AI Consulting and Training Club, Bahria Town Lahore

Category: Courses

  • URDF for Robot Modeling in ROS2 – Basic ROS2

    URDF for Robot Modeling in ROS2: A 4-Month Self-Study Course

    Course Description:

    This comprehensive 4-month self-study course is designed to equip motivated beginners and intermediate learners with the essential skills to master URDF (Unified Robot Description Format) for robot modeling within the ROS2 (Robot Operating System 2) ecosystem. From understanding the fundamental XML structure to implementing complex kinematics, dynamics, and sensor definitions, you will gain hands-on experience in building realistic and functional robot models. By the end of this course, you will be proficient in creating, visualizing, and integrating URDF models into ROS2 simulations and real-world applications, paving the way for advanced robotics development.

    Primary Learning Objectives:
    • Understand the core concepts of URDF and its role in robot modeling.
    • Learn to create and interpret URDF XML files for various robot components.
    • Master the definition of links, joints, and kinematic chains.
    • Integrate visual, collision, and inertial properties into URDF models.
    • Define and incorporate sensors, grippers, and other actuators into robot descriptions.
    • Utilize ROS2 tools for visualizing and debugging URDF models.
    • Develop skills in integrating URDF models with Gazebo simulations.
    • Apply URDF knowledge to build a complete and functional robot model for a final project.
    Necessary Materials:
    • A computer with a Linux operating system (Ubuntu 20.04 or later recommended).
    • ROS2 (Foxy Fitzroy or Humble Hawksbill recommended) installed and configured.
    • A text editor (e.g., VS Code, Sublime Text, GEdit).
    • Gazebo simulation environment (usually installed with ROS2).
    • Rviz (ROS visualization tool, usually installed with ROS2).
    • Internet access for additional resources and documentation.
    Course Content: 14 Weekly Lessons
    Week 1: Introduction to URDF and Robot Modeling
    • Title: The Blueprint of Robotics: Understanding URDF Fundamentals
    • Learning Objectives:
      • Explain what URDF is and why it’s crucial for robotics.
      • Identify the basic structure and key elements of a URDF file.
      • Set up a ROS2 workspace for URDF development.
    • Key Vocabulary:
      • URDF: Unified Robot Description Format, an XML file format for describing robots.
      • ROS2: Robot Operating System 2, a set of software libraries and tools for building robot applications.
      • Link: A rigid body component of a robot.
      • Joint: A connection between two links, defining their relative motion.
      • Kinematic Chain: A series of links connected by joints, describing robot movement.
      • Workspace: A directory containing ROS2 packages.
    • Full Written Content:

      Welcome to the exciting world of robot modeling with URDF in ROS2! URDF is like the blueprint for your robot. It’s an XML-based file format that allows you to describe the physical characteristics of your robot, including its geometry, kinematics (how its parts move relative to each other), and dynamics (how forces affect its motion). This detailed description is essential for simulation, visualization, motion planning, and even real-world robot control.

      Think of a URDF file as a hierarchical tree structure. At the top are the robot tags, which encapsulate the entire robot description. Inside, you’ll define link elements, representing the rigid components of your robot (e.g., a robot’s base, an arm segment, a wheel). These links are then connected by joint elements, which specify the type of connection and the degrees of freedom between the links. Understanding this fundamental link-joint structure is key to building accurate robot models.

      To begin our journey, we need to set up a ROS2 workspace. This is a dedicated directory where you’ll create and manage your ROS2 packages, including those that contain your URDF models. First, ensure you have ROS2 installed on your system. If not, refer to the official ROS2 documentation for installation instructions specific to your operating system. Once ROS2 is installed, you can create a workspace using the colcon build tool.

      A basic URDF file will always start with the <robot> tag, and inside it, you’ll define at least one link and potentially a joint. For example:

      <?xml version="1.0"?>
<robot name="my_simple_robot">
<link name="base_link">
<visual>
<geometry>
<box size="0.1 0.1 0.1"/>
</geometry>
</visual>
</link>
</robot>

      This simple URDF defines a single cubic link named “base_link”. We’ll explore the visual, collision, and inertial tags in later lessons. For now, focus on the fundamental robot and link tags.

    • Practical Hands-on Examples:
      1. Set up your ROS2 workspace: 
        mkdir -p ~/urdf_ws/src
cd ~/urdf_ws
colcon build
source install/setup.bash
      2. Create your first URDF file:

        Inside ~/urdf_ws/src, create a new package:

        ros2 pkg create --build-type ament_cmake my_first_urdf --dependencies rclcpp urdf

        Navigate to ~/urdf_ws/src/my_first_urdf/urdf (create the urdf directory if it doesn’t exist) and create a file named simple_robot.urdf with the XML content provided above.

      3. Visualize your simple robot:

        Open Rviz:

        rviz2

        In Rviz, add a “RobotModel” display. In the “RobotModel” properties, set “Description File” to the full path of your simple_robot.urdf file. You should see a small cube representing your robot’s base.

    Week 2: Defining Links and Joints
    • Title: Anatomy of a Robot: Links, Joints, and the Power of Connection
    • Learning Objectives:
      • Differentiate between various joint types (revolute, prismatic, fixed, etc.).
      • Understand the concept of parent and child links in joint definitions.
      • Define basic links with their dimensions and colors.
    • Key Vocabulary:
      • Revolute Joint: A joint that allows rotation around a single axis.
      • Prismatic Joint: A joint that allows linear motion along a single axis.
      • Fixed Joint: A joint that rigidly connects two links (no relative motion).
      • Parent Link: The link from which a joint extends.
      • Child Link: The link attached to the parent link via a joint.
      • Origin: Specifies the pose (position and orientation) of a joint or visual/collision element relative to its parent frame.
      • Axis: Defines the axis of rotation for revolute joints or translation for prismatic joints.
    • Full Written Content:

      Now that we understand the basic robot and link tags, let’s dive into the crucial elements that bring our robot models to life: joint tags. Joints define how links connect and move relative to one another. Each joint has a type attribute, which is fundamental to defining its behavior.

      The most common joint types you’ll encounter are:

      • revolute: Allows rotation around a single axis. Think of a door hinge.
      • prismatic: Allows linear motion along a single axis. Imagine a piston moving in and out.
      • fixed: Creates a rigid connection between two links, meaning no relative motion is allowed. This is useful for assembling static parts of your robot.
      • continuous: Similar to revolute, but without joint limits, allowing continuous rotation.
      • planar: Allows movement in a 2D plane.
      • floating: Allows full 6-DOF movement between