Lab 1: ROS Nodes, Topics, and Messages

Overview

In this lab, we are going to initialize ROS workspace, create our first ROS package, and create two communicating ROS Nodes.

While following the step-by-step tutorials, please take your time to think about what you are doing and what happens in each step.

Creation of ROS Package

From now on, we assume that you have already installed Ubuntu 20.04 and ROS Noetic.

  • Please open a new terminal, and create a new ROS workspace by the following commands (run them line by line).

mkdir -p ~/catkin_ws/src
cd ~/catkin_ws/src
catkin_init_workspace
cd ../
catkin_make
echo "source ~/catkin_ws/devel/setup.bash" >> ~/.bashrc
source ~/catkin_ws/devel/setup.bash

  • Take a look at catkin_ws directory and see what happens. You can use ls command to see the files in this directory, or use ls -a to see all files including hidden files. Alternatively, you can open File Explorer and navigate to this folder, and use Ctrl + H to see hidden files. The file .catkin_workspace was created to tell the system that this current directory is a ROS workspace.

Note

If you are fresh new to Linux, the instructions might be a bit hard to understand at this moment. No worries. Please just try it for the time being and you will have a better understanding as we move on. You can always ask me any question you want during lab sessions to help you better understand lab materials. (Remember that there is no stupid question; even for those “too simple to ask” questions, We are always happy to answer.)

  • Next let’s create a new ROS package.

cd ~/catkin_ws/src
catkin_create_pkg ee106w26 std_msgs rospy

  • Take a look at your new package ee106w26 and see what happens. You should be able to see a package.xml file and a CMakeLists.txt file. Open them and take a quick look. You may use Google to help you build up a high-level understanding.

  • After creating a new package, we can go back to our workspace and build this package. This is to tell ROS that “Hey, we have a new package here. Please register it into the system.”

cd ~/catkin_ws
catkin_make

  • Now the system knows this ROS package, so that you can have access to it anywhere. Try navigating to different directories first, and then go back to this ROS package by roscd command. See what happens when running the following commands.

cd
roscd ee106w26

cd ~/catkin_ws
roscd ee106w26

cd ~/Documents
roscd ee106w26

  • Congratulations. You have initialized the ROS workspace and created the ee106s25 ROS package! Take some time to think about how the above steps work.

ROS Publisher and Subcriber Python Nodes

The next step is to head to our ROS tutorial and create the ROS publisher and subscriber nodes. The Python scripts can be saved under the ee106w26/src/ folder. To be able to use the developed ROS python nodes, you need to provide execution permissions by,

roscd ee106w26/src/
chmod +x publisher.py
chmod +x subscriber.py

To execute the created ROS nodes, firstly you need to launch the ROS core in a new terminal by doing,

roscore

Then create two separate terminals, and execute,

rosrun ee106w26 publisher.py

and

rosrun ee106w26 subscriber.py

By performing these commands you have successfully created and executed your first ROS application, on which you transfer string data through a ROS topic from the talker to the listener ROS node. To preview the transmitted information through the chatter ROS topic, you can use,

rostopic echo /chatter

The ROS wiki and rospy contain the analytic documentation of the followed steps.

Creation of Custom ROS Message

As mentioned in the class, ROS features a simplified message description language for describing the data values that ROS nodes publish. In our example, we will create a new ROS message, named “EE106lab_custom”, which will be described by the variables,

Header header
int32 int_data
float32 float_data
string string_data

To create this new message type, initially create a folder msg inside the ee106w26 ROS package. Additionally, create a file EE106lab_custom.msg inside the created msg folder, by containing the information depicted above.

To be able to use the new ROS message type, we need to indicate its creation to the ROS workspace and compile it. To achieve this, fistly you need to update the package.xml of ee106w26 and make sure these two lines are in it,

<build_depend>message_generation</build_depend>
<exec_depend>message_runtime</exec_depend>

Additionally, to indicate this modification to the cmake compiler, you need to update the line of CMakeLists.txt of ee106w26 package to contain the message_generation,

# Update the existing line
find_package(catkin REQUIRED COMPONENTS roscpp rospy std_msgs message_generation)

and uncomment this block,

# add_message_files(
#   FILES
#   Message1.msg
#   Message2.msg
# )

to modify it like,

add_message_files(
  FILES
  EE106lab_custom.msg
)

to include the newly created msg type. Similarly, add the message_generation at catkin_package field, as appeared bellow,

catkin_package(
  ...
  CATKIN_DEPENDS rospy std_msgs message_runtime
  ...)

and uncomment the generate_messages cmake field. By performing catkin_make under the ~\catkin_ws\ directory the ROS package is compiled and the EE106lab_custom.msg can be used by any node of any package, as soon as the depedencies are satisfied. This msg structure will be utilized and tested in the submission part of Lab 1. More information about the previous steps can be found in the official ROS msg page.

Submission

  1. Submission: individual submission via Gradescope

  2. Demo: required (Present the subscriber’s additions results in real-time.)

  3. Due time: 11:59pm, Jan 22, Thursday

  4. Files to submit:

    • lab1_report.pdf (A template .pdf is provided for the report. Please include the developed Python code in your report.)

  5. Grading rubric:

    • + 20% Create a new ROS publisher and subscriber node (python scripts). You can use the Python scripts provided at the ee106 class repository.

    • + 20% Create a new ROS message type, named EE106lab_custom_new.msg, that contains a Header and two int32 variables and save it in the msg folder. Build the ROS workspace following the above steps.

    • + 10% Import the EE106lab_custom_new.msg in both publisher and subscriber scripts.

    • + 10% Update the publisher ROS node to send a ROS topic named EE106lab_topic, of EE106lab_custom_new msg type. Send random integers over the ROS topic and update the header with the corresponding timestamp. For the random integer generator you can use random.randint(a,b) function from the random python library.

    • + 10% Update the subscriber ROS node to receive the EE106lab_topic and print the addition of the two int32 variables and the Header timestamp information during the callback.

    • + 30% Write down your lab report, by including comments and screenshots of the following steps, along with terminal results and important findings.