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Jellyfish: an unexpected model for flying robots
Hovering has proven to be a challenge for tiny robots. Efforts over the last two decades have taken the biomimetic approach- imitating wing movement of insects and birds to achieve hovering. Leif Ristroph and Stephen Childress at the New York University have designed a tiny robot, a first of its kind, which weighs just 2 grams, is 4 inches wide and flies by floating like a jellyfish. They designed, constructed, and tested a prototype that hovers using four wings that flap in-and-out similar to a jellyfish swimming in water, as opposed to the motions of insects or birds flying.
Ornithopters, or flapping-wing air crafts, have been conventionally modeled on wing movements of bees, hummingbirds, fruit flies, and moths. The main challenge is that flapping wings are intrinsically unstable. Insects and birds have compensatory sensory-motor systems to keep them upright during hovering, a system that man-made robots lack. State-of-the-art ornithopters are stabilized by additional feedback controls systems or additional tails or sail-like appendages that act as aerodynamic dampers to overcome intrinsic instability. Ristroph and Childress’ robot, however, keeps itself right-side-up during flight using flapping wings alone, without relying on additional aerodynamic surfaces and without feedback control.
Out of the four wings, two wings opposite each other flap simultaneously and the other pair stays out of phase by a quarter cycle. Using high-speed video recordings and motion tracking, the researchers have shown that the robot orientation is stable during ascending, forward and hovering flight modes. These recordings have enabled scientists to develop mathematical models that reveal the aerodynamic basis of stability of these in-out wing-flapping robots. The mathematical models suggest that, unlike the back-and-forth wing motions used in most robots, the in-out wing flapping provides the strong body motion damping needed for stability. The results, published in the Journal of the Royal Society Interface, show that the next-generation of robots might utilize flapping-flight strategies beyond those that directly mimic the back-and-forth wing motions of flying insects and birds.
The prototype robots reported in this study are not autonomous – their wings are connected to a power source via a wire, and hence these robots do not fly entirely on their own. The next stage of development includes autonomous robots and scale-down miniaturization. Such next-gen hovering robots would have potential applications in surveillance, search-and-rescue missions, environmental monitoring of air quality and pollution, amongst many other areas.