Lab 01

During this lab, you should learn how to use the Singularity, the tmux, the terminal commands of Robot Operating System (ROS), and how to program a simple python subscriber/publisher node.

The Singularity is software for virtualization via containers that allows you to program and test ARO homeworks and semestral work without installing all ROS dependencies on your computer.

• You can learn how to use the singularity on Lab PCs and/or details how to install/build it on your computer here.
• To use the Lab computers remotely using ssh or remote desktop refer to this page.
• For now with your notebook with Linux OS, you can install singularity, download the image, and run the image using:

 git clone https://gitlab.fel.cvut.cz/robolab/deploy.git
 ./deploy/scripts/install_singularity
 ./deploy/scripts/download_singularity_aro
 ./deploy/scripts/start_singularity_aro
 source /opt/ros/aro/setup.bash
 

Tmux allows you to switch easily between several windows in one terminal., detach them (they keep running in the background) and reattach them to a different terminal. To start Tmux use:

• tmux - starts a new tmux session
• tmux a - attaches to an existing session

Inside tmux session, press $prefixctrl+b followed by any of the keys below to controll the session:

• $prefix+ d - detach from the session while it keeps running
• $prefix+ c - create new window
• $prefix+ w - list windows with option to select one to see
• $prefix+ n - next window
• $prefix+ p - previous window
• $prefix+ , - name window
• $prefix+ % - split vertically
• $prefix+ “ split horizontally

Robot Operating System (ROS) https://www.ros.org/ is a set of software libraries that allows you to build a complex robotic system. It is not an operating system but rather a very convenient middleware for sharing data (messages) between different parts of the robot's software (nodes). When using ROS you can leverage the work of others and use an already very large code base of implemented packages for, e.g., mapping, localization, control, planning, and sensor interfacing.

Key aspects/buzzwords to know are:

• ROS architecture - ROS uses a central node called master that registers all software modules (nodes) that want to communicate. The modules then communicate directly with each other using assigned ports.
• master - Can be started by calling roscore and needs to be always started first before other nodes. Its address is stored in the environment variable $ROS_MASTER_URI.
• programming - We will use python 3, but ROS also supports programming in c++.
• version - We will use ROS noetic inside Ubuntu 20.04 inside singularity image.
• package - Software in ROS is organized in packages. Usually, they contain individual nodes with different purposes.
• node - Executable program that runs some part of the robot's software and uses ROS to communicate with other nodes. Multiple instances with different name can run simultaneously.
• message - ROS data type used to send information between nodes.
• topic - Named communication channel for particular message types, e.g., camera image topic ”/camera/image“, that the nodes use to send/receive data from.
• publisher - Part of a node that sends a messages of some type to a particular topic. There can be multiple publishers on the same topic.
• subscriber - Part of a node that receives all messages of a particular topic. There can be multiple subscribers on the same topic.
• launch file - Recipe that can start multiple nodes with given params simultaneously.
• workspace - Place for your project with multiple ROS packages.
• catkin - CMake-based build system for ROS that builds your workspace packages. Also, auto-generates specified messages based on their definition.
• bag file - File format in ROS for storing ROS message data allowing to log all transmitted messages within the robotic system.
• rosbag - A command line tool for recording and playing back the recorded bag files.

The code for your projects with ROS is stored in a workspace. You can create your own workspace in some folder by running the following commands in started singularity image:

cd <where_you_want_the_workspace>
source /opt/ros/aro/setup.bash
mkdir -p catkin_ws/src
cd catkin_ws
catkin build
source devel/setup.bash

This code sources the default ROS workspace to make your workspace an extension of the default one. Then it creates the workspace folders and calls the catkin tool to build the workspace. Finally, it sources the workspace to load information about all built packages. Now you can place any of your packages into the src folder and after repeating catkin build and sourcing parts you can use them.

ROS has many handy command line programs that you can learn.

