primary calibration of accelerometers using laser interferometry · 2010. 11. 18. · shaker...
TRANSCRIPT
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Primary Calibration of Accelerometers Using Laser
Interferometry
Jeffrey Dosch
23rd Transducer WorkshopJune 17-18Buffalo NY
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23rd Transducer Workshop 2008
Outline
• “Tour” of PCB’s accelerometer calibration facility.– Motivation for primary calibration.
• Primary calibration system.– Interferometer theory– Shaker design
• Conformance to ISO 17025– Uncertainty– Proficiency testing– Data review
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23rd Transducer Workshop 2008
Production calibration• Sensitivity and Frequency Response• Back-to-back calibration against working standard.
– 5Hz to 15kHz– 100000+ accelerometers calibrated each year.– 16 calibration stations located NY and NC.
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23rd Transducer Workshop 2008
Production Calibration
• Sensitivity and Frequency Response
• Low frequency Calibration (0.5Hz to 1000Hz)– 4 Low frequency calibration
stations
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23rd Transducer Workshop 2008
Production Calibration Other frequency response tests
• Discharge time constant– Approximately -5% at .5/DTC
Computer equipped with DAQ card and softwareTest SignalConditioner
Test Fixture
Test sensor
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23rd Transducer Workshop 2008
Production Calibration Other frequency response tests
• Resonant frequency – Approximately +5% at 1/5 resonant frequency.
StrikingMotion
Test Sensor
Tungsten Mass
Mylar strip with attached steel ball
Test SignalConditioner
Computer equipped with DAQ card and software
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23rd Transducer Workshop 2008
Production Calibration Transverse Sensitivity
COMPUTERA/D AND D/A
AMPLIFIER
x
y
θ
DIRECTION OFTRANSVERSEEXCITATION
SIGNALCONDITIONER
SUT
X ACCELEROMETER
BEAM
X EXCITER
Y EXCITER
BASE
Y ACCELEROMETER
TOP VIEW
SIDE VIEW
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23rd Transducer Workshop 2008
Production Calibration Transverse Sensitivity
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23rd Transducer Workshop 2008
Production calibration process control
• Daily verification sensor.
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23rd Transducer Workshop 2008
Motivation: Better production calibration
• Traceability of Sensitivity [mV/g]
TRANSFERSTANDARD
LASE
R
Primary calibrationWorking standards
Production calibration(approx. 12000/month)
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23rd Transducer Workshop 2008
Motivation
• Customer demand for more accurate calibration– More frequent calibration of transfer standards.– Primary calibration of working standards.
• PCB in-house cal lab uses dozens of working standards.– Primary calibration of in-house verification standards.– Meet demand for calibrations not addressed by national
laboratories• Low uncertainty, frequency, amplitude.
• Feasibility enabled by developments in PC-based acquisition and laser/optic technology.
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23rd Transducer Workshop 2008
Accelerometer calibration system
• 4 Channels– Accelerometer voltage– In-phase photodetector– Quadrature photodetector– Velocity (reference)
LASERINTERFEROMETER
VIBROMETERCONTROLLER
I/QDEMODULATOR
COMPUTER
DAQ
Acceleration Velocity
I
Q
RF
LO
SUT
SHAKERARMATURE
Interferometer
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23rd Transducer Workshop 2008
Interferometer Theory
• Fringe Counting
PCB Piezotronics Engineer Jing LinAccelerometer Calibration, Laser fringe countingNational Vibration Laboratory, Beijing China, c1970
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23rd Transducer Workshop 2008
Interferometer Theory
• Interference of fixed-path reference and target reflected light.– HeNe wavelength λ
accepted constant
• Interference proportional to target displacement
• Acceleration calculated from displacement and frequency [m/s2]
)(2 txm =φ
xfa 2)2( π= [m/s2]
FIXED REFERENCE
x(t)
REFLECTED HeNeWAVE
TAR
GET
φm
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23rd Transducer Workshop 2008
Michelson Interferometer
• Photodetector output, Q, varies by cosine of displacement
• Fringe counting: photodetector peaks per target period.– Fringe =½ λ=316 nm– Requires displacement
large compared to λ.– Less than 1kHz
)(tx
HeNe Laser
Photodetector
ReferenceMirror
INTERFEROMETER
TARGET
)(4cos txQλπ
=
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23rd Transducer Workshop 2008
Michelson Interferometer: Quadrature detector
• Add ¼ retarder• Motion computed
from I and Q:
• Reconstruct waveform– Phase information
• Measures displacement smaller than wavelength
)(tx
HeNe Laser
Photodetectors
ADC ADC
ReferenceMirror
INTERFEROMETER
ACQUISITION
TARGET 1/4 WaveRetarder
)(4cos txQλπ
=)(4sin txIλπ
=
⎟⎠⎞
⎜⎝⎛= −
IQtx 1tan
4)(
πλ
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23rd Transducer Workshop 2008
Heterodyne Interferometer Quadrature detector
• Signal heterodyned on fixed 40MHz carrier.
