What is ROS? R obot O perating S ystem ROS essentially provides two things: – A set of tools that...

What is ROS?

• Robot Operating System• ROS essentially provides two things:

– A set of tools that are generally useful for controlling robots

• E.g. Interfaces to sensors, general-purpose algorithms, etc.

– A communication framework to allow different pieces of your robot brain to talk to one another

Intro to ROS Nodes

cam_data publisher

cam_data subscriber

cam_data subscriber

ROS$ roscore

... logging to ~/.ros/log/9cf88ce4-b14d-11df-8a75-00251148e8cf/roslaunch-machine_name-13039.log

Checking log directory for disk usage. This may take awhile.

Press Ctrl-C to interrupt

Done checking log file disk usage. Usage is <1GB.

… blah blah blah

Master

rosout

$ rosrun package1 first-nodeNote: run this in a

new terminal window

ROS

first-node

Master

Advertise topic1

rosout

$ rosrun package2 second-nodeNote: run this in a

new terminal window

ROS

first-node

Master

rosout second-node

Subscribe to topic1

ROS$ rosrun package2 second-node

Note: run this in a new terminal

window

first-node

Master

rosout second-node

Master connects topic1 publisher to topic1 subscriber

ROS$ rosnode list

/rosout/first-node/second-node

first-node

Master

rosout second-node

topic1

ROS

first-node

Master

rosout second-node

$ rostopic list

/rosout/rosout_agg/topic1

topic1

ROS commands• roscore : Run this first; starts the Master and some other

essential ROS stuff• rosrun [package-name] [node-name] : Use this to start a ROS

node• rosnode :

– rosnode list : List all currently running nodes– rosnode info [node-name] : Get more info about a particular

node– rosnode -h : Learn about more things rosnode can do

• rostopic :– rostopic list : List all topics currently published– rostopic echo : See the messages being published to a

particular topic– rostopic -h : Learn about more things rostopic can do

ROS commands• roslaunch [package-name] [launch-file-name] : Reads

a .launch file, which contains instructions for running multiple nodes, and doing fancy things like passing parameters to nodes, renaming topics, etc.

<launch> <node name="listener-1" pkg="rospy_tutorials" type="listener" /> <node name="listener-2" pkg="rospy_tutorials" type="listener" args="-foo arg2" /> <node name="listener-3" pkg="rospy_tutorials" type="listener" respawn="true" /> <group ns="wg2"> <remap from="chatter" to="hello"/> <node pkg="rospy_tutorials" type="listener" name="listener" args="--test" /> <node pkg="rospy_tutorials" type="talker" name="talker"> <param name="talker_1_param" value="a value" /> <remap from="chatter" to="hello-1"/> <env name="ENV_EXAMPLE" value="some value" /> </node> </group></launch>

#include "ros/ros.h" int main (int argc, char **argv){ ros::init(argc, argv, “name"); ros::NodeHandle n;

ros::Rate loop_rate(10); while (ros::ok()) {

ros::spinOnce(); loop_rate.sleep(); } return 0;}

/* Body of event loop goes here */

ROS Skeleton




Line-by-Line Breakdown

• Include the ROS library






• Program execution always starts at “main”

• main gets passed command line arguments (argc, argv), which you are required to pass into ros::init so it can handle them appropriately






• ros:: is a namespace; a collection of related functions, data, etc. that are all packaged together

• Anything in this namespace has to be accessed by prefixing it with ros::

• ros::init (initialize) must be called before any other ROS functions; it expects to be passed main’s command line arguments (argc, argv) and the name of the current node





Line-by-Line Breakdown• ros::NodeHandle is a class

which allows us to communicate with the ROS system

• You must create a NodeHandle object in order to properly initialize and register your ROS node; when the handle goes out of scope at the end of the program, it will automatically cleanup after itself

• In the body of your ROS node, a NodeHandle can also be used to advertise or subscribe to topics







