Overview and the most important commands in ROS2

Abb 1: Communication in ROS2

Absolute basics

• Each robot-functionality is managed in one entity known as a ROS 2 Node, [here is a great illustration]. ROS2 takes care of the running Nodes: timing, communication,...
• Communication between nodes is managed via interfaces, which are described using a specific ROS2-format [ROS2].

1. Topics: a "Publisher" sends messages and one or many "Subsriber" gets them
2. Service: a "Client" requests informations and a "Server" sends a response
3. Action: ??

• Organization in ROS2:
  • A package contains one or more programs (in C++ or Python). "Entry-Points" in their setup.py show ROS2 how to execute them.
  • Every "program" will start at least one Node. Each Node handles a specific robot function and ROS2 takes care of the running Nodes (timing, messaging,...)
  • A Launch-File launches a number of Nodes (from one or more packages)
• In ROS2 we use multiple terminals
• At the beginning of a terminal, use this so that the linux-terminal recognises all ROS2-commands

source /SomeWorkspace/install/setup.bash, e. g. source ~/ros2_ws/install/setup.bash

• Run execubale files ros2 run <package_name> <executable_file>

• ROS2-workspace: Folder for your ROS2-project, e. g. ros2_ws

• Launch a launch-file ros2 launch <package_name> <launch_file>
• Show all running nodes ros2 node list

Step-by-Step

• Find out your ROS2-Version for the next command
• Turn on the ROS2-environment via source /opt/ros/<ROS2-Version>/setup.bash
• Set up your workspace source ~/ros2_ws/install/setup.bash
• put this command in ~/.bashrc (~ means home-directory in Linux), so that it will be executed each time we start a terminal gedit ~/.bashrc

ROS-packages [ROS2-docs]

What is a package

Every program that we want to execute or create is organized in ROS2-packages. They are ether created in C/C++ or Python.

A Python-package consists of:

1. package.xml - meta-information (maintainer, dependencies, etc.)
2. setup.py - instructions how to compile the package
3. setup.cfg - instructions how to install the package
4. <package_name> - folder whith the name of your package. All our Python scripts etc. go here. Contains an empty __init__.py file at the beginning.

Show ROS2-packages

show all: ros2 pkg list

show only my "package_name": ros2 pkg list | grep <package_name>

Step-by-Step [go here for example-files]

1. Go to the src-directory of your workspace-folder: cd/ros2_ws/src
2. Create your package <package_name>: ros2 pkg create --build-type ament_python --license Apache-2.0 <package_name> --dependencies <package_dependency_1> <package_dependency_2>
3. Create a launch-folder: create folder so that you get src/<package_name>/launch
4. Modify setup.py: In order for ROS2 to find the .launch.py-files, we make sure the data_files in setup.py looks like

   # this is for finding
   import os
   from glob import glob
   #...
   data_files=[
           ('share/ament_index/resource_index/packages',
               ['resource/' + package_name]),
           ('share/' + package_name, ['package.xml']),
           (os.path.join('share', package_name), glob('launch/*.launch.py'))
       ],
   

5. Create a program: Create MyFirstPythonROSProgram.py Example of a Python-File for ROS2 with Important Basic Principles
6. Create a shortcut for the methods in a package: They are called executables/entry points/... Enter the name of the package in "entry_points" in the setup.py of your package

       entry_points={
           'console_scripts': [
               <name_of_executable> = <package_name>.<NewPythonScript>:<SomeMethod>,
               'for_example = somepackage.pythonscriptxy:main',
           ],
       },
   

7. Build package: Execute this command in your workspace-folder (e. g. ros2_ws) to compile all packages colcon build or a specific one colcon build --packages-select <package_name>
8. Refresh: After package-creation always do source ~/ros2_ws/install/setup.bash to let ROS2 recognize all updates
9. Test: ros2 run <package_name> <name_of_executable>

Topics

• Print all available topics ros2 topic list
• Search for specific topic ros2 topic list | grep <topic_name>
• Get information about a topic ros2 topic info <topic_name>
  • Get more details including the Quality of Service (QoS) ros2 topic info <topic_name> -v
  • "Type" is the interface. Show it via ros2 interface show <Type>
  • This gets us a list of "fields" which are the data fields / variables a topic transmits.

