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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.
git clone https://gitlab.fel.cvut.cz/robolab/deploy.git ./deploy/scripts/install_singularity ./deploy/scripts/download_singularity_image ./deploy/scripts/start_singularity_aro
The start_singularity_aro script not only starts the singularity, but also creates appropriate ROS workspace, clones the student packages to the workspace, builds the workspace, and sources the workspace. Details of this process are described here.
After starting the singularity container, try to run the simulation which will be part of your first homework using command:
roslaunch aro_reactive_control hw01_sim.launch
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
tmux a
Inside tmux session, press $prefix (ctrl+b) followed by any of the keys below to controll the session:
ctrl+b
+ d
+ c
+ w
+ n
+ p
+ ,
+ %
+ “
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:
roscore
The code for your projects with ROS is stored in a workspace.
In case you would like to create your own workspace in some folder, you can do it by running the following commands in the started singularity image:
cd <where_you_want_the_workspace> mkdir -p catkin_ws/src cd catkin_ws catkin init catkin config --extend /opt/ros/aro catkin build source devel/setup.bash
Please note that even when you are using Python, which normally does not require a build step, ROS requires you to re-build and re-source the workspace in some cases - most notably when you add a new package.
ROS has many handy command line programs that you can learn.
rosnode list/info <node_name>/ping <node_name>/kill <node_name>
rostopic list/info <topic_name>/hz <topic_name>/echo <topic_name>/type <topic_name>/pub <topic_name> <topic_type> <data>
rosmsg list/show <message_type>/package
roslaunch <package_name> <launch_filename>
rosbag record/play/info
roscd <package_name>
rosrun <package_name> <node_name>
catkin build/clean/init/config
Test the following individual commands. Use the tab for suggestions of node names, topic names, message types, etc.
rosnode
rostopic pub /new std_msgs/String "data: 'whaaat'" -r 10
rosmsg list
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:
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.
chmod u+x <filename>.py
source devel/setup.bash
catkin build
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).
rospy.init_node('my_node')
rospy.spin()
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.
rosmsg package std_msgs
rosmsg info std_msgs/Header
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('message', String, queue_size=10). Then every time you want to send a message you create a new message object msg = String() , then fill it with your data (msg.data=“ahoj”) and publish it to ROS using publisher.publish(msg).
publisher = rospy.Publisher('message', String, queue_size=10)
msg = String()
msg.data=“ahoj”
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('message', String , callback) ,where the callback is a custom function (def callback(msg):) that handles the received message.
subscriber = rospy.Subscriber('message', String , callback)
def callback(msg):
Implement a simple publisher and subscriber nodes as specified below. You can get inspiration on how to write such python nodes in the following ROS tutorial. Both can be placed inside the aro_reactive_control/scripts of the student-packages.
publisher.py
subscriber.py
Follow the assignment of the homework HW01 which requires only some additional work compared to the lab task above.