ROS Selection Task 2024-2025

Problem statement

The objective of the task is to move the turtle inside the turtlesim window in a vertical Candy shape
To acheive this task you are supposed to create a node named /node_turtle_move within a python script, node_turtle_move.py

Dont worry if you are new to ROS or Linux(Ubuntu), the task is fairly simple and we have provided you with ample resource and tutorials in this WIKI to the complete this task so only a strong will and a little bit of brains is required to get the work done. Also even though this just a weekend task we have provided ample amount of time as we also have our midterm exams during this time. So we think a week time is enough so you guys can give your exams freely and manage your time in order to complete the task

Note

All the resources to complete the said task are provided in the ROS section of ATOM WIKI. So make sure to check it out if you are new to ROS2.

Warning

The Deadline for completing the task is 15th October, 2024.

Expected Output

video link

Caution

The CANDY should be VERTICAL. Also the max number of spirals can be of your choice but 3 spirals are required.

Hints

The turtle needs to move in a vertical Candy shape .
You can refer POSE to learn more about pose function.
You can refer TWIST to learn more about twist function.
Use linear velocity and angular velocity to get this done.
Keep tracking the distance travelled so as to know when to stop. You can refer to Overview of rclpy for more hint

Sample Code Snippet

Question: Write a python code to move ROS’s turtlesim bot on a straight path while bot’s distance is less than 3.

#!/usr/bin/env python3

import rclpy
from rclpy.node import Node
from geometry_msgs.msg import Twist
from turtlesim.msg import Pose
import math

class MoveTurtle(Node):

   def __init__(self):
      super().__init__('move_turtle')
      self.start_x = None
      self.start_y = None
      self.target_distance = 3.0  # Distance to move in pixels
      self.lin_vel = 2.0  # Linear velocity

      # Create a publisher for controlling the turtle's velocity
      self.pub = self.create_publisher(Twist, '/turtle1/cmd_vel', 10)
      # Create a subscriber for getting the turtle's position
      self.sub = self.create_subscription(Pose, '/turtle1/pose', self.pose_callback, 10)

   def pose_callback(self, pose):
      if self.start_x is None and self.start_y is None:
            # Set the starting position when the first pose message is received
            self.start_x = pose.x
            self.start_y = pose.y
            self.get_logger().info(f"Starting position set to X = {self.start_x:.2f}, Y = {self.start_y:.2f}")

      # Calculate the distance from the starting position
      distance = math.sqrt((pose.x - self.start_x) ** 2 + (pose.y - self.start_y) ** 2)
      self.get_logger().info(f"Distance from start = {distance:.2f}")

      # Create a Twist message to set the turtle's velocity
      vel = Twist()
      vel.linear.x = self.lin_vel
      vel.linear.y = 0.0
      vel.linear.z = 0.0
      vel.angular.x = 0.0
      vel.angular.y = 0.0
      vel.angular.z = 0.0

      if distance >= self.target_distance:
            self.get_logger().info("Turtle reached the target distance")
            vel.linear.x = 0.0  # Stop the turtle
            self.get_logger().warn("Stopping Turtle")
            self.pub.publish(vel)
            rclpy.shutdown()
      else:
            self.pub.publish(vel)

def main(args=None):
   rclpy.init(args=args)
   move_turtle_node = MoveTurtle()
   rclpy.spin(move_turtle_node)
   move_turtle_node.destroy_node()
   rclpy.shutdown()

if __name__ == '__main__':
   main()

Commands used:

ros2 run turtlesim turtlesim_node
ros2 run package_name script_name

Procedure

Follow the instructions given below to get started with the task.

First, you will need to create a package named selection_task within your ROS workspace. Once your package is created, source and build your workspace.
Within this package, you should have a ‘scripts’ folder inside which you’ll create a python script, named node_turtle_move.py.
Fill the script with proper programming ethics. Doing this will help us understand your code better and quicker than usual.
After completing the python script. Make it executable, if it isn’t already. To do that, enter the following code.

cd ~/turtle_ws
colcon_build
source install/setup.bash

You can either run them in separate terminals or simply create a selection_task.launch file inside the ~/turtle_ws/src/selection_task/launch/ folder. Launch file can run multiple nodes unlike a python/cpp script. Run the launch file, enter, This should run three processes in parallel.
turtlesim_node
node_turtle_move.py