URDF Humanoid Modeling

This section covers creating Unified Robot Description Format (URDF) models for humanoid robots. URDF is XML-based format that describes robot models including kinematic structure, visual and collision properties, and physical properties.

Introduction to URDF

URDF (Unified Robot Description Format) is an XML format used in ROS to describe robot models. It defines the robot's physical structure, including:

Links (rigid bodies)
Joints (connections between links)
Visual and collision properties
Inertial properties

Basic URDF Structure

A basic URDF file follows this structure:

<?xml version="1.0"?>
<robot name="humanoid_robot">
  <!-- Define links -->
  <link name="base_link">
    <visual>
      <geometry>
        <box size="0.5 0.2 0.2"/>
      </geometry>
      <material name="blue">
        <color rgba="0 0 1 1"/>
      </material>
    </visual>
    <collision>
      <geometry>
        <box size="0.5 0.2 0.2"/>
      </geometry>
    </collision>
    <inertial>
      <mass value="1"/>
      <inertia ixx="0.01" ixy="0" ixz="0" iyy="0.01" iyz="0" izz="0.01"/>
    </inertial>
  </link>

  <!-- Additional links and joints would go here -->
</robot>

Link Properties

Links represent rigid bodies in the robot. Each link has several components:

Visual Properties

geometry: Shape of the link for visualization
- <box size="x y z"/>
- <cylinder radius="r" length="l"/>
- <sphere radius="r"/>
- <mesh filename="path/to/mesh.dae"/>
material: Color and texture properties

Collision Properties

Similar to visual but defines collision boundaries
Often simpler geometry for performance

Inertial Properties

Mass and inertia tensor
Used for physics simulation

Joint Properties

Joints connect links together 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="x y z" rpy="roll pitch yaw"/>
  <axis xyz="x y z"/>
  <limit lower="min" upper="max" effort="effort" velocity="vel"/>
</joint>

Joint types include:

revolute: Rotational joint with limits
continuous: Rotational joint without limits
prismatic: Linear sliding joint
fixed: No movement (rigid connection)
floating: 6DOF with no constraints
planar: Movement on a plane

Complete Humanoid Robot Example

Here's a simplified humanoid robot URDF:

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

  <!-- Base link -->
  <link name="base_link">
    <visual>
      <geometry>
        <box size="0.3 0.3 0.3"/>
      </geometry>
      <material name="gray">
        <color rgba="0.5 0.5 0.5 1"/>
      </material>
    </visual>
    <collision>
      <geometry>
        <box size="0.3 0.3 0.3"/>
      </geometry>
    </collision>
    <inertial>
      <mass value="5"/>
      <inertia ixx="0.1" ixy="0" ixz="0" iyy="0.1" iyz="0" izz="0.1"/>
    </inertial>
  </link>

  <!-- Torso -->
  <link name="torso">
    <visual>
      <geometry>
        <box size="0.25 0.2 0.5"/>
      </geometry>
      <material name="red">
        <color rgba="1 0 0 1"/>
      </material>
    </visual>
    <collision>
      <geometry>
        <box size="0.25 0.2 0.5"/>
      </geometry>
    </collision>
    <inertial>
      <mass value="3"/>
      <inertia ixx="0.05" ixy="0" ixz="0" iyy="0.05" iyz="0" izz="0.05"/>
    </inertial>
  </link>

  <joint name="base_to_torso" type="fixed">
    <parent link="base_link"/>
    <child link="torso"/>
    <origin xyz="0 0 0.4"/>
  </joint>

  <!-- Head -->
  <link name="head">
    <visual>
      <geometry>
        <sphere radius="0.1"/>
      </geometry>
      <material name="white">
        <color rgba="1 1 1 1"/>
      </material>
    </visual>
    <collision>
      <geometry>
        <sphere radius="0.1"/>
      </geometry>
    </collision>
    <inertial>
      <mass value="1"/>
      <inertia ixx="0.004" ixy="0" ixz="0" iyy="0.004" iyz="0" izz="0.004"/>
    </inertial>
  </link>

