ROS Introduction¶

Welcome to the ROS section of the course. The purpose of these pages is to give you a compact ROS introduction where you can use in your course project. The documentation in this chapter is a target-based collection of mainly the following very nice ROS tutorials.

What is ROS?¶

ROS: “(Robot Operating System) is an open-source, meta-operating system for your robot. It provides the services you would expect from an operating system, including hardware abstraction, low-level device control, implementation of commonly-used functionality, message-passing between processes, and package management. It also provides tools and libraries for obtaining, building, writing, and running code across multiple computers.” Follow this link for ros.org
- Meta operating system
- Fundamental components

ROS versions

Source: ROS Versions¶

Important Shortcuts¶

Open a new terminal: Ctrl+Alt+T
Copy Paste: Ctrl+Shit+C and Ctrl+Shit+V (regular Ctrl+C and Ctrl+V does not work in terminals)
Tab for auto-complete
Recursive search Ctrl+R

Important Commands¶

roscore: Start ROS master
rosrun my_pkg my_node: Start a node (a .py or .cpp file)
roslaunch my_pkg my_launch.launch: Start a launch file
catkin_make: Compile catkin workspace
rosdep install --from-paths src --ignore-src -r -y: Install dependencies of the packages in src
source ~/catkin_ws/devel/setup.bash: loads the compiled shell environment (use after every catkin_make)
rospack profile: checks and loads new packages (use after a new package is compiled)
rostopic echo/list/info (topic_name): Listen/list/get information about available topics
rosnode list/info (topic_name): List/get information about available nodes
rosmsg info msg_name
roscd: Change directory in ROS packages
rosparam list/load/get/set: Commands for parameter server

Dictionary¶

ROS master: The main node enables other nodes to communicate
node: ROS executable
launch: Multiple ROS executables as well as parameters and arguments
publisher: An executable providing data to the ROS system
subscriber: An executable retrieving data from the ROS system
package: A folder contains several nodes/launchers/resource developed for a specific purpose.
topic: The channel between (a) publisher and (a) subscriber (more like a sophisticated data type definition)
message: The information in a topic
gazebo: A powerful simulator with physical properties (gravity, collision, lights/shadows etc)
rviz: A visualization software without physical properties
rqt: Many useful ROS-Qt packages like rqt_graph, rqt_publisher, rqt_controller_manager, rqt_plot etc.
urdf/sdf: File formats to define a robot
parameter server: A shared dictionary of parameters that nodes store and retrieve at runtime
tf: Stands for Transform and it is a powerful ROS package builds relationships of multiple frames from a given robot model.

Appendix¶

These are extra notes.

ROS Cheat Sheet¶

ROS Cheat Sheet

ROS Ternimology¶

This section explains the most frequently used ROS terms. Use this section as a ROS glossary.

ROS¶

ROS provides standard operating system services such as hardware abstraction, device drivers, implementation of commonly used features including sensing, recognizing, mapping, motion planning, message passing between processes, package management, visualizers and libraries for development as well as debugging tools.

Master¶

The master acts as a name server for node-to-node connections and message communication. The command roscore is used to run the master, and if you run the master, you can register the name of each node and get information when needed. The connection between nodes and message communication such as topics and services are impossible without the master.

Node¶

A node refers to the smallest unit of processor running in ROS. Think of it as one executable program. ROS recommends creating one single node for each purpose, and it is recommended to develop for easy reusability. For example, in case of mobile robots, the program to operate the robot is broken down into specialized functions. Specialized node is used for each function such as sensor drive, sensor data conversion, obstacle recognition, motor drive, encoder input, and navigation.

Package¶

A package is the basic unit of ROS. The ROS application is developed on a package basis, and the package contains either a configuration file to launch other packages or nodes. The package also contains all the files necessary for running the package, including ROS dependency libraries for running various processes, datasets, and configuration file. The number of official packages is about 2,500 for ROS Indigo as of July 2017and about 1,600 packages for ROS Kinetic. In addition, although there could be some redundancies, there are about 4,600 packages developed and released by users.

Metapackage¶

A metapackage is a set of packages that have a common purpose. For example, the Navigation metapackage consists of 10 packages including AMCL, DWA, EKF, and map_server.

Message¶

A node sends or receives data between nodes via a message. Messages are variables such as integer, floating point, and boolean. Nested message structure that contains another messages or an array of messages can be used in the message.

Topic¶

The topic is literally like a topic in a conversation. The publisher node first registers its topic with the master and then starts publishing messages on a topic. Subscriber nodes that want to receive the topic request information of the publisher node corresponding to the name of the topic registered in the master. Based on this information, the subscriber node directly connects to the publisher node to exchange messages as a topic.

Publish and Publisher¶

The term ‘publish’ stands for the action of transmitting relative messages corresponding to the topic. The publisher node registers its own information and topic with the master, and sends a message to connected subscriber nodes that are interested in the same topic. The publisher is declared in the node and can be declared multiple times in one node.

Service¶

The service10 is synchronous bidirectional communication between the service client that requests a service regarding a particular task and the service server that is responsible for responding to requests.

Service Server¶

The service server is a server in the service message communication that receives a request as an input and transmits a response as an output. Both request and response are in the form of messages. Upon the service request, the server performs the designated service and delivers the result to the service client as a response. The service server is implemented in the node that receives and executes a given request.

Service Client¶

The service client is a client in the service message communication that requests service to the server and receives a response as an input. Both request and response are in the form of message. The client sends a request to the service server and receives the response. The service client is implemented in the node which requests specified command and receives results.

