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Blink (example subscriber)

melodic noetic   Show EOL distros: 

Blink: Creating a Subscriber

The Code

Now that we've created a ROS publisher in the previous tutorial, we'll create a subscriber. If you have followed the Arduino IDE Setup tutorial, you'll be able to open the sketch below by choosing ros_lib -> Blink from the Arduino examples menu.

This should open the following code in your IDE:

   1 /*
   2  * rosserial Subscriber Example
   3  * Blinks an LED on callback
   4  */
   5 
   6 #include <ros.h>
   7 #include <std_msgs/Empty.h>
   8 
   9 ros::NodeHandle nh;
  10 
  11 void messageCb( const std_msgs::Empty& toggle_msg){
  12   digitalWrite(13, HIGH-digitalRead(13));   // blink the led
  13 }
  14 
  15 ros::Subscriber<std_msgs::Empty> sub("toggle_led", &messageCb );
  16 
  17 void setup()
  18 {
  19   pinMode(13, OUTPUT);
  20   nh.initNode();
  21   nh.subscribe(sub);
  22 }
  23 
  24 void loop()
  25 {
  26   nh.spinOnce();
  27   delay(1);
  28 }

The Code Explained

Now, let's break the code down.

#include <ros.h>
#include <std_msgs/Empty.h>

As before, we need to include the ros.h as with any other ROS Arduino program. We also include the header files for messages, in this case, the Empty message.

ros::NodeHandle nh;

Next, we need to instantiate the node handle, which allows our program to create publishers and subscribers. The node handle also takes care of serial port communications.

void messageCb( const std_msgs::Empty& toggle_msg){
  digitalWrite(13, HIGH-digitalRead(13));   // blink the led
}

We then create the callback function for our subscriber. The call back function must take a constant reference of a message as its argument. In our callback messageCb, the type of message is std_msgs::Empty and the message name will be toggle_msg.

Inside our callback, we could reference toggle_msg, but since it is empty, there is no need to. We just blink the LED on the Arduino every time we receive a message.

ros::Subscriber<std_msgs::Empty> sub("toggle_led", &messageCb );

We need to instantiate the publishers and subscribers that we will be using. Here we instantiate a Subscriber with a topic name of "toggle_led" and type std_msgs::Empty. With Subscribers, you must remember to template the subscriber upon the message. Its two arguments are the topic it will be subscribing to and the callback function it will be using.

void setup()
{
  pinMode(13, OUTPUT);
  nh.initNode();
  nh.subscribe(sub);
}

In the Arduino setup function you then need to initialize your ROS node handle, advertise any topics being published, and subscribe to any topics you wish to listen to.

void loop()
{
  nh.spinOnce();
  delay(1);
}

Finally, in the loop function we call ros::spinOnce() where all of the ROS communication callbacks are handled. We don't need to do any additional processing in the loop(), since ros::spinOnce() will handle passing messages to the subscriber callback.

Uploading the Code

To upload the code to your Arduino, use the upload function within the Arduino IDE. This is no different from uploading any other sketch.

Running the Code

Now, launch the roscore in a new terminal window:

roscore

Next, run the rosserial client application that forwards your Arduino messages to the rest of ROS. Make sure to use the correct serial port:

rosrun rosserial_python serial_node.py /dev/ttyUSB0

Finally, you can toggle the LED using rostopic:

rostopic pub toggle_led std_msgs/Empty --once

Further Reading

Please see rosserial/Overview for more information on publishers and subscribers. Also see limitations for information about more complex data types.

To generate header files for your custom messages, check rosserial_arduino/Tutorials/Adding Custom Messages tutorial.