  • pose.orientation converted back from yaw to quaternion
• Publish the full list of poses as a nav_msgs/msg/Path on the /path topic

How to run

Make sure the CSV file from Part 1 exists, then run:

ros2 run lab1_logger_exercise path_publisher_node

To visualise the published path, open RViz2 in a separate terminal:

rviz2

In RViz2:

1. Set the Fixed Frame to odom
2. Add a Path display and set its topic to /path

You should see the recorded trajectory rendered in the odom frame.

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Acceptance Criteria

Logger node:

• Subscribes to /odom and writes x, y, yaw rows to a CSV file
• Closes the file and shows a trajectory plot on Ctrl+C

Path publisher node:

• Reads the CSV file and publishes a valid nav_msgs/msg/Path on /path
• Path is correctly visualised in RViz2 in the odom frame

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Useful ROS 2 Commands

# Check that /odom is being published during bag playback
ros2 topic echo /odom

# Verify the path is being published
ros2 topic echo /path

# List all active topics
ros2 topic list

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Project Structure

Path	Description
lab1_logger_exercise/	Main Python package containing node source files
resource/	Package resource marker (required by ament)
test/	Unit and integration tests
package.xml	Package metadata and dependencies
setup.py	Python package build configuration
setup.cfg	Entry points and install settings

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License

This project is licensed under the Apache License 2.0. See LICENSE for details.