Chapter 7: URDF, SDF & Unity Visualization

"A robot is only as good as its model. Accurate representation is the foundation of simulation and control."

Table of Contents

1. Introduction to Robot Description
2. URDF Deep Dive
3. SDF Format
4. Xacro for Modularity
5. Unity for Robotics
6. ROS-Unity Integration
7. Visualization Tools
8. Best Practices

---

Introduction to Robot Description

Robot description formats define the physical and visual properties of robots for simulation and visualization.

Format Comparison

Format	Purpose	Strengths	Weaknesses
URDF	ROS robot description	ROS integration, simple	Limited features, no sensors
SDF	Gazebo simulation	Full simulation support	More complex
Xacro	URDF templating	Reusable, parametric	Processing step needed
Unity	3D visualization	Beautiful graphics	Heavier, game engine

---

URDF Deep Dive

Complete URDF Structure

<?xml version="1.0"?>
<robot name="mobile_manipulator">
  
  <!-- Materials -->
  <material name="blue">
    <color rgba="0 0 0.8 1"/>
  </material>
  
  <material name="black">
    <color rgba="0 0 0 1"/>
  </material>
  
  <material name="white">
    <color rgba="1 1 1 1"/>
  </material>
  
  <!-- Base Link -->
  <link name="base_link">
    <visual>
      <origin xyz="0 0 0" rpy="0 0 0"/>
      <geometry>
        <box size="0.6 0.4 0.2"/>
      </geometry>
      <material name="blue"/>
    </visual>
    
    <collision>
      <origin xyz="0 0 0" rpy="0 0 0"/>
      <geometry>
        <box size="0.6 0.4 0.2"/>
      </geometry>
    </collision>
    
    <inertial>
      <origin xyz="0 0 0" rpy="0 0 0"/>
      <mass value="15.0"/>
      <inertia ixx="0.175" ixy="0" ixz="0"
               iyy="0.495" iyz="0"
               izz="0.62"/>
    </inertial>
  </link>
  
  <!-- Left Wheel -->
  <link name="left_wheel">
    <visual>
      <origin xyz="0 0 0" rpy="1.5708 0 0"/>
      <geometry>
        <cylinder length="0.05" radius="0.1"/>
      </geometry>
      <material name="black"/>
    </visual>
    
    <collision>
      <origin xyz="0 0 0" rpy="1.5708 0 0"/>
      <geometry>
        <cylinder length="0.05" radius="0.1"/>
      </geometry>
    </collision>
    
    <inertial>
      <origin xyz="0 0 0" rpy="0 0 0"/>
      <mass value="1.0"/>
      <inertia ixx="0.00277" ixy="0" ixz="0"
               iyy="0.00277" iyz="0"
               izz="0.005"/>
    </inertial>
  </link>
  
  <!-- Left Wheel Joint -->
  <joint name="left_wheel_joint" type="continuous">
    <parent link="base_link"/>
    <child link="left_wheel"/>
    <origin xyz="0.2 0.25 0" rpy="0 0 0"/>
    <axis xyz="0 1 0"/>
    <dynamics damping="0.1" friction="0.1"/>
  </joint>
  
  <!-- Right Wheel -->
  <link name="right_wheel">
    <visual>
      <origin xyz="0 0 0" rpy="1.5708 0 0"/>
      <geometry>
        <cylinder length="0.05" radius="0.1"/>
      </geometry>
      <material name="black"/>
    </visual>
    
    <collision>
      <origin xyz="0 0 0" rpy="1.5708 0 0"/>
      <geometry>
        <cylinder length="0.05" radius="0.1"/>
      </geometry>
    </collision>
    
    <inertial>
      <origin xyz="0 0 0" rpy="0 0 0"/>
      <mass value="1.0"/>
      <inertia ixx="0.00277" ixy="0" ixz="0"
               iyy="0.00277" iyz="0"
               izz="0.005"/>
    </inertial>
  </link>
  
  <joint name="right_wheel_joint" type="continuous">
    <parent link="base_link"/>
    <child link="right_wheel"/>
    <origin xyz="0.2 -0.25 0" rpy="0 0 0"/>
    <axis xyz="0 1 0"/>
    <dynamics damping="0.1" friction="0.1"/>
  </joint>
  
  <!-- Caster Wheel -->
  <link name="caster">
    <visual>
      <geometry>
        <sphere radius="0.05"/>
      </geometry>
      <material name="white"/>
    </visual>
    
