MAINTAINABILITY DESIGN TECHNIQUES METRIC - AcqNotes.com

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Downloaded from http://www.everyspec.com on 2011-10-29T14:56:01.DOD-HDBK-791(AM)3. Built-in test equipment.Each of these categories is discussed in the paragraphsthat follow.7-3.5.2.1 Logic FlowLogic flow–-a troubleshooting road map is an outlineof the sequence to be followed in the event of a systemfailure; it is essentially a manual procedure. As a maintenanceaid for fault isolation, it provides a step-by-stepdescription of the questions to be asked, tests to be conducted,and continued sequential localization paths to befollowed depending on test results. The “roads” lead fromgeneralized localization to specific fault localization.Specific data required by the technician to enable him tolocalize a fault will allow him to accept or reject eachcomponent tested as the source of the trouble. For example,an overheated engine can result from coolant leaks.low coolant pump pressure, and constricted passages.Measurement of fluid flow and temperature at strategicpoints in the cooling system can aid the technician ineliminating the source of the fault—radiator. hoses, andengine—and zero in on a pump leak or failure. A checkoutof the system after repair usually requires a repeat ofall the checks used for localization.In electronic systems, three data flow patterns aregenerally of diagnostic interest, i.e.,I. Simple series pattern2. Complex—series-parallel—pattern3. Loop pattern.In the simple series pattern the data flow goes throughone element at a time, and the element that standsbetween a good signal and a bad one is the fault source. Inthe complex pattern, places exist where the data flow fansout along parallel paths. In this case the fault may belocalized to a parallel set, and further tests must be conductedto locate the faulty element of the set. In the looppattern, part of the flow returns to an element as feedback.In this case a fault in one loop element can make itmore difficult to distinguish between the element causingthe fault and elements simply reacting to the faulty signal.Logic flow diagrams are very important in designingcomputer software used with ATE. The programmermust match the logic flow of his software with the logicflow of the hardware being tested and with various hardwarefault modes and combinations of some of the morecommon modes. The programmer must take precautionsto code the software for all conceivable circumstances.For example, under some fault circumstances one of thetests may prematurely transfer control to the end of thesoftware loop. Adequate design of test software requiresthat the programmer strive for reliability and that hethoroughly test his program before releasing it for use.The sequence of repair in a flowchart should recommendcorrective actions according to1. Probability of failure2. Accessibility and/or simplicity of repairbut not necessarily in the order given—i.e., even if it isunlikely that Part A failed, if Part A is readily accessible,it might be advantageous to check Part A first.A logic flow for a tracking radar is discussed in par.7-5.5 and is illustrated in Fig. 7-3.7-3.5.2.2 Automatic Test EquipmentAutomatic test equipment (ATE) is defined as equipmentthat is designed to conduct automatically the analysisof functional or static parameters and to evaluate thedegree of the performance degradation of the unit undertest (UUT), and it may be used to perform fault isolationof the UUT malfunction (Ref. 14). The decision making,control. or evaluative functions are conducted with minimumreliance on human intervention and usually aredone under computer control. ATE is used to detect andisolate a fault automatically and to check out the systemfollowing a maintenance action. ATF is separate from theunit under test and is used primarily at the intermediateand depot maintenance levels the UUT is brought to theATE or the ATE is brought to the UUT. If the use of ATFis contemplated, a trade-off analysis must be performedearly in the system acquisition process to determine thevalue of’ the automatic test features in relationship tooperational effectiveness. logistics, maintenance eflectiveness,and life cycle costs. The interdependent parametersof quantity, location, and reliability of ATE can bedetermined by using a generalized cost-effectivenessmodel. ATE is costly and. if not properly integrated intothe overall system, may introduce more reliability andmaintenance problems than it solves. However, the costsassociated with the design and procurement of ATF arenot the complete story. If the ATE results cannot beconfidenty relied upon, false alarms can result in thedisposal of serviceable equipment and in reduced equipmentavailability resulting from unnecessary maintenance.ATE—when it is used properly and when it can berelied upon to yield an accurate diagnosis will reducemaintenance time and the skill level and number of maintenancepersonnel and will increase the availability of thesystem.Selection of test features to be incorporated into anATE system should be based on the following considerations:1. Test Function for Purpose. Is ATE to enhancemaintainability, optimize performance of the system.monitor operational readiness. improve system availability,or is it a combination of these purposes?2. Testing Modes.. Will ATE be used on-line with thesystem operating, on-line with the system under test, oroff-line?3. Level for Defection. At which level will ATE detecta fault or malfunction equipment, subsystem, ormodule?4. Degree of Fault Isolation. At which level willATE isolate a fault—equipment, subsystem, groups ofreplaceable modules, modules, line-replaceable units, orindividual circuits?System level design requirements for the automatic testfeatures should be specified quantitatively and definedquantitatively in the system specification in terms of fail-7-8
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
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