1.ROS Introduction

ROS wiki : http://wiki.ros.org/

ROS teaching：http://wiki.ros.org/ROS/Tutorials

ROS install：http://wiki.ros.org/melodic/Installation/Ubuntu

ROS (Robot Operating System) is an open source operating system suitable for robots. It provides the necessary services for the operating system, including hardware abstraction, underlying device control, implementation of commonly used functions, inter process messaging, and package management. It also provides the tools and library functions needed to obtain, compile, write, and run code across computers.

The main goal of ROS is to provide code reuse support for robot research and development. ROS is a distributed process (also known as "node") framework, which is encapsulated in program and feature packages that are easy to share and publish. ROS also supports a joint system similar to a code repository, which can also achieve engineering collaboration and publishing. This design can enable the development and implementation of a project to make completely independent decisions from the file system to the user interface (not limited by ROS). At the same time, all projects can be integrated with the basic tools of ROS.

1.1 Main characteristics of ROS

(1) Distributed architecture (where each work process is treated as a node and managed uniformly using a node manager),

(2) Multi language support (such as C++, Python, etc.),

(3) Good scalability (you can write a node or organize many nodes into a larger project through Rosslaunch)

(4) Open source (ROS follows the BSD protocol and is completely free for individual and commercial applications and modifications).

1.2 Overall Architecture of ROS

Open source community level: mainly including developer knowledge, code, and algorithm sharing.

File system level: used to describe the code and executable programs that can be found on the hard drive,

Computational graph level: Reflects the communication between processes, as well as between processes and systems.

1.2.1 Calculation diagram level

• node

roscore
rosrun turtlesim turtlesim_node

The following command is the entire one. After entering a part, use the [Tab] key to complete it, and then modify the content

xxxxxxxxxx
rosservice call /spawn "x: 3.0
y: 3.0
theta: 90.0
name: 'my_turtle'" 

Nodes are the main computational execution processes. ROS is composed of many nodes.

xxxxxxxxxx
rqt_graph

When we enter [rosnode] on the command line and double-click the Tab key, we will find that these words appear below the command line

ROS command-line tool rosnode:

When developing and debugging, it is often necessary to view the current node and node information, so please remember these commonly used commands. If you can't remember, you can also check the usage of the rosnode command through the rosnode help.

• message

Nodes achieve logical connections and data exchange between each other through messages.When we enter [rosmsg] on the command line and double-click the Tab key, we will find that these words appear below the command line

ROS command-line tool rosmsg:

• Topic

A topic is a way of delivering messages (publish/subscribe). Every message should be posted to the corresponding topic, and each topic is strongly typed.The topic messages of ROS can be transmitted using TCP/IP or UDP, and the default transmission method used by ROS is TCP/IP. TCP based transmission becomes TCPROS, which is a long connection method; UDP based transmission, known as UDPROS, is a low latency and efficient transmission method, but it is prone to data loss and is suitable for remote operations.When we enter 'rostopic' on the command line and double-click the Tab key, we will find that these words appear below the command line

ROS command-line tool rostopic

• give service to

The service used for the request response model must also have a unique name. When a node provides a service, all nodes can communicate with it by using code written by ROS clients.When we enter 'rossservice' on the command line and double-click the Tab key, we will find that these words appear below the command line

ROS command-line tool roseservice:

• Message logging package

Message recording package is a file format used to save and replay ROS message data, stored in a. bag file. It is an important mechanism for storing data.When we enter [rostag] on the command line and double-click the Tab key, we will find that these words appear below the command line

ROS command-line tool rosbag:

• parameter server

The parameter server is a shared multivariate dictionary that can be accessed through the network and stored on the node manager through keywords.When we enter 'rosparam' on the command line and double-click the Tab key, we will find that these words appear below the command line

ROS command-line tool rosparam:

• Node Manager (Master)

The node manager is used for registering and searching topics and service names. If there is no node manager in the entire ROS system, there will be no communication between nodes.

1.2.2 File System Level

Dependency relationships can be configured between feature packages. If Function Package A relies on Function Package B, then when building the system in ROS, B must be built earlier than A, and A can use the header and library files in B.The concept of file system level is as follows:

• List of feature packages:

This list indicates the dependencies of the feature pack, source file compilation flag information, and so on. The package.xml file in the feature pack is a list of feature packs.

• Feature Pack:

Function packages are the basic form of software organization in ROS systems, including running nodes and configuration files.

ROS package related commands

• Comprehensive function package

Organize several functional packages together to form a comprehensive functional package.

