1、Best Practices Entry: Best Practice Info:a71 Committee Approval Date: 2000-04-11a71 Center Point of Contact: JPLa71 Submitted by: Wilson HarkinsSubject: Voltage it describes in detail the execution of the tests, including exercising the functional characteristics of the design and assessing the asso
2、ciated circuit parameters against the established pass/fail criteria. Potential failure modes must be identified prior to VTMT to ensure that no damage occurs to the unit under test.Typically the operational extremes are extended to demonstrate positive flight margins using temperature as the univer
3、sal test parameter to simulate other parameters such as environmental and end-of-life changes. Thus the item under test is exposed to risk of damage by stress due to high temperature. Hence, the support equipment used to control the temperature and the test parameters must be extremely accurate, esp
4、ecially at maximum temperatures.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Technical Rationale:VTMT has long been an important tool for verification of circuit operational limits that are dependent upon part parameter variations. The following t
5、echniques form the VTMT repertoire: (1) temperature variation, (2) applied voltage variation, and (3) clock frequency variations for digital circuits. The use of VTMT to simulate worst case functional performance is justified because the effects of voltage, temperature, and frequency upon device per
6、formance parameters is similar to the effects of radiation and end-of-life changes. This concept is very well demonstrated quantitatively at the part level, but less quantitatively at the assembly level. The rationale for use of these three test procedures is discussed below.1. Temperature Variation
7、s:As temperature changes, so does the absolute values of the parameters of the individual parts. Temperature is the first order term in almost every variation of part parameters except for initial tolerance variation. Similarly for cables and transmission lines, distributed parameters exist that als
8、o vary with temperature.2. Applied Voltage Variations:Changing the supply voltage to the circuit under test is equivalent to changing the voltage potentials across groups of parts. Thus the potential across each part within a circuit loop changes accordingly whenever the total applied voltage is cha
9、nged.Varying the applied voltage can check the ability of an analog circuit to operate within specifications and generally can be added linearly to the temperature induced performance changes.3. Variation of Clock Frequency for Digital Circuits:Varying the frequency of an input clock or pulse train
10、can simulate changes of digital circuit delay parameters which may occur during flight. The limits of design degradation (and limits of absolute failure) with frequency can be determined. This knowledge can be used to determine if sufficient timing margins exist. Often voltage is reduced during the
11、frequency margin testing to achieve even more margin. Clock frequency changes of 25% are typical in performing the VTMT.In forming combinations of two or more of these test tools, considerable attention is given to simulating the effects of operating life, radiation, and initial tolerance. Preventiv
12、e measures are ascertained from relevant experience, related reliability analyses, or test and manufacturing data that have been obtained on similar units and interfaces.VTMT duration should be sufficient for the devices to reach thermal equilibrium and exhibit steady-Provided by IHSNot for ResaleNo
13、 reproduction or networking permitted without license from IHS-,-,-state operating conditions. The test requires approximately 3 hours at each temperature level over a 24-hour duration. Measurements before and after the tests are recorded and compared with a predetermined deviation, e.g., less than
14、15 percent.The test planner consults the failure prevention plan for the flight project to ensure the safety of the hardware. A test matrix may be used to form a safety prevention checklist of critical conditions, and the matrix may provide a catalogue identifying diagnostics and possible failure ch
15、ain contributors. The VTMT technique is a closed-loop process; it comprises a checklist that is planned, monitored, and evaluated concurrently by quality assurance, reliability, and engineering personnel.Since the voltage, temperature, and frequency variations applied in VTMT can exceed those expect
16、ed during flight, the additional circuit design margins successfully demonstrated by the test are assumed to encompass the parameter margins expected from radiation and operating life. Calculations can be performed to estimate the final margin. Note that although initial tolerances of parameters dis
17、appear when the to-be-flown hardware is the item under test, the initial tolerance variation must be considered in VTMT application to all other units of the same design. Figure 1 is a flow diagram of the VTMT process.Provided by IHSNot for ResaleNo reproduction or networking permitted without licen
18、se from IHS-,-,-refer to D descriptionD ReferencesProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-1. Reliability Analyses Handbook (1990), Jet Propulsion Laboratory, D5703.2. Reliability Assurance Guidelines for Low Cost/Short Duration Missions (1995
19、), Jet Propulsion Laboratory, NASA Technical Memorandum 4629.Impact of Non-Practice: Using neither WCA nor VTMT to simulate circuit functional performance, a unit is subject to functional peril from changes in uncontrolled circuit parameters due to part variations. Inadequate margins can result in s
20、udden failure to operate, or the condition can lead to functional degradation of the circuit with a high probability of catastrophic failure due to drift outside the operational limits of the unit. Lacking insight from test or analysis, the project is likely to lack provisions for detection of these
21、 problematic areas in the failure intervention plan. Without the VTMT or WCA parameter change matrix as a readily available diagnostic tool, failure causes and remedies must be determined under post-failure constraints, where time and hardware limitations may inhibit adequate corrective action.Related Practices: N/AAdditional Info: Approval Info: a71 Approval Date: 2000-04-11a71 Approval Name: Eric Raynora71 Approval Organization: QSa71 Approval Phone Number: 202-358-4738Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-
