41,99 €
Mehr erfahren.
- Herausgeber: Packt Publishing
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
Bring life to your robot using ROS robotic applications
About This Book
- This book will help you boost your knowledge of ROS and give you advanced practical experience you can apply to your ROS robot platforms
- This is the only book that offers you step-by-step instructions to solidify your ROS understanding and gain experience using ROS tools
- From eminent authors, this book offers you a plethora of fun-filled examples to make your own quadcopter, turtlebot, and two-armed robots
Who This Book Is For
If you are a robotics developer, whether a hobbyist, researchers or professional, and are interested in learning about ROS through a hands-on approach, then this book is for you. You are encouraged to have a working knowledge of GNU/Linux systems and Python.
What You Will Learn
- Get to know the fundamentals of ROS and apply its concepts to real robot examples
- Control a mobile robot to navigate autonomously in an environment
- Model your robot designs using URDF and Xacro, and operate them in a ROS Gazebo simulation
- Control a 7 degree-of-freedom robot arm for visual servoing
- Fly a quadcopter to autonomous waypoints
- Gain working knowledge of ROS tools such as Gazebo, rviz, rqt, and Move-It
- Control robots with mobile devices and controller boards
In Detail
The visionaries who created ROS developed a framework for robotics centered on the commonality of robotic systems and exploited this commonality in ROS to expedite the development of future robotic systems.
From the fundamental concepts to advanced practical experience, this book will provide you with an incremental knowledge of the ROS framework, the backbone of the robotics evolution. ROS standardizes many layers of robotics functionality from low-level device drivers to process control to message passing to software package management. This book provides step-by-step examples of mobile, armed, and flying robots, describing the ROS implementation as the basic model for other robots of these types. By controlling these robots, whether in simulation or in reality, you will use ROS to drive, move, and fly robots using ROS control.
Style and approach
This is an easy-to-follow guide with hands-on examples of ROS robots, both real and in simulation.
Sie lesen das E-Book in den Legimi-Apps auf:
Seitenzahl: 477
Veröffentlichungsjahr: 2016
Ähnliche
Table of Contents
ROS Robotics By Example
ROS Robotics By Example
Copyright © 2016 Packt Publishing
All rights reserved. No part of this book may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, without the prior written permission of the publisher, except in the case of brief quotations embedded in critical articles or reviews.
Every effort has been made in the preparation of this book to ensure the accuracy of the information presented. However, the information contained in this book is sold without warranty, either express or implied. Neither the authors, nor Packt Publishing, and its dealers and distributors will be held liable for any damages caused or alleged to be caused directly or indirectly by this book.
Packt Publishing has endeavored to provide trademark information about all of the companies and products mentioned in this book by the appropriate use of capitals. However, Packt Publishing cannot guarantee the accuracy of this information.
First published: June 2016
Production reference: 1240616
Published by Packt Publishing Ltd.
Livery Place
35 Livery Street
Birmingham B3 2PB, UK.
ISBN 978-1-78217-519-3
www.packtpub.com
Credits
Authors
Carol Fairchild
Dr. Thomas L. Harman
Reviewer
Shweta Gupte
Commissioning Editor
Dipika Gaonkar
Acquisition Editor
Vivek Anantharaman
Prachi Bisht
Content Development Editor
Mehvash Fatima
Technical Editor
Devesh Chugh
Copy Editors
Roshni Banerjee
Gladson Monteiro
Rashmi Sawant
Project Coordinator
Kinjal Bari
Proofreader
Safis Editing
Indexer
Hemangini Bari
Graphics
Kirk D'Penha
Production Coordinator
Shantanu N. Zagade
Cover Work
Shantanu N. Zagade
About the Authors
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.
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.
About the Reviewer
Shweta Gupte is an embedded systems engineer with 4 years of experience in the robotics field. She has worked with robots both at the software level and at the low-level hardware interface. She has experience in working with embedded system design, RTOS, microcontroller programming and low-level robot interfaces , motion planning, and computer vision.
Shweta has experience programming in C, embedded C, Java, MATLAB, and microcontroller assembly language. She has worked with low-level controllers, including the Renesas RX62, Renesas QSK, Arduino, PIC, and Rasberry Pi.
