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Understanding and Creating URDF Models

Overview​

Unified Robot Description Format (URDF) is the standard XML-based format used in ROS for describing robot models. URDF allows you to define the physical and visual properties of a robot, including its links (rigid parts), joints (connections between links), and their relationships in a kinematic chain. This lesson will guide you through the fundamentals of creating URDF models for robots.

What is URDF?​

URDF (Unified Robot Description Format) is an XML-based format that describes robot models in ROS. It defines the physical and visual properties of a robot, including:

  • Links: Rigid parts of the robot (e.g., base, arms, wheels)
  • Joints: Connections between links that allow relative motion
  • Visual properties: How the robot appears in simulation and visualization
  • Collision properties: How the robot interacts with the environment in simulation
  • Inertial properties: Mass, center of mass, and moments of inertia for physics simulation

URDF is essential for:

  • Robot simulation in Gazebo
  • Robot visualization in RViz
  • Kinematic calculations
  • Motion planning

Basic URDF Structure​

A basic URDF file has the following structure:

<?xml version="1.0"?>
<robot name="my_robot">
  <!-- Links definition -->
  <link name="base_link">
    <visual>
      <geometry>
        <cylinder length="0.6" radius="0.2"/>
      </geometry>
      <material name="blue">
        <color rgba="0 0 0.8 1"/>
      </material>
    </visual>
    <collision>
      <geometry>
        <cylinder length="0.6" radius="0.2"/>
      </geometry>
    </collision>
  </link>

  <!-- Joints definition -->
  <joint name="joint_name" type="fixed">
    <parent link="base_link"/>
    <child link="another_link"/>
    <origin xyz="0 0 0.5" rpy="0 0 0"/>
  </joint>
</robot>

Robot Element​

The root element of every URDF file is the <robot> tag, which must have a name attribute that uniquely identifies the robot.

A link represents a rigid part of the robot. Each link can contain:

  • Visual: How the link appears in visualization tools like RViz
  • Collision: How the link interacts with the environment in simulation
  • Inertial: Physical properties for physics simulation
  • Material: Color and appearance properties

Joint Elements​

A joint connects two links and defines their relative motion. Key attributes include:

  • name: Unique identifier for the joint
  • type: Type of motion (fixed, revolute, continuous, prismatic, etc.)
  • parent: The link that serves as the parent in the kinematic tree
  • child: The link that serves as the child in the kinematic tree
  • origin: Position and orientation of the joint relative to the parent link

Visual Elements​

The visual element defines how a link appears in visualization and simulation:

<visual>
  <origin xyz="0 0 0" rpy="0 0 0"/>
  <geometry>
    <cylinder length="0.6" radius="0.2"/>
  </geometry>
  <material name="blue">
    <color rgba="0 0 0.8 1"/>
  </material>
</visual>

Geometry Types​

URDF supports several geometry types:

  • Box: Defined by size="x y z"
  • Cylinder: Defined by radius and length
  • Sphere: Defined by radius
  • Mesh: Defined by a file path using package:// notation

Materials​

Materials define the appearance of links:

  • Color: RGBA values (Red, Green, Blue, Alpha)
  • Texture: Optional texture mapping
  • Name: Reference to reusable material definitions

Collision Elements​

The collision element defines the physical shape of the link for collision detection:

<collision>
  <origin xyz="0 0 0" rpy="0 0 0"/>
  <geometry>
    <cylinder length="0.6" radius="0.2"/>
  </geometry>
</collision>

Collision geometry is often simpler than visual geometry for performance reasons.

Inertial Elements​

The inertial element defines physical properties for simulation:

<inertial>
  <mass value="10"/>
  <inertia ixx="1e-3" ixy="0.0" ixz="0.0" iyy="1e-3" iyz="0.0" izz="1e-3"/>
</inertial>

Joint Types​

URDF supports several joint types:

Fixed Joint​

A fixed joint connects two links with no relative motion:

<joint name="fixed_joint" type="fixed">
  <parent link="parent_link"/>
  <child link="child_link"/>
  <origin xyz="0 0 0.5" rpy="0 0 0"/>
</joint>

Revolute Joint​

A revolute joint allows rotation around a single axis with limited range:

<joint name="revolute_joint" type="revolute">
  <parent link="parent_link"/>
  <child link="child_link"/>
  <axis xyz="0 0 1"/>
  <limit lower="-1.57" upper="1.57" effort="100" velocity="1"/>
  <origin xyz="0 0 0" rpy="0 0 0"/>
</joint>

Continuous Joint​

A continuous joint allows unlimited rotation around an axis:

