• Smart Structures and Systems
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• 제8권3호
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• pp.303-320
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• 2011

Comparing to active damage monitoring, impact localization on composite by using time reversal focusing method has several difficulties. First, the transfer function of the actuator-sensor path is difficult to be obtained because of the limitation that no impact experiment is permitted to perform on the real structure and the difficulty to model it because the performance of real aircraft composite is much more complicated comparing to metal structure. Second, the position of impact is unknown and can not be controlled as the excitation signal used in the active monitoring. This makes it not applicable to compare the difference between the excitation and the focused signal. Another difficulty is that impact signal is frequency broadband, giving rise to the difficulty to process virtual synthesis because of the highly dispersion nature of frequency broadband Lamb wave in plate-like structure. Aiming at developing a practical method for on-line localization of impact on aircraft composite structure which can take advantage of time reversal focusing and does not rely on the transfer function, a PZT sensor array based phase synthesis time reversal impact imaging method is proposed. The complex Shannon wavelet transform is presented to extract the frequency narrow-band signals from the impact responded signals of PZT sensors. A phase synthesis process of the frequency narrow-band signals is implemented to search the time reversal focusing position on the structure which represents the impact position. Evaluation experiments on a carbon fiber composite structure show that the proposed method realizes the impact imaging and localization with an error less than 1.5 cm. Discussion of the influence of velocity errors and measurement noise is also given in detail.

• 센서학회지
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• 제4권4호
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• pp.75-80
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• 1995

금속 alkoxide를 출발물질로 PZT-PNN 3성분계 압전박막을 제작하여, 박막의 결정성과 전기적 특성을 평가하였다. 박막의 X-RD 분석결과 $550^{\circ}C$ 소결온도에서 결정성이 가장 양호하였다. D-E 이력곡선의 관측 결과 항전계는 28.8 kV/cm, 잔유분극은 $18.3\;{\mu}C/cm^{2}$ 이었다. 박막의 진성파괴전압은 $76.0\;{\sim}\;27.0\;MV/m$이었고, 소결온도가 상승함에 따라 특성이 저하되었다. 박막의 비유전율은 조성비에 따라 (50:40:10)은 406, (50:30:20)은 1084, (45:35:20)은 723, (40:40:20)은 316이었다.

• 대한기계학회논문집A
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• 제30권1호
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• pp.84-89
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• 2006

A wireless capsule endoscope has been developed to replace the conventional endoscope. However, the commercialized capsule endoscope moves passively by peristaltic waves, which has some limitations for doctors to diagnose more thoroughly and actively. In order to solve this problem, a locomotive mechanism is proposed for wireless capsule endoscopes. Based on the tests of various actuators, a piezo actuator is selected as a micro actuator for capsule endoscope. In general, piezo actuators are known to have limited displacement with high voltage supply. In order to overcome the limitation of common piezo actuator, the impact based piezo actuator, is developed to realize long stroke up 11mm. By using the impact based piezo actuator, a prototype of an earthworm-like locomotive mechanism was developed. In addition, the proposed locomotive mechanism has engraved clamps mimicked the claw of an insect. The earthworm-like locomotive mechanism has 15 mm in diameter and 30mm under retraction stage and 41 mm under elongation stage in total length. Hollow space is allocated to comprise essential endoscope components such as a camera, a communication module, bio sensors, and a battery. For the feasibility test of proposed locomotive mechanism, a series of experiments were carried out including in-vitro tests. Based on results of the experiments, we conclude that the proposed locomotive mechanism is effective to be used for micro capsule endoscopes.

