ROS/ROS2机器人命令（cli）和基础编程（rclpy）的高效学习方法

Posted 2021-10-04 zhangrelay

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啰啰嗦嗦讲满整整51分钟的直播回放如下：

• ROS2官方教程14 (dingtalk.com)https://h5.dingtalk.com/group-live-share/index.htm?encCid=41b6de7bfdba9689f8fc78dd4aafa8c2&liveUuid=b6aecc5b-b447-4593-b9c4-fbd25a805a50#/

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官方-主题教程：

• ros2 topic pub --rate 1 /turtle1/cmd_vel geometry_msgs/msg/Twist "{linear: {x: 2.0, y: 0.0, z: 0.0}, angular: {x: 0.0, y: 0.0, z: 1.8}}"

官方-发布和订阅（Python）：

发布源码：

import rclpy
from rclpy.node import Node

from std_msgs.msg import String


class MinimalPublisher(Node):

    def __init__(self):
        super().__init__('minimal_publisher')
        self.publisher_ = self.create_publisher(String, 'topic', 10)
        timer_period = 0.5  # seconds
        self.timer = self.create_timer(timer_period, self.timer_callback)
        self.i = 0

    def timer_callback(self):
        msg = String()
        msg.data = 'Hello World: %d' % self.i
        self.publisher_.publish(msg)
        self.get_logger().info('Publishing: "%s"' % msg.data)
        self.i += 1


def main(args=None):
    rclpy.init(args=args)

    minimal_publisher = MinimalPublisher()

    rclpy.spin(minimal_publisher)

    # Destroy the node explicitly
    # (optional - otherwise it will be done automatically
    # when the garbage collector destroys the node object)
    minimal_publisher.destroy_node()
    rclpy.shutdown()


if __name__ == '__main__':
    main()

订阅源码：

import rclpy
from rclpy.node import Node

from std_msgs.msg import String


class MinimalSubscriber(Node):

    def __init__(self):
        super().__init__('minimal_subscriber')
        self.subscription = self.create_subscription(
            String,
            'topic',
            self.listener_callback,
            10)
        self.subscription  # prevent unused variable warning

    def listener_callback(self, msg):
        self.get_logger().info('I heard: "%s"' % msg.data)


def main(args=None):
    rclpy.init(args=args)

    minimal_subscriber = MinimalSubscriber()

    rclpy.spin(minimal_subscriber)

    # Destroy the node explicitly
    # (optional - otherwise it will be done automatically
    # when the garbage collector destroys the node object)
    minimal_subscriber.destroy_node()
    rclpy.shutdown()


if __name__ == '__main__':
    main()

通过3年多ROS2教学发现，学生掌握这些非常轻松，但是知识迁移的能力欠缺，此处说明。

比如使用主题发布字符串如何实现：

• 参考python代码思考

from std_msgs.msg import String

msg.data = 'Hello World: %d' % self.i

ros2 topic pub --rate 1 /turtle1/cmd_vel geometry_msgs/msg/Twist "{linear: {x: 2.0, y: 0.0, z: 0.0}, angular: {x: 0.0, y: 0.0, z: 1.8}}"

改为：

字符串消息发布
ros2 topic pub -r 1 /py_msg std_msgs/msg/String "{ data: My Hello World }"

字符串消息订阅
ros2 topic echo /py_msg

同样类比一下：

geometry_msgs/msg/Twist

from geometry_msgs.msg import Twist

速度消息发布如何使用python代码实现

import rclpy
from rclpy.node import Node

# from std_msgs.msg import String
from geometry_msgs.msg import Twist



class MinimalPublisher(Node):

    def __init__(self):
        super().__init__('my_pub')
        self.publisher_ = self.create_publisher(Twist, 'turtle1/cmd_vel', 10)
        timer_period = 1  # seconds
        self.timer = self.create_timer(timer_period, self.timer_callback)
        self.i = 0

    def timer_callback(self):
        vel = Twist()
#        vel.data = 'My Hello World: %d' % self.i
        vel.linear.x=1.0
        vel.angular.z=0.01*self.i
        self.publisher_.publish(vel)
        self.get_logger().info('Publishing: x"%f"' % vel.linear.x)
        self.get_logger().info('Publishing: z"%f"' % vel.angular.z)
        self.i += 1


def main(args=None):
    rclpy.init(args=args)

    minimal_publisher = MinimalPublisher()

    rclpy.spin(minimal_publisher)

    # Destroy the node explicitly
    # (optional - otherwise it will be done automatically
    # when the garbage collector destroys the node object)
    minimal_publisher.destroy_node()
    rclpy.shutdown()


if __name__ == '__main__':
    main()

这种思考和研究的方法，是通用的，掌握了学习官方文档的速度和效率都会快很多。

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