outline.md

Intro

What is ROS?

http://wiki.ros.org/ROS/Introduction

Why ROS?

• open-source
• active community with 3000+ packages
  • Drivers for sensor
  • Modern algorithms/capabilities
• python for accessibility, C++ for performance
• Well thoughtout inframestructure and extensive documentation/examples
• Doesn't support windows (which is a pro)

Concepts Overview

http://wiki.ros.org/ROS/Concepts

• Packages
• Nodes
• Messages
• Master and Communication
• Parameter Server
• Topics
• Services
• Bags

Packages

http://wiki.ros.org/Packages Organization tool

• Dependency during build and run time
• Modularity
• Avoid namespace conflict

Structure (Folders)

• Folders (Shoow tree)
  • src
  • script
  • include
  • launch
  • msg
  • srv
• Files
  • CMakeList.txt (For build, python need this too)
  • package.xml (For dependency and build order)

Tools

• rospack profile
• roscd
• rospkg
• catkin_create_pkg

Example

• roscd duckietown

Nodes and Topics

http://wiki.ros.org/Nodes http://wiki.ros.org/Topics

Nodes and Topics

• Each node has it's own processes
• Communicate with each other through publishing/subscribing to topics
• Example: Part of the ROS diagram
• Nodes can be run on different machines
  • Figures
• There can be multiple nodes of the same type, but their names have to be unique. (More on this later in the namespaces)
• Topic Types (Message)

Commandline Tools and Demos

• rosrun
  • commandline remapping
  • Can be done in launch file to save a lot of typing
• rosnode list
• rosnode info
• rostopic list
• rostopic info
• rqt_graph

Best practice and code snippet

• rospy.subscriber with the ~
• rospy.publisher with the ~
  • Do we need to edit the source code all the time? No, we do remapping at launch time.
  • Might not be a big deal for python, but imagine having to recompile everytime the name of a topic changes.
• rospy.init
• rospy.spin
• code organization

Master

http://wiki.ros.org/Master

• Think of it as telephone operators in the old days.
  • Picture
• One major difference from the operator analogy: the traffic does not go through the master once publisher and subscribers are connected.
  • Node A publishing topic_a
  • Node B want to subscribe to topic_a, it asks the ROS Master for the dedicated address of Node A and topic_a
  • Node B and Node A then communicates directly through TCP/IP (UDP optioins available too).
  • Master sit back and relax

Commandline tools and demos

• roscore
• ROS_MASTER_URI
• ros_out?

Messages

http://wiki.ros.org/msg Messages define topic types with a straight-forward syntax. A topic must have a message type associate with it.

• Common msg types: bool, string, float etc.
• std_msgs, geometry_msgs, nav_msgs, etc
• Header is important.
  • timestamp
  • frame_id
• duckietown_msgs
  • All non standard msgs use in Duckietown are in duckietown_msgs
  • Use composition of existing msgs instead of creating a new one from scratch
  • Example: WheelsCmdStamped.msg?

Commandline tools

• rosmsg show

Launch Files

Node and Topic Namespaces

• Avoid leading / at all cost

Think of them as ROS specific scripts

• tags
  • node
  • launch
  • group
  • arg
  • if and unless

Node

• attributes
  • pkg
  • name
  • ns
  • machine
  • output
• elements
  • remap
  • rosparam
  • param

Duckietown ROS Diagram

Perception

Control

Localization

Planning & Coordination

Duckietown Guidelines

Too much details at this point I think.

Config?

• In duckietown/config/config_name/pkg_name/node_name/param_file_name.yaml
  • You can easily backup a configuration for all nodes by copying the config_name folder
• The name of a node must ends with _node.
• Each node should have a launch file with the same name. This launch file is referred to as the elemental launch file.