This study presents a shoe-mounted nonlinear piezoelectric energy harvester (PEH) with intent to capture energy from human walking. The PEH consists of a piezoelectric cantilever beam magnetically coupled to a ferromagnetic ball and a crossbeam. A sleeve is included to guide the travel of the ball. Experimental measurements and theoretical simulations demonstrate that the proposed design can collect energy from diverse excitation sources with different directions produced by the foot, including vibrations, swing motions, and the compressive force. The ball and the crossbeam sense the swing motion and the compressive force, respectively, and then actuate the piezoelectric beam to function. The piezoelectric beam senses the vibration along the tibial axis and generates electricity. The proposed PEH achieves the superposition of these excitations and generates multiple peaks in voltage output within one gait cycle. The output power generated by the fabricated prototype ranges from 0.03 mW to 0.35 mW when the walking velocity varies from 2 km/h to 8 km/h.

This letter presents a monostable piezoelectric energy harvester (PEH) for achieving enhanced energy extraction from low-level excitations. The proposed PEH is realized by introducing symmetric magnetic attraction to a piezoelectric cantilever beam and a pair of stoppers to confine the maximum deflection of the beam. The lumped parameter model of such a system is presented and experimentally validated. Theoretical simulations and experimental measurements demonstrate that the proposed design can bring about a wider operating bandwidth and higher output voltage than the linear PEH. Under a sinusoidal vibration with an amplitude of 3 m/s², a 54% increase in the operating bandwidth and a 253% increase in the magnitude of output power are achieved compared to its linear counterpart. Moreover, the proposed PEH exhibits rich dynamic features, including the tunable operating bandwidth, adjustable voltage and power levels, and softening hysteresis.

Capturing mechanical energy from our living environment is an effective approach for implementing self-sustained electronics. One of the key challenges is how to generate more electricity from the low-frequency and multi-directional excitations. To address this interesting issue, this paper reports a hybrid energy harvester (HEH) that consists of a piezoelectric energy harvester (PEH) and an electromagnetic energy harvester (EMEH), which can scavenge energy from both low-frequency and bi-directional excitations. The PEH is composed of two piezoelectric cantilever beams, each of which can deliver a peak power of approximately 0.13 mW when excited directly by a harmonic vibration of 0.8 g . A spherical magnet is utilized to not only trigger the PEH to function, which endows the PEH with bi-directional energy-harvesting capability, but also induce the coil to generate electricity, achieving the integration of EMEH with PEH. When the magnet is subjected to a sinusoidal excitation of 1.24 g , each beam of the PEH generates a peak power of 0.058 mW at 7 Hz and the coil outputs a peak power of 2.94 mW at 7.2 Hz. The HEH exhibits a better charging performance than the PEH or EMEH for charging a capacitor, and the fabricated HEH prototype can light up tens of light-emitting diodes (LEDs) when excited by hand-shaking vibrations.