Recently, embedded systems and wireless sensor nodes have been gaining importance. For operating these devices several vibration-based energy harvesters have been successfully developed and reported, such as piezoelectric, electromagnetic, and electrostatic energy harvesters (EEHs). This paper presents the state-of-the-art in the field of vibration-based EEHs. Mainly, two types of EEHs, electret-free and electret-based, are reported in the literature. The developed EEHs are mostly of the centimeter scale. These energy harvesters, with resonant frequencies ranging from 2 Hz to 1.7 kHz, when subjected to excitation on the order of 0.25 g to 14.2 g, generate power that ranges from 0.46 nW to 2.1 mW.

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This paper presents the modeling, simulation, fabrication and experimental results of a vibration-based electromagnetic power generator (EMPG). A novel, low-cost, one-mask technique is used to fabricate the planar coils and the planar spring. This fabrication technique can provide an alternative for processes such as lithographie galvanoformung abformung (LIGA) or SU-8 molding and MEMS electroplating. Commercially available copper foils of 20 μm and 350 μm thicknesses are used for the planar coils and planar spring, respectively. The design with planar coils on either side of the magnets provides enhanced power generation for the same footprint of the device. The harvester's overall volume is 1 cm 3 . Excitation of the EMPG, at the fundamental frequency of 371 Hz, base acceleration of 13.5 g and base amplitude of 24.4 μm, yields an open circuit voltage of 60.1 mV, as well as 46.3 mV load voltage and 10.7 μW power for a 100 load resistance. At a matching impedance of 7.5 the device produced a maximum power of 23.56 μW and a power density of 23.56 μW cm −3 . The simulations based on the analytical model of the device show good agreement with the experimental results.

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For health monitoring of bridges, wireless acceleration sensor nodes (WASNs) are normally used. In bridge environment, several forms of energy are available for operating WASNs that include wind, solar, acoustic, and vibration energy. However, only bridge vibration has the tendency to be utilized for embedded WASNs application in bridge structures. This paper reports on the recent advancements in the area of vibration energy harvesters (VEHs) utilizing bridge oscillations. The bridge vibration is narrowband (1 to 40 Hz) with low acceleration levels (0.01 to 3.8 g). For utilization of bridge vibration, electromagnetic based vibration energy harvesters (EM-VEHs) and piezoelectric based vibration energy harvesters (PE-VEHs) have been developed. The power generation of the reported EM-VEHs is in the range from 0.7 to 1450000 μW. However, the power production by the developed PE-VEHs ranges from 0.6 to 7700 μW. The overall size of most of the bridge VEHs is quite comparable and is in mesoscale. The resonant frequencies of EM-VEHs are on the lower side (0.13 to 27 Hz) in comparison to PE-VEHs (1 to 120 Hz). The power densities reported for these bridge VEHs range from 0.01 to 9539.5 μW/cm3and are quite enough to operate most of the commercial WASNs.

Wireless sensor nodes (WSNs) and embedded microsystems have recently gained tremendous traction from researchers due to their vast sensing and monitoring applications in various fields including healthcare, academic, finance, environment, military, agriculture, retail, and consumer electronics. An essential requirement for the sustainable operation of WSN is the presence of an uninterrupted power supply; which is currently obtained from electrochemical batteries that suffer from limited life cycles and are associated with serious environmental hazards. An alternative to replacing batteries of WSNs; either the direct replacement or to facilitate battery regular recharging, is by looking into energy harvesting for its sustainable drive. Energy harvesting is a technique by which ambient energy can be converted into useful electricity, particularly for low-power WSNs and consumer electronics. In particular, vibration-based energy harvesting has been a key focus area, due to the abundant availability of vibration-based energy sources that can be easily harvested. In vibration-based energy harvesters (VEHs), different optimization techniques and design considerations are taken in order to broaden the operation frequency range through multi-resonant states, increase multi-degree-of-freedom, provide nonlinear characteristics, and implement the hybrid conversion. This comprehensive review summarizes recent developments in VEHs with a focus on piezoelectric, electromagnetic, and hybrid piezoelectric-electromagnetic energy harvesters. Various vibration and motion-induced energy harvesting prototypes have been reviewed and discussed in detail with respect to device architecture, conversion mechanism, performance parameters, and implementation. Overall sizes of most of the reported piezoelectric energy harvesters are in the millimeter to centimeter scales, with resonant frequencies in the range of 2-13 900 Hz. Maximum energy conversion for electromagnetic energy harvesters can potentially reach up to 778.01 μW/cm 3. The power produced by the reported hybrid energy harvesters (HEHs) is in the range of 35.43-4900 μW. Due to the combined piezoelectric-electromagnetic energy conversion in HEHs, these systems are capable of producing the highest power densities.

For portable and embedded smart, wireless electronic systems, energy harvesting from the ambient energy sources has gained immense interest in recent years. Several ambient energies exist in the environment of wireless sensor nodes (WSNs) that include thermal, solar, vibration and acoustic energy. This paper presents the recent development in the field of acoustic energy harvesters (AEHs). AEHs convert the acoustic energy into useful electrical energy for the operation of autonomous wireless sensors. Mainly, two types of AEHs (electromagnetic and piezoelectric based) have been developed and reported in literature. The power produced by the reported piezoelectric AEHs ranges from 0.68 pW to 30 mW; however, the power generation of the developed electromagnetic AEHs is in the range of 1.5–1.96 mW. The overall size of most of the developed piezoelectric and electromagnetic AEHs are quite comparable and in millimeter scale. The resonant frequencies of electromagnetic AEHs are on the lower side (143–470 Hz), than that of piezoelectric AEHs (146 Hz–16.7 kHz).

Advancement in technology has reduced the size and power consumption of wireless sensor nodes (WSNs), which leads to the possibility of a battery's replacement with alternative power sources, such as energy harvesters. For WSNs, harvesting energy from ambient vibration has great promise. This paper reports on the recent advancements in the development of vibration-based, non-resonant energy harvesters (NR-EHs). Based on the transduction mechanism, non-resonant electromagnetic energy harvesters (NR-EMEHs), non-resonant electrostatic energy harvesters (NR-ESEHs), and non-resonant piezoelectric energy harvesters (NR-PEEHs) have been successfully developed and reported. The frequency band of NR-EMEHs, NR-ESEHs, and NR-PEEHs is 0.5 to 140 Hz, 85 to 100 Hz, and 5 to 120 Hz, respectively. Moreover, these NR-EHs are subjected from low to high acceleration levels (0.102 to 16.1 g) during characterization. The overall size of the produced NR-EHs is in meso scale. The power generation of the reported NR-EMEHs is in the range from 0.75 to 2200 μW. NR-ESEHs are reported to produce power from 0.7 to 35.3 μW; however, the power production by the developed NR-PEEHs ranges from 3 to 18.5 μW. These NR-EHs are shown to produce power densities from 0.01103 to 8461.54 μW/cm3 which are reasonably sufficient to operate most of the commercially available wireless acceleration sensor nodes.