ROS

ROS is a widely used framework for developing complex and distributed robotic systems.

Installation

Main article: http://wiki.ros.org/melodic/Installation/Ubuntu

ROS is already installed on the RPi image.

To use ROS on a PC, we recommend using Ubuntu Linux (or a virtual machine such as Parallels Desktop Lite](https://itunes.apple.com/ru/app/parallels-desktop-lite/id1085114709?mt=12) or VirtualBox).

For ROS Melodic distribution, we recommend using Ubuntu 18.04.

Concepts

Nodes

Main article: http://wiki.ros.org/Nodes

ROS node is a special program (usually written in Python or C++) that communicates with other nodes via ROS topics and ROS services. Dividing complex robotic systems into isolated nodes provides certain advantages: reduced coupling of the code, increases re-usability and reliability.

Many robotic libraries and the drivers are executed in the form of ROS-nodes.

In order to turn an ordinary program into a ROS node, include a rospy or roscpp library, and insert the initialization code.

An example of a ROS node in Python:

import rospy

rospy.init_node('my_ros_node')  # the name of the ROS node

rospy.spin() # entering an endless cycle...

Topics

Main article: http://wiki.ros.org/Topics

A topic is a named data bus used by the nodes for exchanging messages. Any node can post a message in a random topic, and subscribe to an arbitrary topic.

An example of std_msgs/String (line) message type posting in topic /foo in Python:

from std_msgs.msg import String

# ...

foo_pub = rospy.Publisher('/foo', String, queue_size=1)  # creating a Publisher

# ...

foo_pub.publish(data='Hello, world!')  # posting the message

An example of subscription to topic /foo:

def foo_callback(msg):
    print(msg.data)

# Subscribing. When a message is received in topic /foo, function foo_callback will be invoked.
rospy.Subscriber('/foo', String, foo_callback)

You can also work with topics using the the rostopic utility. For example, using the following command, you can view messages published in topic /mavros/state:

rostopic echo /mavros/state

The rostopic info command shows the type of messages in the topic, and rostopic hz shows frequency of published messages.

Services

Main article: http://wiki.ros.org/Services

A service can be assimilated to the a function that can be called from one node, and processed in another one. The service has a name that is similar to the name of the topic, and 2 message types: request type and response type.

An example ROS service invoking from Python:

from clover.srv import GetTelemetry

# ...

# Creating a wrapper for the get_telemetry service of the clover package with the GetTelemetry type:
get_telemetry = rospy.ServiceProxy('get_telemetry', srv.GetTelemetry)

# Invoking the service, and receiving the quadcopter telemetry:
telemetry = get_telemetry()

You can also work with the services using the rosservice utility. For instance, you can call service /get_telemetry from the command line:

rosservice call /get_telemetry "{frame_id: ''}"

More examples of using the services for Clover quadcopter autonomous flights are available in the documentation for node simple_offboard.

Working on several PCs

Main article: http://wiki.ros.org/ROS/Tutorials/MultipleMachines.

The advantage of using ROS is the possibility of distributing the nodes across several PCs in the network. For example, a node that recognizes an image may be run on a more powerful PC; the node that controls the copter may be run directly on a Raspberry Pi connected to the flight controller, etc.

ROS

ROS