Get Started with ROS 2

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Robot Operating System 2 (ROS 2) is the second version of ROS, which is a communication interface that enables different parts of a robot system to discover, send, and receive data. MATLAB® support for ROS 2 is a library of functions that allows you to exchange data with ROS 2 enabled physical robots or robot simulators such as Gazebo®. ROS 2 is built on Data Distribution Standard (DDS) which is an end-to-end middleware that provides features such as discovery, serialization and transportation. These features align with the design principles of ROS 2 such as distributed discovery and control over different "Quality of Service" options for transportation. DDS uses Real Time Publish-Subscribe (RTPS) protocol which provides communication over unreliable network protocols such as UDP. For more information, see RTPS.

This example shows how to:

  • Set up ROS 2 within MATLAB

  • Get information about capabilities in a ROS 2 network

  • Get information about ROS 2 messages

To learn about ROS, see Get Started with ROS.

ROS 2 Terminology

  • A ROS 2 network comprises different parts of a robot system (such as a planner or a camera interface) that communicate over ROS 2 network. The network can be distributed over several machines.

  • A ROS 2 node is an entity that contains a collection of related ROS 2 capabilities (such as publishers and subscribers). A ROS 2 network can have many ROS 2 nodes.

  • Publishers and subscribers are different kinds of ROS 2 entities that process data. They exchange data using messages.

  • A publisher sends messages to a specific topic (such as "odometry"), and subscribers to that topic receive those messages. There can be multiple publishers and subscribers associated with a single topic.

  • A Domain is the physical segmentation of network. It is identified by a unique integer value known as Domain ID. By default the Domain ID is 0.

  • Every node in ROS 2 network on creation advertises its presence to other nodes in the same Domain ID only.

  • ROS 2 network is built on Data Distribution Standard (DDS) which makes it possible to connect multiple nodes across distributed network.

  • RTPS (Real Time publisher-subscriber) protocol provides ROS 2 network with capabilities to send messages in unreliable network conditions.

  • ROS 2 offers variety of Quality of Service (QoS) policies that allow you to tune your communication between nodes. For more information, see Manage Quality of Service Policies in ROS 2.

For more information, see Robot Operating System2 (ROS 2) and the Concepts section on the ROS 2 website.

Initialize ROS 2 Network

Unlike ROS, ROS 2 does not require initialization in MATLAB. The ROS 2 network automatically starts with creation of nodes.

Use ros2node to create a node.

test1 = ros2node("/test1")
test1 = 
  ros2node with properties:

    Name: '/test1'
      ID: 0

Use ros2 node list to see all nodes in the ROS 2 network.

ros2 node list
/test1

Use clear to shutdown the node in ROS 2 network.

clear test1

Use exampleHelperROS2CreateSampleNetwork to populate the ROS network with three additional nodes with sample publishers and subscribers.

exampleHelperROS2CreateSampleNetwork

Use ros2 node list again, and observe that there are three new nodes, node_1, node_2, and node_3).

ros2 node list
/node_1
/node_2
/node_3

A visual representation of the current state of the ROS 2 network is shown below. Use it as a reference when you explore this sample network in the remainder of the example.

Topics

Use ros2 topic list to see available topics in the ROS 2 network. Observe that there are four active topics: /pose, /clock, /parameter_events and /scan.The topics /clock and /parameter_events are global topics which always present in the ROS 2 network. Topic /clock is used for synchronization whereas topic /parameter_events is used by nodes to monitor or change parameters in the network. The other two topics /scan and /pose were created as part of the sample network.

ros2 topic list
/parameter_events
/pose
/scan

Each topic is associated with a message type. Use ros2 topic list -t to see the message type of the topics.

ros2 topic list -t
            Topic                       MessageType           
    _____________________    _________________________________

    {'/parameter_events'}    {'rcl_interfaces/ParameterEvent'}
    {'/pose'            }    {'geometry_msgs/Twist'          }
    {'/scan'            }    {'sensor_msgs/LaserScan'        }

Messages

Publishers and subscribers use ROS 2 messages to exchange information. Each ROS 2 message has an associated message type that defines the datatypes and layout of information in that message. For more information, see Work with Basic ROS 2 Messages.

Use ros2 msg show to view the properties of a message type. The geometry_msgs/Twist message type has two properties, Linear and Angular. Each property is a message of type geometry_msgs/Vector3, which in turn has three properties of type double.

ros2 msg show geometry_msgs/Twist
# This expresses velocity in free space broken into its linear and angular parts.

Vector3  linear
Vector3  angular

ros2 msg show geometry_msgs/Vector3
# This represents a vector in free space.

float64 x
float64 y
float64 z

Use ros2 msg list to see the full list of message types available in MATLAB.

Disconnect From ROS 2 Network

Use exampleHelperROS2ShutDownSampleNetwork to remove the sample nodes, publishers, and subscribers from the ROS 2 network. To remove your own nodes, use clear with the node, publisher, or subscriber object.

exampleHelperROS2ShutDownSampleNetwork

Next Steps

ROS Toolbox Documentation

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