ROS Package Beginner Tutorials

About

This Respository consists of a ROS package for beginners for establishing a publisher and a subscriber node. The publisher publishes a string message onto the topic "chatter". The subscriber node subscribes to the corresponding topic. Service calls are made to the publisher to change the string the publisher publishes.

Author

Aditya Khopkar, akhopkar@umd.edu

Tutorial Steps

If you want to create your own ROS package, you may follow the steps as follows:

Step 1: Navigating ROS Wiki
Step 2: Navigating the Filesystem
Step 3: Creating Catkin Package
Step 4: Building the Package
Step 5: Understanding Nodes
Step 6: Understanding Topics
Step 7: Writing Publisher Subscriber
Step 8: Examining Publisher Subscriber
Step 9: Getting started with roswtf
Step 10: Understanding Services and Params
Step 11: Using rqt_console roslaunch

Dependencies

The project depends on the following dependencies

ROS-Melodic - Robotic Operating System is a meta-operating system for robots. The software can be installed from here
catkin - Catkin is build tool which is developed using CMake and GNU Make. You may install catkin here

Instructions

You need a workspace for this package. A workspace is a directory containing all the projects/packages along with their build and dependency files. To run a ROS package, a catkin workspace is recommended. First, we setup the catkin workspace for our ROS package -

$ mkdir -p ~/catkin_ws/src
$ cd ~/catkin_ws
$ catkin_make
$ source devel/setup.bash

Now that we have setup our catkin workspace, we can clone this repository to build the package and execute -

$ cd ~/catkin_ws/src
$ git clone https://github.com/akhopkar01/beginner_tutorials.git
$ cd beginner_tutorials
$ mkdir include
$ cd ~/catkin_ws
$ catkin_make
$ source devel/setup.bash

Usage - roslaunch and rosservice (Week9)

In this subsection, we will now launch our package using roslaunch and call rosservice to change the output string To execute the procedure, we do the following on the terminal:

$ cd ~/catkin_ws
$ catkin_make
$ source devel/setup.bash
$ roslaunch beginner_tutorials week9.launch freq:=<frequency argument>

The launch file takes a floating point argument to set the frequency and can be given to the roslaunch when evoked. For example, roslaunch beginner_tutorials week9.launch freq:=5. Default value of frequency is 0 and the publisher will automatically set the frequency to 1 in such a case as the frequency cannot be 0, else no message will be published.

To invoke service calls to the nodes, you may do the following (NOTE: roslaunch must stay active): On a new terminal:

$ cd ~/catkin_ws
$ source devel/setup.bash
$ rosservice call /ChangeString "<your string input>"

Ex: rosservice call /ChangeString "ENPM808X is AWESOME!!"

You may observe the log messages on the terminal as well as rqt_console. To invoke rqt-console, open a new terminal (NOTE: roslaunch must stay active or alternatively, you may start roscore on a new terminal): rosrun rqt_console rqt_console

Usage - tf, rostest, rosbag, UnitTest(gtest) (Week10)

Perhaps, in this sub-section we will be using rostest, rosbag and Unit testing in our ROS package. The source code is modified and updated to take into account /tf package of ROS.

tf

In this, we implement a transform broadcaster to broadcast a static transform between the talker node (/talk) frame and the /world frame. To understand tf, follow the following steps:

In order to inspect the tf frames, you may do as follows:

$ sudo apt-get install ros-melodic-ros-tutorials ros-melodic-geometry-tutorials ros-melodic-rviz ros-melodic-rosbash ros-melodic-rqt-tf-tree

$ cd ~/catkin_ws
$ catkin_make
$ source devel/setup.bash
$ roslaunch beginner_tutorials week9.launch freq:=1

Open a new terminal and do the following:

$ rosrun rqt_tf_tree rqt_tf_tree

OR, you may simply:

$ rqt &

And then select, Plugins->Visualization->TF_tree. You may view the tf frames by:

$ rosrun tf view_frames
$ evince frames.pdf

You may also execute tf_echo functionality by running the command: rosrun tf tf_echo talk world. Follow Step 1 for further clarification.

The frames.pdf could also be accessed from beginner_tutorials/results/frames.pdf.

rostest

This package includes rostest suite which includes unit-testing framework using g-test. The rostest may be executed and validated as follows, Open a terminal:

$ cd ~/catkin_ws
$ source devel/setup.bash
$ rostest beginner_tutorials test.launch

There are 2 tests, the Summary should suggest RESULT:SUCCESS. You may follow the following links for details regarding rostest:

rosbag

rosbag is a package majorly used to record data and simulate in simulation time, which emulates real-time. The rosbag functionality can be enabled for the package as follows in your catkin workspace:

$ source devel/setup.bash
$ roslaunch beginner_tutorials week9.launch freq:=1 rosbag:=enable

This starts the rosbag. The rosbag data is logged in beginner_tutorials/results/record.bag

In order to play the bag, navigate to the location suggested, Open a new terminal, After starting roscore in a new terminal:

$ cd ~/catkin_ws
$ source devel/setup.bash
$ rosrun beginner_tutorials listener

In the previous terminal, play the bag file using the command: rosbag play record.bag. You will see the listener node generating messages received from the bag file, thus emulating the real functionality of our ROS package.

You can inspect the rosbag by following techniques: rosbag info record.bag To get the info of the recorded bagfile. You can see the /Chatter topic in the info. rosbag check record.bag TO check if the bagfile is playable or not. You may see this link for reference.

NOTE

It can be observed that, everytime a new terminal is opened you have to write the command source devel/setup.bash. It gets really annoying to do so. The following workaround could be used:

$ echo "source ~/catkin_ws/devel/setup.bash" >> ./bashrc

By doing this, you wont have to worry about sourcing the setup file every time for your workspace.

akhopkar01/ros-pub-sub-demo