Vibration Testing at University of Leicester SRC

1. Vibration Testing atUniversity of Leicester SRC Jon Sykes Space Research Centre Department of Physics and Astronomy University of Leicester

2. 2010/2011 Flight Project Vibration Testing

3. Scope of Vibration Testing Activities • Interpretation of specification and derivation of subassembly design/test levels • Sine vibration • Random vibration • Shock • Quasi-static strength testing • Minimise conservatism across assembly levels where possible • Dynamic Analysis (Siemens NX 6, I-DEAS, export models for MSC NASTRAN Compatibility) • Sine response • Random response • Test predictions • Developing notched inputs to reduce conservatism • Demonstrating safety of tests with regard to upper and lower assembly levels • Model correlation/delivery • Damping, stiffness • Demonstrate correlation with test results for delivery of models to higher level • Test procedure/specification/responsibility • Test instrumentation • Test control strategy • Specification of test facilities • Test Director status – responsibility for irreplaceable hardware

4. JWST Mid-Infrared Instrument • Instrument to fly on the James Webb Space Telescope (JWST) • NASA/ESAs largest and most complex joint astronomy project to date. Launch 2015. • SRC are lead mechanical engineers for this instrument • Working with institutes and industry in 12 countries • Mechanical design, analysis assembly and testing at instrument level • Instrument testing completed October 2010 • Review of subassembly hardware design, analysis, testing • Specification of subassembly test and hardware requirements • Supervision of subassembly tests • E.g. Carl Zeiss FM and QM mechanism tests, Oberkochen • Notching to minimise mechanism/optics loads • Represent instrument interests at higher asembly level • Support of NASA testing later in the year at Goddard Space Flight Center SRC staff working at NASA Artist’s Impression of JWST Spacecraft FM ‘Deck’ – Instrument Optical Bench SRC staff with FM at RAL

5. Mechanical loads due to launch environment dominate structural design of most payloads Mechanical test specifications derived from serial analysis campaigns through the assembly levels Simplified models and data analysis Margins applied at each assembly level by enveloping Very conservative results in terms of load specifications Test equipment/conventions add yet more conservatism Result: Over design of mass-critical payloads Compromise of scientific performance Over test of mission critical hardware Unnecessary failures - delays, costs, panic... Mitigation Sensible derivation of loads. Mechanical loads derivation across the ‘hardware responsibility’ barriers. Trade off against management of interfaces. Testing strategies to reduce conservatism Force limiting, moment limiting, acceleration limiting MIRI Subassembly Mechanical Loads Environment – Reducing Conservatism With Force Limiting

6. Enveloping Necessary to allow independent hardware development Adds conservatism, especially if applied serially Enveloping And Shaker Impedance Problems • Example: JWST MIRI Focal Plane Module (FPM) random vibration loads • Derived for NASA-JPL from University of Leicester FE model and STM testing • Avoided extra assembly level • Used notched input loads at instrument interface • Minimise input at FPM resonant frequency ~700Hz • Order of magnitude improvement over serial analysis method • Shaker Mechanical Impedance • Real payload structures absorb energy at resonance • Real payload structures fail due to high amplitude responses at resonance • Force limiting provides much improved representation of interface condition and reduces over testing

7. JWST MIRI Analysis – Test Planning/Predictions • Determine Notch Criteria • Sine tests: • Primary notching for sine testing based upon limits derived by simultaneous application of Design Limit Loads (DLLs): • Summed in-axis force limit • CG acceleration backup limit • Moment limits derived from out of axis forces • Random tests: • Primary notching based on semi-empirical force limit • Based on semi-empirical effective mass method as documented by T. Scharton • Secondary notching based upon subassembly test levels vs GSFC on ISIM predictions for random and acoustic tests. • Notching approach validated by test predictions prior to testing • Respect subassembly test levels. Negotiate notch criteria or accept risk • Ensure notching methods respect higher level requirements • All reviewed and agreed Project/ESA/NASA prior to testing MIRI FE Model Predicted Notched Input Vs Test Data Respect Subassembly Levels and S/C levels

8. JWST MIRI FM Vibration Test • Testing carried out 4-12 October 2010 at RAL, UK • Instrumentation • 9 channels force measurement • (image shown is from STM/ETU test) • Control of test inputs • Mass properties • 12 channels strain gauge measurement • (measurements between axes only) • Primary structure trending • 51 accelerometer channels • Control of test inputs, limits, environments • University of Leicester – Test Director Role • Test Observers • MIRI UK Project • ESA • NASA GSFC • Outcome • Successful application of all test levels • Secondary notching negotiated and agreed on the spot with ESA GSFC - subassembly levels all respected • All health checks nominal • MIRI Instrument qualified to fly on JWST, huge project milestone on a huge project • End of 7 years of design, analysis, documentation, argument, negotiation, testing

9. Non-contact instrumentation – Laser Vibrometry 1st resonant frequency – carrier frame. Measure frequency 1544 Hz, FEA predicted frequency 1320Hz • Used for MIXS optics characterisation • White noise excitation • Scanning laser vibrometer • Mapping of mode shapes • Correlation with model predictions • Many applications for sensitive and low mass items 2nd resonant frequency – MCP optic Measured frequency 2394 Hz FEA predicted frequency 2133Hz.

10. Summary of SRC Approach to Vibration Testing • Avoiding excessive conservatism in load specifications • Sensible approach to subassembly load derivations • Clear management of subassembly interfaces with test strategy in mind • Get early agreement on test control approach (ESA, NASA, ISRO, etc......) • UoL has established excellent working relationships with these partners • Through projects such as JWST MIRI, BepiColumbo, EXOMARS, Astrosat • Through participation in ESA workshops and studies (e.g. Improvement of Force Limited Vibration Testing study) • Extensive use of FE response analysis • SRC analysts perform full sine and random vibration analysis ‘test simulations’. • Notching (to some extent) can compensate for unknowns such as damping • Allows design to benefit from less severe loads, therefore improving performance and/or reducing mass • Test predictions and model correlation studies • Model deliveries to Agency and Industry specifications (e.g. EADS Astrium, Thales Alenia Space) • Use force limiting as notching approach during vibration test • Specify automatic notching where possible (may be test facility dependent) • Take advantage of semi-empirical force limit approach to reduce test responses • Negotiation/development with test facility may be required (instrumentation, data acquisition, data formats) • Use of non-contact instrumentation becoming more relevant (e.g. MIXS optics, detector assemblies)

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