ROS Robotics By Example - Second Edition

4.7 (10 reviews total)
By Carol Fairchild , Dr. Thomas L. Harman
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  1. Getting Started with ROS

About this book

ROS is a robust robotics framework that works regardless of hardware architecture or hardware origin. It standardizes most layers of robotics functionality from device drivers to process control and message passing to software package management.

But apart from just plain functionality, ROS is a great platform to learn about robotics itself and to simulate, as well as actually build, your first robots. This does not mean that ROS is a platform for students and other beginners; on the contrary, ROS is used all over the robotics industry to implement flying, walking and diving robots, yet implementation is always straightforward, and never dependent on the hardware itself.

ROS Robotics has been the standard introduction to ROS for potential professionals and hobbyists alike since the original edition came out; the second edition adds a gradual introduction to all the goodness available with the Kinetic Kame release.

By providing you with step-by-step examples including manipulator arms and flying robots, the authors introduce you to the new features. The book is intensely practical, with space given to theory only when absolutely necessary. By the end of this book, you will have hands-on experience on controlling robots with the best possible framework.

Publication date:
November 2017
Publisher
Packt
Pages
484
ISBN
9781788479592

 

Chapter 1. Getting Started with ROS

In this chapter, we will introduce the Robot Operating System (ROS), which is a collection of software packages to aid researchers and developers using robotic systems. After we discuss the instructions to install ROS on your computer system using the Ubuntu operating system, the ROS architecture and many of its components are discussed. This will aid you in understanding the use of ROS to develop software for robotic applications.

ROS will be introduced in terms of its elements and their functions. An understanding of the ROS vocabulary is necessary to become proficient in using ROS to create programs for the control of real or simulated robots as well as devices, such as cameras.

To make the discussion more concrete, the turtlesim simulator will be presented with various examples of the ROS command usage. This simulator is part of ROS and it provides an excellent introduction to the capabilities of ROS.

In this chapter, we will cover the following topics:

  • What ROS is and which robots use ROS

  • How to install and launch ROS on your computer

  • How to navigate the ROS directories

  • An introduction to ROS packages, nodes, and topics

  • Examples of useful ROS commands

  • How to use ROS commands with the turtlesim simulator

 

What does ROS do and what are the benefits of learning ROS?


ROS is sometimes called a meta operating system because it performs many functions of an operating system, but it requires a computer's operating system such as Linux. One of its main purposes is to provide communication between the user, the computer's operating system, and equipment external to the computer. This equipment can include sensors, cameras, as well as robots. As with any operating system, the benefit of ROS is the hardware abstraction and its ability to control a robot without the user having to know all of the details of the robot.

For example, to move a robot's arms, a ROS command is issued or scripts in Python or C++ written by the robot designers cause the robot to respond as commanded. The scripts can, in turn, call various control programs that cause the actual motion of the robot's arms. It is also possible to design and simulate your own robot using ROS. These subjects and many others will be considered in this book.

In this book, you will learn a set of concepts, software, and tools that apply to an ever-increasing and diverse army of robots. For example, the navigation software of one mobile robot can be used, with a few changes, to work in another mobile robot. The flight navigation of an aerial robot is similar to that of the ground robot and so on. All across the broad spectrum of robotics, system interfaces are standardized or upgraded to support increased complexity. There are readily available libraries for commonly used robotics functions. ROS not only applies to the central processing of robotics but also to sensors and other subsystems. ROS hardware abstraction combined with low-level device control speeds the upgrade toward the latest technology.

ROS is an open source robotic software system that can be used without licensing fees by universities, government agencies, and commercial companies. The advantages of open source software are that the source code for the system is available and can be modified according to a user's needs. More importantly for some users, the software can be used in a commercial product as long as the appropriate licenses are cited. The software can be improved and modules can be added by users and companies.

ROS is used by many thousands of users worldwide and knowledge can be shared between users. The users range from hobbyists to professional developers of commercial robots. In addition to the large group of ROS researchers, there is a ROS-Industrial group dedicated to applying ROS software to robots for manufacturing. Other versions of ROS currently under development include:

  • ROS-M for military robotic systems

  • H-ROS is Hardware ROS for interoperable robot components

  • ROS 2.0 to upgrade ROS with the latest technology and software

Who controls ROS?

A ROS distribution is a set of ROS software packages that can be downloaded to your computer. These packages are supported by the Open Source Robotics Foundation (OSRF), a nonprofit organization. The distributions are updated periodically and given different names by the ROS organization. More details about the ROS organization are available at: http://www.ros.org/about-ros/

 

Which robots are using ROS?


There is a long list of robots on the ROS wiki website, http://wiki.ros.org/Robots, which use ROS. For example, we are using four different robots in this book to provide you with an experience of a wide range of ROS capabilities. These robots are as follows:

  • TurtleBot, a mobile robot

  • Baxter, a friendly two-armed robot

  • Crazyflie and Bebop, flying robots

The images of these robots are in the following figures:

TurtleBot 2 and 3

Of course, not everyone has the opportunity to use real robots such as Baxter (shown in the following image):

Baxter in the authors' laboratory

However, there is good news! Using the ROS Gazebo software, you can simulate Baxter as well as many other robots whose models are provided for Gazebo. We will simulate TurtleBot using Gazebo and actually design our own mobile robot in the upcoming chapters of this book.

