Robust legged locomotion on granular terrain

Develop robust locomotion methods for legged robots operating on deformable granular terrain, where complex, nonlinear contact forces make balancing and walking substantially harder than on rigid ground.

Background

The paper identifies deformable granular terrain—including sand, gravel, and loose soil—as a particularly challenging environment for field-deployed legged systems. Granular media can behave as either solids or fluids depending on loading conditions, producing difficult-to-model contact forces, sinkage, and foot slip.

The paper addresses this challenge with a three-dimensional resistive force theory contact model and a terrain-adaptive teacher-student reinforcement-learning controller. The reported simulation and hardware results demonstrate progress, but the broader problem of reliable legged locomotion on granular terrain remains unresolved.

References

Locomotion on granular terrain therefore remains a significant open problem for legged robots.

— Learning Terrain-Adaptive Humanoid Locomotion on Granular Terrain  (2609.10286 - Kamohara et al., 9 Sep 2026) in Section I, Introduction

We conjecture that this is induced by the terrain encoder that estiamtes terrain stiffness, and baseline PPO without terrain estimation learns energy consuming gait that works for any terrain condition.

— Learning Terrain-Adaptive Humanoid Locomotion on Granular Terrain  (2609.10286 - Kamohara et al., 9 Sep 2026) in Section V-B, Hardware Experiments

However, despite recent progress in agile locomotion, perception, and navigation for terrestrial quadrupeds, a gap remains in understanding which improvements are critical for deployment on the Moon. In particular, open questions remain in how learning-based locomotion policies should account for regolith interaction, how foot designs could affect traction, sinkage, and dust generation, and how perception and navigation stacks should handle low-texture terrain, strong shadows, and changing illumination.

— Toward Lunar Legged Robots: Field Deployment Lessons at LUNA  (2610.12276 - Fuhrer et al., 8 Oct 2026) in Section I, INTRODUCTION