• 비파괴검사학회지
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• 제31권1호
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• pp.1-10
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• 2011

Nonlinear harmonic waves generated at cracked interfaces are investigated both experimentally and theoretically. A compact tension specimen is fabricated and the amplitude of transmitted wave is analyzed as a function of position along the fatigued crack surface. In order to measure as many nonlinear harmonic components as possible a broadband Lithium Niobate ($LiNbO_3$) transducers are employed together with a calibration technique for making absolute amplitude measurements with fluid-coupled receiving transducers. Cracked interfaces are shown to generate high acoustic nonlinearities which are manifested as harmonies in the power spectrum of the received signal. The first subharmonic (f/2) and the second harmonic (2f) waves are found to be dominant nonlinear components for an incident toneburst signal of frequency f. To explain the observed nonlinear behavior a partially closed crack is modeled by planar half interfaces that can account for crack parameters such as crack opening displacement and crack surface conditions. The simulation results show reasonable agreements with the experimental results.

• 한국항공우주학회지
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• 제38권5호
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• pp.445-455
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• 2010

본 연구에서는 실제 로켓엔진 및 가스터빈용 연소기 내부의 열음향 불안정을 효과적으로 예측하기 위하여, 헬름홀츠 방정식과 시간지연모델을 이용한 3차원 유한요소법 해석코드를 개발하였다. 연소응답항에 의해 수치적으로 야기되는 비선형성은 반복법으로 선형화 하였으며, Arnoldi 방법을 사용하여 대용량 고유치 문제를 해석하였다. 해석결과인 복소각주파수와 음향 압력장을 통해 각 음향모드의 공진주파수, 진폭의 증폭/감쇠 여부 그리고 모드 형태를 예측할 수 있다. 이론해가 존재하는 두 가지 문제를 통해 출구 임피던스와 예혼합 화염이 종 방향 음향장에 미치는 영향에 대한 예측 정확도를 평가하였으며, 배플 유무에 따른 횡 방향 음향 모드의 주파수 변이를 상온 음향시험 결과와 비교/검증하였다.

• 비파괴검사학회지
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• 제26권2호
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• pp.77-83
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• 2006

초음파 비선형성은 적층 구조물 내부의 미세크랙이나 계면 들뜸을 검출하기 위한 수단으로 주목받고 있다. 비선형 초음파의 특징적인 현상은 전파 과정중에 고조파가 발생되는 것이다. 그러므로 비선형성의 정량화를 위해서는 수신된 초음파 신호에 포함된 고조파 성분의 검출이 중요하며, 일반적으로 2차 조화 성분과 기본 주파수 성분의 진폭비가 비선형 파라미터로 이용되고 있다. 그러나 이 비선형 파라미터를 정확하게 추정할 수 있는 방법이 확립되어 있지 않기 때문에 현재까지는 현장적용에 어려움을 겪고 있다. 본 연구에서는 초음파 비선형 파라미터의 정밀 추정을 위한 신호 처리 기술을 제안하고자 한다 이 기술은 파워 스펙트럼과 바이스펙트럼 분석에 기초하며, 특히 본 연구에서는 초음파 현미경(SAM, scanning acoustic microscope)에 사용되는 펄스형 신호로부터 고조파를 검출하는데 주목하였다. 제안된 기법의 유효성은 틈새의 크기가 접촉 중심에서 반경 방향으로 일정하게 증가하는 뉴턴링(Newton-ring)과 칩의 윗면에 국부적인 들뜸을 가진 반도체 샘플에 대한 실험을 통해 검증되었다. 결과적으로 제안된 신호처리기법에 의해 획득된 비선형 파라미터는 계면 들뜸과 좋은 상관성을 보였다.

• 대한의용생체공학회:의공학회지
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• 제14권2호
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• pp.125-136
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• 1993

The study of Cochlear biomechanics is to clearly define three biomechanical principles of the Cochlea : Activity, Nonlinearity and Feedback. In this article, the Cochlea is linearly and actively modelled in one dimensional time domain. The sharp tunning of the Basilar Membrane displacement is shown when the amplifying activity of hair cells is added to the model. The amplified energy of the travelling displacement wave is emitted throughout the Cochlear fluid, so that the model becomes unstable. A new technique is introduced to reduce strong echos fro the Helicotrema. It makes the model less unstable. Both pure and click tones are used as input stimuli onto the ear durm. When the model is normal, the click response of the model shows that the backward emission of the amplified fluid pressure has mainly the echos from the Helicotrema. However, when the linear and active model is assumed to be abnormal, that is, some of hair cells are damaged not to produce the active process, the effect of the hair cell damage is resulted in the Oto-acoustic emission. The frequency response of the abnormally emitted sound pressure shows that the Oto-acoustic emission has the information about the characteristic frequency of the damaged hair cell. The main aim of this paper is to demonstrate the active biomechanics of the Chchlea in the time domain.

