当前位置：网站首页>[ROS] (06) ROS Communication - Topic Communication

[ROS] (06) ROS Communication - Topic Communication

2022-08-02 14:18:00 【CynalFly】

文章只是个人学习过程中学习笔记,主要参考ROS教程¹.

1、概念

话题（Topics）：节点可以将消息发布到话题,Or receive messages by subscribing to a topic.A topic is a name used to identify the content of a message.Nodes interested in some kind of data will subscribe to the corresponding topic.A single topic may have multiple concurrent publishers（publishers）和订阅者（subscribers）,And a single node can publish and subscribe to multiple topics at the same time,Individual nodes can also publish or subscribe（监听）multiple topics.通常,发布者和订阅者不知道彼此的存在.There is a decoupling relationship between them.从逻辑上讲,A topic can be thought of as a strongly typed message bus.Each bus has a name,只要类型正确,Anyone can connect to the bus to send or receive messages.
消息（Messages）：订阅或发布话题时所使用的ROS数据类型.Messages are made up of typed fields.Standard primitive types are supported（整数、浮点、布尔值等）,and arrays of primitive types.Messages can include arbitrarily nested structures and arrays（很像C结构）.
话题（Topics)、发布者（Publisher)和订阅者（Subscriber）的概念图

图1-1 话题（Topics）、发布者（Publisher）和订阅者（Subscriber）的概念图

为了让事情更清楚,我将使用 FMA radio system as an analogy：广播电台（node）的发射器（publisher）will be at some fixed frequency（topic）Stream your favorite channels（message）.Suppose you are in a car,Want to listen to your favorite channel（message）.You will have a radio receiver（subscriber）Tune to this frequency（topic）.You neither know the exact location of the radio station,Radio stations also don't want to know where you are.But you can still enjoy music on your favorite channels.

2、话题通信机制

话题通信是ROSOne of the most commonly used in communication is based on发布和订阅How the model communicates.for real-time、周期性、少逻辑处理的数据传输场景.
ROS Node想要通过topicway to deliver messages,首先 publisher 节点和 Subscriber Nodes are comingROS Master（节点管理器）Register the node name in 、话题名称、消息类型、URI地址和端口,然后ROS Master向 Subcriber 节点发送 publisher 节点信息, Subscriber Node according to the received publisher 节点信息,向 publisher The node requests a direct connection,after they are connected, publisher 节点向 Subscriber 节点发送消息.Communication between nodes uses a basedTCP/IP的消息传输,称为TCPROS.

topic通信模型

图2-1 话题（Topics）通信模型

3、话题命令rostopic

rostopic命令工具能让你获取ROS话题的信息.

命令	功能
`rostopic bw`	列出消息发布带宽
`rostopic delay`	display delay of topic from timestamp in header
`rostopic echo`	显示某个话题上发布的数据
`rostopic find`	根据消息类型查找话题
`rostopic hz`	Reports the rate at which data is published
`rostopic info`	获取指定Topic Current published news
`rostopic list`	列出当前所有的Topic
`rostopic pub`	把数据发布到当前某个正在广播的话题上
`rostopic type`	View message types for all posted threads

Tips：使用rostopic <command> -h Help option for more detailed usage.

4、 Topic communication practice – The keyboard controls the turtle（turtlesim）运动

通过ROS内置的turtlesimTo carry out the practice of topic communication,There are two nodes involved in this practice,One is the keyboard node that controls the movement of the turtleturtle_teleop_key,The other is a display node that shows the turtle's movementturtlesim_node,They communicate with each other through topics,turtle_teleop_key（publisher）在话题上发布键盘按下的消息,turtlesim_node（subscriber）则订阅该话题以接收消息,Implement turtle motion.我们可以使用rqt_graph GUI来Displays currently running nodes and topics.

图4-1 使用rqt_graphDisplays currently running nodes and topics

实现的步骤如下：
步骤1：在新终端中启动roscore

roscore

步骤2：在新终端中启动turtlesim_node

rosrun turtlesim turtlesim_node

步骤3：Start the keyboard in a new terminalturtle_teleop_key

rosrun turtlesim turtle_teleop_key

步骤4：在turtle_teleop_key终端中（Be sure to select the terminal window,Make sure keystrokes are captured）,Press the arrow keys on the keyboard to control the turtle movement.
topic turtlesim

图4-2 The keyboard controls the turtle（turtlesim）运动

5、Topic command practice

使用控制乌龟（turtlesim）运动Cases to practice topic commands.

5.1 rostopic list

rostopic list Ability to list all topics that are currently subscribed and published.

# (1) Lists all topics that are currently subscribed and published.
rostopic list
# (2) Lists all published and subscribed topics and their type details.
rostopic list -v

rostopic list

5.2 rostopic type

rostopic type [topic] The command is used to view the message type of the posted topic.

# (1) View posted topics[/turtle1/cmd_vel]的消息类型.
rostopic type /turtle1/cmd_vel
# (2) 使用rosmsgView the details of the message
rosmsg show geometry_msgs/Twist

rostopic type

5.3 rostopic echo

rostopic echo [topic] 可以显示在某个话题上发布的数据.

rostopic echo /turtle1/cmd_vel

当输入命令后,You may see that nothing happens,Because no data has been posted to this topic yet.This can be done by pressing the arrow keys on the keyboardturtle_teleop_key节点发布数据.Remember if you can't control the turtle,请选中turtle_teleop_keyterminal window to ensure that keystrokes are captured.When the right and up keys are pressed,应该看到以下内容：

现在让我们再看一下rqt_graph.First press the refresh button in the upper left corner to display the new node.正如你所看到的,rostopic echo（Shown here in red）Also subscribed nowturtle1/cmd_vel话题.
rqt_graph rostopic echo

5.4 rostopic pub

rostopic pub [topic] [msg_type] [args] Commands can publish data to a topic that is currently being broadcast.

# The command will send a message to turtlesim,告诉它以2.0大小的线速度和1.8大小的角速度移动.
rostopic pub -1 /turtle1/cmd_vel geometry_msgs/Twist -- '[2.0, 0.0, 0.0]' '[0.0, 0.0, 1.8]'

-1：这一选项会让rostopic只发布一条消息,然后退出.
/turtle1/cmd_vel：这是要发布到的话题的名称
geometry_msgs/Twist：这是发布到话题时要使用的消息的类型.
--：这一选项（两个破折号）用来告诉选项解析器,表明之后的参数都不是选项.如果参数前有破折号（-）比如负数,那么这是必需的.

There is also a quick way to type：先输入以下命令,然后双击Tab键,最后使用<-和->键修改linear和angular.
rostopic pub -1 /turtle1/cmd_vel geometry_msgs/Twist 

rostopic pub one message
下面我们用 rostopic pub -r command to issue a steady stream of commands：

# 命令以1Hzfrequency to send messages toturtlesim,Make sure that the turtle continues to do a circular motion
rostopic pub /turtle1/cmd_vel geometry_msgs/Twist -r 1 -- '[2.0, 0.0, 0.0]' '[0.0, 0.0, -1.8]'

rostopic pub continue message