urdf-fundamentals

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Core URDF/Xacro syntax, joint types, kinematic structures, and URDF validation for robot descriptions

Ranch-Hand-Robotics By Ranch-Hand-Robotics schedule Updated 3/5/2026
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name: urdf-fundamentals description: Core URDF/Xacro syntax, joint types, kinematic structures, and URDF validation for robot descriptions

URDF Fundamentals Skill

This skill provides comprehensive guidance on creating, structuring, and validating URDF (Unified Robot Description Format) files for robot descriptions.

When to Use This Skill

Use this skill when:

  • Creating a new URDF from scratch
  • Understanding URDF/Xacro syntax and structure
  • Designing kinematic chains and joint hierarchies
  • Adding links, joints, or coordinate frames
  • Debugging URDF validation errors
  • Implementing inertial properties and mass
  • Understanding and using package paths and mesh references

URDF File Structure

Basic Robot Structure

Every URDF file has this basic structure:

<?xml version="1.0"?>
<robot name="robot_name">
  
  <!-- Define links (rigid bodies) -->
  <link name="base_link">
    <!-- Visual geometry for rendering -->
    <visual>
      <geometry>
        <box size="0.5 0.3 0.1"/>
      </geometry>
      <material name="blue">
        <color rgba="0 0 1 1"/>
      </material>
    </visual>
    
    <!-- Collision geometry for physics -->
    <collision>
      <geometry>
        <box size="0.5 0.3 0.1"/>
      </geometry>
    </collision>
    
    <!-- Inertial properties for dynamics -->
    <inertial>
      <mass value="5.0"/>
      <inertia ixx="0.1" ixy="0" ixz="0" iyy="0.1" iyz="0" izz="0.1"/>
    </inertial>
  </link>
  
  <!-- Define joints (connections between links) -->
  <joint name="base_to_wheel" type="continuous">
    <parent link="base_link"/>
    <child link="wheel_link"/>
    <origin xyz="0.2 0.2 0" rpy="0 1.57 0"/>
    <axis xyz="0 1 0"/>
  </joint>
  
  <!-- Define other links -->
  <link name="wheel_link">
    <!-- ... geometry ... -->
  </link>
  
</robot>

Key Elements

<link> - Represents a rigid body in the robot:

  • name - Unique identifier
  • Usually contains: <visual>, <collision>, <inertial> elements
  • The first link is typically the "base_link" (root of kinematic chain)

<joint> - Represents a connection between two links:

  • name - Unique identifier
  • type - Joint type (revolute, continuous, prismatic, fixed, etc.)
  • <parent> - Parent link name
  • <child> - Child link name
  • <origin> - Relative position and orientation
  • <axis> - Direction and rotation/translation axis
  • <limit> - Joint constraints (for movable joints)

<origin> - Specifies relative pose:

  • xyz - Position (x, y, z in meters)
  • rpy - Orientation (roll, pitch, yaw in radians)

Links in Detail

Visual Geometry

Used for display and rendering:

<link name="example_link">
  <visual>
    <!-- Geometry type and dimensions -->
    <geometry>
      <box size="x y z"/>
      <!-- or -->
      <cylinder radius="r" length="l"/>
      <!-- or -->
      <sphere radius="r"/>
      <!-- or -->
      <mesh filename="package://robot_name/meshes/part.stl" scale="0.001 0.001 0.001"/>
    </geometry>
    
    <!-- Optional: Position and orientation within link -->
    <origin xyz="0 0 0" rpy="0 0 0"/>
    
    <!-- Optional: Material coloring -->
    <material name="blue">
      <color rgba="0 0 1 1"/>  <!-- R G B Alpha (0-1) -->
    </material>
  </visual>
</link>

Geometry types:

  • <box size="x y z"/> - Rectangular box in meters
  • <cylinder radius="r" length="l"/> - Cylinder with radius and length in meters
  • <sphere radius="r"/> - Sphere with radius in meters
  • <mesh filename="path" scale="sx sy sz"/> - External 3D mesh file (STL, DAE, OBJ)

Material colors (RGBA):

  • rgba="1 0 0 1" - Red
  • rgba="0 1 0 1" - Green
  • rgba="0 0 1 1" - Blue
  • rgba="1 1 1 1" - White
  • rgba="0 0 0 1" - Black
  • Last value (alpha) is transparency: 1 = opaque, 0 = transparent

Collision Geometry

Used for physics simulation and collision detection. Should typically be simpler than visual geometry:

