URDF

A link element defines a rigid body segment of the robot. It is the fundamental building block representing a physical part, such as a chassis, arm segment, or wheel.

• Each link contains inertial properties (mass, center of mass, moment of inertia).
• It defines visual geometry (shape, color, texture for rendering).
• It defines collision geometry (a typically simplified shape used for physics calculations).
• Example: <link name="base_link"> describes the robot's main body.

A joint element defines the connection and relative motion between two links. It specifies the kinematic relationship and constraints.

• Key attributes include type (e.g., revolute, prismatic, fixed, continuous).
• It defines the parent and child links.
• It specifies the axis of rotation or translation.
• It can include limits for position, velocity, and effort.
• Example: A <joint type="revolute" name="elbow_joint"> connects an upper arm link to a forearm link.

The visual property of a link describes its appearance in a 3D renderer or visualization tool. It is used for creating a realistic or schematic view of the robot.

• Geometry can be a box, cylinder, sphere, or mesh (imported from a file like .stl or .dae).
• Includes material properties for color (<color rgba="0 0.5 1.0 1.0"/>) or texture.
• Crucially, the visual geometry does not affect physics simulation; it is for display only.
• Example: A complex gripper might use a mesh file for detailed visual representation.

The collision property of a link defines the simplified shape used by the physics engine to compute contacts, collisions, and dynamics.

• Typically uses primitive shapes (box, cylinder, sphere) for computational efficiency, even if the visual geometry is a complex mesh.
• The collision geometry can be a different size and shape than the visual geometry to optimize simulation speed.
• Accurate collision geometry is critical for realistic contact dynamics and manipulation tasks in simulation.
• Example: A robot link shaped like an 'L' might use two overlapping boxes for its collision model.

The inertial tag within a link defines its dynamic properties, which are essential for accurate physics simulation.

• Mass: The weight of the link in kilograms.
• Origin: The pose of the center of mass relative to the link's coordinate frame.
• Inertia Matrix: A 3x3 matrix representing the rotational inertia about the center of mass. This determines how the link resists angular acceleration.
• Omitting or incorrectly specifying inertial properties leads to unrealistic or unstable simulation behavior.
• Example: <inertia ixx="0.001" ixy="0.0" ... />

URDF has limitations, leading to the use of macros and more powerful formats.

• Xacro (XML Macros): A macro language that extends URDF, allowing for parameterization, code reuse, and conditional statements to manage complex robots without repetitive XML.
• SDF (Simulation Description Format): A more feature-rich successor used by simulators like Gazebo and Ignition. SDF supports nested models, plugins, atmosphere and physics properties, and is better suited for describing entire simulated worlds.
• URDF files are often processed by xacro into a flat URDF, which can then be converted to SDF for use in physics simulators.

These are the core computational methods for understanding robot motion, directly informed by the kinematic chain defined in a URDF.

• Forward Kinematics (FK): Calculates the end-effector pose (position and orientation) given a set of joint angles. It answers: "Where is my robot's hand?"
• Inverse Kinematics (IK): Calculates the joint angles required to achieve a desired end-effector pose. It answers: "How do I move my robot's hand here?" The <joint> tags in a URDF (defining axis, limits, and type) provide the essential parameters for these calculations, which are performed by libraries like KDL (Kinematics and Dynamics Library) or TRAC-IK.

While a basic URDF defines joint types and limits, realistic simulation requires detailed models of the actuation and friction dynamics within those joints.

• Actuator Model: Represents motor dynamics—how a commanded voltage or PWM signal translates to torque, accounting for saturation, back-EMF, and rotor inertia. This is critical for simulating torque control.
• Friction Model: Captures resistive forces like Coulomb (dry) friction, viscous damping, and stiction. These non-linear effects are often defined in the SDF or simulator-specific plugins, not the core URDF. Accurate modeling of these dynamics in simulation is essential for sim-to-real transfer, as real motors and gears are never ideal.

A URDF defines two distinct geometries for each link, a critical distinction for simulation and planning.

• <visual> Geometry: Defines the appearance of the link for rendering. It can be high-detail and complex (e.g., a textured mesh of the robot's shell).
• <collision> Geometry: Defines the simplified shape used for physics-based contact and collision detection. It is typically composed of primitive shapes (boxes, cylinders, spheres) or very coarse meshes for computational efficiency. This separation allows a robot to look realistic while maintaining fast and stable physics simulation. Motion planners like MoveIt! use the collision geometry exclusively.