URDF Robot Modeling: Describing Humanoid Structure

Introduction

You've learned how ROS 2 nodes communicate via topics, services, and actions. But how does ROS 2 know what your robot is? How does it understand that your humanoid has two arms, two legs, a torso, and a head—and how they're all connected?

The answer is URDF (Unified Robot Description Format)—an XML-based language for describing a robot's:

• Physical structure (what parts exist)
• Kinematics (how parts move relative to each other)
• Dynamics (mass, inertia, friction)
• Visual appearance (3D models for rendering)
• Collision geometry (shapes for physics simulation)

URDF is the "blueprint" that tells simulators, visualizers, and control systems how your robot is built.

Why URDF Matters for Humanoid Robots

A humanoid robot is a complex kinematic chain—interconnected rigid bodies (links) connected by movable joints. URDF provides a standardized way to describe this structure so that:

1. Simulators (Gazebo, Isaac Sim) can accurately model physics
2. Visualizers (RViz) can display the robot's current pose
3. Motion planners can compute valid joint trajectories
4. Controllers know which actuators to command

The same URDF file can be used across simulation, visualization, and real hardware—ensuring consistency.

URDF Core Concepts

1. Links: Rigid Body Parts

A link represents a rigid body part of the robot that doesn't deform. For a humanoid:

• base_link (torso/pelvis)
• head
• left_upper_arm, left_forearm, left_hand
• right_upper_arm, right_forearm, right_hand
• left_thigh, left_shin, left_foot
• right_thigh, right_shin, right_foot

Each link can have:

• Visual representation: 3D mesh or primitive shape (box, cylinder, sphere)
• Collision geometry: Simplified shape for physics calculations
• Inertial properties: Mass, center of mass, inertia tensor

2. Joints: Connections Between Links

A joint defines how two links connect and move relative to each other. Joint types include:

Joint Type	Degrees of Freedom	Example
revolute	1 (rotation, limited range)	Elbow, knee
continuous	1 (rotation, unlimited)	Wheel axle
prismatic	1 (translation)	Telescoping antenna
fixed	0 (rigid connection)	Head to camera mount
floating	6 (full 3D pose)	Mobile base
planar	3 (2D translation + rotation)	Sliding door

For humanoid robots, most joints are revolute (shoulders, elbows, hips, knees, ankles).

3. Parent-Child Relationships

URDF describes robots as a tree structure:

base_link (root)
  ├── head (via neck joint)
  ├── left_upper_arm (via left_shoulder joint)
  │     └── left_forearm (via left_elbow joint)
  │           └── left_hand (via left_wrist joint)
  ├── right_upper_arm (via right_shoulder joint)
  │     └── right_forearm (via right_elbow joint)
  │           └── right_hand (via right_wrist joint)
  ├── left_thigh (via left_hip joint)
  │     └── left_shin (via left_knee joint)
  │           └── left_foot (via left_ankle joint)
  └── right_thigh (via right_hip joint)
        └── right_shin (via right_knee joint)
              └── right_foot (via right_ankle joint)

Each joint connects a parent link to a child link, forming a kinematic tree.

URDF XML Structure

Let's examine a simplified humanoid arm in URDF:

Example: Humanoid Left Arm

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

  <!-- Base link (torso) -->
  <link name="torso">
    <visual>
      <geometry>
        <box size="0.3 0.2 0.5"/>  <!-- 30cm x 20cm x 50cm torso -->
      </geometry>
      <material name="blue">
        <color rgba="0.2 0.3 0.8 1.0"/>
      </material>
    </visual>
    <collision>
      <geometry>
        <box size="0.3 0.2 0.5"/>
      </geometry>
    </collision>
    <inertial>
      <mass value="10.0"/>  <!-- 10 kg torso -->
      <inertia ixx="0.5" ixy="0.0" ixz="0.0"
               iyy="0.5" iyz="0.0" izz="0.2"/>
    </inertial>
  </link>

  <!-- Upper arm link -->
  <link name="left_upper_arm">
    <visual>
      <geometry>
        <cylinder radius="0.04" length="0.3"/>  <!-- 30cm long, 4cm radius -->
      </geometry>
      <origin xyz="0 0 -0.15" rpy="0 0 0"/>  <!-- Cylinder centered at joint -->
      <material name="grey">
        <color rgba="0.7 0.7 0.7 1.0"/>
      </material>
    </visual>
    <collision>
      <geometry>
        <cylinder radius="0.04" length="0.3"/>
      </geometry>
      <origin xyz="0 0 -0.15" rpy="0 0 0"/>
    </collision>
    <inertial>
      <mass value="2.0"/>  <!-- 2 kg arm -->
      <origin xyz="0 0 -0.15" rpy="0 0 0"/>
      <inertia ixx="0.05" ixy="0.0" ixz="0.0"
               iyy="0.05" iyz="0.0" izz="0.01"/>
    </inertial>
  </link>

