Post on 07-Apr-2018
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Transducer Mounting Mechanical connection method
Stud mount
Adhesive mount
Magnetic mount
Press-fit friction mount
Test parameter considerations
Frequency range Mass loading
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Mechanical Mounting:
Impact on Frequency Range
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Sensit
ivityDeviation
(dB)~
Ref.100Hz
Log Frequency (Hz)
Stud
Mount
Adhesive
Mount
MountingPad
FlatMagnet
Dual RailMagnet
HandProbe
+40
+30
+20
+10
0
-20-10
1.0 10 100 1000 10 000 100 000
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Stud Mount Transducers Best frequency response characteristics
just like the manufacturers cal labs Apply silicon grease at mating surface
Requires surface preparation
Proper torque recommended
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Adhesive Mounting Supplies
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Adhesive Mount Transducers Cyanoacrylate (superglue)
Instant adhesive; strong, but still removable Gel vs liquid depends upon surface flatness
Excellent frequency response characteristics
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Adhesive Mount Transducers Petro wax (bees wax)
Ultra convenient and simple Good for short term testing only
Frequency response characteristics highly
dependent upon surface prep and amount
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Adhesive Mount Transducers Hot glue
Allows attachment to poorly-mated surfaces Good for short term to mid term testing
Frequency response characteristics poor, but
generally good enough for modal apps
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Adhesive Mount Transducers Dental cement / fast-cure epoxy
Allows attachment to poorly-mated surfaces Pseudo-permanent attachment for reference
transducer at shaker input location
Use disposable mounting pad with stud
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Magnetic Mount Transducers Extremely convenient
High attraction forces allow forreasonable high frequency characteristics
Available in dual-rail style for attachment
to curved surfaces
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Press-fit Mount Transducers Extremely convenient and efficient
Designed specifically for low frequency(
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Mass Loading Considerations Acquire FRF with a single accelerometer
Mount a second accelerometer next to thefirst and re-acquire FRF
Compare for measurable differences
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Test Setup Considerations Understand goals/reasons for performing
experimental modal analysis Troubleshooting or failure analysis
Finite element model verification
Finite element model correction
Component substructure / system modeling
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Test Setup Considerations Recognize the 4 primary assumptions of
experimental modal analysis Observability
Time Invariance (Stationarity)
Linearity
Maxwells Reciprocity
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Observability Assumption Response DOF must have adequate
spatial resolution to represent the modesof interest
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Graphics from Agilent
Application Note 243-3
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Observability Assumption
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First bending beam with seven accelerometer measurement points
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Observability Assumption
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If data acquired only at endpoints bending is not observable
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Observability Assumption
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Observability Assumption Forcing function(s) applied at input
location(s) must adequately excite themodes of interest
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Graphic from Agilent
Application Note 243-3
AVOID NODES!
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Modally-Tuned Impact Hammers as Pre-
Test Tool for Evaluating Structures...
