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Connect to ROS Network and Establish Communication

ROS Master and ROS Nodes

Nodes are the basic building blocks of ROS applications, which takes care of the computation. A ROS network consists of a single ROS master and multiple ROS nodes. The ROS master facilitates the communication in the ROS network by keeping track of all active ROS entities. Every node needs to register with the ROS master to be able to communicate with the rest of the network. MATLAB® can start the ROS master, or the master can be launched outside of MATLAB (for example, on a different computer).

Initialize ROS Network

To connect to a ROS network, you can create the ROS master in MATLAB or connect to an existing ROS master. In both cases, MATLAB will also create and register its own ROS node (called the MATLAB global node) with the master. To create the ROS master in MATLAB, call rosinit function without any arguments.

rosinit
Launching ROS Core...
..Done in 2.2595 seconds.
Initializing ROS master on http://172.30.131.170:50105.
Initializing global node /matlab_global_node_62502 with NodeURI http://bat6246win64:54321/ and MasterURI http://localhost:50105.

View ROS Network Information

Use rosnode list to see all nodes in the ROS network. Note that the only available node is the global node created by rosinit.

rosnode list
/matlab_global_node_13423

Use exampleHelperROSCreateSampleNetwork to populate the ROS network with three additional nodes and sample publishers and subscribers.

exampleHelperROSCreateSampleNetwork

Use rosnode list again to see the three new nodes (node_1, node_2, and node_3).

rosnode list
/matlab_global_node_13423
/node_1
/node_2
/node_3

This figure shows the current state of the ROS network. The MATLAB global node is disconnected since it currently does not have any publishers, subscribers or services.

Shutdown ROS Network

Use rosshutdown to shut down the ROS master and the global node

rosshutdown
Shutting down global node /matlab_global_node_62502 with NodeURI http://bat6246win64:54321/ and MasterURI http://localhost:50105.
Shutting down ROS master on http://172.30.131.170:50105.

You can create multiple such nodes and establish communication between them by sending and receiving messages of different interface types.

Connect to External ROS Master

You can also use the rosinit command to connect to an external ROS master (for example –– a ROS Master running on a robot or a virtual machine). You can specify the address of the master in two ways:

  • Use an IP address.

  • Use hostname of the computer that runs the master.

After each call to rosinit, you have to call rosshutdown before calling rosinit with a different syntax. For brevity, these calls to rosshutdown are omitted in these examples.

'master_host' is an example host name and '192.168.1.1' is an example IP address of the external ROS master. Adjust these addresses depending on where the external master resides in your network. These commands will fail if no master is found at the specified addresses.

rosinit('192.168.1.1')
rosinit('master_host')

Both calls to rosinit assume that the master accepts network connections on port 11311, which is the standard ROS master port. If the master is running on a different port, you can specify it as a second argument. To connect to a ROS master running on host name master_host and port 12000, use the following command:

rosinit('master_host',12000)

If you know the entire Uniform Resource Identifier (URI) of the master, you can create the global node and connect to this master using this syntax:

rosinit('http://192.168.1.1:12000')

Communication in ROS Network

Node Host Specification

In some cases, your computer may be connected to multiple networks and have multiple IP addresses, of which, this figure is an example:

ROS node host specification

The computer on the bottom left runs MATLAB and is connected to two different networks. In one subnet, its IP address is 73.195.120.50, and in the other, its IP is 192.168.1.100. This computer wants to connect to the ROS master on the TurtleBot® computer at IP address 192.168.1.1. As part of the registration with the master, the MATLAB global node has to specify the IP address or host name where other ROS nodes can reach it. All the nodes on the TurtleBot will use this address to send data to the global node in MATLAB.

When rosinit is invoked with the master's IP address, it tries to detect the network interface used to contact the master and use that as the IP address for the global node. If this automatic detection fails, you can explicitly specify the IP address or host name by using the NodeHost name-value pair in the rosinit call. The NodeHost name-value pair can be used with any of the other syntaxes already shown.

These commands advertise your computer's IP address to the ROS network as 192.168.1.100.

rosinit('192.168.1.1','NodeHost','192.168.1.100')
rosinit('http://192.168.1.1:11311','NodeHost','192.168.1.100')
rosinit('master_host','NodeHost','192.168.1.100')

Once a node is registered in the ROS network, you can see the address that it advertises by using the command rosnode info <nodename>. You can see the names of all registered nodes by calling rosnode list.

ROS Environment Variables

In advanced use cases, you might want to specify the address of a ROS master and your advertised node address through standard ROS environment variables. The syntaxes that were explained in the previous sections should be sufficient for the majority of your use cases.

If no arguments are provided to rosinit, the function will also check the values of standard ROS environment variables. These variables are ROS_MASTER_URI, ROS_HOSTNAME, and ROS_IP. You can see their current values using the getenv command:

getenv('ROS_MASTER_URI')
getenv('ROS_HOSTNAME')
getenv('ROS_IP')

You can set these variables using the setenv command. After setting the environment variables, call rosinit with no arguments. The address of the ROS master is specified by ROS_MASTER_URI and the global node's advertised address is given by ROS_IP or ROS_HOSTNAME. If you specify additional arguments to rosinit, they override the values in the environment variables.

setenv('ROS_MASTER_URI','http://192.168.1.1:11311')
setenv('ROS_IP','192.168.1.100')
rosinit

You do not have to set both ROS_HOSTNAME and ROS_IP. If both are set, ROS_HOSTNAME takes precedence.

For your ROS connection to work correctly, you must ensure that all nodes can communicate with the master and with each other. The individual nodes must communicate with the master to register subscribers, publishers, and services. They must also be able to communicate with one another to send and receive data. If your ROS network is not set up correctly, it is possible to be able to send data and be unable to receive data (or vice versa).