Engineers at the University of Southampton have made an vibration-powered 50µW AC generator smaller than a sugar cube.

“It is a cantilever-based electromagnetic generator,” researcher Dr Steve Beeby told EW. “The key to its effectiveness is very high power magnets, 12µm diameter wire and tight tolerances.”

Aimed at powering wireless sensing nodes, the mechanics of the device occupy only 150mm sq. which increases to 833mm sq. when packaged.

Extensive modelling and two previous prototype generations have produced a generator which produces enough voltage - 500mV - to feed the following passive rectifier, and resonates at the right frequency. “It is manually adjustable from 47 to 55Hz which suits the vibration we have found on air conditioning units and air compressors.

For example, useful vibration peaks on the compressor which feeds Beeby’s lab were found from 43 to 109Hz, at accelerations of 0.19 to 3.7m/s sq.

Q in the basic generator structure is very high, too high for easy tuning. “Connecting the power electronics has the effect of dropping the Q to give a bandwidth of 1Hz, which is fine from a practical point of view,” said Beeby.

A demonstrator has been built which supplies a university-designed low-power wireless transmitter. “It sends 9-bit peak acceleration data to a receiver,” said Beeby.

The generator was developed with partners as part of the E4.13m EU-funded VIBES (Vibration Energy Scavenging) project.
The University plans to exploit the device through Perpetuum, a vibration harvesting Southampton spin-out formed in 2004.

Technically speaking
Micromachined beryllium copper and stainless steel cantilevers are being use, mono crystalline silicon having proved viable in use, but too brittle for reliable assembly.

The cantilever has a slot in to pass over the coil. All parts of the generator are hand assembled. “Its not the winding that’s the problem, it is connecting to it,” said Beeby.

Four NeFeB magnets are mounted on the cantilever, one above and one below on each side of the coil, and backed by iron pole pieces.

The custom magnets are magnetised and oriented so that the top half of the coil sees north on one side and south on the other, and the bottom half sees the other polarity - maximising flux change in the coil as the cantilever moves.

Shaped tungsten blocks around the magnets increase the moving mass to control resonant frequency.

AC to DC conversion electronics are passive and consist of a Schottky diode Cockcroft-Walton multiplier-rectifier feeding a supercapacitor - the latter chosen for low leakage. “The particular diodes were selected for low leakage and low turn on voltage,” said Beeby.

The mechanics, electromagnetics and electronics were extensively modelled for maximum power output, with 20µW DC available to the load.