ROS2 Example (Turtlesim)
Quickly experience the common basic concepts and functionalities of ROS2 using the Turtlesim example.
This tutorial is based on the ROS2 Humble Docker image but is not limited to the Docker environment. It can be performed in any ROS2 environment.
If you are using a Docker environment, it is recommended to first learn how to use Docker.
Start the Docker Script
Use the following command to start the Docker script: ros_humble.sh is a script created during the ROS2 environment setup.
bash ros_humble.sh
It is recommended to use multiple terminals to access the same Docker container for easier subsequent operations.
Start Turtlesim
Use the following command to start Turtlesim:
ros2 run turtlesim turtlesim_node
This command will create the /turtlesim node and display a window containing a turtle.
Control Turtlesim
Use the following command to control the turtle's movement:
ros2 run turtlesim turtle_teleop_key
This command will create the /teleop_turtle node.
Click and focus on the terminal window running this command. You can control the turtle's movement using the arrow keys on your keyboard.
ROS2 Basic Concepts
Through the Turtlesim example, you can further understand and use the basic concepts and commands of ROS2.
To stop a running ROS2 command, use the Ctrl + C keyboard shortcut.
ROS2 Nodes
Nodes are the basic communication units in ROS2. They interact with other nodes through publishers and subscribers.
Key Components:
- Publisher: Interface for publishing messages to a topic.
- Subscriber: Interface for receiving messages from a topic.
View Nodes
Use the ros2 node list command to view nodes. Currently, only the command to start Turtlesim is running.
ros2 node list
ROS2 Topics
Topics are the publish/subscribe communication mechanism in ROS2, enabling asynchronous data transfer between nodes.
Communication Roles:
- Publisher: Sends messages to a specific topic.
- Subscriber: Subscribes to a topic to receive messages.
View Topics
Use the ros2 topic list command to view topics:
ros2 topic list
Publish a Topic
Use the ros2 topic pub command to publish a velocity control message to move the turtle:
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}}"
This command will move the turtle at a linear velocity of 0.5 and an angular velocity of 0.5.
Subscribe to a Topic
Use the ros2 topic echo command to subscribe to a topic and view its messages:
ros2 topic echo /turtle1/pose --once
This command will print the turtle's position once in the terminal. Remove --once to continuously print the turtle's position.
ROS2 Services
Services are the request/response communication mechanism in ROS2, used for synchronous calls between nodes (e.g., remote function calls).
Interaction Flow:
- Client: Sends a request.
- Server: Processes the request and returns a response.
View Services
Use the ros2 service list command to view services:
ros2 service list
Call a Service
Use the ros2 service call command to call a service and clear the screen:
ros2 service call /clear std_srvs/srv/Empty
This command will clear the background in the Turtlesim window.
ROS2 Actions
Actions are the communication mechanism in ROS2 for handling long-running tasks (e.g., navigation, robotic arm control), supporting real-time feedback.
Three-Phase Interaction:
- Goal: The client initiates a task request.
- Feedback: The server continuously pushes progress updates.
- Result: The server returns the final result upon task completion.
View Actions
Use the ros2 action list command to view actions:
ros2 action list
Send an Action
Use the ros2 action send_goal command to send an action and rotate the turtle to a specified angle:
ros2 action send_goal /turtle1/rotate_absolute turtlesim/action/RotateAbsolute "{theta: 1.57}"
This command will rotate the turtle by 1.57 radians (approximately 90 degrees).
