Vibration Testing at
University of Leicester SRC
Jon Sykes
Space Research Centre
Department of Physics and Astronomy
University of Leicester
www.le.ac.uk
2010/2011 Flight Project Vibration Testing
Project
Description
JWST MIRI
Launch Date
SRC Responsibility
Delivering to
Test Summary
Infra-red Imager and 2015
Spectrometer for
James Webb Space
Telescope
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
Detector
Assembly
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
•
–
–
–
Artist’s Impression of JWST Spacecraft SRC staff working at NASA
Review of subassembly hardware
design, analysis, testing
Specification of subassembly test and
hardware requirements
Supervision of subassembly tests
•
•
–
Instrument testing completed October 2010
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
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)
10
1
ASD (g2/Hz)
0.1
0.01
FPM_random_specification 12.2grms
0.001
FPM_Random_Max_Fn_ver2
MIRIM_Force_Limited_FPM_Env
SMO_Force_Limited_FPM_env
0.0001
FPM_Spec_From_SA_level_31.2grms
0.00001
10
100
1000
Frequency
•
10000
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
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
–
–
•
MIRI FE Model
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
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
–
–
–
–
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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
•
•
•
•
•
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Measured frequency 2394 Hz
FEA 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
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–
•
Get early agreement on test control approach (ESA, NASA, ISRO, etc......)
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•
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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
–
–
–
•
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
–
–
–
•
Sensible approach to subassembly load derivations
Clear management of subassembly interfaces with test strategy in mind
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