<link name="complex_part">
  <visual>
    <!-- Detailed visual mesh -->
    <geometry>
      <mesh filename="package://robot/meshes/detailed_part.stl"/>
    </geometry>
  </visual>
  
  <collision>
    <!-- Simplified collision boxes -->
    <geometry>
      <box size="0.5 0.3 0.1"/>
    </geometry>
  </collision>
</link>

Best practices:

  • Keep collision geometry simple for performance
  • Use multiple collision elements to approximate complex shapes:
<link name="gripper">
  <collision>
    <origin xyz="0 0 0"/>
    <geometry><box size="0.1 0.2 0.05"/></geometry>
  </collision>
  
  <collision>
    <origin xyz="0.08 0.1 0"/>
    <geometry><box size="0.04 0.1 0.05"/></geometry>
  </collision>
</link>

Inertial Properties

Define mass and rotational inertia for dynamics simulation:

<link name="base_link">
  <inertial>
    <!-- Mass in kilograms -->
    <mass value="5.0"/>
    
    <!-- Inertia tensor (moment of inertia) -->
    <!-- ixx, iyy, izz = rotational inertia about x, y, z axes -->
    <!-- ixy, ixz, iyz = products of inertia (usually 0 for symmetric objects) -->
    <inertia ixx="0.1" ixy="0" ixz="0" iyy="0.1" iyz="0" izz="0.2"/>
    
    <!-- Optional: Center of mass offset from link origin -->
    <origin xyz="0 0 0.05" rpy="0 0 0"/>
  </inertial>
</link>

Computing inertia:

  • For a box: I = (1/12) * m * (h^2 + d^2) where h, d are dimensions
  • For a cylinder: I_x = I_y = (1/12) * m * (3*r^2 + h^2), I_z = (1/2) * m * r^2
  • For a sphere: I = (2/5) * m * r^2

Simplified approach: If exact inertia is unknown, use rough estimates:

<inertial>
  <mass value="1.0"/>
  <inertia ixx="0.01" ixy="0" ixz="0" iyy="0.01" iyz="0" izz="0.01"/>
</inertial>

Joints in Detail

Joint Types

1. Fixed Joint

No movement - connects two links rigidly:

<joint name="fixed_joint" type="fixed">
  <parent link="base_link"/>
  <child link="sensor_link"/>
  <origin xyz="0.1 0 0.05" rpy="0 0 0"/>
</joint>

Use for:

  • Sensors mounted on the robot
  • Non-moving parts
  • Rigid attachments

2. Revolute Joint

Rotational movement with limits (like a door hinge):

<joint name="shoulder_joint" type="revolute">
  <parent link="base_link"/>
  <child link="arm_link"/>
  <origin xyz="0 0 0.3" rpy="0 0 0"/>
  <axis xyz="0 1 0"/>  <!-- Rotate around Y axis -->
  <limit lower="-1.57" upper="1.57" effort="50" velocity="1.0"/>
</joint>

Parameters:

  • <axis> - Rotation axis (unit vector, typically one value = 1)
  • <limit>:
    • lower, upper - Min/max angles in radians
    • effort - Maximum torque in Newton-meters
    • velocity - Maximum angular velocity in rad/s

Common axes:

  • xyz="1 0 0" - Rotate around X (roll)
  • xyz="0 1 0" - Rotate around Y (pitch)
  • xyz="0 0 1" - Rotate around Z (yaw)

3. Continuous Joint

Unlimited rotation (like a wheel):

<joint name="wheel_joint" type="continuous">
  <parent link="base_link"/>
  <child link="wheel_link"/>
  <origin xyz="0.2 0.15 0" rpy="0 1.57 0"/>
  <axis xyz="0 1 0"/>
</joint>

Use for:

  • Wheels that spin indefinitely
  • Rotating shafts
  • Continuous rotating joints

Note: No <limit> element needed

4. Prismatic Joint

Linear sliding movement (like a piston):

<joint name="linear_actuator" type="prismatic">
  <parent link="base_link"/>
  <child link="piston_link"/>
  <origin xyz="0 0 0" rpy="0 0 0"/>
  <axis xyz="0 0 1"/>  <!-- Move along Z axis -->
  <limit lower="0" upper="0.2" effort="100" velocity="0.1"/>
</joint>

Parameters:

  • <axis> - Direction of movement (unit vector)
  • <limit>:
    • lower, upper - Min/max position in meters
    • effort - Maximum force in Newtons
    • velocity - Maximum linear velocity in m/s

