URDF+: An Enhanced URDF for Robots with Kinematic Loops

TL;DR

URDF+ enhances the standard robot description format to natively support kinematic loops, enabling better simulation and analysis of closed-chain robotic systems with automated parsing and minimal user effort.

Contribution

This paper introduces URDF+, an extension of URDF that supports kinematic loops and provides automated parsing for closed-chain dynamics, addressing a key limitation of existing robot description formats.

Findings

URDF+ successfully encodes kinematic loops in robot models.

Automated parser groups bodies into local loops for dynamics algorithms.

URDF+ maintains simplicity while supporting complex closed-chain structures.

Abstract

Designs incorporating kinematic loops are becoming increasingly prevalent in the robotics community. Despite the existence of dynamics algorithms to deal with the effects of such loops, many modern simulators rely on dynamics libraries that require robots to be represented as kinematic trees. This requirement is reflected in the de facto standard format for describing robots, the Universal Robot Description Format (URDF), which does not support kinematic loops resulting in closed chains. This paper introduces an enhanced URDF, termed URDF+, which addresses this key shortcoming of URDF while retaining the intuitive design philosophy and low barrier to entry that the robotics community values. The URDF+ keeps the elements used by URDF to describe open chains and incorporates new elements to encode loop joints. We also offer an accompanying parser that processes the system models coming from URDF+ so that they can be used with recursive rigid-body dynamics algorithms for closed-chain systems that group bodies into local, decoupled loops. This parsing process is fully automated, ensuring optimal grouping of constrained bodies without requiring manual specification from the user. We aim to advance the robotics community towards this elegant solution by developing efficient and easy-to-use software tools.