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Life estimation after testing for early failures
World Abstracts on Microelectronics and Reliability operation from device to field are integrated. A common data base can provide effective strategy settlement on the quality and reliability improvement. This report describes the organization of the integrated reliability information system and it's application to the field reliability improvements, along with the corrective actions in manufacturing process.
Simple test of hypotheses on system availability and mean time to repair. SERAFETTINCABUK. IEEE Trans. Reliab. R-35 (5), 581 (1986). A method is developed for testing hypotheses on system availability and mean time to repair, under the assumption that the failure times and repair times are, respectively, independent exponential and gamma random variables. System is assumed to be in a steady state; availability is defined in usual manner as the ratio of mean time to failure to the sum of mean time to failure and mean time to repair. Testing hypotheses on system availability is achieved by simple application of the F distribution, provided: (a) the system mean time to repair is known or determinable from other assumptions, or (b) the ratio of system mean time to failure to the gamma scale parameter is known. Similarly, hypotheses on system mean repair time can be tested using the chi-square distribution, if the gamma scale parameter is known.
High reliability by automation and design quality. TONY D. Cox and E. M. BOHAN. Proc. a. Reliab. Maintainab. Syrup., 108 (1987). This paper will address the attainment of high reliability on digital electronic hardware through the application of automated design techniques, and appropriate product assurance activities. The integration between automated design and product assurance will be addressed and discussed in relation to those activities critical to achieving a high reliability. An algorithm for reducing cut sets in fault-tree analysis. N. LIMNIOS and R. ZIA~. IEEE Trans. Reliab. R-35 (5), 559 (1986). The main goal of a fault-tree algorithm is to obtain the minimal cut sets as quickly as possible. This paper presents a new algorithm for cut-set reduction; it is based on the partition of the cut sets into two families: Those containing repeated events, and others. It is proved that only those containing repeated events need reduction. This algorithm was implemented by a computer program which had been associated with M O C U S algorithm, both developed on a microcomputer. This algorithm can be combined with other reduction algorithms. We believe that this algorithm is efficient.
Operational reliability of the DX 200 switching system. TAPANI PURHO,ZEKROLLAHAFLATUNIand JOUNISOITINAHO. Proc. a. Reliab. Maintainab. Syrup., 38 (1987). Availability performance of telecommunication services is one of the most important factors when the quality of service of a telecommunication network is assessed. This paper summarizes the operational objectives and field results of availability performance of the DX 200 Digital Switching System widely used in the telephone network of the Finnish PTT and private telephone companies. Analysis of the field results from the point of view of quality and reliability improvement is also presented.
Reliability optimization problems with multiple constraints. MAW-SHENG CHERN and RONG-HONG JAN. IEEE Trans. Reliab. R-35 (4), 431 (1986). This paper presents a class of reliability optimization problems with multiple-choice constraints. We assume that at least one design alternative can be chosen as redundancy for each subsystem. A 2-phase solution method is presented for solving these problems. In phase I, we decompose a problem into n subproblems. These subproblems can be solved by dynamic programming, independently. That is, these subproblems can be solved by parallelism. In phase II, we solve a 0-1 multiple-choice ~R 2 7 : 6 - F
1029
knapsack problem which is generated from phase I. We use a combinatorial tree which always satisfies the multiple-choice constraints. The 2-phase solution method is illustrated with a numerical example.
Life estimation after testing for early failures. SAMIR K. BHATTACHARYA and ASHOK SINGH. IEEE Trans. Reliab. R35 (4), 423 (1986). Life estimation based on ordered observations from the exponential distribution is considered for the case when several early failures may be present. The method proposed here requires a preliminary statistical test to decide whether the suspected observations are indeed early failures. The role of the suspected observations (early failures) in the subsequent estimation problem is based on the result of this test. The proposed estimator has, under certain conditions, smaller mean square error than that of the minimum variance unbiased estimator, and its bias remains small.