• roscore - starts the ros master
• rosnode list/info <node_name>/ping <node_name>/kill <node_name> - list shows all running nodes, info gives you information about a specified node, ping checks if you can contact the node, kill terminates the node
• rostopic list/info <topic_name>/hz <topic_name>/echo <topic_name>/type <topic_name>/pub <topic_name> <topic_type> <data> - list shows all currently available topic names, info shows information about a given topic, hz shows frequency of messages published on a topic, echo prints all messages on a given topic, type shows message type of the topic, pub publishes a message to a topic
• rosmsg list/show <message_type>/package - list shows all available messages, show displays the definition of a given message type, package lists messages of a certain package
• roslaunch <package_name> <launch_filename> - launches the <launch_filename> of a given <package_name>
• rosbag record/play/info - record a rosbag, play existing rosbag, info gives you information what is inside a rosbag
• roscd <package_name> - goes into directory of a given package
• rosrun <package_name> <node_name> - starts node <node_name> from package <package_name>
• catkin build/clean/init/config - build workspace you are in, clean workspace you are in, init creates a new workspace in current folder, config - shows configuration of a workspace you are in

Test the following individual commands. Use the tab for suggestions of node names, topic names, message types, etc.

1. Start tmux and run the roscore.
2. Create new tmux window and test all rosnode commands on default /rosout node.
3. Publish std_msgs/String message on topic /new with rate 10Hz using command rostopic pub /new std_msgs/String "data: 'whaaat'" -r 10. Test other rostopic commands on this topic in new tmux window.
4. Test listing all available messages using rosmsg list and find out what messages are in std_msgs package. Show the definition of one of the message type.
5. Download and extract the bagfile for first homework. Test robag info to find its content and then play the rosbag.

In ROS noetic we will use python 3. The most important library that enables interfacing with ROS is rospy. The typical content of a package folder in, e.g., catkin_ws/src/my_package/ is:

• launch/ - folder with launch files
• msg/ - folder with message definitions
• src/ - folder with source codes where you can put your nodes
• CMakeLists.txt - CMake build file
• package.xml - file with information about the package used by the catkin

Always make sure the python node is a runnable script chmod u+x <filename>.py, otherwise you can not start it with rosrun or roslaunch. Always make sure to source devel/setup.bash in your workspace after you build your workspace with catkin build. You need to build the workspace if you have new package, if you change c++ code or, e.g., message definitions. Changing python code does not require rebuilding.

At the beginning of a python script you need to initialize the node such that it is registered within the ROS with rospy.init_node('my_node') command. Afterwards you can initialize all your subscribers, publishers, threaded workers, etc. Finally you need to run rospy.spin() which internally runs the messaging system of the node (especially important for subscribers to run callbacks for new messages).

You can print messages within your package with different severity levels using:

• rospy.loginfo('your message')
• rospy.logwarn('your warning message')
• rospy.logerr('your error message')

Messages are data structures used to send data over topics. They are composed of either simple or complex data types. Simple types are, e.g.: bool, int<N>, uint<N>, float<N> (with N being {8, 16, 32, 64}), String, Time, Duration (see standard messages rosmsg package std_msgs). Complex data types are composed of simple ones or their arrays (see, e.g., rosmsg info std_msgs/Header ). Custom message files can be defined in message files and their respective c++ headers and python objects are generated during build.

Publisher is an object assigned to a specific topic and message type that you can use to send messages from your node to other nodes. You can create publisher object using: publisher = rospy.Publisher('mytopic', AROMessage, queue_size=10). Then every time you want to send a message you create a new message object msg = AROMessage() , then fill it with your data (msg.message=“ahoj”) and publish it to ROS using publisher.publish(msg).

Subscriber is an object that basically assigns a callback function to received messages of a given topic name and message type. You can create the subscriber using: subscriber = rospy.Subscriber('mytopic', AROMessage , callback) ,where the callback is a custom function (def callback(msg):) that handles the received message.

Implement a simple publisher and subscriber nodes as specified below. Follow the TODOs inside the code. You can get inspiration on how to write such python nodes in the following ROS tutorial.

1. First download lab01.zip and unpack it in your workspace's src folder (to have catkin_ws/src/lab01_package/).
2. Implement publisher inside publisher.py that publish AROMessage on topic “messages” every one second.
3. Launch the publisher using the launch file lab01_demo.launch
4. Implement subscriber inside subscriber.py that receives the messages and print them in terminal.
5. Modify the launch file to additionally also start the subscriber node and thus to start both nodes using the same launchfile.
6. Inside subscriber.py implement an additional publisher that publishes on topic “messages_sum” a sum of the numbers received inside a given number of last received AROMessages on topic “messages”.
7. Add parameter num_messages to the subscriber node part of the launch file to be able to change the parameter of how many received messages should be summed.

Follow the assignment of the homework HW1 which requires only some additional work compared to the lab task above.