Hybrid Parallel Compliance Allows Robots to Operate With Sensorimotor Delays and Low Control Frequencies

Abstract

Animals locomote robustly and agile, albeit significant sensorimotor delays of their nervous system and the harsh loading conditions resulting from repeated, high-frequent impacts. The engineered sensorimotor control in legged robots is implemented with high control frequencies, often in the kilohertz range. Consequently, robot sensors and actuators can be polled within a few milliseconds. However, especially at harsh impacts with unknown touch-down timing, controllers of legged robots can become unstable, while animals are seemingly not affected. We examine this discrepancy and suggest and implement a hybrid system consisting of a parallel compliant leg joint with varying amounts of passive stiffness and a virtual leg length controller. We present systematic experiments both in computer simulation and robot hardware. Our system 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 compliance ratio (fraction of physical stiffness of the sum of virtual and physical stiffness) of 0.7. In computer simulations, we report successful drop-landings 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.

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

“…Software online tuning of stiffness and damping has been realized 51,52 , but relies on precise sensing, high-frequency control and strong actuation. Virtual feedback impedance control 53,54 combined with physical springs and dampers provide software control flexibility and fast physical response 5 . With these improvements, we can readily implement controllers and hardware for versatile and robust locomotion in natural terrains such as gravel.…”

“…However, animals' neurotransmission delays slow down sensorimotor information propagation 2 , rendering a neuronal response impossible for as much as 5 to 40% of the stance phase duration, depending on the animal size 1 . How animals are able to produce and maintain highly dynamic movements despite delayed sensorimotor information is, therefore, a central question in neuroscience and biorobotics 1,[3][4][5] .…”

BirdBot achieves energy-efficient gait with minimal control using avian-inspired leg clutching

“…Although this approach achieves smooth and stable gaits, it relies on precise and high-frequency sensor feedback to detect contact and impact events (18,21). Paradoxically, existing robots remain clumsy compared with animals, despite using appreciable faster sensing-and information-transfer rates (22). Sensor data can be noisy, making it hard to reliably detect contact events, particularly in unstructured terrain (20,21).…”

“…However, the robustness and agility of legged robots remain limited. Paradoxically, animals vastly outperform current robots despite considerably slower sensing and information transfer rates ( 10 , 21 , 22 ).…”

Editorial: Biological and Robotic Inter-Limb Coordination

“…Discrete impact with the ground is a major factor of instability during legged locomotion due to their unknown timing and impact magnitude. Ashtiani et al (2021) examined the effect of the combination of passive and active compliance on leg control during the landing event. Simulation and experiment with a single leg robot, followed by simulation with a quadruped robot were conducted.…”