Y H Choi

Publications (22)35.42 Total impact

• Article: Quench and Recovery Characteristics of the Zr-Doped (Gd,Y) BCO Coated Conductor Pancake Coils Insulated With Copper and Kapton Tapes

  D.G. Yang, J.B. Song, Y.H. Choi, M.C. Ahn, J.B. Na, T.K. Ko, H.G. Lee
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  ABSTRACT: Quench protection is one of the most important issues for magnet applications using 2 G HTS tapes because of their non-self-protecting properties. Although many researchers have examined the quench/recovery characteristics of HTS magnets to establish the required stability, new information on the quench/recovery characteristics of the recently commercialized HTS magnet with GdBCO CC is needed. In this study, the quench/recovery characteristics of Zr-doped (Gd,Y)BCO CC pancake coils that were insulated with various materials, such as copper and Kapton tapes, were examined in a liquid nitrogen (LN2) bath. The minimum quench energy (MQE) and normal zone propagation (NZP) velocity of the Zr-doped (Gd,Y)BCO CC pancake coils are discussed in terms of the quench/recovery test results.
  IEEE Transactions on Appiled Superconductivity 07/2011; · 1.04 Impact Factor
• Article: Robust adaptive formation control and collision avoidance for electrically driven non-holonomic mobile robots

  B.S. Park, J.B. Park, Y.H. Choi
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  ABSTRACT: This study proposes a robust adaptive formation controller for electrically driven non-holonomic mobile robots to achieve desired formation tracking and collision avoidance with static and moving obstacles. The projection algorithm is employed to estimate the time-varying velocities of the leader robot. The robust adaptive technique is adopted to deal with parametric uncertainties and external disturbances. Moreover, the avoidance function is used to avoid the collisions even though many obstacles exist within the detection region of the mobile robot. It is shown using Lyapunov stability theory that formation errors are uniformly ultimately bounded and collision avoidance is guaranteed. Simulations are included to illustrate the control method.
  IET Control Theory and Applications 03/2011; · 0.99 Impact Factor
• Article: Displacement sensor circuit using eddy current for automobile brake

  Y.S. Kim, Y.H. Choi, J.M. Lee, J.S. Noh, F. Bien
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  ABSTRACT: A displacement sensing circuit for an automobile brake switch is presented. The circuit generates a control signal for the electronic control unit of an automobile during the brake being pressed. By using the eddy current on the metal plate induced by the oscillation circuit of the sensor, high speed non-contact switching is implemented. The proposed circuit is well suited for the replacement of the conventional metallic switch for the safety of the vehicle.
  Electronics Letters 11/2010; · 0.96 Impact Factor
• Article: Brief paper: adaptive formation tracking control of electrically driven multiple mobile robots

  S.J. Yoo, J.B. Park, Y.H. Choi
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  ABSTRACT: An adaptive formation control method is proposed for multiple uncertain non-holonomic mobile robots at the actuator dynamics level. All parameters of the robot kinematics and dynamics, and actuator dynamics are unknown. The virtual structure with path parameters and the dynamic surface design methodology are combined to design a simpler adaptive formation control scheme than the previous backstepping-based control system. Using the Lyapunov stability theorem, the authors present the adaptation laws for tuning all unknown parameters of multiple mobile robots regardless of considering path parameters in the reference trajectories. In addition, it is proved that all signals in the total closed-loop system are semi-globally uniformly bounded and all formation tracking errors and synchronisation errors of the path parameters converge to an adjustable neighbourhood of the origin. Finally, simulation results demonstrate the effectiveness of the proposed approach.
  IET Control Theory and Applications 09/2010; · 0.99 Impact Factor
• Article: Adaptive formation control in absence of leader's velocity information

