Jae Eun Kim

Power enhancing by reversing mode sequence in tuned mass-spring unit attached vibration energy harvester

We propose a vibration energy harvester consisting of an auxiliary frequency-tuned mass unit and a piezoelectric vibration energy harvesting unit for enhancing output power. The proposed integrated system is so configured that its out-of-phase mode can appear at the lowest eigenfrequency unlike in the conventional system using a tuned unit. Such an arrangement makes the resulting system distinctive: enhanced output power at or near the target operating frequency and very little eigenfrequency separation, not observed in conventional eigenfrequency-tuned vibration energy harvesters. The power enhancement of the proposed system is theoretically examined with and without tip mass normalization or footprint area normalization.

A small-form-factor piezoelectric vibration energy harvester using a resonant frequency-down conversion

While environmental vibrations are usually in the range of a few hundred Hertz, small-form-factor piezoelectric vibration energy harvesters will have higher resonant frequencies due to the structural size effect. To address this issue, we propose a resonant frequency-down conversion based on the theory of dynamic vibration absorber for the design of a small-form-factor piezoelectric vibration energy harvester. The proposed energy harvester consists of two frequency-tuned elastic components for lowering the first resonant frequency of an integrated system but is so configured that an energy harvesting beam component is inverted with respect to the other supporting beam component for a small form factor. Furthermore, in order to change the unwanted modal characteristic of small separation of resonant frequencies, as is the case with an inverted configuration, a proof mass on the supporting beam component is slightly shifted toward a second proof mass on the tip of the energy harvesting beam component. The proposed small-form-factor design capability was experimentally verified using a fabricated prototype with an occupation volume of 20 × 39 × 6.9 mm3, which was designed for a target frequency of as low as 100 Hz.

On the Energy Conversion Efficiency of Piezoelectric Vibration Energy Harvesting Devices

압전 진동 에너지 수확 장치(PEH: piezoelectric vibration energy harvester)의 에너지 변환 효율에 관Key Words: Vibration(진동), Energy Harvesting(에너지 수확), Piezoelectricity(압전), Conversion Efficiency(변환효율), Impedance Matching(임피던스 정합), Electro-Mechanical Coupling Coefficient(전기-역학 연성 계수) 초록: 압전 진동 에너지 수확 장치의 설계 및 성능 평가 시 에너지 변환 효율을 고려하는 것은 매우 당연하다. 본 연구에서 고려하는 에너지 변환 효율은 부하 저항이 부착된 압전 진동 에너지 수확 장치에 입력되는 가진 진동 파워 대비 전기 출력 값으로 정의된다. 기존의 연구에서는 근사적으로 임피던스 정합된 부하 저항에서의 전기 출력을 고려한 반면, 본 연구에서는 최적의 임피던스 정합 값을 사용하여 새롭게 에너지 변환 효율 식을 유도하였다. 유도된 식의 타당성을 검증하기 위해 3개의 서로 다른 전기-역학 연성 계수 값을 갖는 진동 에너지 수확 장치에 대한 유한 요소 해석 결과를 이용하였다. 또한, 부하 저항의 임피던스 정합 방법의 차이에 따른 에너지 변환 및 변환 효율 특성을 살펴보았다. Abstract: To properly design and assess a piezoelectric vibration energy harvester, it is necessary to consider the application of an efficiency measure of energy conversion. The energy conversion efficiency is defined in this work as the ratio of the electrical output power to the mechanical input power for a piezoelectric vibration energy harvester with an impedance-matched load resistor. While previous research works employed the electrical output power for approximate impedance-matched load resistance, this work derives an efficiency measure considering optimally matched resistance. The modified efficiency measure is validated by comparing it with finite element analysis results for piezoelectric vibration energy harvesters with three different values of the electro-mechanical coupling coefficient. New findings on the characteristics of energy conversion and conversion efficiency are also provided for the two different impedance matching methods.

Performance Characteristics of Vibration Energy Harvesting Using [001] and [011]-Poled PMN-PZT Single Crystals

This work investigated the electromechanical performance of a cantilevered vibration energy harvester incorporating the single crystal PMN-PZT, manufactured with the most recent technology of solid-state single crystal growth. Single crystal PMN-PZTs with two different crystallographic axes such as [011] and [001] were considered. For the [011] orientation, because material properties such as the stiffness, piezoelectric strain coefficients are not the same in the directions normal to the crystallographic axis, the effects of the transversely anisotropy on the magnitude and frequency bandwidth of output power were also analyzed.

