• Structural Engineering and Mechanics
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• v.42 no.2
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• pp.229-245
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• 2012

Bridge vibration displacements have been directly measured by LVDTs (Linear Variable Differential Transformers) or laser equipment and have also been indirectly estimated by an algorithm of integrating measured acceleration. However, LVDT measurement cannot be applied for a bridge crossing over a river or channel and the laser technique cannot be applied when the weather condition is poor. Also, double integration of accelerations may cause serious numerical deviation if the initial condition or a regression process is not carefully controlled. This paper presents an algorithm of estimating bridge vibration displacements using vibration strains measured by FBG (Fiber Bragg Grating) sensors and theoretical mode shapes of a simply supported beam. Since theoretically defined mode shapes are applied, even high modes can be used regardless of the quality of the measured data. In the proposed algorithm, the number of theoretical modes is limited by the number of sensors used for a field test to prevent a mathematical rank deficiency from occurring in computing vibration displacements.89The proposed algorithm has been applied to various types of bridges and its efficacy has been verified. The closeness of the estimated vibration displacements to measured ones has been evaluated by computing the correlation coefficient and by comparing FRFs (Frequency Response Functions) and the maximum displacements.

• Structural Engineering and Mechanics
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• v.7 no.2
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• pp.181-202
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• 1999

Dynamic response of axisymmetric arbitrary laminated composite cylindrical shell of finite length, using three-dimensional elasticity equations are studied. The shell is simply supported at both ends. The highly coupled partial differential equations are reduced to ordinary differential equations (ODE) with variable coefficients by means of trigonometric function expansion in axial direction. For cylindrical shell under dynamic load, the resulting differential equations are solved by Galerkin finite element method, In this solution, the continuity conditions between any two layer is satisfied. It is found that the difference between elasticity solution (ES) and higher order shear deformation theory (HSD) become higher for a symmetric laminations than their unsymmetric counterpart. That is due to the effect of bending-streching coupling. It is also found that due to the discontinuity of inplane stresses at the interface of the laminate, the slope of transverse normal and shear stresses aren't continuous across the interface. For free vibration analysis, through dividing each layer into thin laminas, the variable coefficients in ODE become constants and the resulting equations can be solved exactly. It is shown that the natural frequency of symmetric angle-ply are generally higher than their antisymmetric counterpart. Also the results are in good agreement with similar results found in literatures.

• Structural Engineering and Mechanics
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• v.14 no.4
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• pp.417-434
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• 2002

This paper explores the possibility of using a combination of the empirical mode decomposition (EMD) and the Hilbert transform (HT), termed the Hilbert-Huang transform (HHT) method, to identify the modal damping ratios of the structure with closely spaced modal frequencies. The principle of the HHT method and the procedure of using the HHT method for modal damping ratio identification are briefly introduced first. The dynamic response of a two-degrees-of-freedom (2DOF) system under an impact load is then computed for a wide range of dynamic properties from well-separated modal frequencies to very closely spaced modal frequencies. The natural frequencies and modal damping ratios identified by the HHT method are compared with the theoretical values and those identified using the fast Fourier transform (FFT) method. The results show that the HHT method is superior to the FFT method in the identification of modal damping ratios of the structure with closely spaced modes of vibration. Finally, a 36-storey shear building with a 4-storey light appendage, having closely spaced modal frequencies and subjected to an ambient ground motion, is analyzed. The modal damping ratios identified by the HHT method in conjunction with the random decrement technique (RDT) are much better than those obtained by the FFT method. The HHT method performing in the frequency-time domain seems to be a promising tool for system identification of civil engineering structures.

• Structural Engineering and Mechanics
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• v.20 no.6
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• pp.655-672
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• 2005

Seismic random responses due to the wave passage effect are extensively investigated by using the pseudo excitation method (PEM). Two examples are used. The first is very simple but also very informative, while the second is a realistic suspension bridge. Numerical results show that the seismic responses vary significantly with wave speed, especially for low velocity or large span. Such variations are not monotonic, especially for flexible structures. The contributions of the dynamic and quasi-static components depend heavily on the seismic wave velocity and the natural frequencies of structures. For the lower natural frequency cases, the dynamic component has significant effects on the dynamic responses of the structure, whereas the quasi-static component dominates for higher natural frequencies unless the wave speed is also high. It is concluded that if insufficient data on local seismic wave velocity is available, it is advisable to select several possible velocity values in the seismic analysis and to choose the most conservative of the results thus obtained as the basis for design.

• Structural Engineering and Mechanics
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• v.30 no.3
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• pp.369-382
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• 2008

An oscillator of two lumped masses linked through a vertical spring moves forward in the horizontal direction, initially at a certain height, over a horizontal Euler beam and descends on it due to its own weight. Vibration of the beam and the oscillator is excited at the onset of the ensuing impact. The impact produced by the descending oscillator is assumed to be either perfectly elastic or perfectly plastic. If the impact is perfectly elastic, the oscillator bounces off and hits the beam a number of times as it moves forward in the longitudinal direction of the beam, exchanging its dynamics with that of the beam. If the impact is perfectly plastic, the oscillator (initially) sticks to the beam after its first impact and then may separate and reattach to the beam as it moves along the beam. Further events of separation and reattachment may follow. This interesting and seemingly simple dynamic problem actually displays rather complicated dynamic behaviour and has never been studied in the past. It is found through simulated numerical examples that multiple events of separation and impact can take place for both perfectly elastic impact and perfectly plastic impact (though more of these in the case of perfectly elastic impact) and the dynamic response of the oscillator and the beam looks noisy when there is an event of impact because impact excites higher-frequency components. For the perfectly plastic impact, the oscillator can experience multiple events of consecutive separation from the beam and subsequent reattachment to it.

