• The Journal of the Acoustical Society of Korea
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• v.27 no.8
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• pp.411-417
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• 2008

An alternative formulation of the Helmholtz integral equation derived to express the pressure field explicitly in terms of the velocity vector of a radiating surface is used to solve acoustic radiation and fluid/structure interaction problems. This formulation, derived for arbitrary sources, is similar in form to the Rayleigh's formula for planar sources. Because the surface pressure field is expressed explicitly as a surface integral of the surface velocity, which can be implemented numerically using standard Gaussian quadratures, there is no need to use BEM to solve a set of simultaneous equations for the surface pressure at the discretized nodes. Furthermore the non-uniqueness problem inherent in methods based on Helmholtz integral equation is avoided. Validation of this formulation is demonstrated for some simple geometries.

• Journal of the Korean Society of Fisheries and Ocean Technology
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• v.36 no.3
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• pp.257-265
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• 2000

Brushless DC(BLDC) motors are widely used as AC servo motors in factory automation fields because of their quick instantaneous mobility, good energy saving efficiency and easiness of design for control system comparing with induction motors. Recently, a Two-Degree-of-Freedom(2DOF) PI control law has been adopted to some application parts to accomplish an advanced speed control of BLDC motors. The method can treat the two conflicting performances, minimum tracking errors versus reference inputs without large overshoot and rejection of some disturbances including modeling errors, independently. However, the method can not design the optimal system which is able to minimize tracking errors and energy consumption simultaneously. In this paper, a 2DOF integral type optimal servo control method is investigated to promote the speed control performances of BLDC motors considering energy consumption. In order to applicate the method to the speed servo system of the BLDC motor, the motor is modeled in the state space using the vector control and decoupling technique. To verify the validity of the suggested method, some simulations and experiments are performed.

• Korean Journal of Fisheries and Aquatic Sciences
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• v.27 no.6
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• pp.782-786
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• 1994

Integrated finite difference method (IFDM) is used to solve one dimensional free oscillation problem in the coastal area. To evaluate the solution accuracy of IFDM in free oscillation analysis, two finite difference equations based on area discretization method and point discretization method are derived from the governing equations of free oscillation, respectively. The difference equations are transformed into a generalized eigenvalue problem, respectively. A numerical example is presented, for which the analytical solution is available, for comparing IFDM to conventional finite difference equation (CFDM), qualitatively. The eigenvalue matrices are solved by sub-space iteration method. The numerical results of the two methods are in good agreement with analytical ones, however, IFDM yields better solution than CFDM in lower modes because IFDM only includes first order differential operator in finite difference equation by Green's theorem. From these results, it is concluded that IFDM is useful for the free oscillation analysis in the coastal area.

• Geophysics and Geophysical Exploration
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• v.21 no.1
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• pp.1-7
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• 2018

In the two-dimensional (2D) modeling of electrical method, the potential in the space domain is reconstructed with the calculated potentials in the wavenumber domain using inverse Fourier transform. The inverse Fourier integral is numerically evaluated using the transformed potential at different wavenumbers. In order to improve the precision of the integration, either the logarithmic or exponential approximation has been used depending on the size of wavenumber. Two numerical methods have been generally used to evaluate the integral; interval integration and Gaussian quadrature. However, both methods do not consider the distance from the current source. Thus the resulting potential in the space domain shows some error. Especially when the distance from the current source is very small or large, the error increases abruptly and the evaluated potential becomes extremely unstable. In this study, we developed a new method to calculate the integral accurately by introducing the distance from the current source to the rescaled Gauss abscissa and weight. The numerical tests for homogeneous half-space model show that the developed method can yield the error level lower than 0.4 percent over the various distances from the current source.

• Journal of Korea Water Resources Association
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• v.31 no.2
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• pp.177-187
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• 1998

The spatially integrated model (SIM) which can evaluate temporal variation of groundwater quality is proposed in the stream-aquifer setting entered by nonpoint source contaminants. And the developed SIM included unsaturated soil zone and was tested against the spatially distributed model (SDM) of the coupled advection-dispersion and Richards equations for the various hydrologic and aquifer simulating conditions. The result of the comparison showed that the average concentration responses of saturated aquifer and groundwater outflow between the SIM and the SDM was in good agreement, except for the case of the large dispersivity ratio and thick aquifer system. And it is shown that for the cases of the large dispersivity ratio and thick aquifer system the performance of the nonlinear SIM is better than that of the linear SIM for evaluating the average concentration of groundwater outflow response.