<joint name="continuous_joint" type="continuous">
  <parent link="parent_link"/>
  <child link="child_link"/>
  <axis xyz="0 0 1"/>
  <origin xyz="0 0 0" rpy="0 0 0"/>
</joint>

Prismatic Joint​

A prismatic joint allows linear translation along an axis:

<joint name="prismatic_joint" type="prismatic">
  <parent link="parent_link"/>
  <child link="child_link"/>
  <axis xyz="0 0 1"/>
  <limit lower="0" upper="0.5" effort="100" velocity="1"/>
  <origin xyz="0 0 0" rpy="0 0 0"/>
</joint>

URDF Examples​

<?xml version="1.0"?>
<robot name="simple_robot">
  <material name="blue">
    <color rgba="0 0 0.8 1"/>
  </material>

  <link name="base_link">
    <visual>
      <geometry>
        <cylinder length="0.6" radius="0.2"/>
      </geometry>
      <material name="blue"/>
    </visual>
    <collision>
      <geometry>
        <cylinder length="0.6" radius="0.2"/>
      </geometry>
    </collision>
    <inertial>
      <mass value="10"/>
      <inertia ixx="1e-3" ixy="0.0" ixz="0.0" iyy="1e-3" iyz="0.0" izz="1e-3"/>
    </inertial>
  </link>
</robot>
<?xml version="1.0"?>
<robot name="multi_link_robot">
  <material name="blue">
    <color rgba="0 0 0.8 1"/>
  </material>
  <material name="white">
    <color rgba="1 1 1 1"/>
  </material>

  <!-- Base link -->
  <link name="base_link">
    <visual>
      <geometry>
        <cylinder length="0.6" radius="0.2"/>
      </geometry>
      <material name="blue"/>
    </visual>
    <collision>
      <geometry>
        <cylinder length="0.6" radius="0.2"/>
      </geometry>
    </collision>
  </link>

  <!-- Leg link -->
  <link name="leg_link">
    <visual>
      <geometry>
        <box size="0.6 0.1 0.2"/>
      </geometry>
      <origin rpy="0 1.57075 0" xyz="0 0 -0.3"/>
      <material name="white"/>
    </visual>
    <collision>
      <geometry>
        <box size="0.6 0.1 0.2"/>
      </geometry>
      <origin rpy="0 1.57075 0" xyz="0 0 -0.3"/>
    </collision>
  </link>

  <!-- Joint connecting base and leg -->
  <joint name="base_to_leg" type="fixed">
    <parent link="base_link"/>
    <child link="leg_link"/>
    <origin xyz="0 -0.22 0.25"/>
  </joint>
</robot>

Visualizing URDF Models with RViz​

Launching URDF in RViz​

To visualize a URDF model in RViz, you can use the URDF tutorial package:

ros2 launch urdf_tutorial display.launch.py model:=urdf/your_model.urdf

Robot State Publisher​

The robot_state_publisher node is responsible for publishing the robot's joint states and transforms. It reads the URDF and publishes the static transforms between links based on the joint positions.

Best Practices for URDF Creation​

1. Start Simple​

Begin with a basic model and gradually add complexity. Start with visual elements before adding collision and inertial properties.

2. Use Meaningful Names​

Use descriptive names for links and joints that reflect their function in the robot.

3. Organize the Kinematic Tree​

Plan your robot's kinematic structure carefully. Each link (except the base) should have exactly one parent.

4. Consider Performance​

Use simple collision geometries (boxes, cylinders, spheres) when possible to improve simulation performance.

5. Validate Your URDF​

Check your URDF for errors using tools like check_urdf:

check_urdf your_robot.urdf

6. Use Standard Units​

  • Length: meters
  • Mass: kilograms
  • Angles: radians
  • Time: seconds

Common URDF Issues and Solutions​

Issue: Robot appears as a single shape​

Solution: Check that joints are properly defined with correct parent-child relationships.

Issue: Robot parts are not visible​

Solution: Ensure visual elements are properly defined and materials are specified.

Issue: Robot falls through the ground in simulation​

Solution: Check that collision elements are properly defined for the base link.

Issue: Joint limits not working​

Solution: Verify that joint limits are correctly specified with proper units.

Learning Objectives​

By the end of this lesson, you should be able to:

  • Understand the structure and components of URDF files
  • Create basic robot models with links and fixed joints
  • Define visual, collision, and inertial properties for links
  • Understand how to visualize URDF models in RViz
  • Apply best practices for URDF creation
  • Troubleshoot common URDF issues
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