The power of magnetism

UK researchers are developing a more efficient and sensitive power source for wireless sensors activated by magnetic fields, for use inside engines, buildings or the human body.

Wireless sensors can already scavenge power from a magnetic field but the team’s new combination of materials could provide a bigger electrical supply in a weaker magnetic field from further away. The researchers from Cranfield and Heriot-Watt universities plan to couple magnetostrictive with piezoelectric material in a sandwich. Magnetostrictive materials subtly change shape in a magnetic field and if attached will cause a strain in the adjacent piezoelectric material that generates a current.

The micro-power source for use on micro-electronic chips could be as small as 100 microns and generate over 1V of output, said Dr. Paul Record, lecturer in the department of electrical, electronic and computer engineering at Heriot-Watt. ‘Wireless sensors can be distributed anywhere you want, be it in bodies, engines or buildings.’

The team’s power source could significantly improve the sensors’ efficiency and range. ‘Magnetic information storage is another area we could move into. It’s certainly applicable.’ The researchers also believe aerospace or military applications could be explored, such as sensing the presence of explosive mines from a further distance.

The process involves growing films of magnetostrictive nickel ferrite and piezoelectric lead zirconate simultaneously on to a silicon substrate, rather than simply glueing them in a sandwich, which only works on a large scale. By applying different magnetic fields during manufacture, the materials are anisotropic, which allows them to produce power from any orientation of magnetic field applied to the sensor.

Conventional wireless sensors can already be powered by magnetic fields but use tiny coils, which have to be oriented in the right direction to the flux lines to maximise efficiency, and so more than one coil may be needed.

‘RFID tags only work very close up to the source in a particular field direction, and any ferrous metallic object close by distorts the field so they stop functioning,’ Record said. ‘We can make magnetic field sensors sensitive to smaller fields operated from a further distance.’ The Earth’s magnetic field is, however, too weak to power sensors in the near future, he added.