• 대한전자공학회논문지TC
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• 제48권6호
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• pp.22-27
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• 2011

본 논문은 직접 대역 확산(Direct sequence spread spectrum : DS-SS) 시스템에서 적응형 임계값을 적용하여 초기 동기 성능을 향상 시키는 알고리즘을 제안한다. 제안된 알고리즘은 탐색 모드(search mode) 이전에 프리덤프 모드(pre-dump mode)를 추가하고, 상관 에너지를 이용해 탐색 모드와 확인 모드(verification mode)의 임계값을 결정한다. 이를 통해 확인 모드뿐만 아니라 탐색 모드에서까지 거부 성능(rejection performance)을 향상시킬 수 있다. 제안된 방법은 기존 기술과 비교할 때 하드웨어 증가 없이 평균 코드 획득 시간(mean code acquisition time)을 약 40% 단축시킬 수 있다.

• 한국정밀공학회지
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• 제22권6호
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• pp.152-158
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• 2005

The stress conditions acting on the practical structure are complex, and thus most cracks existing in the practical structures are under mixed-mode loading conditions. The effect of shear load component of mixed-mode loading acts more greatly in the stage of crack initiation and initial propagation than crack propagation stage. Hence, research on the behavior in the stage of crack initiation and initial propagation need to be examined in order to evaluate behavior of mixed-mode fatigue cracks. In this study, the crack tip displacement(CTD) was measured by using the direct measuring method(DMM). We examined the behavior at crack tip by determining crack opening load$(P_{op})$. From the test results, the propagation behavior of mixed-mode fatigue cracks was evaluated by considering mixed-mode crack closure. Also, we examined the characteristic of crack propagation under mixed-mode loading with crack propagation direction.

• Journal of Information Processing Systems
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• 제8권4호
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• pp.685-692
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• 2012

B frame bi-directional predictions and the DIRECT mode coding of the H.264 video compression standard necessitate a complex mode decision process, resulting in a long computation time. To make H.264 feasible, this paper proposes an image backtrack-based fast (IBFD) algorithm and evaluates the performances of two promising fast algorithms (i.e., AFDM and IBFD). Evaluation results show that an image backtrack-based fast (IBFD) algorithm can determine DIRECT mode macroblocks with 13% higher accuracy, as compared with the AFDM. Furthermore, IBFD is shown to reduce the motion estimation time of B frames by up to 23% with a negligible quality degradation.

• 한국프랜차이즈경영연구
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• 제4권1호
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• pp.113-129
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• 2013

The realization of agricultural modernization, the solving of the "three rural" issue as well as the increment of farmers' income is one of China's top priority. Thus, under the support and encouragement of government's relevant policies, China has introduced the "Supermarket-Farmer Direct Purchase" which is a new mode for the supply of agricultural products. This is an innovative of agricultural products circulation mode, and the comprehensive promotion will help supermarkets get cheap goods directly from the hands of farmers, thus forming price competitiveness; farmers can also get profits and subsidies from supermarkets which can eliminate middlemen's profit, thus increasing their revenue; consumers can both get a guarantee of food safety and save expenses, and the win-win situation for the supermarkets, farmers, and consumers will be achieved. However, the dilemma between "farmers having difficulty in selling" and "supermarkets having difficulty in buying" is still frequent. So in this thesis, through theoretical research and situation analysis, each relevant part of the "Supermarket-Farmer Direct Purchase" mode will be studied as a whole. Moreover, effectiveness will be evaluated and relevant problems will be identified. Then, based on foreign experience and our national conditions, new modes and advice will be provided for users in different circumstances.

• Journal of Power Electronics
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• 제13권6호
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• pp.1000-1007
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• 2013

This paper proposes a sliding-mode-based direct power control (DPC) method in a three-phase boost rectifier without the use of a voltage sensor. This sliding-mode-based DPC is used to improve transient-state response characteristics. This DPC can eliminate voltage sensors by calculating a voltage using a sensorless method, thus considerably reducing cost. This DPC first presents an effective algorithm that does not significantly affect the previous performance and does not need a voltage sensor. Thereafter, the effectiveness of the algorithm is verified by simulations and experiments.

