transducer mounting and test setup configurations test setup.pdf · transducer mounting and test...
TRANSCRIPT
![Page 1: Transducer Mounting and Test Setup Configurations Test Setup.pdf · Transducer Mounting and Test Setup Configurations 1 ... mass loading on test structure 33 ... • Identifies transducer](https://reader031.vdocument.in/reader031/viewer/2022022500/5aa30ad07f8b9aa0108e08ee/html5/thumbnails/1.jpg)
Transducer Mounting
and Test Setup
Configurations
1
Rick Bono
The Modal Shop
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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
2
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Mechanical Mounting:
Impact on Frequency Range
3
Sen
sitiv
ity D
evia
tio
n
(dB
) ~
Ref.
100
Hz
Log Frequency (Hz)
StudMount
Adhesive
Mount
Mounting
Pad
Flat
Magnet
Dual Rail
Magnet
Hand
Probe
+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 manufacturer’s cal labs
• Apply silicon grease at mating surface
• Requires surface preparation
• Proper torque recommended
4
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Adhesive Mounting Supplies
5
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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
6
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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
7
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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
8
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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
9
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Magnetic Mount Transducers
• Extremely convenient
• High attraction forces allow for
reasonable 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
(<1000 Hz) laboratory modal applications
• Cable base mounts adhesively, modal sensor
mechanically attaches using electrical pins
11
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Mounted Accelerometer
Frequency Response Calibration
Mounted Accelerometer Frequency Response
0.0
50.0
100.0
150.0
200.0
250.0
300.0
0 2000 4000 6000 8000 10000 12000
Frequency (Hz)
Sen
sit
ivit
y (
mV
/g)
Cyanoacrylate
Hot Glue
Putty
Petrowax
Stud (handtight w/out grease)
Stud (w/ grease)
12
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Mass Loading Considerations
• Acquire FRF with a single accelerometer
• Mount a second accelerometer next to the
first and re-acquire FRF
• Compare for measurable differences
13
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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
14
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Test Setup Considerations
• Recognize the 4 primary assumptions of
experimental modal analysis
– Observability
– Time Invariance (Stationarity)
– Linearity
– Maxwell’s Reciprocity
15
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Observability Assumption
• Response DOF must have adequate
spatial resolution to represent the modes
of interest
16
Graphics from Agilent
Application Note 243-3
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Observability Assumption
17
First bending – beam with seven accelerometer measurement points
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Observability Assumption
18
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 the
modes of interest
20
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
22
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
23
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
24
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 in
measurable shifts in resonant frequencies
– Roving accelerometers to acquire data set results in
variable mass loading on test article
25
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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
26
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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 min
4 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
32
• Eases setup troubleshooting
• Eliminates messy “rat’s nest” of cables
• Economical multi-conductor cabling
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Time Invariance Assumption
• Roving accelerometers results in inconsistent
global resonant frequencies due to variable
mass loading on test structure
33
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 to optimize frequency content and signal-to-noise ratio
• Through-hole armature greatly simplifies setup attachment to test structure
35
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Through-Hole Armature
Eases Setup
• Traditional shakers
with tapped armature
connection leave
little tolerance since
setup has tapped
connection at both
ends
36
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Reciprocity Assumption
• Maxwell’s 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
38
Freq Resp 2:1
30dBg/lb
-70
Mag (dB)
Hz500 0 Hz
Freq Resp 2:1
Accelerometer built
into preload stud of
force 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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Transducer Electronic Data Sheet
(TEDS, IEEE 1451.4)
42
TEDS
Memory
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
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Other Transducer Setup
Considerations
• Use PDA scanner with bar-coded TEDS
transducers to ease bookkeeping
43
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44
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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