ENPM702: ROS Programming (Part 3)
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Questions and Answers

What is the primary purpose of a publisher in ROS?

  • To control the actions of the robot directly
  • To retrieve data from other nodes
  • To make information produced by a node available to other nodes (correct)
  • To visualize sensor data in real-time

Which message type is specifically mentioned for controlling robot actions in ROS 2?

  • nav_msgs/msg/Odometry
  • geometry_msgs/msg/Twist (correct)
  • sensor_msgs/msg/Image
  • std_msgs/msg/String

What do subscribers implement in ROS 2 according to the learning objectives?

  • Sending commands to the robot
  • Retrieving LIDAR scans and robot position (correct)
  • Publishing messages to topics
  • Managing the robot's trajectory planning

What are the two main components related to data transmission in ROS discussed in the learning objectives?

<p>Topics and Messages (B)</p> Signup and view all the answers

Which of the following best describes messages in the context of ROS?

<p>Data structures that carry information (B)</p> Signup and view all the answers

What is essential to publish at regular intervals in a publisher implementation?

<p>The frequency of message updates (C)</p> Signup and view all the answers

What should be the focus when writing a subscriber in ROS 2?

<p>Receiving messages from specific topics (C)</p> Signup and view all the answers

What knowledge is expected to be gained regarding topics in this lecture?

<p>How data is structured and transmitted (A)</p> Signup and view all the answers

What characterizes a message in the context of publishers?

<p>A message is a structured data type. (A)</p> Signup and view all the answers

Which of the following statements about publisher-subscriber communication is true?

<p>Publishers can send messages without waiting for any acknowledgement. (A)</p> Signup and view all the answers

What does Quality of Service (QoS) refer to in this messaging context?

<p>Settings that determine reliability and durability of communication. (C)</p> Signup and view all the answers

Which command would you use to see all interfaces installed on your system?

<p>ros2 interface list (C)</p> Signup and view all the answers

What is the first step in using a message in your program?

<p>Include the message structure in your program. (D)</p> Signup and view all the answers

Which of the following is NOT a valid command for interfacing with message structures?

<p>ros2 interface show message (D)</p> Signup and view all the answers

What is the appropriate way to publish a message using the example_interfaces package?

<p>auto message = example_interfaces::msg::String(); (B)</p> Signup and view all the answers

Which of the following best describes the role of a publisher in this messaging system?

<p>To send structured messages over topics. (D)</p> Signup and view all the answers

What does the command ros2 topic list accomplish?

<p>Lists all active topics (D)</p> Signup and view all the answers

Which message type is used for publishing velocity commands on the /cmd_vel topic?

<p>geometry_msgs/msg/Twist (B)</p> Signup and view all the answers

What is the purpose of the ros2 topic echo command?

<p>To display real-time data published on a topic (A)</p> Signup and view all the answers

What should be included in the bot_publisher.hpp file?

<p>The class definition for BotPublisher (D)</p> Signup and view all the answers

What is the significance of the parameter 10 when initializing the velocity publisher in ROS2?

<p>It sets the queue size for the publisher (C)</p> Signup and view all the answers

Which command would you use to find out the type of messages published on a specific topic?

<p>ros2 topic type (A)</p> Signup and view all the answers

What does the command ros2 topic hz measure?

<p>The rate of frequency at which messages are published (D)</p> Signup and view all the answers

In the context of ROS2, what is a topic primarily used for?

<p>As a communication channel for exchanging data (C)</p> Signup and view all the answers

What types of messages are being subscribed to by the BotSubscriber class?

<p>Odometry and Laser (D)</p> Signup and view all the answers

In which file is the BotSubscriber class defined?

<p>bot_subscriber.hpp (A)</p> Signup and view all the answers

What is the purpose of the 'std::bind' function in the subscriber initialization?

<p>To bind callbacks to the subscription messages (D)</p> Signup and view all the answers

How is the main node created in the main.cpp file?

<p>Using a shared pointer (D)</p> Signup and view all the answers

What is the purpose of the callback methods odom_callback and scan_callback?

<p>To log incoming messages (D)</p> Signup and view all the answers

What is specified in the CMakeLists.txt for building the executable?