• Listen to a topic (stop with Cntrl+C) ros2 topic echo <topic_name>
• Listen to a topic once ros2 topic echo <topic_name> --once
• Listen to a field of a topic ros2 topic echo <topic_name> --field <field_name>
• Show camera data in an extra window ros2 run rqt_image_view rqt_image_view <topic_name>/.../image
• Publish a message to a topic ros2 topic pub <topic_name> <interface_name> <message>
• Example
  • Publish a message ros2 topic pub --once /SomeMessageName std_msgs/msg/String "data: 'Hello World'"
  • This topic should show here ros2 topic list
  • Get more information ros2 topic info /SomeMessageName . This way we get the Type=interface std_msgs/msg/String
  • Getting morge info about the interface ros2 interface show std_msgs/msg/String . This gets us at the end all variables, here it's only one string data
  • So in a Python-Skript we have to use

    # Import the String-Definition
    from std_msgs.msg import String
    
    # Subscriber to SomeMessageName
            self.our_new_subscriber = self.create_subscription(
                String,
                '/SomeMessageName',
                self.our_new_subscriber_callback,
                QoSProfile(depth=10, reliability=ReliabilityPolicy.RELIABLE)
            )
    
    # Our Callback-Method accessing the String via "msg.data"
    def our_new_subscriber_callback(self, msg):
            self.get_logger().info("[our_new_subscriber_callback] ...")
            self.get_logger().info(msg.data)
    

Interfaces

• show all ros2 interface list
• a interface has to be created in a CMake-package (there is no way in a Python-package yet)

Step-by-Step

1. Create package ros2 pkg create --build-type ament_cmake --license Apache-2.0 <InterfaceCPackage>
2. Create a msg-directory inside the tutorial_interfaces-folder and a msg-file

   cd ~/ros2_ws/src/<InterfaceCPackage>
   mkdir msg
   touch msg/<SomeInterface>.msg
   

3. Modify the msg-file:

   geometry_msgs/Point center # This custom message uses a message from another message package (geometry_msgs/Point in this case).
   float64 radius # Some new variable
   

4. Add some stuff to the CMakeLists.txt file:

   find_package(geometry_msgs REQUIRED)
   find_package(rosidl_default_generators REQUIRED)
   
   rosidl_generate_interfaces(${PROJECT_NAME}
     "msg/Num.msg"
     "msg/Sphere.msg"
     "srv/AddThreeInts.srv"
     DEPENDENCIES geometry_msgs # Add packages that above messages depend on, in this case geometry_msgs for Sphere.msg
   )
   

5. Add some stuff to the packages.xml file:

   <depend>geometry_msgs</depend>
   <buildtool_depend>rosidl_default_generators</buildtool_depend>
   <exec_depend>rosidl_default_runtime</exec_depend>
   <member_of_group>rosidl_interface_packages</member_of_group>
   

6. Compile in the workspace-folder: colcon build --packages-select custom_interfaces
7. Source (to let ros2 know all changes) source install/setup.bash

Coordinate Systems

coordinate system = coordinate/reference frame = transform = transformation frame (TF)

xyz = red green blue = RGB

View coordinate systems

Show a graph in a pdf-document to analyze the coordinate frame structure of a project:

ros2 run tf2_tools view_frames

Get the same graph, but in a window which you can refresh and see changes in real-time:

ros2 run rqt_tf_tree rqt_tf_tree

Show the link between 2 coordinate systems

ros2 run tf2_ros tf2_echo reference_frame get_frame

View coordinate systems via RViz2

• open RViz rviz2
• set the fixed frame via Global Options/Fixed Frame
• add the frame view via Add/TF
• Add/Image and set up "Description Topic" and "Quality of Service QoS" [in which quality the signal is to be received]
• Add/RobotModel and set up "Description Topic"
• and save the rviz2-config!

Create a coordinate system "NewSystem"

ros2 run tf2_ros static_transform_publisher --x 0 --y 4.0 --z 0 --roll 0 --pitch 0 --yaw 0 --frame-id SomeParentSystem--child-frame-id NewSystem