Design of a cheetah robot with a bio-inspired robotic tail
In late 2020 , the New York Police Department introduced a new tool for assisting its police officers : a 70-pound , four-legged robot manufactured by Boston Dynamics . The dog-shape robot was designed to climb stairs and use its camera to survey crime scenes for potential dangers , but what garnered public attention — and triggered some alarm — was the unnatural way the robot moved . Its legs moved up and down with understandably mechanical precision while data from gyroscopes fed into the control system to orient the “ body .” The reaction led the NYPD to retire the robot dog after a few months , and even the mayor of New York City admitted that the machine was “ creepy ” and “ alienating .”
Engineers for decades have used natural analogues to guide their designs of robots . In addition to Boston Dynamics ’ s robotic dogs , researchers have built mechanical snakes , fish , insects , and other creatures , while private companies have created many iterations of robots that look somewhat like people . Some technologists believe that quadruped robots have significant advantages over wheeled robots for maneuverability over rough terrain .
Until recently , though , most of these robots have lacked a feature that is found again and again in nature — a tail . Studies of animal locomotion and robots in the laboratory indicate that leaving out tails has been a design drawback . In fact , research conducted by our lab at Virginia Tech has shown that an articulated robotic tail can effectively maneuver and stabilize a quadruped both for static and dynamic locomotion .
Adding an articulated tail to something like the NYPD ’ s robot dog might not only make it seem less “ creepy ,” but also could enable it to move more efficiently and better perform its duties .
Why Tails are Useful
Tails on quadrupeds are used for all sorts of functions auxiliary to the legs . For instance , monkeys use them to grasp branches or to provide balance while walking . Kangaroos often power themselves forward , using their tails as an extra limb . Cheetahs in the wild whip their tails to maneuver at high speed while hunting ; by jumping their rear legs in the air and swinging their tail to one side , conservation of momentum pushes the legs to the opposite side to enable fast changes in body orientation .
When researchers first began to study whether adding tails to legged robots would aid in their locomotion , they used the simplest possible abstraction , essentially a stick attached by a pivot to the chassis . And it worked . Although simple , these robotic tails show their effectiveness in mobile robot locomotion , such as helping the robot adjust its airborne orientation , and helped validate some of the hypotheses of how animals use tails .
But most quadrupeds don ’ t have such simple tails . Instead , most animal tails are evolutionary extensions of the spine and have multiple links , each made from a caudal vertebra . This implies that there must be some advantage to this sort of tail , since a multi-link limb consumes more energy to operate and many animals have the ability to evolve long , rigid structures , such as the deer ’ s antlers or the tail fins on a fish . Moreover , apart from some obvious but specialized uses , such as the prehensile , grasping tail of a monkey , there must be a generalized advantage for articulated tails to be seen in many different species .
Attaching multi-link tails to robots could help researchers learn what those advantages are . Research in this area would benefit both biology ( for providing evidence for the tail hypotheses ) and engineering , since any advantages to adding tails to quadruped robots would enhance their agility and versatility .
From the engineering and dynamics perspective , one advantage for an articulated tail is that it can achieve higher speeds and thus generate higher inertia loading in comparison to a single-link structure . This is because a multi-link tail can rotate the same amount at each joint . Therefore , when a multi-link tail and a single-link tail are both executing a bending motion but given the same amount of rotation at the first joint , a multi-link structure undergoes significantly more overall bending compared to a single-link structure . This capability enables a multi-link tail to achieve overall higher speed than a single-link tail . A multi-link structure will also cover a larger area and will therefore have a larger moment of inertia compared to a single-link structure .
In fact , experiments performed in our lab using a multi-link
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