Bebop and Crazyflie

 

Installing and launching ROS


For this book, we assume the reader has a computer with Ubuntu Wily 15.10 or Xenial 16.04 installed. The examples in this book have been developed using ROS Kinetic and this version of ROS is only supported by these two versions of Ubuntu. The instructions for ROS installation provided in this section are for installing Debian (binary) packages. This is the most efficient and preferred way to install ROS.

If you wish to install the ROS Kinetic source code and build the software, refer to the instructions at http://wiki.ros.org/kinetic/Installation/Source. The instructions presented here to install ROS Kinetic with Debian packages can also be found at http://wiki.ros.org/kinetic/Installation/Ubuntu.

If you have any problems while installing ROS, refer to this site and the ROS forum at http://answers.ros.org.

Note

Refer to the Gazebo tutorial at http://gazebosim.org/tutorials/?tut=ros_urdf for a list of these elements and their usage (Elements for Links and Elements for Joints).

Configuring your Ubuntu repositories

To configure your Ubuntu repositories to allow restricted, universe and multiverse, perform the following steps:

  1. Click on the Ubuntu System Settings icon in the menu on the left side of your desktop.

  2. Click on the Software & Updates icon. On the Software & Updates screen, select the appropriate checkboxes to match the following screenshot:

    Ubuntu Software & Updates screen

Setting up your sources.list file

Open a terminal window to set up the sources.list file on your computer to accept software from the ROS software repository at http://packages.ros.org which is the authorized site for the ROS software.

At the $ command prompt, type the following command as one long command:

$ sudo sh -c 'echo "deb http://packages.ros.org/ros/ubuntu
$(lsb_release -sc) main" > /etc/apt/sources.list.d/ros-latest.list'

Note

In copying such two line commands from the electronic version of this book, be sure to delete the Carriage Return at the end of the first line.

This step allows the operating system to know where to download programs that need to be installed on your system. When updates are made to ROS Kinetic, your operating system will be made aware of these updates.

Setting up your keys

Keys confirm the origin of the code and verify that unauthorized modifications to the code have not been made without the knowledge of the owner. A repository and the keys of that repository are added to the operating system's trusted software list. Type the following command:

$ sudo apt-key adv --keyserver hkp://ha.pool.sks-keyservers.net:80 --recv-key 421C365BD9FF1F717815A3895523BAEEB01FA116

Installing ROS Kinetic

Before you begin with the installation, the current system software must be up to date to avoid problems with libraries and wrong versions of software. To make sure your Debian package index is up-to-date, type the following command:

$ sudo apt-get update

Install the desktop-full configuration of ROS. Desktop-full includes ROS, rqt, rviz, robot-generic libraries, 2D/3D simulators, navigation, and 2D/3D perception. In this book, we will be using rqt and rviz for visualization and also the Gazebo 3D simulator, as well as the ROS navigation and perception packages. To install, type the following command:

$ sudo apt-get install ros-kinetic-desktop-full

ROS Kinetic is installed on your computer system when the installation process is complete!

Initialize rosdep

The ROS system may depend on software packages that are not loaded initially. These software packages external to ROS are provided by the operating system. The ROS environment command rosdep is used to download and install these external packages. Type the following commands:

$ sudo rosdep init
$ rosdep update

Environment setup

Your terminal session must now be made aware of these ROS files so that it knows what to do when you attempt to execute ROS command-line commands. Running this script will set up the ROS environment variables:

$ source /opt/ros/kinetic/setup.bash

Alternatively, it is convenient if the ROS environment variables are automatically added to your terminal session every time a new shell is launched. If you are using bash for your terminal shell, do this by typing the following commands:

$ echo "source /opt/ros/kinetic/setup.bash" >> ~/.bashrc
$ source ~/.bashrc

Now when a new terminal session is launched, the bash shell is automatically aware of the ROS environment variables.

Getting rosinstall

The rosinstall command is a command-line tool in ROS that allows you to download ROS packages with one command.

To install this tool on Ubuntu, type the following command:

$ sudo apt-get install python-rosinstall

Troubleshooting – examining your ROS environment

The ROS environment is set up through a number of variables that tell the system where to find ROS packages. Two main variables are ROS_ROOT and ROS_PACKAGE_PATH that enable ROS to locate packages in the filesystem.

To check whether the ROS environment variables are set correctly, use the env command in the following form that lists the ROS environment variables:

$ env | grep ROS

The output of the preceding command is as follows:

ROS_ROOT=/opt/ros/kinetic/share/ros
ROS_PACKAGE_PATH=/opt/ros/kinetic/share
ROS_MASTER_URI=http://localhost:11311
ROSLISP_PACKAGE_DIRECTORIES=
ROS_DISTRO=kinetic
ROS_ETC_DIR=/opt/ros/kinetic/etc/ros

If the variables are not set correctly, you will need to source your setup.bash file, as described in the Environment setup section of this chapter. Check whether the ROS_DISTRO= "kinetic" and ROS_PACKAGE_PATH variables are correct, as shown previously.