Currently, she is working at Mathworks to develop MATLAB and Simulink products for robotics and embedded targets, including developing automated test beds and hardware testing. She has a master's degree in electrical engineering and a bachelor's degree in electronics and telecommunication. Her interests include playing with low-cost microcontroller-based robots and quadrotors. She likes to spend her spare time rock climbing and fostering rescued dogs.
www.PacktPub.com
eBooks, discount offers, and more
Did you know that Packt offers eBook versions of every book published, with PDF and ePub files available? You can upgrade to the eBook version at www.PacktPub.com and as a print book customer, you are entitled to a discount on the eBook copy. Get in touch with us at <[email protected]> for more details.
At www.PacktPub.com, you can also read a collection of free technical articles, sign up for a range of free newsletters and receive exclusive discounts and offers on Packt books and eBooks.
https://www2.packtpub.com/books/subscription/packtlib
Do you need instant solutions to your IT questions? PacktLib is Packt's online digital book library. Here, you can search, access, and read Packt's entire library of books.
Why subscribe?
Preface
Learning ROS and working with ROS robots such as Baxter and TurtleBot is the beginning of a big adventure. The features and benefits of ROS are substantial but the learning curve is steep. Through trial and error, we have foraged a path through many of the ROS applications. In this book, we hope to present to you the best of our knowledge of ROS and provide you with detailed step-by-step instructions for your journey. Our approach centers on using the ROS robots that are featured—TurtleBot, Baxter, Crazyflie, and Bebop as well as simulated robots—Turtlesim and Hector.
This book provides introductory information as well as advanced applications featuring these ROS robots. The chapters begin with the basics of setting up your computer and loading ROS and the packages for ROS robots and tools. Straightforward instructions are provided with troubleshooting steps when the desired results are not achieved. The building blocks of ROS are described first in the simulation, Turtlesim, and then on each of the featured robots. Starting with basic ROS commands, the ROS packages, nodes, topics, and messages are explored to gain an overall knowledge of these ROS robotic systems. Technical information on these example robots is provided to describe the robot's full capabilities.
ROS encompasses a full spectrum of software concepts, implementation, and tools that attempt to provide a homogeneous view of the complex systems and software integration required in robotics. Extensive libraries of sensor and actuator drivers and interfaces are already in place as well as the latest and most efficient algorithms. What ROS doesn't provide directly is imported from other prevailing open source projects such as OpenCV. ROS also possesses a spectrum of time-saving tools to control, monitor, and debug robot applications—rqt, rviz, Gazebo, dynamic reconfigure, and MoveIt to name a few.
In the pages that follow, each of these areas will be incrementally introduced to you as part of the robot examples. With TurtleBot, the subjects of navigation and mapping are explored. Using Baxter, joint control and path planning are described for your understanding. Simple Python scripts are included to provide examples of implementing ROS elements for many of these robots. These robots are all available in simulation to accomplish the exercises in this book. Furthermore, instructions are provided for you to build and control your own robot models in simulation.
The power of ROS, the variety of robots using ROS, and the diversity and support of the widespread ROS community make this adventure worthwhile. Extensive online tutorials, wiki instructions, forums, tips, and tricks are available for ROS. So dive into the pages of this book to begin your adventure with ROS robotics!
What this book covers
Chapter 1, Getting Started with ROS, explains the advantages of learning ROS and highlights the spectrum of robots currently using ROS. Instructions to install and launch ROS on a computer running a Ubuntu operating system are provided. An overview of the ROS architecture is given and its components are described. The Turtlesim simulation is introduced and used to provide a deeper understanding of how the components of ROS work and ROS commands.
Chapter 2, Creating Your First Two-Wheeled ROS Robot (in Simulation), introduces you to the ROS simulation environment of Gazebo. We will lead you through the steps to create your first robot simulation (a two-wheeled differential-drive base) and teach the structure of the Universal Robotic Description Format. The use of the ROS tools, rviz and Gazebo, are detailed to enable you to display your robot and interact with it.
Chapter 3, Driving Around with TurtleBot, introduces you to a real ROS robot, TurtleBot. This mobile base robot can be used in the simulation environment of Gazebo if you do not own one. ROS commands and Python scripts are used to control the TurtleBot through a variety of methods. The ROS tool, rqt, is introduced and subsets of its plugins are used to control TurtleBot and monitor its sensor data.
