Robust bipedal locomotion on flowable slopes via foot-driven terrain manipulation

2026-07-13

AI Summary

This study addresses the instability of bipedal robots on inclined granular media, where foot–ground interactions often induce large terrain deformations and solid-to-fluid phase transitions. To overcome this challenge, the authors propose a limb-centric terrain interaction strategy that employs a toothed foot mechanism with actively adjustable tooth penetration depth. This design optimizes contact force distribution and maintains subsurface stress below the yield threshold, thereby avoiding reliance on torso-based compensation commonly used in prior approaches. Through physical experiments, terradynamics analysis, and cross-scale validation using both 1.4 kg and 15 kg robotic platforms, the method enables robust locomotion on 30-degree granular slopes and demonstrates scalability and generalizability across robot sizes.

Abstract

Bipedal robots are challenging to control because they operate close to instability, where small variations in foot-terrain contact can rapidly destabilize locomotion. On rigid terrain, bipedal robots mitigate this fragility by using well-established contact mechanics and control strategies. On flowable surfaces such as granular slopes, foot contact can induce large surface deformations and solid-fluid-like transitions, coupling terrain effects with robot dynamics, leading to underperformance or failure. This is partly due to the lack of reliable methods to represent the dynamics of flowable terrain, making it difficult to account for terrain effects in locomotion design. Here, we investigate how controlling terrain response can improve bipedal locomotion on granular slopes by studying the terradynamics of cleated feet, thin plates emanating from the foot soles. Systematic studies of a small-scale (1.4 kg) robophysical biped reveal that cleats with sparse and dense spacing lead to excessive terrain yielding and resistance, respectively, degrading performance and leading to failure. An intermediate cleat spacing distributes interaction forces to maintain substrate stresses near (or below) the yield threshold, enabling walking on granular slopes up to 30 degrees. Guided by these principles, we design a foot that actively adjusts cleat depth and accommodates both rigid and granular terrain. We also demonstrate that the principles of effective foot-terrain interaction translate to a larger (15 kg) autonomous biped. Our study presents an alternative to conventional body-centric robot control approaches, which regulate terrain-induced effects through body motion, by instead regulating terrain interactions through limb-centric approach.

Problem

• bipedal locomotion
• flowable terrain
• terrain manipulation
• granular slopes
• foot-terrain interaction

Innovation

• foot-driven terrain manipulation
• bipedal locomotion
• granular media
• cleated feet
• limb-centric control