Analysis of ROS calculation diagram level

Calculation drawing level yes ROS A distributed network that processes data , Used to describe program operation and data communication , When the program is running , All processes and the data they process can be represented through a point-to-point network , The network can be accessed through rqt_graph Visual tools to view

node node

ROS The running instance of the executable in the package is called a node , A node is essentially a process that performs arithmetic tasks . One ROS The system can have multiple nodes when running , Nodes usually have a clear division of labor , For example, a node controls the laser sensor , Another node controls the motor, etc , Nodes can also be distributed on different physical devices .

$rosrun [ Package name ]  [ The node name ]
$roslaunch [ Options ] [ Package name ] <launch file name > [ Variable parameters ]

topic of conversation topic

The communication between nodes can be passed through messages , The message is encapsulated in the topic , The topic is essentially an encapsulation of sockets . News to publish / The mode of subscription , A node can publish messages to a given topic , Another node can subscribe to messages of a specific message type in a given topic . Publishers and subscribers can decouple through topics . It should be noted that , stay ROS in , The topic must be unique , Otherwise, message routing will generate errors .
Topics can be visualized using tools rqt_graph To view the
rostopic Common usage of ：

# Display the receiving speed of topic data, i.e. bandwidth 
$rostopic bw  [ Topic name ]
# Show the delay of the topic 
$rostopic delay [ Options ]  [ Topic name ]
# Show the message content of the topic 
$rostopic echo  [ Options ]  [ Topic name ]
# View specific types of topics 
$rostopic find  [ Topic type ]
# Display the publishing speed of topic data 
$rostopic hz [ Options ]  [ Topic name ]
# Display information about the active topic 
$rostopic info [ Topic name ]
# List activity topics 
$rostopic list
# Publish data to the topic 
$rostopic pub [ Topic name ] [ Message type ] [ Parameters ]
# Print topics / Domain type 
$rostopic type [ Topic name / Domain type ]

# Show a list of all topics 
$rostopic list -v

The output is as follows ：

Published topics:
 * /turtle1/color_sensor [turtlesim/Color] 1 publisher
 * /turtle1/cmd_vel [geometry_msgs/Twist] 1 publisher
 * /rosout [rosgraph_msgs/Log] 3 publishers
 * /rosout_agg [rosgraph_msgs/Log] 1 publisher
 * /turtle1/pose [turtlesim/Pose] 1 publisher
Subscribed topics:
 * /turtle1/cmd_vel [geometry_msgs/Twist] 1 subscriber
 * /rosout [rosgraph_msgs/Log] 1 subscriber

From the output results, you can see the current topic type and the message type delivered by the topic , Such as /turtle1/color_sensor The message type of topic delivery is turtlesim/Color. If you want to view information about a topic , The method is as follows ：

$rostopic info [ Topic name ]

such as , If you want to know the topic of little turtle /turtle1/cmd_vel Information about , The method is as follows ：

$rostopic info /turtle1/cmd_vel

The corresponding output results are as follows ：

Type: geometry_msgs/Twist
Publishers:
 * /teleop_turtle (http://miaozl-ThinkPad-T470p:42883/)
Subscribers:
 * /turtlesim (http://miaozl-ThinkPad-T470p:43015/)

This topic has a publishing node turtlesim And a subscription node teleop_turtle, If you want to know the topic /turtle1/cmd_vel The message type of , The method is as follows ：

$rostopic type  /turtle1/cmd_vel

If you want to view the topic /turtle1/cmd_vel Specific content published on , The method is as follows ：

$rostopic echo /turtle1/cmd_vel

news message

ROS The most important mechanism for communication between nodes is message passing , For message communication between modules implemented in different development languages ,ROS Using the simple 、 The development language independent interface definition language is used to describe the messages passed between modules .
stay ROS in , A message is essentially a data type that passes information between nodes , When a node wants to share information , Nodes can publish messages to corresponding topics , When a node needs to receive information , Nodes can subscribe to corresponding topics .
The node manager will ensure the interoperability of messaging , But the message does not need to be forwarded through the node manager .ROS There are many predefined message types , But developers can use the suffix msg Custom message types for message files ,msg The file should be placed in the... Of the package msg Under the table of contents .

1. Message type

Message types describe the information carried by each message and how it is organized . Each message type belongs to a specific package , The message type is named ：
Package name / Message name
Message type “rosgraph_msgs/Log” Represents a package rosgraph_msgs Of Log Log message , The package name and the message name are separated by “/” separation . This naming method can avoid message type name conflicts .
The declaration of each line of the message type must contain two parts ： Fields and variables ; The field defines the data type of the message , The variable defines the name of the domain . A domain can be an independent domain or a composite domain .
The so-called composite domain refers to a domain composed of one or more simple subdomains , Each of these sub domains can be another composite domain or an independent domain , Independent fields consist of basic data types .ROS The basic data types supported are integer （int8、int16、int32、int64）、 floating-point （float32、float64）、 String type （string）、 Time type （time、duration）、 Other message types 、 Variable array or fixed array, etc .
geometry_msgs/Twist Is a typical message type composed of composite domains , It defines the robot in 3D Linear and angular velocity information in space , The details are as follows ：

geometry_msgs/Vector3 linear
    float64 x
    float64 y
    float64 z
geometry_msgs/Vector3 angular
    float64 x
    float64 y
    float64 z

among , Linear velocity linear、 angular velocity angular The domain of is a composite domain geometry_msgs/Vector3, Indent format indicates x、y、z The domain of is a member of the parent domain . For a robot moving on a plane ,linear Of z and angular Of z Are all 0, For UAVs, etc 3D Robots moving in space ,linear Of z and angular Of z Are not as 0.
Mutable array in ROS It is also widely used in , Variable arrays are made by adding... After the field [] To express , Indicates the message type of the point cloud sensor_msgs/PointCloud A variable array is used , It reads as follows

Header header
geometry_msgs/Point32[] points
ChannelFloat32[] channels

2. view message

utilize rosmsg You can check the message , Common usage is as follows ：

# Displays the message type description 
$rosmsg show [ Message type ]
# The functions and rosmsg show identical 
$rosmsg info [ Message type ]
# List all message types 
$rosmsg list
# Displays the of the message MD5 Signature 
$rosmsg md5
# Lists the message types in the display package 
$rosmsg package [ Message type ]
# List packages that contain specific message types 
$rosmsg packages

If you want to view the topic /turtle1/cmd_vel The message type of geometry_msgs/Twist Details of , The method is as follows ：

$rosmsg show geometry_msgs/Twist

If you want to view the package turtlesim Message types in , The method is as follows ：

$rosmsg package turtlesim

The corresponding output results are as follows ：

turtlesim/Color
turtlesim/Pose

3. Release the news

Most of the time , Publishing messages is mainly carried out in the package , But sometimes you need to post messages manually , The release method of rate mode is as follows ：

$rostopic pub –r rate-in-hz [ Topic name ] [ Message type ] [ news ]

This command repeats the message to the topic at the specified frequency . The example code of a little turtle movement is as follows ：

$rostopic pub -r /turtle1/cmd_vel geometry_msgs/Twist 1--'[2.0, 0.0, 0.0]' '[0.0, 0.0, -1.8]'

Driven by this message , The little turtle will always do a circular motion , Unless you stop executing the command .

service service

action action