Power-generating knee strap hints at end for batteries

June 14, 2012, Institute of Physics

Battery-powered devices could soon be a thing of the past thanks to a group of UK researchers who have created a novel energy harvester to power some of the latest wearable gadgets.

By strapping the energy harvester to the knee joint, a user could power body-monitoring devices such as , pedometers and by simply walking and not have the worry of running out of power and replacing batteries. Soldiers may find this device particularly useful as they often have to carry up to 10kg of power equipment when on foot patrol.

The device has been presented today, 15 June, in journal by researchers from Cranfield University, The University of Liverpool and University of Salford.

The energy harvesting device, which is designed to fit onto the outside of the knee, is circular and consists of an outer ring and . The outer ring rotates as the knee joint goes through a walking motion. The outer ring is fitted with 72 plectra which "pluck" four energy-generating arms attached to the inner hub.

As an individual plectrum deflects off one of the arms – which are called bimorphs – it causes it to vibrate, much 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. Piezoelectric materials have long been used as sensors in SONAR and ultrasound scanners and have recently been the focus of attention in the field of energy harvesting," said lead author of the study Dr Michele Pozzi.

At the moment the device can harvest around two milliwatts of power but the researchers believe that with a few realistic improvements it could exceed 30 milliwatts of power, which could allow new generation GPS tracking, more advanced signal processing and more frequent and longer wireless transmission.

In this study, the energy harvester was tested on a knee motion simulator which reproduced the gait pattern of a human. The researchers were able to accurately control the simulator by examining the intricate detail in the movements of the knee joint by placing a selection of reflective markers on a human subject and using motion capture systems to monitor their walking pattern.

The subject was also fitted with three backpack loads to observe how the knee joint would move under a heavier load.

The knee itself is an ideal starting point for energy generation as it has a large change in angle during walking and does so at significant speeds. A device attached to the joint could therefore generate large amounts of power.

"There is an on-going project looking at manufacturing a more compact and truly wearable harvester. 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," continued Dr Pozzi.

Explore further: Award-winning energy harvester brings practical applications closer

More information: "The pizzicato knee-joint energy harvester: characterisation with biomechanical data and effect of backpack load" Pozzi M et al 2012 Smart Mater. Struct. 7 075023, iopscience.iop.org/0964-1726/21/7/075023

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3.7 / 5 (3) Jun 14, 2012
Soldiers might or might not like this gadget depending on the terrain, the mechanical resistance of the device, and their own physical reserve. You don't get something for nothing.
3.7 / 5 (3) Jun 14, 2012
30 mW isn't exactly enough. To put things into perspective, a cellphone uses 15-30 mW on standby, and around 2 W when you're actually transmitting. Walking around for an hour would give you less than 30 seconds of talk time.

There's no way around physics - you need power to transmit, or it will be very difficult to hear you.
1 / 5 (2) Jun 15, 2012
Well you will get bigger thigh muscles if you do useful work then you wont have to do that particular exercise at the gym :-)
5 / 5 (1) Jun 15, 2012
The thing is, will the weight and the strain of an adequate generator offset the strain and the weight caused by carrying battery packs? I think its important that you should be able to turn the generator in decoupled mode so that you can walk freely if you have enough power.
1 / 5 (1) Jun 15, 2012
They have to make it rugged enough not to break when the solder take a flying dive to the ground when the bullets start flying, waterproof and sand proof. This could add weight and bulk to the device.
5 / 5 (1) Jun 15, 2012
if the load was adjustable this could work similar to regenerative braking when walking downhill. It could take some of the the load of the leg muscles when descending, then be decoupled when going uphill.
5 / 5 (1) Jun 15, 2012
It's a good place to start. Eikka has a point, but the idea is to reduce the amount of weight of the batteries carried in exchange for something that will take less energy to recharge smaller batteries than carrying the original weight would take. The entire system calls for improvements in efficiencies of all components except the batteries, for which progress seems fairly slow, unfortunately. They usually put a large amount of thought into new equipment that they make standard for soldiers, but not always. (Seriously guys, velcro on every pocket? There are times something needs to be retrieved QUIETLY in a hostile environment.)

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