• Smart Structures and Systems
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• v.4 no.4
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• pp.391-406
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• 2008

This study attempts to develop a real-time debonding monitoring system for carbon fiber-reinforced polymer (CFRP) strengthened structures by continuously inspecting the bonding condition between the CFRP layer and the host structure. The uniqueness of this study is in developing a new concept and theoretical framework of nondestructive testing (NDT), in which debonding is detected without relying on previously-obtained baseline data. The proposed reference-free damage diagnosis is achieved based on the concept of time reversal acoustics (TRA). In TRA, an input signal at an excitation point can be reconstructed if the response signal measured at another point is reemitted to the original excitation point after being reversed in the time domain. Examining the deviation of the reconstructed signal from the known initial input signal allows instantaneous identification of damage without requiring a baseline signal representing the undamaged state for comparison. The concept of TRA has been extended to guided wave propagations within the CFRP-strengthened reinforced concrete (RC) beams to improve the detectibility of local debonding. Monotonic and fatigue load tests of large-scale CFRP-strengthened RC beams are conducted to demonstrate the potential of the proposed reference-free debonding monitoring system. Comparisons with an electro-mechanical impedance method and an inferred imaging technique are provided as well.

• Journal of Power Electronics
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• v.16 no.3
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• pp.1152-1162
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• 2016

The unequal impedances of the interconnecting cables between paralleled inverters in the island mode of microgrids cause inaccurate reactive power sharing when the traditional droop control is used. Many studies in the literature adopt low speed communications between the inverters and the central control unit to overcome this problem. However, the losses of this communication link can be very detrimental to the performance of the controller. This paper proposes an improved reactive power-sharing control method. It employs infrequent measurements of the voltage at the point of common coupling (PCC) to estimate the output impedance between the inverters and the PCC and then readjust the voltage droop controller gains accordingly. The controller then reverts to being a traditional droop controller using the newly calculated gains. This increases the immunity of the controller against any losses in the communication links between the central control unit and the inverters. The capability of the proposed control method has been demonstrated by simulation and experimental results using a laboratory scale microgrid.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.24 no.10
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• pp.803-806
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• 2011

1 mol% $Li_2O$ excess $0.9(Na_{0.52}K_{0.48})NbO_3-0.1LiTaO_3$ lead-free piezoelectric multilayer ceramic actuators were investigated to determine their aging properties. To reduce the thermal aging behavior, we applied a rectified unipolar electric field of 5 kV/mm to the specimen to accelerate the electric aging behavior. By employing a rectified unipolar electric field for the piezoelectric actuators, we could remove undesirable heating from the relaxation current in the motion of the ferroelectric domain. To accelerate the aging test, the applied electric fields had a frequency of 900 Hz. To have enough time for charging and discharging, we employed an accurate time constant to design the equivalent circuit model for the aging tester. To extract exact aging behavior, we measured the pseudo-piezoelectric coefficient before and after the aging process. We also measured the electro-mechanical coupling coefficient, the frequency-dependent dielectric permittivity, and the impedance to compare with fresh and aged specimen.

• The Journal of the Acoustical Society of Korea
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• v.30 no.7
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• pp.368-376
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• 2011

Piezoelectric composites have been widely used in broadband acoustic transducers because of their lower acoustic impedance and higher electro-mechanical coupling factor. However, their complex structure has placed many limitations on the design of various transducers. This paper suggests the methodology to substitute the 1-3 piezocomposites by a single-phased material that has properties equivalent to those of the piezocomposites. The resonant method and finite element analysis (FEA) are used to derive the equivalent properties that can accurately depict resonant properties at various vibration modes of the piezocomposites. Validity of the suggested method is confirmed by comparing frequency characteristics of fabricated 1-3 piezocomposite specimens and FEA models. Further, accuracy of the derived material constants is checked by applying the equivalent properties to FEA models of the single phase material for various resonant modes.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.17 no.7
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• pp.1645-1652
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• 2013

In this paper, a new piezoelectric transducer for high performance ultrasonic flaw detector used in non-destructive test (NDT) is implemented. Here, the goals for some major characteristics such as piezoelectric strain constant and electro-mechanical coupling factor are fixed in advanced. Then, the parameters obtained by finite element analysis (FEA) are exploited to design and implement the piezoelectric transducer. As a result of experiments using manufactured samples, it is proved that the new PZT ceramics satisfy the goals very well. It has much improved impedance characteristic at the resonant frequency and generation of ultrasonic signals. In addition, ultrasonic flaw detector with the new transducer provides increased flaw detecting gain than the conventional one. Thus, it is considered that the new flaw detector contributes significantly to improve reliability of the NDT.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.14 no.12
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• pp.1292-1299
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• 2003

