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112 lines
3.7 KiB
Markdown
112 lines
3.7 KiB
Markdown
ROS
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===
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Main article: http://wiki.ros.org
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ROS is a widely used framework for developing complex and distributed robotic systems.
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Installation
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---
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Main article: http://wiki.ros.org/melodic/Installation/Ubuntu
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ROS is already installed on [the RPi image](image.md).
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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](https://www.virtualbox.org)).
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> **Note** For ROS Melodic distribution, we recommend using Ubuntu 18.04.
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Concepts
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---
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### Nodes
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Main article: http://wiki.ros.org/Nodes
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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.
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Many robotic libraries and the drivers are executed in the form of ROS-nodes.
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In order to turn an ordinary program into a ROS node, include a `rospy` or `roscpp` library, and insert the initialization code.
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An example of a ROS node in Python:
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```python
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import rospy
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rospy.init_node('my_ros_node') # the name of the ROS node
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rospy.spin() # entering an endless cycle...
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```
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### Topics
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Main article: http://wiki.ros.org/Topics
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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.
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An example of [`std_msgs/String`](http://docs.ros.org/api/std_msgs/html/msg/String.html) (line) message type posting in topic `/foo` in Python:
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```python
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from std_msgs.msg import String
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# ...
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foo_pub = rospy.Publisher('/foo', String, queue_size=1) # creating a Publisher
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# ...
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foo_pub.publish(data='Hello, world!') # posting the message
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```
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An example of subscription to topic `/foo`:
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```python
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def foo_callback(msg):
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print msg.data
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# Subscribing. When a message is received in topic /foo, function foo_callback will be invoked.
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rospy.Subscriber('/foo', String, foo_callback)
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```
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There is also an opportunity to work with the topics using the `rostopic` utility. For example, using the following command, one can view messages published in topic `/variety of the Aegean sea/state`:
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```(bash)
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rostopic echo /mavros/state
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```
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### Services
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Main article: http://wiki.ros.org/Services
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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.
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An example ROS service invoking from Python:
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```python
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from clover.srv import GetTelemetry
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# ...
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# Creating a wrapper for the get_telemetry service of the clover package with the GetTelemetry type:
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get_telemetry = rospy.ServiceProxy('get_telemetry', srv.GetTelemetry)
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# Invoking the service, and receiving the quadcopter telemetry:
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telemetry = get_telemetry()
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```
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You can also work with the services using the `rosservice` utility. For instance, you can call service `/get_telemetry` from the command line:
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```(bash)
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rosservice call /get_telemetry "{frame_id: ''}"
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```
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More examples of using the services for Clover quadcopter autonomous flights are available in the [documentation for node simple_offboard](simple_offboard.md).
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Working on several PCs
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---
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Main article: http://wiki.ros.org/ROS/Tutorials/MultipleMachines.
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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.
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