Energy harvesting knee strap powers devices through walking

Written by: Laura Hopperton | Published:
Energy harvesting knee strap powers devices through walking

Researchers in the UK have developed a device that can power gadgets such as GPS trackers, heart rate monitors and accelerometers by harvesting energy from users' knees.

Created by a team from the universities of Cranfield, Liverpool and Salford, the circular device is designed to fit onto the outside of the knee and consists of an outer ring and a central hub. 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 – 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 2mW of power, but the researchers believe that with a few realistic improvements it could exceed 30mW of power, which could allow new generation GPS tracking, more advanced signal processing and more frequent and longer wireless transmission.

In the 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.

Dr Pozzi continued: "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."

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