  <joint name="torso_to_head" type="revolute">
    <parent link="torso"/>
    <child link="head"/>
    <origin xyz="0 0 0.3"/>
    <axis xyz="0 0 1"/>
    <limit lower="-1.57" upper="1.57" effort="10" velocity="1"/>
  </joint>

  <!-- Left Arm -->
  <link name="left_upper_arm">
    <visual>
      <geometry>
        <cylinder radius="0.05" length="0.3"/>
      </geometry>
      <material name="blue">
        <color rgba="0 0 1 1"/>
      </material>
    </visual>
    <collision>
      <geometry>
        <cylinder radius="0.05" length="0.3"/>
      </geometry>
    </collision>
    <inertial>
      <mass value="0.8"/>
      <inertia ixx="0.01" ixy="0" ixz="0" iyy="0.01" iyz="0" izz="0.001"/>
    </inertial>
  </link>

  <joint name="torso_to_left_shoulder" type="revolute">
    <parent link="torso"/>
    <child link="left_upper_arm"/>
    <origin xyz="0.15 0 0.1" rpy="0 0 0"/>
    <axis xyz="0 1 0"/>
    <limit lower="-1.57" upper="1.57" effort="10" velocity="1"/>
  </joint>

  <link name="left_lower_arm">
    <visual>
      <geometry>
        <cylinder radius="0.04" length="0.25"/>
      </geometry>
      <material name="blue">
        <color rgba="0 0 1 1"/>
      </material>
    </visual>
    <collision>
      <geometry>
        <cylinder radius="0.04" length="0.25"/>
      </geometry>
    </collision>
    <inertial>
      <mass value="0.5"/>
      <inertia ixx="0.005" ixy="0" ixz="0" iyy="0.005" iyz="0" izz="0.0005"/>
    </inertial>
  </link>

  <joint name="left_elbow" type="revolute">
    <parent link="left_upper_arm"/>
    <child link="left_lower_arm"/>
    <origin xyz="0 0 -0.25" rpy="0 0 0"/>
    <axis xyz="0 1 0"/>
    <limit lower="0" upper="1.57" effort="10" velocity="1"/>
  </joint>

  <!-- Right Arm (similar structure to left arm) -->
  <link name="right_upper_arm">
    <visual>
      <geometry>
        <cylinder radius="0.05" length="0.3"/>
      </geometry>
      <material name="blue">
        <color rgba="0 0 1 1"/>
      </material>
    </visual>
    <collision>
      <geometry>
        <cylinder radius="0.05" length="0.3"/>
      </geometry>
    </collision>
    <inertial>
      <mass value="0.8"/>
      <inertia ixx="0.01" ixy="0" ixz="0" iyy="0.01" iyz="0" izz="0.001"/>
    </inertial>
  </link>

  <joint name="torso_to_right_shoulder" type="revolute">
    <parent link="torso"/>
    <child link="right_upper_arm"/>
    <origin xyz="-0.15 0 0.1" rpy="0 0 0"/>
    <axis xyz="0 1 0"/>
    <limit lower="-1.57" upper="1.57" effort="10" velocity="1"/>
  </joint>

  <link name="right_lower_arm">
    <visual>
      <geometry>
        <cylinder radius="0.04" length="0.25"/>
      </geometry>
      <material name="blue">
        <color rgba="0 0 1 1"/>
      </material>
    </visual>
    <collision>
      <geometry>
        <cylinder radius="0.04" length="0.25"/>
      </geometry>
    </collision>
    <inertial>
      <mass value="0.5"/>
      <inertia ixx="0.005" ixy="0" ixz="0" iyy="0.005" iyz="0" izz="0.0005"/>
    </inertial>
  </link>

  <joint name="right_elbow" type="revolute">
    <parent link="right_upper_arm"/>
    <child link="right_lower_arm"/>
    <origin xyz="0 0 -0.25" rpy="0 0 0"/>
    <axis xyz="0 1 0"/>
    <limit lower="0" upper="1.57" effort="10" velocity="1"/>
  </joint>

