Robotics- Lab Session 1 Solved

This lab session will introduce you to the Robot Operating System (ROS). By the end of this session you will:

·       Understand basic ROS concepts;

·       Learn how to publish/listen ROS messages;

·       Learn useful ROS command-line tools.

This session will introduce you only to the most basic concepts in ROS. You can explore and learn more using online resources, particularly the ROS wiki at: http://wiki.ros.org/ .

What is ROS?
The Robot Operating System (ROS) is a set of software libraries and tools that help you build robot applications.

It is named an "operating system" because its main goal is similar to a regular operating system on a PC: to provide abstraction. On a PC you should not need to re-write the low-level code to control a printer every time you write a new application that has print functionality. Instead, there is a driver for the printer that takes care of the low-level control, and when you write a new application you simply communicate with this driver on a higher abstraction level (e.g. a postscript file). Similarly, on a robot, you should not need to re-write the low-level code to control a wheel motor every time you write a new robot application. Instead, there is a separate process taking care of the low-level motor control, and your robot application communicates with this controller on an abstracted layer (e.g. the desired robot velocity). ROS' main functionality is to provide a mechanism for this abstracted communication between processes. Each such process is named a node in ROS. For example, the motor controller may be a node and your application may be another node. They speak through ROS messages. In this lab session, you will learn how to write two ROS nodes that communicate through a ROS message. There will not be much about robots today; instead we will learn about the basic functionality. Using ROS messages for controlling a robot will be the subject of the next lab session.

While ROS is similar to a regular operating system in terms of the abstraction it provides, the similarities do not go much further. For example, you do not install ROS instead of an OS. On the contrary, ROS needs to be installed on an OS, and ROS is best supported on Ubuntu Linux.

Running Ubuntu Linux on DEC-10/ENIAC computers
Since DEC-10/ENIAC computers are running CentOS Linux, we will use Singularity, a container solution for virtualisation to run Ubuntu Linux on them. (The Turtlebot laptops that you will use on the real robots are running Ubuntu Linux, therefore you will not need to, and should not, run Singularity on them.)

After you login to a DEC-10/ENIAC CentOS computer, open a terminal and execute the following command:

singularity shell --nv /vol/scratch/ros/comp3631.img

(If you are in ENIAC, please remove “--nv” from the command above.)

This will put you in a container running Ubuntu Linux.

Then you can create a new terminal window by running the following command:

xterm &

You should execute all the following commands of this lab session inside Ubuntu terminals. You can create as many terminals as you require using the command above.

Setting up a ROS Catkin workspace
Before you write any ROS code, you need to set up a ROS workspace.

Your code must reside in a workspace to be discovered by ROS. ROS workspace management is performed by a piece of software called "catkin". That is why you will see that these workspaces are sometimes referred to as "catkin workspace".

1. Under your home directory, create a folder to house your catkin workspace (Name it catkin_ws)

mkdir catkin_ws

2. Clone your git repository under your catkin_ws.

cd catkin_ws

git clone https://gitlab.com/comp3631/<YOUR-UNI-USERNAME.git  src

Don’t forget the ‘src’ at the end of the command. It will clone the contents of your git repository under a directory named 'src'.

Go into the src directory, and look at the directories cloned from the git repo.

cd src

ls

You will see one directory for each lab session and the project you will work on. You must put the files you create in a lab session into the corresponding directory. For example in this lab session, you must put all your code under catkin_ws/src/lab1 . You must also add, commit and push all your files back to git, so that you can access them later on the real robots.

3. Now initialise the catkin work space from within the src directory

catkin_init_workspace

4. You can now build (compile) the code in your workspace. Navigate back to the top level of your catkin workspace (e.g. cd ~/catkin_ws) then execute:

catkin_make

source devel/setup.bash

It would also be worth your time to add this last command into your .bashrc to ensure that you do not need to source the workspace every time a new terminal shell is opened. (Of coure, you cannot use the relative path devel/setup.bash in your .bashrc file. Use the absolute path.)

You will use one workspace for all the code you will develop in this module. In other words, you do not need to create a workspace (steps 1 to 3 above) for every different worksheet you will be working on.  You should simply use the lab1, lab2, lab3 … directories under the catkin_ws/src that you created here. You may need to compile your code (step 4 above) every time you change the code in your workspace.