Chapter 4, Navigating the World with TurtleBot, explores visual sensors and the ability for a robot to map its environment. The 3D sensor options for TurtleBot's vision system are described and their setup and operation using ROS enables TurtleBot to navigate autonomously. The knowledge of Simultaneous Localization and Mapping techniques is applied in combination with TurtleBot's navigation stack to move about in the mapped environment.
Chapter 5, Creating Your First Robot Arm (in Simulation), provides a gentle introduction to the complexity of robotic arms. A simulated robot arm is designed and built using the macro language of Xacro. Controllers for the arm are created to operate the arm in Gazebo. Through developing the controllers for this arm, an insight into the mechanics and physics of a simple robot arm is offered.
Chapter 6, Wobbling Robot Arms Using Joint Control, takes a deeper look at the intricacies of controlling robotic arms. Our second ROS robot, the "state-of-the-art" Baxter robot, is introduced. Baxter has two 7 degree-of-freedom arms and a number of other sensors. Baxter Simulator is available as open source software to use for the instructions in this chapter. Examples are provided for the control of Baxter's arms using position, velocity, and torque modes with control for both forward and inverse kinematics. The ROS tool, MoveIt, is introduced for motion planning in simulation and execution on either a real or simulated Baxter.
Chapter 7, Making a Robot Fly, describes a small but growing area of ROS robotics—unmanned air vehicles. This chapter focuses on quadrotors, and an understanding of quadrotor hardware and flight control is provided. Instructions to download and control the simulated quadrotor Hector are supplied. With skills from flying a simulated quadrotor, you can move on to control a real Bitcraze Crazyflie or Parrot Bebop. Quadrotor control is via teleoperation or ROS topic/message commands.
Chapter 8, Controlling Your Robots with External Devices, presents a number of peripheral devices you can use to control a ROS robot. Joystick controllers, controller boards (Arduino and Raspberry Pi), and mobile devices all have ROS interfaces that can be integrated with your robot to provide external control.
Chapter 9, Flying a Mission with Crazyflie, incorporates many of the ROS components and concepts presented in this book into a challenging mission of autonomous flight. The mission involves the Crazyflie quadrotor flying to a "remote" target all mapped through a Kinect 3D sensor. This mission uses ROS message communication and co-ordinate transforms to employ the Kinect's view of the quadrotor and target to orchestrate the flight. Flight control software for the Crazyflie using PID control is described and provided as part of the mission software.
Chapter 10, Extending Your ROS Abilities, provides a few last ROS capabilities to stimulate the further learning of advanced ROS robotic applications. The ability to control a robot with voice commands, the ability for a robot to detect and recognize faces as well as the ability for a robot to speak are all presented.
What you need for this book
The format of this book is intended for you to follow along and perform the instructions as the information is provided. You will need a computer with Ubuntu 14.04 (Trusty Tahr) installed. Other Ubuntu versions and Linux distributions may work as well as Mac OS X, Android, and Windows but documentation for these versions will need to reference the ROS wiki (http://wiki.ros.org/Distributions).
The version of ROS that this book was written around is Indigo Igloo, which is the current release recommended for stability. Its end of life is targeted for April 2019. Other versions of ROS may be used but are untested.
All the software used in this book is open source and freely available for download and use. Instructions to download the software are found in the chapter where the software is introduced. In Chapter 1, Getting Started with ROS, instructions are given to download and set up the ROS software environment.
Our preferred method to download software is the use of Debian packages. Where no Debian packages exist, we refer to downloading the software from repositories such as GitHub.
Gazebo simulation performs intensive graphics processing and the use of a dedicated graphics card is advised but not required.
Peripheral devices such as 3D sensors, Xbox, or PS3 controllers, Arduino or Raspberry Pi controller boards, and Android mobile devices are optional equipment.
Who this book is for
If you are a robotics developer, whether a hobbyist, researcher, or professional, and are interested in learning about ROS through a hands-on approach, then this book is for you. You are encouraged to have a working knowledge of GNU/Linux systems and Python.
Conventions
In this book, you will find a number of text styles that distinguish between different kinds of information. Here are some examples of these styles and an explanation of their meaning.
Code words in text, directory names, filenames, file extensions, and pathnames are shown as follows: "The terminal commands rostopic and rosnode have a number of options".