</robot>

Xacro for Complex Models

For complex humanoid models, it's common to use Xacro (XML Macros), which allows for:

Parameterization
Reusable macros
Cleaner, more maintainable code

<?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="base_size" value="0.3 0.3 0.3" />

  <!-- Macro for creating arm links -->
  <xacro:macro name="arm_link" params="name parent xyz rpy joint_axis joint_limit_lower joint_limit_upper">
    <link name="${name}_upper_arm">
      <visual>
        <geometry>
          <cylinder radius="0.05" length="0.3"/>
        </geometry>
        <material name="blue">
          <color rgba="0 0 1 1"/>
        </material>
      </visual>
      <collision>
        <geometry>
          <cylinder radius="0.05" length="0.3"/>
        </geometry>
      </collision>
      <inertial>
        <mass value="0.8"/>
        <inertia ixx="0.01" ixy="0" ixz="0" iyy="0.01" iyz="0" izz="0.001"/>
      </inertial>
    </link>

    <joint name="${parent}_to_${name}_shoulder" type="revolute">
      <parent link="${parent}"/>
      <child link="${name}_upper_arm"/>
      <origin xyz="${xyz}" rpy="${rpy}"/>
      <axis xyz="${joint_axis}"/>
      <limit lower="${joint_limit_lower}" upper="${joint_limit_upper}" effort="10" velocity="1"/>
    </joint>
  </xacro:macro>

  <!-- Create base link -->
  <link name="base_link">
    <visual>
      <geometry>
        <box size="${base_size}"/>
      </geometry>
      <material name="gray">
        <color rgba="0.5 0.5 0.5 1"/>
      </material>
    </visual>
    <collision>
      <geometry>
        <box size="${base_size}"/>
      </geometry>
    </collision>
    <inertial>
      <mass value="5"/>
      <inertia ixx="0.1" ixy="0" ixz="0" iyy="0.1" iyz="0" izz="0.1"/>
    </inertial>
  </link>

  <!-- Use the macro to create arms -->
  <xacro:arm_link name="left" parent="base_link"
                   xyz="0.15 0 0" rpy="0 0 0"
                   joint_axis="0 1 0"
                   joint_limit_lower="-1.57" joint_limit_upper="1.57"/>

  <xacro:arm_link name="right" parent="base_link"
                   xyz="-0.15 0 0" rpy="0 0 0"
                   joint_axis="0 1 0"
                   joint_limit_lower="-1.57" joint_limit_upper="1.57"/>

</robot>

Validating URDF Models

Before using a URDF model in simulation, validate it:

# Check if URDF is valid
check_urdf path/to/your/model.urdf

# Convert Xacro to URDF
ros2 run xacro xacro path/to/your/model.xacro > output.urdf

# Visualize in RViz
ros2 run rviz2 rviz2
# In RViz, add RobotModel display and set the URDF file

Best Practices

Start Simple: Begin with a basic model and add complexity gradually
Use Consistent Units: Stick to meters for distances, radians for angles
Validate Inertias: Make sure inertia values are reasonable
Collision vs Visual: Use simpler shapes for collision to improve performance
Origin Conventions: Use consistent coordinate frame conventions
Naming Consistency: Use descriptive, consistent names for links and joints

Hands-on Exercise

Create a URDF model for a simple humanoid robot with:

A base/torso link
Head with neck joint
Two arms with shoulder and elbow joints
Two legs with hip, knee, and ankle joints

Test your model using check_urdf and visualize it in RViz. Then create a simple launch file to spawn your robot in Gazebo simulation.

Introduction to URDF​

Basic URDF Structure​

Link Properties​

Visual Properties​

Collision Properties​

Inertial Properties​

Joint Properties​

Complete Humanoid Robot Example​

Xacro for Complex Models​

Validating URDF Models​

Best Practices​

Hands-on Exercise​