<p>The executable target and its dependencies (D)</p> Signup and view all the answers

What function is called to start the ROS node after initializing?

<p>rclcpp::spin() (D)</p> Signup and view all the answers

What are the two main types of messages being processed in BotSubscriber's callbacks?

<p>Odometry and LaserScan (D)</p> Signup and view all the answers

What is the main objective when modifying the BotPublisher class?

<p>To publish a Twist message every 300 ms (C)</p> Signup and view all the answers

What does a subscriber require to function correctly?

<p>A message type and a callback function (A)</p> Signup and view all the answers

Which messages should the created subscribers handle?

<p>nav_msgs/msg/Odometry and sensor_msgs/msg/LaserScan (B)</p> Signup and view all the answers

What function must be used to publish at regular intervals?

<p>create_wall_timer() (D)</p> Signup and view all the answers

Which statement about Quality of Service (QoS) for subscribers is accurate?

<p>Subscribers can define their own QoS settings. (A)</p> Signup and view all the answers

What files are part of the bot_subscriber package?

<p>bot_subscriber.cpp and bot_subscriber.hpp (C)</p> Signup and view all the answers

What is the purpose of the callback function in the subscriber?

<p>To process incoming messages (C)</p> Signup and view all the answers

Which command is used to run the bot_pub executable?

<p>ros2 run bot_publisher bot_pub (A)</p> Signup and view all the answers

What is the purpose of the line 'velocity_publisher_ = this->create_publisher("cmd_vel", 10);'?

<p>To create a publisher for the 'cmd_vel' topic with a queue size of 10. (D)</p> Signup and view all the answers

What does the angular.z parameter in the Twist message control?

<p>The rotational speed around the z-axis. (A)</p> Signup and view all the answers

In which file should the include directories be specified for the publisher node?

<p>CMakeLists.txt (D)</p> Signup and view all the answers

What happens if the message queue exceeds a size of 10 in the publisher?

<p>The oldest messages will be dropped to maintain the queue size. (B)</p> Signup and view all the answers

Which part of the bot node is responsible for initializing the ROS 2 environment?

<p>rclcpp::init(argc, argv); (B)</p> Signup and view all the answers

What is the role of the line 'ament_target_dependencies(bot_pub rclcpp geometry_msgs)' in CMakeLists.txt?

<p>To list dependencies for the target 'bot_pub'. (C)</p> Signup and view all the answers

Which method would be used to publish the velocity message to the 'cmd_vel' topic?

<p>velocity_publisher_-&gt;publish(message); (B)</p> Signup and view all the answers

What does the statement 'auto message = geometry_msgs::msg::Twist();' do?

<p>Instantiates an object of type Twist for robot motion. (C)</p> Signup and view all the answers

Flashcards

ROS Publisher

A node in ROS that sends messages to a topic.

Topic

A channel for message communication in ROS.

Message

Data transmitted over a ROS topic.

Node

A program or component in ROS.

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geometry_msgs/msg/Twist

A ROS data type for vehicle control, typically used to tell a robot how to move its wheels.

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Publisher Implementation

The process of writing a ROS publisher, creating a program that sends messages to a specified topic.

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LIDAR scans

Data collected by a LIDAR sensor.

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Robot position

The location of the robot.

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Asynchronous communication

Publishers send messages without waiting for confirmation from subscribers.

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Publisher

Node that sends messages over a topic.

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Message Structure

Organized format of data within a message.

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Interface Definition Language (IDL)

Language for defining message and service structures.

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ros2 interface list

Command to list all interfaces installed on the system.

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ROS2 Topic

A communication channel for data exchange between ROS2 nodes.

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Topic Name

A unique identifier for a topic, e.g., "/camera/image_raw".

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Message Type

A specification for the data structure transmitted on a topic.

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ros2 topic list

A command to display all active topics in ROS2.

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ros2 topic echo

A ROS2 command that retrieves data currently being published on a topic.

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ros2 topic info

Retrieves details about a specific topic, like its type.

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ros2 topic type

A command to get the message type of a ROS2 topic.

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ros2 topic hz

A command to determine the average publishing rate of a ROS2 topic.

