MAINTAINABILITY DESIGN TECHNIQUES METRIC - AcqNotes.com

Downloaded from http://www.everyspec.com on 2011-10-29T14:56:01.DOD-HDBK-791(AM)CHAPTER 7TESTABILITY AND DIAGNOSTIC TECHNIQUESTestability} and diagnostics are defined, and their differences are discussed. The importance of introducingtestability in early design is presented. The contribution of good diagnostic techniques to facilitate maintenanceis emphasized. Built-in test equipment and automatic test equipment are defined, and examples of theiruse are given. Functional tests—such as detection, isolation, and prediction—are anallyzed. Critical designconsiderations—such as personnel, software, test sequence, and stimuli—are detailed. The application ofartificial intelligence and expert systems to improve programming techniques is presented. Examples of thedesign considerations are provided in several commodity areas. Checklists for testability and diagnosticscomplete the chapter.7-1 INTRODUCTIONThe preponderance of the repair time required for anyitem. subsystem, or system normally is a direct function offault isolation. Rapid advances in system complexityhave aggravated the problem by reducing the effectivenessof conventional testing and diagnostic techniques.During the era when discrete components were used inquantity, it was possible to probe troubleshootelcctronicassemblies to isolate failures. Now there arelayers of hermetic seals and programs stored in memorythat obscure the technician’s view into the physical processesof system operation (Ref. 1). Accordingly, it isimperative that provisions be made for the most effectivediagnostic routines possible. The application of newtechniques is not restricted to electronic systems; newtechniques can also be applied to mechanical systems (seepar. 7-3.4).It is necessary that system testability as a discipline beincorporated into the design process for cost-effectivenessand to insure that diagnostic techniques and supportingtest equipment are mature enough to support materieldelivered to the field. Decisions regarding testing alsoaffect the logistical support plan. Factors involved in thedecision include mission and operational characteristicsfor the equipment, anticipated reliability, maintenancestructure, automatic fault isolation capability of built-intest, built-in test equipment (BIT)/(BITE), equipmentand skill level of personnel available for maintenance,operational environment, development time, and cost.Historically, testability has received a lower priority—effort and funding—than classic performance characteristics(Ref. 2). A change in philosophy is emerging, however,for complex and sophisticated weapon systems asevidenced by the award by the US Air Force of a $3.2million contract to the Boeing Military Aircraft Companyto develop a prototype ground-based diagnosticsystem for the B-lB avionic system (Ref. 3).The paragraphs that follow discuss testability, testabilityanalysis, the relationship of testability to availability,diagnostic techniques and aids, functional testing, anddesign considerations.7-2 TESTABILITY7-2.1 GENERALTestability is defined as a design characteristic thatallows the status of a unit or system to be determined in atimely and cost-effective manner (Ref. 4). This definitiondistinguishes testability from diagnosis which describesthe functions performed and the techniques used indetecting and isolating the cause of a malfunction orfailure, e.g., the application of a physical or electricalstimulus to a device to produce a measurable response.Closely associated with testability and diagnostics are theterms BIT, BITE, and automatic test equipment (ATE).These terms are defined in pars. 7-3.5.2.3 and 7-3.5.2.2,respectively.To illustrate the difference between testability anddiagnostics, consider the function indicator lights on thedashboard of an automobile. The temperature indicator,which lights up when the engine overheats, representscontinuous BITE. It will not isolate which component ofthe cooling system is at fault; it is not a diagnostic tool inthe strictest sense. Now consider a pneumatic tire. The tirevalve represents a designed-in test feature. The determinationof the air pressure inside the tire by a pressure gagerepresents a diagnostic technique.Good testability does not “just happen”; on the contrary,it is achieved by establishing a well-planned testabilityprogram that accomplishes the following generalrequirements (Ref. 4):1. Establishment of sufficient, achievable, and affordabletestability, built-in and off-line test requirements2. Integration of testability into equipment and systemsduring the design process in coordination with themaintainability design process3. Evaluation of the extent to which the design meetstestability requirements4. Inclusion of testability in the program reviewprocess.7-2.2 TESTABILITY ANALYSIS (Ref. 5)Testability analysis is defined as the element ‘in theequipment design analysis effort related to developing the7-1