So why not take inspiration from the humble caterpillar? After all, not only can lepidopteran larvae pass through narrow spaces, but some species also have a rapid escape mechanism known as ballistic rolling. The larva of the mother-of-pearl moth (Pleurotya ruralis), for instance, can anchor its rear end, curl up into a wheel, and spring backward like a gymnast to roll away from danger. Such caterpillars can go from flat and stationary to rolling in about 60 milliseconds, reaching speeds about 40 times their ordinary speed of locomotion. But the ballistic roll is a bit uncontrolled, so it is not useful for everyday locomotion.
A group of researchers have now built a robot that does the same thing. In a report published online April 26 in Bioinspiration & Biomimetics, Huai-Ti Lin, Gary Leisk and Barry Trimmer of Tufts University report that their GoQBot mimics two caterpillar modes of locomotion: inching along like a worm or ballistically rolling at comparatively high speeds. (Unlike a caterpillar, the GoQBot rolls forward, not backward.) The group was funded by the DARPA ChemBots program, a military effort to design flexible robots that can gain access to tight spaces.
The 10-centimeter-long robot has a flexible silicone body with embedded shape-memory alloy coils that act as muscles. When resistively heated with electrical pulses, the shape-memory alloy contracts rapidly, curling the bot into a round wheel. Within about 200 milliseconds, the GoQBot can go from a standstill to speeds of more than 50 centimeters per second, rolling itself forward at more than 200 RPM [see video below]. The robot cannot quite compete with the rapid reaction time of real-life caterpillars, though—the GoQBot takes 50 milliseconds after electrical stimulation before initiating any movement at all, whereas a mother-of-pearl moth larva would be almost completely curled into a wheel shape by that time. The Tufts group attributes the lag to the time it takes for the shape-memory coils to heat and tense up.
Like the caterpillars that inspired it, GoQBot can be a bit erratic when it rolls ballistically. But it covers a great deal more ground than it does by inching along and does so with a proportionally smaller expenditure of energy. The researchers propose that robots that can switch between linear and wheel shapes would be able to worm their way through small openings before curling up into a ball to cover more ground in open spaces.
Photo of GoQBot courtesy of Huai-Ti Lin