Power packed

Electronic engineers at the University of Southampton have developed a tiny vibration-harvesting generator that is claimed to be 10 times more powerful than its predecessors.

The kinetic energy generator, less than a cubic centimetre in size, is placed on a surface that vibrates, and those vibrations transfer through the generator base to excite the moving elements inside. The magnets move relative to the fixed coil to produce electrical energy generated.

Typical applications include wireless sensor nodes set up around a factory for which it would be impractical to change batteries regularly. In the future they could power medical implants such as pacemakers.

Dr Steve Beeby, reader in electronics at Southampton, led the research, part of the €4.13m (£2.79m) EU-funded Vibration Energy Scavenging (VIBES) project. He also helped found spin-out company Perpetuum, which is marketing larger sized energy- harvesting devices.

The new device is a final demonstrator model that consists of the tiny generator and the electronics that go around it to make a wireless sensor node.

According to Beeby, these generators are much more powerful than their predecessors because of a number of unique design factors. ‘We carried out finite element modelling with our partners at the Tyndall National Institute,’ he said. ‘We also used the most powerful rare-earth magnets that you can get, and wound some very fine but high-density coils round them so there are a large number of turns in our coils, using wire that’s just 12µ in diameter. It’s very well designed and well engineered, so the tolerances are very tight.’

The sensors are suited to wireless condition monitoring sensor networks, where there may be up to hundreds of thousands of sensors that detect the status of machinery. They communicate to form a network, across which they relay their information, which can be used to reduce the maintenance burden.

‘Being wireless, you need localised power supplies, and normally you’d use a battery,’ said Beeby. ‘If you’ve got a large number of these sensor nodes in a factory or an oil refinery, it’s impractical to change batteries frequently.

‘But there are often vibrations present in these environments, so you can simply harvest them to power the microsystem via the generator and never have to change the battery. They are designed to operate for years and, because you don’t need to change the battery, you don’t need access to the sensor, which opens up a number of possibilities for actually embedding the sensor inside structures, machines, or even inside the human body, for medical applications.’

The current device is not designed for medical implant applications, but Perpetuum is working on a related project. The basic principle could be used to power low-power applications such as heart pacemakers.

Another potential application is in transport systems, such as wireless devices in cars, trains and on the airframes of aircraft. However, Beeby cautions that this technology could not be used to power a mobile phone, for example, because there is too much difference between the power consumption and the power generated.

One of the major challenges the team faced was to target particular frequencies for the generator to operate at. ‘One of the key frequencies is 50Hz, which coincides with mains frequency, so a lot of electrical equipment has a characteristic vibration of 50Hz or 100 Hz,’ said Beeby. ‘Once tuned to that frequency, the generators need to get power out from very low levels of vibration. The new system works at up to 6 milli g [six thousandths of the force of gravity], which you can barely feel. Perpetuum’s devices work off just 10 milli g and you’re really struggling to sense that.’

The amount of power a single generator can produce depends directly on the power of vibrations. ‘The more you shrink the generators, the less mass they have, so they’re able to store less energy,’ said Beeby. ‘As you go down in size, you’re struggling to make worthwhile amounts of energy. This device can generate 50 microwatts quite comfortably from about 60 milli g, and there’s quite a lot you can do with that amount of power.’

According to Beeby, the new generator could be on the market within six months once it has been redesigned for manufacture and costs are reduced.

Beeby said the future holds opportunities to refine the design of the generator and associated electronics. ‘As the power consumption of the electronics continues to fall, we could reduce the size of the generator further. We could research the frequency behaviour of the device, making it able to track different frequencies and adjust for them so it can address a wider range of applications.’