ROS

From dankwiki

ROS is a network-capable software platform for robotics, capable of managing processes (nodes), and exchanging information between them. Releases are named, with the current 12th release being Melodic Morenia (typically referred to as Melodic, released 2018-05-23).

ROS on Debian

ROS Melodic supports Debian 9 ("stretch") with binary packages. Installation instructions can be found here. Debian Unstable requires building from source.

Concepts

  • Package: defined by a directory containing a file package.xml (REP-0127)
    • msg/MyMessageType.msg defines messages for the package, published to topics
    • srv/MyServiceType.srv defines services for the package, supporting request/reply semantics
  • Node: a named process that performs computation
    • A running node has a type and a graph resource name
    • A node is typically launched standalone using rosrun
    • If a node is started with the name of an existing node, the existing node is stopped
  • Master: implemented by the rosmaster package, usually started via roscore
  • Topic: a named bus on which messages of a single type are published
    • A topic can be latched, meaning that new subscribers get the last message published
  • Service: a named synchronous procedure, implemented by some Node
    • Connections can be persistent according to the client's choice
  • Parameter: a key in the global configuration dictionary
    • XMLRPC data types
  • Bag: a recorded/pregenerated set of messages that can be replayed

Names

Names aka graph resource names are unique for each topic, parameter, node, and service in the ros graph. It is a shared namespace; a topic cannot share a name with a node. The ROS_NAMESPACE environment variable can be used to scope the ros graph namespace. Names can be:

  • relative,
  • /global, and
  • ~private (the node's name replaces the tilde)

Names can furthermore be remapped when a node is started by e.g. rosrun.

  • Special case: launch node into namespace with __ns
  • Special case: change node name with variable __name
    • To launch barnode with the name barnode3: roslaunch foopkg barnode __name:=barnode3

Executables

  • rosrun: launch an executable from a package
  • roslaunch: starts collections of nodes specified in a launch XML file
    • roscore: roslaunch to start core ros functionality
  • rosnode: manage running nodes
    • rosnode list lists running nodes
    • rosndoe info displays information about a running node
  • rostopic: interact with topics using YAML
    • rostopic list will list active topics on a running rosmaster
    • rostopic echo echoes messages from the topic
  • rosservice: interact with services using YAML
  • rosmsg: display information about message types
  • rossrv: display information about services
  • rospack: display information about packages
  • rosversion: display version of available packages
  • rosbag: replay and inspect bags