• 한국지능시스템학회논문지
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• 제17권7호
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• pp.875-880
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• 2007

자기변성유체(magnetorheological fluid)에 관한 연구는 MR 장치의 개발, MR 장치의 수학적 모델링 및 시뮬레이션, 그리고 MR 장치를 채용한 시스템의 제어 알고리즘 개발에 관한 연구로 구분된다. 시뮬레이션을 통한 제어 알고리즘 개발을 위해서는 MR 장치의 비선형 응답을 예상하기 위한 신뢰성 높은 수학적 모델이 요구된다. 또한 MR 장치 시스템을 제어하기 위해서는 제어기에서 요구하는 댐핑력을 출력하기 위한 MR 장치의 전류(또는 전압) 입력 값이 필요하며, 이 입력값을 얻기 위해서는 역댐퍼 모델이 필요하다. 이러한 이유로 MR 장치의 모델링 및 역댐퍼 모델링은 MR 장치개발의 중요한 역할을 담당하며 이에 관한 많은 연구가 요구되고 있다. 본 연구에서는 전단모드 회전형 MR 댐퍼의 모델링을 위해 개발된 MR 댐퍼를 이용하여 동특성 시험기를 제작하였으며, 전단모드 회전형 MR 댐퍼의 특성을 연구하기 위한 실험을 수행하였다. 시험기 시험결과를 통해 모델링에 필요한 시험 데이터들을 획득하였으며 다층신경망을 이용하여 전단모드 회전형 MR 댐퍼의 모델 및 역모델을 구하였다.

• Geomechanics and Engineering
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• 제9권3호
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• pp.345-360
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• 2015

Wave equation analysis programs (WEAP) such as GRLWEAP and TNOWave were primarily developed for pre-driving analysis. They can also be used for post-driving measurement applications with some refinements. In the case of pre-driving analysis, the programs are used for the purpose of selecting the right equipment for a given ground condition and controlling stresses during pile driving processes. Recently, the program is increasingly used for the post-driving measurement application, where an assessment based on a variety of input parameters such as hammer, driving system and dynamic behaviour of soil is carried out. The process of this type of analysis is quite simple and it is performed by matching accurately known parameters, such as from CAPWAP analysis, to the parameters used in GRLWEAP analysis. The parameters that are refined in the typical analysis are pile stresses, hammer energy, capacity, damping and quakes. Matching of these known quantities by adjusting hammer, cushion and soil parameters in the wave equation program results in blow counts or sets and stresses for other hammer energies and capacities and cushion configuration. The result of this analysis is output on a Bearing Graph that establishes a relationship between ultimate capacity and net set per blow. A further application of this refinement method can be applied to the assessment of dynamic formulae, which are extensively used in pile capacity calculation during pile driving process. In this paper, WEAP analysis is carried out to establish the relationship between the ultimate capacities and sets using the various parameters and using this relationship to recalibrate the dynamic formula. The results of this analysis presented show that some of the shortcoming of the dynamic formula can be overcome and the results can be improved by the introduction of a correction factor.

• Smart Structures and Systems
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• 제20권5호
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• pp.619-636
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• 2017

Magnetorheological (MR) damper is a type of controllable device widely used in vibration mitigation. This device is highly nonlinear, and exhibits strongly hysteretic behavior that is dependent on both the motion imposed on the device and the strength of the surrounding electromagnetic field. An accurate model for understanding and predicting the nonlinear damping force of the MR damper is crucial for its control applications. The MR damper models are often identified off-line by conducting regression analysis using data collected under constant voltage. In this study, a MR damper model is integrated with a model for the power supply unit (PSU) to consider the dynamic behavior of the PSU, and then a real-time nonlinear model updating technique is proposed to accurately identify this integrated MR damper model with the efficiency that cannot be offered by off-line methods. The unscented Kalman filter is implemented as the updating algorithm on a cyber-physical model updating platform. Using this platform, the experimental study is conducted to identify MR damper models in real-time, under in-service conditions with time-varying current levels. For comparison purposes, both off-line and real-time updating methods are applied in the experimental study. The results demonstrate that all the updated models can provide good identification accuracy, but the error comparison shows the real-time updated models yield smaller relative errors than the off-line updated model. In addition, the real-time state estimates obtained during the model updating can be used as feedback for potential nonlinear control design for MR dampers.

