• Journal of Electrical Engineering and Technology
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• v.11 no.6
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• pp.1556-1570
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• 2016

In the process of virtual inertia control (VIC), the frequency regulation capability of the directly-driven wind turbine with permanent-magnet synchronous generator (D-PMSG) on wind farm is related to its rotor kinetic energy and capacity margin. This paper proposes the method for assessing the D-PMSG frequency regulation capability and defining its coefficient according to the operating state of wind power generators. In addition, the calculating method of parameters in VIC is also discussed according to the principles of primary frequency regulation and inertia response of synchronous generators. Then, by introducing the capability coefficient into the proportion-differential virtual inertia control (PD-VIC) for power coordination, a coordinated virtual inertia control (C-VIC) strategy is developed, with the consideration of the difference in frequency regulation capability between wind power generators. The proposed control method can not only give full play to the frequency regulation capability of wind power generators, decrease the movements of the pitch angle control system but also bring some self-coordination capability to different wind power generators thus to avoid a secondary drop in system frequency. The simulations and experiments prove the proposed method to be effective and practicable.

• KSII Transactions on Internet and Information Systems (TIIS)
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• v.9 no.12
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• pp.4950-4966
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• 2015

Cloud providers now face the problem of estimating the amount of computing resources required to satisfy a future workload. In this paper, a virtual machine provisioning (VMP) mechanism is designed to adapt workload fluctuation. The arrival rate of forthcoming jobs is predicted for acquiring the proper service rate by adopting an exponential smoothing (ES) method. The proper service rate is estimated to guarantee the service level agreement (SLA) constraints by using a diffusion approximation statistical model. The VMP problem is formulated as a facility location problem. Furthermore, it is characterized as the maximization of submodular function subject to the matroid constraints. A greedy-based VMP algorithm is designed to obtain the optimal virtual machine provision pattern. Simulation results illustrate that the proposed mechanism could increase the average profit efficiently without incurring significant quality of service (QoS) violations.

• ETRI Journal
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• v.36 no.5
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• pp.741-751
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• 2014

Smart TV is expected to bring cloud services based on virtualization technologies to the home environment with hardware and software support. Although most physical resources can be shared among virtual machines (VMs) using a time sharing approach, allocating the proper amount of memory to VMs is still challenging. In this paper, we propose a novel mechanism to dynamically balance the memory allocation among VMs in virtualized Smart TV systems. In contrast to previous studies, where a virtual machine monitor (VMM) is solely responsible for estimating the working set size, our mechanism is symbiotic. Each VM periodically reports its memory usage pattern to the VMM. The VMM then predicts the future memory demand of each VM and rebalances the memory allocation among the VMs when necessary. Experimental results show that our mechanism improves performance by up to 18.28 times and reduces expensive memory swapping by up to 99.73% with negligible overheads (0.05% on average).

• ETRI Journal
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• v.41 no.4
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• pp.473-482
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• 2019

High availability and reliability have been considered promising requirements for the support of seamless network services such as real-time video streaming, gaming, and virtual and augmented reality. Increased availability can be achieved within a local area network with the use of the virtual router redundancy protocol that utilizes backup routers to provide a backup path in the case of a master router failure. However, the network may still lose a large number of packets during a failover owing to a late failure detections and lazy responses. To achieve an efficient failover, we propose the implementation of fast detection with virtual router redundancy protocol (FDVRRP) in which the backup router quickly detects a link failure and immediately serves as the master router. We implemented the FDVRRP using open neutralized network operating system (OpenN2OS), which is an open-source-based network operating system. Based on the failover performance test of OpenN2OS, we verified that the FDVRRP exhibits a very fast failure detection and a failover with low-overhead packets.

• Journal of Power Electronics
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• v.17 no.3
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• pp.766-776
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• 2017

In modern power systems, high penetration of distributed generators (DGs) results in high stress on system stability. Apart from the intermittent nature of DGs, most DGs do not contribute inertia or damping to systems. As a result, a new control method named virtual synchronous machine (VSM) was proposed, which brought new characteristics to inverters such as synchronous machines (SMs). In addition, different active power droop controls for VSMs are being proposed in literatures. However, they are quite different in terms of their dynamic characteristics despite of the similar control laws. In this paper, mathematical models of a VSM adopting different active power droop controls are built and analyzed. The dynamic performance of the VSM output active power and virtual rotor angular frequency are presented for different models. The influences of the damping factor and droop coefficient on the VSM dynamic behaviors are also investigated in detail. Finally, the theoretical analysis is verified by simulations and experimental results.

• Korean Journal of Clinical Laboratory Science
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• v.50 no.4
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• pp.492-500
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• 2018

This study examined the effects of a dual-task virtual reality program on the cognitive function and EEG for patients with mild cognitive impairment. A dual-task virtual reality program was performed in the experimental groups while conventional occupational therapy was carried out in the control group for 30 minutes per session, which was done five days per week for 6 weeks. The results were as follows. First, the memory of the cognitive function and balance was improved significantly in the experimental group with the dual-task virtual reality program compared to the control group with the traditional occupational therapy. Second, EEG was also increased significantly in the experimental group compared to the control group. The results of this study suggest that the dual-task virtual reality program was an effective treatment method for the elderly with mild cognitive impairment and would be a cornerstone of basic data that will be helpful to those suffering from a range of diseases.

• Proceedings of the Korean Society of Broadcast Engineers Conference
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• 1998.06b
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• pp.129-140
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• 1998

There is considerable research activity related to computer graphics and virtual reality within the National Chiao Tung University (NCTU), Hsinchu, Taiwan. In this article, we describe the history of computer graphics at NCTU and report on our current research topics and future works. Besides we also briefly outline the research works on computer graphics and virtual reality within universities in Taiwan.

• Proceedings of the KIEE Conference
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• 2001.07b
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• pp.825-827
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• 2001

In this paper, we made mathematical modeling of BLDC motor system and design of PI controller. Using Apache web-server and PHP web-programming, we embodied virtual laboratory for PI controller design of BLDC Motor system by using proportional constants$(K_P)$ and constants of integration$(K_I)$.

• Proceedings of the Korean Information Science Society Conference
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• 2007.10a
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• pp.189-190
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• 2007

• 한국정보디스플레이학회:학술대회논문집
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• 2005.07a
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• pp.41-45
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• 2005

A new algorithm of the image mapping which is a technique of the elemental image generation is proposed. The proposed method is based on the characteristics of the lens array such as the number, the size and the focal length of the elemental lens. The 3D image generated by 3D graphic API such as OpenGL can be directly adopted without the complex adaptation. Since the image mapping using the proposed method can enhance the speed of the elemental image generation, the computer- generated integral imaging system can be applied to virtual reality system.