• Smart Structures and Systems
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• 제12권2호
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• pp.97-119
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• 2013

This paper presents numerical and experimental results on the use of guided waves for structural health monitoring (SHM) of crack growth during a fatigue test in a thick steel plate used for civil engineering application. Numerical simulation, analytical modeling, and experimental tests are used to prove that piezoelectric wafer active sensor (PWAS) can perform active SHM using guided wave pitch-catch method and passive SHM using acoustic emission (AE). AE simulation was performed with the multi-physic FEM (MP-FEM) approach. The MP-FEM approach permits that the output variables to be expressed directly in electric terms while the two-ways electromechanical conversion is done internally in the MP-FEM formulation. The AE event was simulated as a pulse of defined duration and amplitude. The electrical signal measured at a PWAS receiver was simulated. Experimental tests were performed with PWAS transducers acting as passive receivers of AE signals. An AE source was simulated using 0.5-mm pencil lead breaks. The PWAS transducers were able to pick up AE signal with good strength. Subsequently, PWAS transducers and traditional AE transducer were applied to a 12.7-mm CT specimen subjected to accelerated fatigue testing. Active sensing in pitch catch mode on the CT specimen was applied between the PWAS transducers pairs. Damage indexes were calculated and correlated with actual crack growth. The paper finishes with conclusions and suggestions for further work.

• Smart Structures and Systems
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• 제12권3_4호
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• pp.363-379
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• 2013

Dynamic displacement of structures is an important index for in-service structural condition and behavior assessment, but accurate measurement of structural displacement for large-scale civil structures such as long-span bridges still remains as a challenging task. In this paper, a vision-based dynamic displacement measurement system with the use of digital image processing technology is developed, which is featured by its distinctive characteristics in non-contact, long-distance, and high-precision structural displacement measurement. The hardware of this system is mainly composed of a high-resolution industrial CCD (charge-coupled-device) digital camera and an extended-range zoom lens. Through continuously tracing and identifying a target on the structure, the structural displacement is derived through cross-correlation analysis between the predefined pattern and the captured digital images with the aid of a pattern matching algorithm. To validate the developed system, MTS tests of sinusoidal motions under different vibration frequencies and amplitudes and shaking table tests with different excitations (the El-Centro earthquake wave and a sinusoidal motion) are carried out. Additionally, in-situ verification experiments are performed to measure the mid-span vertical displacement of the suspension Tsing Ma Bridge in the operational condition and the cable-stayed Stonecutters Bridge during loading tests. The obtained results show that the developed system exhibits an excellent capability in real-time measurement of structural displacement and can serve as a good complement to the traditional sensors.

• Smart Structures and Systems
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• 제11권4호
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• pp.411-433
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• 2013

Due to its easy operation and wide applicability, the ambient vibration method is commonly adopted to determine the cable force by first identifying the cable frequencies from the vibration signals. With given vibration length and flexural rigidity, an analytical or empirical formula is then used with these cable frequencies to calculate the cable force. It is, however, usually difficult to decide the two required parameters, especially the vibration length due to uncertain boundary constraints. To tackle this problem, a new concept of combining the modal frequencies and mode shape ratios is fully explored in this study for developing an accurate method merely based on ambient vibration measurements. A simply supported beam model with an axial tension is adopted and the effective vibration length of cable is then independently determined based on the mode shape ratios identified from the synchronized measurements. With the effective vibration length obtained and the identified modal frequencies, the cable force and flexural rigidity can then be solved using simple linear regression techniques. The feasibility and accuracy of the proposed method is extensively verified with demonstrative numerical examples and actual applications to different cable-stayed bridges. Furthermore, several important issues in engineering practice such as the number of sensors and selection of modes are also thoroughly investigated.

• Smart Structures and Systems
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• 제12권3_4호
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• pp.251-269
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• 2013

