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On repairable component fault tree evaluation
756
World Abstracts on Microelectronics and Reliability
connected with the exponential distribution. Various censoring schemes are considered and the likelihood approach is shown to provide a unified framework for treating exact solutions to estimation, prediction, and population-comparison problems. These solutions are provided in a well defined but non-probabilistic form. The classical s-confidence procedures depend strongly on imaginary exact repetitions of the form of the experiment being analyzed. This is often unrealistic and impractical. The data on hand can be directly analyzed by examining the likelihood function.
Improved R, M, and LCC for switching power supplies. D. MONTEITH and W. SHAw. Proc. IEEE Reliability Symposium, Washington D.C., January 23-25, p. 262 (1979). New computer based shipbome defense systems require more than 100 power supplies per system; typical mission requirements call for total system power supply MTBF to be greater than 1000 hr. A prototype switching power supply has been developed with a predicted MTBF greater than 140,000hr. Both functional and component redunt,ancy were required with appropriate fusing to provide fail-safe failure modes. The type of redundant structure employed in this design was of the constantly connected type, which has an increasing failure rate due to greater stress on operating units as a result of failures in redundant elements. The effect of redundancy and fail-safe design was to increase the predicted MTBF by a factor of 1675/o. Life cycle costs of this design were significantly reduced as a result of increased reliability and improved maintainability features. One particular system requires 96 power supplies producing nine different voltages in 13 different configurations. All require factory repair. The Applied Research Laboratories, The University of Texas at Austin (ARL:UT) prototype design for this system requires only ten types of functional modules to meet all the voltage, power, and configuration requirements of this system while providing the capability for shipboard or depot repair. In addition to the commonality of functional modules within the different supplies, built-in test equipment (BITE) is provided to detect component failures which call for corrective maintenance (module replacement) prior to total functional failure. The net effect of the improved maintainability design is to project reduced repair and spare parts costs over the operational lifetime of the system. Compliance test plans for availability. J. L. RISE. Proc. IEEE Reliability Symposium, Washington D.C., January 23-25, p. 368 (1979). Reliability performance of a system is in specifications, often broken into requirements on the system reliability and on the maintainability. An important reason is that we have accepted methods for verifying requirements on MTBF and MTTR whereas no accepted test plans exist for the steady state availability. By specifying reliability performance in terms of the availability we achieve the advantage that the contractor is free to make a cost optimal trade-off between MTBF and MTTR. In this paper we present principles for statistical test plans of availability assuming the system can be treated as either operating or failed, i.e. up or down, and that up- and down-times are independent, gamma distributed random variables. Two types of fixed size plans are considered, namely plans of fixed sample and fixed time sample. Sequential plans are also treated, namely non-truncated, failure/repair- and time-truncated sequential plans. For the first three test plans analytical results are presented, while the truncated sequential plans are treated using simulation. Simulation also indicates that the methods are robust against reasonable deviations in distribution assumptions for the down-times. Justifying vibration monitors in nuclear plants. J. C. ARCHER. Proc. IEEE Reliability Symposium, Washington,
D.C., January 23-25, p. 85 (1979). This analysis explores from an economic standpoint how permanently installed vibration monitoring/analyzing systems can promote increased balance-of-plant equipment availability and hence higher overall plant availability in a hypothetical nuclear facility. The analysis was conducted using existing techniques to calculate equipment downtime, availabilities, and rates of return, and thereby justify vibration monitors for systems which can demonstrate sufficient availability gains.