  <!-- Shoulder joint (connects torso to upper arm) -->
  <joint name="left_shoulder" type="revolute">
    <parent link="torso"/>
    <child link="left_upper_arm"/>
    <origin xyz="0.15 0.0 0.2" rpy="0 0 0"/>  <!-- Offset from torso center -->
    <axis xyz="0 1 0"/>  <!-- Rotate around Y-axis -->
    <limit lower="-1.57" upper="1.57" effort="50.0" velocity="2.0"/>
    <!-- Limits: -90° to +90°, max torque 50 Nm, max speed 2 rad/s -->
  </joint>

  <!-- Forearm link -->
  <link name="left_forearm">
    <visual>
      <geometry>
        <cylinder radius="0.03" length="0.25"/>  <!-- 25cm forearm -->
      </geometry>
      <origin xyz="0 0 -0.125" rpy="0 0 0"/>
      <material name="grey"/>
    </visual>
    <collision>
      <geometry>
        <cylinder radius="0.03" length="0.25"/>
      </geometry>
      <origin xyz="0 0 -0.125" rpy="0 0 0"/>
    </collision>
    <inertial>
      <mass value="1.0"/>  <!-- 1 kg forearm -->
      <origin xyz="0 0 -0.125" rpy="0 0 0"/>
      <inertia ixx="0.02" ixy="0.0" ixz="0.0"
               iyy="0.02" iyz="0.0" izz="0.005"/>
    </inertial>
  </link>

  <!-- Elbow joint (connects upper arm to forearm) -->
  <joint name="left_elbow" type="revolute">
    <parent link="left_upper_arm"/>
    <child link="left_forearm"/>
    <origin xyz="0 0 -0.3" rpy="0 0 0"/>  <!-- End of upper arm -->
    <axis xyz="0 1 0"/>  <!-- Rotate around Y-axis -->
    <limit lower="0.0" upper="2.35" effort="30.0" velocity="2.0"/>
    <!-- Limits: 0° to 135°, max torque 30 Nm -->
  </joint>

</robot>

Breaking Down the URDF

Link Definition

<link name="left_upper_arm">
  <visual>        <!-- How it looks (rendering) -->
  <collision>     <!-- Simplified shape for physics collisions -->
  <inertial>      <!-- Mass and inertia for dynamics -->
</link>

• Visual: What you see in RViz or simulators (can be detailed 3D mesh)
• Collision: Simplified geometry for fast collision detection
• Inertial: Physical properties for realistic simulation

Joint Definition

<joint name="left_shoulder" type="revolute">
  <parent link="torso"/>
  <child link="left_upper_arm"/>
  <origin xyz="0.15 0.0 0.2" rpy="0 0 0"/>  <!-- Position offset -->
  <axis xyz="0 1 0"/>                        <!-- Rotation axis -->
  <limit lower="-1.57" upper="1.57" .../>    <!-- Joint limits -->
</joint>

• Parent/Child: Defines the kinematic tree
• Origin: Where the child link attaches to the parent (translation and rotation)
• Axis: Which direction the joint rotates/translates
• Limit: Safety constraints (angle range, max torque, max velocity)

URDF in Action: Visualization and Simulation

Visualization in RViz

RViz uses URDF's visual elements to display the robot:

# Load URDF and visualize
ros2 launch robot_state_publisher view_robot.launch.py

You can interactively move joints using a GUI, and RViz updates the 3D model in real-time by reading joint states from the /joint_states topic.

Simulation in Gazebo

Gazebo uses URDF's collision, inertial, and visual elements to:

• Render the robot in 3D
• Simulate physics (gravity, collisions, friction)
• Compute realistic dynamics based on mass and inertia

When you command a joint to move, Gazebo calculates the resulting motion considering:

• Joint limits
• Torque constraints
• Gravity effects
• Collisions with environment

Advanced URDF Features

1. Using Meshes for Realistic Appearance

Instead of primitive shapes, you can reference 3D mesh files (STL, DAE, OBJ):

<visual>
  <geometry>
    <mesh filename="package://my_robot/meshes/left_arm.stl" scale="1.0 1.0 1.0"/>
  </geometry>
</visual>

This allows photorealistic rendering while keeping collision geometry simple:

<collision>
  <geometry>
    <cylinder radius="0.04" length="0.3"/>  <!-- Simplified for performance -->
  </geometry>
</collision>

2. Transmissions for Actuators

Transmissions map joints to actuators (motors):

<transmission name="left_shoulder_trans">
  <type>transmission_interface/SimpleTransmission</type>
  <joint name="left_shoulder">
    <hardwareInterface>hardware_interface/EffortJointInterface</hardwareInterface>
  </joint>
  <actuator name="left_shoulder_motor">
    <mechanicalReduction>1</mechanicalReduction>
  </actuator>
</transmission>

This tells the control system how to command the physical motor.