Measurement of deflections of certain bridges is usually hampered by corruption of the GPS signal by multipath associated with passing vehicles, resulting to unrealistically large apparent displacements. Field data from the Gorgopotamos train bridge in Greece and systematic experiments revealed that such bias is due to superimposition of two major effects, (i) changes in the geometry of satellites because of partial masking of certain satellites by the passing vehicles (this effect can be faced with solutions excluding satellites that get temporarily blocked by passing vehicles) and (ii) dynamic multipath caused from reflection of satellite signals on the passing trains, a high frequency multipath effect, different from the static multipath. Dynamic multipath seems to have rather irregular amplitude, depending on the geometry of measured satellites, but a typical pattern, mainly consisting of a baseline offset, wide base peaks correlating with the sequence of main reflective surfaces of the vehicles passing next to the antenna. In cases of limited corruption of GPS signal by dynamic multipath, corresponding to scale distortion of the short-period component of the GPS waveforms, we propose an algorithm which permits to reconstruct the waveform of bridge deflections using a weak fusion of GPS and RTS data, based on the complementary characteristics of the two instruments. By application of the proposed algorithm we managed to extract semi-static and dynamic displacements and oscillation frequencies of a historical railway bridge under train loading by using noisy GPS and RTS recordings. The combination of GPS and RTS is possible because these two sensors can be fully collocated and have complementary characteristics, with RTS and GPS focusing on the long- and short-period characteristics of the displacement, respectively.

• Smart Structures and Systems
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• 제21권3호
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• pp.279-286
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• 2018

The free longitudinal vibration of a circular truncated nanocone is investigated based on the nonlocal elasticity theory. Exact analytical formulations for tapered nanostructures are derived and the nonlinear differential governing equation of motion is developed. The nonlocal small scale effect unavailable in classical continuum theory is addressed to reveal the long-range interaction of atoms implicated in nonlocal constitutive relation. Unlike most previous studies applying the truncation method to the infinite higher-order differential equation, this paper aims to consider all higher-order terms to show the overall nonlocality. The explicit solution of nonlocal stress for longitudinal deformation is determined and it is an infinite series incorporating the classical stress derived in classical mechanics of materials and the infinite higher-order derivative of longitudinal displacement. Subsequently, the first three modes natural frequencies are calculated numerically and the significant effects of nonlocal small scale and vertex angle on natural frequencies are examined. The coupling phenomenon of natural frequency is observed and it is induced by the combined effects of nonlocal small scale and vertex angle. The critical value of nonlocal small scale is defined, and after that a new proposal for determining the range of nonlocal small scale is put forward since the principle of choosing the nonlocal small scale is still unclear at present. Additionally, two different types of nonlocal effects, namely the nonlocal stiffness weakening and strengthening, reversed phenomena existing in nanostructures are observed and verified. Hence the opposite nonlocal effects are resolved again clearly. The nano-engineers dealing with a circular truncated nanocone-based sensors and oscillators may benefit from the present work.

• 한국IT서비스학회지
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• 제15권4호
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• pp.97-109
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• 2016

The rapid deployment of millions of mobile sensors and smart devices has resulted in high demand for opportunistic encounter-based networking. For the cooperative video surveillance of dashboard cameras in nearby vehicles, a fast and energy-efficient asynchronous neighbor discovery protocol is indispensable because a dashboard camera is an energy-hungry device after the vehicle's engine has turned off. In the existing asynchronous neighbor discovery protocols, all nodes always try to discover all neighbors. However, a dashboard camera needs to discover nearby dashboard cameras when an event is detected. In this paper, we propose a fast and energy-efficient asynchronous neighbor discovery protocol, which enables nodes : 1) to have different roles in neighbor discovery, 2) to discover neighbors within a search range, and 3) to report promptly the exact discovery result. The proposed protocol has two modes: periodic wake-up mode and active discovery mode. A node begins with the periodic wake-up mode to be discovered by other nodes, switches to the active discovery mode on receiving a neighbor discovery request, and returns to the periodic wake-up mode when the active discovery mode finishes. In the periodic wake-up mode, a node wakes up at multiples of number ${\alpha}$, where ${\alpha}$ is determined by the node's remaining battery power. In the active discovery mode, a node wakes up for consecutive ${\gamma}$ slots. Then, the node operating in the active discovery mode can discover all neighbors waking up at multiples of ${\beta}$ for ${\beta}{\leq}{\gamma}$ within ${\gamma}$ time slots. Since the proposed protocol assigns one half of the duty cycle to each mode, it consumes equal to or less energy than the existing protocols. A performance comparison shows that the proposed protocol outperforms the existing protocols in terms of discovery latency and energy consumption, where the frequency of neighbor discovery requests by car accidents is not constantly high.

