Hybrid leg compliance enables robots to operate with sensorimotor delays and low control update frequencies

TL;DR

This paper introduces a hybrid robotic leg design with physical and virtual compliance that significantly improves robustness against sensorimotor delays and low control frequencies, mimicking animal locomotion stability.

Contribution

The study presents a novel hybrid leg and controller design combining physical compliance with virtual control, enhancing robustness in legged robots under delays and low update rates.

Findings

Robust drop landings from 1.3 leg lengths high with delays up to 60 ms

Successful simulations of landings from 3.8 leg lengths at 100 Hz control

Hybrid design explains animals' robustness compared to robots

Abstract

Animals locomote robustly and agile, albeit significant sensorimotor delays of their nervous system. The sensorimotor control of legged robots is implemented with much higher frequencies-often in the kilohertz range-and sensor and actuator delays in the low millisecond range. But especially at harsh impacts with unknown touch-down timing, legged robots show unstable controller behaviors, while animals are seemingly not impacted. Here we examine this discrepancy and suggest a hybrid robotic leg and controller design. We implemented a physical, parallel joint compliance dimensioned in combination with an active, virtual leg length controller. We present an extensive set of systematic experiments both in computer simulation and hardware. Our hybrid leg and controller design shows previously unseen robustness, in the presence of sensorimotor delays up to 60 ms, or control frequencies as low as 20 Hz, for a drop landing task from 1.3 leg lengths high and with a passive compliance ratio of 0.7. In computer simulations, we report successful drop-landings of the hybrid compliant leg from 3.8 leg lengths (1.2 m) for a 2 kg quadruped robot with 100 Hz control frequency and a sensorimotor delay of 35 ms. The results of our presented hybrid leg design and control provide a further explanation for the performance robustness of animals, and the resulting discrepancy between animals and legged robots.