Understanding ROS 2 with Turtlesim

1. Introduction

Turtlesim is the Flagship example application for ROS and ROS 2. It demonstrates in simple but effective ways the basic concepts.

This workshop encourages you to refer to the cheat sheet for the syntax and type the commands on your own in order to learn by trial and error. Make use of the --help option for commands as well. However, solutions are also provided in the end. Feel free to approach this any way as you wish.

2. Starting the Turtle simulator

You can start the main application by simply executing two of its nodes. Refer to the cheat sheet for the syntax to execute a node.

The package name you need in this case is turtlesim and the nodes you need to start are turtlesim_node and turtle_teleop_key. Make sure to source ROS 2 and run these nodes in two separate terminals.

Start the 2 nodes of the application

Once you successfully start these nodes, you should see a window popup with a blue background and a random turtle in the middle (this is an RQT pane, if you are interested in knowing!). This little guy is going to help us understand ROS 2. We should treat it as if it is an AGV (Automated Guided Vehicle), and we observe this scene from a top down perspective.

Before moving the turtle, you might find it useful to right click on the title bar of the simulator screen and select Always on top (undo this when you no longer need it).

Keep the simulator window on top

Next, make sure your currently active window is the terminal where you started the teleop node by clicking once on it.

Activate the teleop terminal

Now, as the instructions on the terminal say, you can move the turtle around by pressing the arrow keys on the keyboard, or use the other keys listed to set absolute orientations.

Move the turtle with the arrow keys on the keyboard

Once you are satisfied with playing with the turtle, you can go ahead to the next step.

3. Observing

You can start introspecting at this point and already see many interesting things. Please spend a few minutes trying to list the nodes, parameters, topics, services and actions. Refer to the cheat sheet for the CLI syntax for the list/info/show/echo commands.

List all available entities

Try to get information about each of them as well as the associated types (msg and srv).

Get more information on these entities

You can find out which of the topics are publishers, and listen in on what these topics are currently publishing.

Echo the topics

Perform all of the above introspection activities using RQT wherever applicable. Refer to the cheat sheet to see what plugins are available, or simply explore the RQT Plugins menu!

Use RQT for introspection

NOTE

You might notice some extra topics with the word action in the name when you use RQT Node Graph, which you do not see when you list the topics in CLI. You can safely ignore these for now, as they will be addressed in a later lecture.

3.1 Description of topics

The topic /turtle1/color_sensor tells you the RGB values of the color of the trail left behind when the turtle moves.The topic /turtle1/cmd_vel is used to instruct the turtle to move. Echo this topic in a separate terminal and then use the teleop node to move the turtle to observe how velocity commands are given to it.The topic /turtle1/pose publishes the current pose of the turtle. Echo this topic to see how the pose of the turtle changes as you move it.

3.2 Description of services

The services /kill and /spawn are used to kill and spawn turtles respectively.The service /clear clears the background of trail lines and /reset resets the position of the turtle.The service /turtle1/set_pen sets the color and thickness of the trail line.The services /turtle1/teleport_xx move the turtle instantly.

You can ignore the services with the word parameter in the names for now.

4. Interacting

In this step you will practice interacting with the topics, services, actions and parameters provided by the turtlesim.

4.1 Services

Call the service /turtle1/set_pen from the CLI first by referring to the cheat sheet for the syntax. For selecting the right type, use double tab after typing in the name of the service to retrieve a list of options for the type and use the most appropriate. (Hint: Service types DO NOT have – in their names, and have their names in/hierarchical/format).

In order to find out the right dictionary format to use, inspect the service type from another terminal (Hint: ros2 interface show and ros2 interface proto). The key values take an integer in the range 0-255. Once you call the service correctly, move the turtle with teleop to observe the change in trail color.

Set the pen color of the turtle, and visually verify the change

You can echo /turtle1/color_sensor to verify if this is the same RGB value that you set.

Check the topic to cross verify

You can try to call the same service from RQT as well. Refer to the cheat sheet to select the right plugin.

