Extended URDF: Accounting for parallel mechanism in robot description

TL;DR

The paper addresses the limitation of URDF in describing parallel, closed-loop robots. It introduces a YAML-based extension to URDF that stores closures, actuation data, and joint-type substitutions, preserving backward compatibility. The method supports 6D and 3D closure constraints, expressed via implicit equations $\phi(q)=0$ and $K(q)\dot q=0$ (and $K(q)\ddot q = k(q,\dot q)$), and integrates with Pinocchio for dynamics. Demonstrations on Digit and Kangaroo, together with open-source tooling, show practical benefits in modeling efficiency, reduced redundancy, and usability for advanced robotics, providing a foundation for broader adoption.

Abstract

Robotic designs played an important role in recent advances by providing powerful robots with complex mechanics. Many recent systems rely on parallel actuation to provide lighter limbs and allow more complex motion. However, these emerging architectures fall outside the scope of most used description formats, leading to difficulties when designing, storing, and sharing the models of these systems. This paper introduces an extension to the widely used Unified Robot Description Format (URDF) to support closed-loop kinematic structures. Our approach relies on augmenting URDF with minimal additional information to allow more efficient modeling of complex robotic systems while maintaining compatibility with existing design and simulation frameworks. This method sets the basic requirement for a description format to handle parallel mechanisms efficiently. We demonstrate the applicability of our approach by providing an open-source collection of parallel robots, along with tools for generating and parsing this extended description format. The proposed extension simplifies robot modeling, reduces redundancy, and improves usability for advanced robotic applications.

Figures (3)

• Figure 1: Information needed to fully exploit a parallel mechanism leg, with either 6d or 3d contact
• Figure 2: Digit real and modeled robot. In the model, the 6D contacts constraints are "opened" for better visualization
• Figure 3: Real and modeled robot. The model is presented with open 6D and 3d contact constraint