[ROS] (05) ROS Communication - Node, Nodes & Master

  The article is just a study note in the personal learning process, the main reference is ROS tutorial^1a>.

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1. Concepts

Nodes: A node is an executable file that can communicate with other nodes through ROS.
Node Manager (Master): ROS's naming service, eg to help nodes discover each other.

2, Nodes

    A node (Node) is actually just an executable file in the ROS package (executable file generated by C++ compilation, python script), and the executable file becomes a process after running,This process is called node in ROS..ROS nodes use the ROS client library to communicate with other nodes.Nodes can publish or subscribe to topics, and can also provide or consume services.A ROS control system includes many nodes, such as unmanned vehicles, one node (Node) controls the wheel motor of the unmanned vehicle, one node (Node) obtains positioning information through inertial navigation, one node (Node) obtains image information through the camera, and one node (Node) obtains image information through the camera.Node (Node) integrates various information to execute path planning and so on.

ROS client libraries allow nodes written in different languages ​​to communicate with each other:

• roscpp=C++ client
• rospy=python client

3, Master

   The robot is a systematic implementation with many components, huge functions and high complexity.In practical applications, the robot may integrate various sensors such as lidar, inertial navigation, millimeter-wave radar, camera, GPS, and motion control devices to be responsible for functions such as perception, control, decision-making, and calculation. Each function point is aIndividual Nodes.So how to manage, allocate and control these nodes?This uses the ROS Master, which provides naming and registration services for the rest of the nodes in the ROS system.The role of the Master is to enable each ROS node (Nodes) to locate each other, and then to carry out point-to-point communication.
   

4, rosnode

   rosnode is a command line tool for displaying information about ROS nodes.The commands are:

Command	Features
rosnode ping	Test the connection to the node
rosnode list	List active nodes
rosnode info	Print information about the node
rosnode machine	List nodes on the specified device
rosnode kill	kill a running node
rosnode cleanup	Clear unconnectable nodes

When using ctrl+C to stop the node, the node is not completely cleared, you can use rosnode cleanup to clear the node.

5. Example test

#Step 1: (ROS primary operation) Run the roscore command to start the master, which is responsible for ros node registration, parameter server, topic communication management, etc.

#Step 2: Runrosnode related commands.

/rosout This node is used to collect and log debug output from the node, which is always running.

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1. ROS.otg. ROS Tutorial[EB/OL]. 2020-12-22[2022-7-5].
   http://wiki.ros.org/en/ROS/Tutorials. ︎