• message type

When sending messages between nodes in ROS, message explanations need to be provided in advance. ROS provides standard type messages, which can also be defined by oneself. The description of the message type is stored in the msg file under the feature pack.

• Service Type

Defined the data structure provided by each process in the ROS system regarding service requests and responses.1.2.3. Open source community level

1.2.3. Open source community level

• Distribution: The ROS distribution is a series of comprehensive feature packs that can be installed independently and have version numbers. The ROS distribution plays a similar role as the Linux distribution. This makes ROS software installation easier and enables consistent versions to be maintained through a collection of software.
• Software library: ROS relies on websites or host services that share open source code and software library, where different institutions can publish and share their own robot software and programs.
• ROS Wiki: ROS Wiki is the main forum used to record information about ROS systems. Anyone can register an account, contribute their own files, provide corrections or updates, write tutorials, and engage in other activities.
• Bug Ticket System: If you discover problems or want to propose a new feature, ROS provides this resource to do so.
• Mailing list: ROS user mailing list is the main communication channel about ROS, which can communicate various questions or information from ROS software updates to ROS software usage like a forum
• OS Q&A (ROS Answer): Users can use this resource to ask questions

1.3 Communication Mechanism

1.3.1、Topic

The asynchronous publish subscribe communication mode is widely used in ROS. Topic is generally used for one-way, message flow communication. Topic generally has a strong type definition: a topic of one type can only accept/send messages of a specific data type (message type). Publisher is not required for type consistency, but upon acceptance, the subscriber will check the MD5 of the type and report an error.

1.3.2、Service

Service is used to handle synchronous communication in ROS communication, using server/client semantics. Each service type has two parts: request and response. For servers in the service, ROS does not check for name conflicts. Only the last registered server will take effect and establish a connection with the client.

1.3.3、Action

Action is composed of multiple topics used to define tasks, including goals, feedback on task execution process status, and results. Compiling an action will automatically generate 7 structures: Action, ActionGoal, ActionFeedback, ActionResult, Goal, Feedback, and Result structures.

Characteristics of Action:

• A Question and Answer Communication Mechanism
• With continuous feedback
• Can be terminated during the task
• Implementation of message mechanism based on ROS

Interface for Action:

• goal：Publish Task Objectives
• cancel：Request to cancel task
• status：Notify client of current status
• feedback：Monitoring data for periodic feedback task operation
• result：Send the execution results of the task to the client and publish them only once.

Comparison of communication mode characteristics

1.4 Common Components

Launch startup file; TF coordinate transformation; Rviz； Gazebo； QT toolbox; Navigation； MoveIt!Launch: Launch File is a way in ROS to start multiple nodes simultaneously. It can also automatically start the ROS Master node manager and implement various configurations for each node, providing great convenience for the operation of multiple nodes.TF coordinate transformation: There are often a large number of component elements in the robot body and the working environment of the robot, which involve the position and posture of different components in robot design and application. TF is a functional package that allows users to track multiple coordinate systems over time. It uses a tree data structure to buffer and maintain coordinate transformation relationships between multiple coordinate systems based on time, which can help developers at any time Complete the transformation of points, vectors, and other coordinates between coordinate systems.QT Toolbox: In order to facilitate visual debugging and display, ROS provides a Qt architecture backend graphical tool kit - rqt_ common_ Plugins, which includes many practical tools: log output tool (rqt_console), computational graph visualization tool (rqt_graph), data plotting tool (rqt_plot), parameter dynamic configuration tool (rqt-reconfigure)

Rviz: rviz is a 3D visualization tool, which is well compatible with various robot platforms based on ROS software framework. In rviz, XML can be used to describe the dimensions, masses, positions, materials, joints, and other attributes of robots, surrounding objects, and any other physical objects, and presented in the interface. At the same time, rviz can also display real-time information about robot sensors, robot motion status, and changes in the surrounding environment through graphical means.Gazebo: Gazebo is a powerful 3D physics simulation platform with powerful physics engines, high-quality graphics rendering, convenient programming and graphics interfaces, and most importantly, it has open source and free features. Although the robot model in Gazebo is the same as the model used in rviz, it is necessary to incorporate the physical properties of the robot and its surrounding environment, such as mass, friction coefficient, elasticity coefficient, etc., into the model. The sensor information of the robot can also be added to the simulation environment through plugins and displayed in a visual manner.

Navigation: navigation is the two-dimensional navigation function package of ROS. In brief, it is based on the information flow of odometer and other sensors input and the global position of the robot, and through the navigation algorithm, it can calculate safe and reliable robot speed control commands.Moveit：Moveit！ The feature pack is the most commonly used tool kit, mainly used f