Performance Study of Diagonally Segmented Piezoelectric Vibration Energy Harvester

: 전극 발생 전하량의 허수부 Key Words: Vibration Energy Harvesting(진동 에너지 수확), Piezoelectric(압전), Diagonal Segment(대각선 분할), Mode Sequence Change(모드 순서 변경) 초록: 본 연구에서 제안한 압전 진동 에너지 수확 장치는 기존 외팔보의 직사각형 면이 대각선을 따라 분할되어 2개의 에너지 수확 단위로 구성되어 있다. 부 구조물은 주 구조물이 진동 에너지 원에 부착되는 방향과 반대 방향으로 주 구조물의 끝단에 부착되어 있으며, 각 에너지 수확 단위는 폐회로 상태의 고유 진동수가 일치하도록 설계되었다. 동일한 고유 진동수를 갖는 2개의 구조물이 연결될 때 관찰되는 일반적인 현상과 달리, 제안된 구조에서는 고유 진동수 분리가 작으며, 1차 및 2차 모드의 순서가 바뀌어 나타난다. 이로 인해 출력 전력 역시 특정 주파수 근처에서 집중 생성된다. 상용 유한 요소 해석 소프트웨어를 사용하여 제안된 진동 에너지 수확 장치의 최대 생성 전력이 동일한 설치 영역 및 끝단 질량을 갖는 기존 외팔보 형태의 장치에 비해 실질적으로 향상됨을 보였다. Abstract: This study proposes a piezoelectric vibration energy harvester composed of two diagonally segmented energy harvesting units. An auxiliary structural unit is attached to the tip of a host structural unit cantilevered to a vibrating base, where the two components have beam axes in opposite directions from each other and matched short-circuit resonant frequencies. Contrary to the usual observations in two resonant frequency-matched structures, the proposed structure shows little eigenfrequency separation and yields a mode sequence change between the first two modes. These lead to maximum power generation around a specific frequency. By using commercial finite element software, it is shown that the magnitude of the output power from the proposed vibration energy harvester can be substantially improved in comparison with those from conventional cantilevered energy harvesters with the same footprint area and magnitude of a tip mass.

Mathematical Model for a Mode-sequence Reversed Two-degrees-of-freedom Piezoelectric Vibration Energy Harvester

A cantilevered piezoelectric energy harvester(PEH) and an auxiliary mass-spring unit can be integrated into a novel two-degrees-of-freedom PEH where its lowest eigenmode is not an in-phase modes but an out-of-phase mode. This typical behavior was shown to enhance output power considerably compared with its stand-alone counterpart. The objective of this study is to newly develop a continuum-based mathematical model suitable for efficient analysis of the mode-sequence reversed PEH. Once such a mathematical model is available, various physical behaviors can be analytically investigated for better designs. After a new mathematical model is developed, its validity is checked by using ANSYS results, in terms of resonant frequency, open-circuit voltage, and output power with a specified external resistance.

A Feasibility Study on the Application of the Topology Optimization Method for Structural Damage Identification

A feasibility of using the topology optimization method for structural damage identification is investigated for the first time. The frequency response functions (FRFs) are assumed to be constructed by the finite element models of damaged and undamaged structures. In addition to commonly used resonances, antiresonances are employed as the damage identifying modal parameters. For the topology optimization formulation, the modal parameters of the undamaged structure are made to approach those of the damaged structure by means of the constraint equations, while the objective function is an explicit penalty function requiring clear black-and-white images. The developed formulation is especially suitable for damage identification problems dealing with many modal parameters. Although relatively simple numerical problems were considered in this investigation, the possibility of using the topology optimization method for structural damage identification is suggested through this research.

Development of a Dedicated Algorithm for the Analysis of DC Electrical Outputs of Cantilevered Piezoelectric Vibration Energy Harvesters

For most applications of the vibration energy harvesting technology as in wireless sensor networks for smart buildings and plants, the evaluation of DC output performance of vibration energy harvesters is typically required. However, there is no dedicated algorithm for the evaluation. The lack of a dedicated algorithm results from difficulties in the direct incorporation of nonlinear rectifying and regulating circuitry into finite element models of piezoelectric vibration energy harvesters. In this study, we develop a dedicated algorithm and present software based on it for the evaluation of not only AC but also DC electrical quantities. Here, an equivalent electrical circuit model is employed. The COMSOL multiphysics simulation tool is adopted for extracting equivalent electrical circuit parameters of a piezoelectric vibration energy harvester and MATLAB is used to make a graphical user interface. The AC voltage and power outputs calculated by the proposed algorithm under various conditions are compared with those by a traditional finite element analysis. The DC output voltage and power through a rectifier are obtained for varying values of smoothing capacitance and external resistance.

Finite Element Modeling for the Analysis of In- and Out-of-plane Bulk Elastic Wave Propagation in Piezoelectric Band Gap Structures

This investigation presents a finite element method to obtain the transmission properties of bulk elastic waves in piezoelectric band gap structures(phonon crystals) for varying frequencies and modes. To this end, periodic boundary conditions are imposed on a three-dimensional model while both in-plane and out-of-plane modes are included. In particular, the mode decoupling characteristics between in-plane and out-of-plane modes are identified for each electric poling direction and the results are incorporated in the finite element modeling. Through numerical simulations, the proposed modeling method was found to be a useful, effective one for analyzing the wave characteristics of various types of piezoelectric phononic band gap structures.

Novel topology optimization for the design of multiple coils

The Lorenz-force maximizing coil configuration under a given magnetic field was obtained by the developed coil topology optimization method. The key idea in the developing of this new optimization method was viewing the current-flowing coils as equivalent magnetic dipoles in the first-stage optimization. The optimal distribution of magnetization vectors were determined in the first-stage of the developed optimization method and then the detailed coil shapes, in the second stage using a different method. The results confirmed that the proposed method can successfully generate not only a single coil configuration but also a multiple-coil configuration.