Modeling the Body: URDF for Humanoid Robots

From Software to Structure

You've learned how ROS 2 nodes think (nodes) and communicate (topics). But how does ROS 2 understand a robot's physical body?

Enter URDF (Unified Robot Description Format): an XML-based language that defines:

Links: Rigid body parts (bones)
Joints: Connections between links (muscles/hinges)
Sensors: Cameras, LIDAR, IMUs
Collision Geometry: For physics simulation
Visual Geometry: For 3D visualization

Biological Analogy

If nodes are the nervous system (brain signals), URDF is the skeletal system (bones and joints). Without URDF, ROS 2 can send movement commands, but it doesn't know what it's moving or how those parts are connected.

Why URDF Matters for Physical AI

URDF files serve three critical purposes:

1. Visualization (RViz2)

See your robot in 3D, visualize sensor data, and debug without hardware.

2. Simulation (Gazebo, Isaac Sim)

Physics engines need to know:

Mass and inertia of each link
Joint limits (how far an elbow can bend)
Collision shapes (to prevent self-collision)

3. Motion Planning (MoveIt2)

Inverse kinematics solvers need the kinematic chain:

"To reach this XYZ position, which joints must rotate by how many degrees?"

Real-World Example

Boston Dynamics' Atlas humanoid has 28 joints (shoulders, elbows, hips, knees, ankles). Its URDF file defines every joint's rotation axis, limits, and mass—allowing simulation engines to predict whether a jumping motion will succeed before risking the $2M robot.

URDF Basics: Links and Joints

What is a Link?

A link represents a rigid body part:

For a humanoid: torso, upper arm, forearm, thigh, shin
For a wheeled robot: chassis, wheel
For a drone: frame, propeller

Each link has:

Mass and Inertia (for physics)
Visual Geometry (3D mesh or primitive shape)
Collision Geometry (simplified shape for collision detection)

What is a Joint?

A joint connects two links and defines how they move relative to each other:

Joint Type	Description	Example
Fixed	No movement (welded together)	Torso to pelvis
Revolute	Rotates around axis (1 DOF)	Elbow, knee
Continuous	Rotates 360° infinitely	Wheel axle
Prismatic	Slides along axis (1 DOF)	Telescope, gripper

Degrees of Freedom (DOF)

A humanoid arm typically has 7 DOF: 3 shoulder joints, 1 elbow, 3 wrist joints. This matches human arm mobility and enables complex manipulation tasks.

URDF XML Structure

Here's a minimal URDF file for a single-link robot:

<?xml version="1.0"?>
<robot name="simple_bot">
  
  <!-- Base Link (chassis) -->
  <link name="base_link">
    <visual>
      <geometry>
        <box size="0.5 0.3 0.1"/>  <!-- 50cm x 30cm x 10cm box -->
      </geometry>
      <material name="blue">
        <color rgba="0 0 0.8 1"/>  <!-- Blue color (RGBA) -->
      </material>
    </visual>
    
    <collision>
      <geometry>
        <box size="0.5 0.3 0.1"/>  <!-- Same as visual for simplicity -->
      </geometry>
    </collision>
    
    <inertial>
      <mass value="10.0"/>  <!-- 10 kg -->
      <inertia ixx="0.1" ixy="0.0" ixz="0.0"
               iyy="0.1" iyz="0.0"
               izz="0.1"/>
    </inertial>
  </link>

</robot>

Key Components:

<robot name="...">: Root element, names the robot
<link name="...">: Defines a rigid body
<visual>: What you see in RViz (mesh or primitive shape)
<collision>: Simplified shape for physics (often a box/cylinder for performance)
<inertial>: Mass (kg) and inertia tensor (kg·m²)

Real Example: Robot Leg (Humanoid)

Let's model a single leg with 3 joints: hip, knee, ankle.

<?xml version="1.0"?>
<robot name="humanoid_leg">
  
  <!-- 1. PELVIS (Fixed to world) -->
  <link name="pelvis">
    <visual>
      <geometry>
        <box size="0.2 0.3 0.15"/>  <!-- 20cm x 30cm x 15cm -->
      </geometry>
      <material name="gray">
        <color rgba="0.5 0.5 0.5 1"/>
      </material>
    </visual>
    <collision>
      <geometry>
        <box size="0.2 0.3 0.15"/>
      </geometry>
    </collision>
    <inertial>
      <mass value="5.0"/>
      <inertia ixx="0.05" ixy="0" ixz="0"
               iyy="0.05" iyz="0"
               izz="0.05"/>
    </inertial>
  </link>

