• Journal of Korean Society of Industrial and Systems Engineering
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• v.23 no.57
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• pp.1-10
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• 2000

The purpose of this paper is to propose a method of rest to reduce work load of night shift workers for night shift work. The experiment was carried out 10minutes preparing time, 45minutes first work, 10minutes first rest, 45minutes second work, 10minutes second rest between 2 and 4 o'clock that the lowest physiological function of workers. The methods of rest set up as four patterns (1) non-action rest (2) non-action rest + listening music (3) action rest + non-action rest, (4) action rest + non-action rest + listening music. For the measurements of experiment, heart rates(R-R interval), critical flicker fusion frequency(CFF), blood pressure, oral temperature, reaction time and error rates were considered as criteria for work performance. As a result, action rest + non-action rest and action rest + non-action rest + listening music were more effective to reduce work load additional work than non-action rest and non-action rest + listening music.

• Proceedings of the Korean Magnestics Society Conference
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• 2003.06a
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• pp.60-61
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• 2003

Theoretical considerations on a giant magneto-impedance (GMI) effect in amorphous ribbons (i.e., thin films) have been made in terms of the expressions of effective permeability and impedance derived in the frame of classical electrodynamics and ferromagnetism. The dependence of GMI effect on the external do magnetic field (H$\_$ext/) and the frequency of alternating current are simulated and discussed in the knowledge of energy conversion consisting of the current energy loss, the ferromagnetic energy consumption, and the magnetic energy storage in the film. The obtained results are summarized as follow: (a) As frequency f< 20 ㎒, the real part of effective permeability (${\mu}$′) changes slightly. The peak of the ${\mu}$′curve always locates at H$\_$ext/=H$\_$ani/ - the anisotropy field. However, the peak value of ${\mu}$′ tends to increase with increasing frequency in the frequency range of 11-20 ㎒. (b) In the frequency range, f= 21-23 ㎒, a negative peak additionally appears. Meanwhile, both the positive and negative peak values rapidly increase with increasing frequency and their peak positions shift towards a high H$\_$ext/. (c) The positive peak value of ${\mu}$′ starts to decrease at f= 29 ㎒ and its negative peak does so at about 35 ㎒. Then, both peaks keep such a tendency and their peak positions move to high H$\_$ext/, as increasing frequency. (d) The dependence of the imaginary part of effective permeability (${\mu}$") on the external dc magnetic field and the frequency of the alternating field indicates that there is only one peak involved in ${\mu}$" for the whole frequency range. (e) The impedance vs. magnetic field curves at various frequencies show that there is a critical value of frequency around f= 18-19 ㎒ where the transition between two frequency regimes occurs; the one (low frequency) in which ${\mu}$′ predominantly contributes to the GMI effect and the other (high frequency) in which ${\mu}$" determines the GMI effect.

• The Bulletin of The Korean Astronomical Society
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• v.40 no.2
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• pp.43.3-44
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• 2015

Very Long Baseline Interferometry (VLBI) at millimeter wavelengths results in the highest angular resolutions achieved in astronomy and has a unique access to emission regions that are inaccessible with any other approach or at longer wavelengths. The simultaneous multi-frequency VLBI system in the Korean VLBI Network (KVN) is considered one of the most effective systems for compensating the atmospheric phase fluctuations, which is particularly bothersome at mm-VLBI. We have been demonstrating its performance and uniqueness at mm-VLBI observations. As a results, international VLBI partners from Japan, China, Australia and EU have expressed their interest on the KVN style simultaneous multi-frequency system. In this talk, we will report the activities for extending the simultaneous multi-frequency system to global VLBI network and introduce its science driver, measuring AGN core-shift effects.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.28 no.1
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• pp.16-23
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• 2004

Acoustic characteristics of unbaffled and baffled combustion chamber are experimentally investigated under atmospheric condition to preliminarily determine baffle for mitigation of combustion instability. To investigate the effect of the baffle which has several configurations such as radial baffles and hub/blade baffle, resonant-frequency shift and damping factors of the chamber were analyzed and compared quantitatively with those of the unbaffled combustion chamber. From a view of acoustic characteristics, radial baffles with several configurations have not much difference in resonant-frequency shift and damping factor ratio with each other. On the other hand, hub and blade baffle is very effective to suppress the first tangential mode which was found to be the most harmful acoustic mode in KSR(Korean Sounding Rocket)-III engine. But more study on design parameters such as hub size and axial length should be done for complete optimization of hub and blade baffle. The present study based on linear acoustic analysis is expected to be a useful confirming tool to predict acoustic field and design a passive control devices such as baffle and acoustic cavity.

