• Journal of the Korean Operations Research and Management Science Society
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• v.24 no.1
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• pp.99-117
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• 1999

Since query language is used as a handy tool to obtain information from a database, a more intelligent query answering system is needed to provide user-friendly and fault-tolerant human-machine Interface. Frequently, database users prefer less rigid querying structure, one which allows for vagueness in composing queries, and want the system to understand the intent behind a query. When there is no matching data available, users would rather receive approximate answers than a null information response. This paper presents a knowledge abstraction database that facilitates the development of such a fault-tolerant and intelligent database system. The proposed knowledge abstraction database adepts a multilevel knowledge representation scheme called the knowledge abstraction hierarchy(KAH), extracts semantic data relationships from the underlying database, and provides query transformation mechanisms using query generalization and specialization steps. In cooperation with the underlying database, the knowledge abstraction database accepts vague queries and allows users to pose approximate queries as well as conceptually abstract queries. Specifically. four types of vague queries are discussed, including approximate selection, approximate join, conceptual selection, and conceptual Join. A prototype system has been implemented at KAIST and is being tested with a personnel database system to demonstrate the usefulness and practicality of the knowledge abstraction database in ordinary database application systems.

• 제어로봇시스템학회:학술대회논문집
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• 1997.10a
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• pp.1263-1266
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• 1997

In a fault-tolerant modern manufacturing systms characterized by the configuration, in which automated redundant machines prone to unexpected failures are interconnected with other complex subsystems such as AGV's, robots, computer control systems to produce complete parts, faulures together with repairs and reconfigurations should be considered as the three basic events to be modeled for computing the performance of manufacturing systems. In this papre, transient analysis is applied to modular cell manufacturing systems form a performability viewpoint whose modeling adantage is that various performanc e measures can be evaluated compositely in the context of application. The hypothertical modular cells are modeled firstly with hybrid decomposition method and availability measures as special cases of performability are computed and comments on performabililty modeling analysis are mentioned.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.58 no.3
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• pp.614-620
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• 2009

The force distribution in multi-legged robots is a constrained, optimization problem. The solution to the problem is the set points of the leg contact forces for a particular system task. In this paper, an efficient and general formulation of the force distribution problem is developed using linear programming. The considered walking robot is a quadruped robot with a locked-joint failure, i.e., a joint of the failed leg is locked at a known place. For overcoming the drawback of marginal stability in fault-tolerant gaits, we define safety margin on friction constraints as the objective function to be maximized. Dynamic features of locked-joint failure are represented by equality and inequality constraints of linear programming. Unlike the former study, our result can be applied to various forms of walking such as crab and turning gaits. Simulation results show the validity of the proposed scheme.

• Journal of KIISE:Computer Systems and Theory
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• v.27 no.3
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• pp.313-323
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• 2000

In this paper, we consider ring embedding problem on (n,k)-star graphs. We first present a new ring embedding strategy and also prove the superiority in expandability by showing its application into the fault-tolerant ring embedding problem with edge faults. This result can be applied to the multicating applications that use the underlying cycle properties on the multi-computer system.

• The Transactions of the Korea Information Processing Society
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• v.7 no.4
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• pp.1201-1210
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• 2000

In order to tolerate faults which may occur during the execution of distributed tasks in high-performance parallel computer systems, tasks are duplicated on different processors. In this paper, by utilizing the task duplication based scheduling algorithm, a new task scheduling algorithm which duplicates each task on more than two different processors with the minimum schedule length is presented, and the number of processors required for the duplication is analyzed with the ratio of communication cost to computation time and the workload of the system. A simulation with various task graphs reveals that the number of processors required for the full-duplex fault-tolerant task scheduling with the obtainable minimum schedule length increases about 30% to 75% when compared with that of the task duplication based scheduling algorithm.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.21 no.6
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• pp.1463-1469
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• 1996

A crossed incoplete hypercube structure is based on previous crossed hypercube structure that reduces the diameter about 50% and has the same complexity with general hypercube and is able to expand to any number of nodes. In this study, we proposed a fault-tolerant adaptive routing scheme and broadcasting scheme. Fault-tolerant adaptive routing scheme is resilient to system faults. And performance evaluation is executed on the proposed sturcture. It is enhanced about 30%.

