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3.2 Defining the Body: URDF & SDF

To simulate a robot, we must first define its body. We need to tell the simulator about the robot's physical properties: its shape, size, mass, and how its parts are connected. Just as an architect needs blueprints before building a house, a roboticist needs a description file before simulating a robot. We do this using XML-based description formats.

URDF: The Universal Robot Description Format​

URDF is the standard format in ROS 2 for defining a robot's structure. It describes the robot as a tree of links (rigid bodies) connected by joints (movable connections). Think of links as the bones and joints as the muscles/hinges.

The Building Blocks​

  • <link>: Represents a single physical part of the robot, like a chassis, an upper arm, or a wheel. A link has three main sub-elements:

    • <visual>: Defines what the link looks like to the human eye. This can be a simple primitive (box, cylinder, sphere) or a complex 3D mesh file (STL, DAE). This is purely for rendering and has no effect on physics.
    • <collision>: Defines the simplified geometry used by the physics engine to calculate contacts and collisions. Usually, this is a simpler shape than the visual mesh to save computation power. For example, a complex hand visual mesh might have a simple box as its collision geometry.
    • <inertial>: Defines the mass and the moment of inertia (how hard it is to rotate) of the link. Accurate inertial values are crucial for stable simulation.

  • <joint>: Defines how two links move relative to each other. It connects a "parent" link to a "child" link.

    • Type:
      • revolute: A hinge joint that rotates around an axis (like your elbow). It has upper and lower limits.
      • continuous: A hinge joint that can rotate indefinitely (like a wheel).
      • prismatic: A sliding joint that moves along an axis (like a drawer).
      • fixed: A rigid connection where the two links effectively become one (like bolting a sensor to a chassis).
    • Limits: Defines the physical constraints of the joint, including range of motion (in radians or meters), maximum velocity, and maximum effort (torque or force).

Interactive Explorer​

Use the explorer below to visualize the relationship between the XML code and the physical structure of a simple robot arm. Hover over the code tags to see the corresponding parts on the diagram.

<robot name="arm">
  <link name="base_link">
    <visual>
      <geometry>
        <cylinder length="0.6" radius="0.2"/>
      </geometry>
    </visual>
  </link>
 
  <joint name="shoulder_joint" type="revolute">
    <parent link="base_link"/>
    <child link="upper_arm"/>
  </joint>
 
  <link name="upper_arm">
    <visual>
      <geometry>
        <box size="0.1 0.1 0.5"/>
      </geometry>
    </visual>
  </link>
</robot>
Hover over highlighted lines to see parts
Upper ArmJointBase

SDF: The Simulation Description Format​

While URDF is the standard for ROS 2 tools (like the Rviz visualizer and the robot_state_publisher), it has limitations. It describes a single robot in isolation. It lacks the ability to describe an entire environment—lights, terrain, other objects, and multiple robots interacting.

This is where SDF (Simulation Description Format) comes in. SDF is the native format for the Gazebo simulator. It is more expressive than URDF. It can describe:

  • World properties: Gravity, magnetic fields, lighting conditions.
  • Static objects: Trees, buildings, furniture.
  • Physics engine parameters: Solver types, time step sizes.
  • Sensor noise models: Adding Gaussian noise to camera or laser data.

In a typical ROS 2 workflow, you usually define your robot using URDF (or its more powerful cousin, Xacro, which adds macros and variables to URDF). When you launch a simulation, Gazebo takes your URDF and automatically converts it into SDF behind the scenes to simulate it within a larger world context. This allows you to keep a single "source of truth" for your robot model that works for both visualization (Rviz) and simulation (Gazebo).

References​

[1] "URDF - ROS Wiki," ROS.org. [Online]. Available: http://wiki.ros.org/urdf. [2] "SDF Specification," SDFormat.org. [Online]. Available: http://sdformat.org/spec. [3] "Xacro - ROS Wiki," ROS.org. [Online]. Available: http://wiki.ros.org/xacro.

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