(c)2000 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization.

AfAA AOO-36405

AIAA 2000-3136High Temperature Fiber-Optic PressureSensor with Built-in Temperature Sensor

S. Hachiya, T. lino and F. Kohsaka

Research Institute of Advanced Material Gas-GeneratorTokyo, JAPAN

36th AIAA/ASME/SAE/ASEEJoint Propulsion Conference & Exhibit

17-19 July 2000Huntsville Alabama

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(c)2000 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization.

AiAA-2000-3136

HIGH TEMPERATURE FIBER-OPTIC PRESSURE SENSORWITH BUILT-IN TEMPERATURE COMPENSATION

Satoshi HACHIYA, Toshio UNO and Fusao KOHSAKA

Research Institute of Advanced Material Gas-Generator Co.,Ltd1-13-4 Kita-Ohtsuka, Toshima-ward, Tokyo 170-0004

Phone: +81-422-52-5663Fax: +81-422-52-5946

E-mail: Satoshi_Hachiya@yokogawa.co.jp

ABSTRACTA high temperature fiber-optic pressure sensor for

gas turbine has been researched. The sensor system isbased on Fabry-Perot interferometer with built-intemperature compensation. This sensor is designed tooperate at temperature up to 350°C and to measurestatic pressure. Differing from dynamic pressuresensors, static pressure sensor should obtain an exactpressure value each time the measurement performed.Therefore the sensor must have low drift andinsensitivity to environmental changes other thanpressure.

Since this sensor has a wide range of operatingtemperature, temperature error is the most importantproblem. The temperature error of the sensor isdesigned to be reduced. Residual error is compensatedby temperature sensor.

Pressure is measured by interferometry that iscomposed of metal diaphragm and an optical fiber.Temperature sensor also has been fabricated fromanother optical fiber and in the same sensor head.

The evaluation tests under pressure 0 to 6 MPa andtemperature -50 to 350 °C showed good results. Atroom temperature, pressure non-linearity exhibits+0.32% / -0.35% of full scale (FS) pressure.

Concerning the temperature characteristics,temperature error with compensation showed +2.5% /-2.6% of FS pressure.

Copyright (c) 2000 by The American Institute ofAeronautics and Astronautics, Inc. All rights reserved.

INTRODUCTIONRecently a gas turbine engine that reduces the cost

of operation and influence on the environment hasbeen desired. Especially in future, for low fuelconsumption, the gas-turbine engine operated underhigher temperature and more advanced control isexpected. In addition, low NOx emission isindispensable for that engine. To solve these problemsthe engine should be controlled by newly sensedparameters, therefore novel sensors that can measurethe state of gas-turbine more precisely and/or the statebeing immeasurable previously are needed. Thismeans that, in the field of the measurement, sensorswith high accuracy and/or operated under harshenvironment are required.

In Japan, the Research Institute of AdvancedMaterial Gas-Generator (AMG), which is a joint effortby the Japan Key Technology Center and 14companies, has been conducting technological studieson an innovative gas generator that will use advancedmaterials. The research and development themes inthis program were generated based on three majortargets, which were 70 to 80% reduction of NOxemission, 20% less fuel consumption and 50%reduction of weight compared with the conventionalgas generator l.

In this gas generator, operation under hightemperature and high pressure is indispensable, andthis results in applying of the composite materials.Therefore, the novel sensor that can reduce weight ofwire-harness, operate under high temperature andprovide immunity to electromagnetic interference(EMI) is needed in order to accomplish the major threetargets.

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OBJECTIVESAMG has various research themes. They include the

research of innovative sensor technology applied to thegas turbine engine.

In order to realize the advanced engine control, thesensor target was set for accurate measurement ofstatic pressure in high pressure compressor. Whiledynamic pressure sensors measure only the amount ofpressure change, static pressure sensors should detectthe exact value of pressure each time. To achieveaccurate measurement of static pressure, drift ofmeasurement and temperature error should be reduced.

Considering the above-mentioned requirements,that is: reduced weight, operation under hightemperature and immunity to EMI: fiber optic pressuresensor was adopted.

Fiber-optic sensor is very well suited formeasurements in advanced material gas generator. Itcan potentially operate at high temperature and EMIenvironment. In addition, by using an opticalinterference measurement, extreme highly accuratemeasurement is possible.

Target specifications of pressure sensor weredetermined as table 1.

Tablel Target specification

Pressure range

Operating temperatureNon-linearity

Temperature error (zero)Temperature error (span)

0 to 6 MPastatic pressure-55 to 350 °C

+ 0.30%ofFS pressure±1.0%ofFS pressure±1.0%of FS pressure

Attention should be paid to the following points.•Wide temperature range.• Small non-linearity in spite of wide temperaturerange.