• KSII Transactions on Internet and Information Systems (TIIS)
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• v.13 no.5
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• pp.2434-2453
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• 2019

To improve the separation performance of time-frequency overlapped radar and communication signals from a single channel, this paper proposes an effective separation method based on an improved empirical wavelet transform (EWT) that introduces a fast boundary detection mechanism. The fast boundary detection mechanism can be regarded as a process of searching, difference optimization, and continuity detection of the important local minima in the Fourier spectrum that enables determination of the sub-band boundary and thus allows multiple signal components to be distinguished. An orthogonal empirical wavelet filter bank that was designed for signal adaptive reconstruction is then used to separate the input time-frequency overlapped signals. The experimental results show that if two source components are completely overlapped within the time domain and the spectrum overlap ratio is less than 60%, the average separation performance is improved by approximately 32.3% when compared with the classic EWT; the proposed method also improves the suitability for multiple frequency shift keying (MFSK) and reduces the algorithm complexity.

• The Transactions of the Korean Institute of Power Electronics
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• v.12 no.2
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• pp.107-114
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• 2007

Islanding of PV systems occurs when the utility grid is removed but the PV systems continue to operate and provide power to local loads. Islanding is one of the serious problems in an electric power system connected with dispersed power sources. This can present safety hazards and the possibility of damage to other electric equipments. In the passive method, the voltage and frequency of PCC are measured and it determines islanding phenomena if their values excess the allowed limits. If the real and reactive power of RLC load and those of the PV system are closely matched, islanding phenomena can't be detected by the passive methods. Several active methods were proposed to detect islanding operation in the region where the passive method can not detect it. The most effective method is SFS method which was suggested by Sandia National Laboratory. In this paper, a new islanding detection method using automatic phase-shift is proposed and its validity is verified through the simulation and experimental results.

• Journal of the Korean Society of Manufacturing Technology Engineers
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• v.24 no.5
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• pp.583-587
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• 2015

In this study, a half-duplex mutual communication device via human skin was developed. Frequency-shift keying (FSK) digital modulation and demodulation control were used to transmit and receive data. Data communication through the human body is an effective communication technique and has high-grade security characteristics. In addition, making contact is a natural instinct of humans. Transmitting data through the human body is currently a highly conspicuous technology, because recently a lot of commercial sensors for humans have been developed. A body area network (BAN) can be easily constructed by this communication method on human skin. In this study, a half-duplex FSK mutual communication device was developed using a commercial FSK modulator and demodulator. A special control switching circuit and communication sequence were developed for mutual communication through human skin.

• Journal of the Korean Society for Precision Engineering
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• v.9 no.1
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• pp.126-136
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• 1992

In this paper, a simple structured feedback compensation scheme for a electro-hydraulic servo system to keep the response characteristics unchanged regardless of the load variation is proposed. In electro-hydraulic servo system, servovalve is most important control element. But the relation between input corrent and output flowrate of the servovalve has properties as follows; firstly, in spite of constant input current, output flowrate decreases as load pressure increases, secondly, according to frequency response of typical servovalve, the characteristics of gain and phase shift is something like 2'nd order system. Load pressure feedback compensation method has been applied to eliminate the first influence, the second influence has been improved by phase lead compensation method. As a result of above compensation methods, regardless of variation load condition, spring and inertia load, the compensation scheme has been verified to be effective within the range of frequency less than 25Hz by static response and dynamic response in time domain and frequency domain through experiments.

• Journal of Power Electronics
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• v.19 no.1
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• pp.307-315
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• 2019

Condition monitoring has been recognized as an effective and low-cost method to enhance the reliability and improve the maintainability of power electronic converters. In power electronic converters, high-frequency oscillation occurs during the switching transients of power transistors, which is known as ringing. The ringing frequency mainly depends on the values of the parasitic capacitance and stray inductance in the oscillation loop. Although circuit stray inductance is an important factor that leads to the ringing, it does not change with transistor aging. A shift in either the inside inductance or junction capacitance is an important failure precursor for power transistors. Therefore, ringing frequency can be used to monitor the health of power transistors. However, the switching actions of power transistors usually result in a dynamic behavior that can generate oscillation signals mixed with background noise, which makes it hard to directly extract the ringing frequency. A frequency extraction method based on empirical mode decomposition (EMD) and Fast Fourier transformation (FFT) is proposed in this paper. The proposed method is simple and has a high precision. Simulation results are given to verify the ringing analysis and experimental results are given to verify the effectiveness of the proposed method.

• Proceedings of the KIEE Conference
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• 1994.07a
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• pp.379-381
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• 1994

This paper is concerned with a zero-voltage soft-switching PWM DC-DC high-pelter converter using IGBTs, which Bakes the most of the parastic LC parameters of high-voltage transformer link, for diagnostic X-Ray power generator. The converter circuit basically utilizes phase-shift pulse width modulated series resonant full-bridge PWM DC-DC high-Power converter operating at a constant frequency:20kHz. This technique brings about dramatic decreases in the switching losses of power devices and their electrical stresses as compared with the commonly-used hard-switching PWM DC-DC power converter. The high-frequency switching operation of the converters has some effective advantages, which consist in the physical reduction in size and weight and lowered acoustic noise.