Post on 31-Jan-2021
transcript
Primary Calibration of Accelerometers Using Laser
Interferometry
Jeffrey Dosch
23rd Transducer WorkshopJune 17-18Buffalo NY
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
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.
23rd Transducer Workshop 2008
Production Calibration
• Sensitivity and Frequency Response
• Low frequency Calibration (0.5Hz to 1000Hz)– 4 Low frequency calibration
stations
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
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
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
23rd Transducer Workshop 2008
Production Calibration Transverse Sensitivity
23rd Transducer Workshop 2008
Production calibration process control
• Daily verification sensor.
23rd Transducer Workshop 2008
Motivation: Better production calibration
• Traceability of Sensitivity [mV/g]
TRANSFERSTANDARD
LASE
R
Primary calibrationWorking standards
Production calibration(approx. 12000/month)
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.
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
23rd Transducer Workshop 2008
Interferometer Theory
• Fringe Counting
PCB Piezotronics Engineer Jing LinAccelerometer Calibration, Laser fringe countingNational Vibration Laboratory, Beijing China, c1970
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
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
=
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
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
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
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
23rd Transducer Workshop 2008
Shaker Construction
• 396C10/C11
23rd Transducer Workshop 2008
Shaker Construction
23rd Transducer Workshop 2008
Shaker Construction: Armature
• 396C10 hardcoat aluminum armature.
• 396C11 beryllium armature. INSERT
Armature body
AC Coil
DC Coil
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
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)
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
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
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
23rd Transducer Workshop 2008
Shaker Performance: transverse
• 30 gram tungsten mass simulates SUT
• 356A11 triax accel (5% to 10kHz)• Transverse acceleration
zyx 22
100+
=
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
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
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
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
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]
23rd Transducer Workshop 2008
Performance: rocking
23rd Transducer Workshop 2008
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
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
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
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)
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
23rd Transducer Workshop 2008
Proficiency Testing Low frequency
• 301M26 1 Hz Trend• Change
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
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
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
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
23rd Transducer Workshop 2008
Data review Sanity checks
• Screen shot
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
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
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)
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
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)
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
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