• Smart Structures and Systems
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• v.29 no.3
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• pp.499-512
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• 2022

This work proposes a novel contactless vibration damping and thermal isolation tripod platform based on Superconducting Magnetic Levitation (SML). This prototype is suitable for cryogenic environments, where classical passive, semi active and active vibration isolation techniques may present tribological problems due to the low temperatures and/or cannot guarantee an enough thermal isolation. The levitating platform consists of a Superconducting Magnetic Levitation (SML) with inherent passive static stabilization. In addition, the use of Operational Modal Analysis (OMA) technique is proposed to characterize the transmissibility function from the baseplate to the platform. The OMA is based on the Stochastic Subspace Identification (SSI) by using the Expectation Maximization (EM) algorithm. This paper contributes to the use of SSI-EM for SML applications by proposing a step-by-step experimental methodology to process the measured data, which are obtained with different unknown excitations: ambient excitation and impulse excitation. Thus, the performance of SSI-EM for SML applications can be improved, providing a good estimation of the natural frequency and damping ratio without any controlled excitation, which is the main obstacle to use an experimental modal analysis in cryogenic environments. The dynamic response of the 510 g levitating platform has been characterized by means of OMA in a cryogenic, 77 K, and high vacuum, 1E-5 mbar, environment. The measured vertical and radial stiffness are 9872.4 N/m and 21329 N/m, respectively, whilst the measured vertical and radial damping values are 0.5278 Nm/s and 0.8938 Nm/s. The first natural frequency in vertical direction has been identified to be 27.39 Hz, whilst a value of 40.26 Hz was identified for the radial direction. The determined damping values for both modes are 0.46% and 0.53%, respectively.

• Journal of Mechanical Science and Technology
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• v.20 no.4
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• pp.533-544
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• 2006

The amplitude and phase lag of surface deformation were determined for a compliant coating under the action of turbulent pressure fluctuations. For this purpose, pressure fluctuations were measured experimentally. The amplitude and duration of coherent wave train of pressure fluctuations were investigated using digital filtration. The transient response was calculated for stabilization of forced oscillations of the coating in approximation of local deformation. The response of coating was analyzed with considerations of its inertial properties and limited duration of coherent harmonics action of pressure fluctuations. It is shown that a compliant coating interacts not with the whole spectrum of pressure fluctuations, but only with a frequency range near the first resonance. According to the analysis, with increasing elasticity modulus of the coating material E, deformation amplitude decreases as 1/E, and dimensionless velocity of the coating surface decreases as $1/\sqrt{E}$. For sufficiently hard coatings, deformation amplitude becomes smaller than the thickness of viscous sublayer, while surface velocity remains comparable to vertical velocity fluctuations of the flow.

• Korean Journal of Applied Biomechanics
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• v.14 no.3
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• pp.191-202
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• 2004

The batting performance in baseball is a repetitive movement. In order to make the stabilization of posture and the efficient shift of body weight, both the range of stance and stride are important. The previous studies explained that the consistent stride which included the amount of time, stance, and direction were needed. However, the batting performance is frequently changed according to the several speed of ball. Therefore, this study was to analyze the reaction time, the range of stance, the change of stride, and the change of GRF during the batting movement in three kinds of ball speed (120km/h, 130km/h, & 140km/h). Seven elite players are participated in this study. 1. The reaction time of the stride phase was short whereas the time of the swing phase was long according to the increasing ball speed. 2. The range of the stance was wide and the mediolateral direction of the stride was decreased according to the increasing ball speed. 3. In the three kinds of ball speed, the change of body weight was transferred to the center, the rear foot, and the front foot directions. The ball speed of 130km/h showed the high frequency of the suitable batting. At this ball speed, the movement of the body weight was shifted smoothly and the value of the Ground Reaction Force was large enough.

• Smart Structures and Systems
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• v.18 no.6
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• pp.1203-1215
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• 2016

When strain sensing cables are under long-term stress and cyclic loading, creep may occur in the jacket material and each layer of the cable structure may slide relative to other layers, causing fatigue in the cables. This study proposes a device for testing the fatigue characteristics of three types of cables operating under different conditions to establish a decay model for observing the patterns of strain decay. The fatigue characteristics of cables encased in polyurethane (PU), GFRP-reinforced, and wire rope-reinforced jackets were compared. The findings are outlined as follows. The cable strain decayed exponentially, and the decay process involved quick decay, slow decay, and stabilization stages. Moreover, the strain decay increased with the initial strain and tensile frequency. The shorter the unstrained period was, the more similar the initial strain levels of the strain decay curves were to the stabilized strain levels of the first cyclic elongation. As the unstrained period increased, the initial strain levels of the strain decay curves approached those of the first cyclic elongation. The tested sensing cables differed in the amount and rate of strain decay. The wire rope-reinforced cable exhibited the smallest amount and rate of decay, whereas the GFRP-reinforced cable demonstrated the largest.

