[ROS Q&A] 189 – How to create a custom action message

custom action message for ROS Actions

Written by Alberto Ezquerro



Learn how to create and compile a custom action message for ROS Actions from zero.

You’ll learn

  • How to define the message 
  • How to modify the CMakeLists.txt and package.xml files
  • How to compile and import the message for ROS Actions

Below are the Steps to create the project as shown in the video

Step 1

  • Head to ROS Development Studio and create a new project.
  • Provide a suitable project name and some useful description. (We have named the project how_to_make_custom_ros_action)
  • Load/Start the project (this will take a few seconds).
  • Let’s create a new catkin package in our catkin workspace. Open Tools > Shell and run the following commands

$ cd catkin_ws/src
$ catkin_create_pkg turtlebot3_console_controller actionlib_msgs std_msgs

  • We’ll create an action directory to put the action file. We will create an action file named Motor.action inside the action directory

$ cd turtlebot3_console_controller
$ mkdir action
$ touch Motor.action

  • Use Tools > IDE to browse the directory and file we just created. At this point we should have the following directory structure
    ├── ai_ws
    ├── catkin_ws
    │   ├── build
    │   ├── devel
    │   └── src
    │       ├── CMakeLists.txt
    │       └── turtlebot3_console_controller
    │           ├── action
    │           │   └── Motor.action
    │           ├── CMakeLists.txt
    │           └── package.xml
    ├── how_to_make_custom_ros_action.zip
    ├── notebook_ws
    │   ├── default.ipynb
    │   └── images
    └── simulation_ws
      ├── build
      ├── devel
      └── src

Step 2

  • Open the Motor.action file in IDE and write the following content to it.
# This is an action definition file, which has three parts: the goal
# the result, and the feedback.
# Part 1: the goal.
# The angle in degree the robot to turn, sent by client main
float64 angle
# Part 2: the result, sent by action server unpon completion
# How much time used
duration time_elapsed
# How many updates thrown in total
uint32 updates_n
# Part 3: the feedback,to be sent periodically by server
# The amount of time elapsed from the start
duration time_elapsed
  • The contents of Motor.action defines variables of three categories. The categories are separated by lines. The first category of variables are related to GOAL, second is related to RESULT and third are related to the FEEDBACK. Here we have the following variable in different categories: angle for GOAL, time_elapsed, updates_n for RESULT and time_elapsed for FEEDBACK.
  • Now we have to add the directive to compile action file in CMakeLists.txt file. This file is inside our package directory i.e. ~/catkin_ws/src/turtlebot3_console_controller. Open the CMakeLists.txt file in IDE
  • Add the following code block
  • Also, add this code block
  • Let’s compile the project. Run following commands in the SHELL

    $ cd ~/catkin_ws

    $ catkin_make

  • To test if the action file works we will create a script name node.py inside a new directory within the package. Use the following commands in SHELL

    $ cd ~/catkin_ws/src/turtlebot3_console_controller

    $ mkdir src

    $ touch node.py

  • To create the directory and file we can use IDE tool as well. Let’s add the following code to the node.py file
from turtlebot3_console_controller.msg import MotorGoal, MotorResult, MotorFeedback 

print("OK, it worked!!!")

Let’s run the above script. Enter the following commands in the SHELL

$ cd ~/catkin_ws/src/turtlebot3_console_controller/src

$ ./node.py

After running the above command you should see the reply OK, it worked!!! in the next line.

And that’s it. We have created a basic action message and verified it works.

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Topics: ROS Q&A
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