urdf-sdf-model

name	urdf-sdf-model
description	Expert skill for robot model creation and validation in URDF and SDF formats. Generate URDF files with proper link-joint hierarchy, create Xacro macros, calculate inertial properties, configure joint types, and validate models.
allowed-tools	Bash(*) Read Write Edit Glob Grep WebFetch
metadata	{"author":"babysitter-sdk","version":"1.0.0","category":"robot-modeling","backlog-id":"SK-004"}

urdf-sdf-model

You are urdf-sdf-model - a specialized skill for robot model creation and validation in URDF (Unified Robot Description Format) and SDF (Simulation Description Format).

Overview

This skill enables AI-powered robot modeling including:

• Generating URDF files with proper link-joint hierarchy
• Creating Xacro macros for modular robot descriptions
• Converting between URDF and SDF formats
• Calculating and setting inertial properties (mass, inertia tensors)
• Importing and optimizing mesh files (visual and collision)
• Configuring joint types (revolute, prismatic, continuous, fixed, floating)
• Setting up transmission and actuator definitions
• Adding sensor plugins and attachments
• Validating models with urdfdom and check_urdf
• Visualizing and debugging in RViz

Prerequisites

• ROS/ROS2 with urdf packages
• xacro for macro processing
• urdfdom for validation
• Gazebo for SDF validation
• Mesh tools (MeshLab, Blender) for mesh optimization

Capabilities

1. URDF Generation

Generate URDF files with proper structure:

<?xml version="1.0"?>
<robot name="my_robot" xmlns:xacro="http://www.ros.org/wiki/xacro">

  <!-- Materials -->
  <material name="blue">
    <color rgba="0.0 0.0 0.8 1.0"/>
  </material>

  <!-- Base Link -->
  <link name="base_link">
    <visual>
      <geometry>
        <box size="0.5 0.3 0.1"/>
      </geometry>
      <material name="blue"/>
    </visual>
    <collision>
      <geometry>
        <box size="0.5 0.3 0.1"/>
      </geometry>
    </collision>
    <inertial>
      <mass value="10.0"/>
      <origin xyz= =/>
      
    
  

  
  
    
    
    
    
    
    
  

  
  
    
      
        
      
    
    
      
        
      
    
    
      
      
    
  

2. Xacro Macros

Create modular robot descriptions with Xacro:

<?xml version="1.0"?>
<robot name="my_robot" xmlns:xacro="http://www.ros.org/wiki/xacro">

  <!-- Properties -->
  <xacro:property name="wheel_radius" value="0.05"/>
  <xacro:property name="wheel_width" value="0.02"/>
  <xacro:property name="wheel_mass" value="0.5"/>

  <!-- Inertia Macros -->
  <xacro:macro name="cylinder_inertia" params="m r h">
    <inertia ixx="${m*(3*r*r+h*h)/12}" ixy="0" ixz="0"
             iyy="${m*(3*r*r+h*h)/12}" iyz="0" izz="${m*r*r/2}"/>
  </xacro:macro>

  <xacro:macro name="box_inertia" params="m x y z">
    <inertia ixx="${m*(y*y+z*z)/12}" = =
             = = =/>
  

  
  
    
      
      
      
      
      
      
    

    
      
        
          
        
        
      
      
        
          
        
      
      
        
        
      
    

    
    
      1.0
      1.0
      1e6
      1.0
    
  

  
  
  
  
  

3. Inertia Calculations

Calculate inertia tensors for common geometries:

import numpy as np

def box_inertia(mass, x, y, z):
    """Calculate inertia tensor for a box centered at origin."""
    ixx = mass * (y**2 + z**2) / 12
    iyy = mass * (x**2 + z**2) / 12
    izz = mass * (x**2 + y**2) / 12
    return {'ixx': ixx, 'iyy': iyy, 'izz': izz, 'ixy': 0, 'ixz': 0, 'iyz': 0}

def cylinder_inertia(mass, radius, height):
    """Calculate inertia tensor for a cylinder along z-axis."""
    ixx = mass * (3 * radius**2 + height**2) / 12
    iyy = mass * (3 * radius**2 + height**2) / 12
    izz = mass * radius**2 / 2
    return {'ixx': ixx, 'iyy': iyy, 'izz': izz, 'ixy': 0, 'ixz': 0, 'iyz': 0}

def sphere_inertia(mass, radius):
    """Calculate inertia tensor for a solid sphere."""
    i =  * mass * radius** / 
     {: i, : i, : i, : , : , : }

 ():
    
    
     trimesh
    mesh = trimesh.load(stl_file)
    mesh.density = density  density  mass / mesh.volume
     mesh.moment_inertia

4. Joint Types Configuration

Configure different joint types:

<!-- Revolute Joint (limited rotation) -->
<joint name="arm_joint" type="revolute">
  <parent link="base"/>
  <child link="arm"/>
  <origin xyz="0 0 0.1" rpy="0 0 0"/>
  <axis xyz="0 1 0"/>
  <limit lower="-1.57" upper="1.57" effort="100" velocity="1.0"/>
  <dynamics damping="0.5" friction="0.1"/>
</joint>

<!-- Continuous Joint (unlimited rotation) -->
<joint name="wheel_joint" type="continuous">
  <parent link="base"/>
  <child link="wheel"/>
  <axis xyz="0 0 1"/>
  




  
  
  
  
  




  
  
  

5. Sensor Attachments

Add sensors to the robot model:

<!-- Camera Sensor -->
<link name="camera_link">
  <visual>
    <geometry>
      <box size="0.02 0.05 0.02"/>
    </geometry>
  </visual>
</link>

<joint name="camera_joint" type="fixed">
  <parent link="base_link"/>
  <child link="camera_link"/>
  <origin xyz="0.2 0 0.1" rpy="0 0 0"/>
</joint>

<gazebo reference="camera_link">
  <sensor type="camera" name="camera">
    <update_rate>30.0</update_rate>
    <camera>
      <horizontal_fov>1.3962634</horizontal_fov>
      <>
        640
        480
        R8G8B8
      
      
        0.02
        100
      
    
    
      
        /robot
        image_raw:=camera/image_raw
        camera_info:=camera/camera_info
      
      camera_link
    
  




  
    
      
    
  



  
    0 0 0 0 0 0
    true
    10
    
      
        
          360
          1
          -3.14159
          3.14159
        
      
      
        0.1
        10.0
        0.01
      
    
    
      
        /robot
        ~/out:=scan
      
      sensor_msgs/LaserScan
      lidar_link
    
  

6. Model Validation

Validate URDF models:

# Check URDF syntax
check_urdf robot.urdf

# Process Xacro and check
xacro robot.urdf.xacro > robot.urdf && check_urdf robot.urdf

# Visualize URDF tree
urdf_to_graphviz robot.urdf

# View in RViz
ros2 launch urdf_tutorial display.launch.py model:=robot.urdf.xacro

# Convert URDF to SDF
gz sdf -p robot.urdf > robot.sdf

7. SDF Format

Generate SDF for Gazebo:

<?xml version='1.0'?>
<sdf version='1.7'>
  <model name='my_robot'>
    <link name='base_link'>
      <inertial>
        <mass>10.0</mass>
        <inertia>
          <ixx>0.0833</ixx>
          <iyy>0.2167</iyy>
          <izz>0.2833</izz>
        </inertia>
      </inertial>
      <collision name='base_collision'>
        <geometry>
          <box>
            <size>0.5 0.3 0.1</size>
          </box>
        </geometry>
        <surface>
          <friction>
            <ode>
              <mu>1.0
              1.0
            
          
        
      
      
        
          
            0.5 0.3 0.1
          
        
        
          0.0 0.0 0.8 1
        
      
    
  

MCP Server Integration

This skill can leverage the following MCP servers for enhanced capabilities:

Server	Description	Installation
CAD-Query MCP	Parametric 3D modeling	mcpservers.org
FreeCAD MCP	FreeCAD integration	GitHub
Blender MCP	Mesh creation and editing	blender-mcp.com
OpenSCAD MCP	Parametric modeling	playbooks.com

Best Practices

1. Consistent units - Use SI units (meters, kilograms, radians)
2. Origin placement - Place link origins at center of mass when possible
3. Collision geometry - Use simplified collision meshes for performance
4. Inertia accuracy - Calculate accurate inertia for stable simulation
5. Mesh optimization - Reduce polygon count for collision meshes
6. Modular design - Use Xacro macros for reusable components

Process Integration

This skill integrates with the following processes:

• robot-urdf-sdf-model.js - Primary model creation process
• robot-system-design.js - System architecture with models
• moveit-manipulation-planning.js - MoveIt configuration
• gazebo-simulation-setup.js - Simulation model setup

Output Format

When executing operations, provide structured output:

{
  "operation": "create-urdf",
  "robotName": "my_robot",
  "status": "success",
  "validation": {
    "syntaxValid": true,
    "inertiasValid": true,
    "jointsValid": true
  },
  "artifacts": [
    "urdf/my_robot.urdf.xacro",
    "meshes/base_link.stl",
    "meshes/wheel.stl"
  ],
  "statistics": {
    "links": 5,
    "joints": 4,
    "sensors": 2
  }
}

Constraints

• Verify coordinate frame conventions (REP-103)
• Ensure consistent units throughout model
• Validate inertia tensors are physically plausible
• Check for self-collision in collision geometry
• Respect Gazebo SDF version compatibility