CREATING a free-flying robotic insect is the dearest wish of many an engineer because such a machine would have great potential in surveillance and in seeking out trapped people in search-and-rescue situations. But a curious effect might upset their plans.
Last year, a team at Harvard University released a of a robotic fly they had developed, showing it flapping its wings and levitating up a pair of guide wires.
But Michele Milano of Arizona State University in Tempe wondered whether the wing motion was entirely responsible for giving the robot lift, or whether some other force was involved. 鈥淭he video showed that the guide wires were vibrating significantly when the wings beat,鈥� he told 快猫短视频.
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To find out if these vibrations played a role in the fly鈥檚 upward motion, his team built a vibrating model 鈥渋nsect鈥� with no wings. The balsa-wood contraption consisted of a motor with an off-centre weight on its spindle that produced vibrations, and four metal tubes through which vertical guide wires were threaded (see Diagram). When they set the motor running, the team discovered that the model despite having no wings. They鈥檝e dubbed it the 鈥渇lying brick鈥�.
The researchers suspect that the vibrating motor sets off travelling waves in the guide wires, rather like those produced by plucking guitar strings. Each vibration cycle produces a kink in the wires above the model, which forces the model to travel upwards. Movements of up to 5 centimetres were seen, depending on the wires鈥� tension and the diameters of both the wires and the tubular connectors. The greatest 鈥渇light鈥� effect was achieved when the vibration frequency matched the resonant frequency of the wires (IEEE Transactions in Robotics, vol 25, p 426).
Milano says his experiment is a warning to roboticists. 鈥淭esting hovering using guide wires can produce ambiguous results,鈥� he says. Instead, he suggests testing how much of a robofly鈥檚 lift is due to the wings by replacing them with stiff non-aerodynamic rods of equal mass.
However, roboticists say wire-guided flight is not a definitive lift test. 鈥淭he guide wire demo is used as a dramatic visualisation, rather than quantitative proof of greater-than-weight lift force,鈥� says Ronald Fearing at the University of California, Berkeley. And Robert Wood, who led the Harvard team, is confident his robot insect really flies. 鈥淲e considered this effect while performing our initial take-off experiments. We did replace the wings with rods 鈥� and of course did not attain lift-off without the aerodynamic surfaces,鈥� he says.
