URDF (Unified Robot Description Format)

A link element defines a rigid body segment of the robot, representing a single physical component like a chassis, arm link, or wheel. It contains the component's inertial, visual, and collision properties.

• Inertial: Defines mass and 3D inertia tensor for dynamics simulation.
• Visual: Describes the geometry (box, cylinder, mesh) and material (color, texture) for visualization in tools like RViz.
• Collision: Specifies a simplified geometry, often a convex hull, used for physics engine collision checking. This geometry is typically less detailed than the visual mesh for computational efficiency.

A joint element defines the kinematic connection between two links, specifying how they move relative to each other. It is the primary element that creates a robot's kinematic tree.

• Type: Specifies the joint's degrees of freedom: fixed, revolute, continuous, prismatic, planar, or floating.
• Parent/Child Links: Establishes the hierarchical relationship (e.g., parent="base_link" child="shoulder_link").
• Axis: Defines the vector (x, y, z) around which rotation or along which translation occurs.
• Limits: For revolute and prismatic joints, sets bounds for position, velocity, and effort.

These sub-elements within a link define its shape for rendering and physics.

• Visual Geometry: Used for display. Can be a simple primitive (<box>, <cylinder>, <sphere>) or a complex <mesh> file (e.g., STL, COLLADA).
• Collision Geometry: Used for contact simulation. Often a simpler, convex approximation of the visual mesh to drastically speed up collision detection in physics engines like Gazebo or Bullet. Using a highly detailed visual mesh for collision can make simulation prohibitively slow.

The inertial sub-element within a link is critical for accurate dynamic simulation. It defines the link's mass distribution.

• Mass: The scalar mass value in kilograms.
• Inertia: A 3x3 rotational inertia matrix (often diagonal) defined by the <inertia> tag with attributes ixx, ixy, ixz, iyy, iyz, izz. This matrix describes how the mass is distributed relative to the link's origin.
• Origin: The pose of the inertial frame relative to the link's frame. The center of mass should be at the origin of this frame. Incorrect inertia values can cause unrealistic wobbling or tipping in simulation.

While not part of the core URDF spec, the <transmission> element is a common extension (defined in a separate Xacro macro or URDF extension) that maps a joint to an actuator and defines the mechanical reduction between them.

• Type: Often "SimpleTransmission".
• Actuator: References a <hardwareInterface> like "EffortJointInterface" (for torque control) or "VelocityJointInterface".
• Mechanical Reduction: The gear ratio (e.g., "50" for a 50:1 gearbox). This is essential for the ROS control stack (ros2_control) to correctly command hardware.

Xacro (XML Macros) is not a URDF component but a critical preprocessing language used to create modular, maintainable, and parameterized URDF files. It is the de facto standard for complex robot descriptions.

• Properties & Math: Enables the use of variables (<xacro:property>) and arithmetic operations within the XML.
• Macros: Allows definition of reusable code blocks (<xacro:macro>) to instantiate common components (e.g., a wheel assembly) multiple times with different parameters.
• File Inclusion: Enables modularization by breaking a robot description into multiple files using <xacro:include>. A .xacro file is processed into a single .urdf file before use by ROS nodes.

Xacro (XML Macros) is an XML macro language used to create more manageable and reusable URDF files. It allows developers to avoid repetitive XML code by using constructs familiar to programmers.

Key features include:

• Properties and Variables: Define constants (e.g., $(wheel_radius 0.1)) to be reused throughout the file.
• Mathematical Expressions: Perform calculations directly within property definitions.
• Macros and Code Reuse: Define parameterized blocks of XML (macros) that can be invoked multiple times with different arguments, ideal for defining identical components like multiple wheels.
• Conditional Blocks and Loops: Use if statements and for loops to generate XML conditionally.

A .xacro file is processed by the xacro tool into a plain .urdf XML file before it is used by ROS nodes or simulators, making it the de facto standard for authoring complex robot descriptions.

Within a URDF <link> element, the <visual> and <collision> tags define distinct geometric representations used for rendering and physics, respectively. This separation is a critical concept for simulation fidelity and performance.

• <visual> Geometry: Defines the robot's appearance for rendering in tools like RViz. It can use high-detail meshes (e.g., .dae, .stl) with colors and textures. This geometry is used only for visualization.
• <collision> Geometry: Defines the simplified shape used for physics engine collision detection. It typically uses primitive shapes (boxes, cylinders, spheres) or very low-polygon meshes. Using simplified collision geometry drastically reduces computational cost.

Best Practice: The collision geometry should be a conservative approximation of the visual geometry, ensuring the simulated robot interacts with the world in a physically plausible way without unnecessary computational overhead. A mismatch can lead to objects visually intersecting without colliding.

The URDF <joint> element defines the kinematic relationship between two links. Its type attribute specifies the degrees of freedom, while the optional <dynamics> child tag models physical properties.

Joint Types:

• fixed: No motion (0 DOF). The child link is rigidly attached to the parent.
• revolute: Rotational motion about a single axis (1 DOF), with optional position limits.
• continuous: A revolute joint without position limits (e.g., a wheel).
• prismatic: Linear sliding motion along a single axis (1 DOF).
• planar: Motion in a plane (2 DOF: translation in x, y).
• floating: Full 6-DOF motion (translation and rotation).

Joint Dynamics (<dynamics>): This tag adds physical properties for simulation:

• damping: Viscous friction coefficient (resistance proportional to velocity).
• friction: Static friction coefficient (Coulomb friction).

These parameters are used by simulators like Gazebo to produce realistic motion and are essential for accurate controller tuning.