# STANCE: Locomotion Adaptation over Soft Terrain

## TL;DR

This paper introduces STANCE, a novel soft terrain adaptation algorithm for legged robots that improves stability and performance over unmodeled, compliant terrains by online terrain compliance estimation and optimized whole-body control.

## Contribution

The paper presents STANCE, a new framework combining terrain knowledge and compliance estimation to enable adaptive locomotion over soft terrains, addressing limitations of existing WBC methods.

## Key findings

- Successfully adapted to various terrains with different compliances
- Handled terrain transitions and external disturbances effectively
- Achieved online adaptation without pre-tuning

## Abstract

Whole-body Control (WBC) has emerged as an important framework in locomotion control for legged robots. However, most of WBC frameworks fail to generalize beyond rigid terrains. Legged locomotion over soft terrain is difficult due to the presence of unmodeled contact dynamics that WBCs do not account for. This introduces uncertainty in locomotion and affects the stability and performance of the system. In this paper, we propose a novel soft terrain adaptation algorithm called STANCE: Soft Terrain Adaptation and Compliance Estimation. STANCE consists of a WBC that exploits the knowledge of the terrain to generate an optimal solution that is contact consistent and an online terrain compliance estimator that provides the WBC with terrain knowledge. We validated STANCE both in simulation and experiment on the Hydraulically actuated Quadruped (HyQ) robot, and we compared it against the state of the art WBC. We demonstrated the capabilities of STANCE with multiple terrains of different compliances, aggressive maneuvers, different forward velocities, and external disturbances. STANCE allowed HyQ to adapt online to terrains with different compliances (rigid and soft) without pre-tuning. HyQ was able to successfully deal with the transition between different terrains and showed the ability to differentiate between compliances under each foot.