Use RQT to perform the same service call

You can similarly call the other services as well and observe what changes they make.

Invoke all the other services as well using CLI and/or RQT.

4.2 Parameters

The parameter commands are fairly straightforward. Try to list the params available and see what values they have. Change the background color of the sim by setting one or more of the relevant params. The allowed range of values for this is 0 - 255.

Change background color of the same by setting a parameter

4.3 Actions

Next, you can invoke the action provided by this sim. List the actions available, find out the interface type, and then invoke the action with meaningful values.

In this case, the action server can be invoked by a command line client using the ros2 action send_goal <goal> command. The syntax is similar to calling a service. The input parameter is theta which is an angle measured in radians, i.e. The valid range is -3.14 < theta < 3.14.

Using the --feedback option with the command prints the feedback to the console. The result of the action is always displayed.

Invoke an action call from the terminal

4.4 Topics

Finally, you can publish a velocity command on /turtle1/cmd_vel from CLI and RQT in a similar manner as calling a service, but of course, using the right commands for topics/messages instead. Make sure the teleop node is shut down before attempting this.

(Hint: The msg type is composite in this case, using Vector3. Each Vector3 type has 3 fields: x, y, y. The sub-fields can be accessed with : Ex- linear:x:0.5. Play around with spaces until the command works. The solution is provided in the end.)

Control the turtle from a terminal publisher

5. Using ros2bags

In a separate terminal start recording a ros2bag file with the selected topic /turtle1/cmd_vel. Back in your keyboard teleop terminal, give some velocity commands to make the turtle move. Go back to the ros2bag terminal and hit ctrl+c to kill it and stop recording.

Replay this ros2bag file, and you will notice the turtle moving in the same way as you recorded.

6. Advanced - Remapping and other options

Every ROS 2 command and sub-command has a list of options that you can use to modify its behavior. The list of options can be seen with -h and included as desired. You can experiment with these as well and see how the commands you have already executed so far change.

For example, when you publish a velocity command from CLI, it publishes this message continuously and the turtle keeps moving until you kill the publisher. You could instead give a -r 0.5 option to make it publish at 0.5 Hz. meaning there is a slight pause before every movement.

You could try adding the same rate option command to the spawn service (make sure to remove the name field as this would otherwise cause a name clash error). You will observe that turtles keep spawning until you kill the service caller.

7. Solutions

7.1 Starting the Turtle simulator

ros2 run turtlesim turtlesim_noderos2 run turtlesim turtle_teleop_key

7.2 Observing

ros2 node listros2 topic list -tros2 topic info /turtle1/cmd_velros2 interface show turtlesim/msg/Poseros2 service listros2 interface show turtlesim/srv/Spawnros2 interface proto turtlesim/srv/Spawn

7.3 Interacting

ros2 service call /spawn turtlesim/srv/Spawn "{x: 5,y: 5,theta: 0}"ros2 service call /reset std_srvs/srv/Empty

ros2 topic pub /turtle1/cmd_vel geometry_msgs/msg/Twist "linear:
  x: 0.5
  y: 0.0
  z: 0.0
angular:
  x: 0.0
  y: 0.0
  z: 0.5"

ros2 topic pub -r 0.5 /turtle1/cmd_vel geometry_msgs/msg/Twist "linear:
  x: 0.5
  y: 0.0
  z: 0.0
angular:
  x: 0.0
  y: 0.0
  z: 0.5"

ros2 service call -r 0.5 /spawn turtlesim/srv/Spawn "{x: 5,y: 5,theta: 0}"

ros2 service call /turtle1/set_pen turtlesim/srv/SetPen "r: 100 g: 0 b: 0 width: 0 'off': 0"

ros2 action send_goal /turtle1/rotate_absolute turtlesim/action/RotateAbsolute {'theta: -1.57'} --feedback

ros2 param set /turtlesim background_r 125

7.4 Using ros2bags

ros2 bag record /turtle1/pose -o velocities