URDF — Describing Humanoid Robot Bodies

What is URDF?

URDF (Unified Robot Description Format) is an XML format used to describe a robot's physical structure — its links (rigid bodies), joints (connections), sensors, and visual geometry.

Think of URDF as the robot's "birth certificate" — it tells ROS 2, Gazebo, and NVIDIA Isaac exactly what the robot looks like and how it can move.

URDF Structure

A URDF file describes two things:

1. Links — the rigid bodies (torso, legs, arms, head)
2. Joints — how links connect and move relative to each other

<?xml version="1.0"?>
<robot name="simple_humanoid">

  <!-- Base / Torso -->
  <link name="torso">
    <visual>
      <geometry>
        <box size="0.3 0.2 0.5"/>
      </geometry>
      <material name="blue">
        <color rgba="0 0 1 1"/>
      </material>
    </visual>
    <collision>
      <geometry>
        <box size="0.3 0.2 0.5"/>
      </geometry>
    </collision>
    <inertial>
      <mass value="10.0"/>
      <inertia ixx="0.1" iyy="0.1" izz="0.05" ixy="0" ixz="0" iyz="0"/>
    </inertial>
  </link>

  <!-- Left Hip Joint -->
  <joint name="left_hip" type="revolute">
    <parent link="torso"/>
    <child link="left_thigh"/>
    <origin xyz="-0.1 0 -0.25" rpy="0 0 0"/>
    <axis xyz="1 0 0"/>  <!-- Rotates around X axis -->
    <limit lower="-1.57" upper="1.57" effort="100" velocity="1.0"/>
  </joint>

  <!-- Left Thigh -->
  <link name="left_thigh">
    <visual>
      <geometry>
        <cylinder radius="0.05" length="0.4"/>
      </geometry>
    </visual>
    <inertial>
      <mass value="3.0"/>
      <inertia ixx="0.02" iyy="0.02" izz="0.005" ixy="0" ixz="0" iyz="0"/>
    </inertial>
  </link>

</robot>

Joint Types

Type	Description	Example
revolute	Rotates around an axis (with limits)	Hip, knee, elbow
continuous	Rotates without limits	Wheels
prismatic	Slides along an axis	Linear actuators
fixed	No movement	Camera mount on head
floating	6 DOF (rarely used directly)	Robot base in simulation

Viewing URDF in RViz2

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

# Launch RViz2 with your URDF
ros2 launch urdf_tutorial display.launch.py model:=my_humanoid.urdf

From URDF to Gazebo (SDF)

Gazebo uses SDF (Simulation Description Format) which is more expressive than URDF. You can either:

1. Use URDF directly with Gazebo's URDF → SDF auto-conversion
2. Add <gazebo> extension tags inside your URDF for physics properties

<!-- Add Gazebo-specific properties to your URDF -->
<gazebo reference="left_thigh">
  <material>Gazebo/Blue</material>
  <mu1>0.8</mu1>   <!-- Friction coefficient -->
  <mu2>0.8</mu2>
</gazebo>

Real Humanoid URDFs

For real projects, you don't build URDF from scratch. Download existing models:

# Unitree H1 humanoid
git clone https://github.com/unitreerobotics/unitree_ros

# Robotis OP3
sudo apt install ros-humble-robotis-op3-description

# Generic humanoid template
sudo apt install ros-humble-humanoid-nav-msgs

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Module 1 Complete! Move on to Module 2: Gazebo & Unity →