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Connecting many rovers together

Prerequisites

In this tutorial, we will show you how to configure many rovers in one network, so that they will see each other and will communicate.

In such setup, we need every rover, to have its own robot namespace for ROS nodes and topics, and only one ROS master. We also need to connect all the rovers to the same network, and know their IP addresses.

Once all rovers are connected to the same network, we can start configuring them.

You will need to make almost the same steps on every rover. The only differences are for the rover with ROS master on it, but it will be marked, how given step is different for the main rover (the rover with ROS master).

Getting IP address

First, we need to know the IP address of the rover in the network with all other rovers. To get it, you have to connect to the rover via ssh, and type in the terminal

This will show addresses assigned to all network interfaces, we just need to check the correct interface. If you didn't make many changes to the network setup before, then you need to find the wlan_int interface (if you did some changes and don't have this interface, you need to find the interface connected to the network). There, you can find the address next to inet tag.

You need to remember or write somewhere this address.

You need IP address of the main rover for other rovers, so remember specially what address it has.

Changing setup file

Now, we need to configure some environment variables. We can do this by changing the setup.bash file in the /etc/ros directory. To do so, type:

Here, we need to change a few things:

  • make sure the START_ROSCORE variable is set to true on the main rover, and for other rovers it is set to false
  • uncomment the line with variable ROS_NAMESPACE. You also need to provide the namespace, it can be "leox", where x is a number of the rover in your setup, or "leo_main" for the main rover. We need to change it, as you can't have two nodes with the same name. Setting this variable will give your node name prefix with the namespace.
  • make sure the line with the ROS_HOSTNAME variable is commented
  • uncomment the line with the ROS_IP variable and provide the IP addres of the rover
  • uncomment the line with the ROS_MASTER_URI variable. Now, for the main rover, you need to set it to http://localhost:11311. For other rovers, you need to set it to http://<ip address of main rover>:11311

Changing urdf file

We also need to change the robot.urdf.xacro file in the /etc/ros/urdf directory. If we don't make changes in this file, all rovers will have the same tf frames, which will give some error readings when doing something with tf tree. To edit it, type

In this file, you need to provide the link_prefix variable. This way we will have distinct tf trees for every rover, as the frames will have the prefix. You can set it to the same value that you gave to ROS_NAMESPACE environment variable. You can find the link_prefix variable inside xacro:leo tag.

Changing launch file

In the previous paragraph, we were making changes required for working tf tree, but those changes were not enough. We also need to change the robot.launch file in /etc/ros directory, to make it work correctly. To edit it, type

In this file, we include another launch file. The included launch file has ros argument tf_frame_prefix, so we can set it here in the <include> tag, to our value. To add it, you need to include this line in the file:

Your robot.launch file in the end should look like this:

/etc/ros/robot.launch

You need to set the value of tf_frame_prefix argument in the robot.launch file to the same value, you gave in the link_prefix variable in robot.urdf.xacro file.
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