URDF (Unified Robot Description Format) for Humanoids

URDF (Unified Robot Description Format) is an XML format for representing a robot model. URDF is used extensively in ROS to describe robots in terms of their joints, links, visual and collision properties, and more. When designing humanoid robots, URDF plays a crucial role in specifying the physical structure and kinematic properties.

What is URDF?

URDF stands for Unified Robot Description Format. It is an XML file format used to describe robots in ROS. A URDF file contains information about:

Robot's physical structure (links)
How parts are connected (joints)
Visual appearance (collision and visual properties)
Kinematic properties (inertial properties)
Sensors attached to the robot

URDF Structure for Humanoid Robots

A humanoid robot URDF typically contains multiple limbs (arms, legs), a torso, and a head. Each limb is modeled as a series of links connected by joints.

Basic URDF Elements

<?xml version="1.0"?>
<robot name="humanoid_robot">
  <!-- Links define the physical parts of the robot -->
  <link name="base_link">
    <visual>
      <geometry>
        <box size="0.2 0.1 0.1"/>
      </geometry>
      <material name="blue">
        <color rgba="0 0 1 1"/>
      </material>
    </visual>
    <collision>
      <geometry>
        <box size="0.2 0.1 0.1"/>
      </geometry>
    </collision>
    <inertial>
      <mass value="1"/>
      <inertia ixx="0.1" ixy="0" ixz="0" iyy="0.1" iyz="0" izz="0.1"/>
    </inertial>
  </link>

  <!-- Joints define how links connect -->
  <joint name="joint1" type="revolute">
    <parent link="base_link"/>
    <child link="link1"/>
    <origin xyz="0.1 0 0" rpy="0 0 0"/>
    <axis xyz="0 0 1"/>
    <limit lower="-1.57" upper="1.57" effort="10" velocity="1"/>
  </joint>

  <link name="link1">
    <!-- Link definition similar to base_link -->
  </link>
</robot>

Designing a Humanoid Robot in URDF

Basic Structure

A humanoid robot typically includes:

Torso / Pelvis (base link)
Two legs (hip, knee, ankle, foot)
Two arms (shoulder, elbow, wrist, hand)
Neck and head

Example URDF for a Simple Humanoid

<?xml version="1.0"?>
<robot name="simple_humanoid">
  <!-- Torso / Base Link -->
  <link name="torso">
    <visual>
      <geometry>
        <box size="0.2 0.3 0.4"/>  <!-- Width x Depth x Height -->
      </geometry>
      <origin xyz="0 0 0.2"/>
      <material name="gray">
        <color rgba="0.5 0.5 0.5 1"/>
      </material>
    </visual>
    <collision>
      <geometry>
        <box size="0.2 0.3 0.4"/>
      </geometry>
      <origin xyz="0 0 0.2"/>
    </collision>
    <inertial>
      <mass value="5"/>
      <inertia ixx="0.3" ixy="0" ixz="0" iyy="0.2" iyz="0" izz="0.1"/>
    </inertial>
  </link>

  <!-- Head -->
  <link name="head">
    <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="1"/>
      <inertia ixx="0.02" ixy="0" ixz="0" iyy="0.02" iyz="0" izz="0.02"/>
    </inertial>
  </link>

  <!-- Joint connecting torso and head -->
  <joint name="neck_joint" type="revolute">
    <parent link="torso"/>
    <child link="head"/>
    <origin xyz="0 0 0.4"/>
    <axis xyz="0 0 1"/>
    <limit lower="-0.5" upper="0.5" effort="1" velocity="2"/>
  </joint>

  <!-- Left Arm -->
  <link name="left_shoulder">
    <visual>
      <geometry>
        <box size="0.1 0.15 0.15"/>
      </geometry>
      <material name="gray"/>
    </visual>
    <collision>
      <geometry>
        <box size="0.1 0.15 0.15"/>
      </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 connecting torso and shoulder -->
  <joint name="left_shoulder_joint" type="revolute">
    <parent link="torso"/>
    <child link="left_shoulder"/>
    <origin xyz="0.15 0.1 0.2"/>
    <axis xyz="0 0 1"/>
    <limit lower="-1.57" upper="1.57" effort="5" velocity="3"/>
  </joint>

  <!-- Right arm, legs, and other body parts would follow similar pattern -->
</robot>

Advanced URDF Features for Humanoids

Transmission Elements

To connect the robot to a control system, transmission elements are added:

<transmission name="left_shoulder_trans">
  <type>transmission_interface/SimpleTransmission</type>
  <joint name="left_shoulder_joint">
    <hardwareInterface>PositionJointInterface</hardwareInterface>
  </joint>
  <actuator name="left_shoulder_motor">
    <hardwareInterface>PositionJointInterface</hardwareInterface>
    <mechanicalReduction>1</mechanicalReduction>
  </actuator>
</transmission>

Gazebo-Specific Elements

For simulation in Gazebo, additional elements are often needed:

<gazebo reference="torso">
  <material>Gazebo/Gray</material>
  <mu1>0.2</mu1>
  <mu2>0.2</mu2>
</gazebo>

URDF Tools and Verification

Using check_urdf

After creating a URDF file, verify its validity:

check_urdf path/to/robot.urdf

Using urdf_to_graphiz

Visualize the robot structure:

urdf_to_graphviz path/to/robot.urdf

Best Practices for Humanoid URDF

Use proper units: All dimensions in meters, mass in kilograms
Realistic inertial properties: Accurate mass and inertia values improve simulation
Joint limits: Define appropriate limits for each joint
Collision vs. Visual: Use simple shapes for collision and detailed for visuals
Naming convention: Use consistent naming across all links and joints
Kinematic chain: Ensure proper parent-child relationships
Mounting points: Define attachment points for accessories

Creating URDF with Python

While URDF is an XML format, it can be programmatically generated using Python:

import xml.etree.ElementTree as ET

def create_simple_link(name, mass, geometry_type="box", size=[0.1, 0.1, 0.1]):
    link = ET.Element("link", name=name)
    
    # Visual element
    visual = ET.SubElement(link, "visual")
    vis_geom = ET.SubElement(visual, "geometry")
    ET.SubElement(vis_geom, geometry_type, size=" ".join(map(str, size)))
    
    # Collision element
    collision = ET.SubElement(link, "collision")
    coll_geom = ET.SubElement(collision, "geometry")
    ET.SubElement(coll_geom, geometry_type, size=" ".join(map(str, size)))
    
    # Inertial element
    inertial = ET.SubElement(link, "inertial")
    ET.SubElement(inertial, "mass", value=str(mass))
    ET.SubElement(inertial, "inertia", 
                  ixx="0.01", ixy="0", ixz="0", 
                  iyy="0.01", iyz="0", izz="0.01")
    
    return link

# Example usage
robot = ET.Element("robot", name="humanoid")
torso = create_simple_link("torso", 5.0, "box", [0.2, 0.3, 0.4])
robot.append(torso)

Integration with ROS

Once the URDF is created, it can be used in ROS nodes:

import rclpy
from rclpy.node import Node
from sensor_msgs.msg import JointState
from tf2_ros import TransformBroadcaster
import math

class URDFRobotNode(Node):
    def __init__(self):
        super().__init__('urdf_robot_node')
        
        # Initialize joint state publisher
        self.joint_pub = self.create_publisher(JointState, 'joint_states', 10)
        
        # Initialize TF broadcaster
        self.br = TransformBroadcaster(self)
        
        # Timer for publishing state
        self.timer = self.create_timer(0.1, self.publish_joint_states)
        
        # Sample joint positions
        self.joint_names = ['left_shoulder_joint', 'neck_joint']
        self.joint_positions = [0.0, 0.0]
    
    def publish_joint_states(self):
        # Create joint state message
        msg = JointState()
        msg.name = self.joint_names
        msg.position = self.joint_positions
        msg.header.stamp = self.get_clock().now().to_msg()
        msg.header.frame_id = "base_link"
        
        self.joint_pub.publish(msg)

Summary

URDF is a critical component in modeling humanoid robots for ROS. Properly constructed URDF files enable accurate simulation, kinematic analysis, and motion planning. Understanding how to effectively describe complex human-like structures in URDF is essential for developing humanoid robots in ROS.

Exercises

Create a URDF file for a simplified humanoid with at least 6 degrees of freedom
Add transmission elements to your URDF for position control
Visualize your URDF using RViz
Simulate your humanoid in Gazebo

URDF (Unified Robot Description Format) for Humanoids

What is URDF?​

URDF Structure for Humanoid Robots​

Basic URDF Elements​

Designing a Humanoid Robot in URDF​

Basic Structure​

Example URDF for a Simple Humanoid​

Advanced URDF Features for Humanoids​

Transmission Elements​

Gazebo-Specific Elements​

URDF Tools and Verification​

Using check_urdf​

Using urdf_to_graphiz​

Best Practices for Humanoid URDF​

Creating URDF with Python​

Integration with ROS​

Summary​

Exercises​