On the mean time between failures for repairable systems. MAX ENGELHARDT and LEE J. BAIN. IEEE Trans. Reliab. R35 (4), 419 (1986). Much of the recent work on modeling repairable systems involves Poisson processes with nonconstant intensity functions, viz. nonhomogeneous Poisson processes. Since times between failures are not identically distributed when the process is nonhomogeneous, it is not clear what concept should take the place of the mean time between failures in assessing the reliability of a repairable system. A number of alternate concepts can be found in the literature. We investigate the relationship between two of the most frequently considered alternatives: the reciprocal of the intensity function, and the mean waiting time from t until the next failure. Theorem 1 states a necessary and sufficient condition for the mean time until the next failure to be asymptotically proportional to the reciprocal of the intensity function. Some examples, including the familiar log-linear and powerintensity process satisfy this condition. A monotonicity property is also established between these two concepts which could be used to obtain conservative statistical confidence limits for the mean time until the next failure, based on results which are already available for the intensity function of the power-intensity process. However, further study of concepts such as the rate of convergence would be needed in order to determine the degree of approximation of the nominal confidence level to the actual level. Until more is known about the mean time from t until the next failure, it would be advisable to use the reciprocal of the intensity function, which has been studied more extensively, as the basis of reliability assessment for a repairable system.
Jointly optimal block-replacement and spare provisioning policy. D. ACHARYA, G. NAGABHUSHANAMand S. S. ALAM. IEEE Trans. Reliab. R-35 (4), 447 (1986). This paper deals with the problem of finding a jointly optimal, block preventive replacement and spare provisioning policy for a system consisting of several like equipments. It studies the effect of inventory of spares on the optimal block replacement interval. For a single period replacement model, the jointly optimal preventive replacement interval is appreciably different from the corresponding optimal preventive replacement interval where only the replacement related costs are considered, For a multiperiod replacement model, the jointly optimal preventive replacement interval decreases from its single period optimal interval as the multiplicity of the period increases. This shows that a trade-off exists between the replacement related costs and the inventory related costs. Therefore, we get a replacement interval which is much lower than the optimal replacement interval in a single period model. There exists a finite optimal interval between two inventory replacements not necessarily equal to the basic preventive replacement interval.
Simple test of hypotheses on system availability and mean time to repair. SERAFETTINCABUK. IEEE Trans. Reliab. R-35 (5), 581 (1986). A method is developed for testing hypotheses on system availability and mean time to repair, under the assumption that the failure times and repair times are, respectively, independent exponential and gamma random variables. System is assumed to be in a steady state; availability is defined in usual manner as the ratio of mean time to failure to the sum of mean time to failure and mean time to repair. Testing hypotheses on system availability is achieved by simple application of the F distribution, provided: (a) the system mean time to repair is known or determinable from other assumptions, or (b) the ratio of system mean time to failure to the gamma scale parameter is known. Similarly, hypotheses on system mean repair time can be tested using the chi-square distribution, if the gamma scale parameter is known.
High reliability by automation and design quality. TONY D. Cox and E. M. BOHAN. Proc. a. Reliab. Maintainab. Syrup., 108 (1987). This paper will address the attainment of high reliability on digital electronic hardware through the application of automated design techniques, and appropriate product assurance activities. The integration between automated design and product assurance will be addressed and discussed in relation to those activities critical to achieving a high reliability. An algorithm for reducing cut sets in fault-tree analysis. N. LIMNIOS and R. ZIA~. IEEE Trans. Reliab. R-35 (5), 559 (1986). The main goal of a fault-tree algorithm is to obtain the minimal cut sets as quickly as possible. This paper presents a new algorithm for cut-set reduction; it is based on the partition of the cut sets into two families: Those containing repeated events, and others. It is proved that only those containing repeated events need reduction. This algorithm was implemented by a computer program which had been associated with M O C U S algorithm, both developed on a microcomputer. This algorithm can be combined with other reduction algorithms. We believe that this algorithm is efficient.
