Tail-assisted pitch control in lizards, robots and dinosaurs

Abstract

In 1969, a palaeontologist proposed that theropod dinosaurs used their tails as dynamic stabilizers during rapid or irregular movements, contributing to their depiction as active and agile predators. Since then the inertia of swinging appendages has been implicated in stabilizing human walking, aiding acrobatic manoeuvres by primates and rodents, and enabling cats to balance on branches. Recent studies on geckos suggest that active tail stabilization occurs during climbing, righting and gliding. By contrast, studies on the effect of lizard tail loss show evidence of a decrease, an increase or no change in performance. Application of a control-theoretic framework could advance our general understanding of inertial appendage use in locomotion. Here we report that lizards control the swing of their tails in a measured manner to redirect angular momentum from their bodies to their tails, stabilizing body attitude in the sagittal plane. We video-recorded Red-Headed Agama lizards (Agama agama) leaping towards a vertical surface by first vaulting onto an obstacle with variable traction to induce a range of perturbations in body angular momentum. To examine a known controlled tail response, we built a lizard-sized robot with an active tail that used sensory feedback to stabilize pitch as it drove off a ramp. Our dynamics model revealed that a body swinging its tail experienced less rotation than a body with a rigid tail, a passively compliant tail or no tail. To compare a range of tails, we calculated tail effectiveness as the amount of tailless body rotation a tail could stabilize. A model Velociraptor mongoliensis supported the initial tail stabilization hypothesis, showing as it did a greater tail effectiveness than the Agama lizards. Leaping lizards show that inertial control of body attitude can advance our understanding of appendage evolution and provide biological inspiration for the next generation of manoeuvrable search-and-rescue robots.

More than a safety line: jump-stabilizing silk of salticids

“…Besides adjusting the centre of mass (CoM) close to the vector of propulsive thrust at take-off to avoid later body rotation [7,8], some jumpers actively use dynamic control for in-air stability. Two mechanisms have previously been proposed to counteract unwanted torque in the air: using the inertia of swinging appendages [9][10][11][12] and aerodynamic forces from flapping wings [6,7]. Take-off mechanisms and stability control that evolved in nature have led to significant progress in bioinspired designs for manoeuvrable jumping robots [11,13].…”

Students’ Perception of Different Learning Options and Use of Authentic Research Papers in a First Year Engineering Course

“…They were both chosen because of a clear IMRD-structure and because they aligned nicely with most of the students building a robot with a specified task in the semester project. The third research paper compares the tail-assisted pitch control in lizards, robots and dinosaurs and was published as a letter in Nature a few years ago [37]. Hence this paper does not follow the normal IMRD-format, but was chosen merely because of its relevance for the students' semester projects and for the stimulating and intriguing topic.…”

On designing an active tail for legged robots: simplifying control via decoupling of control objectives

“…Nonetheless there are some notable exceptions: kangaroo rats and pocket mice have been observed performing precise aerial maneuvering using their tail (Bartholomew and Caswell, 1951;Hickman, 1979). Geckos and other lizards have also been shown to use their tails extensively for aerial posture-control, particularly for controlling body-pitch (Jusufi et al, 2008;Libby et al, 2012) and the concept has been demonstrated to be useful in robotics as well (Johnson et al, 2012;Libby et al, 2012;Liu et al, 2014). Among larger terrestrial mammals, cheetahs appear to use their rather large tails to aid in cornering at high speeds, a concept that has been recently explored in robotics (Briggs et al, 2012;Patel and Braae, 2013).…”