URDF Description

URDF (Unified Robot Description Format) is an XML format used to describe robot models in ROS. It defines the physical and visual properties of a robot, including its links, joints, and other components. URDF is essential for simulating robots in Gazebo and visualizing them in RViz.

URDF Basics

A URDF file describes a robot as a collection of rigid bodies (links) connected by joints. The structure typically includes:

Links: Rigid bodies that make up the robot structure
Joints: Connections between links with specific degrees of freedom
Visual: How the link appears in simulation and visualization
Collision: Collision properties for physics simulation
Inertial: Mass, center of mass, and inertia properties

Basic URDF Structure

<?xml version="1.0"?>
<robot name="simple_robot" xmlns:xacro="http://www.ros.org/wiki/xacro">
  <!-- Base link -->
  <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>
    <inertial>
      <mass value="10"/>
      <inertia ixx="1.0" ixy="0.0" ixz="0.0" iyy="1.0" iyz="0.0" izz="1.0"/>
    </inertial>
  </link>

  <!-- Child link connected via joint -->
  <joint name="base_to_top" type="fixed">
    <parent link="base_link"/>
    <child link="top_link"/>
    <origin xyz="0 0 0.3"/>
  </joint>

  <link name="top_link">
    <visual>
      <geometry>
        <box size="0.1 0.1 0.1"/>
      </geometry>
      <material name="red">
        <color rgba="0.8 0 0 1"/>
      </material>
    </visual>
  </link>
</robot>

Links

Links represent rigid bodies in the robot. Each link can have:

Visual: How the link looks in visualization
Collision: How the link interacts in physics simulation
Inertial: Physical properties for dynamics simulation

Visual Properties

<link name="link_name">
  <visual>
    <origin xyz="0 0 0" rpy="0 0 0"/>
    <geometry>
      <!-- Choose one geometry type -->
      <box size="0.1 0.1 0.1"/>
      <!-- <cylinder radius="0.1" length="0.1"/> -->
      <!-- <sphere radius="0.1"/> -->
      <!-- <mesh filename="package://path/to/mesh.stl"/> -->
    </geometry>
    <material name="material_name">
      <color rgba="0.8 0.2 0.2 1.0"/>
    </material>
  </visual>
</link>

Collision Properties

<collision>
  <origin xyz="0 0 0" rpy="0 0 0"/>
  <geometry>
    <box size="0.1 0.1 0.1"/>
  </geometry>
</collision>

Inertial Properties

<inertial>
  <origin xyz="0 0 0" rpy="0 0 0"/>
  <mass value="0.1"/>
  <inertia ixx="0.01" ixy="0.0" ixz="0.0" iyy="0.01" iyz="0.0" izz="0.01"/>
</inertial>

Joints

Joints connect links and define how they can move relative to each other:

<joint name="joint_name" type="joint_type">
  <parent link="parent_link_name"/>
  <child link="child_link_name"/>
  <origin xyz="0 0 0" rpy="0 0 0"/>

  <!-- For revolute joints -->
  <axis xyz="0 0 1"/>
  <limit lower="-3.14" upper="3.14" effort="100" velocity="1"/>

  <!-- For continuous joints (like wheels) -->
  <!-- <mimic joint="other_joint" multiplier="1.0" offset="0.0"/> -->
</joint>

Joint Types

fixed: No movement allowed
revolute: Rotational movement around an axis (limited)
continuous: Rotational movement around an axis (unlimited)
prismatic: Linear movement along an axis (limited)
floating: 6 DOF movement (rarely used)
planar: Movement in a plane (rarely used)

Complete Humanoid Robot Example

Here's a simplified example of a humanoid robot with basic body parts:

<?xml version="1.0"?>
<robot name="simple_humanoid" xmlns:xacro="http://www.ros.org/wiki/xacro">
  <!-- Base body -->
  <link name="base_link">
    <visual>
      <geometry>
        <box size="0.3 0.2 0.5"/>
      </geometry>
      <material name="gray">
        <color rgba="0.5 0.5 0.5 1"/>
      </material>
    </visual>
    <collision>
      <geometry>
        <box size="0.3 0.2 0.5"/>
      </geometry>
    </collision>
    <inertial>
      <mass value="5.0"/>
      <inertia ixx="0.1" ixy="0.0" ixz="0.0" iyy="0.1" iyz="0.0" izz="0.1"/>
    </inertial>
  </link>

  <!-- Head -->
  <joint name="neck_joint" type="revolute">
    <parent link="base_link"/>
    <child link="head_link"/>
    <origin xyz="0 0 0.35" rpy="0 0 0"/>
    <axis xyz="0 1 0"/>
    <limit lower="-0.5" upper="0.5" effort="10" velocity="2"/>
  </joint>

  <link name="head_link">
    <visual>
      <geometry>
        <sphere radius="0.1"/>
      </geometry>
      <material name="skin">
        <color rgba="0.8 0.6 0.4 1"/>
      </material>
    </visual>
    <collision>
      <geometry>
        <sphere radius="0.1"/>
      </geometry>
    </collision>
    <inertial>
      <mass value="0.5"/>
      <inertia ixx="0.001" ixy="0.0" ixz="0.0" iyy="0.001" iyz="0.0" izz="0.001"/>
    </inertial>
  </link>