The tutorial that discusses the ROS environment can be found at: http://wiki.ros.org/ROS/Tutorials/InstallingandConfiguringROSEnvironment

 

Creating a catkin workspace


The next step is to create a catkin workspace. A catkin workspace is a directory (folder) in which you can create or modify existing catkin packages. The catkin structure simplifies the build and installation process for your ROS packages. The ROS wiki website is http://wiki.ros.org/catkin/Tutorials/create_a_workspace.

A catkin workspace can contain up to three or more different subdirectories (/build, /devel, and /src), each of which serve a different role in the software development process.

We will label our catkin workspace catkin_ws. To create the catkin workspace, type the following commands:

$ mkdir -p ~/catkin_ws/src
$ cd ~/catkin_ws/src
$ catkin_init_workspace

Even though the workspace is empty (there are no packages in the src folder, just a single CMakeLists.txt link), you can still build the workspace by typing the following commands:

$ cd ~/catkin_ws/
$ catkin_make

The catkin_make command creates the catkin workspace. If you view your current directory contents, you should now have the build and devel folders. Inside the devel folder there are now several setup.*sh files. We will source the setup.bash file to overlay this workspace on top of your ROS environment:

$ source ~/catkin_ws/devel/setup.bash

Remember to add this source command to your .bashrc file by typing the following command:

$ echo "source ~/catkin_ws/devel/setup.bash" >> ~/.bashrc

To make sure your workspace is properly overlaid by the setup script, make sure the ROS_PACKAGE_PATH environment variable includes the directory you're in by typing the following command:

$ echo $ROS_PACKAGE_PATH

The output of the preceding command should be as follows:

/home/<username>/catkin_ws/src:/opt/ros/kinetic/share

Here, <username> is the name you chose for the user when Ubuntu was installed.

 

ROS packages and manifest


The ROS software is divided into packages that can contain various types of programs, images, data, and even tutorials. The specific contents depend on the application for the package. The site http://wiki.ros.org/Packages discusses ROS packages.

A package can contain programs written in Python or C++ to control a robot or another device. For the turtlesim simulator package, for example, the package contains the executable code used to change the background color or move a turtle around on the screen. This package also contains images of a turtle for display and files used to create the simulator.

There is another class of packages in ROS called metapackages that are specialized packages that only contain a package.xml manifest. Their purpose is to reference one or more related packages, which are loosely grouped together.

ROS manifest

Each package contains a manifest named package.xml that describes the package in the Extensible Markup Language (XML) format. In addition to providing a minimal specification describing the package, the manifest defines properties about the package such as the package name, version numbers, authors, maintainers, and any dependencies on other packages.

Exploring the ROS packages

Occasionally, we would like to find packages that we wish to use and display the files involved. This section introduces several useful ROS commands:

  • rospack used for information about a package

  • roscd used to navigate the ROS directories

  • rosls used to list directories and files in a package directory

The rospack command can be used to list ROS packages, locate packages by name, and determine if a package depends on another package, among other uses. For more information use the following command with the help or -h option in the form:

$ rospack help | less

We will use the turtlesim package for the examples here. To change directories to the location of turtlesim, use the following command:

$ roscd turtlesim

This changes the location on one of the author's workstations as follows:

[email protected]:/opt/ros/kinetic/share/turtlesim$

On your computer, the $ command prompt will be preceded by the information about your computer. Generally, that information for our computers will be deleted in our examples using ROS commands. Once you are in the turtlesim directory, the standard Linux commands can be used with the subdirectories or files, or the ROS commands can be used. To determine the directories and files in the turtlesim directory but without changing to the turtlesim directory, use the following command:

$ rosls turtlesim

Here is the result from the home directory of the author's workstation with ROS installed:

cmake images  msg package.xml srv

To see the filenames of the images loaded with turtlesim, specify the images directory in the package as follows:

$ rosls turtlesim/images

The output of the preceding command is as follows:

box-turtle.png   groovy.png  indigo.svg   palette.png
diamondback.png  hydro.png   jade.png     robot-turtle.png
electric.png     hydro.svg   kinetic.png  sea-turtle.png
fuerte.png       indigo.png  kinetic.svg  turtle.png

There are various turtle images that can be used. The rosls turtlesim command will also work to show the contents of the turtlesim subdirectories: /msg for messages and /srv for services. These files will be discussed later. To see the manifest, type the following commands:

$ roscd turtlesim
$ cat package.xml

This will also show the dependencies, such as roscpp for C++ programs.

rospack find packages

The rospack find <package name> command returns the path to the package named <package name>. For example, type the following command:

$ rospack find turtlesim

The preceding command displays the path to the turtlesim directory.

rospack list

Execute the following command:

$ rospack list

This lists the ROS package names and their directories on the computer. In the case of the workstation mentioned earlier, there are 195 ROS packages listed!

Note

If you really want to see all the ROS packages and their locations, use the following command form:

$ rospack list | less

This form allows paging of the long list of names and directories for the packages. Press Q to quit.

Alternatively, this is the form of the rospack command:

$ rospack list-names

This lists only the names of the packages without the directories. After such a long list, it is a good idea to open a new terminal window or clear the window with the clear command.

