Teaching Assistant: Roi Yehoshua
roiyeho@gmail.com
Course Agenda
• Working with multi robots in ROS
• Handling communication and synchronization
between robots
• Canonical multi-robot problems in ROS
– Patrolling, Formation, Coverage, etc.
• Decision making system
– We will be using CogniTAO, a decision making system
developed by Cogniteam
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Course Requirements
• Course grade will be composed of:
– 40% Assignments
– 40% Project
– 20% Exam
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Who am I?
• My name is Roi Yehoshua
• PhD Student at Bar-Ilan Robotics lab headed by
Prof. Gal Kaminka
• Teaching assistant at the courses:
– Introduction to Robotics (89-685)
– Multi-Robot Systems (89-689)
• My home page: http://u.cs.biu.ac.il/~yehoshr1/
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Today’s Agenda
• Intro to Multi-Robots Systems in ROS
• Multi Turtlesim Demo
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Intro to Multi Robot Systems in ROS
Three options to run a multi-robot system in ROS:
• Running multiple instances of the same node on
the same computer
• Running multiple nodes on different computers
using one common roscore
• Establishing a multi-master network
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Two Turtle Simulators
• Now we will try to run two turtlesim nodes at the
same time
• First start the ROS infrastructure by running in a
terminal the command:
$ roscore
• Then start the first turtle simulator node by
running in a new terminal:
$ rosrun turtlesim turtlesim_node
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Two Turtle Simulators
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Two Turtle Simulators
• Now try to run yet another turtlesim_node in a new
terminal:
$ rosrun turtlesim turtlesim_node
• This naive approach does not work. Instead, the first
turtle simulation terminates with a warning log and
the new simulation node replaces the first one.
• Node names must be unique across the ROS system.
If a second node is started with the same name as
the first, the first will be shutdown automatically.
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Two Turtle Simulators
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Two Turtle Simulators
• To avoid name conflicts, you need to provide a
different node name for the second turtle
simulator.
• The rosrun command does allow to assign a
different name to the node
• You can provide a value to the __name
parameter as the replacement for the default
node name:
$ rosrun turtlesim turtlesim_node __name:=turtlesim2
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Two Turtle Simulators
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Two Turtle Simulators
• In this case the two nodes publish and subscribe
to the same topics
• You can verify this by running the command
rostopic list
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Two Turtle Simulators
• This means you can use the same driver to
control both turtle simulations
• In another terminal type:
$ rosrun turtlesim turtle_teleop_key
• Use the keys to move both turtles together
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Two Turtle Simulators
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ROS Namespaces
• A namespace can be viewed as a directory whose
contents are items of different names.
– These items can be nodes, topics, services or even
other namespaces.
• Each resource in ROS is defined within a
namespace, which it may share with other
resources.
• This encapsulation isolates different portions of
the system from accidentally grabbing the wrong
named resource or globally hijacking names.
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ROS Namespaces
• Examples for resource names:
– / (the global namespace)
– /turtle1
– /bar_ilan/robot/name
– /wg/node1
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ROS Namespaces
• Usually topics related to a specific robot will be
mapped into a namespace.
• For example, robot1 would receive commands
on the topic /robot1/cmd_vel, robot2 on the
topic /robot2/cmd_vel, etc.
• They both could exchange messages via some
topic, such as /shared_topic.
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Changing Node Namespace
• All nodes launch in the global namespace
• You can change the namespace of a node by:
– Setting the ROS_NAMESPACE environment variable
– Using a launch file
• Changing the namespace of a node effectively
remaps all of the names in that node.
– Node name, topics, services and parameters will be
rescoped into a child namespace
– This in effect "pushes it down" into a child
namespace
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Name Resolving
• There are four types of Graph Resource Names in
ROS: base, relative, global, and private, which have
the following syntax:
base
relative/name
/global/name
~private/name
• By default, resolution is done relative to the node's
namespace.
