Example_Turtlesim.md

Hands-on example for the Turtlesim node []

To learn ROS one of the first tutorials that everyone does is turtlesim.

Basically, it is a small graphical interface where a turtle can be moved using the speed commands of any robotic base in ROS. Official TurtleSim tutorial

We will use it as an example to show the models. Let's imagine that we want to create a system in which we have two nodes, one of them is the turtlesim and the other is the node to teleoperate it. The first thing we need to do is to create the models for both components. Both nodes are implemented in the turtlesim package in the ros_tutorials repository.

We can create the component models manually or use our static code analysis tools to get them automatically. Using the second method we obtained the following models.

turtlesim:
  fromGitRepo: "https://github.com/ros/ros_tutorials/"
  artifacts:
    turtle_teleop_key:
        node: turtle_teleop_key
        publishers:
          cmd_vel:
            type: "geometry_msgs/msg/Twist"
        parameters:
          scale_angular:
            type: Double
            default: 2.0
          scale_linear:
            type: Double
            default: 2.0
    turtlesim_node:
      node: turtlesim_node
      publishers:
        color_sensor:
          type: "turtlesim/msg/Color"
        pose:
          type: "turtlesim/msg/Pose"
      subscribers:
        cmd_vel:
          type: "geometry_msgs/msg/Twist"
      serviceservers:
        teleport_absolute:
          type: "turtlesim/srv/TeleportAbsolute"
        spawn:
          type: 'turtlesim/srv/Spawn'
        set_pen:
          type: 'turtlesim/srv/SetPen'
        reset:
          type: "std_srvs/srv/Empty"
        kill:
          type: 'turtlesim/srv/Kill'
        teleport_relative:
          type: 'turtlesim/srv/TeleportRelative'
        clear:
          type: 'std_srvs/srv/Empty'

To import this as a project in the RosTooling you can create a new modeling project. This can be done by pressing the icon "Add new ROS Project".

If the button doesn't work you can also create it manually using the Eclipse menu File -> New -> Other.. and searching for "Ros Model Project".

By default, a new project with a reference to the content of the "de.fraunhofer.ipa.ros.communication.objects" will be created. This new project contains a folder called "rosnodes" to hold the nodes description and a file with the extension .ros2 which will have an error because it is empty. This can be removed for this case.

Once the project is created, you can create a new file my File -> New -> Other -> File. We recommend giving as name to the file the name of the package and it must have the extension .ros2, this means the new file should be called turtlesim.ros2. By creating a file type .ros2, Eclipse will convert the project to an Xtext project. Then copy the previous model content to the new file.

Now that we have already the components we can compose them. For that, we have to create a new .rossystem file. Again go to File -> New -> Other -> File. The new file must have an extension .rossystem.

In RosSystem description we explain the format of a system and the editor will support you to write the model properly.

The first that must be given is a name and then a ":" is required. In the next line, you must add indentation and you can press the keys "Ctrl" + Space bar for help. Firstly, we will add the 2 nodes that compose our system.

So far our file looks like this:

turtlesim_system:
  nodes:
    turtlesim:
      from: "turtlesim.turtlesim_node"
    key_teleop:
      from: "turtlesim.turtle_teleop_key"

The "from" attributes are references to the previous turtlesim.ros2 file nodes. If you change the name of them, the references must also be updated.

Now, we want to expose the ports to be connected. This means the subscriber of the velocity command of the turtle and the publisher from the keyboard teleop:

And the model is updated to:

turtlesim_system:
  nodes:
    turtlesim:
      from: "turtlesim.turtlesim_node"
      interfaces:
        - cmd_subscriber: sub-> "turtlesim_node::cmd_vel"
    key_teleop:
      from: "turtlesim.turtle_teleop_key"
      interfaces:
        - cmd_publisher: pub-> "turtle_teleop_key::cmd_vel"

Again, the interfaces are references to the ones already defined in the turtlesim.ros2 file. In ROS terms, we can only create instantiate topics that are described on my original ROS code (cpp or python).

The last step is to create a connection between the two components.

And the model is updated to:

turtlesim_system:
  nodes:
    turtlesim:
      from: "turtlesim.turtlesim_node"
      interfaces:
        - cmd_subscriber: sub-> "turtlesim_node::cmd_vel"
    key_teleop:
      from: "turtlesim.turtle_teleop_key"
      interfaces:
        - cmd_publisher: pub-> "turtle_teleop_key::cmd_vel"
  connections:
   -[ cmd_publisher , cmd_subscriber]

If you save (Ctrl+S) after the last modifications a new folder "src-gen" will be automatically created. This folder contains a ROS 2 package ready to be executed with a launch file to start the designed system.

For a quick check, if you source a valid ROS installation and call the launch command the turtlesim example will be launched:

source /opt/ros/ROSDISTRO/setup.bash
ros2 launch PATH_TO_LAUNCH_PY_FILE

Using the terminal of the keyboard node you can use the arrows to send new commands to the turtle.

The autogenerated README file of the package will show you how to create the new workspace and call the launch file.