Action¶

The action11 is another message communication method used for an asynchronous bidirectional communication. Action is used where it takes longer time to respond after receiving a request and intermediate responses are required until the result is returned. The structure of action file is also similar to that of service. However, feedback data section for intermediate response is added along with goal and result data section which are represented as request and response in service respectively. There are action client that sets the goal of the action and action server that performs the action specified by the goal and returns feedback and result to the action client.

Action Server¶

The action server is in charge of receiving goal from the client and responding with feedback and result. Once the server receives goal from the client, it performs predefined process.

Action Client¶

The action client is in charge of transmitting the goal to the server and receives result or feedback data as inputs from the action server. The client delivers the goal to the action server, then receives corresponding result or feeedback, and transmits follow up instructions or cancel instruction.

Parameter¶

The parameter in ROS refers to parameters used in the node. Think of it as ** .ini* configuration files in Windows program. Default values are set in the parameter and can be read or written if necessary. In particular, it is very useful when configured values can be modified in real-time. For example, you can specify settings such as USB port number, camera calibration parameters, maximum and minimum values of the motor speed.

Parameter Server¶

When parameters are called in the package, they are registered with the parameter server which is loaded in the master.

Catkin¶

The catkin refers to the build system of ROS. The build system basically uses CMake (Cross Platform Make), and the build environment is described in the ‘CMakeLists.txt’ file in the package folder. CMake was modified in ROS to create a ROS-specific build system. Catkin started the alpha test from ROS Fuerte and the core packages began to switch to Catkin in the ROS Groovy version. Catkin has been applied to most packages in the ROS Hydro version. The Catkin build system makes it easy to use ROS-related builds, package management, and dependencies among packages. If you are going to use ROS at this point, you should use Catkin instead of ROS build (rosbuild).

ROS Build¶

The ROS build is the build system that was used before the Catkin build system. Although there are some users who still use it, this is reserved for compatibility of ROS, therefore, it is officially not recommended to use. If an old package that only supports the rosbuild must be used, we recommend using it after converting rosbuild to catkin.

roscore¶

Roscore is the command that runs the ROS master. If multiple computers are within the same network, it can be run from another computer in the network. However, except for special case that supports multiple roscore, only one roscore should be running in the network. When ROS master is running, the URI address and port number assigned for ROS_MASTER_URI environment variables are used. If the user has not set the environment variable, the current local IP address is used as the URI address and port number 11311 is used which is a default port number for the master.

rosrun¶

Rosrun is the basic execution command of ROS. It is used to run a single node in the package. The node uses the ROS_HOSTNAME environment variable stored in the computer on which the node is running as the URI address, and the port is set to an arbitrary unique value.

roslaunch¶

While rosrun is a command to execute a single node, roslaunch18 in contrast executes multiple nodes. It is a ROS command specialized in node execution with additional functions such as changing package parameters or node names, configuring namespace of nodes, setting ROS_ROOT and ROS_PACKAGE_PATH, and changing environment variables19 when executing nodes. roslaunch uses the ** .launch * file to define which nodes to be executed. The file is based on XML (Extensible Markup Language) and offers a variety of options in the form of XML tags.

bag¶

The data from the ROS messages can be recorded. The file format used is called bag20, and ** .bag * is used as the file extension. In ROS, bag can be used to record messages and play them back when necessary to reproduce the environment when messages are recorded. For example, when performing a robot experiment using a sensor, sensor values are stored in the message form using the bag. This recorded message can be repeatedly loaded without performing the same test by playing the saved bag file. Record and play functions of rosbag are especially useful when developing an algorithm with frequent program modifications.

ROS Wiki¶

ROS Wiki is a basic description of ROS based on Wiki that explains each package and the features provided by ROS. This Wiki page describes the basic usage of ROS, a brief description of each package, parameters used, author, license, homepage, repository, and tutorial. The ROS Wiki currently has more than 18,800 pages of content.

Repository¶

An open package specifies repository in the Wiki page. The repository is a URL address on the web where the package is saved. The repository manages issues, development, downloads, and other features using version control systems such as svn, hg, and git. Many of currently available ROS packages are using GitHub21 as repositories for source code. In order to view the contents of the source code for each package, check the corresponding repository.

Graph¶

The relationship between nodes, topics, publishers, and subscribers introduced above can be visualized as a graph. The graphical representation of message communication does not include the service as it only happens one time. The graph can be displayed by running the ‘rqt_graph’ node in the ‘rqt_graph’ package. There are two execution commands, ‘rqt_graph’ and ‘rosrun rqt_graph rqt_graph’.

Name¶

Nodes, parameters, topics, and services all have names. These names are registered on the master and searched by the name to transfer messages when using the parameters, topics, and services of each node. Names are flexible because they can be changed when being executed, and different names can be assigned when executing identical nodes, parameters, topics, and services multiple times. Use of names makes ROS suitable for large-scale projects and complex systems.

Client Library¶

ROS provides development environments for various languages by using client library23 in order to reduce the dependency on the language used. The main client libraries are C++, Python, Lisp, and other languages such as Java, Lua, .NET, EusLisp, and R are also supported. For this purpose, client libraries such as roscpp, rospy, roslisp, rosjava, roslua, roscs, roseus, PhaROS, and rosR have been developed.

URI¶

A URI (Uniform Resource Identifier) is a unique address that represents a resource on the Internet. The URI is one of basic components that enables interaction with Internet and is used as an identifier in the Internet protocol.

CMakeLists.txt¶

Catkin, which is the build system of ROS, uses CMake by default. The build environment is specified in the ‘CMakeLists.txt’ file in each package folder.

package.xml¶

An XML file contains package information that describes the package name, author, license, and dependent packages.

Message Communication in ROS¶

Here is the ROS message communication.

Source: ROS Robot Programming (Book)¶