Electro-mechanical and electronic integrated harvester for shoes application

Energy harvesting allows making sensors or transmitters electrically autonomous. Several studies have been proposed but in most cases lack of electrical and mechanical parts integration and practical application purpose. Here, with the aim of supply a Bluetooth step-counter placed in the sole of a training shoe, a complete system is presented. It consists of a magnetoinductive transducer embedded with an electronic interface for power conditioning and exploits only the energy recovered by the impact on the ground. A 27 × 16 mm cylindrical device containing the transducer, the electronic interface, the step-counter electronics and the protective shell is developed. Energy recovery derives from the magnet free oscillation following the shoe impact. The proposed interface exploits pulse width modulation to perform transducer output rectification and to emulate optimal load impedance while charging a storage capacitor. Numerical and experimental analysis show effective optimal resistive load emulation and energy recovery gain of 2 compared to the standard AC-DC interface. Prototypes has been manufactured and tested. Considering a sample foot-step, energy recovery is 360 μJ against 400 μJ dissipated on optimal resistor. Mean energy recovery over a complete run is 644 μJ/foot-step. In spite of variable excitation, energy recovery at each foot-step is larger than the required and allows the step-counter transmitting the information.