• 제어로봇시스템학회:학술대회논문집
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• 제어로봇시스템학회 2000년도 제15차 학술회의논문집
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• pp.261-261
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• 2000

This paper analyzes the characteristics of pre-sliding friction of an X-Y table of CNC machining center at velocity reversal, and presents a simple and effective method of friction compensation based on this characteristics. At velocity reversal, a large position tracking error occurs because of the discontinuous change of friction. The relationship between the occurrence time of maximum position tracking error and the acceleration at zero velocity is analyzed by using the spring-like friction model. Furthermore, the experimental observation verifies this relation. From this, the state transition tine from pre-sliding regime into sliding regime can be predicted. Using the predicted transition time, the friction can be effectively compensated and table experimental results show its effectiveness.

• Journal of Mechanical Science and Technology
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• 제15권12호
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• pp.1892-1899
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• 2001

An experimental study was performed for measuring velocity and turbulence intensity in a circular tube for Re= 10,000, 15,000 and 20,000, with swirl and without swirling flow. The velocity fields were measured using PIV techniques and swirl motion was produced by a tangential inlet condition. Some preliminary measurements indicated that over the first 4 diameter, two regions of flow reversal were set up (the so called 2-cell structure). At the highest Reynolds numbers, the maximum values of the measured axial velocity components had moved toward the test tube wall and produce more flow reversal at the conter of the tube. As the Reynolds number increased, the turbulence intensity of swilling flow at the tube inlet also increased.

• 한국안전학회지
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• 제29권6호
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• pp.104-110
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• 2014

The Han River is the only waterway in Korea where estuary is not blocked by dykes so that tidal water is flowing in and out through the tidal reach. The extreme tidal range in the Yellow Sea causes an intense flood current, stretching over horizontal extents of tens of kilometers into the rivers. To elucidate the flow reversal by discharge conditions and transient tidal level in the Han river, numerical simulations were conducted under 7 boundary conditions for two days with 10 minute time step. As the flow conditions changed from low discharge and high tidal difference to high discharge and low tidal difference, the flow reversals became weaker and the velocity of forward flow direction became higher due to the increased flow momentums and decreased tidal differences. In the case of normal flow, the maximum reverse velocity was 0.4 m/s, which was equivalent to the maximum forward velocity. In addition, the pattern of the development and decay of forward and reverse flow was presented.

• PNF and Movement
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• 제13권3호
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• pp.119-125
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• 2015

Purpose: The current study seeks to examine the effects of stabilizing reversal(SR) and rhythmic stabilization(RS) in proprioceptive neuromuscular facilitation (PNF) techniques on static balance in normal female adults. Methods: Subjects included 20 healthy women in their 20s. They were randomly divided into SR and RS groups. To assess static balance before and after PNF intervention, the good balance system was used to measure the interior and exterior center of pressure (COP) movement distance, front and rear COP movement distance before and after the intervention, and velocity moment. Results: Interior and exterior COP movement distance, front and rear COP movement distance, and velocity moment decreased in the SR group, but no statistically significant differences were observed. In the RS group, the interior and exterior COP movement distance declined without any significant difference, while the front and rear COP movement distance and velocity moment significantly decreased. The interior and exterior COP movement distance, front and rear COP movement distance, and velocity moment declined more in the RS group than in the SR group. Conclusion: The findings of the current study indicate that the RS technique is more effective than the SR technique for improving the balance ability of healthy female adults. The results of this study can thus be utilized in the selection of effective interventions for improving the balance ability of healthy female adults.

• 한국신재생에너지학회:학술대회논문집
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• 한국신재생에너지학회 2006년도 춘계학술대회
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• pp.377-381
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• 2006

A seismic survey for gas hydrate have performed over the East sea by the KIGAM since 1997. General indicator of gas hydrate in seismic data is commonly inferred from the BSR(Bottom Simulating Reflector) that occurred parallel to the sea floor, amplitude decrease at the top of the BSR, amplitude blanking at the bottom of the BSR, decrease of the interval velocity and the reflection phase reversal at the BSR. In this paper we had analyzed optimum parameters of the field data to detect the 9as hydrate. Shot delay correction is applied 95ms, spherical divergence correction is applied velocity library 3, bandpass filter is applied 25-30-115-120Hz deconvolution operator length is applied 60ms, lag is 6ms and accurate velocity analysis NMO correction, stack is performed. Geobit 2.11.0 developed by the KIGAM was used for all data processing. Processing results say that the BSR occurred parallel to the sea floor were shown at 3,150m/s of two way travel time from the sea floor through shot point 5,000-5,610, and identified the interval velocity decrease around BSR and the reflection phase reversal corresponding to the reflection at the sea floor.

