IntroductionToROS

ROS Tutorial

Getting Started with ROS

Benjamin Cohen University of Pennsylvania 4/9/2010

Disclaimer! Achtung! 



I am by no means a ROS wizard! I have a lot of experience. That's about it. I can rant on and on about ROS for days. This is a 1.5 hour summary and so I'm leaving some useful things out...

2

Outline 



Part 1: Introduction to ROS 

General Description



Important concepts



Simulation



Debugging Tools

Part 2: Create a node 

(if you are ROS-curious) ROS-curious)

(if you are planning on using ROS immediatly)

 what your basic code should look like



how to build your project



how to run the simulator



bring up the visualizer

3

What is ROS? 





ROS & ROS-Pkgs are a creation of Willow Garage, Inc. ROS is a 'meta-operating system' that provides a message passing system between different processes across a network (Inter-Process Communication (IPC)). Why do I need some sort of IPC? Why not run a big 'for loop' on my robot? 



For loop: 

Simple design & implementation, only uses one core



Works on basic platforms with minimal computing power



Use multiple cores, multiple computers



Modular



ex. plan on an old map while receiving new sensor data



ex. run controller while planning

Parallel:

4

ROS 

Are 'ROS' and 'Gazebo' synonymous? NO 







Linux, MacOS, partial support for Windows

Languages: 



Gazebo is a 3D simulator that originally used 'player' for communication but now uses ROS.

Supported Platforms 



ROS is not a simulator.

C/C++,python,octave,lisp,~java

Suggested Hardware: 

many cores!



nVidia video card for simulation

Network Support: 

TCP/IP

5

High Level Diagram

6

Nodes 









is a process that performs some function. nodes communicate with each other using topics & services. nodes are assigned unique names nodes are intended to be modular and 'operate on the fine-grained scale' a robotic system should be made up of many many nodes – it provides some fault tolerance when an isolated node crashes 7

Nodes 

below is a diagram displaying the breakdown of the functionality of a navigation system into nodes

8

Messages 

nodes communicate by passing around messages



a message is a data structure with typed fields







many standard messages already exist, new messages can be defined  with a simple text file a message can be comprised c omprised of other messages ROS generates a data structure for new message that contains many standard stl type of functions (size(), resize(),etc.) mapping_msgs/CollisionMap CollisionMap.msg #header for interpreting box positions Header header #boxes for use in collision testing OrientedBoundingBox[] boxes

Header header uint32 seq time stamp string frame_id OrientedBoundingBox[] boxes geometry_msgs/Point32 center float32 x float32 y float32 z geometry_msgs/Point32 extents float32 x float32 y float32 z float32 angle

9

Params 



a parameter server that stores parameter strings & value pairs  which are normally passed as input to a program some params can be viewed by other nodes 

great way to pass around a name of a topic or other info multiple nodes might need to know



can put XML & YAML files onto server



ex. ”shoulder_pan_max_vel” → '0.7'



ex. ”camera_resolution_640_480” → 'true' (bool)



ex. ”type_of_planner” → ”ARA” (string)

(double)

10

Topics 

Method of passing messages



Node 'N' advertises that it is publishing Topic 'A'



Topic B subscribes to Topic 'A' (so (so do topics 'C','D'...) 'C','D'...)



Whenever 'A' publishes message, all of its subscribers receive it and their callback functions are executed



Asynchronous



Many-to-many



Not appropriate for request/reply interaction



Callback function is multi-threaded



ex. 'base_scan' is a publisher that publishes laser scans at 10 hz. Global planner, Controller & localization nodes subscribe to 11 base_scan.

Services 







Request/Reply interaction between nodes Node 'A' advertises service 'S', Node 'B' sends request and waits for response. Node 'A' sends back a reply No topic callbacks are issued during service call (service request is blocking)



one-to-one



Used as a remote procedure call



ex. request a motion plan

12

Launch Nodes 

A launch file is a convenient way to bringup many different nodes at once



Written in XML



Asynchronous execution



Can put parameters on server

ld.launch"/> unch"/> ht_arm.launch"/>

13

ROS Master 





The 'roscore' is the master process that establishes connections between nodes (provides lookup info similar to DNS) Node 'N' wants to subscribe to topic 'T'. First a request for a connection is made, then the master establishes a connection over an agreed connection protocol between the nodes. ** Messages are not passed through the master to the receiving node. Nodes connect to other nodes directly **



Stores topics & services registration info for ROS nodes



Updates nodes about other nodes' statuses



Uses TCP/IP, many computers can connect to one master



Environment Variable ”ROS_MASTER_URI” ”ROS_MASTER_URI” defines the address & port number of ROS master

14

Packages 

Willow Garage created 'ROS' as well as ~600 rospackages that are available for your use. 



ex. laser pipeline, motor controllers, localization, localization, SLAM, forward kinematics, Hokuyo driver...

A folder that contains your code, build files, launch files, etc.



Should only contain code that is related



Can contain any number of nodes





'manifest.xml' – lists the ROS dependencies & system deps (some pkgs have up to 150 deps!) IMPORTANT: A package will not be found by ROS if it is not in your 'ROS_PACKAGE_PATH' 15

Build System 









Need to specify in 'Cmakelists.txt' how to build the source code of a package No need for a handwritten Makefile Compile just package itself OR compile package + all dependencies 

'rosmake': compile pkg + deps



'make': compile pkg

Can download system dependencies if not installed Compile multiple pkgs in parallel. ROS resolves deps first. 'ROS_PARALLEL_JOBS' = # of cores 16

Command Line Tools General Use 

Many commands support ROS tab-completion!



