www.le.ac.uk

Vibration Testing atUniversity of Leicester SRC

Jon Sykes

Space Research CentreDepartment of Physics and Astronomy

University of Leicester

 

2010/2011 Flight Project Vibration TestingProject Description Launch Date SRC Responsibility Delivering to Test Summary

JWST MIRI Infra-red Imager and Spectrometer for James Webb Space Telescope

2015 Mechanical/Structural Lead

NASA via ESA Sine and Random. Force limited and acceleration limited with secondary notching. Instrument level and subassembly level testing. Minimise loads on mechanisms

MIXS Imaging X-ray Spectrometer for Bepi-Columbo mission to Mercury

2014 Instrument Lead ESA via EADS Astrium Sine and Random. Acceleration limited and non-contact measurement. Minimise loads on optics

Astrosat X Ray Telescope Focal Plane Camera

2011 Instrument Lead ISRO Sine and Random. Acceleration limited. Minimise loads on detector assembly

XRD X Ray Diffraction Instrument For EXOMARS

2018 DetectorAssembly

ESA via Thales Alenia Space (Milan)

Sine and Random. Press the button and stand back

LMC Life Marker Chip Experiment for EXOMARS

2018 Instrument Lead Institute ESA via Thales Alenia Space (Torino)

Sine and Random. Control strategy TBD. Instrument level and subassembly level testing. Minimise loads on – (TBD)

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

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

Artist’s Impression of JWST Spacecraft SRC staff working at NASA

FM ‘Deck’ – Instrument Optical Bench

SRC staff with FM at RAL

MIRI Subassembly Mechanical Loads Environment – Reducing Conservatism With Force Limiting

• 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

Enveloping And Shaker Impedance Problems

• Enveloping– Necessary to allow independent hardware development– Adds conservatism, especially if applied serially

FPM Random Max Fn (XYZ)

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FPM_random_specification 12.2grms

FPM_Random_Max_Fn_ver2

MIRIM_Force_Limited_FPM_Env

SMO_Force_Limited_FPM_env

FPM_Spec_From_SA_level_31.2grms

• 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

• 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

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

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

Non-contact instrumentation – Laser Vibrometry

1st resonant frequency – carrier frame.

Measure frequency 1544 Hz,FEA predicted frequency 1320Hz

2nd resonant frequency – MCP optic

Measured frequency 2394 HzFEA predicted frequency 2133Hz.

• 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

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, Bepi Columbo, 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)

End