• Small temperature error.

SENSOR DESIGN

Sensor system descriptionThe fiber-optic pressure sensor system is designed

as shown in Fig. 1. To satisfy the target specificationand to keep robust to the perturbations in the fiber,Fabry-Perot interferometer was adopted. The lightfrom laser diode travels in a single mode fiber (SMF),

and reflects at the sensor head. The sensor headconsisted of metal diaphragm and metal housing.These construct extrinsic Fabry-Perot interferometer.By employing the extrinsic interferometer, reductionof temperature error is possible. Optical signal isdetected by photo diode.

Applied pressure was measured by the phase changeof interference wave. Using this method, moreaccurate sensor system can be realized than the systemdetected by optical intensity. Among the sensorsmeasured by optical intensity there were microbendpressure sensor and reflective sensor 2>3. Such sensorsrequire the means of intensity calibration becauseintensity changes by aging, connection of fiber andcircumstance of the light source.

On the other hand, concerning the measurement bythe interferometer, it is impossible to determine thedirection of pressure change by detecting only phasechange 4. In this sensor, however, modulating thewavelength of the light source makes it possible todetect the pressure change direction. The interferencebeat is inducted by the modulation even under constantpressure, so direction is simply detected by observingthe phase shift of beat wave.

An 850nm DBR-LD (Distributed Bragg Reflector-Laser Diode) was used as a light source, because it waspossible to modulate the wavelengh widely andcontinuously 5.

Sensor headFig.2 shows a fabricated sensor head of the pressure

sensor. The sensor head consists of three parts: metalhousing, metal diaphragm and fiber assemblies. Thesensing cavity is formed between the Fresnel reflectionat the cleaved end of SMF and reflective diaphragm.

In measuring the phase change, it is impossible tospecify the real phase change in case that amount ofphase change exceeds 2n. For example, if real phasechange was 3n, it is impossible to distinguish n, 3rc and5it, etc. This means that in starting measurement at anypressure value, it is possible to measure pressurechange from starting value but impossible to determinethe static pressure value. Target of this sensor ismeasurement of static pressure value in any time.

For high accurate measurement under wide pressurerange, two probes were used as shown in Fig.2.

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Modu-lation

\Phase

detector

Laserdiode

controller

Band-passfilter

Laserdiode

Cc

Photodiode

Signal processing circuit

Opticalcoupler

Optical

Fig.l Block diagram of sensor system. The light from laser diode travels in an optical

fiber and reflects at sensor head. Returned light is detected at photo diode.

Absoluteprobe (AP)

Pressureprobe (PP)

Temperatureprobe (TP)

SUS flange

Zirconiaferrule

Metalhousing

Optical fiber

Metaliaphragm

Fig.2 Sensor head. Extrinsic Fabry-Perot interferometer is made between cleaved end of

fiber and reflective diaphragm. Li: housing length, Ls: flange length, Lz: ferrule length.

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Approximate pressure value was measured byabsolute probe (AP) and exact value was measured bypressure probe (PP)6.

4m3(l-u2)a4

X 16Eh3 (4)

Sensor head configuration is described by a flatcircular diaphragm clamped at its circumference.Deflection at center of the diaphragm is indicated asEq.(l).

d 3(1 -u2)a' (1)16 Eh*

whered diaphragm deflection,P pressure,« Poisson's ratio,E Young's modulus,a diaphragm radius,h diaphragm thickness.

On the other hand, the phase of interference wavedetected by photo diode is given as Eq.(2).

8;r/m A vnL 4jinL(2)c A

= cot + <t>where

fm modulation frequency,d v modulation amplitude of light frequency,n cavity refractive index,L cavity length,c light velocity,A light source wavelength.

Applied pressure makes the change of cavity length,so pressure can be measured by detecting the change ofthe second term of Eq.(2). The change of cavity lengthequals to the deflection of the diaphragm, so thechange of & is given by Eq.(3) and (4).

A0-A

4mdA

(3)

As a result, phase change is proportional to pressurevalue.

Temperature sensorTemperature probe is shown in Fig.3. Fiber intrinsic

Fabry-Perot interferometer was constructed betweenreflection at splice end and cleaved end 7. The changeof cavity length due to temperature makes the phasechange. As similar to Eq.(3), the phase change ofinterference beat is described as following:

(5)

where<t>T phase change of temperature sensor,na refractive index of glass fiber,LT cavity length of temperature sensor.

By differentiating Eq.(5),

dL, T dncA dT dT (6)

Eq.(6) shows that the temperature value is measuredby the phase change and that the amount of the phasechange can be controlled by cavity length LT.

Fiber jacket Optical fiber

S p l i c e d C leaved

Fig.3 Temperature probe.

