ROS Beginners Tutorial

Description

This repository will cover the installation of ROS with basic implementation of publisher and sbuscriber nodes where we'll have a publisher node(/talker) and a subscriber node(/listner) and the transfer of data(string messages) will be over a topic(/chatter).

Overview

One package is built(beginner_tutorials) which has two nodes which are defined in a C++ source code. The Publisher node publishes a string message on the topic and the subscriber node reads the string message from that topic. Following is the graphical representation.

One of the node has a service to change the string which is published which depends on the input given by the user. Also, there is a launch file generated which acts as a script for firing up all the nodes mentioned in the script at once which frees us from opening the terminal one by one and input rosrun for individual nodes.

Dependencies

Ubuntu 16.04
ROS Kinetic
python (if not already installed)
catkin_pkg (if not already installed while installing ROS) For installing catkin_pkg and directly adding it to PYTHONPATH.

pip install catkin_pkg

Check if the catkin_pkg path is added to PYTHONPATH by using the following command

echo $PYTHONPATH

Installation

ROS Installation: Install the ROS Kinetic for Ubuntu and it's dependencies using the link

Build Instructions

Run the following commands for building and running the Publisher and Subscriber Nodes:

To make the catkin workspace:

--skip command 2(sudo rm -R ~/catkin_ws) if no such folder is found


sudo rm -R ~/catkin_ws
source /opt/ros/kinetic/setup.bash 
mkdir -p ~/catkin_ws/src
cd ~/catkin_ws/
cd src/
git clone -b Week10_HW https://github.com/KrishnaBhatu/beginner_tutorials.git
cd ..
catkin_make

Now the package is ready to use

The above steps are same for bothe the methods (with and without launch files)

Run without using launch file

We have to open up 3 terminals to run the master, publisher and subscriber nodes.

In first terminal

source /opt/ros/kinetic/setup.bash
roscore

In second terminal

cd ~/catkin_ws
source devel/setup.bash
rosrun beginner_tutorials beginner_tutorialsPublisher

In third terminal

cd ~/catkin_ws
source devel/setup.bash
rosrun beginner_tutorials beginner_tutorialsSubscriber

Thus we will see the string messages which are published on the /chatter topic in the third terminal.

Now we can run the service to modify the string that is published by using the changeString service.

Open up a new terminal for calling the service.(Press: ctrl+alt+T)

cd ~/catkin_ws
source devel/setup.bash
rosservice call changeString "<any new string the user wants to enter>"

Now we observe that the published message changes to our new input string message on the listener terminal.

Run with using launch file

Open up one terminal and follow the following steps:

cd ~/catkin_ws
source devel/setup.bash
roslaunch beginner_tutorials changeString.launch frequency:=<integer value for frequency loop>

The frequency parameter is optional. Now we can see that all the nodes along with the roscore are fired up.

Now open up new terminal to call the service and follow the folowing instructions.

cd ~/catkin_ws
source devel/setup.bash
rosservice call changeString "<any new string the user wants to enter>"

Now we observe that the published message changes to our new input string message on the listener terminal.