Taking steps to power wearable tech

Two shoe-worn devices have shown promise in harnessing power from a person’s walking gait in order to provide power to next-generation, wearable electronics.

The first, dubbed shock harvester, generates power when the heel of a shoe strikes the ground whilst the so-called swing harvester generates power when the foot is swinging. Details of both are published today in Smart Materials and Structures.

Lead author Klevis Ylli, from the Hahn-Schickard-Gesellschaft Institute for Micromachining and Information Technology, told The Engineer that shock harvester was developed as part of the EU project 9DSense, which aimed to enhance the battery life of an indoor navigation system (INS).

‘The INS uses a three-axis accelerometer, gyroscope and magnetic field sensor,’ Ylli said via email. ‘Specific algorithms calculate the path the user has travelled from this sensor data. Rescue units could, for example, track their motion within a building they don’t know on their handheld devices.’

He added that the swing harvester has been developed to power a self-lacing shoe with one possible user group being found amongst the elderly.

‘Sensors detect you have stepped into the shoe and the shoe closes,’ he said. ‘With a more recent version of the harvester…it is expected that within less than one hour of normal walking - at about 4km/h - enough energy would have been stored in a battery to allow one closing process. Future harvester iterations and the optimisation of the closing procedure should further reduce this charging time.’

The energy-harvesting devices generate power by utilizing the motion between magnets and coils: as the magnetic field of a moving magnet passes by a stationary coil, a voltage is induced and an electric current is generated.

In the swing device, the swinging of the foot accelerates a stack of 14 magnets through a set of coils. The shock harvester device is placed in the heel of a shoe and when it strikes the ground a spring-loaded set of magnets begins to vibrate.

Ylli conceded that the durability of both systems is yet to be investigated, but that both were designed with longevity in mind.

He said: ‘The shock harvester, for example, is suspended magnetically and thus contains no magnetic springs that could break. The only mechanical part that is under load is the bearing of the swing.

‘The swing harvester has only one moving part, which is a magnet stack. The only damaging factor can be the friction between the magnet and the plastic channel it moves through, but this is not considered to be a problem. Our best guess is that the devices should be able to withstand the average lifetime of a shoe of a couple of years.’

The swing harvester is 41mm x 70mm 15mm, weighs approximately 25g in one iteration and generates an average power output of 0.84mW. The shock harvester has dimensions of 40mm x 60mm x 20mm, weighs 150g and has been able to generate a maximum of 4.13mW of power when a test subject was travelling at 5km/h on solid ground.

Both were used to power a temperature sensor in a shoe, generating enough power for the temperature readings to be wirelessly transmitted over 10m to a handheld device.

‘As a demonstrator we have added simple power-management electronics to the harvesters which only rectify the voltage and immediately use all available energy from the harvester to measure the temperature and transmit the data wirelessly to a mobile device,’ said Ylli. ‘This was to prove that the immediate power output of the devices suffices for wireless transmissions. An improved choice of geometrical parameters can further improve the power output.’

Looking ahead, the researchers plan to develop the devices further by testing them on a larger set of subjects and optimising the device parameters to fully capitalise on the large amount of power generated by the human walking gait.

If the shoe fits…installing energy harvesting devices into shoes

It is claimed that the shock and swing energy-harvesting devices can fit comfortably into the sole of an ordinary shoe.

Researcher Klevis Ylli, from the Hahn-Schickard-Gesellschaft Institute for Micromachining and Information Technology, said shoes fitted with the devices feel no different to other shoes with a comparatively stiff sole.

He told The Engineer: ‘The mounting into the shoe sole was very basic and the same for both harvesters.

’The sole was removed and an appropriate space created within it. Since a lot of sole material was removed to accommodate the harvester, a few millimeters of sole material were added (in height) to retain the user comfort.

’If the sole had been designed with energy harvesting in mind, an appropriate structure could have been developed from the beginning in order to keep the sole size constant and the same as in normal shoes.

’A partner institute of ours, which also works on the self-lacing shoe project, has used a special 3D printer to print rubber soles in the past. At the moment they are trying to develop a sole that can accommodate the harvester right from the beginning.’