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The article presents the development of a self-powered rectified electromagnetic energy harvester (EMEH) under low frequency excitations. To overcome the drawback of low output voltage across the small optimal load, it is proposed to use the transistor-based rectifier biased by the self-powered SECE-based piezoelectric energy harvester (PEH). In addition, the buck-boost converter controlled by the self-powered SSHI-based PEH is im- plemented for the maximum power point tracking. A semi-analytic model is developed for predicting the peak power and the optimal load used for designing the buck-boost converter. The prediction is then validated by experiment showing 1.5 mW optimal output power. Further, it is found that the 0.22 low rectified voltage is increased up to 2.5 V by the proposed SSHI-based voltage boosting technique. It offers advantages of zero quiescent power dissipation and the ease of tuning the input impedance of the buck-boost converter by varying the SSHI load impedance.

Many biological materials such as bone, teeth and nacre exhibit superior mechanical strength and toughness. These materials share similar hierarchical arrangement that stiff blocks are embedded in a soft matrix. For example, the sophisticated brick-and-mortar design in nacres can effectively increase the fracture toughness by a factor of 3,000 compared to its major component – mineral. Although extensive studies have been done attempting to understand the toughening of the nacre-like materials, the underlying mechanisms associated with crack behaviors are not fully clear yet. This study applies the phase-field method for crack behaviors in both layered structure and nacre-like materials to distinguish the importance of the commonly observed toughening mechanisms. First, we investigate the toughening of a simple layered structure, where the surfing boundary condition is imposed to suppress the crack deflection. We compute the maximum energy release rate using the J-integral technique to find out that the effective fracture toughness is much less than the experimentally measured fracture toughness of these bio- composite materials. Then we investigate the crack growth in a nacre-like material to obtain a phase diagram summarizing four different modes of crack growth: straight crack, interface crack, branching, and crack arrest for a range of structural parameters relevant to nacres, including the aspect ratio, volume fraction of mineral, elastic modulus mismatch and fracture resistance mismatch. Our results clarify the relation between the complex hierarchical microstructure and the toughening mechanisms, such as crack bridging, microcracking and tablet sliding.

The article presents the study of a novel self-powered SECE (synchronized electric charge extraction) circuit for scavenging piezoelectric energy under low-frequency shock excitation. Specifically, the device consists of a piezoelectric cantilever beam whose tip magnet is impulsively excited by a rotating magnet. It is attached to a load-independent SECE circuit with the advantage of power enhancement at low-level output voltage. The proposed circuit includes an envelope detector for voltage detection, a voltage divider for minimizing the switching delay and a diode for recovering the charge on the detection capacitance. The result shows that the predicted harvested power agrees well with the experimental observations. In addition, the power ripples are significantly reduced due to the larger electric-induced damping drawn from the SECE technique. Finally, the load-independent property makes the observed SECE power remain constant within the output voltage operated at around 2-5 volt, and therefore outperforms the standard DC power.

The article investigates various cantilever configurations for energy harvesting from rotational motion. A piezoelectric cantilever beam is mounted radially on a rotating body with different configurations. A harvester of type A (type B) refers to an outward (inward) configuration of beam with the direction of the transverse vibration perpendicular to the rotational axis. On the other hand, a rotary harvester of the type C is an outward beam with the transverse vibration in parallel to the rotational axis. A unified approach based on the Hamiltonian principle is employed and the methodology for deriving the formulations is based on the distributed parameter method. The result shows that both the type A and type C enjoy the feature of passive self-tuning of resonance. But the type C exhibits better ability in tuning than the type A. In addition, the type B shows significant harvested power at the cost of loss of tuning ability

The paper is focuses on the development of a theoretical framework together with an experimental validation to investigate rotational piezoelectric energy harvesting. The proposed device includes an electrically rectified piezoelectric bimorph mounted on a stationary base with a magnet attached to its free end. Energy is harvested by vibration of beam induced by non-contact rotary magnetic plucking. The DC power frequency response is predicted and found to be in good agreement with experiment. It shows that the harvested DC power is around 1 mW in average with the rotational frequency ranging from 5 Hz to 14 Hz. In addition, the parallel connection of two piezoelectric oscillators with respective electrical rectification is considered. It is observed that the power output of the array is the addition of the response from each individual piezoelectric oscillator.

This article proposes a framework for determining the types of nonlinearity observed in the frequency response of microscale energy harvesters made of a piezoelectric film deposited on a stainless-steel substrate. The model accounts for inertial, geometrical and material nonlinearities due to amplified excitation and induced hysteresis. The simulations based on the multiple scale analysis reveals the softening type of nonlinearity for the case of a 15 μm PZT thick film deposited on a 60 μm stainless-steel substrate. They agree quite well with the experimental observations. In addition, the further investigation shows the existence of the critical film thickness such that the hardening (softening) nonlinearity is observed if the film thickness is below (above) this critical value. It is also found that such a key parameter is mainly affected by the ratio of the bending stiffness due to material nonlinearity to that based on linear moduli. Finally, the hardening type of nonlinearity was also observed in different samples with very small film thickness, as predicted by the proposed framework.

In the conventional model of general vibration energy harvesters, the harvesting effect was regarded as only the electrically induced damping. Such intuition has overlooked the detailed dynamic contribution of practical power conditioning circuits. This paper presents an improved impedance model for the electromagnetic energy harvesting (EMEH) system considering the detailed dynamic components, which are introduced by the most extensively used full-wave bridge rectifier. The operation of the power electronics is studied under harmonic excitation. The waveforms, energy cycles, and impedance picture are illustrated for showing more information about the EMEH system. The theoretical prediction on harvesting power can properly describe the changing trend of the experimental result.