  K. Choi, S.J. Yoo, J.B. Park, Y.H. Choi
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  ABSTRACT: In the leader-following approach, it has been commonly assumed that the leader robot's velocity information may be either measured or telecommunicated. However, in some circumstances, it is difficult to satisfy it because the follower robots may not have the sensor or the communication device to obtain accurate enough leader robot's velocity. Therefore this study proposes an adaptive formation control approach for multiple mobile robots in the absence of the velocity information of the leader robot. The proposed control system only requires a knowledge of the leader'follower relative distance and the bearing angle. The smooth projection algorithm is employed to estimate time-invariant or time-varying velocities of the leader robot. Using Lyapunov stability theory, it is shown that the formation errors are uniformly ultimately bounded and converge to an adjustable neighbourhood of the origin. Simulation and experiment results are provided to demonstrate the performance and stability of the proposed formation scheme.
  IET Control Theory and Applications 05/2010; · 0.99 Impact Factor
• Conference Proceeding: Utility interactive PV system with improved peak-cut characteristics

  H.S. Heo, G.H. Choe, Y.H. Choi, H.S. Kim, J.C. Kim
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  ABSTRACT: This paper describes Utility Interactive PV(UIPV) system that is able to improve peak-cut characteristic without major alteration of power circuit structure which adds energy storage device like batteries and alters software application algorithm. A new power circuit and application algorithm has been simulated to prove this.
  Power Electronics, 2007. ICPE '07. 7th Internatonal Conference on; 11/2007
• Article: Microwave properties of large YBCO films on as-prepared and annealed MgO substrates

  J.H. Lee, J. Lim, S.H. Moon, J.H. Yun, Y.H. Choi, B. Oh, Sang Young Lee
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  ABSTRACT: We have studied the microwave surface impedance of large YBCO films on as-prepared MgO substrates and on annealed MgO substrates with 50 mm in diameter. YBCO films were prepared by pulsed laser deposition. The surface resistance (R_S), homogeneity in the R_S, and structural properties of YBCO on annealed MgO appeared improved compared with those of YBCO on as-prepared MgO. The reduced R_S of YBCO films on annealed MgO is attributed to improved in-plane orientation of the YBCO grains in the films with local variations in the in-plane orientation of YBCO grains correlated well with the R_S for both YBCO films on as-prepared MgO and those on annealed MgO.
  IEEE Transactions on Appiled Superconductivity 07/2003; · 1.04 Impact Factor
• Article: Double-sided YBa2Cu3O7-δ thin films made by off-axis pulsed laser deposition

  B C Min, Y H Choi, S H Moon, S M Lee, Sang Young Lee, B Oh
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  ABSTRACT: Double-sided YBa2Cu3O7-δ (YBCO) thin films were prepared on LaAlO3 substrates 50 mm in diameter by an off-axis pulsed laser deposition method. The surface of the YBCO films appeared to have low a boulder density with a surface roughness of about 3 nm. Spiral growth was observed at both sides of the thin films. X-ray diffraction data showed that the YBCO thin films had grown epitaxially on LaAlO3 with c-axis orientation and a full width at half maximum for the (005) rocking curve of 0.15°. The transition temperature was in the range of 86-88 K and the critical current density at 77 K was 2.4-3.7×106 A cm-2 at various positions on both sides of the film. The surface resistance of the YBCO films was estimated to be as low as ~1.0 mΩ at 19.57 GHz and 77 K on both sides of the YBCO films, which corresponds to a value of ~250 µΩ at 10 GHz and 77 K.
  Superconductor Science and Technology 07/2001; 14(8):543. · 2.66 Impact Factor
• Article: Narrowband Y-Ba-Cu-O filter with quasi-elliptic characteristic

  I.B. Vendik, A.N. Deleniv, V.O. Sherman, A.A. Svishchev, V.V. Kondratiev, D.V. Kholodniak, A.V. Lapshin, P.N. Yudin, B.-C. Min, Y.H. Choi, B. Oh
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  ABSTRACT: The compact meander-shape configuration of planar resonators is suggested for applications in narrowband filters with quasi-elliptic characteristic. The quasi-elliptic frequency response with two transmission zeroes at the edges of the passband is provided by the specific symmetry of the coupled resonators and is controlled by an appropriate choice of the resonator dimensions and the couplings between the resonators. The 12-pole quasi-elliptic filter with 0.5% fractional bandwidth at 1.775 GHz has been designed using meander-line microstrip resonators and manufactured on a double-sided YBCO film on a LaAlO₃ substrate. The filter exhibited the in-band insertion loss less than 0.5 dB at T=60 K, the return loss of 20 dB, the out-of-band rejection better than 75 dB, and 40 dB/MHz slope of the characteristic
  IEEE Transactions on Appiled Superconductivity 04/2001; · 1.04 Impact Factor
• Article: Cross-coupled band-pass filter using HTS microstrip resonators