• Smart Structures and Systems
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• 제16권2호
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• pp.281-294
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• 2015

Bolted joint connection is the most commonly used connection element in structures and devices. The loosening due to external dynamic loads cannot be observed and measured easily and may cause catastrophic loss especially in an extreme requirement and/or environment. In this paper, an innovative Real-time Cross-Correlation Method (RCCM) for monitoring of the bolted joint loosening was proposed. We apply time reversal process on stress wave propagation to obtain correlation signal. The correlation signal's peak amplitude represents the cross-correlation between the loosening state and the baseline working state; therefore, it can detect the state of loosening. Since the bolt states are uncorrelated with noise, the peak amplitude will not be affected by noise and disturbance while it increases SNR level and increases the measured signals' reliability. The correlation process is carried out online through physical wave propagation without any other post offline complicated analyses and calculations. We implemented the proposed RCCM on a single bolt/nut joint experimental device to quantitatively detect the loosening states successfully. After that we implemented the proposed method on a real large structure (reaction wall) with multiple bolted joint connections. Loosening indexes were built for both experiments to indicate the loosening states. Finally, we demonstrated the proposed method's great anti-noise and/or disturbance ability. In the instrumentation, we simply mounted Lead Zirconium Titanate (PZT) patches on the device/structure surface without any modifications of the bolted connection. The low-cost PZTs used as actuators and sensors for active sensing are easily extended to a sensing network for large scale bolted joint network monitoring.

• 한국음향학회지
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• 제9권4호
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• pp.18-32
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• 1990

최근 신호 처리 기기와 센서로서 각광을 받고 있는 SAW Device 는 때론 목적에 따라 수중에서 사용해야 할 때가 있다. 그러나 유체내의 고체 표면을 전파하는 표면파의 경우, 유체내로의 에너지 손실로 인해 설계상에 많은 어려움을 주고 있다. 따라서 본 연구에서는 이러한 어려움을 극복할 수 있는 최적 설계법으로서, 컴퓨터 모형해석을 통해 수중에서 압전물질에 의한 표면파의 최대 발진 효율, 최소 전파 감쇄율, 그리고 pure mode 전파를 이룰 수 있는 SAW Device 의 최적 geometry, 즉 초적 압전 결정 평면, 표면과 전파 방향, 그리고 무차원 전파 계수 들을 구하였다. 본 논문에서는 표면파가 전파하는 고체 재료로서 PZT와 PVDF 적층, 그리고 쇠 하부층을 사용하였으나, 이 설계법은 임의의 유체층과 고체층의 조합에도 적용할 수 있다. 동일한 기술은 수중음향 계측기, antifouling, 그리고 산업 및 의료 분야 등에 쓰이는 센서와 발진기의 설계에도 바로 응용할 수 있다.

• 한국전산구조공학회논문집
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• 제24권5호
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• pp.553-567
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• 2011

최근 3년간 판 구조물 건전성 감시분야에서 시간반전램파(Time reversal Lamb waves)의 가능성이 주목받고 있다. 이 연구에서는 직사각형 평판에서 시간반전램파의 공간모임을 적은 연산비용으로 효과적으로 모사할 수 있는 수치기법을 제안한다. 제안된 기법에서는 파 반사에 의해 발생하는 실제 탐지자의 거울상 효과와 등가를 이루는 활성 가상탐지자를 이용하여 시간반전과정을 주파수 영역에서 정식화한다. 시간반전램파의 공간모임 모사에서 필요한 순방향 및 역방향 파 전달은 스칼라 형태의 함수로 표현한다. 제안된 방법을 웨이퍼 형태의 압전소자가 병치된 직사각형 평판에서 시간반전램파의 공간 모임 문제에 적용하고 유한요소해석 결과와 비교하여 타당성을 검증한다.

