URDF+: An Enhanced URDF for Robots with Kinematic Loops

TL;DR

The paper addresses the limitation of URDF which cannot model kinematic loops by introducing URDF+, an augmented format with <loop> and coupling constructs plus an independent coordinates option. A fully automated parser based on Jain's constraint-embedding and strongly connected components constructs loop-aggregated connectivity graphs (LACG) to enable recursive dynamics for closed-chain systems while preserving backward compatibility with URDF. Key contributions include new URDF+ data structures, an automated parser that derives optimal sub-groupings, and illustrative examples (e.g., wrist and belt-transmission) demonstrating practical closed-chain modeling within the URDF ecosystem. The work aims to accelerate adoption of closed-chain dynamics tools by providing accessible software that integrates with existing robotics workflows.

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.

Figures (5)

• Figure 1: Robots employing kinematic loops to achieve proximal actuation. Left to right: LIMS2-AMBIDEX song2018development, MIT Humanoid chignoli2021humanoid, Kangaroo roig2022hardware, Hermes wang2015hermes, Digit digit.
• Figure 2: Exploded view of a four-bar mechanism showing its coordinate frames and the transforms between them. Tree joint quantities are shown in blue, and loop joint quantities are shown in green.
• Figure 3: Illustrative connectivity graphs for a system with kinematic loops resulting in multiple aggregate links.
• Figure 4: Schematic of the 2-DOF wrist joint for the LIMS2-AMBIDEX robot song2018development and its corresponding connectivity graphs.
• Figure 5: CAD view of the parallel belt transmission in the MIT Humanoid chignoli2021humanoid and its corresponding connectivity graphs.