MAINTAINABILITY DESIGN TECHNIQUES METRIC - AcqNotes.com

Downloaded from http://www.everyspec.com on 2011-10-29T14:56:01.DOD-HDBK-791(AM)ure detectability; false alarm rate; degree of fault isolationdesired; fail-safe provisions; and the reliability and maintainabilityof the sensors, interface hardware, and theATE itself. Some general-purpose ATE now exist, e.g.,AN/USM-410 (EQUATE) and GENRAD. Accordingly,new systems, where apropos, should be designed to becompatible with these ATE systems. Artificial intelligence,described in par. 7-3.5.3, may be used beneficiallyin the design of ATE.7-3.5.2.3 Built-In Test EquipmentBuilt-in test (BIT) is defined as an integral capability ofthe mission equipment that provides an on-board, automatedtest capability to detect, diagnose, or isolate systemfailures (Ref. 14). The fault detection and, possibly, isolationcapability are used for periodic or continuous monitoringof the operating condition of a system and forobservation and, possibly, diagnosis as a prelude to maintenance.BIT may be of several types (Ref. 14), i.e.,1. Active BIT. A type that is temporarily disruptiveto the prime system operation through the introduction oftest stimuli into the system.2. Continuous BIT. A type that continually monitorssystem operation for errors. Examples include parity—maintenance of a sameness of level or count, i.e., keepingthe same number of binary ones in a computer word andthus be able to perform a check based on an even or oddnumber for all words under examination—and othererror-detecting codes.3. Initiated BIT. A type that is executed only afterthe occurrence of an external event such as an action byan operator.4. Passive BIT. A type that is nondisruptive andnoninterfering to the prime system.5. Periodic BIT. A type that is initiated at somefrequency. An example is BIT software executing duringplanned processor idle time.6. Turn-On BIT. A specific type of initiated BIT thatis exercised each time power is applied to the unit orsystem.These definitions indicate that BIT equipment may beautomatically or manually triggered.A BIT capability, as a means of attaining the requiredlevel of failure detection capability, must be relied upon asan automatic diagnostic tool because of the ever-increasingcomplexity of modern weapon systems. The need for BITis driven by operational availability requirements that donot permit the lengthy mean-time-to-repair associatedwith detecting and isolating failure modes in microcircuittechnology equipment. Since BIT equipment operateswithin the prime system and at the same functional speed,it has the capability to detect and isolate failures thatconventional test equipment and techniques could notprovide. A well-designed BIT system can reduce substantiallythe need for highly trained field and intermediatelevel maintenance personnel by permitting less skilledpersonnel to locate failures and send them to centralizedintermediate and depot repair facilities equipped to diagnoseand repair defective hardware.A BIT capability is not a comprehensive solution to allsystem maintenance problems, but rather a necessary toolto deal with the complex it y of modern electronic systems.Despite the advantages of BIT to provide a fast troubleshootingcapability, it does have disadvantages. Theprimary disadvantage is that the BIT test elements mustbe made an integral part of the prime unit. This additionincreases size, weight, complexity, and cost and imposesan extra maintenance burden when the BITE fails. If aBIT capability is to be employed, the decision must bemade in the early stage of development and the BITEmust mature together with the prime equipment. Some ofthe basic BIT decisions to be made early in system designare whether testing should be manual or automatic andwhether testing should occur without interrupting thenormal operation of the system or should be programmedat intervals when the prime system can be released fromits operational commitment to conduct the test. Otherconsiderations relative to the selection of features to beincorporated into BIT equipment are similar to thosedescribed for ATE in par. 7-3.5.2.2. Artificial intelligence,described in par. 7-3.5.3, may be used beneficially in thedesign of BITE.Par. 7-6.2 provides an example of how BITE has beenapplied to an advanced attack helicopter to solve a criticaldiagnostic problem.7-3.5.3 Artificial IntelligenceArtificial intelligence (AI) is a field aimed at pursuingthe possibility that a computer can be made to behave in amanner that humans recognize as intelligent behavior ineach other (Ref. 15). In a more restrictive sense AI couldbe considered a study of techniques for more effective useof digital computers through improved programmingtechniques. A further extension of the AI concept is thedevelopment of an expert system, i.e., an intelligent computersystem that uses knowledge and inference proceduresto solve problems that are difficult enough torequire significant human expertise in their solution (Ref.15). The consensus of experts—resulting from case studiesof BIT and ATE associated with existing weaponsystems—is, that despite the abundance of automateddiagnostic aids and trainers, there must be radical changesin maintainability technology in order to achieve significantimprovement in maintenance procedures (Ref. 10).AI technology lends itself to the reduction of humanworkload in complex weapon systems and to the enhancementof training and maintenance and thus enables lowerskill level technicians to maintain complex defense systemsmore efficiently. For example, AI technology couldbe applied to the design of very large-scale integratedcircuits (VLSIC) and very high-speed integrated circuits(VHSIC) to design in testability and fault tolerance (Ref.10). AI and expert system applications are not confinedexclusively to electronic systems. An example of theapplication of an expert system to a mechanical system isthat developed by the General Electric Company to assistrailroad maintenance personnel in the repair of GE’sdiesel-electric locomotives. The program is referred to asDELTA (Diesel-Electric Locomotive Troubleshooting7-9