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)6. Cost. Cost is always a factor in any trade-off studyof BITE versus ATE. It is generally more cost-effective toprovide specialized diagnostics for BITE and to providegeneral-purpose diagnostics for ATE.7-5.4 DIAGNOSTIC SOFTWAREDiagnostic software designers should consider1. Structured Programming. Structured programming—aset of programming principles that producessoftware modules that are easily understood and maintained—shouldbe used. In applying the programming,unconditional reverse transfers are to be avoided to eliminatelooping transfers.2. Language Selection. Higher level languages,approved by the Army, may be necessary to developcomputer programs that simulate human behavior or toemploy artificial intelligence or expert systems (see par.7-3.5.3) in problem solving. The higher level languagesalso are more readily understandable, but the lower levellanguages (assembler) are often more efficient. ATLAS(Abbreviated Test Language for All Systems) is an exampleof a high-level directed standard test language whosemajor features are (Ref. 14)a. Standard testing terminology to reduce erroneousinterpretationsb. UUT-oriented test statements to increase portability,i.e., ability to be used with different test equipmentconfigurations.3. Debugging. The debugging of diagnostic software, can be a lengthy and tedious process because of thenumerous ways in which fault sources can occur andbecause some sources appear infrequently but haveserious consequences when they do occur. Thus it isimportant that the diagnostic system mature with thedevelopment of the major system, i.e., being employedthroughout the phases of system development and notwhen the prime system is released to troops.4. Automatic Inspection, Diagnostic, and PrognosicSystem (AIDAPS) Software. AIDAPS software resultingfrom various contracts have resulted in developingdiagnostic computer programs that can be transferredfrom one application to another (Ref. 18).7-5.5 TEST POINT IDENTIFICATIONThe identification of test points involves the followingsteps in the order indicated:1. List system parameters to be measured.2. List the output signals that, if present and withintolerance, would indicate that the system is in normaloperating condition.3. Identify internal functions that could cause thesystem output signal to fail and, therefore. should bemonitored.4. Decide on required test points and their locutions.Each of the selected test points with its output signalshould be evaluated for its compatibility with the diagnostictechnique—manual, BIT, or ATE—to determinethose signals adaptable to direct measurement and thoserequiring signal conditioning via a transducer.Fig. 7-1 (Ref. 19) depicts a simplified example of a testFigure 7-1. Liquid Coolant Subsystem Showing Transducers and Parameters of Interest (Ref.19)7-12
Downloaded from http://www.everyspec.com on 2011-10-29T14:56:01.DOD-HDBK-791(AM)point selection process for a liquid coolant subsystem.Apply the guide for test point selection previously stated:1. Step 1. Output parameters to be measured aretemperature and flow rate.2. Step 2. Output reading signals area. Specific temperatures plus tolerance, i.e., T i°C-±t i, degreesb. Specific flow rate Fi, i.e., meters per second3. Step 3. Internal functions that could cause theouput signal to fail area. Coolant loss or reduced flow rate between(1) Cabinet and radiator(2) Radiator and cabinet(3) Cabinet and power amplifierb. Coolant temperature rise due to defectiveradiatorc. Temperature rise due to defective liquid coolantcabinet.4. Step 4. To monitor the parameter and fault conditions,test points must be locateda. Between cabinet and radiator, both inflow andoutflow, for temperature and flow rateb. Between cabinet and power amplifier fortemperature and flow rate.To satisfy these locations and conditions, three temperaturetransducers and two flow rate transducers werechosen for the selected test locations. The transduceroutputs were examined and found to be compatible withthe test equipment. These points will produce stable voltageoutputs with the desired time constants to provideaccurate real-time readouts of the parameters that governthe system.7-5.6 TEST SEQUENCE CONSIDERATIONSBefore fixing the diagnostic plan and its associated testequipment, it is necessary to determine a logical testsequence to be followed in the event of system failure. Theanalysis should reveal1. Sequence or order of test2. System or function to be tested3. Prior tests required for measurement4. System or function required for test5. Parameters to be measured6. Processing required for interpretation.Approaches for implementing the diagnostic analysisare1. First. test those components or units—powersupplies, power amplifiers, and high-voltage modulators—known to exhibit the highest failure rate. Table 7-2 illustratesthis procedure.2. Second, test the system function by function in asequence corresponding to the normal flow of signalsthrough the system. This approach, though frequentlytime-consuming, is more logical and consequently moreeasily learned by maintenance personnel.If testing is to be fully automatic, the difference in diagnostictime between the two approaches is negligible.The complexity of the automatic system usually can bereduced greatly if self-isolating units, i.e., those units of asystem with a BIT capability, are scanned before initiatinga fault isolation procedure. The diagnostics associatedwith the prime system can be further simplified by groupingcomponents of like functions into a self-isolatingmodule. If the module is not of the throwaway type, theBIT equipment output must be compatible with the ATEat the intermediate or depot level for further diagnosticsand repair. The self-contained concept is illustrated in thetest sequence shown in Fig. 7-2 (Ref. 19) for the liquidcoolant subsystem depicted in Fig. 7-1.The example test sequence of Fig. 7-2 begins with adetermination of whether the coolant temperature T 1isbelow the established maximum T max, and whether thecoolant flow F 1exceeds the specified minimum value F min.When both answers are affirmative, the test equipmentwill indicate that the system is functioning properly.However, if T l> T maX, a test of the heat exchange radiatorinput temperature T 3and output temperature T 2must bemade. If T 2and T 3are within the specified range, the faultmust be in the coolant cabinet. If T 2and T 3are not withinspecified range and the flow rate F 2into the radiator isadequate, then the heat exchange radiator is suspect. Ifthe flow rate F 1into the power amplifier is not adequate,TABLE 7-2. TYPICAL TEST SEQUENCE (Ref. 19)7-13
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