Turtle Controller - ROS 2 Humble Tutorial Package

A comprehensive beginner-friendly ROS 2 package demonstrating fundamental concepts using the turtlesim simulator. This package includes examples of publishers, subscribers, services, clients, and launch files.

Overview

This package is designed as a complete learning resource for ROS 2 beginners. It demonstrates:

• Topic Publishers - Sending velocity commands to control turtle movement
• Topic Subscribers - Receiving and displaying turtle pose information
• Service Servers - Providing status checking services
• Service Clients - Calling status checking services
• Launch Files - Starting multiple nodes simultaneously

You should see output similar to: ros2 cli version: 0.18.x

Package Structure

turtle_controller/
├── launch/
│   ├── turtle_system_launch.py          # Basic launch file
├── turtle_controller/
│   ├── __init__.py
│   ├── velocity_publisher.py            # Publishes velocity commands
│   ├── pose_subscriber.py               # Subscribes to turtle pose
│   ├── turtle_status_service.py         # Service server for status checking
│   └── turtle_status_client.py          # Service client for status checking
├── package.xml                          # Package metadata
├── setup.py                             # Python package setup
├── setup.cfg                            # Package configuration
└── README.md                            # This file

Quick Start

1. Create Workspace and Clone Package

# Create workspace
mkdir -p ~/turtle_ws/src
cd ~/turtle_ws/src

# Clone or copy this package into src/
# (If you created it manually, skip this step)

cd ~/turtle_ws

2. Install Dependencies

rosdep install --from-paths src --ignore-src -r -y

3. Build the Package

cd ~/turtle_ws
colcon build
source install/setup.bash

4. Launch the Complete System

ros2 launch turtle_controller turtle_system_launch.py

You should see:

• Turtlesim window opens
• Turtle moves in a circle
• Pose information printed to terminal

Nodes Description

0. Run the turlesim simulation

ros2 run turtlesim turtlesim_node

1. Velocity Publisher (velocity_publisher)

Publishes Twist messages to /turtle1/cmd_vel to make the turtle move in a circular pattern.

Run standalone:

ros2 run turtle_controller velocity_publisher

Topics Published:

• /turtle1/cmd_vel (geometry_msgs/msg/Twist) - Velocity commands

2. Pose Subscriber (pose_subscriber)

Subscribes to /turtle1/pose and displays the turtle's position and orientation.

Run standalone:

ros2 run turtle_controller pose_subscriber

Topics Subscribed:

• /turtle1/pose (turtlesim/msg/Pose) - Turtle position and orientation

3. Turtle Status Service (turtle_status_service)

Provides a service to check if the turtle is near the center of the window.

Run standalone:

ros2 run turtle_controller turtle_status_service

Services Provided:

• /check_turtle_status (std_srvs/srv/Trigger) - Check if turtle is near center

Call the service manually:

ros2 service call /check_turtle_status std_srvs/srv/Trigger

4. Turtle Status Client (turtle_status_client)

A command-line client that calls the turtle status service.

Usage:

ros2 run turtle_controller turtle_status_client

Example output:

✓ Turtle is near center! Position: (5.54, 5.54)

Usage Examples

Example 1: Manual Turtle Control with Pose Monitoring

Terminal 1 - Start turtlesim:

ros2 run turtlesim turtlesim_node

Terminal 2 - Monitor pose:

ros2 run turtle_controller pose_subscriber

Terminal 3 - Control with keyboard:

ros2 run turtlesim turtle_teleop_key

Example 2: Automatic Movement with Status Checking

Terminal 1 - Launch system:

ros2 launch turtle_controller turtle_system_launch.py

Terminal 2 - Check turtle status:

ros2 run turtle_controller turtle_status_client

Terminal 3 - Move turtle manually:

ros2 run turtlesim turtle_teleop_key

Terminal 2 - Check status again after moving:

ros2 run turtle_controller turtle_status_client

Development

Building After Changes

After modifying any Python files:

cd ~/turtle_ws
colcon build --packages-select turtle_controller
source install/setup.bash

Adding New Nodes

1. Create your Python script in turtle_controller/
2. Make it executable: chmod +x your_script.py
3. Add entry point in setup.py:

   entry_points={
       'console_scripts': [
           'your_node = turtle_controller.your_script:main',
       ],
   },

4. Rebuild the package

Testing Individual Components

Test publisher only:

# Terminal 1
ros2 run turtlesim turtlesim_node

# Terminal 2
ros2 run turtle_controller velocity_publisher

Test service:

# Terminal 1
ros2 run turtlesim turtlesim_node

# Terminal 2
ros2 run turtle_controller turtle_status_service

# Terminal 3
ros2 run turtle_controller turtle_status_client

Troubleshooting

Issue: "Package 'turtle_controller' not found"

Solution:

cd ~/turtle_ws
source install/setup.bash

Issue: "No executable found"

Solution:

1. Check that entry points are correctly added in setup.py
2. Rebuild: colcon build --packages-select turtle_controller
3. Source: source install/setup.bash

Issue: Service or topic not found

Solution:

1. Ensure turtlesim is running: ros2 run turtlesim turtlesim_node
2. List available topics: ros2 topic list
3. List available services: ros2 service list

Issue: "Permission denied" when running scripts

Solution:

chmod +x ~/turtle_ws/src/turtle_controller/turtle_controller/*.py

Issue: Import errors

Solution:

# Install missing dependencies
rosdep install --from-paths src --ignore-src -r -y
colcon build
source install/setup.bash

Useful Commands

Inspect Running System

# List all nodes
ros2 node list

# List all topics
ros2 topic list

# See topic info
ros2 topic info /turtle1/cmd_vel

# Echo topic messages
ros2 topic echo /turtle1/pose

# List services
ros2 service list

# See service type
ros2 service type /rotate_turtle

# View computational graph
rqt_graph

Debugging

# Run with debug output
ros2 run turtle_controller velocity_publisher --ros-args --log-level debug

# Check node info
ros2 node info /velocity_publisher

# Monitor topic frequency
ros2 topic hz /turtle1/cmd_vel