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What is the role of the velocity publisher?

The velocity publisher sends messages to a topic called 'cmd_vel' which controls a robot's linear and angular velocities.

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What does the 'cmd_vel' topic represent?

The 'cmd_vel' topic carries messages about a robot's desired linear and angular velocities.

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What is the message type used by the velocity publisher?

The message type used is 'geometry_msgs::msg::Twist', which represents linear and angular velocities.

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What is the 'create_publisher' method used for?

The 'create_publisher' method creates a publisher object with a specified topic name, message type, and QoS profile.

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What is QoS profile?

QoS profile refers to the Quality of Service settings for a publisher, defining the buffer size and how messages are handled.

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What is the purpose of the 'keep_last' QoS policy?

The 'keep_last' policy sets the publisher to maintain a buffer of the last 'n' messages (where 'n' is specified as the queue size).

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What happens when the message queue size is exceeded?

Older messages are dropped from the queue when the size is exceeded, prioritizing recent data.

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What is the importance of the 'main' function in a ROS publisher?

The 'main' function initializes ROS, creates a publisher node, and runs the spin loop, which continuously checks for new messages to be sent.

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BotSubscriber Class

A C++ class that defines a ROS subscriber node dedicated to receiving and processing data from specific topics (e.g., 'odom' and 'scan')

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Subscriber Objects

Objects of the rclcpp::Subscription::SharedPtr type that represent connections to specific ROS topics. These objects handle incoming messages

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Odom Callback

A function executed by the BotSubscriber class whenever a new message arrives on the 'odom' topic. This function processes the message containing information about the robot's position and orientation

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Scan Callback

A function executed by the BotSubscriber class whenever a new message arrives on the 'scan' topic. This function processes the message containing data from the robot's LIDAR sensor

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Creating Subscribers

The process of initializing and configuring subscriber objects within the BotSubscriber class to bind them to specific topics

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ROS Subscriber Node

A ROS node responsible for receiving messages from specific topics. Examples include the BotSubscriber class

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rclcpp::init()

A function from the ROS 2 library that initializes the ROS system. Usually called at the start of your main function

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rclcpp::spin()

A function that keeps your ROS node running and processing incoming messages

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What is a ROS Subscriber?

A ROS node that receives messages from a topic. It defines the message type it expects and has a callback function to process incoming data.

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What are the key components of a ROS Subscriber?

A subscriber consists of three main components: a topic to receive messages from, a message type to define the expected data, and a callback function to handle incoming data.

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What does QoS stand for in ROS?

Quality of Service. This refers to the reliability and consistency of communication between nodes in a ROS system. It defines things like how messages are sent and received, and how data is prioritized.

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What is the purpose of a Callback Function in a Subscriber?

A callback function is executed whenever a new message arrives on the topic the subscriber is listening to. It processes the data from the message and allows the subscriber to react to new information.

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Create a Subscriber: BotSubscriber

Implement a subscriber node called BotSubscriber that receives data from two topics: '/odom' (robot's position) and '/scan' (LiDAR data) using the nav_msgs/msg/Odometry and sensor_msgs/msg/LaserScan message types.

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What is the purpose of the 'bot_subscriber' package?

This package houses the subscriber node 'BotSubscriber' to receive and process data from topics like '/odom' and '/scan' using the message types nav_msgs/msg/Odometry and sensor_msgs/msg/LaserScan respectively.

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How does a subscriber work in ROS?

Subscribers listen for messages on specified topics. When a new message arrives, their callback function is triggered, allowing them to process the received data according to their needs.

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Study Notes

Course Information

  • Course: ENPM702: Introductory Robot Programming
  • Lecture: L11: ROS (Part 3)
  • Version: 1.1
  • Lecturer: Z. Kootbally
  • School: University of Maryland
  • Semester/Year: Fall 2024
  • Date: 2024/11/13

Table of Contents

  • Learning Objectives

    • Messages and Topics: Gain knowledge about message structures and how data is transmitted through topics
    • Publishers: Learn to write and implement a publisher in ROS 2 to control robot actions, including publishing specific data types like geometry_msgs/msg/Twist
    • Subscribers: Learn to write and implement a subscriber in ROS 2 to retrieve LIDAR scans and the robot position

  • Publishers

    • Topics: Publishers send messages on topics
    • Messages: Data sent by publishers are structured as messages
    • Asynchronous: Publishers can keep generating and sending messages without waiting for subscribers
    • Quality of Service (QoS): QoS settings determine how communication is handled (reliability, durability, etc.)

