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Directed graph techniques for the analysis of fault trees
312
\i’orld Abstracts
on MIcroelectronic\
(MMICS). It describes MMICS in a historlcal perspective: it identifies problems which must be addressed in the modernization process; and. it presents II Imaintenance management modernization concept that has been proposed by the author-s. The vleu$ and opinrons expressed in this paper should not in an) way be construed to reflect an) endorsement or confirmation by the Department of Defense or any other agency of the U.S. Government. Quality Assurance for reliable software. PA~u. P. HOWLEY. JK. Proc. IEEE /l. Rrliirb. Mai/rt~ri,~trh. S~np.. Los Angeles. I7 I9 January 1978. p. 73. Concern is being expressed over software reliability b? the aerospace systems agencies procuring large systems with embedded software. While there is disagreement among various advocates about exactly what constitutes software reliability and how it is defined, there is an acl\nowledged need for the application of methods to achieve reliability in software. A number of techniques and tools do exist that are being practiced lo differing degrees among the various firms debeloping large systems software. However. consistency in concept and methodology is not evident. The methodology proposed in this paper is that of Software Ouality Assurance (SWQA). This methodology provides the conceptual and organirational bases for estabhshing and controlling the implementation of software quality goals. It is applied throughout the software life cycle, from the point of system definition through development. extending out through the operaticma life of the software system.
and Reliabilltc
high levels of system ava~lablllty, in the order 01‘ Y).ox”,,. while minimizing system design and operations costs ‘I.116 paper presents the rcliabilitj consideration\ utilired in the design and implementation of the GTE Satellite (‘~~I-~oI-ation (GSAT) earth stations. The design ohlcctl\c of 90.98” (1 system availability was achieved through 111c LISZ ot”: redundant configurations. proven commercial equipmen1, environmental control. equipment accessihillt). ;~nd reliance on equipment supplier< for reliability input da18
Software
Risk reduction by design. PAIJI. F.. IEEE 4. Krlilrh. Mtrinroi,ltrh. Sytnp..
~‘OLNGL~AHI.. Pror~.
Los Angeles. 17- I9 January 107X. p 360. Risk reduction is a goal in many contexts Including risk reduction of personal injury from machines or products. An engineer, using all the tools available, should make a hazard and risk analysis :IS part of an orderly approach to the design of a reasonably safe machine. Several examples in mechanical engineering illustrate the application of the principles. Directed graph techniques for the analysis of fault M,AKTIN F. CHAMOW. 1EEE 7’1~1s. Kelitrb. R-27,
trees.
(1) 7 (April 1978). Since the introduction of the fault tree method for system aafety and reliability analysis more than a decade ago. the method has gained considerable acceptance for qualitative analyses. It has also gained a degree of acceptance for quantitative analyses, despite difficulties encountered in performing the probabilistic evaluations using available methods. Some of the difficulties encountered with previous evaluation methods are avoided by the methods of this paper. The new methods involve the use of directed graphs (digraphs) and related matrix methods, and solutions for paths in a manner similar to that for conventional digrapha. Most of the attractiveness stems from the fundamental philosophy of speedily transforming the graphics into corresponding matrices. This puts the bulk of the solution elfort into the mathematics where it belongs. The ma,jor benefit arises because the mathematical solutions are readily performed by standard matrix techniques, which can be implemented either manually or with the aid of a computer. The neu methods have been used on various hypothetical logic combinations plus actual fault trees of typical sizes. reliability of GSAT Earth Station. WIL.I IAhl J. and GLENN A. ALLFN. Proc. IEEE 4. Relicrh. Mtrimc~imh. Symp.. Los Angeles, 17-19 January 1978, p. 465. Does :I 99.9X”8I 0 system availability design objective for a satellite communications system require an exhaustive and expensive reliability program’? GTE’s experience has indicated that commercially-proven equipment can yield Operational SANKO
An evaluation of two model specification techniques for a lognormal distribution. DIANE I. GIBBONS. IEEE /-~oII,\. Relitrh. R-27, (I) 60 (April 147X). Two frequently used
statistical estimation techniques are applied to the lognormal distribution with unit shape-parameter 1-11~ fbt lechnique involves comparing sample estimates of \he\$ nc~ and kurtosis with their corresponding population value\ in order to determine the suitability of the di\tribution as a model for a set of empirical data. The sample rhcwness and kurtosis for samples of moderate G/c pro\idc biased downward estimates of the population calueh. Thus bias can be considered if this technique IS applied IO ;I data set by using results presented in this paper. The second technique ib probability plotting. .The recommended plotting position is (i - 0.51’1r. An evaluation i>f the praphical eatimatorh sho\\s that they are suhstantiall~ I inferior to the best linear unhlased estlmatorh and Blom’\ estimators. The result3 are applied to ;I data SCI ct)nhisting of automoti\~e emissions.
