Something in the air

New renewable power sources for electronic circuits are to be developed by using background vibrations to power wireless sensors.

Bristol University researchers will investigate how to improve the performance of vibration-powered generators by using resonant systems with non-linear elements.

Current generators usually only operate at one frequency, but incorporating non-linear elements has the potential to make the generator work over a wide range of frequencies.

Vibration-powered generators use a resonant mass on the end of a spring to amplify small, ambient vibrations to a useable level. The amplitude of vibrations is dependent on the application but may only be tenths of a millimetre — this must be amplified to several millimetres to extract power.

Current designs of vibration-powered generators only respond to vibrations at a particular frequency, in the same way that a guitar string produces a single note. If the frequency of the vibration source changes, the generator can no longer produce power. An example of this would be vibrations from a car engine.

The team plans to explore ways in which to extend the spectrum of frequencies — the bandwidth — that a device can be configured to respond to, which would increase the number of uses for the technology. Other types of energy-harvesting technologies suitable for low power levels include solar, heat differential, pressure difference and sound waves.



Stumbling block

According to the researchers, although a great deal of progress has been made in finding renewable power sources at high levels of power, such as wind and solar, there have been fewer developments in renewable power sources for low power levels.

‘The powering of small electrical devices is a stumbling block at the moment,’ said Dr Stephen Burrow, who specialises in low power electronics at Bristol.

‘We’re trying to provide an alternative to batteries at very low power and in doing that we’re hoping to enable technology that will power what are known as distributed electrical systems.’

Power can be extracted from vibrations in the environment, such as mains-powered equipment on a small scale or on a larger scale, from a helicopter in flight. The extracted power then has the potential to be used in place of a battery.

‘We have miniaturised lots of technology and we have a good, reliable wireless link,’ said Burrow. ‘But if you think of your mobile phone or your laptop, the limiting factor is the battery. So we are trying to address that through harvesting energy.’

In many environments there are sufficient vibrations to generate powers of up to a few milliwatts, without the generator becoming too bulky, and Burrow and his team hope to ‘harvest’ low levels of energy such as this to power wireless sensors for a range of applications.

A typical use of wireless sensors would be to monitor aspects of the human body, such as temperature or blood pressure, or to observe changes in the conditions of a large building.

The advantages of a wireless system include a reduction in the number of cables required, which allows for a higher number of sensors measuring different bodily functions to be attached to a person.

A large number of sensors could also be incorporated within the structure of a building during construction.

These sensors could detect and pinpoint where unusual stress is occuring, making it easier for engineers to identify and resolve problematic structures.

To further illustrate this application, to determine if an aircraft is suffering corrosion, it will usually be periodically stripped and closely examined. If there were wireless sensors in the body of the aircraft, however, the time and economic costs of stripping an entire aircraft could be avoided and problems identified at an early stage.



Advantages

Burrow also compared the advantages of using renewable energy such as that extracted from vibrations, with batteries that traditionally power small wireless electronic circuits.

‘Often batteries can be smaller and lighter than the energy harvesting solution,’ he said. ‘But of course, the latter has the potential to last forever, and doesn’t have either dangerous or polluting chemicals, and there are no disposal issues at the end.’

Due to the nature of these uses, vibration-powered generators need to be small. Yet size is a problem because a small generator may not produce sufficient energy to power a device, and this is something the researchers plan to rectify.

The government-funded research will start in September and is expected to be completed by August 2010.

Anh Nguyen