This is the form of the rospack command:

$ rospack list-names | grep turtle

This lists the packages with turtle in the name.

More information on commands that are useful to navigate the ROS filesystem is available at the ROS website http://wiki.ros.org/ROS/Tutorials/NavigatingTheFilesystem

 

ROS nodes, topics, and messages


Here we will explore some of the main components of ROS. One of the primary purposes of ROS is to facilitate communication between the ROS nodes. These nodes represent the executable code. The code can reside entirely on one computer, or nodes can be distributed between computers or between computers and robots. The advantage of this distributed structure is that each node can control one aspect of a system.

For example, one node can capture the images from a camera and send the images to another node for processing. After processing the image, the second node can send a control signal to a third node for controlling a robotic manipulator in response to the camera view.

The main mechanism used by ROS nodes to communicate is by sending and receiving messages. The messages are organized into specific categories called topics. Nodes may publish messages on a particular topic or subscribe to a topic to receive information.

ROS nodes

Basically, nodes are processes that perform some computation or task. The nodes themselves are really software processes but with the capability to register with the ROS Master node and communicate with other nodes in the system. The ROS design idea is that each node is independent and interacts with other nodes using the ROS communication capability. The Master node is described in the ROS Master section to follow.

One of the strengths of ROS is that a particular task, such as controlling a wheeled mobile robot, can be separated into a series of simpler tasks. The tasks can include the perception of the environment using a camera or laser scanner, map making, planning a route, monitoring the battery level of the robot's battery, and controlling the motors driving the wheels of the robot. Each of these actions might consist of a ROS node or a series of nodes to accomplish the specific tasks.

A node can independently execute code to perform its task but can also communicate with other nodes by sending or receiving messages. The messages can consist of data, commands, or other information necessary for the application.

ROS topics

Some nodes provide information for other nodes, as a camera feed would do, for example. Such a node is said to publish information that can be received by other nodes. The information in ROS is called a topic. A topic defines the types of messages that will be sent concerning that topic.

The nodes that transmit data publish the topic name and the type of message to be sent. The actual data is published by the node. A node can subscribe to a topic and transmitted messages on that topic are received by the node subscribing.

Continuing with the camera example, the camera node can publish the image on the camera/image_raw topic.

Image data from the camera/image_raw topic can be used by a node that shows the image on the computer screen. The node that receives the information is said to subscribe to the topic being published, in this case camera/image_raw.

In some cases, a node can both publish and subscribe to one or more topics.

ROS messages

ROS messages are defined by the type of message and the data format. The ROS package named std_msgs, for example, has messages of type String which consist of a string of characters. Other message packages for ROS have messages used for robot navigation or robotic sensors. The turtlesim package has its own set of messages that pertain to the simulation.

We will see in the section, Turtlesim – the first ROS robot simulation that the turtlesim simulator has two nodes that are created when turtlesim is executed. Turtlesim has relatively few topics and messages so it is ideal for the initial study of ROS.

The ROS site http://www.ros.org/core-components/ describes the communication and robot-specific features of ROS. Here, we will explore some of the main components of a ROS system including ROS nodes and the ROS Master. It is important for you to understand the ROS nodes, topics, and messages as they are involved in almost every ROS activity.

ROS Master

The ROS nodes are typically independent programs that can run concurrently on several systems. The ROS Master provides naming and registration services to the nodes in the ROS system. It tracks publishers and subscribers to the topics. Communication is established between the nodes by the ROS Master.

The role of the Master is to enable individual ROS nodes to locate one another. The most often used protocol for connection is the standard Transmission Control Protocol/Internet Protocol (TCP/IP) or Internet Protocol called TCPROS in ROS. Once these nodes are able to locate one another, they can communicate with each other peer-to-peer.

One responsibility of the Master is to keep track of nodes when new nodes are executed and come into the system. Thus, the Master provides a dynamic allocation of connections. The nodes cannot communicate however until the Master notifies the nodes of each other's existence. A simple example is shown at http://wiki.ros.org/Master.

Invoking the ROS Master using roscore

roscore starts processes that you must have running in order for ROS nodes to communicate. When it executes, roscore will start the following:

  • A ROS Master

  • A ROS Parameter Server

  • A rosout logging node

The roscore command creates the Master so that nodes can register with the Master. You can view the ROS tutorial for roscore at http://wiki.ros.org/roscore.

Issue the following command to start the Master in a new terminal window and observe the output:

$ roscore

The output of the preceding command is as follows:

... logging to /home/linux/.ros/log/9c3776b4-09cd-11e7-bb39-1866da2351d7/roslaunch-D158-45929-29790.log
Checking log directory for disk usage. This may take a while.
Press Ctrl-C to interrupt
Done checking log file disk usage. Usage is <1GB.

started roslaunch server http://D158-45929:34807/
ros_comm version 1.12.7


SUMMARY
========

PARAMETERS
 * /rosdistro: kinetic
 * /rosversion: 1.12.7

NODES

auto-starting new master
process[master]: started with pid [29802]
ROS_MASTER_URI=http://D158-45929:11311/

setting /run_id to 9c3776b4-09cd-11e7-bb39-1866da2351d7
process[rosout-1]: started with pid [29815]
started core service [/rosout]

In the preceding screen output, you will see information about the computer, parameters that list the name (kinetic) and version number of the ROS distribution, and other information. The Master is defined by its Uniform Resource Identifier (URI). This identifies the location of the Master; in this case, it is running on the workstation used to execute the roscore command.

