• Restorative Dentistry and Endodontics
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• v.38 no.3
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• pp.146-153
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• 2013

Objectives: Aluminum step wedge (ASW) equivalent radiodensity (eRD) has been used to quantify restorative material's radiodensity. The aim of this study was to evaluate the effects of image acquisition control (IAC) of a digital X-ray system on the radiodensity quantification under different exposure time settings. Materials and Methods: Three 1-mm thick restorative material samples with various opacities were prepared. Samples were radiographed alongside an ASW using one of three digital radiographic modes (linear mapping (L), nonlinear mapping (N), and nonlinear mapping and automatic exposure control activated (E)) under 3 exposure time settings (underexposure, normal-exposure, and overexposure). The ASW eRD of restorative materials, attenuation coefficients and contrasts of ASW, and the correlation coefficient of linear relationship between logarithms of gray-scale value and thicknesses of ASW were compared under 9 conditions. Results: The ASW eRD measurements of restorative materials by three digital radiographic modes were statistically different (p = 0.049) but clinically similar. The relationship between logarithms of background corrected grey scale value and thickness of ASW was highly linear but attenuation coefficients and contrasts varied significantly among 3 radiographic modes. Varying exposure times did not affect ASW eRD significantly. Conclusions: Even though different digital radiographic modes induced large variation on attenuation of coefficient and contrast of ASW, E mode improved diagnostic quality of the image significantly under the underexposure condition by improving contrasts, while maintaining ASW eRDs of restorative materials similar. Under the condition of this study, underexposure time may be acceptable clinically with digital X-ray system using automatic gain control that reduces radiation exposure for patient.

• Transactions of the Korean Society of Automotive Engineers
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• v.18 no.6
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• pp.130-137
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• 2010

Due to test facility and specialty, it is physically difficult to conduct the modal tests of large-sized structures such as truck, bus and airplane. So, in case of a large-sized truck, the mode analysis on a full vehicle model comprised of reliable cabin, frame, and deck has been generally performed. However, the reliability of overall vibrational characteristics of the analytic model has not been fairly guaranteed by the testified models of each subsystem owing to the existence of cab suspension and the nonlinear mounting between a chassis frame and a special deck system. In this paper, a method to find out the modal characteristics of a large-sized military truck is presented. New modal test equipment is developed to set the boundary conditions of three military truck variants as close as a free-free condition. And the mode analysis method using coupled structure and dynamic models is established to consider the above-mentioned dynamic non-linearities of the vehicle itself. The usefulness of the suggested method is verified by comparing with the modal test results. Finally, the modal parameters of the final variant are extracted using the proved analytic method.

• Journal of the Korean Society for Precision Engineering
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• v.13 no.3
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• pp.102-109
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• 1996

In this paper, we present a new approach for the automation of deburring process. An algorithm for teaching skills of a human expert to a robot manipulator is developed. This approach makes use of TSK fuzzy mode that can wxpress a highly nonlinear functional relation with small number of rules. Burr features such as height, width, area, grinding area are extracted from image processing by use of the vision system. Grinding depth, repetitive number and normal grinding force are chosen as control signals representing actions of the human expert. It is verified that our proposed fuzzy model can accurately express the skills of human experts for the deburring process.

• Computers and Concrete
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• v.28 no.5
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• pp.521-532
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• 2021

Present paper focuses on the modeling of size effect on the compressive strength of normal concrete with the application of Discrete Element Method (DEM). Test specimens with different size and shape were cast and uniaxial compressive strength test was performed on each sample. Five different concrete mixes were used, all belonging to a different normal strength concrete class (C20/25, C30/37, C35/45, C45/55, and C50/60). The numerical simulations were carried out by using the PFC 5 software, which applies rigid spheres and contacts between them to model the material. DEM modeling of size effect could be advantageous because the development of micro-cracks in the material can be observed and the failure mode can be visualized. The series of experiments were repeated with the model after calibration. The relationship of the parallel bond strength of the contacts and the laboratory compressive strength test was analyzed by aiming to determine a relation between the compressive strength and the bond strength of different sized models. An equation was derived based on Bazant's size effect law to estimate the parallel bond strength of differently sized specimens. The parameters of the equation were optimized based on measurement data using nonlinear least-squares method with SSE (sum of squared errors) objective function. The laboratory test results showed a good agreement with the literature data (compressive strength is decreasing with the increase of the size of the specimen regardless of the shape). The derived estimation models showed strong correlation with the measurement data. The results indicated that the size effect is stronger on concretes with lower strength class due to the higher level of inhomogeneity of the material. It was observed that size effect is more significant on cube specimens than on cylinder samples, which can be caused by the side ratios of the specimens and the size of the purely compressed zone. A limit value for the minimum size of DE model for cubes and cylinder was determined, above which the size effect on compressive strength can be neglected within the investigated size range. The relationship of model size (particle number) and computational time was analyzed and a method to decrease the computational time (number of iterations) of material genesis is proposed.

• The Journal of the Korea institute of electronic communication sciences
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• v.18 no.5
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• pp.859-866
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• 2023

This paper focuses on configuring a user-friendly system of LED systems by applying improvement measures such as gamma correction, non-flicker, and driving noise removal using MCUs and LED drivers. As a result of the experiment, the 22kHz PWM mode of the LED driver generated noise outside the audible frequency range, making it practically imperceptible to users. The appropriate pull-up resistor values within the normal operating delay ratio of 5% were found to be 1kΩ to 10kΩ for the 3kHz PWM mode and 1kΩ to 2kΩ for the 22kHz PWM mode. In addition, gamma correction can be optimized for nonlinear human visual systems to express accurate contrast and as a result, it is expected to develop an LED system that can be expressed more naturally and accurately than conventional LED systems and improve users' visual experience.