  B.-C. Min, Y.H. Choi, S.K. Kim, B. Oh
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  ABSTRACT: We have fabricated microstrip band-pass filters based on the quadruplet geometry using high-temperature superconductor. Every half-wavelength resonator in the filter consists of a loop-like inductor and two patch capacitors at both ends. The inductive coupling in between the loop-like inductors of non-adjacent resonators produces transmission zeros in the frequency response. The transmission zero can be allocated by changing the polarity and the strength of the cross coupling. We have designed a 9-pole filter for personal communication services (PCS) application using combination of a CAD tool and a full-wave analysis EM simulator. We have fabricated filters using double-sided YBa₂Cu₃O_7-δ (YBCO) thin films on LaAlO₃ substrates. The filter has 1.784 GHz center frequency, 11 MHz 3-dB bandwidth, and 0.8 dB insertion loss at 60 K. Due to the transmission zeros near the passband, the steep skirt characteristic is exhibited. The attenuation at 1 MHz away from the edge of the passband is -34 dB
  IEEE Transactions on Appiled Superconductivity 04/2001; · 1.04 Impact Factor
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  Conference Proceeding: Movie-based VR therapy system for treatment of anthropophobia

  H.J. Jo, J.H. Ku, D.P. Jang, B.H. Cho, H.B. Ahn, J.M. Lee, Y.H. Choi, I.Y. Kim, S.I. Kim
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  ABSTRACT: The fear of public speaking is a kind of social phobia. The patients having the fear of public-speaking show some symptoms like shame and timidity in the daily personal relationship. They are afraid that the other person would be puzzled, feel insulted, and they also fear that they should be underestimated for their mistakes. For the treatment of the fear of public speaking, the cognitive-behavioral therapy is generally used. The cognitive-behavioral therapy is the method that makes the patients gradually experience some situations inducing the fears and overcome those at last. But if the real situations inducing fears cause dangerous symptoms or the patients have difficulty in imagining the situations, the effect of this method is notably reduced. And making the situations inducing the fears to patients requires a vast amount of effort and time. In this study, we developed the public-speaking simulator and the virtual environment for the treatment of the fear of public speaking. The head-mounted display, the head-tracker and the 3-dimensional sound system were used for immersing in the virtual environment. The virtual environment of this system is suggested in a seminar room where 6 virtual audiences are seated. The virtual audiences were made with real movies and inserted into the virtual environment. The patient speaks in front of these virtual audiences and the therapist can make virtual audience members respond with some motions. Moreover, clinical tests have been made to verify the effectiveness of the treatment.
  Engineering in Medicine and Biology Society, 2001. Proceedings of the 23rd Annual International Conference of the IEEE; 02/2001
• Article: Seven-pole HTS microstrip bandpass filter using non-adjacent coupling of pseudo-lumped element resonator

  B.K. Jeon, J.H. Kim, C.J. Lee, B.C. Min, Y.H. Choi, S.K. Kim, B. Oh
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  ABSTRACT: A new HTS microstrip bandpass filter structure with a quasi-elliptic response is realised using a pseudo-lumped element resonator. A seven-pole bandpass filter on an LaAlO₃ substrate is designed and fabricated. The filter has an insertion loss of 0.8 dB at 20 K, a bandwidth of 8 MHz at a centre frequency of 1774 MHz, and an attenuation of 33 dB for a cutoff band of 1 MHz
  Electronics Letters 09/2000; · 0.96 Impact Factor
• Conference Proceeding: A novel HTS microstrip quasi-elliptic function bandpass filter using pseudo-lumped element resonator

