ROS tutorial

Introduction

This is a brief tutorial for how to setup a ROS package. It discusses the how to setup Robotic operating system (ROS) and create nodes and packages.

Currently this only gives an overview on the C++ elements of ROS. It will be altered in the future to include python based ROS.

Important

One very important note for everything thing to run smoothly. This package, like all other ROS packages. Must be within a Catkin workspace. This is discussed in greater detain on the ROS wiki. The parent directory must be ~/catkin_ws/src. It is standard notation to have your workspace simply called catkin_ws

To quickly do this, run the following commands.

mkdir -p ~/catkin_ws/src
mv mv ../ROS_tutorials ~/catkin_ws/src
cd ~/catkin_ws/src/ROS_tutorials

ROS Install

Currently is it set to install ROS for ubuntu 16.04 and 18.04. These are ROS versions kinetic and melodic respectively. A script has been created to install any required ROS programs with minimal user interaction. To run the script enter the following commands into terminal.

chmod +x ros_insall.sh
./ros_install.sh

The above script will also build your packages within your workspace and run it. If it has been successful it should show a continuous stream of Install Successful

Alternatively, you can install ROS by following the steps found on the ROS website.

Building the packages

Now that ROS is running. Any time that you make any changes to any of your workspace you must now build the package. This can be done when inside of the catkin workspace by running the following command.

catkin build

If there are any compile errors with your code. These will be displayed as with any GCC errors.

Running your code

There are 2 main method for running ROS packages and each with their own benefits.

1. The first method is using rosrun. This will run a single node and is very beneficial for the testing process of new nodes. One important point with rosrun is that you will need a second terminal with the command roscore running.

When this is done you will be able to run your code. The base structure of a rosrun is rosrun package node param:=value. To run our example code run the following

rosrun ROS_tutorials ROS_tutorials_node

For more information on rosrun fo to the following link

2. The second method that can be used to run ROS nodes is roslaunch. The benefit of this over its run counterpart. Is firstly that you do not have to run a roscore along side this (but this is sometimes advised). Secondly as it you can run multiple nodes from one command.

Any launch file that you create should be placed in the launch folder. To run a launch file, the command is similar to that of rosrun. The structure is roslaunch package launch_file.launch. To run our example run the following command

roslaunch ROS_tutorials example_launch_file.launch

For more detail on roslaunch go to the following link

Further Points

Now that you have ROS working here are a couple of things might help

Useful commands

These are just a couple of useful commands that you can use for making you life slightly easier.

• roscd is a command similar to that of cd except it will go directly to a ROS package. The usage of it is roscd package. A detailed description can be found here
• rostopic allows you to subscribe and publish to any topic that is running on the current roscore. This is extremely beneficial for the debugging process. There are a number of sub commands that run alongside rostopic. A comprehenistve description of rostopic here
  • list will list all current topics. The structure of the commange is rostopic list
  • echo will subscribe to a particular topic and display the message when one is published. The structure of the command is rostopic echo <topic>
  • info will give information about a particular topic. This includes its message type. As well as nodes that publish and subscribe to it. The structure of the command is rostopic info <topic>
  • pub will allow you to publish to a topic which can be incredibly useful for debugging. The structure of the command is rostopic pub <topic> <message type> <message>
• roscore begins the core ROS module which is required for you to run nodes. This is not necessary when using roslaunch, but can be beneficial if running multiple at once. This will run a roscore as part of the first launch file that is ran. Therefore when it stops the roscore will also stop, breaking your whole system. It is therefore beneficial to have a roscore constantly running in the background.
• roslaunch already discussed about is used to run launch files
• rosrun already discussed. Runs individual nodes but requires a roscore running in the background.