Parameter Server

The Parameter Server is a shared dictionary of parameters that nodes store and retrieve at runtime. The Parameter Server runs inside the Master and parameters are globally viewable so that nodes can access the parameters.

In the preceding screen output from the roscore command, the parameters associated with the Master are as follows:

* /rosdistro: kinetic
* /rosversion: 1.12.7

Kinetic is the ROS distribution release that we are using. As Kinetic is changed or packages are added, numbered versions such as 1.12.7 are released. Issuing the roscore command is a way to determining the version of ROS running on your computer.

Whenever ROS is executing, it is possible to list the nodes that are active and the topics that are used for communication. We will explore the information in the roscore output in more detail by invoking useful ROS terminal commands.

ROS commands to determine the nodes and topics

Three commands used extensively in ROS are as follows:

  • roscore to start the Master and allow nodes to communicate

  • rosnode list to list the active nodes

  • rostopic list to list the topics associated with active ROS nodes

After the roscore command is executed, the terminal window used to execute roscore must remain active, but it can be minimized. In another terminal window, the rosnode list command will cause a list of the ROS nodes that are active to be displayed on the screen. After the command for roscore is executed, only one node rosout will be listed as an active node if you type the following command:

$ rosnode list

The output of the preceding command is as follows:

/rosout

In the second terminal window, list the active topics by typing:

$ rostopic list

The output of the preceding command is as follows:

/rosout
/rosout_agg

Notice that the /rosout node and the /rosout topic have the same designation. In ROS terms, the rosout node subscribes to the /rosout topic. All the active nodes publish their debug messages to the /rosout topic. We will not be concerned with these messages here; however, they can be useful to debug a program. For an explanation refer to the ROS wiki at http://wiki.ros.org/rosout.

The rosout node is connected to every other active node in the system. The /rosout_agg topic receives messages also, but just from the rosout node so it does not have to connect to all of the nodes and thus saves time at system startup.

The rostopic and rosnode terminal commands have a number of options that will be demonstrated by various examples in this book.

Note

Most of the ROS commands have help screens that are usually helpful. Type the following command for the command options:

$ rosnode -h

For more detailed usage, use the subcommand name, for example:

$ rosnode list –h

This will list the subcommands and the options for the rosnode command.

There are a number of other important ROS terminal commands that you should know. They are introduced and explained using the turtlesim simulator in the upcoming section.

 

Turtlesim – the first ROS robot simulation


A simple way to learn the basics of ROS is to use the turtlesim simulator that is part of the ROS installation. The simulation consists of a graphical window that shows a turtle-shaped robot. The background color for the turtle's world can be changed using the Parameter Server. The turtle can be moved around on the screen by ROS commands or using the keyboard.

Turtlesim is a ROS package, and the basic concepts of package management were presented in the Exploring the ROS packages section, as discussed earlier. We suggest that you refer to this section before continuing.

We will illustrate a number of ROS commands that explore the nodes, topics, messages, and services used by the turtle simulator. We have already covered the roscore, rosnode, and rostopic commands. These commands will be used with turtlesim also.

Other important ROS terminal commands that will be covered in this section are as follows:

  • rosrun: Finds and starts a requested node in a package

  • rosmsg: Shows information about messages

  • rosservice: Displays runtime information about nodes and can pass data between nodes in a request/response mode

  • rosparam: Used to get and set parameters (data) used by nodes

Starting turtlesim nodes

To start turtlesim with ROS commands, we need to open two separate terminal windows. First, issue the following command in the first window if the Master is not already running:

$ roscore

Wait for the Master to complete startup. You can minimize this window but do not close it because the Master must run to allow the nodes to communicate.

The result on your screen will resemble the output discussed previously in the Invoking the ROS Master using roscore section, where roscore was described.

rosrun command

To display the turtle on the screen, use the rosrun command. It takes the arguments [package name][executable name], and in this case, turtlesim as the package and turtlesim_node as the executable program.

In the second terminal window, issue the following command:

$ rosrun turtlesim turtlesim_node

You will see an output similar to this:

[ INFO] [1489616730.714683337]: Starting turtlesim with node name /turtlesim
[ INFO] [1489616730.727083554]: Spawning turtle [turtle1] at x=[5.544445], y=[5.544445], theta=[0.000000]

Wait for the display screen to appear with the image of a turtle at the center, as shown in the turtlesim screen in the following screenshot. The terminal window can be minimized, but keep the turtle display screen in view. The turtle is called turtle1 since this is the first and only turtle in our display.

After you have started turtlesim by executing the rosrun command, you will see information about the turtle's position on the screen. The /turtlesim node creates the screen image and the turtle. Here, the turtle is in the center at about x = 5.5, y = 5.5 with no rotation since angle theta is zero. The origin (0, 0) is at the lower-left corner of the screen:

Turtlesim screen

Let's study the properties of the nodes, topics, services, and messages available with the turtlesim package in another terminal window. Thus, at this point, you will have three windows active but the first two can be minimized or dragged off to the side or the bottom. They should not be closed.

