Micro-generator feeds on good vibrations

A sugar-cube-sized electric generator that feeds on environmental vibrationshas been developed. It could power swarms of wireless sensors or even medical implants, researchers claim.
The new micro-generator harvests power electromagnetically, exploiting the wobbling of several magnets attached to a millimetre-sized cantilever. It measures just 7.0 millimetres by 7.0 mm by 8.5 mm, and the team behind it say it is the most efficient micro-generator yet developed.
The generator converts 30% of environmental kinetic energy into electrical power, and could keep all sorts of low-power devices running without batteries – particularly when alternatives like solar power are not an option.
Steve Beeby, an engineer at the University of Southampton, UK, led development of the device. He says it could power devices attached to bridges, large buildings and other structures that experience vibration.
Beeby notes, for example, that small battery-powered accelerometers are already attached to the Golden Gate Bridge in San Francisco, US. These monitor movement of the bridge to help engineers predict structural problems, but their batteries must be recharged regularly.
"Vibration energy harvesting is receiving a considerable amount of interest as a means for powering wireless sensor nodes," Beeby says. "By removing wires and batteries, there is the potential for embedding sensors in previously inaccessible locations."
Shake it all over
To demonstrate a potential application for their micro-generator, Beeby's team used it to power a small wireless accelerometer (see image, right).
A larger version of the same design – about the size of a coffee cup – is already sold by a company called Perpetuum, a spin-off from the Southhampton University research.
That device is used primarily to provide power for wireless sensors attached to oil refineries. The new micro-generator, called Mk2, was developed as part of a wider European project called Vibration Energy Scavenging (VIBES).
Mk2 contains four magnets made of neodymium iron boron, each measuring 1.0 mm by 1.0 mm by 1.5 mm. These are attached to the end of a cantilever a few millimetres long – two on top and two underneath. The cantilever is forked at one end, so that the magnets sit either side of a fixed copper coil, which is wound around a disc.
As the cantilever moves up and down in response to shaking, the magnets move and the magnetic field interacts with the disc to generate electrical power. The magnetic field produced by the arrangement of magnets is compact, meaning it is not easily affected by external magnetic fields.
Playing by heart
In testing with the sort of vibration levels you might expect on a structure such as a bridge, the generator produced up to 46 microwatts of electrical power. Beeby says this is easily enough to power small devices such as wireless environmental sensors.
And while this is not enough to drive anything like an MP3 player or cellphone, he says it could certainly provide enough power for medical implants such as pacemakers – in this case, the beating of the heart providing the necessary vibrations.
The team compared the efficiency of their device to various other energy-harvesting micro-generators, ranging in size from a few cubic centimetres to less than a micron in width. Some of these devices use alternative methods to produce electrical power, exploiting piezoelectric or electrostatic processes instead.
In order to compare efficiencies accurately, the researchers had to create a mathematical graph that accounted for differences in the acceleration experienced. This showed their generator to be the most efficient yet.
"The results obtained are remarkable, obtaining a good voltage output in a very small volume," says Francesc Moll, who researches energy harvesting at the Polytechnic University of Catalonia in Spain. "The device is promising."
But Moll also cautions that such a generator would only be suitable for particularly low-power devices. "In the future, there may be electronic circuits that consume such low power levels, but nowadays the applicability is very limited," he told New Scientist. Another issue, he says, is that the power supplied may not be continuous.