• Journal of the Korean Society for Precision Engineering
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• v.16 no.2 s.95
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• pp.218-224
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• 1999

In the feed drive systems of machine tools that consist of many mechanical components such as motor, coupling, ballscrew, nut or table, a torsional vibration is often generated because of its elastic elements in torque transmission. Generally, the accuracy of motion control system is strongly influenced by the dynamic behavior of coupled transmission components. Especially, a torsional vibration caused by the elasticity of mechanical elements might deteriorate the quick movement of system and lead to shorten the life time of the mechanical transmission elements. So, it is necessary to analyze the electromechanical system mathematically to optimize the dynamic characteristics of the feed system. In this paper, the mathematical model of a feed drive system was developed and its mechanical characteristics were analyzed on the basis of the proposed model. The design concepts of speed control loop to stabilize a feed drive system were also proposed.

• Journal of the Korean Society of Visualization
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• v.3 no.2
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• pp.51-56
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• 2005

Acoustic excitations were imposed to coaxial air jet of non-premixed jet flame with hydrogen gaseous injected axially in the center of the flow. The frequencies of excitation were three dominant resonant frequencies at 1L, 2L, 3L. modes including specially 514 Hz (2L-mode) which was estimated theoretically as longitudinal mode of combustor characteristics. The mixing enhancement by acoustic forcing has been investigated quantitatively using PLIF and PIV. The effect of acoustic excitation on combustion process was significant to enhance mixing rate that coincides with specific resonant frequencies. And the behavior of vortex-interaction on flame structure was a good evidence to investigate the phenomenon of shear/mixing layer of fuel-air jet structure. The results obtained in this study concludes that generated streamwise vortex by acoustic excitation has a potential to enhance the mixing rate and abating NOx emissions.

• Structural Engineering and Mechanics
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• v.46 no.2
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• pp.153-177
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• 2013

Pendulums can be used as passive vibration control devices in several structures and machines. In the present work, the nonlinear behavior of a pendulum-tower system is studied. The tower is modeled as a bar with variable cross-section with concentrated masses. First, the vibration modes and frequencies of the tower are obtained analytically. The primary structure and absorber together constitute a coupled system which is discretized as a two degrees of freedom nonlinear system, using the normalized eigenfunctions and the Rayleigh-Ritz method. The analysis shows the influence of the geometric nonlinearity of the pendulum absorber on the response of the tower. A parametric analysis also shows that, with an appropriate choice of the absorber parameters, a pendulum can decrease the vibration amplitudes of the tower in the main resonance region. The results also show that the pendulum nonlinearity cannot be neglected in this type of problem, leading to multiplicity of solutions, dynamic jumps and instability. In order to improve the effectiveness of the control during the transient response, a hybrid control system is suggested. The added control force is implemented as a non-linear variable stiffness device based on position and velocity feedback. The obtained results show that this strategy of nonlinear control is attractive, has a good potential and can be used to minimize the response of slender structures under various types of excitation.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.11 no.6
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• pp.447-452
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• 1998

The selective dry etching of GaAs to Al\ulcornerGa\ulcornerAs using $BCI_3/SF_6$ gas mixture in electron cyclotron resonance(ECR) plasma is investigated. A selectivity of GaAs to AlGaAs of more than 100 and maximum etch rate of GaAs are obtained at a gas ratio $SF_6/BCI_3+SF_6$ of 25%. We verified the formation of $AlF_3$ on $Al_{0.25}Ga_{0.75}As$from the Auger spectra which enhanced the etch selectivity. In order to investigate surface damage of AlGaAs caused by ECR plasma, we performed a low temperature photoluminescence(PL) measurement as a function of RF power. As the RF power. As the RF power increases, the PL intensity decreases monotonically from 50 to 100 Wand then repidly decreases until 250 W. This behavior is due to surface damage by plasma treatment. This dry etching technique using $BCI_3/SF_6$ gas mixture in ECR plasma is suitable for gate recess formation on the GaAs based pseudomorphic high electron mobility transistor(PHEMT)

• International Journal of Naval Architecture and Ocean Engineering
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• v.8 no.2
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• pp.127-136
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• 2016

Springing is the resonance phenomenon of a ship hull girder with incoming waves having the same natural frequency of the ship. In this study, a simple and reliable calculation method was developed based on quadratic strip theory using the Timoshenko beam approach as an elastic hull girder. Second-order hydrodynamic forces and Froude-Krylov forces were applied as the external force. To improve the accuracy of the strip method, the variation in the added mass along the ship hull longitudinal direction, so called tip-effect, was considered. The J-factor was also employed to compensate for the effect of three-dimensional fluid motion on the two-node vibration of the ship. Using the developed method, the first- and second-order vertical bending moments of the Flokstra ship were compared. A comparative study was also carried out for a uniform barge ship and a 10,000 TEU container ship with the respective methods including the J-factor and tip-effect.

