Odometry

• Odometry message is used for /odom topic.
  1. The Robot’s “belief” of where it is, and what it is doing.
  2. Based on sensors in the base, including possibly the motors, a GPS, a magnetometer etc.
  3. Odometry is published as topic /odom and the message definition is called nav_msgs/Odometry
• nav_msgs/Odometry
  1. Header - standard header
  2. pose - where does the robot thinks that it is (including Covariance - how sure is it?)
  3. twist - how does it think it is moving (also with a covariance)

catkin

• catkin is the ROS build system
• It’s quite complicated and related to CMAKE
• Treat it like a black box

Workspaces

• A subdirectory in which you will be doing your ROS work
• You need only one and it will be called catkin_ws/
• You should already have it on your linux install, but here’s how you would create it otherwise:

$ cs
$ catkin_init_workspace

• From time to time you have to build things, called “doing a build or make”
• Use the alias cm

.bashrc

• A “hidden” file in your home directory
• Contains a series of shell commands that are executed each time you start a shell
• It is populated with lots of useful stuff by the installation of ROS
• Any commands you want run each time will be placed there
• source ~/.bashrc command reads the file and executes it as if you typed it into bash
• Key environment variables have to be defined in .bashrc

example

$ source ~/catkin_ws/devel/setup.bash
$ export ROS_MASTER_URI=http://localhost:11311
$ export ROS_HOSTNAME=localhost
$ export TB3_MODEL=burger
$ export TURTLEBOT3_MODEL=burger

ROS Packages

• A complete “application” or “tool”, stored as a directory tree in a standard layout
• Analogous to a gem in ruby, or a library in Java
• Can be created like this:

$ catkin_create_pkg my_new_package rospy

Contents of a package directory

• package.xml # describes all the things that are in the package
• CMakeLists.txt # describes how to “build”
• ./messages # descriptors of the messages
• ./scripts # contains all the python source files
• ./launch # contains all the .launch files

Python programs

• Most of your python code will implement nodes
• You should put them into the ./scripts directory
• Name them *.py and give them execution file property: chmod +x myfile.py

rosrun and roslaunch

# run a certain program from a certain package
$ rosrun package program.py

# "execute" that launch file
$ roslaunch package file.launch 

Names, Namespaces and Remapping

• Everything in the ros world has a name that looks like a file path
• For example: /cmd_vel is a topic name
• To allow flexibility,m you can remap a name to another name on the command line, e.g.

$ ./image_view image:=right/image

roslaunch

• Many real ROS applications have multiple nodes
• To save time having to start each one separately
• (And for many other reasons)
• A .launch file contains instructions on what to launch and in what order
• You run it with the roslaunch command. It will start roscore if necessary

tf: Coordinate Transforms

• See TF Tutorial
• Coordinate system
• There will/can be many
• Some of the components might move (an arm for example)
• tf system periodically broadcasts the current relationship between coordinate systems
• Quaternions

Poses, Positions, and Orientations

• Handy and very common term to mean: 3d position and 3d orientation of something
• (Not velocity, just the position)
• Usually the transform between the robot coordinate frame and the maps coordinate frame

tf

• Frames: Essential and complicated concept
• Find the change in position and orientation between two coordinate frames
• High level: a way to compute relative coordinate systems
• tf is a central package of ROS which computes transformations for Frames
• If I know a position relative to the room, what is that position relative to the robot?