In this paper, we fabricated new GaAs-based dielectric-supported air gapped microstriplines(DAMLs) using the surface MEMS and the LPF for Ka-band using the fabricated DAMLs. We elevated the signal lines from the substrate, in order to reduce the substrate dielectric loss and obtain low losses at millimeter-wave frequency band with wide impedance range. We fabricated LPF with DAMLs for Ka-band. Due to reducing the dielectric loss of DAMLs, the insertion loss of LPF can be reduced. Miniature is essential to integrate LPF with active devices, so that we fabricated LPF with the slot on the ground to reduce the size of the LPF. We compared a characteristic to LPF with the slot and LPF without the slot.

• Journal of the Institute of Electronics Engineers of Korea SD
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• v.41 no.8
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• pp.37-44
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• 2004

This paper describes a new GaAs-based surface-micromachined microstrip line supported by dielectric post and air-gapped signal line with ground metal. This new type of dielectric-supported air-gapped microstripline(DAML) structure is developed using surface micromachining techniques to provide easy means of airbridge connection between the signal lines and to archive low losses at millimeter-wave frequency band with wide impedance range. Each DAMLs with the length of 5 mm are fabricated and the measured characteristics are compared with those of the conventional microstrip transmission line. These transmission lines are composed of 10 ${\mu}{\textrm}{m}$ height of signal line, post size of 10 ${\mu}{\textrm}{m}$ ${\times}$ 10 ${\mu}{\textrm}{m}$ and post height of 9 ${\mu}{\textrm}{m}$. By elevating the signal lines from the substrate using the micromachining technology, the substrate dielectric loss can be reduced Compared with of the conventional microstrip transmission line showing 7.5 dB/cm loss at 50 GHz, the loss can be reduced to 1.1 dB/cm loss at 50 GHz.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.12 no.1
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• pp.18-26
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• 2001

In this paper, the resonant characteristics and modes of the film bulk acoustic wave resonator (FBAR) used in 1~2 GHz frequency region are analyzed by it's input impedance which was calculated by three dimensional finite element method formulated as eigenvalue problem using electro-mechanical wave equation and boundary condition. It was extracted that the resonant and the spurious characteristics considering the effects of electrode area and shape variation and unsymmetry of upper and lower electrode. Those effects couldn't be analyzed by on dimensional analysis, e.g. Mason equivalent model. The simulation result was confirmed by comparing with the simulation data from Mason model analysis and the measured data of the ZnO FBAR fabricated using micro-machining technique. Also, through the simulation of the area variations of FBAR, it was obtained that the optimum ratio of length and thickness is 20:1 and the minimum ratio is 5:1 to operate thickness vibration mode.

• Journal of the Korean Ceramic Society
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• v.38 no.11
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• pp.980-986
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• 2001

Bragg reflector type FBAR was fabricated on the Si(100) substrate. We measured a frequency response of the resonator at 5.2 GHz and analyzed it by numerical calculation considering actual acoustic losses of each layer in the structure. We fabricated nine layer Bragg reflector of W-SiO$_2$pairs using r.f. sputtering method and fabricated AlN piezoelectric and Al electrodes using pulsed dc sputtering. The return loss(S$_{11}$) of the fabricated Bragg reflector type FBAR was 12 dB at 5.38 GHz and the series resonance frequency(f$_{s}$) was 5.376 GHz and the parallel resonance frequency(f$_{p}$) was 5.3865 GHz. Effective electro-mechanical coupling constant (K$_{eff{^2}}$) and Quality factors(Q$_{s}$), the Figures of Merit of the resonator, were about 0.48% and 411, respectively. We extracted acoustic parameters of AlN piezoelectric and reflection coefficient of the Bragg reflector by numerical calculation. We could know that material acoustic impedance and wave velocity of AlN piezoelectric decreased for intrinsic value and the electromechanical coupling constant(K$_2$) value was very low owing to the poor quality of the AlN piezoelectric. Reflection coefficient of Bragg reflector was 0.99966 and reflection band was very wide from 2.5 to 9.5 GHz.