To use ROS in an Arduino script you first have to include some libraries :

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

You have to replace «Type» with the type of the std message you are using. Here are the different types of std_msgs : http://wiki.ros.org/std_msgs.

You can also use different kind of messages. For instance the ultrasound sensors use :

#include <sensor_msgs/Range.h>

Then you have to start a ROS node (takes care of serial port communications, allows to create publishers and subscribers) with this line :

ros::NodeHandle nh;

You can now create a message object that you will later fill with data and this is the message you will finally publish :

std_msgs::String msg;

where msg is the name of the object. And of course you can choose the type of the message depending on the type of the transferred message.

Then you have to choose if you want to create a publisher, a subscriber or even both.

To create a publisher :

ros::Publisher chatter("chatter", &msg);

This tells that we are going to be publishing a message of the type of the msg variable (with std_msgs::String type) on the topic «chatter». This lets the master tell any nodes listening on «chatter» that we are going to publish data on that topic.

You can now add lines in the void setup() of the Arduino script. You first initialize the node :

nh.initNode()

Next, the following call connects to the master to publicize that the node will be publishing messages on the given topic :

nh.advertise(chatter);

So finally this is what the void setup() in your Arduino code should look like :

void setup()
{
   nh.initNode();
   nh.advertise(chatter);
}

To fill the message object with the data you just have to do this :

msg.data = hello;

where hello is the data you put in your message and hello must have the same type as the msg variable.

You can then publish you message on the topic. To do so you can either publish it once or continually in the void loop() of your Arduino code like this :

void loop()
{
   chatter.publish( &str_msg );
}

Finally you have to add a last line of code in the loop that will call all the callbacks waiting to be called at that point in time :

nh.spinOnce();

Now that you know how each part of the code works you can test a «Hello World» example available on the wiki.ros.org website http://wiki.ros.org/rosserial_arduino/Tutorials/Hello%20World :

/*
 * rosserial Publisher Example
 * Prints "hello world!"
 */
// Use the following line if you have a Leonardo or MKR1000 
//#define USE_USBCON 

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

ros::NodeHandle nh;

std_msgs::String str_msg;
ros::Publisher chatter("chatter", &str_msg);

char hello[13] = "hello world!";

void setup()
{
  nh.initNode();
  nh.advertise(chatter);
}

void loop()
{
  str_msg.data = hello;
  chatter.publish( &str_msg );
  nh.spinOnce();
  delay(1000);
}

You have to upload this code on the Arduino board before connecting it to the Raspberry and before starting the test on ROS.

To test it you first have to connect the Arduino board to the Raspberry. Then you launch the terminal and execute the following commands :

on a first window you type :

roscore

on another window :

/dev/ttyXXXX is the usb port the Arduino is connected to and you’ll have to find the exact name in order to replace the XXXX by the name of the port (examples : ttyUSB0, ttyACM0, ttyACM1, ...). You can find the name of the port ttyXXXX in the Arduino IDE or you can find it in the terminal by typing :

ls /dev/tty*

you should first execute this previous command and only then plug the Arduino in the Raspberry usb port and execute this command again to see which port has been added to the list displayed on the terminal and thus know which one is the Arduino board.

The first time you use the usb port the Arduino is connected to, you have to give permissions to use that port. To do so, you have to type :

sudo chmod 666 /dev/ttyXXXX

Now that you know the port you are using and you have permissions, you can type this :

rosrun rosserial_python serial_node.py /dev/ttyXXXX

Finally you can see what the Arduino is publishing on a specific topic by typing on another terminal window :

rostopic echo /topic_name

where you have to replace topic_name by the topic your Arduino is publishing on. In the example above, the «hello world!» is published on the topic «chatter». So you can type :

rostopic echo /chatter