本文是ROS2基础知识的综合小应用，练习如何创建工作包，创建Node，定义Topic和Service，以及通过LaunchFile启动多个节点。基础知识可以参考：ROS2基础编程，ROS2 Topics和Services，ROS2 LaunchFile和Parameter
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创建工作包

ros2 pkg create turtlesim_greedy_turtle --build-type ament_python

创建节点

touch turtle_controller.py
touch turtle_spawner.py
chmod +x turtle_controller.py
chmod +x turtle_spawner.py

添加依赖

<depend>rclpy</depend>
<depend>turtlesim</depend>
<depend>geometry_msgs</depend>
<depend>my_robot_interfaces</depend>

创建Msg
Turtle.msg

string name
float64 x
float64 y
float64 theta

TurtleArray.msg

Turtle[] turtles

创建Srv
CatchTurtle.srv

string name
---
bool success

添加节点控制Turtle
turtle_controller.py

#!/usr/bin/env python3
import rclpy
import math
from functools import partial
from rclpy.node import Node
from turtlesim.msg import Pose
from geometry_msgs.msg import Twist
from my_robot_interfaces.msg import Turtle
from my_robot_interfaces.msg import TurtleArray
from my_robot_interfaces.srv import CatchTurtle


class TurtleControllerNode(Node):
  def __init__(self):
    super().__init__("turtle_controller")

    self.pose = None
    self.turtle_to_catch = None

    self.pose_subscriber = self.create_subscription(Pose, "turtle1/pose", self.callback_turtle_pose, 10)
    self.cmd_vel_publisher = self.create_publisher(Twist, "turtle1/cmd_vel", 10)
    self.alive_turtles_subscriber = self.create_subscription(TurtleArray, "alive_turtles", self.callback_alive_turtles, 10)

    self.control_loop_timer = self.create_timer(0.01, self.control_loop)
  
  def callback_turtle_pose(self, msg):
    self.pose = msg

  def control_loop(self):
    if self.pose == None or self.turtle_to_catch == None:
      return
    # calcu the distance between current position and target position
    dist_x = self.turtle_to_catch.x - self.pose.x
    dist_y = self.turtle_to_catch.y - self.pose.y
    distance  = math.sqrt(dist_x * dist_x + dist_y * dist_y)
    # construct cmd_vel msg to control the turtle
    msg = Twist()
    if distance <= 0.5:  # target reached
      msg.linear.x = 0.0   # stop the turtle
      msg.angular.z = 0.0
      self.call_catch_turtle_server(self.turtle_to_catch.name)
      self.turtle_to_catch = None
    else:                # move to the target position
      msg.linear.x = 2 * distance  # optimize
      goal_theta = math.atan2(dist_y, dist_x)  # orientation
      diff = goal_theta - self.pose.theta
      if diff > math.pi:
        diff -= 2 * math.pi
      elif diff < -math.pi:
        diff += 2 * math.pi
      msg.angular.z = 6 * diff  # optimize

    self.cmd_vel_publisher.publish(msg)

  def callback_alive_turtles(self, msg):
    if len(msg.turtles) > 0:
      self.turtle_to_catch = msg.turtles[0]

  def call_catch_turtle_server(self, turtle_name):
    client = self.create_client(CatchTurtle, "catch_turtle")
    while not client.wait_for_service(1.0):
      self.get_logger().warn("Waiting for server...")
    
    request = CatchTurtle.Request()
    request.name = turtle_name
    future = client.call_async(request)
    future.add_done_callback(partial(self.callback_call_catch_turtle, turtle_name=turtle_name))

  def callback_call_catch_turtle(self, future, turtle_name):
    try:
      response = future.result()
      if not response.success:
        self.get_logger().error("Turtle " + str(turtle_name) + " could not be caught")
    except Exception as e:
      self.get_logger().error("Service call failed %r" & (e,))
  

def main(args=None):
  rclpy.init(args=args)
  node = TurtleControllerNode()
  rclpy.spin(node)
  rclpy.shutdown()


if __name__ == "__main__":
  main()