– For example, the node /wg/node1 has the namespace
/wg, so the name node2 will resolve to /wg/node2.
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Name Resolving
• Names that start with a "/" are global - they are
considered fully resolved.
– Global names should be avoided as much as possible
as they limit code portability.
• Names that start with a "~" are private. They
convert the node's name into a namespace.
– For example, node1 in namespace /wg/ has the
private namespace /wg/node1.
– Private names are useful for passing parameters to a
specific node via the parameter server.
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Name Resolution Examples
Node
Relative (default)
Global
Private
/node1
bar -> /bar
/bar -> /bar
~bar -> /node1/bar
/wg/node2
bar -> /wg/bar
/bar -> /bar
bar -> /wg/node2/bar
/wg/node3
foo/bar -> /wg/foo/bar /foo/bar ->
/foo/bar
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~foo/bar ->
/wg/node3/foo/bar
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Two Turtle Simulators With Two Drivers
• We illustrate the concept of namespace with two
turtle simulators driven by two instances of
the draw_square driver.
• We will use a launch file that will run two
instances of turtle_sim in two different
namespaces
• This will ensure that the nodes have different
names and also that within each simulation,
nodes use different topic names.
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Launch Files
• Launch files (.launch) are XML configuration files
that specify the parameters to set and nodes to
launch, as well as the machines that they should
be run on.
• roslaunch is the command-line tool to run the
launch file
• If you use roslaunch, you do not have to run
roscore manually
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Launch File
• Create the file multi_turtlesim.launch
<launch>
<group ns="sim1">
<node name="turtle" pkg="turtlesim" type="turtlesim_node"/>
<node name="controller" pkg="turtlesim" type="draw_square"/>
</group>
<group ns="sim2">
<node name="turtle" pkg="turtlesim" type="turtlesim_node"/>
<node name="controller" pkg="turtlesim" type="draw_square"/>
</group>
</launch>
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The <group> Tag
• The <group> tag in a launch file is equivalent to
the top-level <launch> tag and simply acts as a
container for the tags within.
– This means that you can use any tag as you would
normally use it within a <launch> tag.
• It has an ns attribute that lets you push the
group of nodes into a separate namespace.
• The namespace can be global or relative, though
global namespaces are discouraged.
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Two Turtle Simulators With Two Drivers
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Two Turtle Simulators With Two Drivers
• rqt_graph shows the active nodes and topics:
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Remapping Arguments
• Any ROS name within a node can be remapped
when it is launched at the command-line.
• This is a powerful feature of ROS that lets you
launch the same node under multiple
configurations from the command-line.
• The names that can be remapped include the
node name, topic names, and Parameter names.
• Remapping arguments can be passed to any
node and use the syntax name:=new_name.
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Remapping Arguments
• For example, to configure turtle_teleop_key
node to publish command velocities to the first
turtle simulator, we first have to find out the
name of the topic it publishes to
• This can be done by running rosnode info
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Remapping Arguments
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Remapping Arguments
• Now remap the name of the topic
/turtle1/cmd_vel to /sim1/turtle1/cmd_vel
$ rosrun turtlesim turtle_teleop_key /turtle1/cmd_vel:=/sim1/turtle1/cmd_vel
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Remapping Arguments
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The <remap> Tag
• Remapping arguments can also be done in the
launch file using the <remap> tag
• Attributes:
from="original-name“
to="new-name“
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Remapping Arguments In Launch File
<launch>
<group ns="sim1">
<node name="turtle" pkg="turtlesim" type="turtlesim_node"/>
</group>
<group ns="sim2">
<node name="turtle" pkg="turtlesim" type="turtlesim_node"/>
</group>
<node pkg="turtlesim" name="teleop" type="turtle_teleop_key"
output="screen">
<remap from="/turtle1/cmd_vel" to="/sim1/turtle1/cmd_vel"/>
</node>
</launch>
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Multi Robot Systems in ROS
Three options to run a multi-robot system in ROS:
• Running multiple instances of the same node on
the same computer
• Running multiple nodes on different computers
using one common roscore
• Establishing a multi-master network
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Network Setup
• A running ROS system can comprise dozens of
nodes, spread across multiple machines.