Turtlesim nodes

In the third window, issue the rosnode command to determine information about any node. First, list the active nodes, using the following command:

$ rosnode list

The output is as follows:

/rosout
/turtlesim

We will concentrate on the /turtlesim node. Note the difference in notation between the /turtlesim node and the turtlesim package.

To see the publications, subscriptions, and services of the turtlesim node, type the following command:

$ rosnode info /turtlesim

The output of the preceding command is as follows:

Node [/turtlesim]
Publications:
 * /turtle1/color_sensor [turtlesim/Color]
 * /rosout [rosgraph_msgs/Log]
 * /turtle1/pose [turtlesim/Pose]

Subscriptions:
 * /turtle1/cmd_vel [unknown type]

Services:
 * /turtle1/teleport_absolute
 * /turtlesim/get_loggers
 * /turtlesim/set_logger_level
 * /reset
 * /spawn
 * /clear
 * /turtle1/set_pen
 * /turtle1/teleport_relative
 * /kill


contacting node http://D158-45929:38895/ ...
Pid: 29981
Connections:
 * topic: /rosout
    * to: /rosout
    * direction: outbound
    * transport: TCPROS

The following diagram represents a graphical illustration of the relationship of the turtlesim node in elliptical shapes and topics in the rectangular boxes:

/turtlesim node

The graph was created using the rqt_graph command as described in the Introducing RQT tools section in Chapter 3, Driving Around with TurtleBot.

We read the output of the rosnode info command and the graph of the turtlesim node and topics in the preceding diagram as follows (ignoring the logging services and the /rosout and /rosout_agg nodes):

  • The /turtlesim node publishes to two topics. These topics control the color of the turtle's screen and the position of the turtle on the screen when messages are sent from /turtlesim:

    • /turtle1/color_sensor with the message type [turtlesim/Color]

    • /turtle1/pose with the message type [turtlesim/Pose]

  • The /turtlesim node subscribes to the turtle1/cmd_vel topic. The /turtlesim node is waiting for another node to publish on the turtle1/cmd_vel topic.

  • There are a number of services associated with the /turtlesim node. The services can be used to move the turtle around (teleport), clear the screen, kill the nodes, and perform other functions. The services will be explained later in the section ROS services to move turtle.

Turtlesim topics and messages

A ROS message is a strictly typed data structure. In fact, the message type is not associated with the message contents. For example, one message type is string, which is just text. Another type of message is uint8, which means that it is an unsigned 8-bit integer. These are part of the std_msg package or standard messages. The command form rosmsg list lists the type of messages on your system; it is a long list! There are packages for messages of the type control, type geometry, and type navigation, among others to control robot actions. There are sensor message types used with laser scanners, cameras, and joysticks to name just a few of the sensors or input devices possible with ROS. Turtlesim uses several of the message types to control its simulated robot—the turtle.

For these exercises, keep the roscore and /turtlesim windows active. Open other terminal windows as needed. We will concentrate on the turtle1/color_sensor topic first. You will be typing in the third window.

Note

If the screen gets too cluttered, remember the $ clear command.

Use the $ history command to see the commands you have used.

rostopic list

For the /turtlesim node, the topics are listed using the following command:

$ rostopic list

The output of the preceding command is as follows:

/rosout
/rosout_agg
/statistics
/turtle1/cmd_vel
/turtle1/color_sensor
/turtle1/pose

rostopic type

The topic type can be determined for each topic by typing the following command:

$ rostopic type /turtle1/color_sensor
turtlesim/Color

The message type in the case of the /turtle1/color_sensor topic is turtlesim/Color. This is the format of ROS message type names:

[package name]/[message type]

If a node publishes a message, we can determine the message type and read the message.

rosmsg list

The turtlesim package has two message types that are found with the following command:

$ rosmsg list | grep turtlesim

The output is as follows:

turtlesim/Color
turtlesim/Pose

The rosmsg list command displays all of the messages. Adding | grep turtlesim shows all messages of the turtlesim package. There are only two in the turtlesim package. From the message type, we can find the format of the message. Make sure that you note that Color in the message type starts with a capital letter.

rosmsg show

The rosmsg show <message type> command displays the fields in a ROS message type. For the turtlesim/Color type, the fields are integers:

$ rosmsg show turtlesim/Color

Output of the preceding command is as follows:

uint8 r
uint8 g
uint8 b

The format of values designating colors red (r), green (g), and blue (b) is unsigned, 8-bit integer.

To understand the message, it is necessary to find the message type. For example, turtlesim/Color is a message type for turtlesim that has three elements that define the color of the background. For example, the red color in the background is defined by uint8 r. This indicates that if we wish to modify the red value, an 8-bit unsigned integer is needed. The amount of red in the background is in the range of 0–255.

In general, the formats of numerical data include integers of 8, 16, 32, or 64 bits, floating point numbers, and other formats.

rostopic echo

To determine the color mixture of red, green, and blue in the background of our turtle, use the rostopic echo [topic name] command in the form, as follows:

$ rostopic echo /turtle1/color_sensor

Output of the preceding command is as follows:

r: 69
g: 86
b: 255
---

Press Ctrl + C to stop the output.