Reliability
modeling and analysis of fault-tolerant
memories.
W. Cox and B. D. C.\KKOII.. IEEE Trrorc. Relioh. R-27, (I) 49 (April 1978). A memory array rehability model is developed that can be applied to a uide range of including random-acccsa memory organizationa memories (RAM) and read-only memories (ROM). The model is particularly useful for computing the reliability of fault-tolerant memories that employ technlquca vuch as hardware redundancy, error-correcting codes, and auftuarc error-correcting algorithms. The model accommodates the effect of faults masked by data. Reliability models that incorporate the array model arc given for 21 simplex RAM. an N-modular-redundant R4M. a spared RAM, a single-error-correcting RAM. a multipleerror-correcting RAM. and a ROM. Reliabillt) chnracteristics of these memories are compared. The rei;ult\ suggest that memories with error-correcting capability and >pare hit-planes provide the hest reliability. Memories with sparing at the array level are nest best followed b! NMR and simplex organizations. ROM reliability i& shown to be more optimistic when masked faults are conridered GLENN
Optimal reliability allocation under preventive maintenance schedule. T. INAGARI, K. 14o~r. and H. AKASIII. IEEE Trtrrls. Rditrh. R-27, (I) 39 (April 1078). This paper con-
siders a series system of components with time-dependent reliability and gives a nen formulation of an optlmal rehability allocation problem where an optimal preventive maintenance (PM) schedule is determined simlIIt;~neousl~. The importance of this formulation is shown in COIIIparison with a conventional formulation where PM schedule is not taken into account. The optimization problem becomes :I nonlinear mixed-integer programming problem. A simple approximate solution algorithm 15 given on the basis of a nonlinear programmlng (NLP) algorithm. The procedure is illustrated bq use of :I numerical example. Though we restrict out attentmn to the case where a preventive replacement is adopted HIS it PM policy. a similar discussion is possible for the asgood-as-new repair.
\i’orld Abstracts
on MIcroelectronic\
(MMICS). It describes MMICS in a historlcal perspective: it identifies problems which must be addressed in the modernization process; and. it presents II Imaintenance management modernization concept that has been proposed by the author-s. The vleu$ and opinrons expressed in this paper should not in an) way be construed to reflect an) endorsement or confirmation by the Department of Defense or any other agency of the U.S. Government. Quality Assurance for reliable software. PA~u. P. HOWLEY. JK. Proc. IEEE /l. Rrliirb. Mai/rt~ri,~trh. S~np.. Los Angeles. I7 I9 January 1978. p. 73. Concern is being expressed over software reliability b? the aerospace systems agencies procuring large systems with embedded software. While there is disagreement among various advocates about exactly what constitutes software reliability and how it is defined, there is an acl\nowledged need for the application of methods to achieve reliability in software. A number of techniques and tools do exist that are being practiced lo differing degrees among the various firms debeloping large systems software. However. consistency in concept and methodology is not evident. The methodology proposed in this paper is that of Software Ouality Assurance (SWQA). This methodology provides the conceptual and organirational bases for estabhshing and controlling the implementation of software quality goals. It is applied throughout the software life cycle, from the point of system definition through development. extending out through the operaticma life of the software system.
and Reliabilltc
high levels of system ava~lablllty, in the order 01‘ Y).ox”,,. while minimizing system design and operations costs ‘I.116 paper presents the rcliabilitj consideration\ utilired in the design and implementation of the GTE Satellite (‘~~I-~oI-ation (GSAT) earth stations. The design ohlcctl\c of 90.98” (1 system availability was achieved through 111c LISZ ot”: redundant configurations. proven commercial equipmen1, environmental control. equipment accessihillt). ;~nd reliance on equipment supplier< for reliability input da18
Software
Risk reduction by design. PAIJI. F.. IEEE 4. Krlilrh. Mtrinroi,ltrh. Sytnp..
~‘OLNGL~AHI.. Pror~.