• 한국음향학회지
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• 제16권2호
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• pp.10-15
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• 1997

수중 청음기는 진동하는 물체 위에 설치되어 다양한 외부 잡음 원이 유입되는 환경에 노출되어 있다. 외부 잡음 원으로는 수중 청음기가 설치된 구조물 자체의 진동, 프로펠러 잡음, 그리고 유동 유기 잡음들이 있고, 이들 외부 잡음원은 실제 강도가 상당히 높아서 센서의 정확한 작동에 장애가 되고 있다. 본 연구에서는 외부 잡음에 무관한 고 정밀도, 저 잡음 특성을 갖는 수중 청음기를 개발하기 위하여 유한 요소법(FEM)을 사용하여 잡음 전달 특성의 분석 및 air pocket과 음향 감쇠층의 다양한 조합으로 이루어진 개선된 구조의 수중 청음기를 설계하고, 내 잡음성 평가를 하였다. 그 결과 센서 측면 하단부에 잡음 원이 위치할 경우 가장 큰 잡음 신호로 작용하므로 구조를 변경한 결과 기존 수중 청음기에 비해 59% 이상 내 잡음성을 증진 시켰다.

• Structural Engineering and Mechanics
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• 제39권1호
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• pp.1-20
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• 2011

This study investigates the feasibility of detecting structural damage using the HHT method. A damage detection index, the ratio of bandwidth (RB) is proposed. This index is highly correlated or approximately equal to the change of equivalent damping ratio for an intact structure incurring damage from strong ground motions. Based on an analysis of shaking table test data from benchmark models subjected to adjusted Kobe and El Centro earthquakes, the damage detection index is evaluated using the Hilbert-Huang Transform (HHT) and the Fast Fourier Transform (FFT) methods, respectively. Results indicate that, when the response of the structure is in the elastic region, the RB value only slightly changes in both the HHT and the FFT spectra. Additionally, RB values estimated from the HHT spectra vs. the PGA values change incrementally when the structure response is nonlinear i.e., member yielding occurs, but not in the RB curve from the FFT spectra. Moreover, the RB value of the top floor changes more than those from the other floors. Furthermore, structural damage is detected only when using the acceleration response data from the top floor. Therefore, the ratio of bandwidth RB estimated from the smoothed HHT spectra is an effective and sensitive damage index for detecting structural damage. Results of this study also demonstrate that the HHT is a powerful method in analyzing the nonlinear responses of steel structures to strong ground motions.

• 한국재료학회:학술대회논문집
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• 한국재료학회 2009년도 춘계학술발표대회
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• pp.3.1-3.1
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• 2009

Normal sintering process of producing porous ceramics is not to sinter perfectly, i.e., stop sintering in middle-process. Our porous ceramic materials are a product of complete sintering. For example if one want to make a porous carborundum, raw carborundum powder is sintered at either lower temperatures than normal sintering temperature or shorter sintering periods than normal sintering time to obtain incompletely sintered materials, i.e., porous carborundum. This implies normally sintered porous ceramic materials can mot be used in high vacuum conditions due to dust coming out from uncompleted sintering. We could produce completely sintered porous ceramic materials. For example, we can produce porous carborundum material by using carborundum particles bonded by glassy material. The properties of this material are similar to carborundum. We could make quasi-zero thermal expansion porous material by using carborundum and particles of negative thermal expansion materials bonded by the glassy material. We apply to sinter them also by microwave to sinter quickly. We also use HIP process to introduce closed pores. We could sinter them in large size to produce $2.5m{\times}2.5m$ ceramic plate to use as a precision plate for flat display industries. This flat ceramic plate is the world largest artificial ceramic plate. Precision plates are basic importance to any advanced electronic industries. The produced precision plate has lower density, lower thermal expansivity, higher or similar damping properties added extra properties such as vacuum vise, air sliding capacity. These plates are highly recommended to use in flat display industries. We could produce also cylindrical porous ceramics materials, which can applied to precision roller for polymer film precision motion for also electronic industries.

