ROS: A Minimum Overview

What's ROS? †

• A middleware
  that helps to make a big system
  by combining small programs (nodes).
• ROS provides:
  • Communication ways between nodes.
  • Tools to create nodes and ROS system.
• Why do we use ROS?
  • For using a lot of packages developed by other developers.
  • For using ROS-compatible robots and sensors
    • Baxter, Sawyer
    • PR2
    • Motoman
    • Dynamixel
    • Atlas
    • MegaBots
    • ...
  • For making collaboration with others easy.
  • For integrating C++ and Python programs.

Concepts †

• Node
  is an executable whose code is written in C++, Python, etc.
  It should contain some "ROS magic code" to become a ROS node.
  • Nodes can be executed on different computers.
• Two communication ways of nodes:
  • Data-sending type:
    Any type of data can be sent from a node to other nodes.
    A publisher is a ROS magic (or a "port") on a sending node to send data.
    A subscriber is a ROS magic (or a "port") on a receiving node to receive data.
    Each node has any number of publishers and subscribers.
    The data is identified by a topic (name), and its type (integer, string, etc.) is called a message.
    node_A.publisher_a ---<topic>--→ node_B.subscriber_b
  • Function-call type:
    A node can call a function of another node. There may be a return value.
    The function is called a service.
    A service server is a ROS magic (or a "port") on a node that provides the service.
    A service client is a ROS magic (or a "port") on a node that uses the service.
    Each node has any number of service servers and service clients.
    A service consists of a request (parameters) and a response (return value).
    node_A.service_client_a ---<request>--→ node_B.service_server_b
    node_A.service_client_a ←--<response>--- node_B.service_server_b
• Launch file
  is a script (XML) to execute multiple nodes at once.
• Package
  is a unit of related things of ROS.
  • Package is a set of node source codes, definitions of messages and services, launch files, package descriptions, and some extra files.
  • ROS identifies a node type, a launch file, a message, and a service by /PACKAGE_NAME/NODE_NAME and so on.
  • Package is used to distribute a ROS software. See the list of ROS packages.

Tools †

The following programs are command-line tools for ROS. Execute a command with -help option to see the usage.

• roscore: A core program of ROS, which launches a ROS master. Execute roscore before executing nodes.
• rosrun: Execute a node.
• roslaunch: Execute a launch file. roslaunch automatically executes roscore.
• rosnode: Print information about ROS nodes.
• rostopic: Print information about ROS topics.
• rosservice: Print information about ROS services.
• rosmsg: Print information about ROS message types.
• rossrv: Print information about ROS service types.
• rospack: Print information about ROS packages.
• roscreate-pkg: Create a new ROS package.
• rosbag: Record and play ROS topics.