Unified Robot Description Format (URDF)

A link element represents a rigid body segment of the robot with physical properties. It is the fundamental building block of a URDF model.

• Attributes: Each link has a unique name attribute.
• Sub-elements: Contains child elements that define its characteristics:
  • <visual>: Defines the shape, color, and texture for simulation and visualization (e.g., a <mesh> file or primitive <box>).
  • <collision>: Defines a (often simplified) geometry used by the physics engine for contact and collision detection.
  • <inertial>: Contains the link's mass, center of mass (<origin>), and inertia tensor (<inertia>), which are critical for dynamic simulation.

A joint element defines the kinematic relationship and constraints between two links. It specifies how one link (the child) moves relative to another (the parent).

• Types: The type attribute specifies the joint's degrees of freedom:
  • revolute: Rotational movement about a single axis (e.g., an elbow joint).
  • prismatic: Linear movement along a single axis (e.g., a sliding piston).
  • fixed: No movement; rigidly connects two links.
  • continuous: Unlimited rotation about an axis (e.g., a wheel).
• Key Properties:
  • <origin>: The pose of the child link frame relative to the parent link frame.
  • <axis>: The unit vector defining the axis of rotation or translation.
  • <limit>: For revolute and prismatic joints, specifies operational bounds (lower, upper) and effort/velocity limits.

The robot element is the root XML tag of a URDF file. It encapsulates the entire robot description.

• Attributes: Typically includes a name attribute for the overall model.
• Structure: It acts as a container for all <link> and <joint> elements that constitute the robot. The kinematic tree is formed by linking child links to parent links via joints. A properly formed URDF must have one more link than joint, resulting in a connected tree structure without kinematic loops (which require SDF format).

These three properties are defined within a <link> and serve distinct, critical purposes in simulation and control.

• <visual>: Purely for rendering. Can be high-detail meshes for realism. Does not affect physics.
• <collision>: Used by the physics engine (e.g., Gazebo, Bullet). Geometry is often simplified (convex hulls of meshes) for faster collision checking.
• <inertial>: Defines dynamic properties. This is non-optional for any link involved in dynamic simulation. An object with zero mass will not respond to forces or gravity correctly.

Best Practice: The <origin> tags within these elements allow you to offset each geometry from the link's origin, enabling alignment of visual, collision, and inertial frames.

A transmission element maps an actuator (defined outside the URDF, in robot control software) to a joint. It defines the mechanical relationship between the actuator's output and the joint's movement.

• Purpose: Bridges the gap between the kinematic/dynamic model (URDF) and the actuation model. It tells the controller how to apply torque or position commands.
• Common Type: <type>transmission_interface/SimpleTransmission</type>.
• Key Sub-elements:
  • <actuator name="motor1">: Declares the actuator with its mechanicalReduction and hardware limits.
  • <joint name="joint1">: Specifies which joint the actuator drives.
• Usage: Essential for using ROS Control, as the ros_control framework uses transmission definitions to instantiate hardware interfaces.

Collision Geometry in a URDF defines the simplified shapes (e.g., boxes, cylinders, meshes) used by motion planners and physics simulators to detect and avoid intersections between the robot and its environment or itself.

• Purpose: Enables collision checking, a critical function for safe motion planning and realistic physics simulation.
• Best Practice: Collision geometries are typically much simpler (using primitive shapes) than the detailed visual geometry meshes used for rendering, as this drastically reduces computational cost for real-time planning.

A Kinematic Chain is a series of rigid links connected by joints that define the motion capabilities of a robot. The URDF explicitly models this chain, which is fundamental to calculating robot kinematics.

• Forward Kinematics: Uses the chain defined in the URDF (link lengths, joint angles) to compute the position and orientation of the end-effector.
• Inverse Kinematics: Solves for the joint angles required to achieve a desired end-effector pose, relying on the kinematic model described in the URDF.
• Tree Structure: A robot's URDF defines a kinematic tree, with the base_link as the root and branches extending through joints to end-effectors.