ROS

Main documentation: https://wiki.ros.org.

ROS is a widely used framework for developing complex and distributed robotic systems. The Clover autonomous flights platform is based on ROS.

Installation

ROS is already installed on the RPi image.

To install ROS on your PC you may address the official installation documentation. For a quick start it's recommended to use the virtual machine image with ROS and Clover simulator.

Concepts

Nodes

Main article: https://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, increased reusability and reliability.

Many robotic libraries and drivers are made as ROS nodes.

In order to turn an ordinary program into a ROS node, include the rospy (Python) or roscpp (C++) 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 infinite loop...

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Any autonomous flight script for Clover is a ROS node.

Topics

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

A topic is a named data bus used by the nodes for exchanging messages. Any node can publish a message to any topic, and subscribe to any topic.

Для каждого созданного топика должен быть задан тип сообщений, которые по нему передаются. ROS включает в себя большое количество стандартных типов сообщений, покрывающих различные аспекты робототехники, но при необходимости возможно создание собственных типов сообщений. Примеры стандартных типов сообщений:

Each topic has the a of messages it passes. ROS include a lot of standard message types, covering different aspects of robotics. Creating custom message types is also possible. Example of standard message types:

Message type	Description
std_msgs/Int64	Integer number.
std_msgs/Float64	Double-precision floating-point number.
std_msgs/String	String.
geometry_msgs/PoseStamped	Position and orientation of an object in a given coordinate system and a time stamp (widely used for passing the robot pose or some robot's part pose).
geometry_msgs/TwistStamped	Linear and angular velocity of an object in a given coordinate system and a time stamp.
sensor_msgs/Image	Image (see the article on working with the camera).

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See the rest of standard message types in packages: common_msgs, std_msgs, geometry_msgs, sensor_msgs, and others.

Example of publishing a message of type String in a topic /foo in Python:

from std_msgs.msg import String

rospy.init_node('my_ros_node')

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

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

Example of subscription to a topic /foo:

import rospy
from std_msgs.msg import String

rospy.init_node('my_ros_node')

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 read a topic message once, using wait_for_message function:

msg = rospy.wait_for_message('/foo', String, timeout=3)  # wait for a message in /foo topic with timeout of 3 seconds

You can also work with topics using 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.

Also you can monitor and visualize topics using graphical tools of ROS.

Services

Main article: https://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.

Thus, ROS services implement remote procedure call (RPC) pattern.

Example of invoking a ROS service in Python:

import rospy
from clover.srv import GetTelemetry

rospy.init_node('my_ros_node')

# 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 getting 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.

Names

Main article: https://wiki.ros.org/Names.

Any topic, service or a parameter is identified with a unique name. A ROS name is hierarchical structure with a / symbol as a separator (which is close to a file name in a file system).

Examples of ROS names:

• / (global namespace)
• /foo
• /stanford/robot/name
• /wg/node1

This names are global (close to global names in a file system). In practice, it's recommended to use private or relative names.

Private name

Each node can use its own private namespace (corresponding its name) for its resources. For example, aruco_detect node may publish such topics:

• /aruco_detect/markers
• /aruco_detect/visualization
• /aruco_detect/debug

When a node is referring its private resource, instead of /aruco_detect/ namespace it may use ~ symbol:

• ~markers
• ~visualization
• ~debug

Thus, creating a foo topic and the private namespace would look like this:

private_foo_pub = rospy.Publisher('~foo', String, queue_size=1)

Relative name

Several nodes may group into a common namespace (for example, when there are several robots in the network). For referring topics and services in the current namespace, the opening / symbol is omitted.

Example of create a foo topic in the current namespace:

relative_foo_pub = rospy.Publisher('foo', String, queue_size=1)

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Generally, it's recommended to use private or relative names instead of global ones.

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.