U01-L01-Topics-and-Transfer.md

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Objective: Unit 1 Lesson 1 Topics and Transfer

• Understand ROS messages
• Understand the concept of a publish / subscribe systems.
• Understand ROS middlewares.
• Identify ROS namespaces.
• Understand and use the ros CLI for ros2 topic.

ROS Messages

• A key feature of ROS is its message system.
• ROS Messages are like a standard sized mail envelope. They hold a value.
• A message is simply a collection of data, arranged as a set of fields with a name and type.
  • The position of a robot, or an image are examples of a message.
  • Messages can be composed from other messages, e.g. a robot's state message may be composed of a position message and an orientation message.
  • These messages are similar to Python's dict type or C++'s struct.
• Each ROS package can define a set of messages or use messages defined in another package.
• ROS has many prefined messages for common robot component.
  • These shared messages are what make ROS so powerful!

ROS Messages in Practices

• Let's look at an example from TB4.
• Message files use the *.msg suffix. Let's look at a few.
• Left pane: We used find to list all of the *.msg files in the TB4 sources.
• Right pane: We used less to view the UserLed.msg message files.
  • An LED is a light emitting diode, a small light bulb.
• We can see that messages are simply types (e.g. uint8) and a name. There are also comments telling us what the messages represent.
• The UserLed message defines which led you want to color and the color you want to make it.

Moving Messages

• In the abstract, ROS moves messages using a concept called publish / subscribe, often abbreviated pub/sub.
• It is easier to think of publishing as sending, and subscribing as recieving.
• In ROS messages are published and subscribed to topics.
• Topics are like a phone number, or a mail box, they allow the same message type to be used multiple times.
• ROS topics are hierarchical, they can be grouped in a logical hierarchy.
• This hierarchy is denoted using the forward slash, "/", similarly to your computer's hard disk.

Where This Analogy Fails

• Pub/sub systems aren't exactly like the mail in some respects. In some ways they're more like broadcast towers!
• In the mail, a letter goes from exactly one house to exactly one other house. We call this one-to-one transfer.
• In a pub/sub system messages are moved in a many-to-many fashion.
• In a pub/sub system multiple people can send and recieve messages to a topic.
• When a program subscribes to a topic it is asking to recieve messages from a topic.
• When a program publishes to a topic it is asking to send messages to everyone subscribed to a topic.
• In some ways a topic is like a public TV station, where anyone can make a program and anyone can watch that show (if they want).

How does ROS Move Messages?

• ROS doesn't actually move messages published to a topic!
• Instead, ROS has a generic interface for moving messages, and you choose how you want them to be moved.
• This generic layer is called a "ROS Middleware Layer" or RMW.
• An RMW is like an adapter that let's you plug ROS into lots of different software protocols and hardware transport layers.
  • If you want to move ROS to publish topics over carrier pigeon or e-mail it is possible.
• Most ROS 2 robots use DDS as their RMW. DDS stands for Data Distribution Service.
• DDS is a generic protocol for moving data..
  • DDS can supports a variety of transport protocols like WiFi, 4G, 5G, and even USB.
• There are a number of vendors of DDS software. Some of them are open-source and others are proprietary.

Practical RMW / DDS

• Each ROS distro ships with a default RMW implementation.
• Just like any software, there are trade-offs with each RMW implementation. For each ROS distro the default is selected after a thorough vetting process.
• On the TurtleBot4, the TurtleBot4 simulation, and Create3, we're using CycloneDDS, but you can select other DDS vendors.
• The TB4 has two places where we can check and change our RMW.
• One the Create3
  • Log into the iRobot console by entering it in your browser and then go to Application->Configuration. (see above).
  • There is a drop down to select the RMW.
  • On the Turtlebot / desktop you can check the RMW_IMPLEMENTATION by using the printenv command. Detailed instructions are here.
  • If no value is set, assume you are using the default.
  • Otherwise the RMW is the value listed in the environment.

The ROS Command Line

• To see how ROS topics work in action we need to learn the ROS 2 command line interface (CLI).
• If you installed ROS yourself you're already familiar with a command line application. Commands like ls, cd, and mv are all command line interfaces.
• A command line interface is very simply a set of text-based programs for working with ROS.
• The ROS 2 CLI has a very particular grammar. Commands always start with ros2 followed by a noun like topic or node and then subsuquent commands or parameters.
• The ROS 2 CLI is self documenting, meaning each command contains its own little help file. For any command you can add --help to the command and it will list its options.

Before You CLI

• ROS has two concepts you need to be aware of at this point:
  • Workspaces -- these are folders that are a collection of ROS code and executables. They usually live in a folder that ends with _ws.
  • Overlays -- Overlaying is the process of combining ROS code. We say that one workspace overlays another workpace.
  • We'll cover these in detail later on.
• Roughly, on your desktop or laptop, you should currently have two workspace.
  • The system level workspace (e.g. the whole computer with ROS installed).
  • The TB4 Simulator workspace (turtlebot4_ws).
  • We say that the turtlebot4_ws overlays your system level ROS installation.
• If you want to use ROS generally, you can tell your terminal about ROS by calling:
  • source /opt/ros/<ros distro>/setup.bash
  • Our ROS distro is galactic.
• If you are working on specific ROS project in a folder you need to setup the workspace.
  • e.g. in ~/turtlebot4_ws call source ~/install/setup.bash
• Before using the ROS 2 CLI you need to tell your terminal which workspace you want to use.

ROS 2 CLI in Action

• Let's explore the ROS 2 CLI together. You can follow along.
• If you want to use your robot to follow along:
  • ssh into your robot, and then start byobu
  • ssh [email protected] -- where XXX is your robot's IP address.
  • On the robot you simply need to source the system bash file.
    • source /opt/ros/galactic/setup.bash
• If you want to use the TB4 simulator:
  • If you're using a VM or container, start it up.
  • Now run source /opt/ros/galactic/setup.bash
  • Move to the simulator workspace and run source ./install/setup.bash
  • Next start the sim, ros2 launch turtlebot4_ignition_bringup ignition.launch.py
  • Finally open a new terminal and source the system bash file again.
• From here the steps should be similar.

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