MAINTAINABILITY DESIGN TECHNIQUES METRIC - AcqNotes.com

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