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.
apm install @a5c-ai/urdf-sdf-model
[](https://apm-p1ls2dz87-atlamors-projects.vercel.app/packages/@a5c-ai/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
<?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="0 0 0" rpy="0 0 0"/>
<inertia ixx="0.0833" ixy="0" ixz="0"
iyy="0.2167" iyz="0" izz="0.2833"/>
</inertial>
</link>
<!-- Wheel Joint -->
<joint name="wheel_joint" type="continuous">
<parent link="base_link"/>
<child link="wheel_link"/>
<origin xyz="0.2 0.15 -0.05" rpy="-1.5708 0 0"/>
<axis xyz="0 0 1"/>
<limit effort="10" velocity="10"/>
<dynamics damping="0.1" friction="0.1"/>
</joint>
<!-- Wheel Link -->
<link name="wheel_link">
<visual>
<geometry>
<cylinder radius="0.05" length="0.02"/>
</geometry>
</visual>
<collision>
<geometry>
<cylinder radius="0.05" length="0.02"/>
</geometry>
</collision>
<inertial>
<mass value="0.5"/>
<inertia ixx="0.0003" ixy="0" ixz="0"
iyy="0.0003" iyz="0" izz="0.0006"/>
</inertial>
</link>
</robot>
```
### 2. Xacro Macros
Create modular robot descriptions with Xacro:
```xml
<?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}" ixy="0" ixz="0"
iyy="${m*(x*x+z*z)/12}" iyz="0" izz="${m*(x*x+y*y)/12}"/>
</xacro:macro>
<!-- Wheel Macro -->
<xacro:macro name="wheel" params="prefix parent x_offset y_offset">
<joint name="${prefix}_wheel_joint" type="continuous">
<parent link="${parent}"/>
<child link="${prefix}_wheel_link"/>
<origin xyz="${x_offset} ${y_offset} 0" rpy="-1.5708 0 0"/>
<axis xyz="0 0 1"/>
<limit effort="10" velocity="10"/>
<dynamics damping="0.1" friction="0.1"/>
</joint>
<link name="${prefix}_wheel_link">
<visual>
<geometry>
<cylinder radius="${wheel_radius}" length="${wheel_width}"/>
</geometry>
<material name="black"/>
</visual>
<collision>
<geometry>
<cylinder radius="${wheel_radius}" length="${wheel_width}"/>
</geometry>
</collision>
<inertial>
<mass value="${wheel_mass}"/>
<xacro:cylinder_inertia m="${wheel_mass}" r="${wheel_radius}" h="${wheel_width}"/>
</inertial>
</link>
<!-- Gazebo friction -->
<gazebo reference="${prefix}_wheel_link">
<mu1>1.0</mu1>
<mu2>1.0</mu2>
<kp>1e6</kp>
<kd>1.0</kd>
</gazebo>
</xacro:macro>
<!-- Instantiate wheels -->
<xacro:wheel prefix="front_left" parent="base_link" x_offset="0.15" y_offset="0.12"/>
<xacro:wheel prefix="front_right" parent="base_link" x_offset="0.15" y_offset="-0.12"/>
<xacro:wheel prefix="rear_left" parent="base_link" x_offset="-0.15" y_offset="0.12"/>
<xacro:wheel prefix="rear_right" parent="base_link" x_offset="-0.15" y_offset="-0.12"/>
</robot>
```
### 3. Inertia Calculations
Calculate inertia tensors for common geometries:
```python
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 = 2 * mass * radius**2 / 5
return {'ixx': i, 'iyy': i, 'izz': i, 'ixy': 0, 'ixz': 0, 'iyz': 0}
def mesh_inertia_from_stl(stl_file, mass, density=None):
"""Estimate inertia from STL mesh using convex hull approximation."""
# Use trimesh or similar library for accurate calculation
import trimesh
mesh = trimesh.load(stl_file)
mesh.density = density if density else mass / mesh.volume
return mesh.moment_inertia
```
### 4. Joint Types Configuration
Configure different joint types:
```xml
<!-- 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"/>
<limit effort="10" velocity="10"/>
</joint>
<!-- Prismatic Joint (linear motion) -->
<joint name="slider_joint" type="prismatic">
<parent link="base"/>
<child link="slider"/>
<origin xyz="0 0 0"/>
<axis xyz="0 0 1"/>
<limit lower="0" upper="0.5" effort="50" velocity="0.5"/>
</joint>
<!-- Fixed Joint (no motion) -->
<joint name="sensor_mount" type="fixed">
<parent link="base"/>
<child link="sensor"/>
<origin xyz="0.1 0 0.05" rpy="0 0 0"/>
</joint>
```
### 5. Sensor Attachments
Add sensors to the robot model:
```xml
<!-- 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>
<image>
<width>640</width>
<height>480</height>
<format>R8G8B8</format>
</image>
<clip>
<near>0.02</near>
<far>100</far>
</clip>
</camera>
<plugin name="camera_plugin" filename="libgazebo_ros_camera.so">
<ros>
<namespace>/robot</namespace>
<remapping>image_raw:=camera/image_raw</remapping>
<remapping>camera_info:=camera/camera_info</remapping>
</ros>
<frame_name>camera_link</frame_name>
</plugin>
</sensor>
</gazebo>
<!-- LiDAR Sensor -->
<link name="lidar_link">
<visual>
<geometry>
<cylinder radius="0.03" length="0.04"/>
</geometry>
</visual>
</link>
<gazebo reference="lidar_link">
<sensor type="ray" name="lidar">
<pose>0 0 0 0 0 0</pose>
<visualize>true</visualize>
<update_rate>10</update_rate>
<ray>
<scan>
<horizontal>
<samples>360</samples>
<resolution>1</resolution>
<min_angle>-3.14159</min_angle>
<max_angle>3.14159</max_angle>
</horizontal>
</scan>
<range>
<min>0.1</min>
<max>10.0</max>
<resolution>0.01</resolution>
</range>
</ray>
<plugin name="lidar_plugin" filename="libgazebo_ros_ray_sensor.so">
<ros>
<namespace>/robot</namespace>
<remapping>~/out:=scan</remapping>
</ros>
<output_type>sensor_msgs/LaserScan</output_type>
<frame_name>lidar_link</frame_name>
</plugin>
</sensor>
</gazebo>
```
### 6. Model Validation
Validate URDF models:
```bash
# 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
<?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</mu>
<mu2>1.0</mu2>
</ode>
</friction>
</surface>
</collision>
<visual name='base_visual'>
<geometry>
<box>
<size>0.5 0.3 0.1</size>
</box>
</geometry>
<material>
<ambient>0.0 0.0 0.8 1</ambient>
</material>
</visual>
</link>
</model>
</sdf>
```
## 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](https://mcpservers.org/servers/rishigundakaram/cadquery-mcp-server) |
| FreeCAD MCP | FreeCAD integration | [GitHub](https://github.com/bonninr/freecad_mcp) |
| Blender MCP | Mesh creation and editing | [blender-mcp.com](https://blender-mcp.com/) |
| OpenSCAD MCP | Parametric modeling | [playbooks.com](https://playbooks.com/mcp/jhacksman-openscad) |
## 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:
```json
{
"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