You can find more information about ROS Catkin workspaces here: http://wiki.ros.org/catkin/Tutorials/create_a_workspace

Catkin Packages
A package houses related code and files in a named directory. Each of the lab1, lab2, … directories under your catkin_ws/src are packages.

Navigate into the src folder within the lab1 package, this is where you will edit your first python scripts.

cd ~/catkin_ws/src/lab1/src

You can find more information about creating ROS packages here: http://wiki.ros.org/catkin/Tutorials/CreatingPackage

A simple publisher and subscriber
You can use Python or C++ to write ROS nodes. Here, we will use Python. There will be two nodes, one talker.py and one listener.py. The talker will publish a simple message and the listener will subscribe to that message and print what it hears to the terminal.

In case they are not, make these scripts executable:

chmod +x talker.py listener.py

(Whenever you create or download a python script, you will need to make them executable by running chmod +x on them, as above. Please keep this in mind for the rest of the semester.)

Before you run these scripts, inspect them. Go to this link

http://wiki.ros.org/ROS/Tutorials/WritingPublisherSubscriber(python)

and read the section "1.2 The code explained" which explains talker.py. Scroll down, there is another section named "2.2 The code explained" which explains listener.py. Read both sections and make sure you understand what is happening inside both the talker and the listener.

You can use ROS commands to run your nodes. However for the nodes to be able to communicate, a ROS master must be available. So in a new terminal window, we first run a ROS master using the roscore command:

roscore

Now use rosrun to run the publisher node in a separate terminal window:

rosrun lab1 talker.py

(If you got the error: "[rospack] Error: package 'lab1' not found", then you probably did not put "source devel/setup.bash" into your .bashrc yet, and you have not executed it in your new terminal window either. You have to do at least one of these for ROS to be able to find your packages.)

You should notice the publisher logging its outputs in the terminal you ran it in.

Open another terminal window and rosrun the subscriber node:

rosrun lab1 listener.py

Launch files
A robot runs tens, sometimes hundreds of nodes like this. As you can imagine getting the robot operational by running each node separately would be far too tedious to handle. Therefore ROS employs launch files to run many nodes in one go.

Now Crtl-c your roscore, publisher and listener, and we will re-launch them this time using a single launch file.

The .launch file specifies nodes from packages and any parameters they may need to be launched when the .launch file is ran. The .launch files follow a specific syntax, a form of XML. A launch file always begins with a <launch tag and ends with a  </launch tag. Declarations of nodes to be launched are enclosed within a <node tag for example:

<node pkg="lab1" type="talker.py" name="talker"/

pkg refers to the package we are launching a node from. type refers to the exact node file we wish to launch. The name parameter allows us to name the node; you can change the name if you wish. Under lab1/launch, you should find the launch file that launches the two nodes: lab1.launch. Inspect lab1.launch file and see how it refers to the talker and listener scripts.

Then in a terminal window roslaunch the launch file:

roslaunch lab1 lab1.launch

OR use the absolute/relative path of the launch file:

roslaunch path/to/lab1.launch

(Note that we did not run roscore this time. The command ‘roslaunch’ handles that for us.)

You are now running the talker and listener.

Useful ROS commands
ROS provides many command line tools to inspect the state of the ROS environment. For example, while your launch file is still running, in a new terminal window run:

     rosnode list

This will list the names of all the nodes running currently in the ROS environment.

You can also get information about a specific node. For example, run:

     rosnode info talker

Notice that, ROS gives you information about the type of this node, but also about the topics it is publishing to.

Try running rosnode info for another running node and see what information you get.

Similar to rosnode, there is also rostopic to get information about topics. Run:

      rostopic list

This will list all the active topics in your ROS environment.

Again, you can get information about a specific topic. For example, run:

      rostopic info /chatter

Notice that ROS gives you information about the type of the topic and the nodes publishing to and listening to this topic.

Try running rostopic info on other topics you have seen in the rostopic list output.

Using rostopic echo you can also listen to a particular topic. Execute:

rostopic echo /chatter

One final useful command is roscd. This is like regular cd in Linux, but allows you to navigate between ros packages without knowing the full path. For example, go to any random directory on your system, and then execute:

     roscd lab1

Notice that this takes you to your package's directory.