  B.K. Jeon, J.H. Kim, C.J. Lee, B.C. Min, Y.H. Choi, S.K. Kim, B. Oh
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  ABSTRACT: This paper presents a new HTS microstrip bandpass filter structure using pseudo-lumped resonator. Quasi elliptic function filter characteristics were obtained using nonadjacent couplings among the resonators. The new structure has the advantage of compactness and simplicity for the design and the realization. In this paper, a seven-pole quasi-elliptic function bandpass filter on LaAlO₃ substrate is designed and fabricated. The filter has an insertion loss of 0.8 dB at 20 K, a bandwidth of 8 MHz at the center frequency of 1774 MHz, and an attenuation of 33 dB for the cut-off-band of 1 MHz
  Microwave Symposium Digest. 2000 IEEE MTT-S International; 02/2000
• Article: YBa2Cu3O7-δ/NdBa2Cu3O7-δ/YBa2Cu3O7-δ edge junctions and SQUIDS

  B. Oh, Y. H. Choi, S. H. Moon, H. T. Kim, B. C. Min
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  ABSTRACT: YBa 2 Cu 3 O 7-δ (YBCO) Josephson junctions in a ramp edge geometry with NdBa 2 Cu 3 O 7-δ (NBCO) barriers were fabricated by pulsed laser deposition on (100) SrTiO 3 substrates. The barrier layer thicknesses were d=100, 200, and 300 Å . The I–V characteristics of YBCO/NBCO/YBCO junctions changed from the resistively shunted junction type to flux‐creeplike behavior as temperature decreased. Shapiro steps due to the ac Josephson effect clearly developed under microwave irradiation. The interface between the superconducting (YBCO) and the normal layer (NBCO) turned out to be fairly clean with small interface resistance. The mean value of the measured I cR n product for junctions with 100‐Å‐thick barriers was 71±34 μV at 77 K. The normal coherence length of the NBCO barrier material was ∼40 Å. The SQUIDs, made of superconductor–normal metal–superconductor junctions, showed voltage modulation at 77 K. © 1996 American Institute of Physics.
  Applied Physics Letters 11/1996; · 3.84 Impact Factor
• Article: A compact two-pole X-band high-temperature superconducting microstrip filter

  B. Oh, H.T. Kim, Y.H. Choi, S.H. Moon, P.H. Hur, M. Kim, S.Y. Lee, A.G. Denisov
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  ABSTRACT: In this paper we examine a microstrip line filter with three gaps separated by approximately a quarter wavelength. Two filters, one with gold and the other with YBCO were fabricated. The thickness of top conducting microstrips was 600 nm. The ground plane under the 0.5 mm-thick lanthanum aluminate (LaAlO₃) substrate was implemented with a 2 micron-thick gold layer for both filters. These filters showed similar frequency responses with 5% 3-dB bandwidth. The insertion loss, however, improved from 2.8 dB to 1.5 dB by using the superconductor. The performance of the filter was also tested by employing the three dimensional microwave FEM software, and the simulation result compared well with the measured data.
  IEEE Transactions on Appiled Superconductivity 07/1995; · 1.04 Impact Factor
• Article: Exploration of entire range of III–V semiconductors and their device applications

  M. Razeghi, Y. H. Choi, X. He, C. J. Sun
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  ABSTRACT: The most recent advances in III–V semiconductors, from wide band gap AlN to the new narrow band gap InTlSb material, are examined. Specifically, results for AlN, GaN, and their alloys are examined with a view to applications in the blue–ultraviolet spectral range. For operation in the long wavelength infrared range (8–12 μm), the potential and realisation of a new III–V alloy, InTlSb, are presented. The GaInAsP/GaAs system has also been investigated as an alternative to the widely used AlGaAs/GaAs system for device applications in the intermediate spectral range. Attention is given to the crystal growth technique and physical properties of these III–V semiconductors. Demonstrations and the performance in service of several devices based on these material systems, such as high power GaInAsP diode lasers and InTlSb infrared detectors, are described.MST/3220
  Materials Science and Technology 12/1994; 11(1):3-30. · 0.77 Impact Factor
• Article: Detailed analysis of carrier transport in InAs0.3Sb0.7 layers grown on GaAs substrates by metalorganic chemical‐vapor deposition