  <!-- Left Arm -->
  <joint name="left_shoulder_joint" type="revolute">
    <parent link="base_link"/>
    <child link="left_upper_arm"/>
    <origin xyz="0.2 0 0.1" rpy="0 0 0"/>
    <axis xyz="0 1 0"/>
    <limit lower="-1.57" upper="1.57" effort="10" velocity="2"/>
  </joint>

  <link name="left_upper_arm">
    <visual>
      <geometry>
        <cylinder length="0.3" radius="0.05"/>
      </geometry>
      <material name="gray"/>
    </visual>
    <collision>
      <geometry>
        <cylinder length="0.3" radius="0.05"/>
      </geometry>
    </collision>
    <inertial>
      <mass value="0.5"/>
      <inertia ixx="0.005" ixy="0.0" ixz="0.0" iyy="0.005" iyz="0.0" izz="0.0005"/>
    </inertial>
  </link>

  <joint name="left_elbow_joint" type="revolute">
    <parent link="left_upper_arm"/>
    <child link="left_lower_arm"/>
    <origin xyz="0 0 -0.3" rpy="0 0 0"/>
    <axis xyz="0 1 0"/>
    <limit lower="-1.57" upper="1.57" effort="10" velocity="2"/>
  </joint>

  <link name="left_lower_arm">
    <visual>
      <geometry>
        <cylinder length="0.3" radius="0.05"/>
      </geometry>
      <material name="gray"/>
    </visual>
    <collision>
      <geometry>
        <cylinder length="0.3" radius="0.05"/>
      </geometry>
    </collision>
    <inertial>
      <mass value="0.3"/>
      <inertia ixx="0.003" ixy="0.0" ixz="0.0" iyy="0.003" iyz="0.0" izz="0.0003"/>
    </inertial>
  </link>
</robot>

Xacro for Complex Models

Xacro is an XML macro language that makes URDF more manageable for complex robots:

<?xml version="1.0"?>
<robot xmlns:xacro="http://www.ros.org/wiki/xacro" name="humanoid_with_xacro">
  <!-- Define properties -->
  <xacro:property name="M_PI" value="3.1415926535897931" />
  <xacro:property name="body_width" value="0.3" />
  <xacro:property name="body_depth" value="0.2" />
  <xacro:property name="body_height" value="0.5" />

  <!-- Macro for creating limbs -->
  <xacro:macro name="simple_arm" params="name parent *origin">
    <joint name="${name}_shoulder_joint" type="revolute">
      <xacro:insert_block name="origin"/>
      <parent link="${parent}"/>
      <child link="${name}_upper_arm"/>
      <axis xyz="0 1 0"/>
      <limit lower="-1.57" upper="1.57" effort="10" velocity="2"/>
    </joint>

    <link name="${name}_upper_arm">
      <visual>
        <geometry>
          <cylinder length="0.3" radius="0.05"/>
        </geometry>
        <material name="gray">
          <color rgba="0.5 0.5 0.5 1"/>
        </material>
      </visual>
    </link>
  </xacro:macro>

  <!-- Use the macro -->
  <link name="base_link">
    <visual>
      <geometry>
        <box size="${body_width} ${body_depth} ${body_height}"/>
      </geometry>
    </visual>
  </link>

  <xacro:simple_arm name="left" parent="base_link">
    <origin xyz="${body_width/2} 0 ${body_height/4}" rpy="0 0 0"/>
  </xacro:simple_arm>
</robot>

Working with URDF in ROS 2

To use URDF in ROS 2 applications:

Robot State Publisher: Publishes the robot's joint states and transforms
TF2: Handles coordinate transformations between links
Gazebo Integration: Use the robot in simulation

Launching with Robot State Publisher

from launch import LaunchDescription
from launch.substitutions import Command
from launch_ros.actions import Node
from ament_index_python.packages import get_package_share_directory
import os

def generate_launch_description():
    pkg_share = get_package_share_directory('your_robot_description')
    urdf_file = os.path.join(pkg_share, 'urdf', 'your_robot.urdf')

    robot_state_publisher_node = Node(
        package='robot_state_publisher',
        executable='robot_state_publisher',
        parameters=[{'robot_description': Command(['xacro ', urdf_file])}]
    )

    return LaunchDescription([robot_state_publisher_node])

Best Practices

Use consistent naming conventions for links and joints
Keep visual and collision geometries as simple as possible for performance
Use Xacro for complex robots to reduce redundancy
Include proper inertial properties for accurate simulation
Validate URDF files using tools like check_urdf
Use appropriate joint limits based on physical constraints

Exercise

Create a URDF file for a simple humanoid robot with:

A body, head, and 4 limbs (arms and legs)
Proper joint connections with realistic movement ranges
Visual and collision properties for each link
Use Xacro macros to avoid redundancy in similar limbs

URDF Basics​

Basic URDF Structure​

Links​

Visual Properties​

Collision Properties​

Inertial Properties​

Joints​

Joint Types​

Complete Humanoid Robot Example​

Xacro for Complex Models​

Working with URDF in ROS 2​

Launching with Robot State Publisher​

Best Practices​

Exercise​