• Displacement proportional to carrier phase.– Velocity proportional to
frequency (Vibrometer).
• I/Q signal processing same as quadrature Michelson.
)(sin)( ii ttI ϕ=
)(tx
HeNe Laser
Bragg Cell
Photodetector
tu cBragg ω2sin=
tu cc ωsin=))(sin( ttu c ϕω +=
Splitter
PhaseShift 0°
90°
Mixer
ADCLPFilter
ADCLPFilter )(cos)( ii ttQ ϕ=
INTERFEROMETER
DEMODULATOR ACQUISITION
RF
LO
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23rd Transducer Workshop 2008
Accelerometer calibration system
• 4 Channels– Accelerometer voltage– In-phase photodetector– Quadrature photodetector– Velocity (reference)
LASERINTERFEROMETER
VIBROMETERCONTROLLER
I/QDEMODULATOR
COMPUTER
DAQ
Acceleration Velocity
I
Q
RF
LO
SUT
SHAKERARMATURE
Shaker
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23rd Transducer Workshop 2008
Shaker Performance
• Number of shaker characteristics will influence calibration accuracy– Armature transverse motion
• Influence on SUT– Armature “rocking” motion
• Influence on laser– Grounding.
• Ground loops• Coil capacitive coupling.
– Insufficient excitation level.– Distortion– Magnetic field– Mounting adaptors
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23rd Transducer Workshop 2008
Shaker Construction
• 396C10/C11
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23rd Transducer Workshop 2008
Shaker Construction
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23rd Transducer Workshop 2008
Shaker Construction: Armature
• 396C10 hardcoat aluminum armature.
• 396C11 beryllium armature. INSERT
Armature body
AC Coil
DC Coil
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23rd Transducer Workshop 2008
Shaker Construction: Armature
• Materials
Material Modulus E
(GPa)
Density ρ
(kg/m3)
c ρE
(m/s) Beryllium 303 1840 12800 Aluminum 69 2700 5050 Titanium 114 4540 5011
Alumina (ceramic) 375 3900 9805
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23rd Transducer Workshop 2008
Shaker Construction: Insert
• Removable insert– Off-ground from armature– Shear quartz accelerometer reference.– Variety of mounting threads (10-32, ¼-28, M5)
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23rd Transducer Workshop 2008
Shaker Construction: AC Coil
• “AC Coil” provides dynamic force to armature (F = BIL; B is constant).
0
1000
2000
3000
4000
5000
6000
7000
8000
0.00 1.00 2.00 3.00 4.00
Distance (cm)
Flux
den
sity
(Gau
ss)
a b
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23rd Transducer Workshop 2008
Shaker Construction: DC Coil
• DC Coil Provides force for electrical spring (F = BIL where B varies linearly).
0
1000
2000
3000
4000
5000
6000
7000
8000
0.00 1.00 2.00 3.00 4.00
Distance (cm)
Flux
den
sity
(Gau
ss)
dc
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23rd Transducer Workshop 2008
ShaerPerformance: Benchmarking
• ISO 16063-21:2003(E), “Methods for the calibrations of vibration and shock transducers – Part 21: Vibration calibration by comparison to a reference transducer,”
• Tested a number of shakers– “50 lb” calibration shaker of modal pedigree– Laboratory beryllium air bearing shaker– 396C10/C11
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23rd Transducer Workshop 2008
Shaker Performance: transverse
• 30 gram tungsten mass simulates SUT
• 356A11 triax accel (5% to 10kHz)• Transverse acceleration
zyx 22
100+
=
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23rd Transducer Workshop 2008
Shaker Performance: transverse
• Triax Validated against laser– Reasonable to 15kHz– Triax over estimates transverse
Transverse Laser
0
10
20
30
40
50
0 5000 10000 15000 20000
Frequency (Hz)
Tran
sver
se (%
)
TransTrans xTrans y
Transverse Triax
0
10
20
30
40
50
0 5000 10000 15000 20000
Frequency (Hz)
Tran
sver
se (%
)
TransTrans xTrans y
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23rd Transducer Workshop 2008
Shaker Performance: transverse
• “50 lb” calibration shaker
0102030405060708090
100
0 2000 4000 6000 8000 10000
Frequency (Hz)
Tran
sver
se M
otio
n (%
of a
xial
)
Shaker response
ISO recommendedlimit
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23rd Transducer Workshop 2008
Shaker Performance: Transverse
• Air bearing shakers
0
5
10
15
20
25
30
35
40
45
50
0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000
Frequency (Hz)
Tran
sver
se M
otio
n (%
of a
xial
)
396C10
ISO recommended limit
396C11
Be Lab
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23rd Transducer Workshop 2008
Shaker Performance: rocking modes
• SUT calibration is based on motion of surface under SUT.– Primary calibration: laser is directed adjacent to SUT.
