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Chapter 1: URDF for Robot Modeling

Welcome to Module 2, where we delve into the world of digital twins. Our journey begins with understanding how to describe a robot's physical structure and kinematics using the Unified Robot Description Format (URDF). URDF is an XML format used in ROS to describe all elements of a robot.

What is URDF?​

URDF (Unified Robot Description Format) is an XML file format that describes a robot's physical and kinematic properties. It's used to represent a robot's structure, including its rigid bodies (links) and the connections between them (joints). This description is crucial for various robotics tasks, such as simulation, motion planning, and visualization.

A URDF file typically contains:

  • Links: Represent the rigid bodies of the robot (e.g., base, arm segments, end-effector). They have properties like mass, inertia, visual appearance, and collision geometry.
  • Joints: Represent the connections between links. They define the type of motion allowed (e.g., revolute, prismatic, fixed) and their limits.

Structure of a URDF File​

A basic URDF file starts with a <robot> tag, which contains multiple <link> and <joint> tags.

<robot name="my_robot">

  <!-- Link definitions -->
  <link name="base_link">
    <visual>
      <geometry>
        <box size="0.1 0.1 0.1"/>
      </geometry>
    </visual>
  </link>

  <!-- Joint definitions -->
  <joint name="base_to_arm_joint" type="revolute">
    <parent link="base_link"/>
    <child link="arm_link"/>
    <origin xyz="0 0 0.1" rpy="0 0 0"/>
    <axis xyz="0 0 1"/>
    <limit lower="-3.14" upper="3.14" effort="100" velocity="100"/>
  </joint>

  <link name="arm_link">
    <visual>
      <geometry>
        <cylinder length="0.2" radius="0.02"/>
      </geometry>
    </visual>
  </link>

</robot>

In this example:

  • base_link and arm_link are two rigid bodies (links).
  • base_to_arm_joint is a revolute joint connecting base_link to arm_link. It rotates around the Z-axis.

Key Elements in URDF​

  • name: A unique identifier for the link.
  • <visual>: Defines the visual properties of the link, such as its geometry (box, cylinder, mesh) and color.
  • <collision>: Defines the collision properties, which are used by physics engines for collision detection.
  • <inertial>: Defines the mass and inertia tensor of the link, crucial for accurate physics simulation.

<joint> Tag​

  • name: A unique identifier for the joint.
  • type: Specifies the type of joint. Common types include:
    • revolute: A rotational joint with a single axis of rotation and limits.
    • continuous: A rotational joint with a single axis of rotation, no limits.
    • prismatic: A linear joint with a single axis of translation and limits.
    • fixed: A joint that rigidly connects two links, allowing no relative motion.
  • <parent link="PARENT_LINK_NAME"/>: Specifies the parent link of the joint.
  • <child link="CHILD_LINK_NAME"/>: Specifies the child link of the joint.
  • <origin xyz="X Y Z" rpy="ROLL PITCH YAW"/>: Defines the transform from the parent link's origin to the child link's origin. xyz specifies the translation and rpy (roll, pitch, yaw) specifies the rotation in radians.
  • <axis xyz="X Y Z"/>: Defines the axis of rotation for revolute/continuous joints or the axis of translation for prismatic joints.
  • <limit>: Defines the upper and lower limits, velocity limits, and effort limits for the joint.

In the next section, we will see a complete example of a simple robot arm described using URDF.