This paper presents analytical models for studying the transient behavior of several power harvesting circuit topologies for use with piezoelectric bending transducers. Specifically, the problem of charging a large storage capacitor, which is inherently a time-varying process, is considered. Three circuit designs are studied-direct charging, synchronized switching and discharging to a storage capacitor, and synchronized switching and discharging to a storage capacitor through an inductor (SSDCI)-and they are compared to a matched resistive load case. Analytical models are developed for these cases to predict the charging rates and output power for various values of storage capacitance and quality factor. Experimental circuit designs are given and their results are compared to the theoretical predictions. It is shown that these predictions are accurate when the losses in the circuit are considered in the model. In spite of these losses, it is demonstrated that the SSDCI design can produce about 200% the output power of the idealized, matched resistive load case throughout the charging process and substantially reduce the charging time of the storage capacitor.
This paper focuses on comparing the effects of varying degrees of electromechanical coupling in piezoelectric power harvesting systems on the dynamics of charging a storage capacitor. In order to gain an understanding of the behavior of these dynamics, a transducer whose vibrational dynamics are impacted very little by electrical energy extraction is compared to a transducer that displays strong electromechanical coupling. Both transducers are cantilevered piezoelectric beams undergoing base excitation whose harvested electrical energy is used to charge a storage capacitor. The transient dynamics of the coupled system are studied in detail with an emphasis on their charging power curves and the time to charge the storage capacitor to a specified voltage. An analytic model for the system is derived that takes into consideration the reduction in vibration amplitude of the beam caused by the removal of electrical energy. Although this model makes the typical assumption that the beam is vibrating at its open-circuit resonance, it is shown to predict the charging behavior of the system accurately when compared to experimental results and a complete, nonlinear simulation without this simplification. Finally, the simplifications and discrepancies created by several types of modeling assumptions for a highly coupled energy harvesting system are discussed.
Through finite element analysis and a 3D printing assisted experimental study, we demonstrate a design of mechanical metamaterials for simultaneous mechanical wave filtering and energy harvesting. The mechanical metamaterials compromise a square array of free-standing cantilevers featuring piezoelectric properties being attached to a primary structural frame. A complete bandgap has thus been created via the strong coupling of the bulk elastic wave propagating along the structural frame and the distributed local resonance associated with the square array of piezoelectrically active cantilevers. Operating within the stop-band, external vibration energy has been trapped and transferred into the kinetic energy of the cantilevers, which is further converted into electric energy through mechano-electrical conversion of its integrated piezoelectric elements. Therefore, two distinct functions, vibration isolation and energy harvesting, are achieved simultaneously through the designed mechanical metamaterials.
wileyonlinelibrary.com COMMUNICATIONcloaking devices was then reported by Wegener's group using directly laser writing process employing two-photon polymerization and later stimulated emission depletion principle in achieving deep sub-wavelength resolution. [11,27] However, the issues with these fabrication techniques have intrinsically been associated with nonscalability as the sophisticate 3D structures are fabricated in a point-by-point serial fashion. Therefore, efficient 3D scalable fabrication techniques are needed to implement intricate GRIN optical elements for increased scalability and extension of working frequencies to broader ranges in the electromagnetic spectrum. Commercial 3D printers have been used widely in manufacturing industries to make parts with millimeter features. However, they still lack the capability to create GRIN metamaterials, with both the optically large overall size and strictly accurate sub-wavelength structures.In this paper, we present a powerful additive manufacturing technique-projection microstereolithography (PμSL) for creation of 3D GRIN metamaterials that can operate at the terahertz (THz) frequency band. The THz band is one of the most important but underdeveloped spectrum regions. Its spectra can carry unique absorption fingerprints characterizing molecular vibrational modes and provide information that is not available in the rest of the electromagnetic spectrum. [28] The PμSL technique enables a scalable and rapid way to produce feature sizes down to tens of microns that meets the demand of THz metamaterials. To illustrate these claims, we demonstrate a THz GRIN lens that possesses a continuous index variation from 1.1 to 1.64 and delivers a resolution close to the diffraction limit from 0.4 to 0.6 THz, which are sufficient for broadband THz TO devices. The index variation is fully controlled by accurately constructing the polarization-independent woodpile-like structure in a layer-by-layer fashion under the effective media approximation. The experimental results show the lens effectively improves the imaging capability compared to a homogeneous spherical lens with nonignorable aberration. Our work represents the convergence of TO designed metamaterials and additive manufacturing. The integration sets the stage for realization of a variety of intriguing THz applications with unprecedented functionalities.In THz optics, the design and manufacture of optical elements, such as lenses, plays a critical role in controlling THz waves. Many plastics make suitable THz lenses, but they are difficult to fabricate with the tight tolerances necessary for highquality imaging. Semiconductors and other dielectrics, such as silicon, are also difficult to manufacture and generally suffer from high Fresnel losses due to their high dielectric permittivity. Usually such dielectrics have fixed permittivity and the entire operation of the optics relies on careful crafting of its shape. The undesired imaging aberration is particularly recognizable Transformation optics (TO) provides a gener...
The modeling of nanopaddle bridges is studied in this article by proposing a lumped-parameter mathematical model which enables structural characterization in the resonant domain. The distributed compliance and inertia of all three segments composing a paddle bridge are taken into consideration in order to determine the equivalent lumped-parameter stiffness and inertia fractions, and further on the bending and torsion resonant frequencies. The approximate model produces results which are confirmed by finite element analysis and experimental measurements. The model is subsequently utilized to quantify the amount of mass which attaches to the bridge by predicting the modified resonant frequencies in either bending or torsion.
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