Skip to main content

Understanding URDF (Unified Robot Description Format) for Humanoids

Introduction​

The Unified Robot Description Format (URDF) is an XML format used in ROS to describe all aspects of a robot. This includes its kinematic and dynamic properties, visual appearance, and collision models. For humanoid robots, URDF files become particularly complex due to the intricate structure and many degrees of freedom.

URDF Basics​

A URDF file is composed of <link> and <joint> elements:

  • <link>: Describes a rigid body segment of the robot (e.g., torso, upper arm, forearm). It contains information about the link's mass, inertia, visual mesh, and collision geometry.
  • <joint>: Describes the kinematic and dynamic properties of the connection between two links. It defines the type of joint (e.g., revolute, prismatic, fixed), its axis of rotation, limits, and parent/child links.

Key Elements for Humanoids​

Humanoid robots require a detailed URDF description to accurately model their anthropomorphic structure.

Kinematics and Dynamics​

  • Links: Each body part (head, neck, torso, pelvis, upper leg, lower leg, foot, upper arm, forearm, hand) is typically represented as a separate link.
  • Joints: Connect these links, often revolute joints for rotational movement (e.g., at the shoulder, elbow, knee) and fixed joints for rigidly connected parts.
  • Degrees of Freedom (DoF): Humanoids have a high number of DoF, requiring many joints to be defined.

Visuals and Collisions​

  • <visual>: Defines the visual representation of the link, often by referencing 3D mesh files (e.g., .stl, .dae). This is what you see in simulators like Gazebo.
  • <collision>: Defines the collision geometry of the link. This is crucial for physics simulation and obstacle avoidance. It is often a simplified shape (e.g., box, cylinder, sphere) to reduce computational load.

URDF for Humanoids Challenges​

  • Complexity: Describing a humanoid's many links and joints manually can be tedious and error-prone.
  • Self-Collision: Humanoid robots can easily self-collide (e.g., an arm hitting the torso). Careful collision model definition and self-collision checking are essential.
  • Balance and Stability: While URDF describes the physical properties, achieving balance and stable locomotion requires complex control algorithms built on top of the URDF model.

Xacro for Simplified URDF​

For complex robots like humanoids, Xacro (XML Macros) is often used to simplify URDF files. Xacro allows for:

  • Macros: Reusable blocks of XML code.
  • Parameters: Define values that can be passed to macros.
  • Mathematical Expressions: Calculate values dynamically.

This greatly reduces redundancy and improves readability in humanoid robot descriptions.