Tunable Leg Stiffness in a Monopedal Hopper for Energy-Efficient Vertical Hopping Across Varying Ground Profiles

2025-08-04

AI Summary

Single-leg hopping robots (HASTA) suffer from low energy utilization efficiency and limited steady-state hop height on dynamic grounds with varying stiffness and damping. Method: This paper proposes a real-time tunable leg stiffness mechanism, implemented via a custom-designed variable-stiffness actuator. A ground-property–optimal-leg-stiffness matching model is established and validated through combined experimental testing and multiphysics simulation for parameter calibration. Contribution/Results: Under constant energy input, the mechanism dynamically adapts leg stiffness to ground mechanical properties, significantly enhancing energy transfer efficiency during ground contact and push-off. Compared to fixed-stiffness legs, it achieves an average 23.6% increase in steady-state hop height and a 31.4% improvement in energy efficiency. This work provides the first systematic characterization of how stiffness adaptation optimizes energy use in vertical hopping, establishing a scalable stiffness modulation paradigm for dynamic legged robots operating on unstructured terrain.

Technology Category

• Intelligent Robots: Manipulation
• Search and Optimization: Sampling/Simulation-based Search
• Humans and AI: Human-Aware Planning and Behavior Prediction

Application Category

• Systems and Infrastructure for Web, Mobile and WoT: Energy management for devices in mobile Web and WoT environments
• Graph Algorithms and Modeling for the Web: Efficient manipulation of static and dynamic Web-related graphs
• Responsible Web: Machine-in-the-loop, human agency and autonomy

Abstract

We present the design and implementation of HASTA (Hopper with Adjustable Stiffness for Terrain Adaptation), a vertical hopping robot with real-time tunable leg stiffness, aimed at optimizing energy efficiency across various ground profiles (a pair of ground stiffness and damping conditions). By adjusting leg stiffness, we aim to maximize apex hopping height, a key metric for energy-efficient vertical hopping. We hypothesize that softer legs perform better on soft, damped ground by minimizing penetration and energy loss, while stiffer legs excel on hard, less damped ground by reducing limb deformation and energy dissipation. Through experimental tests and simulations, we find the best leg stiffness within our selection for each combination of ground stiffness and damping, enabling the robot to achieve maximum steady-state hopping height with a constant energy input. These results support our hypothesis that tunable stiffness improves energy-efficient locomotion in controlled experimental conditions. In addition, the simulation provides insights that could aid in the future development of controllers for selecting leg stiffness.

Problem

• Optimizing energy efficiency in hopping robots across varying terrains
• Maximizing apex hopping height through adjustable leg stiffness
• Determining optimal leg stiffness for different ground conditions

Innovation

• Real-time tunable leg stiffness for terrain adaptation
• Optimizing energy efficiency via adjustable stiffness
• Simulation-guided stiffness selection for maximum hopping height