Last year, a team at Harvard University released a video demonstration (below) of a robotic fly they had developed, showing it flapping its wings and levitating up a pair of guide wires. But a curious effect might upset their plans.
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 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. "The video showed that the guide wires were vibrating significantly when the wings beat," he told New Scientist.
To find out if these vibrations played a role in the fly's upward motion, his team built a vibrating model "insect" 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
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' tension and the diameters of both the wires and the tubular connectors. The greatest "flight" 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. "Testing hovering using guide wires can produce ambiguous results," he says. Instead, he suggests testing how much of a robofly's 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. "The guide wire demo is used as a dramatic visualisation, rather than quantitative proof of greater-than-weight lift force," says Ronald Fearing at the University of California, Berkeley. And Robert Wood, who led the Harvard team, is confident his robot insect really flies. "We considered this effect while performing our initial take-off experiments. We did replace the wings with rods - and of course did not attain lift-off without the aerodynamic surfaces," he says.
This article originally appeared on New Scientist.
This was first published in August 2009