• Journal of Power Electronics
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• 제9권4호
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• pp.582-592
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• 2009

This paper proposes a new speed sensorless direct torque and flux controlled interior permanent magnet synchronous motor (IPMSM) drive. Closed-loop control of both the torque and stator flux linkage are achieved by using two proportional-integral (PI) controllers. The reference voltage vectors are generated by a SVM unit. The drive uses an adaptive sliding mode observer for joint stator flux and rotor speed estimation. Global asymptotic stability of the observer is achieved via Lyapunov analysis. At low speeds, the observer is combined with the high frequency signal injection technique for stable operation down to standstill. Hence, the sensorless drive is capable of exhibiting high dynamic and steady-state performances over a wide speed range. The operating range of the direct torque and flux controlled (DTFC) drive is extended into the high speed region by incorporating field weakening. Experimental results confirm the effectiveness of the proposed method.

• Journal of Electrical Engineering and Technology
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• 제9권5호
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• pp.1569-1576
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• 2014

This paper mainly proposes a direct torque control strategy to minimize torque ripple in brushless DC (BLDC) motor. BLDC motor has large current and torque ripple when one voltage vector applied in one cycle due to its low inductance. Hence, this paper proposed a hysteresis torque control with PWM mode to control the resultant torque. Moreover, when the direct torque control system is operating during the two-phase half-bridge $120^{\circ}$ conduction mode, large torque ripple in commutation area appears every 120 electrical degree. Based on analyzing the root of torque ripple in detail, lookup tables of switching devices states for new half-bridge modulation mode in the positive and negative reference torque put forwarded. Finally, simulations by MATLAB software and experiment results from DSP are presented to verify the feasibility and effectiveness of the proposed strategy operating in four-quadrant operation.

• 전력전자학회논문지
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• 제21권3호
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• pp.238-248
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• 2016

In this study, an automatic command mode transition strategy of direct power control (DPC) is proposed for permanent magnet synchronous generators (PMSGs) medium-voltage (MV) offshore wind turbines (WTs). Benchmarking against the control methods are performed based on a three-level neutral-point-clamped (NPC) back-to-back type voltage source converter (VSC). The ramping rate criterion of complex power is utilized to select the switching vector in DPC for a three-level NPC converter. With a grid command and an MPPT mode transition strategy, the proposed control method automatically controls the generated output power to satisfy a grid requirement from the hierarchical wind farm controller. The automatic command mode transition strategy of DPC is confirmed through PLECS simulations based on Matlab. The simulation result of the automatic mode transition strategy shows that the proposed control method of VOC and DPC achieves a much shorter transient time of generated output power than the conventional control methods of MPPT and VOC under a step response. The proposed control method helps provide a good dynamic performance for PMSGs MV offshore WTs, thereby generating high quality output power.

• 한국지진공학회논문집
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• 제22권5호
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• pp.299-309
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• 2018

Now that problems with force-based seismic design have been clearly identified, design is inclined toward displacement-based methods. One such widely used method is Direct-Displacement-Based Design (DDBD). Yet, one of the shortcomings of DDBD is considering higher-mode amplification of story shear, moments, and displacements using equations obtained from limited parametric studies of regular planar frames. In this paper, a different approach to account for higher-mode effects is proposed. This approach determines the lateral secant stiffness of the building frames that fulfill the allowable inter-story drift without exceeding the desired story displacements. Using the stiffness, an elastic response spectrum analysis is carried out to determine elastic higher-mode force effects. These force effects are then combined with DDBD-obtained first-mode force effects using the appropriate modal superposition method so that design can be performed. The proposed design procedure is verified using Nonlinear Time History Analysis (NTHA) of twelve planar frames in four categories accounting for mass and stiffness irregularity along the height. In general, the NTHA response outputs compared well with the allowable limits of the performance objective. Thus, it fulfills the aim of minimizing the use of NTHA for planar frame buildings, thereby saving computational resources and effort.

• 전력전자학회논문지
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• 제5권5호
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• pp.427-434
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• 2000

본 논문은 유도전동기의 직접벡터 제어 시스템에서 전동기가 기동 시 토크 충격없이 부드럽게 기동할 수 있는 기동기법을 제시한다. 정지모드에서는 고정자 전류만으로 고정자자속을 계산하는 방식을 사용하며, 기동 후 직접 벡터제어 모드에서는 프로그램어블 3-단계 저역필터로 고정자자속을 계산한다. 정지모드에서 직접벡터제어 모드로 변환 시 3-단계 저역필터의 시간지연으로 고정자 자속이 급격히 감소되어 토크 역시 갑자기 감소된다. 본 논문에서는 전동기가 기동 시 토크 충격을 방지하기 위한 고정자 자속의 피이드포워드 제어 기법을 제시하고, 시뮬레이션 및 실험을 통하여 이 기법의 타당성을 확인한다.