    <collision>
      <geometry>
        <sphere radius="0.05"/>
      </geometry>
    </collision>
    
    <inertial>
      <mass value="0.5"/>
      <inertia ixx="0.0005" ixy="0" ixz="0"
               iyy="0.0005" iyz="0"
               izz="0.0005"/>
    </inertial>
  </link>
  
  <joint name="caster_joint" type="fixed">
    <parent link="base_link"/>
    <child link="caster"/>
    <origin xyz="-0.2 0 -0.05"/>
  </joint>
  
  <!-- Arm Links -->
  <link name="arm_base">
    <visual>
      <geometry>
        <cylinder length="0.1" radius="0.05"/>
      </geometry>
      <material name="blue"/>
    </visual>
    
    <collision>
      <geometry>
        <cylinder length="0.1" radius="0.05"/>
      </geometry>
    </collision>
    
    <inertial>
      <mass value="0.5"/>
      <inertia ixx="0.001" ixy="0" ixz="0"
               iyy="0.001" iyz="0"
               izz="0.001"/>
    </inertial>
  </link>
  
  <joint name="arm_base_joint" type="revolute">
    <parent link="base_link"/>
    <child link="arm_base"/>
    <origin xyz="0 0 0.15" rpy="0 0 0"/>
    <axis xyz="0 0 1"/>
    <limit lower="-3.14" upper="3.14" effort="10" velocity="1.0"/>
  </joint>
  
  <!-- Add more arm links... -->
  
</robot>

Joint Types

Type	DOF	Description	Use Case
fixed	0	No movement	Sensors, static attachments
revolute	1	Rotation with limits	Robot joints
continuous	1	Unlimited rotation	Wheels
prismatic	1	Linear motion	Linear actuators
planar	2	Plane motion	Mobile platforms
floating	6	Free motion	Underwater robots

Inertia Calculations

# Inertia calculator utilities
import math

def box_inertia(mass, x, y, z):
    """Calculate inertia for box"""
    ixx = (mass / 12.0) * (y**2 + z**2)
    iyy = (mass / 12.0) * (x**2 + z**2)
    izz = (mass / 12.0) * (x**2 + y**2)
    return {'ixx': ixx, 'iyy': iyy, 'izz': izz}

def cylinder_inertia(mass, radius, length):
    """Calculate inertia for cylinder"""
    ixx = (mass / 12.0) * (3 * radius**2 + length**2)
    iyy = ixx
    izz = 0.5 * mass * radius**2
    return {'ixx': ixx, 'iyy': iyy, 'izz': izz}

def sphere_inertia(mass, radius):
    """Calculate inertia for sphere"""
    i = 0.4 * mass * radius**2
    return {'ixx': i, 'iyy': i, 'izz': i}

# Example usage
box = box_inertia(15.0, 0.6, 0.4, 0.2)
print(f"Box inertia: ixx={box['ixx']:.3f}, iyy={box['iyy']:.3f}, izz={box['izz']:.3f}")

---

SDF Format

SDF vs URDF

<?xml version="1.0"?>
<sdf version="1.8">
  <model name="mobile_robot">
    
    <!-- Model pose -->
    <pose>0 0 0.1 0 0 0</pose>
    
    <!-- Links -->
    <link name="base_link">
      <pose>0 0 0 0 0 0</pose>
      
      <inertial>
        <mass>15.0</mass>
        <inertia>
          <ixx>0.175</ixx>
          <ixy>0</ixy>
          <ixz>0</ixz>
          <iyy>0.495</iyy>
          <iyz>0</iyz>
          <izz>0.62</izz>
        </inertia>
      </inertial>
      
      <visual name="base_visual">
        <geometry>
          <box>
            <size>0.6 0.4 0.2</size>
          </box>
        </geometry>
        <material>
          <ambient>0 0 0.8 1</ambient>
          <diffuse>0 0 0.8 1</diffuse>
          <specular>0.1 0.1 0.1 1</specular>
        </material>
      </visual>
      
      <collision name="base_collision">
        <geometry>
          <box>
            <size>0.6 0.4 0.2</size>
          </box>
        </geometry>
        <surface>
          <friction>
            <ode>
              <mu>0.6</mu>
              <mu2>0.6</mu2>
            </ode>
          </friction>
        </surface>
      </collision>
      
      <!-- Sensors in SDF -->
      <sensor name="imu_sensor" type="imu">
        <always_on>true</always_on>
        <update_rate>100</update_rate>
        <pose>0 0 0 0 0 0</pose>
      </sensor>
      