System maintainability verification--the paired time comparison (PTC).method. W. g. DOWNS. Proc. IEEE Reliability Symposium, Washington D.C., January 23-25, p. 280 (1979). The Paired Time Comparison (PTC) method of Maintainability Demonstration was developed at McDonnell Douglas Astronautics Company, Huntington Beach. The method is graphical in nature, possessing an inherent proof of significance by pairing observations (Ref. 1). This maintainability demonstration method shows that the anticipated maintenance tasks can be performed within the estimated task times. This method tests the accuracy of task time estimates used to develop a system prediction and verifies satisfaction of requirements of the total data base by comparing estimated and observed maintenance task times on a task-by-task basis. The demonstration of achieving system requirements is thus provided by the updated system prediction for the maintainability parameters of interest. The method evaluates task times for either preventive or corrective maintenance. It is independent of the underlying distribution of the individual maintenance task times and does not seek mean, median, variance, or other statistical values. A control chart, prepared from information available through the task time estimation process, provides the means for determining the "accept" or "reject" status of each task time estimate that is tested. The demonstration is conducted concurrently with equipment performance evaluation testing, making use of equipment malfunctions as they occur, or special tests, as preferred. This demonstration may be continued into early operational activities or until enough information has been collected. On repairable component fault tree evaluation. B. S. DHILLON, C. L. PROCTOR and A. M. KOTHARI. Proc. IEEE Reliability Symposium, Washington D.C., January 23-25, p. 190 (1979). This paper presents a brief background on fault trees and an analytical technique for evaluating the reliability of a system with repairable components. The major advantages and disadvantages of this technique are briefly discussed. The technique illustrates how the top or intermediate level steady state unavailability, limiting mean failure rate, limiting mean repair rate, and steady state failure rate results may be obtained for the redundancy free repairable component fault tree. A numerical example is presented to demonstrate applicability of this technique to an event tree composed of OR and AND-gates. The effect of endless burn-in on reliability growth projections. A. G. BEZAT and L. L. MONTAGUE. Proc. IEEE Reliability Symposium, Washington D.C., January 23-25, p. 392 (1979). Current reliability prediction methods treat solid-state electronics equipment on the basis of a constant failure rate throughout the operating life of the equipment. This paper presents an extensive data base (over 19 x l09 part hours), supported by theory, that shows failure rate of solid-state electronics to be a decreasing function with operational age of individual equipments. Data is presented in graphical form, including a best-fit equation for the data. The mechanics of prediction methods are shown, along with examples. The entire concept is summarized as a family of growth curves, wherein the applic-
World Abstracts on Microelectronics and Reliability
connected with the exponential distribution. Various censoring schemes are considered and the likelihood approach is shown to provide a unified framework for treating exact solutions to estimation, prediction, and population-comparison problems. These solutions are provided in a well defined but non-probabilistic form. The classical s-confidence procedures depend strongly on imaginary exact repetitions of the form of the experiment being analyzed. This is often unrealistic and impractical. The data on hand can be directly analyzed by examining the likelihood function.
Improved R, M, and LCC for switching power supplies. D. MONTEITH and W. SHAw. Proc. IEEE Reliability Symposium, Washington D.C., January 23-25, p. 262 (1979). New computer based shipbome defense systems require more than 100 power supplies per system; typical mission requirements call for total system power supply MTBF to be greater than 1000 hr. A prototype switching power supply has been developed with a predicted MTBF greater than 140,000hr. Both functional and component redunt,ancy were required with appropriate fusing to provide fail-safe failure modes. The type of redundant structure employed in this design was of the constantly connected type, which has an increasing failure rate due to greater stress on operating units as a result of failures in redundant elements. The effect of redundancy and fail-safe design was to increase the predicted MTBF by a factor of 1675/o. Life cycle costs of this design were significantly reduced as a result of increased reliability and improved maintainability features. One particular system requires 96 power supplies producing nine different voltages in 13 different configurations. All require factory repair. The Applied Research Laboratories, The University of Texas at Austin (ARL:UT) prototype design for this system requires only ten types of functional modules to meet all the voltage, power, and configuration requirements of this system while providing the capability for shipboard or depot repair. In addition to the commonality of functional modules within the different supplies, built-in test equipment (BITE) is provided to detect