5. Planar Joint

Movement in a 2D plane (advanced, less common)

6. Floating Joint

Free 6-DOF movement (for flying robots or underwater vehicles)

Joint Limits and Effort

Always specify limits for revolute joints:

<!-- Limited rotation: safe joint -->
<joint name="elbow" type="revolute">
  <axis xyz="0 1 0"/>
  <limit lower="-2.0" upper="2.0" effort="30" velocity="1.5"/>
  <!-- Can rotate -2 to +2 radians (~-114 to +114 degrees) -->
  <!-- Max torque: 30 Nm, Max speed: 1.5 rad/s -->
</joint>

If you forget limits: Many simulators will refuse to load the file!

Kinematic Structures

Building a Kinematic Chain

A robot is organized as a tree of links connected by joints:

                  base_link (root)
                      |
                      | shoulder_joint (revolute)
                      |
                  arm_link
                      |
                      | elbow_joint (revolute)
                      |
                forearm_link
                      |
                      | wrist_joint (revolute)
                      |
                  hand_link

URDF representation:

<robot name="robot_arm">
  <!-- Root link -->
  <link name="base_link">
    <visual>
      <geometry><box size="0.3 0.3 0.1"/></geometry>
    </visual>
  </link>
  
  <!-- First joint and child link -->
  <joint name="shoulder_joint" type="revolute">
    <parent link="base_link"/>
    <child link="arm_link"/>
    <origin xyz="0 0 0.15"/>
    <axis xyz="0 0 1"/>
    <limit lower="-1.57" upper="1.57" effort="50" velocity="1"/>
  </joint>
  
  <link name="arm_link">
    <visual>
      <geometry><cylinder radius="0.05" length="0.4"/></geometry>
    </visual>
  </link>
  
  <!-- Continue chain... -->
</robot>

Important Kinematic Rules

  1. Each link (except root) must have exactly ONE parent joint
  2. A link can have multiple child joints (branching is okay)
  3. No cycles allowed - Structure must be a tree
  4. Root link - Convention is to name it base_link
  5. Unique names - Every link and joint must have a unique name

Example: Quadruped Robot

            base_link (root)
         /      |      \      \
        /       |       \      \
  fl_hip  fr_hip  rl_hip  rr_hip
   /        /      /        /
fl_udeg  fr_uleg  rl_uleg  rr_uleg
 /        /        /        /
fl_leg  fr_leg   rl_leg   rr_leg

Coordinate Frames and Transforms

Understanding <origin>

The xyz and rpy in <origin> define the transform from parent to child:

<joint name="example" type="fixed">
  <parent link="base_link"/>
  <child link="camera_link"/>
  <origin xyz="0.1 0 0.2" rpy="0 0 0"/>
  <!-- camera_link is positioned 0.1m forward, 0.2m up from base_link -->
  <!-- With no rotation -->
</joint>

Rotations (rpy - Roll Pitch Yaw)

Three rotations applied in order: X (roll), Y (pitch), Z (yaw):

<!-- 90 degree pitch (rotate around Y axis) -->
<origin xyz="0 0 0" rpy="0 1.57 0"/>

<!-- 45 degree roll and yaw (rotate around X and Z) -->
<origin xyz="1 2 3" rpy="0.785 0 0.785"/>

Converting degrees to radians: radians = degrees * π / 180

  • 90° = 1.57 rad
  • 45° = 0.785 rad
  • 180° = 3.14 rad

Visual Origin vs Joint Origin

These are different:

<link name="camera">
  <visual>
    <!-- Position of visual geometry WITHIN the link -->
    <origin xyz="0 0 0" rpy="0 0 0"/>
    <geometry><box size="0.05 0.05 0.05"/></geometry>
  </visual>
</link>

<joint name="camera_mount" type="fixed">
  <!-- Position of camera_link RELATIVE TO parent (base_link) -->
  <origin xyz="0.1 0 0.2" rpy="0 0 0"/>
  <parent link="base_link"/>
  <child link="camera"/>
</joint>

Package and Mesh References

Using ROS Packages

Mesh files and resources use package-relative paths:

<!-- Correct: package:// URI (portable across machines) -->
<mesh filename="package://my_robot/meshes/wheel.stl"/>

<!-- Wrong: Absolute path (not portable) -->
<mesh filename="/home/user/ros_ws/my_robot/meshes/wheel.stl"/>

<!-- Wrong: Relative path (fragile) -->
<mesh filename="meshes/wheel.stl"/>

Package Discovery

The extension finds packages by:

  1. Scanning workspace for package.xml files
  2. Using ROS package commands if available
  3. Building a package name → path mapping

Best practice: Always use package:// URIs in your URDF files.