Use '--help' to learn about numerous options

Command

Description

roscd

Change directory to specified ros-package

rosls

List contents of a ros-package

rosmake

Build all of the ros packages that a package depends on '--pre-clean': first run 'make clean' in each package then run 'make' '--rosdep-install': '--rosdep-install': install system dependencies first then compile all

●

●

roslaunch

Launch a '.launch' file (looks in 'launch' directory for file)

roscreate-pkg

Create a ro ros-package State the name & dependencies Automatically Automatically generates the directory, manifest.xml, manifest.xml, Makefile, etc. Can always change anything later!

●

●

●

rosdep

State dependencies of a package, Find out what depends on a specific package, Capable of output in tree format

roscp

Copy files from one package to another

rosed

Bring up your default text editor and edit file ”ros_package filename.txt” filename.txt”

t

t

E

t

i

t

t fil

17

Command Line Tools Debugging



rxgraph: displays a visualization of the ROS graph – the ROS nodes that are currently running and the topics that connect them

18

Command Line Tools Debugging 

rxplot: plot data from one or more ROS topic fields that are currently being published.

19

Command Line Tools Debugging 



rxconsole: brings up a viewer that displays any messages being published to 'rosout' can display the filename & line number of a message 

useful for debugging code you are unfamiliar unfamiliar with

20

Command Line Tools Debugging Command

Description

*** rostopic rostopic *** displays displays a lot lot of informati information on about about a topic topic lists all topics available (list') echo messages of a specified topic ('echo') get publishing publishing rate of a topic ('hz') get bandwidth used by a topic ('bw') get info about a topic (subscribers, node) ('info') publish data to a topic ('pub') get the type of message of the topic ('type') ●

●

●

●

●

●

●

roswtf

outputs po possible er errors/warnings wi with th the cu current ro roscore ba based on on a set of defined rules

rosnode

displays a list of of no nodes, ca can ge get de detailed in information ab about th them

rosservice

same as rostopic but for services

rostopic echo

21

Command Line Tools Documentation 



rosmsg: get field names and field types of a message rossrv: get the field names and field types of a service request/reply message

rosmsg show

22

Simulation: Gazebo 



open source 3D simulator has been around for a while but has now been converted to using ROS for communication, communication, is a ros package offered by WG



uses ODE library for ridgid body dynamics



supports multiple robots







good for experiments that include motion planning, control & perception of ground vehicles less good for experiments that require precise contact between robot & environment such as manipulation  world and objects in world are described in an XML '.world' file



offers fake localization & fake sensors



fully integrated into ROS



 we will discuss in more in the more technical half of the presentation

23

Simulation: Gazebo Screenshots

24

Simulation: Stage 

open source 2D simulator



great for multi-agent experiments



great for use with an integrated video card & slower machines



fully integrated into ROS

25

Visualizers: rviz 











3D visualizer -

not a

simulator

capable of displaying all 'visualizable' messages without extra coding can change reference frame of visualization supports 'visualization markers', user created shapes to represent 'unvisualizable' data - great for debugging most vital ROS related tool available. note: 'nav_view' is a 2D version of rviz (nav_view is to stage as rviz is to gazebo)

26

Visualizers: rviz

gazebo simulation

Visualization markers accelerate debugging debugging!! red: padding around obstacles orange: obstacle cuboids in occupancy grid blue cubes: poses of randomly generated high-level states small green cubes: poses of low-level states expanded large green cubes: poses of high-level states expanded

rviz visualization

27

Other 

tf library 





'transform' library can fetch a transform from one frame to another frame at a specific point in time



uses description of robot, the state of joints



can also transform from one point in time to another point in time

ROS Play/Record 

can record any information passed over ROS to a 'bag' file



the file can be played back later



ex. log sensor data for later analysis



ex. great for debugging hard to recreate situations

glc-record 



record gazebo & rviz windows at the same time to create a multi-window video can easily record many OpenGL apps simultaneously.

28

Documentation 



 www.ros.org 

documentation has been improving



there are many tutorials available

many schools have made public their own rospackage repository (there is a list at https://code.ros.org/gf/) 

drivers for lots of common hardware are available

29

Summary 

ROS 



easy build system



easy to navigate in huge code library





a message passing system between processes on a single computer or on a network

valuable debugging tools are critical to hunting down bugs in complicated systems

ROS-Packages 



Simulators: Gazebo & Stage many other packages are available that perform functions you might want

30

Part 2 



In part 2 we'll create a new ros package that contains a basic node Feel free to run if you're ROSed-out

31

Part 2: Create a simple node 



create a new package  write your code (usually as a class)



create a main function that instantiates class



list the dependencies



describe how it should be built



build it



create a launch file



use rviz to inspect it's working correctly 32

create a new package roscreate-pkg meam620_tutorial

33

create a new package (cont'd) 

Does 'rospack' find my new package? 



test: open a new terminal, then run 'roscd meam620_tutorial' if it can't be found then make sure you created the package in your ROS_PACKAGE_PATH 



run 'echo $ROS_PACKAGE_PATH' to check the path move the folder to within the path

34

start coding 

Why contain your node's functionality in a class? 





you will have many shared variables that you don't  want to pass around as parameters between functions (publishers,subscribers,transforms, (publishers,subscribers,transforms, node handles)  works best with specific pieces of functionality  written as classes so that they can build on top of each other nicely and be reused have a main function that instantiates the class, and then calls ros::spin() 35

using messages 



use 'rosmsg show …' to remind yourself of field names and types (or go to ros.org) remember to include the message header file  with the correct case 





package name: all lowercase message name: first letter of each word is capitalized

36

launching stuff 





running many nodes requires opening many terminals launching many nodes from one launch file reduces the number of terminals you will have to open but is not a good idea to include a node that you are debugging in that file try not to copy and paste exact duplicates of launch files, but rather 'include' another launch file and it will be launched from its location 37

Play! 

At this point we can continue from the terminal...

38