Developing Custom ROS Nodes and Packages

This module dives into the core of building custom functionalities within the Robot Operating System (ROS). We'll explore how to create your own ROS nodes, the fundamental building blocks of ROS applications, and how to organize them into reusable packages. This is crucial for implementing specific robot behaviors, sensor processing, and control algorithms in advanced robotics and industrial automation.

Understanding ROS Nodes

A ROS node is a process that performs computation. Nodes are the fundamental units of computation in ROS. They can be as simple as publishing sensor data or as complex as controlling a robot's arm. Nodes communicate with each other by sending and receiving messages via topics, services, and actions.

Nodes are the executable processes that perform specific tasks in ROS.

Think of nodes as individual workers on a factory floor, each responsible for a specific job like picking, placing, or inspecting. They communicate with each other to achieve a larger goal.

In ROS, a node is typically a compiled program written in C++ or Python. Each node has a specific purpose, such as reading data from a camera, controlling a motor, or planning a robot's path. Nodes communicate by publishing messages to topics, subscribing to topics, offering services, or calling services. This modular approach allows for flexible and scalable robot systems.

What is the primary function of a ROS node?

A ROS node is a process that performs computation and executes specific tasks within the ROS ecosystem.

Creating Your First ROS Node

Let's walk through the basic steps to create a simple ROS node. We'll use Python for this example, as it's often more accessible for beginners.

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The process involves creating a ROS package, writing the node's source code (e.g., a Python script), and then building and running it.

ROS Packages: Organizing Your Code

ROS packages are the primary mechanism for organizing software in ROS. A package is a directory that contains ROS-specific files, such as source code, configuration files, launch files, and message definitions. They allow for modularity, reusability, and easy distribution of robot software.

Packages bundle related ROS nodes and resources for better organization and reusability.

Imagine a toolbox for your robot. A ROS package is like a compartment in that toolbox, holding all the tools (nodes) and instructions (launch files, configs) for a specific task or robot component.

A ROS package typically includes a package.xml file (describing the package's metadata, dependencies, and build type) and a CMakeLists.txt file (for C++ packages) or a setup.py file (for Python packages) to manage building and installation. This structure ensures that ROS can discover, build, and run your code effectively.

Key Components of a ROS Package

File/Directory	Purpose	Example
`package.xml`	Package metadata, dependencies, author information.	Defines package name, version, author, and build dependencies.
`CMakeLists.txt` (C++)	Build instructions for C++ nodes and libraries.	Specifies executables to build and link libraries.
`setup.py` (Python)	Build and installation instructions for Python packages.	Manages Python dependencies and entry points.
`src/`	Source code for nodes and libraries.	Contains `.cpp` or `.py` files.
`include/`	Header files for C++ libraries.	Contains `.h` or `.hpp` files.
`scripts/`	Executable Python scripts (often nodes).	Contains runnable Python scripts.
`launch/`	Launch files to start multiple nodes.	XML files defining node execution.

Building and Running Custom Packages

Once your package is structured correctly, you'll use ROS build tools to compile your code and then the

code

rosrun

code

roslaunch

commands to execute your nodes.

The catkin_make (or colcon build for ROS 2) command is essential for building ROS packages. It reads the package.xml and build files (CMakeLists.txt or setup.py) to compile source code, generate libraries, and install executables. After building, you source your workspace's setup file (e.g., source devel/setup.bash) to make your new nodes and packages available to the ROS environment. Then, rosrun <package_name> <node_executable> starts a single node, while roslaunch <package_name> <launch_file.launch> starts a set of nodes defined in a launch file.

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What command is used to build ROS packages, and what command is used to run a specific node from a package?

The catkin_make (or colcon build) command builds packages. The rosrun <package_name> <node_executable> command runs a specific node.

Best Practices for Package Development

Adhering to best practices ensures maintainable, efficient, and collaborative ROS development.

Keep nodes focused on a single responsibility. This makes them easier to test, debug, and reuse.

Use clear and descriptive names for packages, nodes, topics, and parameters. Document your code thoroughly using comments and README files. Leverage ROS launch files to manage complex system configurations and node startup parameters. Consider creating custom ROS messages or services when existing ones don't meet your needs.

Learning Resources

ROS Wiki: Creating Packages(documentation)

The official ROS Wiki page detailing the process of creating new ROS packages, including essential file structures and commands.

ROS Wiki: Nodes(documentation)

An overview of ROS nodes, their lifecycle, and how they interact within the ROS graph.

ROS Tutorial: Writing a Simple Publisher and Subscriber (Python)(tutorial)

A step-by-step guide to creating your first ROS nodes in Python that publish and subscribe to messages.

ROS Tutorial: Writing a Simple Publisher and Subscriber (C++)(tutorial)

A step-by-step guide to creating your first ROS nodes in C++ that publish and subscribe to messages.

ROS Wiki: catkin(documentation)

Documentation for catkin, the build system used in ROS Groovy and later, explaining how to build packages.

ROS Wiki: roscpp(documentation)

The C++ client library for ROS, essential for writing C++ nodes and understanding its API.

ROS Wiki: rospy(documentation)

The Python client library for ROS, crucial for developing Python-based nodes and interacting with ROS topics and services.

ROS Wiki: Launch Files(documentation)

Information on ROS launch files, which are used to start multiple nodes and configure their parameters.

The Construct: ROS Package Creation Tutorial(tutorial)

A practical tutorial from The Construct on creating and managing ROS packages, often with a focus on practical application.

ROS Answers: Best Practices for Package Structure(blog)

A discussion thread on ROS Answers covering community-driven best practices for organizing ROS packages.