1、Lessons Learned Entry: 2036Lesson Info:a71 Lesson Number: 2036a71 Lesson Date: 2009-01-13a71 Submitting Organization: JPLa71 Submitted by: David Oberhettingera71 POC Name: John C. Pearson; Leslie Calluma71 POC Email: John.C.Pearsonjpl.nasa.gov; Leslie.N.Callumjpl.nasa.gova71 POC Phone: 818-354-6822
2、(Pearson); 818-354-3039 (Callum)Subject: Provide a Design Capability Allowing Key Mechanical Adjustments After Subsystem Integration Abstract: The Tunable Laser Spectrometer (TLS) are sensitive to optical standing wave contamination that can be ameliorated by making small adjustments in instrument a
3、lignment after system integration. However, the Mars Science Laboratory (MSL) TLS optical components had been staked prior to Integration and Test (I&T) and could not safely be realigned. For all precision assemblies subject to correction by mechanical adjustments, provide a design capability that a
4、llows adjustments for key mechanical parameters both before and after integration. Do not permanently fasten assembly components until it is demonstrated that functionality and performance requirements can be met with adequate margin during I&T.Description of Driving Event: The Tunable Laser Spectro
5、meter (TLS) is one of three instruments that make up the Sample Analysis at Mars (SAM) instrument suite within the Mars Science Laboratory (MSL) spacecraft payload. In developing and operating the MSL TLS, the NASA/Caltech Jet Propulsion Laboratory (JPL) seeks to understand Martian atmospheric and g
6、eophysical processes by measuring methane, water, and carbon dioxide abundances in the Martian atmosphere and soil with unprecedented accuracy. For any TLS, the backscattering of the instruments primary laser beam from reflective internal component surfaces produces secondary laser beams, and pairs
7、of these scattering surfaces can set up “optical standing waves.“ When combined with the primary laser beam at the detector, this Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-interference can reduce the sensitivity of these instruments to a level
8、below performance requirements. Because its compact design places various internal surfaces in close proximity, the JPL TLS design is especially vulnerable to standing wave contamination or “fringes.“ Using the TLS Development Model (DM) as a testbed (Figure 1), JPL employed a systematic approach to
9、 assessing the potential impact of each optical surface on standing wave amplitudes. Realignment of optical elements within the DM was successful in attenuating optical standing waves to acceptable levels. Figure 1 is a color photograph of a number of components laid out linearly on a perforated tab
10、le. The component at the top of the photo is an assembly that appears as a clear plastic disk, labeledFigure 1. The JPL TLS DM unit provided an ability to adjust and realign optical elements to optimize instrument performance.When the flight unit of the MSL TLS opto-mechanical system was first integ
11、rated and performance-tested in August 2007, however, optical standing waves were observed that greatly exceeded those previously observed in the DM unit (Reference (1). This threat to mission performance was mitigated by adding mylar sheet material to the MSL flight TLS foreoptics that attenuated t
12、he amplitude of the fringes. The reduction of standing wave fringe amplitudes in tunable laser spectrometers is typically accomplished by making small adjustments in instrument alignment after the system integration. On the JPL TLS flight unit, however, the optical elements (that were adjusted in th
13、e DM unit) had been staked and bonded prior to integration and test (I&T) (Reference (2). Although staking of flight units is useful in preventing unintentional shifts in component alignment, it greatly restricted the Provided by IHSNot for ResaleNo reproduction or networking permitted without licen
14、se from IHS-,-,-ability to make any optical alignment adjustments at the laser/foreoptics end of the instrument after system integration. References: 1. “TLS Science Standing Waves,“ JPL Problem/Failure Report No. 6617, August 17, 2007.2. Robert T. Menzies, “Report on the TLS Standing Waves Tiger Te
15、am Activity: Findings & Recommendations,“ December 7, 2007.Lesson(s) Learned: Unless precision assemblies are specifically designed to permit mechanical adjustment of components following subsystem integration, it may be infeasible to “tweak“ an assembly during I&T to meet functionality or performan
16、ce requirements without risk of damage. For example, if machining to realign the MSL TLS optical elements had been performed after staking, it could have led to localized deformation or delamination of these elements.Recommendation(s): 1. For all precision assemblies subject to correction by mechani
17、cal adjustments, provide a design capability that allows adjustments for key mechanical parameters both before and after integration. For example, the use of attenuator material should be accommodated in the design of a TLS, and the ability to perform TLS laser beam position adjustment and angular a
18、djustment should be retained during the I&T period.2. Staking or fastening actions that limit the ability to make necessary alignment adjustments within assemblies should not be employed until it is demonstrated that functionality and performance requirements can be met with adequate margin during I
19、&T.Evidence of Recurrence Control Effectiveness: JPL has referenced this lesson learned as additional rationale and guidance supporting Paragraph 8.3.3.6 (“System Assembly, Integration and Test: System Functional Verification - System Alignments“) in the JPL standard “Design, Verification/Validation
20、 and Operations Principles for Flight Systems (Design Principles),“ JPL Document D-17868, Rev. 3, December 11, 2006.Documents Related to Lesson: N/AMission Directorate(s): Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-a71 Aeronautics Researcha71 Ex
21、ploration Systemsa71 ScienceAdditional Key Phrase(s): a71 0.a71 0.a71 1.Planning of requirements verification processesa71 1.Spacecraft and Spacecraft Instrumentsa71 0a71 1a71 0.a71 0.a71 1.Flight Equipmenta71 1.Hardwarea71 1.Payloadsa71 1.Risk Management/Assessmenta71 1.Test & Verificationa71 1.Test ArticleAdditional Info: a71 Project: Mars Science LaboratoryApproval Info: a71 Approval Date: 2009-03-25a71 Approval Name: mbella71 Approval Organization: HQProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-