      <sensor name="camera" type="camera">
        <pose>0.3 0 0.2 0 0 0</pose>
        <camera>
          <horizontal_fov>1.047</horizontal_fov>
          <image>
            <width>1920</width>
            <height>1080</height>
          </image>
          <clip>
            <near>0.1</near>
            <far>100</far>
          </clip>
        </camera>
        <always_on>true</always_on>
        <update_rate>30</update_rate>
      </sensor>
    </link>
    
    <!-- Joints -->
    <joint name="left_wheel_joint" type="revolute">
      <parent>base_link</parent>
      <child>left_wheel</child>
      <pose>0 0 0 0 0 0</pose>
      <axis>
        <xyz>0 1 0</xyz>
        <dynamics>
          <damping>0.1</damping>
          <friction>0.1</friction>
        </dynamics>
      </axis>
    </joint>
    
    <!-- Plugins -->
    <plugin name="differential_drive_controller" 
            filename="libgazebo_ros_diff_drive.so">
      <update_rate>50</update_rate>
      <left_joint>left_wheel_joint</left_joint>
      <right_joint>right_wheel_joint</right_joint>
      <wheel_separation>0.5</wheel_separation>
      <wheel_diameter>0.2</wheel_diameter>
      <max_wheel_torque>20</max_wheel_torque>
      <command_topic>cmd_vel</command_topic>
      <odometry_topic>odom</odometry_topic>
    </plugin>
    
  </model>
</sdf>

---

Xacro for Modularity

Xacro Properties

<?xml version="1.0"?>
<robot xmlns:xacro="http://www.ros.org/wiki/xacro" name="modular_robot">
  
  <!-- Properties -->
  <xacro:property name="wheel_radius" value="0.1"/>
  <xacro:property name="wheel_width" value="0.05"/>
  <xacro:property name="wheel_mass" value="1.0"/>
  <xacro:property name="wheel_separation" value="0.5"/>
  
  <xacro:property name="base_length" value="0.6"/>
  <xacro:property name="base_width" value="0.4"/>
  <xacro:property name="base_height" value="0.2"/>
  <xacro:property name="base_mass" value="15.0"/>
  
  <!-- PI constant -->
  <xacro:property name="pi" value="3.14159265359"/>
  
  <!-- Calculated properties -->
  <xacro:property name="wheel_offset_y" value="${wheel_separation/2}"/>
  
</robot>

Xacro Macros

<!-- Wheel macro -->
<xacro:macro name="wheel" params="prefix reflect">
  
  <link name="${prefix}_wheel">
    <visual>
      <origin xyz="0 0 0" rpy="${pi/2} 0 0"/>
      <geometry>
        <cylinder length="${wheel_width}" radius="${wheel_radius}"/>
      </geometry>
      <material name="black"/>
    </visual>
    
    <collision>
      <origin xyz="0 0 0" rpy="${pi/2} 0 0"/>
      <geometry>
        <cylinder length="${wheel_width}" radius="${wheel_radius}"/>
      </geometry>
    </collision>
    
    <inertial>
      <origin xyz="0 0 0" rpy="0 0 0"/>
      <mass value="${wheel_mass}"/>
      <xacro:cylinder_inertia m="${wheel_mass}" 
                              r="${wheel_radius}" 
                              h="${wheel_width}"/>
    </inertial>
  </link>
  
  <joint name="${prefix}_wheel_joint" type="continuous">
    <parent link="base_link"/>
    <child link="${prefix}_wheel"/>
    <origin xyz="0.2 ${reflect * wheel_offset_y} 0" rpy="0 0 0"/>
    <axis xyz="0 1 0"/>
    <dynamics damping="0.1" friction="0.1"/>
  </joint>
  
  <gazebo reference="${prefix}_wheel">
    <mu1>1.0</mu1>
    <mu2>1.0</mu2>
    <kp>10000000.0</kp>
    <kd>1.0</kd>
    <material>Gazebo/Black</material>
  </gazebo>
  
</xacro:macro>

<!-- 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>

<xacro:macro name="sphere_inertia" params="m r">
  <inertia ixx="${2*m*r*r/5}" ixy="0" ixz="0"
           iyy="${2*m*r*r/5}" iyz="0"
           izz="${2*m*r*r/5}"/>
</xacro:macro>

