Multi Robot Setup

How to namespace multiple robots and have them on the same roscore

To have multiple robots on the same ROS core and each of them listen to a separate node, namespacing would be an easy and efficient way to help.

Set namespace on robots' onboard computers with environment variable

Boot up the robot and ssh into it
On the robot's onboard computer, open the terminal and type in

nano ~/.bashrc

Then add the following line to the end of the file, using the robots name as the namespace

export ROS_NAMESPACE={namespace_you_choose}

Also make the following other changes:

alias bu='roslaunch turtlebot3_bringup turtlebot3_robot.launch'
export IP="$(ifconfig | sed -En 's/127.0.0.1//;s/.*inet (addr:)?(([0-9]*\.){3}[0-9]*).*/\2/p')"
export ROS_IP=$IP
export ROS_MASTER_URI=http://roscore1.cs.brandeis.edu:11311
export ROS_NAMESPACE=roba
export TB3_MODEL=burger
export TURTLEBOT3_MODEL=burger

Save the file and don't forget to do source ~/.bashrc

Set namespace on your laptop with environment variable

Now that the robot is configured properly with its own unique name space, how do we talk to it?
There are three ways:
Configure your laptop to be permanently associated with the same name space
Set a temporary environment variable that specifies the name space
Add the namespace to a launch file
Use the __ns parameter for roslaunch or rosrun (not recommended)

Permanently associate your laptop with the name space

Use the same steps above. Make sure your namespace is exactly the same as the namespace of the robot you want to control.
From now on, whenever you do, e.g. a cmd_vel, it will be directed just to your robot.

Use an environment variable

Set namespace for a termimal with temporary environment variable.
To set a namespace temporarily for a terminal, which will be gone when you close the termial, just type in export ROS_NAMESPACE={namespace_you_choose} directly in your terminal window.
You can use echo $ROS_NAMESPACE to check it.

Use the .launch file

To set namespace for a node in launch file, Use the attribute ns, for example:

<node name="listener1" pkg="rospy_tutorials" type="listener.py" ns="{namespace_you_choose}" />

Not recommended: add __ns to the run or launch command

Launch/Run a file/node with namespace in terminal. Add a special key __ns (double underscore here!) to your command, for example: roslaunch turtlebot3_teleop turtlebot3_teleop_key.launch __ns:={namespace_you_choose} However,
Use of this keyword is generally not encouraged as it is provided for special cases where environment variables cannot be set. (http://wiki.ros.org/Nodes)

Publishing/Subsribing topics in other namespace

Do a rostopic list, you will find out topics under a namespace will be listed as /{namespace}/{topic_name}

Make changes to turtlebot3_navigation package on your laptop

Type roscd turtlebot3_navigation to go to the package directory.
Type cd launch to go to the folder that stores .launch files.
Open the turtlebot3_navigation.launch file with nano or your favorite code editor.
You will see the scripts for launching move base and rviz:

  <!-- move_base -->
  <include file="$(find turtlebot3_navigation)/launch/move_base.launch">
    <arg name="model" value="$(arg model)" />
    <arg name="move_forward_only" value="$(arg move_forward_only)"/>
  </include>

  <!-- rviz -->
  <group if="$(arg open_rviz)">
    <node pkg="rviz" type="rviz" name="rviz" required="true"
          args="-d $(find turtlebot3_navigation)/rviz/turtlebot3_navigation.rviz"/>
  </group>

Change move base arguments

Add another argument cmd_vel_topic to the four arguments in the  section:

  <!-- Arguments -->
  <arg name="model" default="$(env TURTLEBOT3_MODEL)" doc="model type [burger, waffle, waffle_pi]"/>
  <arg name="map_file" default="$(find turtlebot3_navigation)/maps/map.yaml"/>
  <arg name="open_rviz" default="true"/>
  <arg name="move_forward_only" default="false"/>
  <arg name="cmd_vel_topic" default="/{namespace_you_choose}/cmd_vel"/>

Pass the new argument to move base launch file:

<!-- move_base -->
<include file="$(find turtlebot3_navigation)/launch/move_base.launch">
 <arg name="model" value="$(arg model)" />
 <arg name="move_forward_only" value="$(arg move_forward_only)"/>
 <arg name="cmd_vel_topic" value="$(arg cmd_vel_topic)"/>
</include>

These changes will make move base publish to /{namespace_you_choose}/cmd_vel instead of /cmd_vel. Open the move_base.launch file in the launch folder, you will see why it worked.

Change rviz arguments

Type roscd turtlebot3_navigation to go to the package directory.
Type cd rviz to go to the folder that has turtlebot3_navigation.rviz. It stores all the arguments for rviz.
Open turtlebot3_navigation.rviz with VSCode or other realiable code editors, since this file is long and a bit messy.
In the .rviz file, search for all the lines that has the part Topic:
Add your namespace to the topics you found. For example, change

  Topic: /move_base/local_costmap/footprint
  `

to

  Topic: /roba/move_base/local_costmap/footprint

These changes will make rviz subsribe to topics in your namespace.