• The main body of a ROS program is a loop

• The loop keeps running as long as this ROS node hasn’t been asked to shutdown (i.e. as long as ros::ok() returns true)

• At a high level, a typical loop might look like this:– Do some work– Publish results– Handle any incoming

messages– Sleep for a little while, then

repeat





• ros::spinOnce() checks to see if there are any new incoming messages from other ROS nodes, and if so, it invokes callbacks to handle them appropriately

• We haven’t subscribed to any topics in this example, but we’ll see shortly how this works





Line-by-Line Breakdown• ros::Rate is essentially a

timer class that helps to keep your main loop running at a reasonable frequency

• We first declare loop_rate(10), which is a 10Hz timer

• At the end of our main loop, we call loop_rate.sleep(); if our loop took less than 0.1s (1/10th of a second), loop_rate.sleep() will wait until the full 0.1s has passed

• This way, our main loop always runs at a (roughly) consistent frequency


Now, let’s subscribe to a topic


ros::Subscriber sub = n.subscribe(“topic1", 1000, callbackFunction);

ros::Rate loop_rate(10); while (ros::ok()) { ros::spinOnce(); loop_rate.sleep(); } return 0;}

ROS Subscriber• Recall, `n` is our NodeHandle• Subscribe to a topic using

n.subscribe(...)– “topic1” is the name of the topic to

subscribe to– 1000 is the number of incoming

messages to hold on to before deleting some (so, if we cannot process messages fast enough, and get more than 1000 messages behind, we will start dropping old messages)

– callbackFunction is the name of the function to invoke when a new message is received

• n.subscribe(…) returns a ros::Subscriber object; as long as we hold on to this object, we will remain subscribed to “topic1”

// somewhere else in the program …

void callbackFunction(const std_msgs::String::ConstPtr& msg){ printf("I heard: [%s]", msg->data.c_str());}

ROS Subscriber

• This function will be invoked whenever a new “topic1” message is processed by ros::spinOnce()

• Note the signature of the function: it returns void, and it accepts a single parameter “msg”, which is a pointer to the message received– The type of “msg” will naturally vary, depending on the type of

messages that you are subscribed to. In our case, messages published to “topic1” are string messages, and have type std_msgs::String

– Note this is not the same thing as a C++ string object; it is a ROS message which contains a C++ string

Common std_msgsMessage Type Include Callback signature Usagestd_msgs::String #include

“std_msgs/String.h”void callback( const::std_msgs::String::ConstPtr & msg)

string data = msg->data

std_msgs::Bool #include “std_msgs/Bool.h”

void callback(const::std_msgs::Bool::ConstPtr & msg)

bool data = msg->data

std_msgs::Int32 #include “std_msgs/Int32.h”

void callback(const::std_msgs::Int32::ConstPtr & msg)

int data = msg->data

std_msgs::Float64

#include “std_msgs/Float64.h”

void callback(const::std_msgs::Float64::ConstPtr & msg)

double data = msg->data

Let’s publish to a topic

#include "ros/ros.h“

int main (int argc, char **argv){ /* … blah blah blah … */

ros::Publisher pub = n.advertise<std_msgs::String> (“topic1”, 1000); ros::Rate loop_rate(10); while (ros::ok()) { std_msgs::String msg; msg.data = “Foobar”; pub.publish(msg);


ROS Publisher• Advertise a topic using

n.advertise(...)– Unlike with subscribers, where the type

of the message can be inferred from the callback function, we need to explicitly tell ROS what type of message to expect using <> triangle brackets

– “topic1” is the name of the topic to advertise

– 1000 is the number of outgoing messages to hold on to before deleting some (so, if we cannot send messages fast enough, and get more than 1000 messages behind, we will start dropping old messages)

• n.advertise(…) returns a ros::Publisher object; we send a message by calling its .publish(…) method and passing in a ROS message of the appropriate type

Okay, we’ve got a complete (trivial) program – let’s build it!

