Energy harvester could reduce the battery burden on soldiers

A new body-worn energy harvester could reduce the need for soldiers to carry large batteries to power their equipment.

Researchers from Cranfield, Liverpool and Salford universities have developed a device that is strapped to the outside of the knee and uses piezoelectric material to generate electricity from the user’s walking movement.

The device can harvest around two milliwatts of power, enough to supply body monitoring devices such as heart rate monitors, pedometers and accelerometers with energy.

But the researchers believe this could easily be increased to 30 milliwatts, allowing it to power new-generation GPS devices, more advanced signal processing and more frequent and longer wireless transmission.

The circular device consists of a central hub and an outer ring, which rotates as the knee joint goes through a walking motion, and is fitted with 72 plectra that ‘pluck’ four energy-generating arms attached to the inner hub.

As each plectrum hits one of the arms — known as bimorphs — it causes it to vibrate like a guitar string and generates the electrical energy.

‘A bimorph is a type of piezoelectric device capable of converting mechanical energy, such as the vibrating caused by the plectra, into electrical energy, and vice versa,’ said Cranfield’s Dr Michele Pozzi, lead author of a paper on the research.

The energy harvester was tested on a knee motion simulator that reproduced the gait pattern of a human, using data from a motion capture system that tracked markers on a human subject wearing different backpack loads to simulate different conditions.

The knee is particularly suited to energy generation because there is a large change in its angle during walking, which occurs at significant speeds, and so a device attached to the joint can generate relatively large amounts of power.

‘There is an ongoing project looking at manufacturing a more compact and truly wearable harvester,’ said Pozzi.

‘At the moment, we are using precise but cost-effective manufacturing techniques for the plectra and casing and anticipate that remaining parts will be moulded industrially, slashing the cost. I’d put a cost tag of less than £10 for each harvester on a large-scale production.’

The research, published in the Institute of Physics’ Smart Materials and Structures journal, was initially funded by the EPSRC and the government’s Defence Science and Technology Laboratory (DSTL) as part of a joint project to reduce the heavy battery burden on dismounted soldiers.