ROS Nodes and Topics (Thu Sep 5, lect 3) | previous | next | slides |

Reading: Read PRR Chapter 3

Reminder: Readings are your responsibility. You will be expected to come to class prepared, having read the material, and ready to participate in the discussion

Quiz today!

• The quiz will take around 40 minutes

Logistics

• Any questions at the start?
• Our Autonomous Robotics Lab Notebook

Programming Assignment Discussion

• I corrected some things in the online description. Please ASK QUESTIONS if something is confusing
• Review first programming assignment: PA Basic Control

Chapter 3 Review

Introduction to Nodes and Topics

• ROS is a distributed operating system
• Meaning it controls potentially (and typically) multiple computers
• One of them is in the robot itself
• Almost always there is at least one more which is more powerful
• It could be your laptop or your web workspace which runs on our rover cluster
• The computers have to be on the same network

Communication model

• Message based
• Most important model is “publish/subscribe”
• publish a topic, subscribe to a topic
• Many to many (Nodes can publish to many topics and subscribe to many topics)
• Topics have a name (e.g. /cmd_vel)
• Topics have an associated message
• Important to understand the difference between Topics and Messages
• Think of the topic as a function name, and the Message as argument list.

ROS Nodes

• A ROS application is designed as a collection of separate ROS Nodes that communicate with each other
• ROS Nodes are separate processes and they can run on the different computers
• ROS Nodes are nothing more than programs written in C++ or Python
• What makes them a “node” is that they communicate with other programs in very specific ways

roscore

• roscore is a special program provided by ROS
• It is the “traffic cop” or “name service”
• It is the center of the ROS universe
• It is run on one of the computers
• Whenever a new ROS node starts, the first thing it does is to announce itself to roscore
• That way roscore always has a picture of the graph of ROS Nodes

ROS topics

• ROS Nodes are separate processes (which can even be running on separate computers)
• A function, method or class created in one Node is invisible and cannot be used or called by another
• They are really really separate
• ROS Nodes talk to each other with ROS topics

Topics

• Topics have a name and a message type
• Two of the most common basic ones: /cmd_vel and /odom
• Topics can be published and subscribed
• Which sends a receives messages of a certain message type
• cmd_vel: sent to a robot to command it to move (locomotion)
• odom: received from a robot to report where (it thinks) it is

Message Types

• Twist message is used for cmd_vel
• Expresses a velocity in linear and angular aspects
  1. Note our robot can only move forward and backward (x axis)
  2. And it can only rotate around (z axis)
  3. Twist doesn’t say anything about where the robot (thinks it) is
  4. “cmd_vel” is the topic to directly command the base (people often refer to the robot as a whole as the base) to move. The message type for cmd_vel is Twist

Publishing and Subscribing

• A node can “publish” a ROS topic
• Another node can “subscribe” to a ROS topic
  • The two ROS Nodes don’t know about each other
  • A node can subscribe to a ROS topic that no one is publishing
  • Two ROS Nodes can publish the same ROS topic with no one subscribing
  • etc. etc. etc.
• What is being published and subscribed?
  • messages
  • A message is a data structure with named and typed fields
  • A collection of messages already exists, e.g. Int32, which is message with one element, a 32 bit integer.

ROS Topic /cmd_vel

• A common ROS topic that you will become very familiar with
• When a node (for example one you write) publishes cmd_vel it is asking the robot to move
• The message will include speed and direction
• There is a node (e.g. motor_control) which subscribes to cmd_vel
• motor_control comes with the robot and is run automatically
• motor_control is connected to the motors.
• motor_control exists and runs even if no other node is publishing cmd_vel
• Or if more than one node publishes cmd_vel
• They are totally decoupled!

ROS Topic /odom

• Another very common ROS topic that you will become very familiar with
• Odometry is the term for data coming from the wheels which allow the robot to estimate its location and speed.
• In this case, ROS is publishing the ROS topic odom constantly (say 30 times per second)
• Any node can subscribe to that ROS topic if it needs to know the current Odometry values
• Here is the message definition
• Here is a diagram

subs = rospy.Subscriber("/odom", Odometry, odom_callback)

def odom_callback(msg):
    x_position = msg.pose.pose.position.x

ROS Topic /scan

• Another very common ROS topic that you will learn and use a lot
• /scan contains information coming from the LIDAR sensor on a robot
• It also is published constantly, say 30 times per second
• It contains data about the surrounding area and obstacles seen by the robot
• Any node can subscribe to that ROS topic if it needs to know the current values
• Here is the message definition
• Here is a diagram

def scan_callback(self, msg):
  indices = list(range(350, 360)) + list(range(0, 11))
  relevant_ranges = [msg.ranges[i] for i in indices]
  valid_ranges = [r for r in relevant_ranges if msg.range_min < r < msg.range_max]
  if valid_ranges:
    self.average_distance = sum(valid_ranges) / len(valid_ranges)

def main():
    rospy.init_node('scan_subscriber', anonymous=True)
    rospy.Subscriber("/scan", LaserScan, scan_callback)
    rospy.spin()

Takeaways

• ROS: distributed operating system
• Node: One process which can run on any computer
• roscore: Single process which is the master controller
• Topic: Messages sent between nodes

Thank you. Questions?  (random Image from picsum.photos)