Centerline acceleration must be interpolated.
SHAKERARMATURE
LAS
ER
A
SUTLA
SE
R B
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23rd Transducer Workshop 2008
Shaker Rocking modes
• Operating deflection shape measured with laser– Acceleration frequency response acquired at 6 points on
armature [g/V].
LASE
R1 2 3 4 5 6
Aref [V]
Ai [g]
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23rd Transducer Workshop 2008
Performance: rocking
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23rd Transducer Workshop 2008
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23rd Transducer Workshop 2008
Rocking modes
• Laser cal of reference insert based on two diametrically opposed positions, 1 inch diameter (100 Hz to 50kHz).
0
2
4
6
8
10
12
14
100 1000 10000 100000Frequency (Hz)
Acc
eler
atio
n D
iffer
ence
(%)
396C11396C10
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23rd Transducer Workshop 2008
Performance: magnetic
• Flux density B decreases with distance from gap
• Accelerometer responds to changes in B
0
1000
2000
3000
4000
5000
6000
7000
8000
0.00 1.00 2.00 3.00 4.00
Distance (cm)
Flux
den
sity
(Gau
ss)
xBSa Berror ˆ= [m/s2 rms]
B gradientSensitivity to B displacement
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23rd Transducer Workshop 2008
Shaker Performance: magnetic
• 396C10/C11 B gradient at mounting surface
• Influence for typical accelerometer
B̂
xBSa Berror ˆ=
= 2160 Gauss/meter
BS = 100E-6 m/s2/Gauss
Frequency (Hz) Error (% of axial
acceleration) 10 .0055 100 .0006 1000 .0000
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23rd Transducer Workshop 2008
Confidence in Measurement• Accredited PCB for conformance to ISO 17025 by
A2LA.• ISO 17025 describes requirements to for laboratory
competence. Comprehensive, management/technical.– Management/ organization– Quality systems. Document control.– Equipment/ environmental conditions. Personnel.– Traceability to NMI– Calibration Methods and validation
• Uncertainty estimates• Proficiency testing• Calibration process monitoring• Data review (sanity checks)
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23rd Transducer Workshop 2008
Proficiency testing Trend data
• 160 Hz trend; X353M295 SN 2• Trend shows -.06% change/year.
X353M295 SN 2, 160 Hz Sensitivity
10.180
10.200
10.220
10.240
10.260
10.280
10.300
Apr-01
Sep-02
Jan-04
May-05
Oct-06
Feb-08
Jul-09
Cal Date
Sens
itivi
ty (m
V/g)
PCB PrimaryPTB PrimaryNIST
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23rd Transducer Workshop 2008
Proficiency Testing Low frequency
• 301M26 1 Hz Trend• Change
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23rd Transducer Workshop 2008
Proficiency Testing Low frequency
• 301M26 Frequency Response; Oct 2004
301M26 SN 1702 Oct 2004
495496497498499500501502503
0 1 2 3 4 5 6
Frequency (Hz)
Sens
itivi
ty (m
V/g)
PTBPCB
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23rd Transducer Workshop 2008
Proficiency testing Frequency Response
• X353M295 SN 2; Nov 2002
10.0
10.5
11.0
11.5
12.0
12.5
1 10 100 1000 10000 100000
Frequency (Hz)
Sen
sitiv
ity (m
V/g
)
PTB CalibrationPCB CalibrationNIST Calibration
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23rd Transducer Workshop 2008
Proficiency testing Frequency Response
• X353M295 SN2; Nov 2002
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
1 10 100 1000 10000 100000
Frequency (Hz)
Dev
iatio
n Fr
om P
TB (%
)
PCB CalibrationNIST Calibration
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23rd Transducer Workshop 2008
Proficiency testing Frequency Response
• 353M304; 11/2004Deviation From PTB; 353M304 11/2004
-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
10 100 1000 10000
Frequency (Hz)
Dev
iatio
n (%
)
PCB
NIST
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23rd Transducer Workshop 2008
Data review Sanity checks
• Screen shot
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23rd Transducer Workshop 2008
Review of data