Mesh File Paths

Typical project structure:

my_robot/
├── package.xml
├── urdf/
│   └── my_robot.urdf  (or .xacro)
└── meshes/
    ├── base.stl
    ├── wheel.stl
    └── gripper/
        ├── finger_left.stl
        └── finger_right.stl

References in URDF:

<mesh filename="package://my_robot/meshes/base.stl"/>
<mesh filename="package://my_robot/meshes/wheel.stl"/>
<mesh filename="package://my_robot/meshes/gripper/finger_left.stl"/>

Mesh Scaling

Mesh units may differ from URDF (which uses meters):

<!-- STL exported in millimeters, convert to meters -->
<mesh filename="package://my_robot/meshes/part.stl" scale="0.001 0.001 0.001"/>

<!-- Scale by 50% -->
<mesh filename="package://my_robot/meshes/part.stl" scale="0.5 0.5 0.5"/>

Creating Complex Meshes with OpenSCAD

For complex geometry that cannot be created with basic shapes (boxes, cylinders, spheres), use OpenSCAD:

⚠️ IMPORTANT: Unit Mismatch

  • OpenSCAD uses millimeters (mm)
  • URDF uses meters (m)
  • When referencing the STL in URDF, you MUST scale by 0.001 to convert mm → m

Workflow:

  1. Create a .scad file with your geometry logic in millimeters (e.g., meshes/robot_foot.scad)
  2. The VS Code extension automatically converts .scad → .stl when saved
  3. Reference the generated .stl file in your URDF/Xacro (not the .scad)
  4. Always apply scale="0.001 0.001 0.001" to convert from mm to meters

Example:

Create meshes/robot_foot.scad (dimensions in millimeters):

// robot_foot.scad - Complex foot geometry
// All dimensions are in MILLIMETERS
module robot_foot(length=100, width=50, height=20) {
  difference() {
    // Main foot body: 100mm × 50mm × 20mm
    cube([length, width, height]);
    
    // Mounting holes: 5mm radius
    translate([20, width/2, 0])
      cylinder(h=height, r=5);
    translate([length-20, width/2, 0])
      cylinder(h=height, r=5);
  }
}

// Generate foot with dimensions in mm
robot_foot(length=120, width=60, height=25);

Reference in URDF (with 0.001 scale to convert mm → m):

<link name="foot">
  <visual>
    <geometry>
      <!-- 
        Reference the generated STL file (not the .scad).
        Scale 0.001 converts from OpenSCAD millimeters to URDF meters.
        The foot will be 0.12m × 0.06m × 0.025m in the robot.
      -->
      <mesh filename="package://my_robot/meshes/robot_foot.stl" scale="0.001 0.001 0.001"/>
    </geometry>
  </visual>
  <collision>
    <geometry>
      <mesh filename="package://my_robot/meshes/robot_foot.stl" scale="0.001 0.001 0.001"/>
    </geometry>
  </collision>
</link>

Important notes:

  • Always use scale="0.001 0.001 0.001" when referencing OpenSCAD-generated STL files
  • OpenSCAD source (.scad) stays in millimeters - this is standard CAD convention
  • Reference the .stl file in URDF, never the .scad
  • The .scad file is the source; the .stl is what renders
  • When you save the .scad file, the extension automatically regenerates the .stl
  • You can see a live preview of the geometry using the preview command
  • For parametric geometry reuse, create OpenSCAD modules that other .scad files can reference

URDF Validation and Best Practices

Common Errors

Error: "Duplicate link/joint name"

  • Every link and joint name must be unique
  • Check for typos or copy-paste mistakes

Error: "Parent link not found"

<!-- Wrong: Parent link doesn't exist -->
<joint name="wheel" type="continuous">
  <parent link="baes_link"/>  <!-- Typo! -->
  <child link="wheel"/>
</joint>

Error: "Joint limits missing"

<!-- Wrong: revolute joint without limits -->
<joint name="arm" type="revolute">
  <axis xyz="0 1 0"/>
  <!-- Missing <limit> tag -->
</joint>

<!-- Correct: Always specify limits -->
<joint name="arm" type="revolute">
  <axis xyz="0 1 0"/>
  <limit lower="-1.57" upper="1.57" effort="50" velocity="1"/>
</joint>