添加节点产生或移除Turtle

#!/usr/bin/env python3
import rclpy
import random
import math
from functools import partial
from rclpy.node import Node
from turtlesim.srv import Spawn
from turtlesim.srv import Kill
from my_robot_interfaces.msg import Turtle
from my_robot_interfaces.msg import TurtleArray
from my_robot_interfaces.srv import CatchTurtle


class TurtleSpawnerNode(Node):
 def __init__(self):
    super().__init__("turtle_spawner")
    self.declare_parameter("spawn_frequency", 1.0)
    self.declare_parameter("turtle_name_prefix", "turtle")

    self.spawn_frequency = self.get_parameter("spawn_frequency").value
    self.turtle_name_prefix = self.get_parameter("turtle_name_prefix").value
    self.turtle_counter = 0
    self.alive_turtles = []

    self.spawn_turtle_timer = self.create_timer(1.0 / self.spawn_frequency, self.spawn_new_turtle)
    self.alive_turtles_publisher = self.create_publisher(TurtleArray, "alive_turtles", 10)
    self.catch_turtle_service = self.create_service(CatchTurtle, "catch_turtle", self.callback_catch_turtle)

  def spawn_new_turtle(self):
    self.turtle_counter += 1
    name = self.turtle_name_prefix + "_" + str(self.turtle_counter)
    x = random.uniform(0.0, 10.0)
    y = random.uniform(0.0, 10.0)
    theta = random.uniform(0.0, 2 * math.pi)
    self.call_spawn_server(name, x, y, theta)
  
  def call_spawn_server(self, turtle_name, x, y, theta):
    client = self.create_client(Spawn, "spawn")
    while not client.wait_for_service(1.0):
      self.get_logger().warn("Waiting for server...")
    
    request = Spawn.Request()
    request.name = turtle_name
    request.x = x
    request.y = y
    request.theta = theta

    future = client.call_async(request)
    future.add_done_callback(partial(self.callback_call_spawn, turtle_name=turtle_name, x=x, y=y, theta=theta))

  def callback_call_spawn(self, future, turtle_name, x, y, theta):
    try:
      response = future.result()
      if response.name != "":
        self.get_logger().info("Turtle " + response.name + " is now alive")
        new_turtle = Turtle()
        new_turtle.name = turtle_name
        new_turtle.x = x
        new_turtle.y = y
        new_turtle.theta = theta
        self.alive_turtles.append(new_turtle)
        self.publish_alive_turtles()
    except Exception as e:
      self.get_logger().error("Service call failed %r" % (e,))

  def publish_alive_turtles(self):
    msg = TurtleArray()
    msg.turtles = self.alive_turtles
    self.alive_turtles_publisher.publish(msg)

  def callback_catch_turtle(self, request, response):
    self.call_kill_server(request.name)
    response.success = True
    return response

  def call_kill_server(self, turtle_name):
    client = self.create_client(Kill, "kill")
    while not client.wait_for_service(1.0):
      self.get_logger().warn("Waiting for server...")
    
    request = Kill.Request()
    request.name = turtle_name
    future = client.call_async(request)
    future.add_done_callback(partial(self.callback_call_kill, turtle_name=turtle_name))

  def callback_call_kill(self, future, turtle_name):
    try:
      future.result()
      for (i, turtle) in enumerate(self.alive_turtles):
        if turtle.name == turtle_name:
          del self.alive_turtles[i]
          self.publish_alive_turtles()
          break
    except Exception as e:
      self.get_logger().error("Service call failed %r" & (e,))


def main(args=None):
  rclpy.init(args=args)
  node = TurtleSpawnerNode()
  rclpy.spin(node)
  rclpy.shutdown()


if __name__ == "__main__":
  main()

创建LaunchFile

不要忘记更新my_robot_bringup工作包的package.xml添加依赖

通过LaunchFile启动

ros2 launch my_robot_bringup turtlesim_greedy_turtle.launch.py

最终的效果：

如有错误，欢迎留言或来信指正：hbin6358@163.com