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Downloaded from http://www.everyspec.com on 2011-10-29T14:56:01.DOD-HDBK-791(AM)The system designer responsible for the automatic testfunction must consider the use environment when establishingthe limits and thresholds for the parameters to bemeasured; this will insure that only true malfunctions areisolated. For example, if a stabilized inertial tank platformis being checked in a laboratory, the parametersbeing monitored would be stable and noise free. In thetrue tank environment, however, the same stabilized platformwould experience tank motion and gyroscopic torquing,which would result in less stable parameters in anoisier background.As a general guideline, the test designer should providefor all data manipulation within the test equipment tominimize the number of arithmetic operations required oftest personnel. For example, if power is the measuredparameter, the technician should not be required to readthe voltage and current—and then multiply the twoparameters—to determine power.7-5.7 SELECTION OF TRANSDUCER ORSENSORWhen the parameters to be tested can be readily measuredwithout conversion, e.g., voltage, they do notrequire parameter modification by a transducer. A parameter,such as electric current, can be transduced by meansof a transformer and a known resistance to a desiredvoltage. However, temperature, strain, vibration, pressure,and power parameters require more sophisticatedtransducers. The selection of transducers is a task requiringknowledge of measurement requirements and transducerresponse characteristics. The five characteristics oftransducers that are of particular interest to the maintainabilityengineer are1. Slability. The ability to produce a constant outputfor constant input within a given time constant2. Repeatability. The ability to duplicate the outputafter a change in input and then return to the previousvalue3. Procurability. The ability to be procured involume on the open market4. Calibration. Should not be required except as asimple, scheduled, preventive maintenance task5. Output. Should be amenable to multiplexingwhen the feature will simplify data transmittal; transducerimpedance should be low enough to prevent noisecoupling.The electronics associated with many transducers consistsof a bridge circuit in which the transducer is one legof the bridge. The output voltage is then the voltagerequired to balance the bridge. A useful secondary outputfrom such a circuit is the null voltage, i.e., the minimumoutput of a circuit as a function of an adjusting device.Although it is not good design practice to sense a “noreading” value, this bridge null voltage can validate theoutput as well as provide a point at which to begin faultisolation within the automatic test system. The transducersrequired to implement the logic of Fig. 7-2, for example,may vary from simple temperature transducers tocomplex subsystems. The measurement of phase noisecould require a phase-to-amplitude modulation (P/AM)conversion and then a determination of the AM level,which results in a multistage transducer.A synchronized detection, sample, and hold device isrequired to monitor the radio frequency (RF) levels in theexample. A common circuit for all these functions, withappropriate attenuation to account for all stages of amplification,would reduce the number of different transducersand simplify calibration.7-5.8 STIMULI SELECTIONStimuli are the inputs or signals furnished by the testsystem to the UUT to allow simulation of its operation.The selection of the test set stimuli must be made simultaneouslywith transducer selection to insure that the typeand magnitude of the stimulus are compatible with theprime system. For example, if the stimuli introduced is anelectric current to check circuit continuity, the magnitudeof the current must be limited to avoid the unwantedoperation or activation of a circuit component.The application of the stimuli also must be carefullyconsidered relative to the functions exercised in the UUT.For example, in the track radar it is desirable to knowwhether the radar is operating within specified performancelimits. This information can be provided by introducingan RF pulse into the antenna immediately afterthe radar pulse at a time when the normal radar return isnot expected. This test pulse would then be processed andrecovered by a special gate in the video circuit and testedfor amplitude and delay. Although this example procedurewould test a major portion of the radar, it would nottest the status or dynamic characteristic of the rangegatingcircuitry or transmitter synchronization, both ofwhich are vital to the satisfactory operation of the radar.A second test step should be introduced into the logic toassess these functions. In the track radar transmitterexample, normal signal generation and pulse sources areused to relieve the need for auxiliary stimulus sources.This enhances the simplicity of the automatic testcircuitry.7-5.9 OUTPUT FORMATThe output of the test equipment may take variousforms—from the simple to the complex—e.g., “go”/“nogo” light, meter reading or indication, analog display, orcomputer-generated printout. Regardless of the form, thereadout must be both readable and understandable to theuser. Consider the elementary case of a single meter usedto display the readout from various inputs; the readershould not be required to introduce a scale factor tointerpret correctly the meter readout from differentsources. This practice can result in confusion or errors.The display or readout format may be dictated by thecriticality of the data and the sampling rate. To assurecrew safety and/ or mission success, it may be necessary tosample data at the microsecond or nanosecond rate; inthis case an analog display of the parameters indicatingsystem status would be appropriate. The contribution tothe system failure probability of latent faults. near coincidentfaults, fault coverage, and fault recovery times allwill determine sample rate and output form.7-16
