MAINTAINABILITY DESIGN TECHNIQUES METRIC - AcqNotes.com

Downloaded from http://www.everyspec.com on 2011-10-29T14:56:01.DOD-HDBK-791(AM)Figure 7-2. Test Sequence for Liquid Coolant Subsystem (Ref. 19)the flow in the closed loop system is being blocked. Sincethere are no pressure differentials built into the system,manual procedures must be used for fault isolation.A possible sequence for the maintenance testing of atrack radar transmitter is shown in Fig. 7-3 (Ref. 19). Thelogic shown in Fig. 7-3 is illustrative only and is notintended to be complete. Detailed design of the sensorsand equipment to measure pulse shape, frequency spectrum,and output products is a complex task. The sixparameters—Power Supplies 1 and 2, oscillator outputand frequency, amplifier phase, and noise output aretested in rapid succession. If a failure is detected. theself-isolating units are scanned first for probability failureindication. If none is detected, the particular failure logicis followed to isolate the failed unit. This method, i.e.,progressing from failure indication to the survey of self-isolating units and then to a specific failure logic, usuallypermits the design of less complex test equipment. Thetest sequence should be initiated by an internal clock tosynchronize the steps and insure that required outputs areavailable when required for further sequence decisions.The other circuitry would be conventional logic circuitry,threshold detectors, sample and hold circuits, or simpleanalog circuits.If a centralized automatic test system is used, the testsequence is usually computer controlled and thus requiresthat the programs be developed in close coordinationwith engineers responsible for design of the authomatic testsystem. Similarly, changes to the test sequence necessitatedby hardware changes are implemented by modificationto the test program structured programming simplifiesthe required changes.7-14