• Smart Structures and Systems
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• 제22권2호
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• pp.133-137
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• 2018

Increasing interest in prognostics and health management has heightened the need for wireless sensor networks (WSN) with efficient power sources. Piezoelectric energy harvesters using Pb(Zr,Ti)O3 (PZT) are one of the candidate power sources for WSNs as they efficiently convert mechanical vibration energy into electrical energy. These types of devices are resonated at a specific frequency, which has a significant impact on the amount of energy harvested, by external vibration. Hence, precise prediction of mechanical deformation including modal analysis of piezoelectric devices is crucial for estimating the energy generated under specific conditions. In this study, an experimental vibrational system capable of controlling a wide range of frequencies and accelerations was designed to generate mechanical vibration for piezoelectric energy harvesters. In conjunction with MATLAB, the system automatically finds the resonance frequency of harvesters. A small accelerometer and non-contact laser displacement sensor are employed to investigate the mechanical deformation of harvesters. Mechanical deformation under various frequencies and accelerations were investigated and analyzed based on data from two types of sensors. The results verify that the proposed system can be employed to carry out vibration experiments for piezoelectric harvesters and measurement of their mechanical deformation.

• 센서학회지
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• 제10권6호
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• pp.286-294
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• 2001

본 연구에서는 자체 제작한 3-3형 복합압전체 초음파 트랜스듀서와 SOFM(Self Organizing Feature Map) 신경회로망을 이용한 수중 3차원 물체인식특성에 대해 연구하였다. 자체 제작한 3-3형 복합압전체 소자는 수중 초음파 트랜스듀서 재료로서의 요구조건을 비교적 잘 만족하였다. 자체 제작한 3-3형 복합압전체 트랜스듀서와 SOFM 신경회로망을 이용하여 얻어진 4종의 인식물체(정사각기둥, 직사각기둥, 원통, 정삼각기둥)에 대한 전체적인 수중 물체인식률은 학습데이터인 경우에는 100%, 시험데이터는 94.0%를 나타내었다. 이들 결과로부터 자체 제작한 3-3형 복합압전체 초음파 트랜스듀서는 수중 물체인식용 트랜스듀서로서 응용될 수 있음을 알 수 있었다.

• 한국음향학회지
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• 제30권8호
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• pp.436-445
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• 2011

본 논문에서는 2-2형 압전복합체의 등가 물성 도출에 관해 연구하였다. 먼저 2-2 압전복합체의 특성 변화를 유한요소법으로 해석하였고, 해석 결과와 2-2 압전복합체 시제작품의 특성 측정 결과를 비교하여 유한요소해석의 타당성을 검증하였다. PZT-5H와 고분자재료로 구성된 압전복합체의 단일상 등가물성은 Asymptotic Averaging Method를 이용하여 도출하였고, 도출된 물성을 다양한 진동 모드의 단일상 공진자 유한요소 모델에 대입한 후 완전한 2-2 압전복합체와의 임피던스 스펙트럼 차이를 최소 자승법을 이용하여 보정함으로써, 도출된 등가 물성값의 정확성을 향상시켰다. 본 연구에서 도출한 2-2 압전복합체의 등가 물성은 향후 다양한 음향 센서의 설계에 유용하게 사용될 수 있을 것이다.

• Smart Structures and Systems
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• 제17권2호
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• pp.167-194
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• 2016

This paper investigates the Synergetics based Damage Detection Method (SDDM) for frame structures by using surface-bonded PZT (Lead Zirconate Titanate) patches. After analyzing the mechanism of pattern recognition from Synergetics, the operating framework with cooperation-competition-update process of SDDM was proposed. First, the dynamic identification equation of structural conditions was established and the adjoint vector (AV) set of original vector (OV) set was obtained by Generalized Inverse Matrix (GIM).Then, the order parameter equation and its evolution process were deduced through the strict mathematics ratiocination. Moreover, in order to complete online structural condition update feature, the iterative update algorithm was presented. Subsequently, the pathway in which SDDM was realized through the modified Synergetic Neural Network (SNN) was introduced and its assessment indices were confirmed. Finally, the experimental platform with a two-story frame structure was set up. The performances of the proposed methodology were tested for damage identifications by loosening various screw nuts group scenarios. The experiments were conducted in different damage degrees, the disturbance environment and the noisy environment, respectively. The results show the feasibility of SDDM using piezoceramic sensors and actuators, and demonstrate a strong ability of anti-disturbance and anti-noise in frame structure applications. This proposed approach can be extended to the similar structures for damage identification.