A resistive force model of legged locomotion on granular media

TL;DR

This paper introduces a resistive force model for legged locomotion on granular media, enabling better prediction of ground reaction forces and improving the design of legged robots for traversing sandy and gravelly terrains.

Contribution

It develops a novel multi-segment resistive force model for complex limb geometries in granular media, validated through experiments and dynamic simulations.

Findings

The model accurately predicts forces on complex limb shapes.

Simulations match observed robot speeds on granular surfaces.

The approach outperforms simpler force models.

Abstract

Compared to agile legged animals, wheeled and tracked vehicles often suffer large performance loss on granular surfaces like sand and gravel. Understanding the mechanics of legged locomotion on granular media can aid the development of legged robots with improved mobility on granular surfaces; however, no general force model yet exists for granular media to predict ground reaction forces during complex limb intrusions. Inspired by a recent study of sand-swimming, we develop a resistive force model in the vertical plane for legged locomotion on granular media. We divide an intruder of complex morphology and kinematics, e.g., a bio-inspired robot L-leg rotated through uniform granular media (loosely packed ~ 1 mm diameter poppy seeds), into small segments, and measure stresses as a function of depth, orientation, and direction of motion using a model leg segment. Summation of segmental forces over the intruder predicts the net forces on both an L-leg and a reversed L-leg rotated through granular media with better accuracy than using simple one-dimensional penetration and drag force models. A multi-body dynamic simulation using the resistive force model predicts the speeds of a small legged robot (15 cm, 150 g) moving on granular media using both L-legs and reversed L-legs.