  <!-- 2. THIGH (Upper leg) -->
  <link name="thigh">
    <visual>
      <geometry>
        <cylinder radius="0.05" length="0.4"/>  <!-- 5cm diameter, 40cm long -->
      </geometry>
      <material name="black">
        <color rgba="0.1 0.1 0.1 1"/>
      </material>
    </visual>
    <collision>
      <geometry>
        <cylinder radius="0.05" length="0.4"/>
      </geometry>
    </collision>
    <inertial>
      <mass value="3.0"/>
      <inertia ixx="0.04" ixy="0" ixz="0"
               iyy="0.04" iyz="0"
               izz="0.001"/>
    </inertial>
  </link>

  <!-- 3. SHIN (Lower leg) -->
  <link name="shin">
    <visual>
      <geometry>
        <cylinder radius="0.04" length="0.35"/>  <!-- 4cm diameter, 35cm long -->
      </geometry>
      <material name="black">
        <color rgba="0.1 0.1 0.1 1"/>
      </material>
    </visual>
    <collision>
      <geometry>
        <cylinder radius="0.04" length="0.35"/>
      </geometry>
    </collision>
    <inertial>
      <mass value="2.0"/>
      <inertia ixx="0.02" ixy="0" ixz="0"
               iyy="0.02" iyz="0"
               izz="0.001"/>
    </inertial>
  </link>

  <!-- 4. FOOT -->
  <link name="foot">
    <visual>
      <geometry>
        <box size="0.12 0.06 0.04"/>  <!-- 12cm x 6cm x 4cm -->
      </geometry>
      <material name="gray">
        <color rgba="0.3 0.3 0.3 1"/>
      </material>
    </visual>
    <collision>
      <geometry>
        <box size="0.12 0.06 0.04"/>
      </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 1: Hip (Pelvis to Thigh) -->
  <joint name="hip_joint" type="revolute">
    <parent link="pelvis"/>
    <child link="thigh"/>
    <origin xyz="0 0 -0.075" rpy="0 0 0"/>  <!-- Position: 7.5cm down from pelvis center -->
    <axis xyz="0 1 0"/>  <!-- Rotates around Y-axis (forward/backward) -->
    <limit lower="-1.57" upper="1.57" effort="100" velocity="2.0"/>
    <!-- Limits: -90° to +90°, max torque 100 Nm, max speed 2 rad/s -->
  </joint>

  <!-- JOINT 2: Knee (Thigh to Shin) -->
  <joint name="knee_joint" type="revolute">
    <parent link="thigh"/>
    <child link="shin"/>
    <origin xyz="0 0 -0.2" rpy="0 0 0"/>  <!-- 20cm down from thigh center -->
    <axis xyz="0 1 0"/>  <!-- Rotates around Y-axis -->
    <limit lower="0" upper="2.36" effort="80" velocity="2.0"/>
    <!-- Limits: 0° to 135° (knee only bends one way), max torque 80 Nm -->
  </joint>

  <!-- JOINT 3: Ankle (Shin to Foot) -->
  <joint name="ankle_joint" type="revolute">
    <parent link="shin"/>
    <child link="foot"/>
    <origin xyz="0 0 -0.175" rpy="0 0 0"/>  <!-- 17.5cm down from shin center -->
    <axis xyz="0 1 0"/>  <!-- Rotates around Y-axis -->
    <limit lower="-0.79" upper="0.79" effort="50" velocity="2.0"/>
    <!-- Limits: -45° to +45°, max torque 50 Nm -->
  </joint>

</robot>

Breaking Down the Leg Model

Link Definitions

Thigh Link (Lines 28-50):

Geometry: Cylinder (approximates human thigh shape)
Dimensions: 5cm radius, 40cm length
Mass: 3.0 kg (realistic for adult human)
Inertia: Higher along length axis (cylinder rotates differently than sphere)

Joint Definitions

Hip Joint (Lines 111-117):

<joint name="hip_joint" type="revolute">
  <parent link="pelvis"/>
  <child link="thigh"/>
  <origin xyz="0 0 -0.075" rpy="0 0 0"/>
  <axis xyz="0 1 0"/>
  <limit lower="-1.57" upper="1.57" effort="100" velocity="2.0"/>
</joint>

Key Attributes:

parent/child: Kinematic chain (pelvis → thigh)
origin: Position offset (7.5cm below pelvis center)
axis: Rotation axis (Y-axis = pitch/forward-backward)
limit:
- lower/upper: -90° to +90° (in radians)
- effort: Max torque motor can apply (100 Nm)
- velocity: Max rotational speed (2 rad/s ≈ 115°/s)

Joint Limits

Setting accurate limits is critical:

Too wide: Robot tries impossible movements and breaks simulation
Too narrow: Robot can't perform required tasks Always reference real hardware specs (e.g., Unitree G1 hip joint: ±120°).