Operational reliability of the DX 200 switching system. TAPANI PURHO,ZEKROLLAHAFLATUNIand JOUNISOITINAHO. Proc. a. Reliab. Maintainab. Syrup., 38 (1987). Availability performance of telecommunication services is one of the most important factors when the quality of service of a telecommunication network is assessed. This paper summarizes the operational objectives and field results of availability performance of the DX 200 Digital Switching System widely used in the telephone network of the Finnish PTT and private telephone companies. Analysis of the field results from the point of view of quality and reliability improvement is also presented.
Reliability optimization problems with multiple constraints. MAW-SHENG CHERN and RONG-HONG JAN. IEEE Trans. Reliab. R-35 (4), 431 (1986). This paper presents a class of reliability optimization problems with multiple-choice constraints. We assume that at least one design alternative can be chosen as redundancy for each subsystem. A 2-phase solution method is presented for solving these problems. In phase I, we decompose a problem into n subproblems. These subproblems can be solved by dynamic programming, independently. That is, these subproblems can be solved by parallelism. In phase II, we solve a 0-1 multiple-choice ~R 2 7 : 6 - F
1029
knapsack problem which is generated from phase I. We use a combinatorial tree which always satisfies the multiple-choice constraints. The 2-phase solution method is illustrated with a numerical example.
Life estimation after testing for early failures. SAMIR K. BHATTACHARYA and ASHOK SINGH. IEEE Trans. Reliab. R35 (4), 423 (1986). Life estimation based on ordered observations from the exponential distribution is considered for the case when several early failures may be present. The method proposed here requires a preliminary statistical test to decide whether the suspected observations are indeed early failures. The role of the suspected observations (early failures) in the subsequent estimation problem is based on the result of this test. The proposed estimator has, under certain conditions, smaller mean square error than that of the minimum variance unbiased estimator, and its bias remains small.
On the mean time between failures for repairable systems. MAX ENGELHARDT and LEE J. BAIN. IEEE Trans. Reliab. R35 (4), 419 (1986). Much of the recent work on modeling repairable systems involves Poisson processes with nonconstant intensity functions, viz. nonhomogeneous Poisson processes. Since times between failures are not identically distributed when the process is nonhomogeneous, it is not clear what concept should take the place of the mean time between failures in assessing the reliability of a repairable system. A number of alternate concepts can be found in the literature. We investigate the relationship between two of the most frequently considered alternatives: the reciprocal of the intensity function, and the mean waiting time from t until the next failure. Theorem 1 states a necessary and sufficient condition for the mean time until the next failure to be asymptotically proportional to the reciprocal of the intensity function. Some examples, including the familiar log-linear and powerintensity process satisfy this condition. A monotonicity property is also established between these two concepts which could be used to obtain conservative statistical confidence limits for the mean time until the next failure, based on results which are already available for the intensity function of the power-intensity process. However, further study of concepts such as the rate of convergence would be needed in order to determine the degree of approximation of the nominal confidence level to the actual level. Until more is known about the mean time from t until the next failure, it would be advisable to use the reciprocal of the intensity function, which has been studied more extensively, as the basis of reliability assessment for a repairable system.
Jointly optimal block-replacement and spare provisioning policy. D. ACHARYA, G. NAGABHUSHANAMand S. S. ALAM. IEEE Trans. Reliab. R-35 (4), 447 (1986). This paper deals with the problem of finding a jointly optimal, block preventive replacement and spare provisioning policy for a system consisting of several like equipments. It studies the effect of inventory of spares on the optimal block replacement interval. For a single period replacement model, the jointly optimal preventive replacement interval is appreciably different from the corresponding optimal preventive replacement interval where only the replacement related costs are considered, For a multiperiod replacement model, the jointly optimal preventive replacement interval decreases from its single period optimal interval as the multiplicity of the period increases. This shows that a trade-off exists between the replacement related costs and the inventory related costs. Therefore, we get a replacement interval which is much lower than the optimal replacement interval in a single period model. There exists a finite optimal interval between two inventory replacements not necessarily equal to the basic preventive replacement interval.