Simulation & transfer
URDF
Unified Robot Description Format
URDF, the Unified Robot Description Format, is an XML format used in ROS to describe a robot as links connected by joints, together with properties such as geometry, collision shapes, inertial parameters, limits, and visual materials. A URDF describes a robot model; it is not by itself a controller, simulator, CAD assembly, or complete record of the physical machine.
Also known as: URDF model, URDF file
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Links and joints form the model
ROS describes URDF as a format for specifying robot geometry and organisation. Links represent rigid bodies. Joints connect a parent link to a child link and specify the transform, axis, type and limits that define permitted relative motion.
A link can reference separate visual and collision geometry. It can also contain mass, centre of mass and inertia for dynamics. These roles should not be conflated: a detailed visual mesh may be unsuitable for collision detection, while a coarse collision shape may not match appearance closely enough for rendering or synthetic perception data.
A valid file can still be an incomplete robot description
The urdfdom schema defines elements such as links, joints, inertial properties, geometry and materials. It does not make those values correct. Missing inertia, unrealistic collision shapes, incorrect joint axes or stale limits can produce a model that loads successfully but behaves unlike the hardware.
URDF models also form a tree, so closed kinematic loops and simulator-specific behaviour often require extensions or another format. Transmissions, sensors, actuators, contact parameters and plugins may live in additional ROS or simulator configuration. The robot's firmware, controller gains, delays and safety logic are not recovered from link and joint geometry.
Version the model with the data
For a robot dataset, a URDF can make joint names, frames, kinematic structure and geometry interpretable. It is especially useful when poses, torques, camera calibration or retargeted motion refer to named links. But a generic model downloaded later may not match the robot revision used for collection.
Publish the exact file or source commit, mesh assets, units, calibration changes and hardware modifications with the dataset. State which parameters were measured, identified or approximated. When simulation generated the data, retain any converted model and simulator-specific additions as well as the source URDF.
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Related terms
Hardware & control
Robot embodiment
A robot embodiment is the particular body and sensorimotor interface through which a robot perceives and acts. It includes morphology and kinematics, actuators, end effectors, sensors, physical limits, and the observation and action conventions exposed to a controller or learned policy. Two robots can perform the same task while having different embodiments.
Hardware & control
Robot kinematics
Robot kinematics describes the geometric relationship between a robot’s joint configuration and the position, orientation and velocity of its links or end-effector, without modelling the forces that cause the motion. Forward kinematics computes pose from joint values; inverse kinematics searches for joint values that achieve a requested pose.
Hardware & control
Robot dynamics
Robot dynamics describes the relationship between forces and torques acting on a robot and the motion they produce, accounting for mass, inertia, gravity, velocity-dependent effects and external contact. Forward dynamics predicts acceleration from applied forces; inverse dynamics computes forces or torques for a specified motion.
Hardware & control
Coordinate frame
A coordinate frame is a defined origin and set of oriented axes used to express positions, orientations, motions, forces, or other spatial quantities. A value has no complete geometric meaning until its frame and convention are known. Transformations relate measurements expressed in frames such as world, robot base, camera, end effector, object, or sensor.
Hardware & control
Degree of freedom
A degree of freedom (DoF) is one independent parameter needed to specify a robot’s configuration; equivalently, a robot’s DoF is the dimension of its configuration space. It describes possible motion, not the number of motors. Joint constraints, closed kinematic chains and environmental contacts can make joint count, actuator count and controllable motion differ.
Simulation & transfer
Digital twin
A digital twin is a fit-for-purpose digital representation of a specific physical robot, asset or process that is kept synchronised with its real counterpart through operational data. It may contain geometry, dynamics and simulation models, but the maintained link to an identified real system distinguishes it from an ordinary, standalone simulator.