<!-- Use macros -->
<xacro:wheel prefix="left" reflect="1"/>
<xacro:wheel prefix="right" reflect="-1"/>

Including External Files

<?xml version="1.0"?>
<robot xmlns:xacro="http://www.ros.org/wiki/xacro" name="complete_robot">
  
  <!-- Include common properties -->
  <xacro:include filename="$(find my_robot_description)/urdf/common_properties.xacro"/>
  
  <!-- Include wheel macro -->
  <xacro:include filename="$(find my_robot_description)/urdf/wheel.xacro"/>
  
  <!-- Include sensor macros -->
  <xacro:include filename="$(find my_robot_description)/urdf/sensors.xacro"/>
  
  <!-- Use included macros -->
  <xacro:wheel prefix="left" reflect="1"/>
  <xacro:wheel prefix="right" reflect="-1"/>
  
  <xacro:lidar_sensor parent="base_link" xyz="0.3 0 0.2"/>
  <xacro:camera_sensor parent="base_link" xyz="0.3 0 0.25"/>
  
</robot>

Processing Xacro Files

# Convert xacro to URDF
ros2 run xacro xacro robot.urdf.xacro > robot.urdf

# With parameters
ros2 run xacro xacro robot.urdf.xacro \
  wheel_radius:=0.15 \
  wheel_separation:=0.6 \
  > robot_custom.urdf

# Check URDF validity
check_urdf robot.urdf

# Visualize URDF tree
urdf_to_graphiz robot.urdf

---

Unity for Robotics

Unity Robotics Hub

Unity provides photorealistic visualization and simulation capabilities.

Installation:

# Install Unity Hub
# Download from unity.com

# Install Unity Robotics packages via Package Manager:
# - ROS TCP Connector
# - URDF Importer
# - Robotics Visualizations

URDF Import to Unity

Unity URDF Importer Settings:

// RobotImporter.cs
using UnityEngine;
using Unity.Robotics.UrdfImporter;

public class RobotImporter : MonoBehaviour
{
    public string urdfPath = "Assets/URDF/robot.urdf";
    
    void Start()
    {
        // Import URDF
        GameObject robot = UrdfRobotExtensions.CreateRuntime(urdfPath);
        
        // Configure physics
        ConfigurePhysics(robot);
        
        // Add visualization components
        AddVisualization(robot);
    }
    
    void ConfigurePhysics(GameObject robot)
    {
        // Set up articulation bodies
        foreach (var joint in robot.GetComponentsInChildren<ArticulationBody>())
        {
            joint.solverIterations = 20;
            joint.solverVelocityIterations = 20;
        }
    }
    
    void AddVisualization(GameObject robot)
    {
        // Add custom shaders, effects
        var renderer = robot.GetComponentInChildren<Renderer>();
        if (renderer != null)
        {
            renderer.material.shader = Shader.Find("Standard");
        }
    }
}

Unity Graphics Settings

{
  "render_pipeline": "URP",
  "quality": {
    "anti_aliasing": "4x MSAA",
    "shadows": "Hard Shadows",
    "texture_quality": "Full Res",
    "vsync": true,
    "target_fps": 60
  },
  "post_processing": {
    "ambient_occlusion": true,
    "bloom": true,
    "color_grading": true,
    "depth_of_field": false
  }
}

---

ROS-Unity Integration

ROS TCP Endpoint

Unity → ROS 2 Communication:

using UnityEngine;
using Unity.Robotics.ROSTCPConnector;
using RosMessageTypes.Geometry;

public class RobotController : MonoBehaviour
{
    private ROSConnection ros;
    private string topicName = "/cmd_vel";
    
    void Start()
    {
        ros = ROSConnection.GetOrCreateInstance();
        ros.RegisterPublisher<TwistMsg>(topicName);
    }
    
    void Update()
    {
        // Get input
        float linear = Input.GetAxis("Vertical");
        float angular = Input.GetAxis("Horizontal");
        
        // Create message
        TwistMsg twist = new TwistMsg
        {
            linear = new Vector3Msg { x = linear, y = 0, z = 0 },
            angular = new Vector3Msg { x = 0, y = 0, z = angular }
        };
        