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Heat resistanceTo operate under high temperature (350 °C), heat

resistance of the sensor head is necessary. Thefollowing items should be taken into consideration:structure and material of the sensor head, heat-prooffixed method of the optical fiber and heat resistance ofsurface of the reflective diaphragm.

As mentioned previously, interference is madebetween the reflection at cleaved end of optical fiberand reflective diaphragm. Therefore, the displacementof the cleaved end of the fiber under high temperatureleads to the drift of measured value. Since phasechange 2it equals to 425nm change (that is a half oflight source wavelength) of the cavity length as shownin Eq.(3), even a slight displacement is not allowed.

Concerning diaphragm, not to reduce the reflectiveintensity, the surface was coated by heat proof plating.

Temperature errorTarget Specification of temperature error is ± 1% of

FS pressure as shown in Table 1. To accomplish thishigh target specification, the following means wastaken: the reduction of temperature errors by thedesign of the structure and compensation by built-intemperature sensor.

Temperature errors are divided into two factors:temperature error at pressure span and temperatureerror at pressure zero. The former means the change ofdeflection sensitivity of the diaphragm due to thechange of Young's modulus and the thermal expansion.The latter means the change of optical path length dueto the change of the refractive index and the thermalexpansion in the cavity.

In general, Young's modulus of a metal becomessmall with high temperature. Therefore, the diaphragmdeflection becomes large. This leads to temperatureerror at pressure span. However, it is possible to reducetemperature error by selecting suitable metal as adiaphragm material. In this sensor head Inconel alloywas adopted.

On the other hand, the optical path of the cavity alsochanges owing to change of cavity refractive index andthermal expansion. This leads to temperature error atpressure zero. This is given below by differentiatingEq-(3),

d<f) 4jm dL 4nL dn~dT~~^~df*~^llT (7)

where the first term is dominant in the phase change.The second term is the change of the refractive index inthe cavity. This value is known in case the cavity isfilled by air7. Cavity expansion is designed as,

dL r 7 r— = aiLi+asLs-azLzdT (8)

wherecei expansion coefficient of inconel housing,

J housing length,as expansion coefficient of stainless steel flange,

flange length,z expansion coefficient of zirconia ferrule,

ferrule length.

La

Lz

Selecting the adequate parameters of Eq.(8) reducesthe first term of Eq.(7).

LABORATORY TEST RESULTSSensor system shown in Fig. 1 and sensor head

shown in Fig.2 were fabricated and laboratory testswere performed.

Signal waveformThe beat signal wave shown in Fig.4 was observed.

This waveform was the output of the band-pass filter inthe block diagram in Fig.l. The beat signal was a sinewave synchronized with the modulation frequency (atriangular wave). The phase shifts subject to thepressure change. The phase change is measured bytime lag to the modulation wave.

interferencewave

Fig.4 Beat Signal waveform.

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Pressure characteristicsFig.5 shows the phase change applied 0 to 6 MPa

pressure at room temperature. A linear relationshipbetween pressure and phase change was exhibited inboth pressure probe (PP) and absolute probe (AP).

1 2 3 4 5Applied pressure [MPa]

Fig.5 Phase change versus pressure.

Temperature sensorThe principle of the temperature sensor is shown in

Eq.(6). The second term of Eq.(6) in the used fiber wasunknown, so the temperature sensor was fabricatedunder cavity length Lr=0.55mm. Measured data isshown in Fig.7, From this result, the temperaturecoefficient of refractive index (dnG/dT) wasdetermined +10.6 ppm/°C. The result exhibited aninsignificant non-linearity but it agreed with designedvalue. This is due to non-linearity of expansioncoefficient of glass fiber.

By calculating from this result, temperatureaccuracy was ±5°C. Strictly speaking, since the phasechange exceeds 2it in Fig.7, temperature measurementis impossible by detecting only phase change. In thiscase, however, the principal purpose of this evaluationtest was to confirm the principle of temperaturesensing and temperature compensation. The result waswell agreed with designed value shown in Eq.(6) andthe principle of temperature sensing was confirmed.

Fig.6 shows a non-linearity of pressure probe (PP),which determine the accuracy of sensor system. Goodresults were obtained as followings: non-linearity+0.32%/-0.35% of full scale pressure, hysteresis0.22% and repeatability 0.30%.

-50 50 150 250 350Temperature [°C]

Fig.7 Phase change versus temperature.

Applied pressure [MPa]

Fig.6 Non-linearity of pressure probe.