Los Angeles. 17- I9 January 107X. p 360. Risk reduction is a goal in many contexts Including risk reduction of personal injury from machines or products. An engineer, using all the tools available, should make a hazard and risk analysis :IS part of an orderly approach to the design of a reasonably safe machine. Several examples in mechanical engineering illustrate the application of the principles. Directed graph techniques for the analysis of fault M,AKTIN F. CHAMOW. 1EEE 7’1~1s. Kelitrb. R-27,
trees.
(1) 7 (April 1978). Since the introduction of the fault tree method for system aafety and reliability analysis more than a decade ago. the method has gained considerable acceptance for qualitative analyses. It has also gained a degree of acceptance for quantitative analyses, despite difficulties encountered in performing the probabilistic evaluations using available methods. Some of the difficulties encountered with previous evaluation methods are avoided by the methods of this paper. The new methods involve the use of directed graphs (digraphs) and related matrix methods, and solutions for paths in a manner similar to that for conventional digrapha. Most of the attractiveness stems from the fundamental philosophy of speedily transforming the graphics into corresponding matrices. This puts the bulk of the solution elfort into the mathematics where it belongs. The ma,jor benefit arises because the mathematical solutions are readily performed by standard matrix techniques, which can be implemented either manually or with the aid of a computer. The neu methods have been used on various hypothetical logic combinations plus actual fault trees of typical sizes. reliability of GSAT Earth Station. WIL.I IAhl J. and GLENN A. ALLFN. Proc. IEEE 4. Relicrh. Mtrimc~imh. Symp.. Los Angeles, 17-19 January 1978, p. 465. Does :I 99.9X”8I 0 system availability design objective for a satellite communications system require an exhaustive and expensive reliability program’? GTE’s experience has indicated that commercially-proven equipment can yield Operational SANKO
An evaluation of two model specification techniques for a lognormal distribution. DIANE I. GIBBONS. IEEE /-~oII,\. Relitrh. R-27, (I) 60 (April 147X). Two frequently used
statistical estimation techniques are applied to the lognormal distribution with unit shape-parameter 1-11~ fbt lechnique involves comparing sample estimates of \he\$ nc~ and kurtosis with their corresponding population value\ in order to determine the suitability of the di\tribution as a model for a set of empirical data. The sample rhcwness and kurtosis for samples of moderate G/c pro\idc biased downward estimates of the population calueh. Thus bias can be considered if this technique IS applied IO ;I data set by using results presented in this paper. The second technique ib probability plotting. .The recommended plotting position is (i - 0.51’1r. An evaluation i>f the praphical eatimatorh sho\\s that they are suhstantiall~ I inferior to the best linear unhlased estlmatorh and Blom’\ estimators. The result3 are applied to ;I data SCI ct)nhisting of automoti\~e emissions.
Reliability
modeling and analysis of fault-tolerant
memories.
W. Cox and B. D. C.\KKOII.. IEEE Trrorc. Relioh. R-27, (I) 49 (April 1978). A memory array rehability model is developed that can be applied to a uide range of including random-acccsa memory organizationa memories (RAM) and read-only memories (ROM). The model is particularly useful for computing the reliability of fault-tolerant memories that employ technlquca vuch as hardware redundancy, error-correcting codes, and auftuarc error-correcting algorithms. The model accommodates the effect of faults masked by data. Reliability models that incorporate the array model arc given for 21 simplex RAM. an N-modular-redundant R4M. a spared RAM, a single-error-correcting RAM. a multipleerror-correcting RAM. and a ROM. Reliabillt) chnracteristics of these memories are compared. The rei;ult\ suggest that memories with error-correcting capability and >pare hit-planes provide the hest reliability. Memories with sparing at the array level are nest best followed b! NMR and simplex organizations. ROM reliability i& shown to be more optimistic when masked faults are conridered GLENN
Optimal reliability allocation under preventive maintenance schedule. T. INAGARI, K. 14o~r. and H. AKASIII. IEEE Trtrrls. Rditrh. R-27, (I) 39 (April 1078). This paper con-
siders a series system of components with time-dependent reliability and gives a nen formulation of an optlmal rehability allocation problem where an optimal preventive maintenance (PM) schedule is determined simlIIt;~neousl~. The importance of this formulation is shown in COIIIparison with a conventional formulation where PM schedule is not taken into account. The optimization problem becomes :I nonlinear mixed-integer programming problem. A simple approximate solution algorithm 15 given on the basis of a nonlinear programmlng (NLP) algorithm. The procedure is illustrated bq use of :I numerical example. Though we restrict out attentmn to the case where a preventive replacement is adopted HIS it PM policy. a similar discussion is possible for the asgood-as-new repair.