Design and fabrication of a fiber-optic interferometric accelerometer system.

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by  Chipkevich, Mary Beth A.;Gardner, David L.;Brown, David Alan.

"

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Naval Postgraduate School (ifapplicable) 33 Naval Postgraduate School
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iAiCTiCtleE(EinEclRudOesMecFurTityFcRlassSifYicSatTionE)MDESIGN AND FABRICATION OF A FIBER-OPTIC INTERFEROMETRIC
2 PersonalAuthor!si Man BethA. Chipkevich
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Master'sThesis From To March 1992
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rtionofthe Department ofDefenseorthe U.S. Government.
CosatiCodes 18SubjectTermsicontinueonreverseifnecessaryandidentifybyblocknumber;
Group Subgroup Fiberoptic, Interferometric. Mach-Zender, Flexural Disk
19AbTstwroactfiibcoenrt-ionputeiocnarcevceerlseerifonmeecteessrardyeasnidgnidsentairfeybpyreblsoecnktneudm.berT)he first accelerometer was constructed using two flexural disks
between which is a center spindle. Six uniformly spaced screwsfasten the disk assembly to acircular, hollow base housing
fiber-opticcouplers and splicesand providingfiberaccess. The disk edgeiselasticallyrestrained. Aflat spirallywoundcoil
ofoptical fiberisbonded to one surfaceofeachdisk. Disksurfacestrains,causedbyacceleration,aredetectedinapush-pull
fashion by fibercoilscomprisingthelegsofa Mach-Zenderopticalinterferometer. Thissensordemonstratedanacceleration
sensitivityof2.3 + 0.1 rad goverabandwidthof 150-475 Hz. Theresonancefrequencywas 1.22 ±0.05kHz.
Thesecondaccelerometerdesigndescribedexploitsthebenefitsofusingadiskmaterialhavingasignificantlyslowersound
speedthanaluminum; in general,aslowersound speedincreasesthe acceleration sensitivity and decreasestheresonance fre-
quency. One polycarbonate disk with a simply supportededge condition isfastened betweenanaluminumcapandanalu-
minum housingbase secured with six uniformly spaced screws. One flat fibercoil in each Mach-Zender interferometer leg
was bonded to each disk surface. This sensordemonstratedanacceleration sensitivity of56.9+ 4.0rad/goverabandwidth
of30-950 Hz. Theresonancefrequency was 1.7 + 0.05kHz.
Both sensors are compatible with the symmetric 3x3 demodulator developed at the Naval Postgraduate School. The
second sensoranddemodulatorhave been integratedandconstituteasensors>stem,but testinghasnot beencompleted.
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Design and Fabrication ofa Fiber-Optic Interferometric
Accelerometer System
by
Mary BethA. Chipkevich
Lieutenant ,United States Navy
B.S., United States NavalAcademy, 1984
Submitted in partial fulfillment ofthe
requirements forthe degree of
MASTER OF SCIENCE IN ENGINEERING ACOUSTICS
fromthe
NAVAL POSTGRADUATE SCHOOL
March 1992
ABSTRACT
Two fiber-optic accelerometer designs are presented. The first accelerometer was
constructed using two fiexural disks between which is a center spindle. Six uniformly
spaced screws fasten the disk assembly to a circular, hollow base housing fiber-optic
couplers and splices and providing fiber access. The disk edge is elastically restrained.
A fiat spirally wound coil ofoptical fiber is bonded to one surface ofeach disk. Disk
surface strains, caused by acceleration, are detected in a push-pull fashion by fiber coils
comprising the legs ofa Mach-Zender optical interferometer. This sensordemonstrated
an acceleration sensitivity of2.3 ± 0.1 rad g over a bandwidth of150-475 Hz. The res-
onance frequencywas 1.22 ±0.05 kHz.
The second accelerometer design described exploits the benefits ofusing a disk ma-
terial having a significantly slower sound speed than aluminum; in general, a slower
sound speed increasestheacceleration sensitivity anddecreasestheresonancefrequency.
One polycarbonate disk with a simply supported edge condition is fastened between an
aluminum cap and an aluminumhousing base securedwith six uniformly spaced screws.
One fiat fiber coil in each Mach-Zender interferometer leg was bonded to each disk
surface. This sensor demonstrated an acceleration sensitivity of56.9 + 4.0 rad'g over a
bandwidth of30-950 Hz. The resonance frequency was 1.7 + 0.05 kHz.
Both sensors are compatible with the symmetric 3x3 demodulator developed at the
Naval Postgraduate School. The second sensor and demodulator have been integrated
and constitute a sensor system, but testing has not been completed.
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ft
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TABLE OF CONTENTS
INTRODUCTION
I. 1
A. SCOPE 1
B. CONVENTIONAL ACCELERATION SENSORS 1
1. Background 1
2. Conventional PiezoelectricAccelerometers 2
3. Conventional Piezoresistive Strain Gage Accelerometers 5
4. Conventional Strain Gage Accelerometers 6
C. FIBER-OPTIC INTERFEROMETRIC SENSORS 6
1. Michelson Fiber-Optic Interferometer 7
2. Mach-Zender Fiber-Optic Interferometer 8
3. Fiber-Optic Accelerometers 9
II. MECHANICAL THEORY OF FLEXLRAL DISKS 11
A. BOUNDARY CONDITIONS OF THIN DISKS 11
1. Simply Supported 12
2. Clamped 12
3. Free 12
4. Flastically Restrained 12
5. Comparisons Between Boundary Conditions 12
B. SURFACE STRAIN DEFORMATION OF THIN DISKS 14
1. Simply Supported 15
2. Clamped 15
RESONANCE FREQUENCY
C. 16
1. Simply Supported 16
2. Clamped 16
3. Diskwith Stepped Thickness and;orAdded Mass 17
a. Radius and Mass Ratio's Approach 18
b. Continuity ofZones Approach 21
c. Ritz Method Approach 22
D. PHASE MODULATIONS INDUCED IN OPTICAL-FIBERS 24
1. Optical Fiber Strain Induced by Flexural Disk Strain 24