ROS Package

workspace

ROS Package

art

workspace

ROS Package

localplanner

art

artsystem imu

ucontroller

ROS Package

src

localplanner

include bin

CMakeLists.txt manifest.xml

ROS Packagelocalplanner Source files go in

here

src include bin


src include bin

ROS Packagelocalplanner Header files go in

here


src include bin

ROS Packagelocalplanner Executables will

be built here


src include bin

ROS Packagelocalplanner Build instructions

go here


cmake_minimum_required(VERSION 2.4.6)include($ENV{ROS_ROOT}/core/rosbuild/rosbuild.cmake)

# This is a comment

rosbuild_init()

rosbuild_gen_msg()

rosbuild_add_executable(localplanner src/main.cpp)

ROS PackageCMakeLists.txt

cmake_minimum_required(VERSION 2.4.6)include($ENV{ROS_ROOT}/core/rosbuild/rosbuild.cmake)

# This is a comment

rosbuild_init()

rosbuild_gen_msg()

rosbuild_add_executable(localplanner src/main.cpp)

ROS PackageCMakeLists.txt

You don’t have to worry about most of this junk …

But this line is important. ROS doesn’t magically know where your source files are and which ones you want to package into an executable. So you need to say

rosbuild_add_executable(executable_name source.cpp)orrosbuild_add_executable(executable_name source1.cpp source2.cpp source3.cpp)

ROS Package

src

localplanner

include bin


src include bin

ROS Packagelocalplanner Package description

goes here


<package>

<description brief=“localplanner”>Local Planner

</description><author>Your Name Here</author><license>BSD</license><review status=“unreviewed” notes=“”/><url>http://ros.org/wiki/localplanner</url>

<depend package=“roscpp”/><depend package=“artsystem”/><depend package=“nav_msgs”/><depend package=“ucontroller”/>

</package>

ROS Packagemanifest.xml

<package>

<description brief=“localplanner”>Local Planner

</description><author>Your Name Here</author><license>BSD</license><review status=“unreviewed” notes=“”/><url>http://ros.org/wiki/localplanner</url>

<depend package=“roscpp”/><depend package=“artsystem”/><depend package=“nav_msgs”/><depend package=“ucontroller”/>

</package>

ROS Packagemanifest.xml

You need to list your dependencies here! So if you #include something, and ROS is complaining, one of the first things that you should check is whether you need to add another dependency to the manifest.

$ roscd localplanner in general, roscd [package-name]

src

localplanner

include bin

CMakeLists manifest

ROS will find the specified package and will change your working directory to be the top-level package directory

roscd

$ rosmake localplanner in general, rosmake [package-name]or, when in a package directory, just rosmake

ROS will build the specified package

rosmake

$ rosmake localplanner

[ rosmake ] rosmake starting …[ rosmake ] Packages requested are: [‘localplanner’][ rosmake ] Logging into directory /home/sandro/.ros/rosmake/rosmake_output-20130921-135115[ rosmake ] Expanded args [‘localplanner’] to:[‘localplanner’][ rosmake-0] Starting >>> roslang [ make ][ rosmake-1] Starting >>> geometry_msgs [ make ]

… blah blah blah …

[ rosmake-0 ] Finished <<< ucontroller [PASS] [ 1.53 seconds ][ rosmake-0] Finished <<< localplanner [PASS] [ 1.02 seconds ][ rosmake ] Results:[ rosmake ] Built 8 packages with 0 failures.[ rosmake ] Summary output to directory[ rosmake ] /home/sandro/.ros/rosmake/rosmake_output-20130921-135115

Output of rosmake





Output of rosmake

You could potentially see a lot of stuff here, because rosmake actually goes and builds all of your package’s dependencies for you.





Output of rosmake

[PASS] means that rosmake successfully built a package. [FAIL] means there were errors.





Output of rosmake

0 failures is what you’re looking for (obviously). If there were failures, you’ll also see a list of error messages.

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