Unified Robot Description Format

An XML file format that describes a robot's links, joints, geometry, and mass, used as a standard across ROS.

URDF is the XML format used across the ROS ecosystem to describe a robot model. In the file, each link element describes one rigid body's visual mesh, collision shape, and mass/inertia, and each joint element describes how two links connect — including the joint type (revolute, prismatic, fixed, and so on), its axis, and its range of motion — together forming a kinematic tree. With a URDF, RViz can display the robot, kinematics and dynamics libraries can compute forward and inverse kinematics from it, and simulators such as Isaac Sim, MuJoCo, and PyBullet can import it directly. Its limitations are that it can only describe a single robot, can only express a tree structure, and handles closed kinematic chains poorly; writing it by hand is also long and repetitive, which is why the Xacro macro system is commonly used to simplify it. MuJoCo has its own separate format, MJCF, which can encode more simulation parameters.

ExampleRobot manufacturers typically ship a URDF and mesh files with the robot; importing that URDF into Isaac Lab is enough to start training a policy for that robot in simulation.

Also called

URDF, URDF File

Related

Xacro (XML Macros) · MJCF (MuJoCo XML Format) · Link · Revolute Joint · Kinematic Tree · Mesh File (STL / OBJ / DAE / glTF)

Sources

ROS Wiki: urdf
ROS 2 Documentation: URDF tutorials
Gazebo Classic tutorial: URDF in Gazebo（URDF 只能描述单个机器人、不能表达闭链等局限） (Chinese)