Robophysical study of jumping dynamics on granular media

TL;DR

This study explores how deformable robots jump on granular media, revealing complex interactions involving friction, hydrodynamics, and added mass, which influence performance and inform principles for movement in such environments.

Contribution

It introduces a robophysical approach to study shape-changing impacts on granular media, highlighting the role of added mass and nonlinear forces in jumping dynamics.

Findings

Added mass significantly affects jumping performance.

Optimal timing during push-off is influenced by deformation and added mass.

Passive and active shape changes alter interaction forces and efficiency.

Abstract

Characterizing forces on deformable objects intruding into sand and soil requires understanding the solid and fluid-like responses of such substrates and their effect on the state of the object. The most detailed studies of intrusion in dry granular media have revealed that interactions of fixed-shape objects during free impact (e.g. cannonballs) and forced slow penetration can be described by hydrostatic and hydrodynamic-like forces. Here we investigate a new class of granular interactions: rapid intrusions by objects that change shape (self-deform) through passive and active means. Systematic studies of a simple spring-mass robot jumping on dry granular media reveal that jumping performance is explained by an interplay of nonlinear frictional and hydrodynamic drag as well as induced added mass (unaccounted by traditional intrusion models) characterized by a rapidly solidified region of grains accelerated by the foot. A model incorporating these dynamics reveals that added mass degrades the performance of certain self-deformations due to a shift in optimal timing during push-off. Our systematic robophysical experiment reveals both new soft matter physics and principles for robotic self-deformation and control, which together provide principles of movement in deformable terrestrial environments.