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Temperature error at pressure spanPhase sensitivity of pressure probe was observed

with various ambient temperatures. The sensitivitymeans the amount of phase change when 1 MPapressure applied. Fig.8 shows the sensitivity at eachtemperature. The sensitivity was almost constant in thewhole range of temperature. The change of sensitivitywas only 3% of full scale pressure even withouttemperature compensation. This result was smallenough compared with the temperature error atpressure zero.

o.

0.8

0.6

0.4

0.2

50 150 250Temperature[°C]

350

Fig.8 Pressure sensitivity change with temperature.

Fig.9 Phase change with temperature under constant

i -3.0 ^ Measured ——— Designed

-5.0-50

pressure.

50 150 250Tempe ratu re [°C3

350

Temperature compensationAs mentioned previously, temperature error at

pressure zero required compensation by built-intemperature sensor. Compensated temperature error atpressure zero is shown in Fig. 10. The result oftemperature error was +2.5%/-2.6% of full scalepressure.

This result exceeded the target specification,however, it was considered to be good in comparisonwith the conventional sensors.

Moreover, the result can be reduced by selectingadequate parameters in Eq.(8).

Temperature error at pressure zeroThe phase change with change of ambient

temperature under constant pressure is shown in Fig.9.The amount of phase change was ±3.2 [x2it rad] andthis value was ±59% of full scale pressure. This resultwas not a small value but showed good agreement withdesigned value shown in Eq.(7). Since phase changecan be reduced by selecting parameters in Eq.(8),reduction method of temperature error was confirmed.

To achieve the target specification of temperatureerror, compensation by built-in temperature sensor isnecessary. The result of compensation is given in thenext section.

£ to

£ w<D LJLa. u.£ °<a 2

10 I-""

-5

-10

--H-HH-50 50 150 250

Temperature[°C]

350

Fig.10 Temperature error of zero after compensation.

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CONCLUSIONSA fiber optic pressure sensor based on Fabry-Perot

interferometer with built-in temperature compensationhas been developed. Evaluation tests were performedat static pressure up to 6 MPa and temperature to350°C.1) Pressure characteristics:

It was possible to measure 0 to 6 MPa static pressurevalue. At room temperature, sensor linearity,hysteresis and repeatability of pressure probe overthe full pressure range were +0.32%/-0.35%, 0.22%and 0.30% of full scale pressure, respectively.

2) Temperature sensor:The temperature test was performed at range of-50to 350 °C . Temperature sensing accuracy oftemperature sensor was ±5°C. The principle oftemperature sensing was confirmed.

3) Temperature errors:There was no problem in operation of sensor headunder high temperature. Temperature error atpressure zero was +2.5% / -2.6% of FS pressure,which exceeds the target specification. However, theresult was considered to be good in comparison withthe conventional sensors.

In future, the following two items will be tackled.•To minimize the temperature error at pressure zero byselecting suitable parameters in Eq.(8).

•To reduce the phase change of temperature sensorwithin 2n.

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Material Gas-Generator Research andDevelopment Project", Proceedings of theYokohama International Gas Turbine Congress,1, pp.203-210,1995

[2] J.W.Berthold, W.L.Ghering, and D.Varshneya,"Design and Characterization of a HighTemperature Fiber-Optic Pressure Transducer",Journal of Lightwave Technology, Vol.LT-5,No.?, pp.870-875,1987

[3] T.Poorman, S.Kalashnikov and M.T.Wlodarczyk, "Commercially Available Low-Cost Fiber-Optic Combustion Pressure Sensor",Proceedings SPIE, Vol.2510, pp.194-204,1995

[4] W.N.Macpherson, J.M.Kilpatrick, J.S.Bartonand D.C.Jones, K.S.Ghana, J.S.Anderson,T.VJones, D.R.Buttsworth, "Miniature fibreoptic pressure sensor for high resolutionmeasurements in turbomachinery applications",Proceedings SPIE, Vol.3483, pp.200-204,1998

[5] T.Hirata, M.Maeda, M.Suehiro andH.Hosomatsu, "Fabrication and Characteristicsof GaAs-AlGaAs Tunable Laser Diodes withDBR and Phase-Control Sections Integrated byCompositional Disordering of a Quantum Well",IEEE Journal of Quantum Electronics, Vol.27,No.6, pp.307-313,1991

[6] T.Ueda, F.Kohsaka, T.Iino, K.Kazami,H.Nakayama, "Optical Absolute Encoder UsingSpatial Filter", Proceedings SPIE, Vol.814,pp.217-221, 1987

[7] C.E.Lee, H.F.Taylor, A.M.Marks and E.Udd,"Optical-fiber Fabry-Perot Embedded Sensor",Optics Letters, Vol.14, No.21, pp.1225-1227,1989

[8] G.W.C.Kaye and T.H.Laby, "Tables of physicaland chemical constants and some mathematicalfunctions", Longman, London. 1986

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