3. Gazebo-Specific Extensions

URDF can include Gazebo-specific tags for sensors and plugins:

<gazebo reference="head">
  <sensor type="camera" name="head_camera">
    <update_rate>30.0</update_rate>
    <camera>
      <horizontal_fov>1.3962634</horizontal_fov>
      <image>
        <width>800</width>
        <height>600</height>
      </image>
    </camera>
  </sensor>
</gazebo>

This adds a camera sensor to the head link in simulation.

Xacro: Parameterized URDF

For complex robots, writing raw URDF XML is tedious and error-prone. Xacro (XML Macros) lets you:

• Define constants and variables
• Create reusable macros (e.g., "leg" macro used for both legs)
• Use mathematical expressions

Example Xacro snippet:

<xacro:property name="arm_length" value="0.3"/>
<xacro:property name="arm_radius" value="0.04"/>

<xacro:macro name="arm" params="prefix">
  <link name="${prefix}_upper_arm">
    <visual>
      <geometry>
        <cylinder radius="${arm_radius}" length="${arm_length}"/>
      </geometry>
    </visual>
  </link>
</xacro:macro>

<!-- Use the macro for both arms -->
<xacro:arm prefix="left"/>
<xacro:arm prefix="right"/>

Xacro files (.urdf.xacro) are processed into standard URDF at build time.

Best Practices for Humanoid URDF

1. Start Simple, Iterate

Begin with primitive shapes (boxes, cylinders), validate kinematics, then add detailed meshes.

2. Accurate Inertial Properties

Incorrect mass or inertia causes unrealistic simulation behavior (falling over, unstable walking). Measure or estimate carefully.

3. Consistent Coordinate Frames

Follow ROS conventions:

• X: forward
• Y: left
• Z: up

4. Test in Simulation First

Validate URDF in Gazebo before deploying to hardware. Check for:

• Joint limits preventing self-collision
• Realistic mass distribution (center of mass)
• Proper coordinate frame alignment

5. Use Meaningful Names

<!-- Good -->
<link name="left_shin"/>
<joint name="left_knee"/>

<!-- Avoid -->
<link name="link_7"/>
<joint name="joint_3"/>

URDF and ROS 2 Integration

URDF files are published to the /robot_description topic by the robot_state_publisher node:

ros2 run robot_state_publisher robot_state_publisher --ros-args -p robot_description:="$(xacro my_robot.urdf.xacro)"

Other nodes (visualizers, planners, controllers) subscribe to this topic to learn the robot's structure.

The joint_state_publisher node sends current joint positions to the /joint_states topic, and robot_state_publisher computes forward kinematics to determine where each link is in 3D space.

Summary

URDF is the standardized language for describing robot structure in ROS 2:

• Links: Rigid body parts (arms, legs, torso, head)
• Joints: Connections enabling relative motion (revolute, prismatic, fixed)
• Kinematics: Parent-child tree structure defining how parts connect
• Dynamics: Mass, inertia, friction for realistic physics simulation
• Visualization: 3D meshes or primitive shapes for rendering
• Collision: Simplified geometry for efficient collision detection

For humanoid robots, URDF enables:

• Accurate simulation in Gazebo and Isaac Sim
• Real-time visualization in RViz
• Motion planning with collision avoidance
• Consistency between simulation and real hardware

The same URDF file flows through your entire development pipeline—from simulation to visualization to real robot control.

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Next Steps: You now understand ROS 2's core concepts (nodes, topics, services, actions) and how URDF describes robot structure. Next, explore Module 2: Simulation and Digital Twins to see these concepts in action!

References

Open Robotics. (2024). ROS 2 Documentation: URDF. https://docs.ros.org/en/humble/Tutorials/Intermediate/URDF/URDF-Main.html

Open Robotics. (2024). URDF Specification. http://wiki.ros.org/urdf/XML

Open Robotics. (2024). Using Xacro to Clean Up Your URDF. https://docs.ros.org/en/humble/Tutorials/Intermediate/URDF/Using-Xacro-to-Clean-Up-a-URDF-File.html