(20240911).

How To create a customized ROS msg in a catkin_ws?

In ROS, custom message types are defined using .msg files, which are stored in a package’s msg folder. These message types allow you to define custom data structures that can be used for inter-node communication. Below is a guide on how to create and use custom message types in a catkin_ws workspace.

Steps to Customize a msg Folder and Files in a catkin_ws.

1. Create a ROS Package

First, create a package in your catkin_ws workspace. This package will hold your custom messages.

cd ~/catkin_ws/src
catkin_create_pkg teng4pkg_msgs std_msgs rospy roscpp

Here, teng4pkg_msgs is the package name, and it depends on std_msgs, rospy, and roscpp.

2. Create a msg Directory

Inside your package, create a msg folder where your custom message types will be defined.

cd ~/catkin_ws/src/teng4pkg_msgs
mkdir msg

3. Create a ‘.msg’ File

Inside the msg folder, create a file with a .msg extension. This file defines your custom message type.

Example: PoseCartesian6DOF.msg

gedit msg/PoseCartesian6DOF.msg

Inside PoseCartesian6DOF.msg, define the structure of the message. For example, for a 6D cartesian pose:

float64 x
float64 y
float64 z
float64 a
float64 b
float64 g

Each line defines a field in the message, with the type followed by the field name.

4. Modify CMakeLists.txt

You need to modify your package’s CMakeLists.txt file to include the message generation.

• Find and uncomment (or add) message_generation as a required component in find_package:

find_package(catkin REQUIRED COMPONENTS
  roscpp
  rospy
  std_msgs
  message_generation
)

• Add the custom message files under add_message_files:

add_message_files(
  FILES
  PoseCartesian6DOF.msg
)

• Add the following after add_message_files() to actually generate the messages. Add generate_messages() and ensure std_msgs and other dependencies are listed:

generate_messages(
  DEPENDENCIES
  std_msgs
)

• Ensure the catkin_package includes message_runtime:

catkin_package(
  CATKIN_DEPENDS message_runtime std_msgs
)

5. Modify package.xml

In your package’s package.xml, ensure you have the following dependencies for message generation:

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

6. Build the Package

After modifying the files, go back to the root of your catkin_ws workspace and build the workspace.

cd ~/catkin_ws
catkin_make

If everything is set up correctly, the custom message types will be generated and ready for use.

7. Using the Custom Message

Now, you can use your custom message type in your ROS nodes.

C++ Example:

// teng4note, the Position.h may not be easy to find or even not exist. But it does not matter.
#include "teng4pkg_msgs/PoseCartesian6DOF.h"

// Publishing a custom message
teng4pkg_msgs::PoseCartesian6DOF test1_msg;
test1_msg.x = 1.11;
test1_msg.y = 1.11;
test1_msg.z = 1.11;
test1_msg.a = 1.11;
test1_msg.b = 1.11;
test1_msg.g = 1.11;
pub_test1_pose6dof.publish(test1_msg);

Python Example:

from teng4pkg_msgs.msg import PoseCartesian6DOF

# Publishing a custom message
test1_msg = Position()
test1_msg.x = 1.11;
test1_msg.y = 1.11;
test1_msg.z = 1.11;
test1_msg.a = 1.11;
test1_msg.b = 1.11;
test1_msg.g = 1.11;

8. Verify the Custom Message

Now, we can verify the custom message in a terminal. First you need to run your file.

echo $ROS_PACKAGE_PATH

// May need to source the Workspace Again
source devel/setup.bash

// note, the Terminal needs `cd` to the correct catkin_ws, in order to be able to echo the customized msg.
cd ~/catkin_ws
rostopic list
rostopic list -v

// Check Message Availability, suppose msg route is /teng4pkg_msgs/PoseCartesian6DOF
rosmsg show teng4pkg_msgs/PoseCartesian6DOF

// echo the rostopic, suppose the publisher is /teng4/test1_pose6dof
rostopic echo /teng4/test1_pose6dof

// some other cmd for retrieving the ROS param.
rosparam list
rosparam get /PSM1/rate
cd catkin_ws
echo $ROS_PACKAGE_PATH
source devel/setup.bash

9. Error Debugging.

You may encounter some errors when rostopic echo /your_topic the customized ROS msg. For example,

$ rostopic echo /teng4/test1_pose6dof

ERROR: Cannot load message class for [teng4pkg_msgs/PoseCartesian6DOF]. Are your messages built?

SOLUTION: This error is caused by wrong catkin_ws in terminal. You need to get into the correct catkin_ws2 ($cd catkin_ws2; $cd ~/catkin_ws2), and then “rostopic echo /teng4/test1_pose6dof “ will work.

$ cd ~/catkin_ws2

$ source devel/setup.bash

$ echo $ROS_PACKAGE_PATH

$ rostopic list

$ rostopic echo /teng4/test1_pose6dof

Summary

• Create a ROS package in catkin_ws/src.
• Create a msg directory and define your custom .msg file.
• Update the CMakeLists.txt and package.xml to include message generation.
• Build your workspace with catkin_make.
• Use the custom message in your ROS nodes for inter-node communication.

This setup allows you to define custom data structures for your ROS applications.

Created on 2024-09-11.