A miniature device that harnesses ambient vibrations and converts the energy into electricity has been developed by engineers in the United Kingdom. They claim that the energy-scavenging machine is considerably more efficient than similar devices and capable of generating 10 times more power.

"The motivation is to power wireless devices," says Steve Beeby, an engineer at the University of Southampton, in the United Kingdom. "It's a parasitic energy source." The device, which generates power using the natural vibrations going on around it, could be embedded in sensors in bridges or the airframes of planes. It would be particularly useful in situations in which it would be difficult to access power wires or replace batteries.

The device, which measures less than a cubic centimeter, has shown that it can generate 46 microwatts when vibrated at just 52 hertz. This is typical of the kinds of vibrations found at an industrial plant, and it would be enough power to run a device like a temperature or pressure sensor.

It's not the first time such devices have been made. Larger cup-size commercial generators that monitor equipment in industrial plants or in oil refineries are being produced by some companies, including Perpetuum, a spinout firm from the Southampton lab. Similarly, some wristwatches are now powered by the movement of the wearer's hand or by heat from his or her skin.

But making small generators capable of creating significant amounts of power has been problematic, partly because electromagnetic devices don't scale well, says Beeby. "The smaller you go, the less power you get."

"The biggest challenge is that the power levels are very small," says Eric Yeatman, an engineer at Imperial College London, in the United Kingdom, who works on similar devices. One reason is that the amount of motion being harvested and the available frequencies of the vibrations tend to be low, which means the available energy is pretty limited to start with.

Beeby's device works by having four small, high-performance magnets made out of neodymium iron boron attached to the end of a springy cantilever. The magnets are arranged around a fixed disc-like ring made of coiled copper wire. When the device is shaken, the cantilever oscillates, causing the magnets to move backward and forward across the coil. Their fluctuating magnetic fields induce an electrical current in the coils in much the same way that regular electrical generators work.

Other researchers have tried similar approaches in the past but struggled to generate decent amounts of power, says Yeatman. "Previously, they have tended to have quite low output voltages," he says.

Beeby says he and his team solved this problem by making their coil out of extremely narrow copper wire measuring just 12 thousandths of a millimeter in diameter. They are able to wind it very tightly, squeezing 2,300 turns onto a coil just 2.5 millimeters in diameter. The voltage output of generators is very much dependent on the number of turns in a coil, says Beeby.