• 한국전산구조공학회논문집
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• 제22권1호
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• pp.89-103
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• 2009

이 연구에서는 얇은 판형구조물의 손상탐지에 널리 사용되어오는 램파에 시간반전(time reversal)개념의 적용성을 이론적으로 규명한다. 고전적 시간반전 음향학에 의하면, 센서에서의 출력신호를 시간영역에서 반전 후 재입사시켜 원래의 가진점으로 돌려보내면, 그 가진점에서 원래 입력신호가 복원된다. 그러나 램파에 시간반전과정을 적용하게 되면 램파 고유의 분산성과 판 경계에서의 파 반사로 인해 시간반전성이 복잡한 양상을 띠게 된다. 이러한 램파의 시간반전성을 보다 잘 이해하기 위해 이 연구에서는 램파의 시간반전과정을 이론적으로 규명한다. 특히, 램파의 내부모드분산, 다중모드분산, 그리고 판 경계면에서의 램파의 반사가 시간반전성에 미치는 영향을 정식화하였다 간단한 수치예제를 통해 이 연구에서 제시된 이론적 발견들의 타당성을 검증한다.

• 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.

• Smart Structures and Systems
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• 제33권1호
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• pp.41-53
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• 2024

Grouting sleeves are an essential connecting component of prefabricated components, and the quality of grouting has a significant influence on structural integrity and seismic performance. The embedded grouting sleeve (EGS)'s grouting defects are highly undetectable and random, and no effective monitoring method exists. This paper proposes an ultrasonic guided wave method and provides a set of guidelines for selecting the optimal frequency and suitable period for the EGS. The optimal frequency was determined by considering the group velocity, wave structure, and wave attenuation of the selected mode. Guided waves are prone to multi-modality, modal conversion, energy leakage, and dispersion in the EGS, which is a multi-layer structure. Therefore, a time-reversal (TR)-based multi-mode focusing and dispersion automatic compensation technology is introduced to eliminate the multi-mode phase difference in the EGS. First, the influence of defects on guided waves is analyzed according to the TR coefficient. Second, two major types of damage indicators, namely, the time domain and the wavelet packet energy, are constructed according to the influence method. The constructed wavelet packet energy indicator is more sensitive to the changes of defecting than the conventional time-domain similarity indicator. Both numerical and experimental results show that the proposed method is feasible and beneficial for the detection and quantitative estimation of the grouting defects of the EGS.

• 지구물리와물리탐사
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• 제2권4호
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• pp.184-190
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• 1999

가스 하이드레이트는 전세계적으로 새로운 에너지 자원으로 활용 가능성을 포함하고 있어 연구가 활발하게 진행되고 있다. 한국자원연구소에서는 1997년 부터 동해에서 메탄 하이드레이트 부존 잠재력 규명을 위한 탄성파 탐사를 하고 있다. 탄성파 자료에서 하이드레이트 부존을 의미하는 일반적인 특성은 해저면과 평행하게 나타나는 BSR(Bottom Simulating Reflection)과 BSR 상부에서 보이는 진폭감소 그리고 BSR 하부에서 보이는 진폭증가와 구간속도의 감소 그리고 BSR에서 반사파의 역전현상 등이 있다. 따라서 위와 같은 하이드레이트 부존특성을 탐지하기 위한 목적으로 실시되는 자료처리는 진 진폭을 유지하는 자료처리, 정밀 속도분석 및 AVO분석 등이 요구된다. 본 연구는 1998년 동해에서 취득된 탄성파 탐사자료를 처리하여 하이드레이트 부존 가능성을 확인하고자 하였다. 적용된 자료처리 공정은 구형확산 보정과 주파수 필터링, 공심점 분류, 정밀 속도분석 공정 등이다. AVO분석은 이용된 현장자료가 AVO를 분석할 정도의 입사각을 유지하고 있지 않아 제외하였다. 정밀 속도분석은 반복적으로 속도 스펙트럼을 구하는 방법으로 정확한 중합속도 결정이 가능한 XYA를 이용하였으며 자료처리의 모든 공정은 국내고유의 탄성파 자료처리 소프트웨어인 Geobit 2.9.5 를 이용하였다. 자료처리 결과 음원위치 $1650\~1900$에서 해저면으로 부터 약 $367\~477m$ 깊이(왕복주시 약 1800ms)에 해저면과 평행하게 발달한 BSR을 확인할 수 있었으며, BSR부근에서 구간속도 감소 뿐만 아니라 해저면 반사파의 위상과 반대인 반사파 역전현상도 확인할 수 있었다.

• 대한전자공학회논문지SD
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• 제44권3호
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• pp.20-25
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• 2007

본 논문에서는 $LiNbO_3$의 선택적 영역을 도메인 반전을 수행하였으며, 이를 대역변조기 및 SSB 광변조기 제작에 응용하였다. 인가전압에 대한 회로적인 응답전류를 분석 및 고려함으로써 도메인 벽의 이동속도를 정확히 제어할 수 있었다. 과도한 도메인의 벽 이동속도에 의한 도메인 반전 형상을 확인하였고, 또한 도메인 벽의 진행방향에 따라 그 속도의 차이가 발생함을 알 수 있었다. 제작된 대역변조기는 30.3 GHz를 중심주파수로 하여 5.1GHz의 3dB 대역폭을 보였고. SSB 광변조기의 변조 스펙트럼으로부터 19dBm의 5.8GHz RF 입력신호에 대해 USB가 LSB에 비해 33dB정도 억제됨을 확인할 수 있었다.