• Macromolecular Research
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• v.10 no.2
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• pp.85-90
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• 2002

Poly (ethylene oxide)/polylatide/poly(ethylene oxide) (PEO/PL/PEO) tri-block copolymers, which each block is connected by ester bond, were synthesized by coupling reaction of PL with PEO in the presence of pyridine. PL/PEO/PL tri-block copolymer was synthesized by ring opening polymerization of L-lactide initiated by PEO in the presence of stannous octoate. Degradation behavior of the copolymers was investigated in a pH 7.4 phosphate buffer saline (PBS) at 37$\pm$1 $^{\circ}C$. Gel permeation chromatography (GPC) and $^1$H-nuclear magnetic resonance (NMR) were used to monitor the change of mass loss, molecular weight and composition of copolymers. In hydrolytic degradation, the PEO/PL/PEO tri-block copolymer with high PEO contents affected the increase of its mass loss, and resulted in the decrease of its molecular weight as well as PEO composition. However, when PL/PEO/PL and PEO/PL/PEO tri-block copolymers had similar PEO contents, PEO/PL/PEO decreased faster in molecular weight and PEO composition than PL/PEO/PL.

• Macromolecular Research
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• v.15 no.4
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• pp.324-329
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• 2007

An octa-functional alkoxyamine initiator, with the 2,2,6,6-tetramethyl-l-piperidinyloxy (TEMPO) free radical, was synthesized based on resorcinarene, with its efficiency to initiate the nitroxide-mediated free radical copolymerization of styrene and methyl methacrylate (MMA) described. A difunctional analogue of this initiator was also synthesized, using resorcinol as the core molecule. The structures of the resulting initiators were confirmed by homolysis studies based on electron spin resonance spectroscopy and molecular modeling. The polymerization behavior and characteristics of the polymers obtained using these two initiators were also compared. Well-defined star-shaped and linear random copolymers, with low polydispersities and controlled molecular weights, were prepared. The efficiencies of these initiators towards copolymerization, as well as the parameters permitting the formation of well-defined polymers, were also investigated. The reactivity ratios were $r_a=0.42(a=styrene)\;and\;r_b=0.33(b=MMA)$ for the octa-functional initiator system and $r_a=0.45\;and\;r_b=0.39$ for the difunctional initiator system.

• Macromolecular Research
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• v.15 no.4
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• pp.385-391
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• 2007

Block copolymer micelles are generally formed via the self-assembly of amphiphilic block copolymers in an aqueous medium. The hydrophilic and hydrophobic blocks form shell and core micelles, respectively. The block copolymers of methoxy poly(ethylene glycol) (MPEG)-b-poly(2-diisopropylamino)ethyl methacrylate (PDPA) were synthesized via atom transfer radical polymerization, with the macro initiator synthesized by the coupling of 2-bromoisobutyryl bromide with MPEG in the presence of a triethyl amine base catalyst. The atom transfer radical polymerization of 2-diisopropylamino)ethyl methacrylate was performed in conjunction with an N,N,N',N",N"-pentamethyl-diethylenetriamine/copper bromide catalyst system, in DMF, at $70^{\circ}C$. The pH induced micellization/demicellization was studied using fluorescence, with a pyrene probe. Furthermore, the pH dependent micellization was confirmed using the microviscosity method, with a dipyme fluorescence probe. The pH dependant micelle size distribution was studied using dynamic light scattering. The characterization of the synthesized polymers was established using gel permeation chromatography and from the $^1H-nuclear$ magnetic resonance spectroscopy.

• Journal of the Korea Institute of Building Construction
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• v.12 no.6
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• pp.607-614
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• 2012

In this study, the evolution of the electro-mechanical impedance (EMI) of a piezoelectric (PZT) patch embedded in fresh cement paste was investigated to discuss the possibility of monitoring the setting process of cement-based materials using an EMI sensing technique. A tailored thin square PZT patch was embedded in cement paste before casting, and EMI signatures of the embedded patch were continuously measured from casting up to 12 hours. A standard penetration resistance test was performed to compare and correlate the evolution of EMI during the setting process. The results showed that EMI responses differ according to the age of the cement paste, and that the behavior of the EMI resonance peak has a clear correlation with the penetration resistance of the cement paste. Based on the results, it is concluded that an EMI sensing technique using embedded PZT patch can be effectively applied to monitor the setting process of cement-based materials.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.19 no.1
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• pp.29-34
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• 2006

The effects of $B_2O_3$ addition on the low-temperature sintering behavior and microwave dielectric properties of $(Zn_{0.8}Mg_{0.2})TiO_3$ ceramic system were investigated. Highly dense samples were obtained at the sintering temperatures below $900^{\circ}C$. The $Q{\times}f_o$ values were determined by the microstructures and sintering shrinkages which are affected by the amount of $B_2O_3$ and sintering temperature. Temperature coefficient of resonance frequency($T_f$) changes to a positive value with increasing the amount of $B_2O_3$ due to the increased amount of rutile phase which is one of the reaction products between $(Zn_{0.8}Mg_{0.2})TiO_3$ and $B_2O_3$. For $6.19 moi.{\%}B_2O_3$-added $(Zn_{0.8}Mg_{0.2})TiO_3$ system, it exhibits ${\epsilon}_r$ = 23.5, $Q{\times}f_o$ = 53,000 GHz, and $T_f$ = 0 ppm/$^{\circ}C$ when sintered at $900^{\circ}C$ for 5 h.