• Journal of the Korean Institute of Telematics and Electronics C
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• v.35C no.9
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• pp.64-72
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• 1998

In this paper, the design of a current integrator for low-voltage, low-power, and high frequency applications using complementary high swing cascode current-mirror is presented. The proposed integrator decreases output current errors due to non-zero input resistance and non-infinite output resistance of the simple current integrator. As a design example, the 3rd order Butterworth lowpass filter is designed by a leapfrog method. Also, we apply the predistortion design method to reduce the magnitude distortion which occurs at a cutoff frequency by the undesirable phase shift of a lossless current integrator. The designed current-mode filter is simulated and examined by SPICE using 0.8$\mu\textrm{m}$ CMOS n-well process parameters. The simulation results show 20MHz cutoff frequency and 615㎼ power dissipation with a 2V power supply. And the cutoff frequency of the filters can be easily changed by the DC bias current.

• The Journal of the Korea institute of electronic communication sciences
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• v.16 no.3
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• pp.503-510
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• 2021

This study intends to apply the mathematical method of fractional order differentiation to a controller that controls the response of the system. Therefore, we design integrator for the fractional index by converting it into discrete time to construct a controller. The IP controller composes an integral controller for errors and the proportional controller applies only the system output. The controller is designed by using the fractional order integrator to the integral controller of the IP controller. First, the performance of the PI controller and the IP controller is compared, and the designed controller is applied to the speed control of the motor. As a result, the motor output speed was uniformed and precise control performance could be obtained. It was confirmed that the speed error in the steady state is within 0.1 [%], and it has precise and uniform speed control performance without overshoot.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.11 no.1
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• pp.17-27
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• 1991

The determination of the stress intensity factors is investigated by means of a surface integral defined around the crack tip of the structure. It has been shown in this work that this integral is derived from the standard path integral J. The use of the surface integral has also been extended to the case where body forces act. Computer program obtaining the stress intensity factors $K_I$ and $K_{II}$ is developed, which prepares input variables from the result of the conventional finite element analysis. This paper provides a parabolic smooth curve function. By the use of the function and conventional element meshes in which the aspect ratio (element length/crack length)is about 25 percent, relatively correct $K_I$ and $K_{II}$ values can be obtained for the exterior radius ranging from 1/3 to 1 of the crack length.

• Proceedings of the Computational Structural Engineering Institute Conference
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• 2009.04a
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• pp.304-307
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• 2009

본 논문에서는 3차원 모델링을 이용하여 원공을 갖는 알루미늄 판의 패치 보강효과에 대해 알아보고자 한다. 구조물의 노후화로 인해 높은 응력을 받는 부재의 응력 특이점에서 내구력이 급격하게 저하되거나 때로는 부재의 정적파괴를 유발시키는 원인을 제공한다. 이로 인해 과거에는 손상된 모재에 보강 재료를 연결시키기 위하여 리벳 또는 볼트와 같은 기계적 연결을 통해 보강하였으나 최근에는 접착패치보강 기법이 그 주류를 이루고 있다. 패치 보강시 일면 패치 보강으로 인하여 면외 휨 효과가 발생된다. 판의 두께 방향에 따른 응력집중계수를 별도로 분석하였다. 기존의 3차원 솔리드 요소는 해의 정확성은 뛰어난 반면에 상당한 컴퓨터 시간을 요구하는 단점을 가지고 있다. 이러한 문제를 극복하기 위해서, 본 논문에서는 각 층의 변위장을 2차원 형상함수와 1차원 형상함수의 조합으로 구성하여, 면내거동에 대한 p-세분화와 면외거동에 대한 p-세분화를 분리시키는 방식을 취한다. 또한, 에너지 함수의 적분시 Gauss-Lobatto 적분법을 사용하여 절점의 위치에서의 응력점을 구하는 경우, 외삽과정을 계산하는 단계를 생략하면서도, 해의 정확성 측면에서는 거의 차이가 없기 때문에 좀 더 효율적인 수치적분이 될 수 있다.

• Journal of the Earthquake Engineering Society of Korea
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• v.7 no.5
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• pp.11-17
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• 2003

It is well known that the double integration of measured acceleration records is one of the most difficult signal processing, particularly in the measurements on civil engineering structures, The measured accelerations of civil engineering structures are usually non-stationary and contain non-gaussian low-frequency noises, which can be significant causes of numerical instabilities in double Integration, For the de-noising of this kind of signals, wavelet transform can be very effective because of its inherent processing features for non-stationary signals, In this paper, the de-noising algorithm for the double integration is proposed using the modified wavelet transform, which is extended version of ordinary wavelet transform to process non-gaussian and low-frequency noises, using the median filter concept, The example studies show that the integration can be improved by the proposed method.