• Earthquakes and Structures
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• 제12권4호
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• pp.425-436
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• 2017

Vibration control using a tuned mass damper (TMD) is an effective technique that has been verified using analytical methods and experiments. It has been applied in mechanical, automotive, and structural applications. However, the damping of a TMD cannot be adjusted in real time. An excessive mass damper stroke may be introduced when the mass damper is subjected to a seismic excitation whose frequency content is within its operation range. The semi-active tuned mass damper (SATMD) has been proposed to solve this problem. The parameters of an SATMD can be adjusted in real time based on the measured structural responses and an appropriate control law. In this study, a stiffness-controllable TMD, called a leverage-type stiffness-controllable mass damper (LSCMD), is proposed and fabricated to verify its feasibility. The LSCMD contains a simple leverage mechanism and its stiffness can be altered by adjusting the pivot position. To determine the pivot position of the LSCMD in real time, a discrete-time direct output-feedback active control law that considers delay time is implemented. Moreover, an identification test for the transfer function of the pivot driving and control systems is proposed. The identification results demonstrate the target displacement can be achieved by the pivot displacement in 0-2 Hz range and the control delay time is about 0.1 s. A shaking-table test has been conducted to verify the theory and feasibility of the LSCMD. The comparisons of experimental and theoretical results of the LSCMD system show good consistency. It is shown that dynamic behavior of the LSCMD can be simulated correctly by the theoretical model and that the stiffness can be properly adjusted by the pivot position. Comparisons of experimental results of the LSCMD and passive TMD show the LSCMD with less demand on the mass damper stroke than that for the passive TMD.

• Smart Structures and Systems
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• 제24권3호
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• pp.303-317
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• 2019

Magneto-rheological fluid (MRF) rotatory dampers are normally used for controlling the constant rotation of machines and engines. In this research, such a device is proposed to act as variable stiffness device to alleviate the rotational oscillation existing in the many engineering applications, such as motor. Under such thought, the main purpose of this work is to characterize the nonlinear torque-angular displacement/angular velocity responses of an MRF based variable stiffness device in oscillatory motion. A rotational hysteresis model, consisting of a rotatory spring, a rotatory viscous damping element and an error function-based hysteresis element, is proposed, which is capable of describing the unique dynamical characteristics of this smart device. To estimate the optimal model parameters, a modified whale optimization algorithm (MWOA) is employed on the captured experimental data of torque, angular displacement and angular velocity under various excitation conditions. In MWOA, a nonlinear algorithm parameter updating mechanism is adopted to replace the traditional linear one, enhancing the global search ability initially and the local search ability at the later stage of the algorithm evolution. Additionally, the immune operation is introduced in the whale individual selection, improving the identification accuracy of solution. Finally, the dynamic testing results are used to validate the performance of the proposed model and the effectiveness of the proposed optimization algorithm.

• 한국지진공학회논문집
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• 제11권5호
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• pp.23-32
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• 2007

본 논문은 형상기억합금 바의 휨 거동 특성 파악하기 위하여 여러 가지의 휨 거동 실험을 수행하였으며, 형상기억합금 바의 휨 거동분석을 통하여 지진 시에 적용 가능성을 규명하는데 목적이 있다. 이를 위해 단순 휨 및 이중 휨 실험을 재하속도 및 최대변위를 변수로 수행하였다. 힘-변위 곡선에서 추정한 재하 및 제하 시의 강성이 추정되었으며, 등가의 감쇠비도 각 실험결과에서 추정되었다. 단순 휨에서 형상기억합금 바는 32 mm 변위 이후에 강성증가현상을 나타냈으며, 이것은 SIM(Stress-Induced-Martensite) 현상에 의해서 발생하는 것으로 추정된다. 재하속도의 증가는 형상기억합금 휨 강성 증가에 영향을 주지 않는 것으로 나타났다. 이중 휨 거동에서 형상기억합금 바는 단순 휨에 비해 강성이 약 5배정도 크게 나타났으며, 감쇠비는 유사하게 나타났다. 휨 거동의 형상기억합금 바를 지진 변위 제어장치로 사용하여 3경간 단순지지 교량에 적용하여 지진해석을 수행하였다. 이러한 지진변위 제어장치는 매우 효과적인 것으로 나타났으며, 실용적인 것으로 판단된다. 본 논문의 의미는 형상기억합금 바의 휨 거동에 대한 기초 지식을 제공하는데 있다.