• The Transactions of the Korean Institute of Power Electronics
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• v.6 no.5
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• pp.416-422
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• 2001

The BLDC(Brushless DC) Motor is characterized by linear torque to current and speed to voltage. It has low acoustic noise and fast dynamic response. Moreover, it has high power density with high proportion of torque to inertia in spite of small size drive. However, when armature current is commutated, the current ripple is generated by the motor inductance components in stator windings and back-EMF. This current ripple caused to torque ripple. Therefore, it is difficult to apply the BLDC motor to a precision servo drive system. In this paper, a new current control algorithm using fourier series coefficients is proposed. This proposed algorithm can minimize torque ripple due to the phase current commutation of BLDC motor. Simulation and Experimental results prove the effectiveness at the Proposed algorithm through comparison with the conventional unipolar PWM method.

• The Journal of Korea Institute of Information, Electronics, and Communication Technology
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• v.13 no.5
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• pp.344-352
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• 2020

Covertness and performance are very important design goals in the underwater communications. To satisfy both of them, we proposed efficient underwater communication model which combined multiband and direct sequence spread spectrum method in order to improve performance and covertness simultaneously. Turbo coding method with 1/3 coding rates is used for channel coding algorithm, and turbo equalization method which iterately exchange probabilistic information between equalizer and decoder is used for receiver side. After optimal threshold value was set in Rake processing, this paper analyzed the performance by varying the number of chips were 8, 16, 32 and the number of bands were from 1 to 4. Through the simulation results, we confirmed that the performance improvement was obtained by increasing the number of bands and chips. 2~3 dB of performance gain was obtained when the number of chips were increased in same number of bands.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.8 no.3
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• pp.686-693
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• 2004

We have studied a method to fabricated a Surface Acoustic Waves (SAW) filter for Wide band Codeless Division Multiple Access(WCDMA) was formed on the Langasite substrate and was evaporated by Aluminum-Copper alloy and then we developed a simulator using the mathematica package. And, we can design and fabricate the Slanted finger Inter-digital Transducer (SFIT) for the purpose to decreased the ultimate rejections on side of the electrodes, and performed computer-simulation by simulator. Also, we have employed that the block weighted type Inter-digital Transduce(IDT) as input transduce of the filter and the withdrawal weighted type IDT as an output transducer of the filter in order to minimize effect of diffractions. We have employed that the number of pairs of the input and output IDT are 50 pairs and the thickness and the width of reflector are $5000\AA$, and $1\lambda/4(\cong3.6{\mu}m)$, respectively. Also the width of IDT' finger and the space between IDT' finger and reflector are $1\lambda$/16 and 1\lambda$/8, respectively. Frequency response of the fabricated SAW bandpass filter has the property that center frequency is about 190MHz, bandwidth at the 3dB is probably 4MHz and out-band attenuation is -60dB approximately.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.24 no.3
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• pp.188-193
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• 2011

The osseointegration of dental implant is influenced by many factors such as surface geometry, loading and the amount of bone. Thus, stability of the dental implant should be checked periodically. In order to test the stability of dental implant by using resonance frequency analysis, we designed a structure of transducers and fabricated a piezoelectric devices. Using finite element analysis, the thickness and length of piezoelectric device and transducers were tailorized and the optimized frequency of 10 kHz was obtained. The resonance frequency from simulation analysis and evaluation was estimated to be similar as 10 kHz. The osseointegration was further enhanced with increasing frequency from the evaluation result of the finite element analysis.

• Journal of the Korean Society for Nondestructive Testing
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• v.36 no.1
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• pp.27-34
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• 2016

The type of ultrasound transducer used influences the quality of a reconstructed ultrasound image. This study analyzed the effect of transducer type on ultrasound computed tomography (UCT) image quality. The UCT was modeled in an ultrasound simulator by using a 5 cm anatomy model and a ring-shape 5 MHz 128 transducer array, which considered attenuation, refraction, and reflection. Speed-of-sound images were reconstructed by the Radon transform as the UCT image modality. Acoustic impedance images were also reconstructed by the delay-and-sum (DAS) method, which considered the speed of sound information. To determine the optimal combination of transducers in observation, point-source, flat, and focused transducers were tested in combination as trasmitters and receivers; UCT images were constructed from each combination. The combination of point-source/flat transducer as transmitting and receiving devices presented the best reconstructed image quality. In UCT implementation, the combination of a flat transducer for transmitting and a point transducer for receiving permitted acceptable image quality.

