ROS 2 (Robot Operating System)

Overview

ROS 2 is middleware for exchanging typed data between nodes. A node is a named participant in the ROS graph; one process can host one or more nodes. Nodes communicate through topics, each of which has a message type that defines the data exchanged between publishers and subscribers.

General Components

ROS 2 discovery

ROS 2 does not use the ROS 1 master or roscore. On the Duckiedrone, ROS 2 components run in Docker containers. The zenoh-router container runs rmw_zenohd; the current ROS 2 containers use ROS_DOMAIN_ID=42 and communicate through this router. See Duckiedrone Containers for the current container reference.

ROS 2 nodes

ROS 2 nodes may publish to and subscribe from topics. The processes that host Duckiedrone nodes run inside containers rather than GNU Screen windows. For example, ros2-camera republishes the camera feed as a ROS 2 topic, while ros2-mavros connects the flight controller to ROS 2.

Messages

ROS 2 messages are structured data types. A message can contain primitive fields, such as numbers and strings, as well as nested message types. To exchange data on a topic, publishers and subscribers must use the same message type and compatible quality-of-service (QoS) settings.

Topics

Topics are named, typed channels over which nodes exchange messages. A topic can have multiple publishers and subscribers. The Duckiedrone’s ROS 2 bridge containers expose data such as camera and Time-of-Flight readings to ROS 2 consumers.

Publishers

Publishers send a specific message type to a topic. Sensor bridges publish readings, and control nodes can publish command information for the rest of the flight stack.

Subscribers

Subscribers receive messages from a topic. A subscriber specifies the topic, message type, and QoS settings, then reacts when a compatible publisher sends a new message.