• Depending on how the system is configured, any
node may need to communicate with any other
node, at any time.
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Network Setup
• When running only one master, ROS has certain
requirements of the network configuration:
– There must be complete, bi-directional connectivity
between all pairs of machines, on all ports.
– Each machine must advertise itself by a name that all
other machines can resolve.
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Network Setup
• Find your IP address by typing the command
ifconfig and looking at inet addr
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Network Setup
• Assume that we want to run a ROS system on
two machines, with the following IP addresses:
– 192.168.118.151 (machine A)
– 192.168.118.154 (machine B)
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Basic Check
• Ping between machines
– Ping machine A from machine B and vice versa
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ROS_MASTER_URI
• ROS_MASTER_URI is a required setting that tells
nodes where they can locate the master.
• It should be set to the XML-RPC URI of the
master.
• The default value is http://localhost:11311
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ROS_MASTER_URI
• Change ROS_MASTER_URI in machine B to point
to machine A IP address
$ export ROS_MASTER_URI=http://192.168.118.151:11311
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ROS_MASTER_URI
• Run the master and turtlesim on machine A
$ roscore
$ rosrun turtlesim turtlesim_node
• Now if you type rostopic list on machine B, you
should see the topics advertised from the nodes
in machine A
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ROS_MASTER_URI
• However, seeing the topics only shows you one
direction of communication.
• Every computer needs to be able to talk to each
other (from/to) using whatever name this is
known by to ROS.
• The host name used by ROS is defined by ROS_IP
and ROS_HOSTNAME environment variables
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Setting ROS_IP
• ROS_IP and ROS_HOSTNAME are optional
environment variable that sets the declared
network address of a ROS Node or tool.
– When a ROS component reports a URI to the master
or other components, this value will be used.
• Use ROS_IP for specifying an IP address,
and ROS_HOSTNAME for specifying a host name.
– The options are mutually exclusive, if both are
set ROS_HOSTNAME will take precedence.
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Setting ROS_IP
• In machine A add to your .bashrc:
function get_ip_address { ifconfig | fgrep -v 127.0.0.1 | fgrep
'Mask:255.255.255.0' | egrep -o 'addr:[^ ]*' | sed 's/^.*://'; }
export ROS_IP=$( get_ip_address )
• Open a new terminal and make sure that ROS_IP
is set properly
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Setting ROS_IP
• You must set ROS_IP also on machine B
• Add the following to your .bashrc on machine B
function get_ip_address { ifconfig | fgrep -v 127.0.0.1 | fgrep
'Mask:255.255.255.0' | egrep -o 'addr:[^ ]*' | sed 's/^.*://'; }
export ROS_IP=$( get_ip_address )
export ROS_MASTER_URI=http://192.168.118.151:11311
• Note that ROS_MASTER_URI on machine B
points to the ROS_IP of machine A
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Network Setup Summary
• Both computers should have ROS_IP set to their
own IP address.
• The one on which the master runs should have
ROS_MASTER_URI=http://localhost:11311 (the
default)
• The one that connects to the master should have
ROS_MASTER_URI=http://[IP_OF_MASTER_MAC
HINE]:11311
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Testing Network Setup
To test your network setup:
• Run roscore on machine A
• Run turtlesim node on machine A
• Now you will be able to publish messages to
topics on machine A from machine B
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Testing Network Setup – Machine A
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Testing Network Setup – Machine B
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Homework (not for submission)
• Run three instances of turtlesim at the same time
• Control the first turtle with the keyboard
• Make the second and the third turtles follow the
first one (use the mimic node)
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