Publisher & Subscriber

In this exercise, we will explore the concept of ROS messages and topics. The first type of ROS communication that we will explore is a one-way communication called messages which are sent over channels called topics. Typically one node publishes messages on a topic and another node subscribes to messages on that same topic.

In the following we first complete couple of ROS tutorials to get familiar with how subscribers and publishers are implemented in ROS. After practicing, we add a publisher to our fake_sonar_driver_node.

  1. First, create a new package into your catkin_ws. Name it beginner_tutorials and make it depend on std_msgs, rospy, and roscpp packages.
  2. Complete the Writing a Simple Publisher and Subscriber (C++) tutorial.
  3. Now that you got the nodes communicating, it is time to add a publisher to our sonar_driver package. Check out the documentation of the sensor_msgs/Range message. This message is being used for devices that measure distance to an object while having a specific field of view (for example, ultrasonic, infrared, etc sensors).
  4. Navigate back to the fake_sonar_driver package and open the fake_sonar_driver_node.cpp for editing.
  5. Add the following functionality to our fake sonar node:
  • the node publishes messages on a topic /sonar_distance
  • the type of messages is sensor_msgs/Range
  • the range field in the messages is set to 1.5

  1. Test the publisher with rostopic echo /sonar_distance and show the result to the instructor.

  2. Improve the fake_sonar_driver_node to make it outputing oscilating signal in range [0-2] meters. For that you can use a count variable as was in the beginners_tutorials. Simply scale it down a bit and use it as an argument of a sine() function:

    x = sine(count / 100.0f) + 1;

Tip

Don’t forget the #include <math.h> to bring in the trigonometry functions to your project.

  1. Let’s visualize the distance value on a graph. For that, ROS has a visualization toolkit called rqt. Enter this as a command in a new terminal window.
    • In the upper menu bar, click Plugins –> Visualization –> Plot
    • Specify the topic as /sonar_distance/range and you should see the range field plotted.