ENPM702: ROS Programming (Part 3)
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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</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</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</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</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</p> Signup and view all the answers

    What characterizes a message in the context of publishers?

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

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

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

    What does the command ros2 topic list accomplish?

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

    What should be included in the bot_publisher.hpp file?

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

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

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

    In which file is the BotSubscriber class defined?

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

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

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

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

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

    What does a subscriber require to function correctly?

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

    Which messages should the created subscribers handle?

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

    What function must be used to publish at regular intervals?

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

    What files are part of the bot_subscriber package?

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

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

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

    Which command is used to run the bot_pub executable?

    <p>ros2 run bot_publisher bot_pub</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.</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.</p> Signup and view all the answers

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

    <p>CMakeLists.txt</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.</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);</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'.</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);</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.</p> Signup and view all the answers

    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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