Programming Robots Study Group Session 1

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What is ROS?

  • ROS stands for Robot Operating System
  • BSD-licensed system for controlling robotic components.
  • Some definitions:
    • ROS is a software framework for robot software development.
    • ROS is a collection of software components you can plug together so you don’t have to program everything yourself from scratch. No more reinventing the wheel.
    • ROS is a bunch drivers for robotics hardware.
    • ROS is a peer-to-peer network.
    • ROS controls robotics with software.
    • ROS can interoperate on a heterogeneous network.
    • Some might say generalized federated automation platform.
  • A system running ROS is composed of master, nodes, packages, services and logging.

What ROS is not

  • ROS isn’t an IDE (integrated development environment). Don’t worry, many popular IDEs support the languages used in ROS. The ROS wiki has notes for Eclipse, CodeBlocks, EMacs, Vim, NetBeans and QtCreator.
  • ROS isn’t a programming language. ROS libraries exist for C++, Java and Python. Components can be written in just about any language.

Why ROS?

  • While ROS 1.0 is somewhat complex, many consider it the premier robotics environment.

We are going to look at ROS from 2 aspects:

  • File system artifacts
  • Graph system

File system level

Packages

ROS software is organized into packages. Packages are an elemental compile unit.
Packages are the most granular build and release item.
Packages may contain:

  • ROS runtime processes i.e. nodes
  • A ROS-dependent library
  • Datasets
  • Configuration files
  • Other related data and arifacts

Metapackages

  • Metapackages are specialized Packages which represent a group of related packages.
  • Most commonly metapackages are used as a backwards compatible place holder for converted rosbuild Stacks.

Package Manifests

The package.xml package manifest is defined in REP-0127.
Manifests (package.xml) provide metadata about a package:

  • Name
  • Version
  • Description
  • License information
  • Dependencies
  • Other meta information like exported packages.

Message (msg) types

Message type descriptions are stored SomePackage/msg/SomeMessageType.msg.
Message types define the data structures for messages sent in ROS.

Service (srv) types

Service descriptions, stored in my_SomePackage/srv/SomeServiceType.srv, define the request and response data structures for services in ROS.

Repositories

A repository is collection of packages which share a common VCS system.
Packages which share a VCS share the same version and can be released together using the catkin release automation tool bloom.
Repositories often map to converted rosbuild Stacks.
Repositories may contain only one package.

ROS Computation Graph Level

The Computation Graph is embodied as a peer-to-peer network of ROS processes that are processing data together. ROS Master, Nodes, Topics, etc.

ROS Master

  • Principally a Name service for ROS.
  • Master uses XMLRPC
  • ROS Master provides
    • Registration services
    • Node and service lookup

Rather than ‚ÄúSend this sensor data to that computer at 127.0.0.1.”
We tell Node 1 to send messages to Node 2.

Parameter Server

The Parameter Server allows data to be stored by key in a central location. It is currently part of the Master.

ROSCore contains

  • a ROS Master
  • a ROS Parameter Server
  • a rosout logging node

Nodes

  • ROS systems are made from independent nodes.
  • Nodes communicate with each other using publish/subscribe messaging to topics.
  • Node communication is one to many.
  • Nodes communicate with Master to discover other nodes.
  • Nodes are typically executables in a ROS package.
  • ROS has a client library which nodes can employ to communicate.
  • Nodes may provide or use a Service.

Messages

  • Nodes use messages to talk to each other.
  • A message is a data structure made of typed fields.
  • Data types include (integer, floating point, boolean, etc.) and arrays.
  • Messages may be nested to any level.

Topics

We already know ROS uses publish and subscribe. Topics are part of that concept.

  • Topics are named buses.
  • Topics have names which are used to identify the topic when subscribing, monitoring, etc.
  • A given topic may have multiple publishers.
  • A given topic may have multiple subscribers.

Services

  • Services are request/reply.
  • Each of request and reply have a message structure.
  • A service runs on a node.
  • A caller makes a request to the service and waits for the reply.

Bags

  • Bags are a data storage format.
  • Bags store ROS messages.
  • Bags can be ‘played back’ on ROS as messages.

Resources

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