        // Publish
        ros.Publish(topicName, twist);
    }
}

ROS 2 → Unity (Subscriber):

using Unity.Robotics.ROSTCPConnector;
using RosMessageTypes.Sensor;

public class SensorVisualizer : MonoBehaviour
{
    private ROSConnection ros;
    
    void Start()
    {
        ros = ROSConnection.GetOrCreateInstance();
        ros.Subscribe<LaserScanMsg>("/scan", OnScanReceived);
    }
    
    void OnScanReceived(LaserScanMsg scan)
    {
        // Visualize LiDAR data
        for (int i = 0; i < scan.ranges.Length; i++)
        {
            float angle = scan.angle_min + i * scan.angle_increment;
            float range = scan.ranges[i];
            
            Vector3 point = new Vector3(
                range * Mathf.Cos(angle),
                0,
                range * Mathf.Sin(angle)
            );
            
            Debug.DrawLine(Vector3.zero, point, Color.red, 0.1f);
        }
    }
}

Launch ROS TCP Endpoint

# ros_tcp_endpoint.launch.py
from launch import LaunchDescription
from launch_ros.actions import Node

def generate_launch_description():
    return LaunchDescription([
        Node(
            package='ros_tcp_endpoint',
            executable='default_server_endpoint',
            parameters=[{
                'ROS_IP': '192.168.1.100',
                'ROS_TCP_PORT': 10000
            }],
            output='screen'
        )
    ])

---

Visualization Tools

RViz2

# Launch RViz2
rviz2

# With config file
rviz2 -d robot.rviz

# From launch file
Node(
    package='rviz2',
    executable='rviz2',
    arguments=['-d', rviz_config_path],
    output='screen'
)

RViz Config Example:

Panels:
  - Class: rviz_common/Displays
  - Class: rviz_common/Views
  - Class: rviz_common/Time

Visualization Manager:
  Displays:
    - Class: rviz_default_plugins/RobotModel
      Robot Description: robot_description
      Visual Enabled: true
      
    - Class: rviz_default_plugins/TF
      Frames:
        All Enabled: true
      
    - Class: rviz_default_plugins/LaserScan
      Topic: /scan
      Size: 0.05
      Color: 255; 0; 0
      
    - Class: rviz_default_plugins/Camera
      Topic: /camera/image_raw

Joint State Publisher GUI

# Install
sudo apt install ros-humble-joint-state-publisher-gui

# Launch
ros2 run joint_state_publisher_gui joint_state_publisher_gui

Launch with robot:

Node(
    package='joint_state_publisher_gui',
    executable='joint_state_publisher_gui',
    output='screen'
),
Node(
    package='robot_state_publisher',
    executable='robot_state_publisher',
    parameters=[{'robot_description': robot_description}]
)

---

Best Practices

URDF Best Practices

✅ DO:

• Use realistic mass and inertia values
• Add collision meshes separate from visual
• Use xacro for modularity
• Define materials consistently
• Include proper joint limits

❌ DON'T:

• Use zero or very small masses
• Ignore collision geometry
• Hardcode values (use properties)
• Forget to define axes
• Use complex meshes for collision

File Organization

robot_description/
├── urdf/
│   ├── robot.urdf.xacro          # Main file
│   ├── common_properties.xacro   # Shared properties
│   ├── wheel.xacro               # Wheel macro
│   ├── sensors.xacro             # Sensor macros
│   └── gazebo.xacro              # Gazebo-specific
├── meshes/
│   ├── visual/                   # High-poly meshes
│   │   ├── base_link.dae
│   │   └── arm_link.dae
│   └── collision/                # Low-poly meshes
│       ├── base_link.stl
│       └── arm_link.stl
├── config/
│   └── robot.rviz
└── launch/
    └── display.launch.py

Performance Tips

Aspect	Recommendation
Mesh complexity	Use simplified collision meshes
Material count	Minimize unique materials
Joint count	Only model necessary DOFs
Sensor rate	Match actual hardware rates
Update frequency	Balance accuracy vs performance

---

Summary

Robot description formats are the foundation of simulation and visualization. Master URDF/SDF for accurate robot modeling, use xacro for maintainability, and leverage Unity for beautiful visualization.

Key Takeaways:

1. ✅ URDF for ROS integration
2. ✅ SDF for Gazebo simulation
3. ✅ Xacro for modular, reusable descriptions
4. ✅ Unity for photorealistic visualization
5. ✅ Proper inertia and physics properties are critical
6. ✅ Organize models for maintainability

Next Chapter:

Chapter 8 will introduce NVIDIA Isaac Sim, a cutting-edge photorealistic simulation platform with GPU acceleration.

---

This chapter covered robot description formats and visualization. You can now create accurate robot models for simulation and beautiful visualizations for presentation.