A block of code is set as follows:
To avoid repeating previous code blocks but provide placement of new code blocks, previous code left for reference is abbreviated and new code is highlighted as follows:
Any command-line input is written as follows:
Output from command is written as:
[ INFO] [1427212356.117628994]: Starting turtlesim with node name /turtlesim
New terms and important words are shown in bold.
Words that you see on the screen, for example, in menus or dialog boxes, appear in the text like this: "By clicking the Add button on the Displays panel."
URL references are shown as: http://www.ros.org/about-ros/
Note
Warnings or important notes appear in a box like this.
Tip
Tips and tricks appear like this.
Reader feedback
Feedback from our readers is always welcome. Let us know what you think about this book—what you liked or disliked. Reader feedback is important for us as it helps us develop titles that you will really find useful and enjoyable.
To send us general feedback, simply e-mail <[email protected]>, and mention the book's title in the subject of your message.
If there is a topic that you have expertise in and you are interested in either writing or contributing to a book, see our author guide at www.packtpub.com/authors.
Customer support
Now that you are the proud owner of a Packt book, we have a number of things to help you to get the most from your purchase.
Downloading the example code
You can download the example code files for this book from your account at http://www.packtpub.com. If you purchased this book elsewhere, you can visit http://www.packtpub.com/support and register to have the files e-mailed directly to you.
You can download the code files by following these steps:
Once the file is downloaded, please make sure that you unzip or extract the folder using the latest version of:
The code bundle for the book is also hosted on GitHub at https://github.com/PacktPublishing/ROS-Robotics-By-Example. We also have other code bundles from our rich catalog of books and videos available at https://github.com/PacktPublishing/. Check them out!
Downloading the color images of this book
We also provide you with a PDF file that has color images of the screenshots/diagrams used in this book. The color images will help you better understand the changes in the output. You can download this file from http://www.packtpub.com/sites/default/files/downloads/ROSRoboticsByExample_ColorImages.pdf.
Errata
Although we have taken every care to ensure the accuracy of our content, mistakes do happen. If you find a mistake in one of our books—maybe a mistake in the text or the code—we would be grateful if you could report this to us. By doing so, you can save other readers from frustration and help us improve subsequent versions of this book. If you find any errata, please report them by visiting http://www.packtpub.com/submit-errata, selecting your book, clicking on the Errata Submission Form link, and entering the details of your errata. Once your errata are verified, your submission will be accepted and the errata will be uploaded to our website or added to any list of existing errata under the Errata section of that title.
To view the previously submitted errata, go to https://www.packtpub.com/books/content/support and enter the name of the book in the search field. The required information will appear under the Errata section.
Piracy
Piracy of copyrighted material on the Internet is an ongoing problem across all media. At Packt, we take the protection of our copyright and licenses very seriously. If you come across any illegal copies of our works in any form on the Internet, please provide us with the location address or website name immediately so that we can pursue a remedy.
Please contact us at <[email protected]> with a link to the suspected pirated material.
We appreciate your help in protecting our authors and our ability to bring you valuable content.
Questions
If you have a problem with any aspect of this book, you can contact us at <[email protected]>, and we will do our best to address the problem.
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 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 is 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.
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/
This book is written using Ubuntu 14.04 as the operating system andROS Indigo as the version of the ROS distribution. Always make sure that you check for any updates for the Ubuntu or ROS versions you are using.
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:
The images of these robots are in the following figures:
TurtleBot 2
Baxter in the authors' laboratory
Of course, not everyone has the opportunity to use real robots such as Baxter. 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 Saucy 13.10 or Trusty 14.04 installed. The examples in this book have been developed using ROS Indigo 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 method is the most efficient and preferred way to install ROS.
If you wish to install the ROS Indigo source code and build the software, refer to instructions at http://wiki.ros.org/indigo/Installation/Source. The instructions presented here to install ROS Indigo with Debian packages can also be found at http://wiki.ros.org/indigo/Installation/Ubuntu.
If you have any problems installing ROS, refer to this site and the ROS forum at http://answers.ros.org.