The website describing the RGB Color Codes Chart and the meaning of the numerical color values can be found at http://www.rapidtables.com/web/color/RGB_Color.htm.

The chart explains how to mix the red, green, and blue (rgb) color values to achieve any desired color. The color values are parameters that can be changed.

A simple table can clarify the relationship between the topics and the messages for the /turtlesim node:

Topics and messages for the /turtlesim node

Topic name

Topic type

Message format

Message

$ rostopic list

$ rostopic type

[topic name]

$ rosmsg show

[topic type]

$ rostopic echo

[topic name]

/turtle1

/color_sensor

turtlesim/Color

uint8 r

uint8 g

uint8 b

r: 69

g: 86

b: 255

/turtle1/pose

turtlesim/Pose

float32 x

float32 y

float32 theta

float32 linear_velocity

float32 angular_velocity

x: 5.54444456

y: 5.54444456

theta: 0.0

linear_velocity: 0.0

angular_velocity: 0.0

The table of Topics and messages shows the topics, types, message formats, and data values for the two topics of the /turtlesim node that we explored. The commands to determine the information are also shown in the table.

Move the turtle by publishing /turtle1/cmd_vel

Start the Master and execute the turtlesim_node by typing:

$ roscore

In a second terminal window, issue the following command:

$ rosrun turtlesim turtlesim_node

Once the turtle screen is visible, there are a number of ways to move the turtle around.

The turtlesim_node subscribes to the /turtle1/cmd_vel topic, so the turtle can be commanded to move by sending messages on this topic. First, determine the type of messages for the topic by typing:

$ rostopic type /turtle1/cmd_vel

which displays the message type as the Twist message from the geometry_msgs package:

geometry_msgs/Twist

Next, the format of the message can be determined with the command:

$ rosmsg show geometry_msgs/Twist

The output shows that the message format allows six floating-point values which determine the linear and angular velocity of the turtle:

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

For the turtle in turtlesim that moves in a 2D space, the only motion allowed is forward motion in the turtle's x direction or rotation about the turtle's z axis that would extend out from the screen.

To move the turtle in a circle, the command

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

causes motion forward at 2.0 meters/second as well as rotation at 1.8 radians/second. The command specifies the topic, type of message, the repeat option (-r) and the data values of the velocities. The data arguments are actually in YAML syntax, which is described in the YAML Command Line documentation at http://wiki.ros.org/ROS/YAMLCommandLine.

Note

The ROS and Ubuntu command-line commands have tab-completion capability. For the previous rostopic pub command for example, much typing can be avoided by typing part of the command and hitting the Tab key. In particular, typing out the command up to the data values and hitting the Tab key will yield the format of the data that can be filled in by backspacing and entering the appropriate values.

Move the turtle using the keyboard or joystick

After the turtle screen is visible, type the command:

$ rosrun turtlesim turtle_teleop_key

activates the keyboard control of the turtle with this output:

        
Reading from keyboard
---------------------------
Use arrow keys to move the turtle.
Up arrow       Turtle up
Down arrow     Turtle down
Right arrow    Rotate CW
Left  arrow    Rotate CCW

In Chapter 8, Controlling Your Robots with External Devices in the section Controlling Turtlesim with a custom game controller interface, code is given to allow control of the turtle with a game controller (joystick).

Parameter Server of Turtlesim

The Parameter Server maintains a dictionary of the parameters. Thus, the /turtlesim node can read and write parameters held by the Parameter Server.

rosparam help

Use the help option to determine the form of the rosparam command:

$ rosparam help

Output of the preceding command is as follows:

rosparam is a command-line tool for getting, setting, and deleting parameters from the ROS Parameter Server.   Commands:
rosparam set       set parameter
rosparam get       get parameter
rosparam list      list parameter names
<Edited>

rosparam list for the /turtlesim node

To list the parameters for the /turtlesim node, we will use the following command:

$ rosparam list

Output of the preceding command is as follows:

/background_b
/background_g
/background_r
/rosdistro
/roslaunch/uris/host_d158_45929__34807
/rosversion
/run_id

Note that the last four parameters were created by invoking the Master with the roscore command, as discussed previously. Also, the list defines the characteristics of the parameter but not the data value.

Change parameters for the color of the turtle's background

To change the background color parameters for turtlesim, let's change the turtle's background to red. To do this, make the blue and green data values equal to zero and saturate red = 255 using the rosparam set command. Note that the clear option from rosservice must be executed before the screen changes color.

rosparam get

The default turtle screen is blue. You can use rosparam get / to show the data contents of the entire Parameter Server:

$ rosparam get /

Output of the preceding command is as follows:

background_b: 255
background_g: 86
background_r: 69
rosdistro: 'kinetic

  '
roslaunch:
uris: {host_d158_45929__34807: 'http://D158-45929:34807/'}
rosversion: '1.12.7

  '
run_id: 9c3776b4-09cd-11e7-bb39-1866da2351d7
rosparam set

You can change the colors of the turtle's screen to a full red background using the rosparam set commands:

$ rosparam set background_b 0
$ rosparam set background_g 0
$ rosparam set background_r 255
$ rosservice call /clear

You will see a red background on the turtle screen. To check the numerical results, use the rosparam get / command.