Error: "Mesh file not found"

  • Verify mesh file exists
  • Check spelling and path
  • Ensure package name is correct

Validation Workflow

  1. Syntax check - XML structure is valid
  2. Preview - Use "URDF: Preview" to visualize
  3. Take screenshot - Verify geometry matches expectations
  4. Check kinematic tree - Ensure links and joints form valid tree
  5. Inspect console - Look for warning messages

Naming Conventions

Follow ROS naming standards:

<!-- ✓ Good: lowercase with underscores -->
<link name="base_link"/>
<link name="left_front_wheel"/>
<joint name="base_to_wheel"/>
<joint name="shoulder_joint"/>

<!-- ✗ Bad: CamelCase or mixed case -->
<link name="BaseLink"/>
<link name="leftFrontWheel"/>
<joint name="baseToWheel"/>

Comments and Documentation

<?xml version="1.0"?>
<robot name="my_robot" xmlns:xacro="http://www.ros.org/wiki/xacro">
  
  <!-- Root link representing the robot base -->
  <link name="base_link">
    <visual>
      <geometry>
        <!-- 50cm wide, 30cm deep, 10cm tall -->
        <box size="0.5 0.3 0.1"/>
      </geometry>
      <material name="base_color">
        <color rgba="0.8 0.8 0.8 1"/>  <!-- Light gray -->
      </material>
    </visual>
    
    <!-- Collision geometry approximates base as simple box -->
    <collision>
      <geometry>
        <box size="0.5 0.3 0.1"/>
      </geometry>
    </collision>
  </link>
  
  <!-- Front-left wheel connection -->
  <joint name="base_to_fl_wheel" type="continuous">
    <parent link="base_link"/>
    <child link="fl_wheel"/>
    <!-- Position: 0.15m forward, 0.15m left, 0.05m down from base center -->
    <origin xyz="0.15 0.15 -0.05" rpy="0 1.5708 0"/>
    <!-- Wheel rotates around Y axis (left-right) -->
    <axis xyz="0 1 0"/>
  </joint>
  
  <link name="fl_wheel">
    <visual>
      <geometry>
        <!-- 10cm diameter wheel, 5cm wide -->
        <cylinder radius="0.05" length="0.05"/>
      </geometry>
      <material name="black">
        <color rgba="0.1 0.1 0.1 1"/>
      </material>
    </visual>
  </link>
  
</robot>

Using URDF in the VS Code Extension

Preview Commands

  • Right-click URDF file → "URDF: Preview" to open 3D viewer
  • Right-click URDF file → "URDF: Take Screenshot" for documentation
  • Right-click Xacro file → "URDF: Export" to generate expanded URDF

Validation Features

  • XML schema automatically validates syntax
  • Hover over elements for documentation
  • Go to Definition (F12) jumps to referenced links/joints
  • Diagnostics panel shows errors and warnings

Mesh File Viewer

The extension includes a custom 3D viewer for mesh files:

  • Opens .stl, .dae, .glb, .gltf files in 3D
  • Supports interactive rotation, zoom, pan
  • Displays mesh statistics and properties

Quick Reference: Joint Type Selection

Joint Type Movement Use Case
Fixed None Sensors, non-moving attachments
Revolute Rotational (limited) Robot arms, articulated segments
Continuous Rotational (unlimited) Wheels, rotating shafts
Prismatic Linear (limited) Actuators, sliding mechanisms
Planar 2D planar Advanced wheeled robots
Floating Free 6-DOF Flying/underwater robots

Best Practices Summary

✅ DO:

  1. Start with base_link as the root
  2. Use package:// URIs for mesh files
  3. Specify joint limits for revolute joints
  4. Use simple collision geometry
  5. Include inertial properties for dynamics
  6. Use meaningful link and joint names
  7. Take screenshots to verify geometry
  8. Add XML comments explaining design choices
  9. Validate in the extension's preview
  10. Use Xacro macros for repeated structures

❌ DON'T:

  1. Use absolute file paths
  2. Forget joint limits
  3. Make collision geometry too complex
  4. Reference non-existent mesh files
  5. Create cycles in the kinematic tree
  6. Use inconsistent naming conventions
  7. Skip inertial properties if doing simulation
  8. Make geometry without visualizing it first
  9. Assume mesh scales are in meters
  10. Leave namespacing issues unresolved
Install via CLI
npx skills add https://github.com/Ranch-Hand-Robotics/rde-urdf --skill urdf-fundamentals
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