Enabling Dynamic Behaviors With Aerodynamic Drag in Lightweight Tails

Many agile legged animals employ lightweight, furry tails to regulate orientation during running, leaping, and turning. Most robots attempting the same tasks either lack a tail or employ one with high inertia, which can induce impractical payload and energy costs. Inspired by nature's solution to this tradeoff, we explore the use of aerodynamic drag tails in reorientation tasks. In this article, we present a model of the aerodynamic drag and from this derive a metric that allows for direct comparison between aerodynamic and inertial tails. Motivated by this model, we construct a tail to maximize this effectiveness while minimizing inertia. We demonstrate the utility of this tail for two dynamic behaviors executed on a quadrupedal robot. First, in aerial reorientation the robot achieves a 90^° rotation within one body length of fall at the same performance as an inertial tail but with just 37% of the normalized inertia. Second, the forward acceleration of the robot is improved by 12% despite increasing the system mass by 10% over a tailless version. These results show that aerodynamic drag can provide significant control authority for a robot while decreasing the payload and energy cost.

I. Introduction

Terrestrial animals often use tails to help a wide variety of behaviors that are traditionally challenging for robots. Agama lizards use their long, heavy tails to reorient in mid-air after unexpected foot slip, a failure mode which is often fatal for legged robots [1], [2]. Kangaroos use their tails for stability when hopping and even support themselves with their tails while walking [3], [4]. Although humans do not have tails, we swing our arms to the same effect while walking and running, increasing both lateral balance and energy efficiency [5].