  • Messages

    • Interface Definition Language (IDL)
      • Messages are structured data types for communication between nodes over topics, services, or actions.

  • Commands

    • Listing all interfaces: ros2 interface list
    • Showing interface structure: ros2 interface show <message type> (e.g., geometry_msgs/msg/Twist)
    • Showing interfaces from a package: ros2 interface package <package name> (e.g., geometry_msgs)

  • Usage

    • Understanding message structure: ros2 interface show example_interfaces/msg/String
    • Including the message: #include <example_interfaces/msg/String.hpp>
    • Instantiate a message object: auto message = example_interfaces::msg::String();
    • Fill out message fields: message.data = "Hello";
    • Publish the message: publisher_->publish(message);

  • Topics

    • Structure:
      • Name: Unique identifier (e.g., /camera/image_raw)
      • Message Type: Specific message type (e.g., sensor_msgs/msg/Image) defines the structure of data

  • Commands

    • Listing active topics: ros2 topic list
    • Showing data from a topic: ros2 topic echo <topic name>
    • Getting topic information: ros2 topic info <topic name>
    • Getting a topic's type: ros2 topic type <topic name>
    • Getting average publishing rate: ros2 topic hz <topic name>

  • Write a Simple Publisher

    • Objective: Publish a single Twist message (linear.x = 1.0 and angular.z = 0.5) to cmd_vel
      • Package: bot_publisher
      • Files: bot_publisher.cpp and bot_publisher.hpp
      • Class: BotPublisher
      • Executable: bot_pub
      • Topic: /cmd_vel
      • Message: geometry_msgs/msg/Twist

  • Writing the code (simple publisher): bot_publisher.hpp

    • Creating a publisher attribute: rclcpp::Publisher<geometry_msgs::msg::Twist>::SharedPtr velocity_publisher_;
    • Initializing the publisher: velocity_publisher_ = this->create_publisher<geometry_msgs::msg::Twist>("cmd_vel", 10);
    • Building the message and publishing it: auto message = geometry_msgs::msg::Twist(); message.linear.x = 1.0; message.angular.z = 0.5; velocity_publisher_->publish(message);

  • Initializing velocity publisher

  • Other instructions and details on the topic

  • Write the main function in main.cpp

  • Edit CMakeLists.txt

  • Compile and execute

  • colcon build --packages-select bot_publisher

  • ros2 run bot_publisher bot_pub

  • Introspect

  • ros2 topic echo cmd_vel

  • rqt_graph

  • Publish at Regular Intervals

    • Objective: Publish a Twist message to /cmd_vel every 300 ms

  • Write Subscribers

    • Objective: Create two subscribers
      • Retrieve turtlebot's position from /odom (nav_msgs/msg/Odometry) and receive LiDAR data from /scan (sensor_msgs/msg/LaserScan).
      • Package: bot_subscriber
      • Files: bot_subscriber.cpp and bot_subscriber.hpp
      • Class: BotSubscriber
      • Executable: bot_sub
      • Topics: /odom & /scan
      • Message: nav_msgs/msg/Odometry & sensor_msgs/msg/LaserScan

  • Code for creating and initializing the subscribers

  • Code for main function in main.cpp and CMakeLists.txt

  • Exercise

    • Objective: Integrate publisher and subscribers into bot_controller package to move robot forward at 0.1 m/s and stop if obstacle detected within 0.2 m.

Next Lecture

  • Lecture 12: ROS (Part 4)

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ENPM702 Introductory Robot Programming L11 PDF

Description

This quiz covers the concepts of message structures, publishers, and subscribers within the Robot Operating System (ROS) for robot programming. You will learn how to implement these elements in ROS 2 and understand the role of Quality of Service settings in communication. Test your understanding of how data is transmitted through topics and how robot actions are controlled.

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