Configuring your Ubuntu repositories
To begin, configure the Ubuntu repositories to allow restricted, universe, and multiverse. Click on the Ubuntu Software Center icon in the launch menu on the left side of your desktop. From the Software Center's top menu bar, navigate to Edit, then to the drop-down menu, and select Software Sources. On the Software & Updates screen, select checkboxes to match the following screenshot:
Ubuntu Software Center Screen
Setting up your sources.list
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:
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 Indigo, 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 the operating system's trusted software list. Type the following command:
Installing ROS Indigo
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:
Note
Warning: If you are using Ubuntu Trusty14.04.2 and experience dependency issues during the ROS installation, you may have to install some additional system dependencies.
Do not install these packages if you are using 14.04, it will destroy your X server:
Alternatively, try installing just this to fix dependency issues:
For more information on this issue, refer to the following sites:
http://answers.ros.org/question/203610/ubuntu-14042-unmet-dependencies/
https://bugs.launchpad.net/ubuntu/+source/mesa-lts-utopic/+bug/1424059
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:
ROS Indigo is installed on your computer system when the installation 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 command:
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:
Alternately, 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 command:
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:
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 export command in the following form that lists the ROS environment variables:
The output of the preceding command is as follows:
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= "indigo" and the 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) which each 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:
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 command:
The catkin_make command creates the catkin workspaces. 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:
Remember to add this source command to your .bashrc file by typing the following command:
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:
The output of the preceding command should be as follows:
Here, <username> is the name you chose as 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 manifestnamed package.xml that describes the package in theExtensible 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 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:
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:
Tip
There are many options for this command, so piping with the less option shows one screen at a time. For many commands, using the power of Linux to make the output readable is necessary. Use the Q key to quit (exit) the mode.
We will use the turtlesim package for the examples here. To change directories to the location of turtlesim, use the following command:
This yields the location to one of the author's laptops as follows:
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:
Here is the result from the home directory of the author's laptop with ROS installed:
To see the filenames of the images loaded with turtlesim, specify the images directory in the package as follows:
The output of the preceding command is as follows:
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 command:
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:
The preceding command displays the path to the turtlesim directory.
rospack list
Execute the following command:
This lists the ROS package names and their directories on the computer. In the case of the laptop mentioned earlier, there are 225 ROS packages listed!
Tip
If you really want to see all the ROS packages and their locations, use the following command form:
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:
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:
This lists the four 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 and ROS Master
One of the primary purposes of ROS is to facilitate communication between the ROS modules called nodes. These nodes represent the executable code and 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. One node can capture and output camera images, and another node can control a robot's manipulator in response to the camera view. We will see that the turtlesim simulator has two nodes that are created when turtlesim is executed.
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 and Master, so they will be defined later through discussions and examples.
ROS nodes
Basically, nodes are processes that perform some action. The nodes themselves are really software modules 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 an independent module that interacts with other nodes using the ROS communication capability.
The nodes can be created in various ways. From a terminal window a node can be created directly by typing a command after the command prompt, as shown in the examples to follow. Alternatively, nodes can be created as part of a program written in Python or C++. In this book, we will use both the ROS commands in a terminal window and Python programs to create nodes.
Nodes can publish and nodes can subscribe
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 series of nodes to accomplish the specific tasks.
A node can independently execute code to perform its task but can communicate with other nodes by sending or receiving messages. The messages can consist of data, or commands, or other information necessary for the application.
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, which will be explained later. 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. A number of examples later in the book will demonstrate this functionality.
ROS Master
The ROS nodes are typically small and independent programs that can run concurrently on several systems. Communication is established between the nodes by the ROS Master. The ROS Master provides naming and registration services to the nodes in the ROS system. It tracks publishers and subscribers to the topics. 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 others' existence. A simple example is shown at: http://wiki.ros.org/Master.
Invoking the ROS Master using roscore
roscore is a collection of nodes and programs that you must have running for ROS nodes to communicate. After it is launched, roscore will start the following:
The roscore command starts ROS and 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:
The output of the preceding command is as follows:
In the preceding screen output, you will see information about the computer, parameters that list the name (indigo) 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 laptop and is 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:
Indigo is the ROS distribution release that we are using. As Indigo is changed or packages are added, numbered versions such as 1.11.13 are released. Issuing the roscore command is a way to determine the release 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:
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:
The output of the preceding command is as follows:
In the second terminal window, list the active topics by typing:
The output of the preceding command is as follows:
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 some 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.
Tip
Most of the ROS commands have help screens that are usually helpful. Type the following command for the command options:
For more detailed usage, use the subcommand name, for example:
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.