• Journal of the Korean Institute of Intelligent Systems
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• v.21 no.3
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• pp.323-328
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• 2011

In real system application, the 3-D obstacle avoidance system for the autonomous control of the underwater flight vehicle (UFV) operates with the following problems: the sonar offers the range/bearing information of obstacles in a local detection area, it requires the system that has reduced acoustic noise and power consumption in terms of the autonomous underwater vehicle (AUV), it has the UFV operation constraints such as maximum pitch and depth, and it requires an easy design procedure in terms of its structures and parameters. To solve these problems, an intelligent 3-D obstacle avoidance algorithm using the evolution strategy (ES) and the fuzzy logic controller (FLC), is proposed. To verify the performance of the proposed algorithm, the 3-D obstacle avoidance of UFV is performed. Simulation results show that the proposed algorithm effectively solves the problems in the real system application.

• Journal of the Korean Institute of Intelligent Systems
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• v.19 no.5
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• pp.635-640
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• 2009

In real system application, the obstacle avoidance system for the autonomous control of the underwater flight vehicle (UFV) operates with the following problems: it has local information because the sonar can only offer the obstacle information in a local detection area, it requires a continuous control input because the system that has reduced acoustic noise and power consumption is necessary, and further, it requires an easy design procedure in terms of its structures and parameters. To solve these problems, an intelligent obstacle avoidance algorithm using the evolution strategy (ES) and the fuzzy logic controller (FLC), is proposed. To verify the performance of the proposed algorithm, the obstacle avoidance of UFV is performed. Simulation results show that the proposed algorithm effectively solves the problems in the real system application.

• The Journal of the Acoustical Society of Korea
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• v.38 no.5
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• pp.580-586
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• 2019

In this paper, we propose a biomimetic communication method using a dolphin whistle to covertly transmit the communication signal. A conventional CSS (Chirp Spread Spectrum) modulation technique divides dolphin whistle into several slots and modulates with up and down chirp signals. That causes the time-frequency characteristic difference between the original dolphin whistle and the camouflage performance is degraded. In this paper, we propose a delay based modulation scheme to eliminate distortions. The simulation results show that the bit error rate of the proposed method is better performance than that of the conventional CSS modulation method by about 3.5 dB to 8 dB. And the camouflage performance that evaluated through the cross correlation in the time-frequency domain is also better than that of the CSS modulation method.

• The Journal of the Acoustical Society of Korea
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• v.40 no.4
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• pp.370-381
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• 2021

This paper presented an efficient transceiver structure of multiband Frequency Shift Keying (FSK) signals with direct sequence spread spectrum for maintaining covertness and performance. In aspect to covertness, direct sequence spread spectrum method, which multiplying by Pseudo Noise (PN) codes whose rate is much higher than that of data sequence, is employed. In aspect to performance, in order to overcome performance degradation caused by multipath and Doppler spreading, we applied multiband, turbo equalization, and weighting algorithm are applied. Based on the simulation results, by applying 4 number of multiband and number of chips are 8 and 32, experiments were conducted in a lake with a distance of moving from 300 m to 500 m between the transceivers. we confirmed that the performance was improved as the number of bands and chips are increased. Furthermore, the performance of multiband was improved when the proposed weighting algorithm was applied.

• Geomechanics and Engineering
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• v.24 no.2
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• pp.179-191
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• 2021

In this study, the strength and failure mechanism of red sandstones with combined defects were investigated by uniaxial compression tests on red sandstones with different crack angles using two-dimensional particle flow code numerical software, and their mechanical parameters and failure process were studied and analyzed. The results showed that the mechanical characteristics such as peak strength, peak strain, and elastic modulus of the samples with prefabricated combined defects were significantly inferior than those of the intact samples. With increasing crack angle from 15° to 60°, the weakening area of cracks increased, elastic modulus, peak strength, and peak strain gradually reduced, the total number of cracks increased, and more strain energy was released. In addition, the samples underwent initial brittle failure to plastic failure stage, and the failure form was more significant, leading to peeling phenomenon. However, with increasing crack angle from 75° to 90°, the crack-hole combination shared the stress concentration at the tip of the crack-crack combination, resulted in a gradual increase in elastic modulus, peak strain and peak strength, but a decrease in the number of total cracks, the release of strain energy reduced, the plastic failure state weakened, and the spalling phenomenon slowed down. On this basis, the samples with 30° and 45° crack-crack combination were selected for further experimental investigation. Through comparative analysis between the experimental and simulation results, the failure strength and final failure mode with cracks propagation of samples were found to be relatively similar.