Smart shoe devices are generally sports shoes that generate power by exploiting different characteristics of human gait. Human gait is the movement of limbs and other body parts that results in locomotion. There are four natural gaits namely walking, jogging, running and sprinting and many specialized gaits generally used in martial arts. Smart shoes have mechanisms that convert the energy lost during natural human gaits into electrical energy. They can store this energy and even power electronic devices.

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Need for smart shoes:

The increase in mobility of human beings has led to the popularity of wireless devices. Wireless devices are becoming smaller, lighter and powerful with time. Many wireless devices available today are wearable which has opened doors for many innovative applications. A hindrance to making wearable devices more useful is lack of a continuous source of energy that is as small and lightweight like these devices. A smart shoe device solves this problem by making use of energy harvesting systems that can fit inside a shoe.

Energy Harvesting:

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Energy harvesting is a process by which small amounts of power can be derived from natural sources and stored for use in small devices with minimal power requirements. There are many methods that can be adopted to harvest energy. An example is the use of pedal power in bicycles. This method has a disadvantage because it demands extra effort to be put in order to generate power. Other methods that do not need extra effort to generate power come under the category of parasitic energy harvesting. Energy harvesting mechanisms used in smart shoes falls into the category of parasitic energy harvesting.

Parasitic energy harvesting in smart shoes:

Parasitic energy harvesting is desirable because it does not require too much resource to be spent. While it is carried out in smart shoes the only expenditure is the energy a human being spends daily during gaits. There are basically three conditions in human gait that can be exploited for parasitic energy harvesting. They are the heel strike while the feet lands on the ground, the weight of human body upon a foot and the swinging of legs.

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A challenge to energy harvesting from human gait is the low excitation frequency. The conditions mentioned above are the sources of excitation for energy harvesters. These conditions occur rather slowly because even during fast gaits the speed is achieved more by increasing the frequency of steps than their frequency. This means that achieving a resonance condition is difficult. The challenge is overcome using frequency up-conversion techniques or non-resonant techniques.

Methods for harvesting energy from smart shoes:

There are different methods to harvest energy from smart shoes. One important constraint is that the device should be small enough to fit inside the shoe and be light enough so that normal gaits of a human being are not disturbed. The most common methods use two technologies – electromagnetism and piezoelectricity.

Piezoelectric Smart Shoe Devices:

Piezoelectric effect is the phenomenon by which electrical energy can be generated from a piezoelectric crystal when mechanical energy is applied on it and vice versa. There are many naturally occurring crystals which demonstrate piezoelectricity that have been in use for a very long time. Artificial sources for generating piezoelectricity have also been developed.

One method to harvest energy from piezoelectricity makes use of the bending of shoe sole. In smart shoes using this method the shoe sole itself is made from a stave of a piezoelectric polymer PVDF (polyvinylidine flouride). The PVDF stave is thin enough to be cut into a sole and placed inside the shoe along with some other elements. The bending of the sole results in piezoelectricity in the PVDF stave and the resultant voltage is captured using silver-inked electrodes.

Alternatively the pressure exerted on the heels can be utilized for harvesting energy. To implement energy harvesting using this method, a flexible ceramic piezoelectric material may be used. Recent innovations in such smart shoes include USB power banks to charge devices like smart watches. An alternative to the ceramic piezoelectric is using polymer piezoelectric materials which have properties similar to the materials used in shoes.

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Electromagnetic generators in Smart Shoes:

Electromagnetic generators have been the dominant source of power in the modern era but their integration in a smart shoe was challenging. The reason is simple and obvious – the size of these generators. This challenge is being overcome with the use of smaller harvesters but their application is limited due to the low magnitudes of power generated.

The harvesters have magnets moving inside coils which results in a generation of voltage by virtue of change in direction of magnetic flux. The advantage of these devices is that they can be designed to exploit both the heel strike and the swinging motion of legs. Two devices, one of them harvesting power from heel strike and the other from the swinging of legs can be used in smart shoes.

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The swinging motion is exploited by using a multi-coil system in which multiple magnets are stacked concentrically with ferromagnetic steel spacers between them. Coils connected in series are placed at equal distances in accordance with the separation between the magnets. While the legs are in motion the magnets move inside the coils causing variation in flux and consequently production of emf.

The downward acceleration produced from heel-strike is exploited by exciting a spring-loaded magnetic device into resonance. Due to the previously mentioned limitation of excitation frequency techniques for frequency up-conversion need to be employed.

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Applications of Smart Shoes:

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Smart shoes find applications in delivering power to the wearable wireless gadgets that are becoming popular. They can offer a consistent power supply and avoid the use of batteries. A good application is indoor navigation in which the module for the same can be embedded in the shoe itself.

Electromagnetic harvesters can generate enough power that can be used for the working of sensors that generally have low power requirements. Piezoelectric harvesters are known to generate significant amount of power. They are known to harvest power to charge smart watches by using USB power banks.