• 제어로봇시스템학회:학술대회논문집
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• 제어로봇시스템학회 1996년도 한국자동제어학술회의논문집(국내학술편); 포항공과대학교, 포항; 24-26 Oct. 1996
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• pp.445-448
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• 1996

For a lightly damped system, IIR based filter can have better performance than FIR filter as an adaptive filter in ANC algorithm. IIR based filter which has an infinite impulse response can model a lightly damped acoustic system with a small number of weights compared that of FIR filter and has no nonlinearity and instability problems on weight updating process which are associated to the conventional IIR filters. There are, however, some drawbacks such as design parameters to be determined earlier to get better performance and much increased computational power especially in the presence of error path. In this study, base filter parameters are determined in systematic manner, with the knowledge of the nominal impulse response of the system which should be identified, by Prony's method. Three methods reducing the computational load are proposed and their performance and application limits are discussed. Simulation and experimental results demonstrate the feasibility of the proposed method.

• 한국정밀공학회:학술대회논문집
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• 한국정밀공학회 1997년도 춘계학술대회 논문집
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• pp.817-821
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• 1997

Ever since the nonlinearity of machine tool dynamics was established, researchers attempted to make use of this fact to devise better monitoring, diagnostics and system, which were hitherto based on linear models. Theory of chaos, which explains many nonlinear phenomena comes handy for furthering the analysis using nonlinear model. In this study, measuring system will be constructed using multi-sensor (Tool Dynamometer, Acoustic Emission) in end millingprocess. Then, it will be verified that cutting force is low-dimensional deterministic chaos calculating Lyapunov exponents, Fractal dimension, Embedding dimension. Aen it will be investigated that the relations between characteristic parameter caculated form sensor signal and tool wear.

• 비파괴검사학회지
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• 제33권3호
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• pp.276-282
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• 2013

This paper proposes an ultrasonic method for measurement of linear and hysteretic interfacial stiffness of contacting surfaces between two steel plates subjected to nominal compression pressure. Interfacial stiffness was evaluated by the reflection and transmission coefficients obtained from three consecutive reflection waves from solid-solid surface using the shear wave. A nonlinear hysteretic spring model was proposed and used to define the quantitative interfacial stiffness of interface with the reflection and transmission coefficients. Acoustic model for 1-D wave propagation across interfaces is developed to formulate the reflection and transmission waves and to determine the linear and nonlinear hysteretic interfacial stiffness. Two identical plates are put together to form a contacting surface and pressed by bolt-fastening to measure interfacial stiffness at different states of contact pressure. It is found from experiment that the linear and hysteretic interfacial stiffness are successfully determined by the reflection and transmission coefficient at the contact surfaces through ultrasonic pulse-echo measurement.

• 한국정밀공학회지
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• 제14권11호
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• pp.93-101
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• 1997

Ever since the nonlinearity of machine tool dynamics was established, researchers attempted to make use of this fact to devise better monitoring, diagnostics and control system, which were hitherto based on linear models. Theory of chaos which explains many nonlinear phenomena comes handy for furthering the analysis using nonlinear model. In this study, measuring system will be constructed using multi-sensor (Tool Dynamometer, Acoustic Emission) in end milling process. Then, it will be verified that cutting force is low-dimensional chaos by calculating Lyapunov exponents. Fractal dimension, embedding dimension. And it will be investigated that the relation between characteristic parameter calculated from sensor signal and tool wear.

• 음성과학
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• 제2권
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• pp.125-134
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• 1997

Human beings can perceive various aspects of sound including loudness, pitch, length, and timber. Recently many studies were conducted to clarify complex auditory scales of the human ear. This study critically reviews some of these scales (decibel, sone, phon for loudness perception; mel and bark for pitch) and proposes to apply the scales to normalize acoustic correlates of human speech. One of the most important aspects of human auditory perception is the nonlinearity which should be incorporated into the linear speech analysis and synthesis system. Further studies using more sophisticated equipment are desirable to refine these scales, through the analysis of human auditory perception of complex tones or speech. This will lead scientists to develop better speech recognition and synthesis devices.

• 조명전기설비학회논문지
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• 제28권7호
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• pp.48-54
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• 2014

This paper discusses study of torque ripple minimization employing an improved TDF(torque distribution function)-based instantaneous torque control to reduce acoustic noise and vibration problem of the SRM. As the flux linkage of the SRM is a nonlinear function of phase current and rotor position, design of optimal controller for the SRM is quite complicated. Hence, an accurate mathematical model considering the nonlinearity of the SRM is required. An improved TDF based torque control has been proposed in order to reduce the toque ripple at high speed operation. Dynamic simulation using Matlab/Simulink as well as Finite Element Analysis is presented. A prototype SRM for electric vehicle traction has been manufactured to validate the experimental results comparing the dynamic simulation results.