• Journal of the Korean Institute of Illuminating and Electrical Installation Engineers
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• v.16 no.5
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• pp.53-60
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• 2002

This paper presents exact autotransformer-fed AC electric railroad system modeling using constant current mode, and single-phase STATCOM(Static Synchronous Compensator) which has an effect on electric railroad system. An AC electric railroad is rapidly changing single-phase feeding electric power. To avoid voltage fluctuation under single phase loads, electric power should be received from a large source. The system modeling theory is based on the solution of algebraic. The AC electric railroad load model is nonlinear. Therefore this paper is considered nonlinear load using PSCAD/EMTDC. And the proposed modeling method is considered the line self-impedances and mutual-impedances that techniques for the AC electric railroad system modeling analysis, and that single-phase STATCOM can reliably compensate the voltage drop. In the case study, the allowance range of feeding voltage is 22.5∼27.5 kV, AT-fed AC electric railroad system circuit is analyzed by loop equation both normal and extension modes. The simulation objectives are to calculate the catenary and rail voltages with respect to ground, as the train moves along a section of line between two adjacent ATs. The results show that single-phase STATCOM can reduce the voltage drop in the feeding circuit and improve the power quality at AC electric railroad system by compensating the reactive power.

• Journal of the Korea Institute of Military Science and Technology
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• v.10 no.4
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• pp.81-87
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• 2007

In this study, analysis of structural design criteria for the canopy actuating device has been conducted considering the aerodynamic breakaway capabilities of jettisonable canopy system. Unsteady aerodynamic loads for the opened canopy configuration at passively controlled jettision mode were computed using CFD method. The general purpose multi-body finite element code, SAMCEF Mecano, is used in the implemented analyses for the passive jettision condition. The recommended altitude and speed of aircraft was suggested as design criteria of aerodynamic breakaway capability of jettisonable canopy system as a bakup egress method when normal canopy jettison sequence malfunctioned. Aerodynamic breakaway condition of jettisonable canopy was also simulated and the fracture load conditions of canopy actuator were investigated.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.13 no.2
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• pp.209-220
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• 2000

For non-linear analysis of stiffened shell structures, the total Lagrangian formulation is presented based upon the degenerated shell element. Geometrically correct formulation is developed by updating the direction of normal vectors and taking into account second order rotational terms in the incremental displacement field. Assumed strain concept is adopted in order to overcome shear locking phenomena and to eliminate spurious zero energy mode. The post-buckling behaviors of stiffened shell structures are traced by modeling the stiffener as a shell element and considering general transformation between the main structure and the stiffener at the connection node. Numerical examples to demonstrate the accuracy and the effectiveness of the proposed shell element are presented and compared with references' results.

• Journal of Electrical Engineering and Technology
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• v.13 no.2
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• pp.781-789
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• 2018

This paper proposes a new approach utilizing empirical mode decomposition (EMD) reconstruction to process vibration signals of a transformer under DC bias caused by high voltage direction transmission (HVDC), which is the potential cause of additional vibration and noise from transformer. Firstly, the Calculation Method is presented and a 3D model of transformer is simulated to analyze transformer deformation characteristic and the result indicate the main vibration is produced along axial direction of three core limbs. Vibration test system has been built and test points on the core and shell of transformer have been measured. Then, the signal reconstruction method for transformer vibration based on EMD is proposed. Through the EMD decomposition, the corrupted noise can be selectively reconstructed by the certain frequency IMFs and better vibration signals of transformer have been obtained. After EMD reconstruction, the vibrations are compared between transformer in normal work and with DC bias. When DC bias occurs, odd harmonics, vibration of core and shell, behave as a nonlinear increase and the even harmonics keep unchanged with DC current. Experiment results are provided to collaborate our theoretical analysis and to illustrate the effectiveness of the proposed EMD method.

• Advances in aircraft and spacecraft science
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• v.9 no.3
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• pp.263-278
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• 2022

The time-varying structural reliability of an aeroelastic launch vehicle subjected to stochastic parameters is investigated. The launch vehicle structure is under the combined action of several stochastic loads that include aerodynamics, thrust as well as internal combustion pressure. The launch vehicle's main body structural flexibility is modeled via the normal mode shapes of a free-free Euler beam, where the aerodynamic loadings on the vehicle are due to force on each incremental section of the vehicle. The rigid and elastic coupled nonlinear equations of motion are derived following the Lagrangian approach that results in a complete aeroelastic simulation for the prediction of the instantaneous launch vehicle rigid-body motion as well as the body elastic deformations. Reliability analysis has been performed based on two distinct limit state functions, defined as the maximum launch vehicle tip elastic deformation and also the maximum allowable stress occurring along the launch vehicle total length. In this fashion, the time-dependent reliability problem can be converted into an equivalent time-invariant reliability problem. Subsequently, the first-order reliability method, as well as the Monte Carlo simulation schemes, are employed to determine and verify the aeroelastic launch vehicle dynamic failure probability for a given flight time.