  C. Besikci, Y. H. Choi, G. Labeyrie, E. Bigan, M. Razeghi, J. B. Cohen, J. Carsello, V. P. Dravid
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  ABSTRACT: InAs 0.3 Sb 0.7 layers with mirrorlike morphology have been grown on GaAs substrates by low‐pressure metalorganic chemical vapor deposition. A room‐temperature electron Hall mobility of 2×10⁴ cm²/V s has been obtained for a 2‐μm‐thick layer. Low‐temperature resistivity of the layers depended on TMIn flow rate and layer thickness. Hall mobility decreased monotonically with decreasing temperature below 300 K. A 77 K conductivity profile has shown an anomalous increase in the sample conductivity with decreasing thickness except in the near vicinity of the heterointerface. In order to interpret the experimental data, the effects of different scattering mechanisms on carrier mobility have been calculated, and the influences of the lattice mismatch and surface conduction on the Hall measurements have been investigated by applying a three‐layer Hall‐effect model. Experimental and theoretical results suggest that the combined effects of the dislocations generated by the large lattice mismatch and strong surface inversion may lead to deceptive Hall measurements by reflecting typical n‐type behavior for a p‐type sample, and the measured carrier concentration may considerably be affected by the surface conduction up to near room temperature. A quantitative analysis of dislocation scattering has shown significant degradation in electron mobility for dislocation densities above 10⁷ cm^-2. The effects of dislocation scattering on hole mobility have been found to be less severe. It has also been observed that there is a critical epilayer thickness (∼1 μm) below which the surface electron mobility is limited by dislocation scattering.
  Journal of Applied Physics 12/1994; · 2.17 Impact Factor
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  Article: Characterization of InTlSb/InSb grown by low‐pressure metal‐organic chemical vapor deposition on a GaAs substrate

  Y. H. Choi, P. T. Staveteig, E. Bigan, M. Razeghi
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  ABSTRACT: Optical properties of InTlSb, a new long wavelength infrared material, are investigated. InTlSb/InSb epilayers grown by low‐pressure metal‐organic chemical vapor deposition on semi‐insulating GaAs substrates were characterized using Auger electron spectroscopy and Fourier transform infrared spectroscopy. Auger electron spectra confirm the presence of thallium. Transmission measurements at 77 K indicate an absorption shift from 5.5 μm for InSb up to 8 μm for InTlSb that is confirmed by photoconductivity measurements.
  Journal of Applied Physics 04/1994; · 2.17 Impact Factor
• Article: Photoconductance measurements on InTlSb/InSb/GaAs grown by low‐pressure metalorganic chemical vapor deposition

  P. T. Staveteig, Y. H. Choi, G. Labeyrie, E. Bigan, M. Razeghi
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  ABSTRACT: We report infrared photoconductors based on InTlSb/InSb grown by low‐pressure metalorganic chemical vapor deposition on semi‐insulating GaAs substrates. The photoresponse spectrum extends up to 8 μm at 77 K. The absolute magnitude of the photoresponse is measured as a function of bias. The specific detectivity is estimated to be 3×10⁸ Hz^1/2 cm W^-1 at 7 μm wavelength.
  Applied Physics Letters 02/1994; · 3.84 Impact Factor
• Article: Transport properties in n‐type InSb films grown by metalorganic chemical vapor deposition

  S. N. Song, J. B Ketterson, Y. H. Choi, R. Sudharsanan, M. Razeghi
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  ABSTRACT: We have measured the temperature and magnetic field dependence of the Hall mobility and transverse magnetoresistance in n‐type InSb films epitaxially grown on GaAs substrates by metalorganic chemical vapor deposition. The films show a giant magnetoresistance: e.g., at 240 K the resistivity increases over 20 times at a magnetic field of 5 T; the low field coefficient of resistivity at 77 K is as high as 47.5 μΩ cm/G. The Hall mobility decreases with magnetic field and saturates at higher fields. By taking the interface carrier transport into account, the observed field dependence of the Hall mobility and magnetoresistance may be understood based on a two‐layer model.
  Applied Physics Letters 09/1993; · 3.84 Impact Factor