• 한국정보통신학회논문지
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• 제21권8호
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• pp.1560-1566
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• 2017

최근 사물 인터넷 센서가 설치된 스마트 돈사의 보급을 통해 돈사 관련 빅데이터 축적이 가능해졌고, 다양한 기계 학습방안들이 수집된 데이터에 적용되어 축산농가의 생산성을 향상시키고 있다. 본 연구에서는 다양한 기계학습 방안을 이용하여 돈사관리에서 가장 중요한 요소 중 하나인 급수량을 예측하였다. 구체적으로 실제 돈사에서 수집된 데이터에 회귀 방안인 선형회귀, 회귀트리 및 아다부스트 회귀 방안과 분류 방안인 로지스틱 분류, 결정트리 및 서포트 벡터 머신 (SVM) 분류방안을 적용하여 돈사의 온도와 습도를 기반으로 급수량을 예측하였다. 성능 분석을 통해서 제안한 방안이 높은 정확도로 급수량을 예측하는 것을 확인할 수 있었다. 제안한 방안은 돈사의 급수시설 이상을 조기에 파악하는데 활용되어 가축을 폐사를 막고 돈사 생산성을 높이는데 활용될 수 있다.

• 한국정보통신학회:학술대회논문집
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• 한국정보통신학회 2017년도 추계학술대회
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• pp.723-724
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• 2017

최근 다양한 센서와 디바이스를 활용하여 스마트 환경을 구축하고 사물 간 통신으로 사용자에게 맞춤형 서비스를 제공하는 연구가 진행되고 있다. 그러나 기존 시스템은 측정되는 센서 데이터를 실시간으로 서버에 전송하여 일괄로 처리하는 중앙 집중형 방식을 활용하기 때문에 시스템이 확장될수록 고사양의 서버를 구성해야 하는 문제점이 있었다. 본 논문에서는 이러한 문제점을 해결하기 위해 IoT 분산 처리 환경을 위한 자원 할당 알고리즘을 설계하였다. 디바이스가 동작하기 위해 필요한 자원을 서버로 전송하고 서버에서는 동작중인 작업과 비교하여 자원을 할당한다. 이를 통해 서버의 데이터 처리량을 감소시키고 낮은 사양의 서버에서도 여러 디바이스들을 구성할 수 있을 것으로 사료된다.

• 한국산학기술학회논문지
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• 제10권7호
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• pp.1556-1567
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• 2009

자동차 주요 생산국인 우리나라 보행자의 교통사고 사망률은 인구 10만 명 당 5.28명으로서 OECD 평균의 약 2.5배에 달한다. 보행자를 감지하고 운전자에게 경보를 보내주는 시스템이 개발되어 보행자 교통사고를 조금이라도 줄일 수 있다면, 그 자체만으로도 보행자 감지 시스템의 가치는 충분하기 때문에 PDS에 대한 관심이 높아지고 있다. 보행자 교통사고율은 야간에 더 높기 때문에, 야간 보행자 감지 시스템에 주요 자동차 회사들이 관심을 두고 있으나, 그들은 일반적으로 고가의 나이트비젼 또는 복합적 센서를 사용하는 장비를 채택하고 있다. 본 논문에서는 PDS에서 나이트비젼 대신에, 넓은 동적 범위를 갖는 가시 스펙트럼 대역 흑백 카메라 한 대만을 사용하는 야간 보행자 감지 기법을 제안한다. 서로 다른 환경에서 촬영된 야간 동영상들에 대해 실험한 결과, 제안 알고리듬이 에지가 어느 정도 정확하게 검출되는 상황이라면 정확한 보행자 검출 성능을 보였다.

• 정보과학회 컴퓨팅의 실제 논문지
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• 제21권5호
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• pp.351-358
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• 2015

최근 다양한 센서를 내장한 스마트폰의 발달로 인해 웨어러블 기기를 사용한 동작 인식 연구가 늘어나는 추세이다. 기존의 동작 인식 연구는 사용자 개인의 동작 인식에만 국한되어 있다. 따라서 본 논문에서는 인간의 집단 개념동작을 인식하는 기법을 제안한다. 인식에 앞서 장소 별 집단 동작의 특징을 분석하여 데이터를 생성한다. 강의실 환경에서의 집단 개념동작을 중점적으로 수업하기, 발표하기, 회의하기로 세 가지 동작을 연구한다. 본 연구에서 제안한 알고리즘을 적용하여 96% 이상의 높은 인식률을 도출하였다. 실시간으로 활용한다면 자동적으로 강의실의 사용률 및 사용 목적을 쉽게 분석할 수 있다. 나아가 분석된 데이터를 통해 장소 활용도를 높일 수 있다. 향후 다른 장소에 대한 집단 동작 인식을 연구하여 집단 동작 인식 시스템을 개발할 것이다.