primary calibration of accelerometers using laser interferometry · 2010. 11. 18. · shaker...

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


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.


Production Calibration

• Sensitivity and Frequency Response

• Low frequency Calibration (0.5Hz to 1000Hz)– 4 Low frequency calibration

stations


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


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


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


Production Calibration Transverse Sensitivity


Production calibration process control

• Daily verification sensor.


Motivation: Better production calibration

• Traceability of Sensitivity [mV/g]

TRANSFERSTANDARD

LASE

R

Primary calibrationWorking standards

Production calibration(approx. 12000/month)


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.


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


Interferometer Theory

• Fringe Counting

PCB Piezotronics Engineer Jing LinAccelerometer Calibration, Laser fringe countingNational Vibration Laboratory, Beijing China, c1970


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


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

=


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


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


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


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


Shaker Construction

• 396C10/C11


Shaker Construction


Shaker Construction: Armature

• 396C10 hardcoat aluminum armature.

• 396C11 beryllium armature. INSERT

Armature body

AC Coil

DC Coil


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


Shaker Construction: Insert

• Removable insert– Off-ground from armature– Shear quartz accelerometer reference.– Variety of mounting threads (10-32, ¼-28, M5)


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


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


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


Shaker Performance: transverse

• 30 gram tungsten mass simulates SUT

• 356A11 triax accel (5% to 10kHz)• Transverse acceleration

zyx 22

100+

=



• 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



• “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


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


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


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]


Performance: rocking


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


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


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


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)


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


Proficiency Testing Low frequency

• 301M26 1 Hz Trend• Change


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


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



• 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



• 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


Data review Sanity checks

• Screen shot


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


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


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)


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


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)


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


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

primary calibration of accelerometers using laser interferometry · 2010. 11. 18. · shaker...

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