Chang-Hoon Lie

Estimating parameters of intensity function and maintenance effect for repairable unit

A data-analytic procedure for successive inter-failure times (IFT) of repairable units is investigated. Specifically, a method of data analysis for the estimation of parameters of the failure process and the maintenance effect for repairable units is proposed. Maliks proportional age reduction (PAR) model for imperfect maintenance is utilized. The PAR model is reformulated in terms of the properties of non-homogeneous Poisson processes (NHPP) with piecewise continuous intensity function. The joint distribution of the IFTs is also derived. Two parametric families of intensity functions are considered, power-law and log-linear. Maximum likelihood method is employed to estimate the maintenance effect and the intensity function simultaneously. Two models are considered; one with effective corrective maintenance (CM) without preventive maintenance (PM) and the other with effective PM with minimal CM. Simulation results are presented to illustrate the accuracy of the estimation and the properties of the models. An application example for a set of failure and maintenance data of a central cooler in a nuclear power plant is also presented.

Joint reliability importance of k-out-of-n systems

Abstract Joint Reliability Importance (JRI) is investigated to provide information on the type and the degree of interactions between two components by identifying the sign and the size of it. In specific, JRI is analyzed for k -out-of- n systems with (i) independently identically distributed (IID) components, (ii) non-identical and independent components and (iii) pair-wise dependent components. The closed form solution of JRI is derived for IID components. By using the solution, the variations of JRI are investigated with respect to the level of redundancy ( n / k ). In non-identical and independent case, the point of sign change of JRI is obtained. It is also shown that the sign of JRI can be determined by the relationships between the Schur-convexity (concavity) and the JRI. For dependent case, only the pair-wise dependence is considered. As a result, the error caused by assuming statistical independence between pair-wise dependent components is shown to be measured by their covariance and JRI.

Economic-statistical design of 2-of-2 and 2-of-3 runs rule scheme

In this paper, we investigate the economic-statistical design method for the 2-of-2 runs rule and the 2-of-3 runs rule. The Markov chain approach is used to obtain the average run length and the process cycle time. In addition, a simplified algorithm is presented to search the optimal setting of the design parameters. A numerical example and sensitivity analysis are also provided to compare the performances of the runs rules. The results show that the use of runs rule scheme can reduce operating cost comparing with the Shewhart control chart while maintaining a good statistical performance. Copyright

Computation of joint reliability importance of two gate events in a fault tree

Abstract Joint reliability importance (JRI) of two gate events (GEs) is investigated along with its properties in a fault tree (FT). Computation of JRI of two GEs involves complexity because statistical dependency between GEs exists as a result of replication of basic events (BEs). To avoid complexity in computing the JRI, the topological relationships of two GEs in a FT are identified and classified into three classes. The explicit formula for the JRI of the two GEs is derived. JRI for each class is shown to be computed by using a conventional FT algorithm. By illustrating min. cut set representation of the bridge network, it is also shown that the degree of relationship between two min. cut sets can be measured by the JRI of the corresponding two GEs.

Stress-reducing preventive maintenance model for a unit under stressful environment

We develop a preventive maintenance (PM) model for a unit operated under stressful environment. The PM model in this paper consists of a failure rate model and two cost models to determine the optimal PM scheduling which minimizes a cost rate. The assumption for the proposed model is that stressful environment accelerates the failure of the unit and periodic maintenances reduce stress from outside. The failure rate model handles the maintenance effect of PM using improvement and stress factors. The cost models are categorized into two failure recognition cases: immediate failure recognition and periodic failure detection. The optimal PM scheduling is obtained by considering the trade-off between the related cost and the lifetime of a unit in our model setting. The practical usage of our proposed model is tested through a numerical example.

AN OPTIMAL BLOCK PREVENTIVE MAINTENANCE POLICY FOR A MULTI-UNIT SYSTEM CONSIDERING IMPERFECT MAINTENANCE

A block preventive maintenance (PM) policy for a multi-unit system composed of identical units is investigated in this paper. The block PM model of this paper considers periodic inspection and periodic imperfect maintenance. The imperfect maintenance effects are formulated using improvement factor and age reduction model. We define variables of maintenance policy as the inspection period and the PM period and obtain an optimal maintenance policy which minimizes the average total cost. A numerical example is presented, and some sensitivity analyses for model parameters are also investigated.

Quality of service estimation for soft handoff region ratio and call admission control in CDMA cellular systems

In the operational stage of code division multiple access (CDMA) systems, the soft handoff region ratio (SHRR) is among the important parameters that guarantee the quality of service (QoS) and resource allocation. This paper analytically investigates the effects of the SHRR on QoS. In doing so, we simultaneously take into account the capacity increasing factor and traffic load variation. For this purpose, we propose a new traffic load estimation model using a semi-Markov chain in order to consider the call arrivals influenced by the SHRR under general resident time condition. We also present the calculation algorithm of the QoS measures such as the new call blocking probability (NBP) and forced termination probability (FTP) by applying the proposed model to the prioritized call admission control (CAC). The numerical result illustrates the effects of various soft handoff system parameters. Likewise, it shows that the value of SHRR has a significant impact of the QoS under the normal propagation conditions. Sensitivity analysis is also performed to show the effect of exponential cell resident time assumption. The proposed modeling is expected to provide a practical guide to improve the QoS of CDMA soft handoff systems.

A Dynamic Anchor-Area Scheme Using a Timer Value and a Movement List for PCS Networks

A dynamic anchor-area (DAA) scheme is proposed to reduce the location management cost for PCS networks. Based on an Intra-LA location update (LU) scheme, the proposed DAA scheme utilizes a given timer value and a movement list. The DAA scheme considers a wider variety of mobile terminals movement pattern than the previous intra-LA LU scheme. Simulations confirm that the proposed scheme offers a significant reduction in the location management cost.

Design of location areas with QoS considerations in a cellular network

In this paper, we present a location area (LA) planning model which explicitly has more QoS-conscious constraints than any other models. In order to specify an acceptable level of QoS, paging delay time (PDT) and paging blocking rate (PBR) are introduced. The paging process is described by the M/M/c/K queuing system and queuing analysis is carried out to obtain PDT and PBR. Also, in the proposed model, the time-varying call arrival rates are used to define the paging load on each cell, which helps to reflect the pattern of incoming calls more precisely and subsequently to improve the QoS level. To evaluate the proposed model, some examples are solved along with the average rate model and the peak rate model. The results show that the proposed model has the capability to maintain the acceptable level of QoS with the reasonable location management cost, compared to other models. Also, the sensitivity analysis is performed to see the effect of PDT and PBR in the formulation and it turns out that both PBR and PDT are important elements of the QoS measure and influential to the location management cost.

Analysis of the M/G/1 Priority Queue with vacation period depending on the Customer`s arrival

M/G/1 queue with server vacations period depending on the previous vacation and customer’s arrival is considered. Most existing studies on M/G/1 queue with server vacations assume that server vacations are independent of customers’ arrival. However, some vacations are terminated by some class o f customers’ arrival in certain queueing systems. In this paper, therefore, we investigate M/G/1 queue with server vacations where each vacation period has different distribution and vacation length is influenced by customers’ arrival. Laplace-Stieltjes transform of the waiting time distribution and the distribution of number of customers waiting for each class of customers are respectively derived. As performance measures, mean waiting time and average number of customers waiting for each class of customers are also derived.