• The Transactions of the Korean Institute of Power Electronics
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• v.18 no.3
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• pp.279-288
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• 2013

This paper proposes a improved speed control system for five-phase squirrel-cage induction motor(IM) injecting 3rd. current harmonic components with field oriented control (FOC) A five-phase IM drives present unique characteristics due to the additional degrees of freedom and also drives possess many others advantage compared with the traditional three-phase motor drive system, such as reducing a amplitude of torque pulsation at low frequency and increasing the reliability. In order to maximize the torque per ampere, the proposed motor has concentrated windings. The produced back-electromotive force is almost trapezoidal, and the motor is supplied with the combined sinusoidal plus third harmonic of currents. There is necessary to controlled 3rd harmonic current in order to high response characteristics. For presenting the superior performance of the proposed the speed control system, experimental results are presented using a 32-bit fixed point TMS320F2812 DSP with 1.5[kW] induction motor.

• Proceedings of the Korean Society of Broadcast Engineers Conference
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• 2010.11a
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• pp.63-66
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• 2010

본 논문은 차세데 3D-TV 방송을 위해 수정된 ATSC (Modified Advanced Television Systems Committee) 시스템 [1]에서 파일럿 (Pilot)을 이용한 채널 추정 대신, PN 시퀀스 (Pseudo-Noise Sequence)를 이용한 채널 추정 방식의 문제점에 대하여 2가지 방법으로 연구하였다. PN 시퀀스를 이용하여 채널을 추정하는 TDS-OFDM (Time Domain Synchronous - Orthogonal Frequency Division Multiplexing)시스템은 QAM (Quadrature Amplitude Modulation) 변조 방식을 사용하기 때문에 수신측 PN 시퀀스의 위상 변화가 일어나지 않는다. 하지만 수정된 ATSC 시스템에서 사용하는 VSB (Vestigial Side Band)변조 방식에서 직교위상 (Quadrature) 채널을 통해 전송되는 값은 동위상 (In-Phase) 채널을 통해 전송되는 값의 단순한 힐버트 변환 (Hilbert Transform)에 의해 생성되어 불규칙한 위상 변화가 발생한다. 따라서 수정된 ATSC 시스템에서 채널 추정을 위해 PN 시퀀스를 사용하게 된다면 상관 (Correlation) 특성을 잃어버릴 것이며 올바른 채널 임펄스 응답 (Channel Impulse Response)을 얻을 수 없다.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.21 no.12
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• pp.1112-1119
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• 2011

The goal of this study is the stability evaluation of a vacuum pump through modal test and rotor dynamics. Roots type vacuum pump, which is a dry vacuum pump, is necessary for the manufacturing process of the semiconductor and the display. Eigenvalue was solved by the finite-element method(FEM) using 2D and 3D models, then the modal test result was compared with the FEM result. According to the comparison, the analysis result using the 2D was more accurate than the 3D model. Therefore, rotor dynamics was performed by the 2D model. Campbell diagram and root-locus maps, which were calculated by complex-eigenvalue analysis, were used to evaluate the stability of the rotors of the vacuum pump. And displacement solved by unbalance response analysis was compared with the minimum clearance between two rotors of the vacuum pump. Thus, the vacuum pump is assumed operated under steady state through the evaluation of the rotor dynamics.

• Journal of the Korea Safety Management & Science
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• v.15 no.4
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• pp.131-136
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• 2013

Automobile was first introduced in 1903 in South Korea, the masses of the car was carried out rapidly compared to other countries. However, many people were killed in a traffic accidents and economic loss was occurred due to the spread of the automobile. In South Korea, 2012, traffic accident occurred 223,656 times, 5,392 fatality, 344,565 injured people. In the last five years, about 224,000 accidents per year were occurring. In other words, 610 traffic accidents occur and about 15 people pass away in one day. In addition, the proportion of traffic accidents is first place in the OECD countries and it is very high in the world. Understand occurrence tendency of traffic accident, accident frequency rate of the driver who drives more than 10 years was higher than the novice driver. In addition, as a result of examining the cause of the traffic accident, breach of safe driving obligation appears highest case (125,391 times), and followed by signal violation, break safety distance. Therefore, the majority of traffic accidents occurred by the lack of safety awareness of the driver. In this study, prevent the loss of human life and property in traffic disaster, by establish disaster prevention measures that investigated by questionnaire survey and statistical data of the state of consciousness and driving posture in response to the driving history of the driver.

• Journal of the Korean Home Economics Association
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• v.50 no.7
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• pp.59-66
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• 2012

The purpose of this study was to identify a causal relationship in the career preparation behavior, parental social support, career decision making self-efficacy and the career maturity of the pre-service elementary school teachers. A total of 374 questionnaires were used for data analysis, excluding the 23 copies deemed insincere in response. To ensure the reliability and validity of the questions, technical statistics of the frequency, ratio, average, standard deviation, skewness, and kurtosis via PASW 18.0, item-total correlation, the totality, and the reliability analysis. The structural analysis via AMOS 7.0 in the bootstrapping method was undertaken to perform the path analysis among the variables and to assess the suitability of the model. The findings of the study led to the following conclusions: First, the causal model for the career preparation behavior, parental social support, career decision making self-efficacy, and the career maturity of the pre-service elementary school teachers is suitable to empirical analysis on research variables. Second, the career decision making self-efficacy of pre-service elementary teachers has direct effect on career preparation behavior positively. Third, parental social support of the pre-service elementary teachers has indirect effects on the career preparation behavior positively.