This article presents the modeling of nonlinear response of micro piezoelectric energy harvesters under amplified base excitation. The micro transducer is a composite cantilever beam made of the PZT thick film deposited on the stainless-steel substrate. The model is developed based on the Euler-Bernoulli beam theory considering geometric and inertia nonlinearities, and the reduced formulation is derived based on the Hamiltonian variational principle. The harmonic balance method is used to simulate the nonlinear frequency response under various magnitudes of excitation and electric loads. The hardening type of nonlinearity is predicted and is found to be in good agreement with experiment. However, the softening response is also observed in different samples fabricated under different conditions. Such disagreement is under investigation.

This article reports a novel finite element model of piezoelectric energy harvesters accounting for the effect of nonlinear interface circuits. The idea is to replace the energy harvesting circuit in parallel with the parasitic piezoelectric capacitance by an equivalent load impedance. This approach offers many advantages. First, the model itself can be implemented conveniently in commercial finite element softwares. Second, it directly provides system-level designs on the whole without resorting to circuit solvers. Third, the extensions to complicated structures such as array configurations are straightforward. The proposed finite element model is validated by considering the case of an array system endowed with the standard, parallel-/series-SSHI (synchronized switch harvesting on inductor) interfaces. Good agreement is found between simulation results and analytic estimates.

This article reports the modeling of the parallel connection of multiple piezoelectric oscillators with respective electrical rectification. Such an array structure offers advantages of boosting power output and exhibiting broadband energy harvesting. The theoretical estimates are proposed for different choices of electronic interfaces, including the standard and parallel-/series-SSHI (synchronized switch harvesting on inductor) circuits. It is shown that the electrical response is governed by a set of simultaneous nonlinear equations with constraints indicating blocking by rectifiers. Finally, the validation is carried out by circuit simulations and shows good agreement.

The electrical response of multiple piezoelectric oscillators connected in parallel and endowed with various energy harvesting circuits is investigated here. It is based on the idea of equivalent load impedance of piezoelectric capacitance coupled with harvesting circuits. The main result is the matrix formulation of generalized Ohm's law whose impedance matrix is explicitly expressed in terms of load impedance. It is validated numerically through standard circuit simulations.

This article analyzes the electrical behavior of an array of piezoelectric energy harvesters endowed with several interfacing circuits, including the standard AC/DC circuit and parallel/series SSHI (synchronized switch harvesting on inductor) circuits. The harvesters are classified according to the connection to a single or multiple rectifiers. The analytic estimates of harvested power are derived explicitly for different cases. The results show that DC power output changes from the power-boosting mode to the wideband mode according to various degrees of differences in the parameters of harvesters. In particular, the system with multiple rectifiers exhibits more bandwidth improvement than that with a single rectifier. Finally, it is shown that the electrical performance of an SSHI array system enjoys both power boosting and bandwidth improvement.

Advances in piezoelectric energy harvesting have motivated many research efforts to propose suitable electronic interfaces for power optimization. Two kinds of electronic interfaces are currently used in literature. The common one is the standard interface which includes an AC/DC rectifier followed by a filtering capacitance. Another recently emerged one is the 'synchronized switch harvesting on inductor' (SSHI) which is added to the piezoelectric element together with the standard DC technique. It has been shown that the latter has several advantages over the former; however, the effect of frequency deviation from resonance on the electrical behavior of an SSHI system is not taken into account from the original analysis. We here propose several improved estimates for both Parallel- and Series-SSHI interfaces accounting for this effect, and make comparisons between these two. It shows that both Parallel- and Series-SSHI systems exhibit significant improved bandwidth, while the electrical behavior of the Parallel-SSHI (Series-SSHI) system is similar to that of a strongly coupled electromechanical standard system operated at the short (open) circuit resonance.

This article provides an analysis for the performance evaluation of a piezoelectric power harvesting system using either the standard or SSHI electronic interfaces. Instead of using the un-coupled and in-phase assumptions, an analytic expression of harvested power is proposed for the SSHI circuit based on the improved analysis. It is shown that the behavior of an ideal SSHI system is similar to that of strongly coupled electromechanical system using the standard interface operated at the short circuit resonance. In addition, the performance evaluation of a SSHI circuit is classified according to the relative magnitudes of electromechanical coupling coefficient and the mechanical damping ratio. It is found that the best use of the SSHI harvesting circuit is for the system with the medium range of electromechanical coupling. The performance degradation due to the non-perfect voltage inversion is not pronounced in this case, and a new finding shows that the average harvesting power is much less sensitive in frequency compared to that using the standard interface.

We propose a multiscale framework to study the behavior of pressurized shape-memory thin films. These alloy films are typically heterogeneous and contain three length scales including film thickness, grain size and microstructure scale. We show that the effective behavior of shape-memory film exhibits strong size effect due to the interaction among these dimensional and material length scales. In addition, we apply the thin-film theory to predict maximum recoverable deflection for various shape-memory diaphragms with different textures. We find that recoverable deflection is not sensitive to common film textures in Ti-Ni films while it is sensitive in Cu-based shape-memory films. It turns out that Cu-based films may have better behavior than sputtered Ti-Ni films in view of large recoverable deflection. We conclude with comparison with experiment.