ROS services to move turtle

Another capability of nodes is to provide what in ROS terms is called a service. This feature is used when a node requests information from another node. Thus, there is a two-way communication between the nodes.

You can check the turtle's pose using the /turtle1/pose topic and the message type, /turtlesim/Pose. Carefully note the different notations and meanings. To determine the message type, run the following command:

$ rostopic type /turtle1/pose

The output is as follows:

turtlesim/Pose

To determine the format and meaning of the fields in the message, type:

$ rosmsg show turtlesim/Pose

The output is as follows:

float32 x
float32 y
float32 theta
float32 linear_velocity
float32 angular_velocity

We can find the turtle's position, orientation in angle (theta), and its velocity using the rostopic echo command:

$ rostopic echo /turtle1/pose

The output is as follows:

x: 5.544444561
y: 5.544444561
theta: 0.0
linear_velocity: 0.0
angular_velocity: 0.0

This command outputs the result continuously and is stopped by pressing the Ctrl + C keys. The result will show that the turtle is at the center of its screen with no rotation at angle zero and no movement since the velocities are zero.

rosservice call

The turtle can be moved using the rosservice teleport option. The format of the command is rosservice call <service name><service arguments>. The arguments here will be the turtle's position and orientation as x, y, and theta. The turtle is moved to position [1, 1] with theta = 0 by running the following command:

$ rosservice call /turtle1/teleport_absolute 1 1 0

The result can be seen in the following screenshot:

turtle after an absolute move

The relative teleport option moves the turtle with respect to its present position. The arguments are [linear distance, angle]. Here, the rotation angle is zero. The command for relative movement is as follows:

$ rosservice call /turtle1/teleport_relative 1 0

Your turtle should now move to x=2 and y=1.

 

ROS commands summary


If you are communicating with ROS via the terminal window, it is possible to issue commands to ROS to explore or control nodes in a package from the command prompt, as listed in the following table:

Command

Action

Example usage and subcommand examples

roscore

Starts the Master

$ roscore

rosrun

Runs an executable program and creates nodes

$ rosrun [package name] [executable name]

rosnode

Shows information about nodes and lists the active nodes

$ rosnode info [node name]

$ rosnode<subcommand>

Subcommand: list

rostopic

Shows information about ROS topics

$ rostopic<subcommand><topic name>

Subcommands: echo, info, and type

rosmsg

Shows information about the message types

$ rosmsg<subcommand> [package name]/ [message type]

Subcommands: show, type, and list

rosservice

Displays the runtime information about various services and allows the display of messages being sent to a topic

$ rosservice<subcommand> [service name]

Subcommands: args, call, find, info, list, and type

rosparam

Used to get and set parameters (data) used by nodes

$ rosparam<subcommand> [parameter]

Subcommands: get, set, list, and delete

The website (http://wiki.ros.org/ROS/CommandLineTools) describes many ROS commands. The table lists some important ones. However, these examples only cover a few of the possible variations of the commands.

 

Summary


In this chapter, you first learned how to install and launch ROS. We discussed the ROS architecture and ROS packages, nodes, topics, messages, and services. To apply the knowledge, we used the turtlesim simulator was used to illustrate many ROS commands. For additional control of the turtlesim turtle, see Chapter 8, Controlling Your Robots with External Devices. There the turtle is controlled with a custom game controller.

In Chapter 2, Creating Your First Two-Wheeled ROS Robot (in Simulation), we will show you how to build a robot model that ROS uses to display the robot and allows you to control it in a simulation. The chapter introduces the visualization tool called rviz to display the robot and the simulation tool Gazebo that includes the physics of the robot as you move it around in a simulated environment.

About the Authors

  • Carol Fairchild

    Carol Fairchild is the owner and principal engineer of Fairchild Robotics, a robotics development and integration company. She is a researcher at Baxter's Lab at the University of Houston–Clear Lake (UHCL) and a member of the adjunct faculty. Her research involves the use of Baxter for expanded applications. Ms. Fairchild has been involved in many aspects of robotics from her earliest days of building her first robot, a Heathkit Hero. She has an MS in computer engineering from UHCL and a BS in engineering technology from Texas A&M. Ms. Fairchild has taught middle-school robotics, coached FLL, and volunteered for FIRST Robotics in Houston.

    Browse publications by this author
  • Dr. Thomas L. Harman

    Dr. Thomas L. Harman is the chair of the engineering division at UHCL. His research interests are control systems and applications of robotics and microprocessors. Several of his research papers with colleagues involve robotic and laser applications in medicine. In 2005, he was selected as the UHCL Distinguished Professor. He has been a judge and safety advisor for the FIRST robotic contests in Houston. Dr. Harman has authored or coauthored 18 books on subjects including microprocessors, MATLAB and Simulink applications, and the National Electrical Code. His laboratory at UHCL has a Baxter two-armed robot and several TurtleBots as well as other robots.

    Browse publications by this author

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