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for Optimizing Reference
Locations and Ensuring Observability
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Freq Resp 2:1 82798009.DAT
40dBg/lb
-60
Mag (dB)
kHz10 Hz
Freq Resp 2:1 82798229.DAT
Good Input Location
Bad Input Location
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for Determining Optimal
Frequency Range
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Empty
40dBg/lb
-60
Mag (dB)
kHz1.60 Hz
Freq Resp 2:1
Assure adequate spatial resolution to observe:
- Important, dominant modes- Necessary modal density
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for Testing Boundary Conditions
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Graphic from Agilent
Application Note 243-3
Rule of Thumb: 5-10x separation between rigid
body and flexible modes
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Time Invariance Assumption Test article (and its boundary conditions) must
exhibit stationarity
Parameter estimation algorithms assume consistent
global modal properties throughout data set
Environmental changes during data acquisition cause
shifts in stiffness/damping properties resulting inmeasurable shifts in resonant frequencies
Roving accelerometers to acquire data set results in
variable mass loading on test article
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Time Invariance Assumption DATA CONSISTENCY
DATA CONSISTENCY DATA CONSISTENCY
i.e. acquire entire data set simultaneously
(single snapshot) or at least as fast as
possible
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Time Invariance Assumption Test methodology to achieve best data
consistency Simultaneous MIMO/SIMO testing
Automated bankswitching
Manual bankswitching
Roving accelerometers
Impact testing
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Benefits of Bank-Switching
Bank-switch, 288 to 112 ch
No. of Test
Configurations
PreSetup
Time
Acquisition
Time
Cost
Estimate
1 9 hrs 17 min 60%
2 17 min
3 17 min
4 17 min
Total Time
Allotted
10 hrs
8 min
Roving, 112 ch
No. of Test
Configurations
PreSetup
Time
Acquisition
Time
Cost
Estimate
1 3 hrs 6 hrs 17 min 40%
2 6 hrs 17 min
3 6 hrs 17 min4 6 hrs 17 min
Total Time
Allotted
28 hrs8 min
Simultaneous, 288 ch
No. of TestConfigurations
PreSetupTime
AcquisitionTime
CostEstimate
1 9 hrs 5 min 100%
2 5 min
3 5 min
4 5 min
Total Time
Allotted
9 hrs
20 min
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Bank-Switching Example Inputs: 2 vertical, 1 lateral, 1 skewed
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Bank-Switching Example Response points: 17 patch panels, each
bank of 16 accelerometers
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Bank-Switching Example Bank-switch patches of data (3 x 96 ch)
into smaller data acquisition system
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Modular Cabling / Patch Panel
System for Clean Setup
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Eases setup troubleshooting
Eliminates messy rats nest of cables
Economical multi-conductor cabling
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Time Invariance Assumption Roving accelerometers results in inconsistent
global resonant frequencies due to variablemass loading on test structure
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Graphic from Agilent
Application Note 243-3
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Linearity Assumption Input and output characteristics remain
proportional within measurement range Confirm using precisely controlled inputs
from shaker(s) across a range force levels
Impact testing technique poorly suited
when dealing with nonlinear test
structures
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Electrodynamic Modal Shakers as
Excitation Source for MIMO Allows best control of
input forcing function tooptimize frequencycontent and signal-to-noise ratio
Through-hole armaturegreatly simplifies setupattachment to teststructure
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Reciprocity Assumption Maxwells Theory of Reciprocity states
that FRF matrix is symmetric FRF between input A and output B is the
same as output A and input B
Confirm using multiple shaker locations
and impedance heads for driving point
measurement
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Impedance Heads for Verifying
Reciprocity Assumption
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Freq Resp 2:1
30dBg/lb
-70
Mag (dB)
Hz5000 Hz
Freq Resp 2:1
Accelerometer built
into preload stud offorce transducer
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Other Pre-Test Considerations Free Boundary Conditions
Shock Cord Foam Rubber
Air Suspension
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Other Pre-Test Considerations Fixed Boundary Conditions
Realistic Boundary Conditions Match Impedance(s) at Boundaries
Mass Loaded Boundary Conditions
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Other Pre-Test Considerations Transducer selection
Single axis vs triaxial package Sensitivity, measurement range & resolution
Frequency range & mass
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TransducerElectronicData Sheet
(TEDS, IEEE 1451.4)
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TEDSMemory
To ICP Supply
4ma
-5 V
Identifies transducer (type, serial number, location)
Stores calibration data
Automates book keeping, reducing errors
Uses reverse bias scheme
to access digital memory
O h T d S
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Other Transducer Setup
Considerations Use PDA scanner with bar-coded TEDS
transducers to ease bookkeeping
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Final Channel Setup Definition Combine Data From Geometry, PDA, and
TEDS Complete Test Set-up Information
Defined in Universal Files
Virtual Channel Table (1807) Channel Table (1808)
Geometry (15)
ParameterStored In
TEDS
Stored In
PDA
Stored On
Host (PC)
Calibration X
Model / Serial No. X X
Direction X
Node No. X X
Meas. Ch. X
Geometry X
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Thank you for your time.
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