tags: ROS Raspberry Pi

Publish/Subscribe a Message

Message Setup

You can learn more about msg and srv in ROS wiki: Creating a ROS msg and srv.

1.Create a msg

First, let's create a *.msg file. Let me take an example for message Motor:

$ roscd <Package Name>
$ mkdir msg
$ touch msg/Motor.msg

Motor.msg

float32 direction
float32 speed

2.Edit package.xml

Make sure these two lines are uncommented:

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

3.Edit CMakeLists.txt

You have to edit CMakeLists.txt file to activate your *.msg file.

Make sure you add std_msgs and message_generation to the list of COMPONENTS and export message_runtime dependency in catkin_package:

find_package(catkin REQUIRED COMPONENTS
   roscpp
   rospy
   std_msgs
   message_generation
)

...

catkin_package(
    ...
    CATKIN_DEPENDS ... message_runtime ...
    ...
)

Next, edit add_message_files and generate_messages blocks like this:

add_message_files(
    FILES
    Motor.msg
)

...

generate_messages(
  DEPENDENCIES
  std_msgs
)

Finally, you have to run catkin_make

$ cd ~/catkin_ws
$ catkin_make

Publisher/Subscriber Node (Python)

Let's create publisher/subscriber nodes with Python. In this section, we implement this:

In this example, node Controller is a Publisher and node Motor is a Subscriber. Node Controller doesn't subscribe any topic here.

Publiser (Controller.py)

#!/usr/bin/python
# -*- coding: utf-8 -*-

import rospy
from <Package Name>.msg import Motor

def Controller():

    rospy.init_node('controller', anonymous=True, disable_signals=True)
    # Publish message to Topic named '/motor'
    pub = rospy.Publisher('motor', Motor, queue_size=10)

    msg = Motor()

    msg.direction = 0
    msg.speed = 0.5

    r = rospy.Rate(10)
    while not rospy.is_shutdown():
        pub.publish(msg)
        r.sleep()

if __name__ == "__main__":
    try:
        Controller()
    except rospy.ROSInterruptException:
        pass

Subscriber (Motor.py)