• Bulletin of the Korean Chemical Society
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• v.30 no.9
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• pp.1967-1972
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• 2009

The electrospinnability of pitch was improved by blending in a solution of polyacrylonitrile (PAN) resulting in the reduction of the average fiber diameter from 2000 to 750 nm. Activated carbon fibers (ACFs) derived by stabilization, carbonization and steam activation at 700, 800, and 900 ${^{\circ}C}$ of the PAN/pitch electrospun fibers for 60 min were investigated as electrodes for supercapacitors. The Brunauer, Emmett, Teller (BET) specific surface area ranged from 732 to 1877 $m^2g^{-1}$ and the specific capacitance from 75.5 to 143.5 $Fg^{-1}$, depending on the activation conditions. Electrodes from the electrospun web activated at 900 ${^{\circ}C}$ exhibited a particularly quick response showing a high frequency of 5.5 Hz at a phase angle of ‒$45^o$ of the impedance spectroscopy.

• Korean Journal of Optics and Photonics
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• v.13 no.1
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• pp.58-64
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• 2002

Using Mach-Zehnder type intensity modulator, we stabilized a 10 GHz harmonically mode-locked dispersion-compensated fiber ring laser using a feedback controlling system, and we measured its stability. The laser was stabilized for more than 16 hours by controlling the cavity length to suppress the relaxation oscillation frequency component which had caused the laser output instability. The ms timing jitter and ms amplitude noise were measured to be 260-524 fsec and 4~11.5%, respectively, and BER test measurement showed a value of 10$^{-13}$ .

• The Transactions of the Korean Institute of Electrical Engineers P
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• v.66 no.4
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• pp.213-218
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• 2017

The DC electrolytic capacitor for DC-link of power converter is widely used in various power electronic circuits and system application. Its functions include, DC Bus voltage stabilization, conduction of ripple current due to switching events, voltage smoothing, etc. Unfortunately, DC electrolytic capacitors are some of the weakest components in power electronics converters. Many papers have proposed different algorithms or diagnosis method to determinate the ESR and tan ${\delta}$ capacitance C for fault alarm system of the electrolytic capacitor. However, both ESR vary with frequency and temperature. Accurate knowledge of both parameters at the capacitors operating conditions is essential to achieve the best reference data of fault alarm. According to parameter analysis, the capacitance increases with temperature and the initial ESR decreases. Higher frequencies make the reference ESR with the initial ESRo value to decrease. Analysis results show that the proposed DC Bus electrolytic capacitor reference ESR model setting technique can be applied to advanced reference signal of capacitor diagnosis systems successfully.

• Journal of Astronomy and Space Sciences
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• v.29 no.3
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• pp.269-274
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• 2012

Kinematic global positioning system precise point positioning (GPS PPP) technology is widely used to the several area such as monitoring of crustal movement and precise orbit determination (POD) using the dual-frequency GPS observations. In this study we developed a kinematic PPP technology and applied 3-pass (forward/backward/forward) filter for the stabilization of the initial state of the parameters to be estimated. For verification of results, we obtained GPS data sets from six international GPS reference stations (ALGO, AMC2, BJFS, GRAZ, IENG and TSKB) and processed in daily basis by using the developed software. As a result, the mean position errors by kinematic PPP showed 0.51 cm in the east-west direction, 0.31 cm in the north-south direction and 1.02 cm in the up-down direction. The root mean square values produced from them were 1.59 cm for the east-west component, 1.26 cm for the south-west component and 2.95 cm for the up-down component.

• Journal of the Institute of Electronics and Information Engineers
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• v.50 no.1
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• pp.246-253
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• 2013

This paper addresses the technology of active flow control for stabilizing a flow field. In order for flow field modeling from the control point of view, the huge-data set from CFD(computational fluid dynamics) are reduced by using a POD(Proper Orthogonal Decomposition) method. And then the flow field is expressed with dynamic equation by low-order modelling approach based on the time and frequency domain analysis. A neural network flow estimator from the pressure information measured on the surface is designed for the estimation of the flow state in the space. The closed-loop system is constructed with feedback flow controller for stabilizing the vortices on the flow field.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 1997.04a
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• pp.842-846
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• 1997

Micro-hole drilling (holes less than 0.5 mm in diameter with aspect ratios larger than 10) is gaining increased attention in a wide spectrum of precision production industries. Alternative methods such as EDM, laser drilling, etc. can sometimes replace mechanical micro-hole drilling but are not acceptable in PCB manufacture because they yield inferior hole quality and accuracy. The major difficulties in micro-hold drilling are related to wandering motions during the inlet stage, high aspect ratios, high temperature,etc. However, of all the difficulties, the most undesirable one is the increase of drilling force as the drill penetrates deeper into hold. This is caused mainly by chip related effects. Peck-drilling is thus widely used for deep hole drilling despite the fact that it leads to low productivity. Therefore, in this paper, a method of cutting force regulation is proposed to achieve continuous drilling. A proportional plus derivative (PD) and a sliding modecontrol algorithm will be implemented for controlling the spinle rotational frequeency. Experimental results will show that sliding mode control reduces the nominal cutting force and its variation better than the PD control, resulting in a number of advantages such as an increase in drill life, fast stabilization of the wandering motion, and precise positioning of the hole.