component failures which call for corrective maintenance (module replacement) prior to total functional failure. The net effect of the improved maintainability design is to project reduced repair and spare parts costs over the operational lifetime of the system. Compliance test plans for availability. J. L. RISE. Proc. IEEE Reliability Symposium, Washington D.C., January 23-25, p. 368 (1979). Reliability performance of a system is in specifications, often broken into requirements on the system reliability and on the maintainability. An important reason is that we have accepted methods for verifying requirements on MTBF and MTTR whereas no accepted test plans exist for the steady state availability. By specifying reliability performance in terms of the availability we achieve the advantage that the contractor is free to make a cost optimal trade-off between MTBF and MTTR. In this paper we present principles for statistical test plans of availability assuming the system can be treated as either operating or failed, i.e. up or down, and that up- and down-times are independent, gamma distributed random variables. Two types of fixed size plans are considered, namely plans of fixed sample and fixed time sample. Sequential plans are also treated, namely non-truncated, failure/repair- and time-truncated sequential plans. For the first three test plans analytical results are presented, while the truncated sequential plans are treated using simulation. Simulation also indicates that the methods are robust against reasonable deviations in distribution assumptions for the down-times. Justifying vibration monitors in nuclear plants. J. C. ARCHER. Proc. IEEE Reliability Symposium, Washington,
D.C., January 23-25, p. 85 (1979). This analysis explores from an economic standpoint how permanently installed vibration monitoring/analyzing systems can promote increased balance-of-plant equipment availability and hence higher overall plant availability in a hypothetical nuclear facility. The analysis was conducted using existing techniques to calculate equipment downtime, availabilities, and rates of return, and thereby justify vibration monitors for systems which can demonstrate sufficient availability gains.
System maintainability verification--the paired time comparison (PTC).method. W. g. DOWNS. Proc. IEEE Reliability Symposium, Washington D.C., January 23-25, p. 280 (1979). The Paired Time Comparison (PTC) method of Maintainability Demonstration was developed at McDonnell Douglas Astronautics Company, Huntington Beach. The method is graphical in nature, possessing an inherent proof of significance by pairing observations (Ref. 1). This maintainability demonstration method shows that the anticipated maintenance tasks can be performed within the estimated task times. This method tests the accuracy of task time estimates used to develop a system prediction and verifies satisfaction of requirements of the total data base by comparing estimated and observed maintenance task times on a task-by-task basis. The demonstration of achieving system requirements is thus provided by the updated system prediction for the maintainability parameters of interest. The method evaluates task times for either preventive or corrective maintenance. It is independent of the underlying distribution of the individual maintenance task times and does not seek mean, median, variance, or other statistical values. A control chart, prepared from information available through the task time estimation process, provides the means for determining the "accept" or "reject" status of each task time estimate that is tested. The demonstration is conducted concurrently with equipment performance evaluation testing, making use of equipment malfunctions as they occur, or special tests, as preferred. This demonstration may be continued into early operational activities or until enough information has been collected. On repairable component fault tree evaluation. B. S. DHILLON, C. L. PROCTOR and A. M. KOTHARI. Proc. IEEE Reliability Symposium, Washington D.C., January 23-25, p. 190 (1979). This paper presents a brief background on fault trees and an analytical technique for evaluating the reliability of a system with repairable components. The major advantages and disadvantages of this technique are briefly discussed. The technique illustrates how the top or intermediate level steady state unavailability, limiting mean failure rate, limiting mean repair rate, and steady state failure rate results may be obtained for the redundancy free repairable component fault tree. A numerical example is presented to demonstrate applicability of this technique to an event tree composed of OR and AND-gates. The effect of endless burn-in on reliability growth projections. A. G. BEZAT and L. L. MONTAGUE. Proc. IEEE Reliability Symposium, Washington D.C., January 23-25, p. 392 (1979). Current reliability prediction methods treat solid-state electronics equipment on the basis of a constant failure rate throughout the operating life of the equipment. This paper presents an extensive data base (over 19 x l09 part hours), supported by theory, that shows failure rate of solid-state electronics to be a decreasing function with operational age of individual equipments. Data is presented in graphical form, including a best-fit equation for the data. The mechanics of prediction methods are shown, along with examples. The entire concept is summarized as a family of growth curves, wherein the applic-