Self-propulsion via slipping: frictional swimming in multi-legged locomotors

TL;DR

This study reveals that multi-legged terrestrial robots can achieve effective locomotion through slip mechanisms similar to swimming, with a simplified low-dimensional model capturing the complex dynamics and informing control strategies.

Contribution

The paper introduces a novel geometric model that simplifies multi-legged locomotion to a low-dimensional framework, highlighting the role of slip and body undulation in complex terrains.

Findings

Multi-legged robots can swim-like through slip in terrestrial environments.

A low-dimensional model captures complex multi-legged dynamics effectively.

Body undulation improves performance in obstacle-rich terrains.

Abstract

Locomotion is typically studied either in continuous media where bodies and legs experience forces generated by the flowing medium, or on solid substrates dominated by friction. In the former, centralized coordination is believed to facilitate appropriate slipping through the medium for propulsion. In the latter, slip is often assumed minimal and thus avoided via decentralized controls. We discover in laboratory experiments that terrestrial locomotion of a meter scale multi-segmented/legged robophysical model resembles undulatory fluid swimming. Experiments varying waves of limb stepping and body bending reveal how these parameters result in effective terrestrial locomotion despite seemingly ineffective isotropic frictional contacts. Dissipation dominates over inertial effects in this macroscopic-scaled regime, resulting in essentially geometric locomotion akin to microscopic-scale swimming. Theoretical analysis demonstrates that the high-dimensional multi-segmented/legged dynamics can be simplified to a centralized low-dimensional model, which reveals an effective Resistive Force Theory with an acquired viscous drag anisotropy. We extend our low-dimensional, geometric analysis to illustrate how body undulation can aid performance in non-flat obstacle-rich terrains and also use the scheme to quantitatively model how body undulation affects performance of biological centipede locomotion (the desert centipede S. polymorpha) moving at relatively high speeds (~0.5 body lengths/sec). Our results could facilitate control of multilegged robots in complex terradynamic scenarios.