• “Sanity check” of expected accelerometer response– Low frequency response should follow time constant
– Mid frequency should follow crystal characteristics• -2.5%/decade piezoceramic charge• 0 to +1%/decade piezoceramic voltage• +0.2%/decade ICP quartz• Flat charge quartz
– High frequency should follow second order system
22
2
2 2)(
nn
n
sssH
ωζωω
++=
sssHτ+
=1
)(1
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23rd Transducer Workshop 2008
Review of data
• High frequency theory vs back-to-back calibration– Model:
Resonance = 55 kHz; “droop” +0.3% decade
356B21
00.5
11.5
22.5
33.5
44.5
5
100 1000 10000
Frequency (Hz)
dev
ref 1
00H
z (%
)
back2backtheory
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23rd Transducer Workshop 2008
Review of data
• Low frequency calibration: Theory vs. primary calibration– Model: Time constant = 6.1 seconds;“droop” –1.0%/decade
393M81
9800
9850
9900
9950
10000
10050
10100
10150
0.1 1 10 100Frequency (Hz)
Sens
itivi
ty (m
V/g)
Laser calibration
Theory (ref 100 Hz)
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23rd Transducer Workshop 2008
Review of data
• Potential shaker/reference influences– Transverse motion
• Glitches at discrete frequencies corresponding to shaker resonances
– Magnetic field• Low frequency trend deviation from expected response.
– Grounds • Capacitive coupling of power will cause high frequency trend
deviation from expected.– Cable strain reference
• Low or high frequency glitch– Base strain
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23rd Transducer Workshop 2008
Review of data
• Calibration of internal reference on “laboratory” beryllium air bearing shaker.– Low frequency response does not follow theory
10 0 0 A D S N 16 0 : I N FLU EN C E OF A I R B EA R I N G P R ES S U R E
60.065.070.075.080.085.090.095.0
100.0105.0
0 5 10 15 20
Frequency (Hz)
Sens
itivi
ty (m
V/g)
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23rd Transducer Workshop 2008
Review of data
• Reference accelerometer sensitive to air-bearing pressure.
10 0 0 A D S N 16 0 : I N FLU EN C E OF A I R B EA R I N G P R ES S U R E
60.065.070.075.080.085.090.095.0
100.0105.0
0 5 10 15 20
Frequency (Hz)
Sens
itivi
ty (m
V/g)
1 psi
2.5 psi
5 psi
10 psi
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23rd Transducer Workshop 2008
Summary
• PCB has been performing A2LA accredited primary calibration of accelerometers since 2002.
• Beryllium air bearing 5 Hz to 15 kHz– Uncertainty = 0.2% (k = 2) at 100 Hz
• Long stroke 0.5 Hz to 10 Hz – Uncertainty = 0.3% (k =2) at 1 Hz
Primary Calibration of Accelerometers Using Laser InterferometryOutlineProduction calibrationProduction CalibrationProduction Calibration� Other frequency response testsProduction Calibration� Other frequency response testsProduction Calibration�Transverse SensitivityProduction Calibration�Transverse SensitivityProduction calibration� process controlMotivation:�Better production calibrationMotivationAccelerometer calibration systemInterferometer TheoryInterferometer Theory Michelson InterferometerMichelson Interferometer:�Quadrature detectorHeterodyne Interferometer�Quadrature detectorAccelerometer calibration systemShaker PerformanceShaker ConstructionShaker ConstructionShaker Construction: Armature Shaker Construction: ArmatureShaker Construction: InsertShaker Construction: AC CoilShaker Construction: DC CoilShaerPerformance: BenchmarkingShaker Performance: transverseShaker Performance: transverseShaker Performance: transverseShaker Performance: TransverseShaker Performance: rocking modesShaker Rocking modesPerformance: rockingSlide Number 35Rocking modesPerformance: magneticShaker Performance: magneticConfidence in MeasurementProficiency testing �Trend dataProficiency Testing�Low frequencyProficiency Testing�Low frequencyProficiency testing�Frequency ResponseProficiency testing �Frequency ResponseProficiency testing �Frequency ResponseData review� Sanity checksReview of dataReview of dataReview of dataReview of dataReview of dataReview of dataSummary