ROS2 Test Project: Sensor Data Evaluation

Introduction

This test of the second week is an introduction to the ROS2 framework. The objective is to get hands-on experience with ROS2's core communication concepts, including nodes, topics, and the publish/subscribe model. The test consists of implementing a ROS2 package with two main components: a publisher node and a subscriber node, using both C++ or Python.

Objectives of the Test

The primary goal was to create a ROS2 package named sensor_data_evaluation that includes:

• A publisher node that generates random sensor data (temperature, humidity, and pressure) and publishes it every 0.5 seconds on a topic named /sensor_data.
• A subscriber node that listens to this topic and checks whether the received data falls within predefined valid ranges:
  • Temperature: 15°C to 35°C
  • Humidity: 30% to 70%
  • Pressure: 950 hPa to 1050 hPa Additionally, a launch file was required to run both nodes simultaneously.

Skills and Concepts Learned

Through this test, we learn how to :

• Creat and organize a ROS2 package.
• Write ROS2 nodes in C++.
• Publish and subscribe to topics.
• Work with custom data structures and message types.
• Use a launch files to manage and execute multiple nodes.

Bonus: Multimachine Communication with ROS2 (Fast DDS)

Introduction

As a bonus to this test, we explore Multimachine communication using ROS2, where ROS2 nodes can run on different physical machines and communicate with each other over a local network. This is made possible by DDS (Data Distribution Service), which is the default middleware in ROS2 (Fast DDS). It allows decentralized, serverless communication through a publish/subscribe model.

Concepts of DDS in ROS2

• Automatic Discovery: Nodes automatically detect each other on the network. There is no need for a central master or roscore.
• Direct Communication: Messages are sent directly from the publisher to the subscriber.
• Quality of Service (QoS): The behavior of message transmission (reliability, persistence, frequency) can be finely tuned using QoS profiles.

Steps to Set Up Multimachine ROS2 Communication

1. Ensure All Devices Are on the Same Network

Make sure that all the computers (e.g., PC1 and PC2) are connected to the same local network (Wi-Fi or Ethernet). You can verify this by checking that their IP addresses are in the same subnet.

# On each machine, get your IP address:
ip a   # or use `ifconfig` if available

2 Network Connectivity

Check that each machine can reach the other

# From PC1 to PC2:
ping <IP_ADDRESS_OF_PC2>

# From PC2 to PC1:
ping <IP_ADDRESS_OF_PC1>

If the ping is successful, then the machines are properly connected.

3. Set the ROS Domain ID

To isolate ROS2 communication, each machine must have the same ROS_DOMAIN_ID. You can set it temporarily with:

export ROS_DOMAIN_ID=0

On PC1 (Publisher) Launch the publisher node that sends sensor data:

ros2 run sensor_data_evaluation sensor_publisher

On PC2 (Subscriber) Launch the subscriber node that receives and checks the data:

ros2 run sensor_data_evaluation sensor_subscriber

You should now see the subscriber receiving messages published from the other machine.

4. Architecture Diagram

            ┌────────────┐                          ┌────────────┐
            │   PC1      │                          │   PC2      │
            │------------│                          │------------│
            │ Publisher  │── /sensor_data topic ──► │ Subscriber │
            └────────────┘                          └────────────┘
                   ▲                                      ▲
                   │                                      │
                   ▼                                      ▼
            ┌────────────┐                          ┌────────────┐
            │   PC3      │                          │   ...      │
            │------------│                          │------------│
            │ Subscriber │                          │ Subscriber │
            └────────────┘                          └────────────┘

     All machines on the same local network and sharing ROS_DOMAIN_ID=0

Multimachine communication in ROS2 is straightforward when the following conditions are met:

• All devices are on the same local network.
• They share the same ROS_DOMAIN_ID.
• ROS2 is installed and sourced correctly on each machine.
• Firewall rules do not block DDS traffic.

Useful link

https://roboticsbackend.com/ros2-multiple-machines-including-raspberry-pi/