• Publications of The Korean Astronomical Society
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• v.26 no.2
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• pp.71-87
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• 2011

Gravitational waves are predicted by the Einstein's theory of General Relativity. The direct detection of gravitational waves is one of the most challenging tasks in modern science and engineering due to the 'weak' nature of gravity. Recent development of the laser interferometer technology, however, makes it possible to build a detector on Earth that is sensitive up to 100-1000 Mpc for strong sources. It implies an expected detection rate of neutron star mergers, which are one of the most important targets for ground-based detectors, ranges between a few to a few hundred per year. Therefore, we expect that the gravitational-wave observation will be routine within several years. Strongest gravitational-wave sources include tight binaries composed of compact objects, supernova explosions, gamma-ray bursts, mergers of supermassive black holes, etc. Together with the electromagnetic waves, the gravitational wave observation will allow us to explore the most exotic nature of astrophysical objects as well as the very early evolution of the universe. This review provides a comprehensive overview of the theory of gravitational waves, principles of detections, gravitational-wave detectors, astrophysical sources of gravitational waves, and future prospects.

• Journal of the Korean Society of Surveying, Geodesy, Photogrammetry and Cartography
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• v.38 no.6
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• pp.717-723
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• 2020

Determination of local tie vectors between the space geodetic techniques such as VLBI (Very Long Baseline Interferometer), SLR (Satellite Laser Ranging), DORIS (Doppler Orbit determination and Radiopositioning Integrated on Satellite), GNSS (Global Navigation Satellite System) is essential for combination of ITRF (International Terrestrial Reference Frame). Therefore, it is required to compute IVP (Invariant Point) position of each space geodetic technique with high accuracy. In this study, we have computed Sejong VLBI IVP position by using updated mathematical model for adjustment computation so that the improvement on efficiency and reliability in computation are obtained. The measurements used for this study are the coordinates of reflective targets on the VLBI antenna and their accuracies are set to 1.5 mm for each component. The results show that the position of VLBI IVP together with its standard deviation is successfully estimated when they are compared with those of the results from previous study. However, it is notable that additional terrestrial surveying should be performed so that realistic measurement errors are incorporated in the adjustment computation process.

• Journal of the Korean Society for Precision Engineering
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• v.13 no.10
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• pp.154-160
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• 1996

The control of diamond turning is usually achieved through a laser-interferometer feedback of slide position. The limitation of this control scheme is that the feedback signal does not account for additional dynamics of the tool post and the material removal process. If the tool post is rigid and the material removal process is relatively static, then such a non-collocated position feedback control scheme may surfice. However, as the accuracy requirement gets tighter and desired surface cnotours become more complex, the need for a direct tool-tip sensing becomes inevitable. The physical constraints of the machining process prohibit any reasonable implementation of a tool-tip motion measurement. It is proposed that the measured force normal to the face of the workpiece can be filtered through an appropriate admittance transfer function to result in the estimated dapth of cut. This can be compared to the desired depth of cut to generate the adjustment control action in additn to position feedback control. In this work, the design methodology on the admittance model-based control with a conventional controller is presented. The recursive least-squares algorithm with forgetting factor is proposed to identify the parameters and update the cutting process in real time. The normal cutting forces are measured to identify the cutting dynamics in the real diamond turning process using the precision dynamoneter. Based on the parameter estimation of cutting dynamics and the admitance model-based nanodynamic control scheme, simulation results are shown.

• Composites Research
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• v.37 no.1
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• pp.21-31
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• 2024

This review paper investigates the use of shockwave attenuating materials within composite structures to enhance personnel protection against blast-induced traumatic brain injury (bTBI). This paper also introduces experimental methodologies exploited in the generation and measurement of shockwaves to evaluate the performance of the shock dissipating composites. The generation of shockwaves is elucidated through diverse approaches such as high-energy explosives, shock tubes, lasers, and laser-flyer techniques. Evaluation of shockwave propagation and attenuation involves the utilization of cutting-edge techniques, including piezoelectric, interferometer, electromagnetic induction, and streak camera methods. This paper investigates phase-separated materials, including polyurea and ionic liquids, and provides insight into composite structures in the quest for shockwave pressure attenuation. By synthesizing and analyzing the findings from these experimental approaches, this review aims to contribute valuable insights to the advancement of protective measures against blast-induced traumatic brain injuries.

• Journal of the Institute of Electronics and Information Engineers
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• v.50 no.8
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• pp.253-263
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• 2013

Frequency Scanning Interferometry(FSI) system, one of the most promising optical surface measurement techniques, generally results in superior optical performance comparing with other 3-dimensional measuring methods as its hardware structure is fixed in operation and only the light frequency is scanned in a specific spectral band without vertical scanning of the target surface or the objective lens. FSI system collects a set of images of interference fringe by changing the frequency of light source. After that, it transforms intensity data of acquired image into frequency information, and calculates the height profile of target objects with the help of frequency analysis based on Fast Fourier Transform(FFT). However, it still suffers from optical noise on target surfaces and relatively long processing time due to the number of images acquired in frequency scanning phase. 1) a Polarization-based Frequency Scanning Interferometry(PFSI) is proposed for optical noise robustness. It consists of tunable laser for light source, ${\lambda}/4$ plate in front of reference mirror, ${\lambda}/4$ plate in front of target object, polarizing beam splitter, polarizer in front of image sensor, polarizer in front of the fiber coupled light source, ${\lambda}/2$ plate between PBS and polarizer of the light source. Using the proposed system, we can solve the problem of fringe image with low contrast by using polarization technique. Also, we can control light distribution of object beam and reference beam. 2) the signal processing acceleration method is proposed for PFSI, based on parallel processing architecture, which consists of parallel processing hardware and software such as Graphic Processing Unit(GPU) and Compute Unified Device Architecture(CUDA). As a result, the processing time reaches into tact time level of real-time processing. Finally, the proposed system is evaluated in terms of accuracy and processing speed through a series of experiment and the obtained results show the effectiveness of the proposed system and method.

• The Journal of the Acoustical Society of Korea
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• v.33 no.1
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• pp.31-39
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• 2014

In this paper, two calibration methods for angular vibration pickups using a precision rotary encoder are proposed. The KRISS (Korea Research Institute of Standards and Science) primary angular vibration calibration system and the calibration procedures are briefly explained. The rotary encoder is shown to be calibrated in two methods: The one is to use the laser interferometer to calibrate the rotary encoder under test and the other is to exploit the certificate of the encoder supplied. Complex sensitivities measured from the first are shown to be less than 0.1 % difference in magnitude and $0.01^{\circ}$ difference in phase shift in reference to those of the primary calibration system. Their expanded uncertainties were observed to be less than 0.6 % in magnitude and $0.4^{\circ}$ in phase shift over the range of 0.4 to 200 Hz. Under the same calibration conditions, complex sensitivities evaluated by the second method are shown be 0.1 % difference in magnitude and $0.6^{\circ}$ difference in phase shift in reference to those of the primary calibration system. Their expanded uncertainties were seen to be less than 4.8 % in magnitude and $2.8^{\circ}$ in phase shift.