• 제어로봇시스템학회:학술대회논문집
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• 2000.10a
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• pp.88-88
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• 2000

As more and more critical commercial applications move on the Internet, providing highly available servers becomes increasingly important. One of the advantages of a clustered system is that it has hardware and software redundancy. High availability can be provided by detecting node or daemon failure and reconfiguring the system appropriately so that the workload can be taken over bi the remaining nodes in the cluster. This paper presents how to provide the guaranteeing high availability of clustering web server. The load balancer becomes a single failure point of the whole system. In order to prevent the failure of the load balancer, we setup a backup server using heartbeat, fake, mon, and checkpointing fault-tolerance method. For high availability of file servers in the cluster, we setup coda file system. Coda is a advanced network fault-tolerance distributed file system.

• Proceedings of the Korean Information Science Society Conference
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• 2004.10a
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• pp.661-663
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• 2004

본 논문에서는 SAN 기반 공유 파일 시스템인 SANfs의 fault-tolerance를 보장받기 위해 fault-tolerant server인 shadow server를 구현하였다 SANfs(1)는 SAN에서 Network-attached storage에 접근하는 여러 클라이언트가 서로의 데이터를 공유할 수 있도록 도와주는 파일시스템이다. SANfs에서 파일 관리를 위해 meta server를 두고 있으며, 이 서버에서 네트워크를 통해 접근하는 털러 클라이언트의 request를 관리한다. SAMfs에서는 meta server를 통해 중앙 집중식으로 파일시스템을 관리하고 있기에 meta server가 fault가 나게 되면 전체 시스템의 동작이 멈추게 되는 single point-of-failure의 문제가 생기게 된다. 본 논문에서는 meta server가 fault가 났을 경우에도 지속적으로 서비스를 할 수 있도록 shadow server를 두었으며. 이 서버가 meta server의 이상 시 그 기능을 대행하도록 하였다. 본 논문의 shadow server는 평상시에 meta server와 파일시스템의 metadata의 동기를 맞추고 있으며, 이 정보를 가지고 meta server로 그 기능을 전환하였을 때 서비스를 해 주도록 하고 있다. 상대 서버의 이상 유무의 판단은 heartbeat를 통해 이루어지고 있으며, meta server로의 failover는 heartbeat의 주기에 영향을 받음을 실험을 통해 알게 되었다.

• Journal of Power Electronics
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• v.10 no.6
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• pp.702-708
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• 2010

In this paper, a fault diagnosis algorithm for brushless DC (BLDC) motor drives is proposed to maintain control performance under switch open-damage. The proposed fault diagnosis algorithm consists of a simple algorithm using measured phase current information and it detects open-circuit faults based on the operating characteristic of BLDC motors. The proposed algorithm quickly recovers control performance due to its short detection time and its reconfiguration of the system topology. It can be embedded into existing BLDC drive software as a subroutine without additional sensors. The feasibility of the proposed fault diagnosis algorithm is proven by simulation and experimental results.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 1998.06a
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• pp.515-520
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• 1998

In this paper, we propose a new fault tolerant architecture for high availability systems, where for module internal operations both processor modules perform the same tasks at the same time independently of each other while for module external operations both processor modules act actively. That is, operations of synchronization between dual processor modules except clock synchronization are requested only when module external operations are executed. The architecture can not only improve system availability by reducing system reintegration time but also reduce performance degradation problem due to frequent synchronization between dual processor modules. The clock unit consists of a clock generator and a clock synchronization circuit. This supplies a stable clock signal under clock unit disorder of any processor module or rapid clock signal variation. And this architecture achieves system availability and data credibility by designing as symmetrical form.