Visualizing URDF in RViz

Once you have a URDF file, visualize it:

Step 1: Launch RViz with URDF

# Install joint_state_publisher_gui (if not already installed)
sudo apt install ros-humble-joint-state-publisher-gui

# Launch visualization
ros2 launch urdf_tutorial display.launch.py model:=./humanoid_leg.urdf

Step 2: Interact with Joints

A GUI window appears with sliders for each joint:

Move the hip slider → thigh rotates
Move the knee slider → shin bends
Move the ankle slider → foot tilts

Step 3: View in RViz

RViz shows the 3D model with adjustable joint positions in real-time.

URDF for Complete Humanoids

A full humanoid URDF typically includes:

Component	Links	Joints	Description
Torso	1-3	0-2	Chest, spine (sometimes articulated)
Arms	6 (2×3)	14 (2×7)	Shoulder (3 DOF), elbow (1 DOF), wrist (3 DOF)
Legs	6 (2×3)	12 (2×6)	Hip (3 DOF), knee (1 DOF), ankle (2 DOF)
Hands	30+	20+	Fingers (often simplified to 1 DOF per finger)
Head	1-2	2-3	Neck pan/tilt, cameras

Total for Humanoid:

~45 links
~50 joints
URDF file size: 500-2000 lines (depending on sensor detail)

Example: Unitree G1 URDF

The Unitree G1 humanoid (released 2024) has a public URDF file with:

23 actuated joints (simplified hands)
6 sensors (IMU, cameras, force-torque)
Full collision meshes for self-collision avoidance
Used in Gazebo and Isaac Sim simulations

Advanced: Using Meshes Instead of Primitives

For realistic visuals, use 3D mesh files (.stl, .dae, .obj):

<visual>
  <geometry>
    <mesh filename="package://my_robot_description/meshes/thigh.stl" scale="0.001 0.001 0.001"/>
    <!-- Scale converts mm (CAD files) to meters (ROS 2) -->
  </geometry>
</visual>

Workflow:

CAD Design: Model robot in Fusion 360, SolidWorks, Blender
Export Mesh: Save as .stl (collision) or .dae (visual with textures)
Reference in URDF: Use <mesh filename="..."/>
Optimize: Collision meshes should be low-poly (< 1000 triangles) for performance

Hands-On Exercise: Add an Arm

Challenge: Extend the humanoid_leg.urdf to include a right arm with:

Upper arm: 30cm cylinder, 2kg
Forearm: 25cm cylinder, 1.5kg
Shoulder joint: Revolute, rotates around X-axis, ±90°
Elbow joint: Revolute, rotates around Y-axis, 0° to 150°

Starter Code:

<!-- Add to <robot> tag after leg links/joints -->

<link name="upper_arm">
  <!-- YOUR CODE HERE -->
</link>

<joint name="shoulder_joint" type="revolute">
  <parent link="pelvis"/>
  <child link="upper_arm"/>
  <!-- YOUR CODE HERE -->
</joint>

Verification:

ros2 launch urdf_tutorial display.launch.py model:=./humanoid_leg_and_arm.urdf

You should see 5 joint sliders: hip, knee, ankle, shoulder, elbow.

Key Takeaways

✅ URDF is XML format for defining robot structure
✅ Links are rigid body parts (bones)
✅ Joints connect links and define motion constraints
✅ Revolute joints are most common for humanoids (hinges)
✅ RViz visualizes URDF models interactively
✅ Accurate mass/inertia/limits are critical for simulation fidelity

What's Next?

You've mastered the robot's body definition. The next module covers Module 2: Simulation with NVIDIA Isaac Sim—where you'll bring URDF models to life in a photorealistic physics simulator and train AI policies with millions of parallel experiences.

From Software to Structure​

Why URDF Matters for Physical AI​

1. Visualization (RViz2)​

2. Simulation (Gazebo, Isaac Sim)​

3. Motion Planning (MoveIt2)​

URDF Basics: Links and Joints​

What is a Link?​

What is a Joint?​

URDF XML Structure​

Real Example: Robot Leg (Humanoid)​

Breaking Down the Leg Model​

Link Definitions​

Joint Definitions​

Visualizing URDF in RViz​

Step 1: Launch RViz with URDF​

Step 2: Interact with Joints​

Step 3: View in RViz​

URDF for Complete Humanoids​

Advanced: Using Meshes Instead of Primitives​

Hands-On Exercise: Add an Arm​

Key Takeaways​

What's Next?​

Further Reading​