Slack-based tunable damping leads to a trade-off between robustness and efficiency in legged locomotion

TL;DR

This paper introduces a tunable slack damper system for legged robots that enhances robustness to terrain perturbations by auto-engaging damping, revealing a trade-off between robustness and energy efficiency, inspired by animal tendons.

Contribution

The paper presents a novel adaptive slack damper mechanism that improves perturbation recovery in legged robots, offering insights into biological tendon functions.

Findings

Slack damper improves perturbation recovery by up to 170%.

Damping increases energetic cost by 27%.

Auto-engagement of damping enhances robustness.

Abstract

Animals run robustly in diverse terrain. This locomotion robustness is puzzling because axon conduction velocity is limited to a few ten meters per second. If reflex loops deliver sensory information with significant delays, one would expect a destabilizing effect on sensorimotor control. Hence, an alternative explanation describes a hierarchical structure of low-level adaptive mechanics and high-level sensorimotor control to help mitigate the effects of transmission delays. Motivated by the concept of an adaptive mechanism triggering an immediate response, we developed a tunable physical damper system. Our mechanism combines a tendon with adjustable slackness connected to a physical damper. The slack damper allows adjustment of damping force, onset timing, effective stroke, and energy dissipation. We characterize the slack damper mechanism mounted to a legged robot controlled in open-loop mode. The robot hops vertically and planar over varying terrains and perturbations. During forward hopping, slack-based damping improves faster perturbation recovery (up to 170%) at higher energetic cost (27%). The tunable slack mechanism auto-engages the damper during perturbations, leading to a perturbation-trigger damping, improving robustness at minimum energetic cost. With the results from the slack damper mechanism, we propose a new functional interpretation of animals' redundant muscle tendons as tunable dampers.