Change Linear Speed And Angular Velocity Based On User Input For Node In Turtlesim In ROS2

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Introduction

In this article, we will explore how to modify the linear speed and angular velocity of a turtle in the turtlesim package in ROS2. The turtlesim package is a popular simulation tool used in robotics and computer science education to teach the basics of ROS and robotics. By default, the turtle's speed and turning rate are fixed, but we can modify these values to create a more engaging and interactive experience.

Understanding Linear Speed and Angular Velocity

Before we dive into the code, let's briefly discuss the concepts of linear speed and angular velocity.

  • Linear Speed: Linear speed refers to the rate at which an object moves in a straight line. In the context of turtlesim, linear speed determines how fast the turtle moves forward or backward.
  • Angular Velocity: Angular velocity, on the other hand, refers to the rate at which an object rotates around a central axis. In turtlesim, angular velocity determines how fast the turtle turns.

Modifying Linear Speed and Angular Velocity

To modify the linear speed and angular velocity of the turtle, we need to create a custom node that subscribes to the /cmd_vel topic and publishes modified velocity commands to the turtle.

Step 1: Create a Custom Node

First, let's create a custom node that will subscribe to the /cmd_vel topic and publish modified velocity commands to the turtle. We will use the rclpy library to create the node and subscribe to the topic.

import rclpy
from rclpy.node import Node
from geometry_msgs.msg import Twist

class CustomNode(Node):
def init(self):
super().init('custom_node')
self.subscription = self.create_subscription(
Twist,
'/cmd_vel',
self.listener_callback,
10)
self.get_logger().info('Subscribed to /cmd_vel topic')


def listener_callback(self, msg):
    # Modify linear speed and angular velocity here
    msg.linear.x = 0.5  # Set linear speed to 0.5 m/s
    msg.angular.z = 0.2  # Set angular velocity to 0.2 rad/s
    self.get_logger().info('Modified velocity command published')
    self.publisher_.publish(msg)



def main(args=None):
rclpy.init(args=args)
custom_node = CustomNode()
rclpy.spin(custom_node)
custom_node.destroy_node()
rclpy.shutdown()


if name == 'main':
main()

Step 2: Modify Linear Speed and Angular Velocity

In the listener_callback method, we can modify the linear speed and angular velocity of the turtle by changing the values of msg.linear.x and msg.angular.z. In this example, we set the linear speed to 0.5 m/s and the angular velocity to 0.2 rad/s.

Step 3: Run the Custom Node

To run the custom node, save the code in a file named custom_node.py and run it using the following command:

ros2 run custom_node custom_node

Step 4: Use Teleop Key to Control the Turtle

To control the turtle using the teleop key, run the following command:

ros2 run turtlesim turtlesim_node
ros2 run teleop_twist_keyboard teleop_twist_keyboard

This will launch the turtlesim node and the teleop key node. You can now use the teleop key to control the turtle, and the custom node will modify the linear speed and angular velocity of the turtle based on your input.

Conclusion

In this article, we learned how to modify the linear speed and angular velocity of a turtle in the turtlesim package in ROS2. We created a custom node that subscribes to the /cmd_vel topic and publishes modified velocity commands to the turtle. We also learned how to use the teleop key to control the turtle and modify its speed and turning rate. With this knowledge, you can create a more engaging and interactive experience with the turtlesim package.

Future Work

In the future, we can explore more advanced topics such as:

  • Implementing a PID controller: We can implement a PID controller to control the turtle's speed and turning rate based on its position and velocity.
  • Adding obstacle avoidance: We can add obstacle avoidance to the turtle by using sensors and algorithms to detect and avoid obstacles.
  • Creating a more complex simulation: We can create a more complex simulation by adding multiple turtles, obstacles, and other features to the turtlesim package.

References

Q: What is the purpose of this article?

A: The purpose of this article is to provide a step-by-step guide on how to modify the linear speed and angular velocity of a turtle in the turtlesim package in ROS2. We will create a custom node that subscribes to the /cmd_vel topic and publishes modified velocity commands to the turtle.

Q: What are the benefits of modifying the linear speed and angular velocity of the turtle?

A: Modifying the linear speed and angular velocity of the turtle can create a more engaging and interactive experience with the turtlesim package. It can also be useful for teaching robotics and computer science concepts, such as control systems and motion planning.

Q: What is the difference between linear speed and angular velocity?

A: Linear speed refers to the rate at which an object moves in a straight line, while angular velocity refers to the rate at which an object rotates around a central axis. In the context of turtlesim, linear speed determines how fast the turtle moves forward or backward, while angular velocity determines how fast the turtle turns.

Q: How do I create a custom node in ROS2?

A: To create a custom node in ROS2, you can use the rclpy library and create a new Python file with the following code:

import rclpy
from rclpy.node import Node
from geometry_msgs.msg import Twist

class CustomNode(Node):
def init(self):
super().init('custom_node')
self.subscription = self.create_subscription(
Twist,
'/cmd_vel',
self.listener_callback,
10)
self.get_logger().info('Subscribed to /cmd_vel topic')


def listener_callback(self, msg):
    # Modify linear speed and angular velocity here
    msg.linear.x = 0.5  # Set linear speed to 0.5 m/s
    msg.angular.z = 0.2  # Set angular velocity to 0.2 rad/s
    self.get_logger().info('Modified velocity command published')
    self.publisher_.publish(msg)



def main(args=None):
rclpy.init(args=args)
custom_node = CustomNode()
rclpy.spin(custom_node)
custom_node.destroy_node()
rclpy.shutdown()


if name == 'main':
main()

Q: How do I run the custom node?

A: To run the custom node, save the code in a file named custom_node.py and run it using the following command:

ros2 run custom_node custom_node

Q: How do I use the teleop key to control the turtle?

A: To use the teleop key to control the turtle, run the following command:

ros2 run turtlesim turtlesim_node
ros2 run teleop_twist_keyboard teleop_twist_keyboard

This will launch the turtlesim node and the teleop key node. You can now use the teleop key to control the turtle, and the custom node will modify the linear speed and angular velocity of the turtle based on your input.

Q: What are some potential future developments for this project?

A: Some potential future developments for this project include:

  • Implementing a PID controller: We can implement a PID controller to control the turtle's speed and turning rate based on its position and velocity.
  • Adding obstacle avoidance: We can add obstacle avoidance to the turtle by using sensors and algorithms to detect and avoid obstacles.
  • Creating a more complex simulation: We can create a more complex simulation by adding multiple turtles, obstacles, and other features to the turtlesim package.

Q: Where can I find more information about ROS2 and turtlesim?

A: You can find more information about ROS2 and turtlesim on the official ROS2 documentation website: https://docs.ros.org/en/humble/Tutorials/Turtlesim/Turtlesim.html

Q: What are some common issues that may arise when working with ROS2 and turtlesim?

A: Some common issues that may arise when working with ROS2 and turtlesim include:

  • Subscription and publication issues: Make sure that the subscription and publication are properly configured and that the topics are correctly published and subscribed to.
  • Node initialization issues: Make sure that the node is properly initialized and that the necessary dependencies are installed.
  • Topic and message issues: Make sure that the topics and messages are correctly defined and that the necessary data types are used.

By following the steps outlined in this article and addressing the common issues that may arise, you can successfully modify the linear speed and angular velocity of a turtle in the turtlesim package in ROS2.