Exercises
1. Create a copy of talker.py under lab1/src and save it as a new file numeric_talker.py. Edit this file to include an additional publisher publishing to a new /numeric_chatter topic with message type Int8 (an 8-bit integer type). Your talker should keep a counter starting from 0, increment it every iteration of the while loop (resetting back to zero at 127, which is the max value for a signed 8-bit integer), and publish the value of the counter to the /numeric_chatter topic. You will need to import the new message type to be able to use it. (Go to http://wiki.ros.org/std_msgs to see all the standard message types defined in ROS.) Do not remove the String publisher to the /chatter topic! A node can publish to multiple topics.

2. Create a copy of listener.py under lab1/src and save it as numeric_listener.py. Edit this file to include an additional subscriber to the /numeric_chatter topic and print the received number in its callback function. Do not remove the subscriber to the /chatter topic! A node can listen to multiple topics.

3. Add, commit, and push your new files and changes to the git repository.

cd ~/catkin_ws/src/lab1/src/

git add <list-of-files-you-want-to-push-to-git

git commit -m “your message”

git push

Lab Session 1 – Real Turtlebot Lab
This section should be followed only if you are working on a Turtlebot laptop.

Please keep your laptops in the lockers and please plug them in so that they get charged.

You can login to laptops using

Username: robot

Password: robot

You can connect to the internet through eduroam.

Since you will be working on this laptop with your group’s members, please configure the wireless eduroam settings such that it does not save your password. You can do this by clicking on the WiFi symbol at the top right of the Ubuntu menu, and choosing “Edit connections…”. Then, in the new window, select “eduroam” and click Edit. Then choose “Wi-Fi Security” and check the box saying “Ask for this password every time”. Also go to “IPv6 Settings” and change the Method to “Ignore”. Then Save and close the window. You can ignore any messages about certificates.

 
Initializing your group’s git repo
When you work on the simulation lab, you work individually, and you push your files to your individual git repo. When you work on the real Turtlebots, you work as a group, and you will use another git repo as a group.

Before you start working on a lab worksheet as a group, you should copy the files from one of your individual git repos to your group’s git repo. You can do this by calling a script:

copy_student_files_to_group_repo <student-user-name <group-name <lab

 

For example, let’s say you are Group1, and a member of your team has the username sc20ab and you want to initialize your group’s git repo with his/her files for lab1 (maybe he/she made the most progress during the simulation lab1 and therefore you want to start working with his/her files). Then, in a terminal window, execute:

 

copy_student_files_to_group_repo sc20ab Group1 lab1

 

This will take the lab1 files from sc20ab’s git repo, and copy them to your group’s git repo. Note that, this happens on the gitlab servers. There is no local change on the laptop yet.

 

Also note that you should do this only once for each lab (i.e. each of lab1, lab2, lab3, lab4, lab5, project) when you first start working on that lab session on the Turtlebot laptop.

 
 

Setup Turtlebot laptop for your group
The Turtlebot laptops do not keep your files on disk. Therefore, everytime you log in, you will need to retrieve any files you want to work with from your group’s git repo. To configure the laptop for a specific group, simply open a terminal window, and run the command below with your group’s name:

setup_group <your-group-name

For example:

setup_group Group1

This script does something simple: it clones your group’s git repo under ~/catkin_ws on the Turtlebot laptop. You can see the directories and files under  ~/catkin_ws/src .

Note that you will need to call this setup script every time you logon to the Turtlebot laptop.

!!! IMPORTANT !!!: Turtlebot laptops erase all changes and revert to a clean state once you log off. Therefore, if you edit/create any files on the Turtlebot laptop, and if you do not want to lose your changes/files, then push them to your group’s git repo before logging off. We recommend pushing your changes to git frequently, in case the laptop accidentally turns off, e.g. in case the battery is drained, since you will lose your changes on the laptop if they are not pushed to git.

For example, if you want to push your changes as you work on lab1:

cd ~/catkin_ws/src/lab1/src/

git add <list-of-files-you-want-to-push-to-git

git commit -m “your message”

git push

 

Exercises
1.     Run the talker and listener scripts, and confirm they are working correctly.

2.     Run the numeric_talker and numeric_listener scripts, and confirm they are working correctly.

3.     If you want to save any of your changes, add, commit, and push them to the git repository.