Downloaded from http://www.everyspec.com on 2011-10-29T14:56:01.DOD-HDBK-791(AM)If a printout or permanent record is generated by thetest equipment, the record should be easily readable witha minimum of interpretation. Structure the software sothat the parameters or components are identified byname, rather than a numerical code. The output alsoshould be easy to repproduce i.e., avoid nonreproducibleink colors, odd-sized output forms, and punched cards ortape. Where the output data from one test systembecomes the input data to a second test system, insure thatthe software programs and language are compatible. Adata translation process is costly and subject to error.Moreover, data translation becomes impractical if thedata must be translated in real time.As indicated in Fig. 7-3, the output consists of severaldisplays. A printout of the unit failures, with time anddate markings, may be desirable if the size of the systemwere large enough to justify the added complexity. Theprintouts would provide audit trails of system and componentreliability and would serve as a basis for inventorycontrol and to support warranty claims. Power outputand noise level in this example would be recorded or readfrom a meter at regular intervals.7-5.10 DIAGRAMSPrevious paragraphs have provided guidance to thedesigner of test equipment to assist him in establishingdiagnostic features that achieve the maintainability objectivesthat are compatible with the system being tested. Thefact that an automatic test system might be fashionableshould not be the deciding factor in pursuing thisapproach the complexity of the prime system, reliabilityof the system components and test equipment, and costmust enter into the decision. To facilitate the technician’stask regardless of whether the test system selected ismanual, BIT, and/or ATE—maximum use of block diagrams,flow diagrams, schematics, logic diagrams, andfront panel layouts should be developed to assist with thenecessary measuring, switching, and control circuits.7-6 EXAMPLESThree examples are presented1. likelihood Analysis Computer Program2. Advanced Attack Helicopter3. T700 Gas Turbine Enginewhich illustrate the applications of the design considerationspresented in par. 7-5.7-6.1 LIKELIHOOD ANALYSIS COMPUTERPROGRAMThe application of the state-of-the-art diagnostics isnot limited exclusively to electronic components andassociated circuitry; techniques have been developed formechanical systems (see par. 7-3.4). one of the importantdiagnostic techniques resulting from Ref. 20 was the LikelihoodAnalysis Computer Program which detects differencesin vibration signatures between rotating parts thatare operating satisfactorily and those that have a faultthat signals impending failure. Under the AIDAPS Program,vibration signature data for gearboxes, transmis-sions, and similar rotating items were developed. The testdata were gathered for helicopters, but the principles,diagnostic techniques, and software can be adapted toArmy vehicles in general. To assist in applying the techniques,computer programs for diagnostic evaluationwere developed. The program uses Fast Fourier Transform(FFT) a mathematical tool for analyzing a periodicfunction—to reduce the analog data to digital form.The raw data are also categorized in power spectraldensitybands. A program for likelihood analysis computesthe statistical means and variances for referencedata signatures and for test data signatures; it then computesprobabilities that the signature populations in variousfrequency bands are identical within a prescribednumber of standard deviations. By this procedure themechanical replaceable unit is classified as “satisfactoryoperation” or “impending failure”. Signatures for theimpending failure classes for specific components aredeveloped by sensing and processing specific failurerelateddata. Fig. 7-4 illustrates the diagnostic logic usedfor the analysis of vibration signatures of rotating components.7-6.2 ADVANCED ATTACK HELICOPTERSThe Advanced Attack Helicopter (A AH) was designedto surpass any existing Army aircraft in weaponry sophistication(Ref. 21). This aircraft provides an excellentexample of how BITE was applied to solve critical diagnosticproblems.The Fault Detection Location Subsystem (FD/LS)for the AAH employs a combination of BITE ATE,ground test equipment, technical manuals, and manualdiagnostic procedures to accomplish its function. Table7-3 provides a list of the systems that can be tested on theA AH by BITE. This list is part of the operator’s checklist.The on-aircraft fault isolation is provided for replaceableunits in the following subsystems:1.2.3.4.5.6.7.8.9.10.11.12.13.14.15.16.17.18.In theFD/LS itselfArmamentFlight controlAuxiliary powerPilot’s night visionIntegrated helmet sight and displayFuelEnvironmental controlAntiice and deiceFire controlInstrumentsNavigationAvionicsMultiplexElectric powerFire detection and extinguishingIntermediate gearboxTail rotor gearbox.area of electronics, the FD/LS detects and isolatesfaults to the repairable module level. The replaceableunits have sufficient test provisions to detect and isolate98% or more of all failures to the repairable module. Fig.7-5 shows the data entry keyboard used by the crew to7-17
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