Opticsand Lasersin Engineering16 (1992)105—118

Optically Powered and Interrogated Rotary PositionSensorfor Aircraft Engine Control Applications

W. B. Spiliman, Jr., D. H. Crowne

BFGoodrichAerospaceDivision, SimmondsPrecisionAircraft Systems,PantonRoad,Vergennes,Vermont05491,USA

&

D. W. Woodward

Departmentof ElectricalEngineering,Cornell University, Ithaca, New York 14853,USA

(Received7 November1990;revisedversionreceivedand accepted13 March 1991)

ABSTRACT

A throttle levelangle (TLA) sensingsystemis describedthat utilizes acapacitance based rotary position transducer that is powered andinterrogatedvia light from a singlemultimodeopticalfiber. The systemincorporates a unique GaAs device that serves as both a powerconverter and optical data transmitter. Design considerations arediscussed,and thefabrication andperformanceof thesensorsystemaredetailed.

INTRODUCTION

Commercialaircraft now on the drawingboardswill employ fiber opticsensorsto provide benefits in cost, performanceor both.12 Saving incable weight due to the inherent EM! immunity of fiber optictechnologyprovides the strongest impetus for its use in aerospaceapplications.A principal applicationfor the use of electricallypassivefiber optic compatiblesensorswill be to provide datafrom the cockpitto the control unit mounted on the engine. It is essentialthat thecockpit crew be able to maintainenginecontrol evenif other aircraftsystems are disabled. The engine provides electrical power in adedicated and shielded manner to its control unit. All that is thenrequired is an independentdata transmissionand sensing system

105Opticsand LasersinEngineering0143-8166/92/$05.0O© 1992ElsevierSciencePublishersLtd, England. Printedin NorthernIreland

106 W. B. Spiliman. Jr. et a!.

Fiber OpticLink

Fig. 1. Functional diagramof engine/airframeinterface using a fiber optic link.

originatingat the enginecontrol unit andextendingto the cockpit. Theextremely high EMI environment of the engine makes fiber optictelecommunicationan attractivechoice to link the enginecontrol unitwith the cockpit controls. A systemof this type is shown in Fig. I. Inparticular,the useof fiber optics to interrogatethe cockpit throttle leverangle (TLA) position sensoris being pursuedby the major airframemanufacturers.This sensoris requiredto operateover a dynamicrangeof ±600with an accuracyof 0~2%full scale.

A number of pure optical rotary displacementsensorscompatiblewith fiber optic telecommunicationshave beendemonstratedover thepast few years. Early work to develop fiber optic angular positionsensorsinvolved the useof reflective codeplates.3Since that time thereflective code plate concepthasbeenused in combinationwith polar-ization modulation,4time division multiplexing5andwavelengthdivisionmultiplexing!’ Rotatingoptical retardationplateshave alsobeenusedas fiber optic rotary displacementsensors.7’8It is possiblethat one ofthesesensorswill be usedin the first productionapplicationof a fiberoptic TLA sensor.A reflectivecodeplatetype sensorhas alreadybeensuccessfullyflight testedon aBoeing 757 test aircraft.~It is not clear,however,that thesetypesof sensorsrepresentthe optimum solution foraircraft sensingneeds.In general,existingsensorsperform their tasksadmirably, with improvementsbeing desired primarily in the weightand EMI susceptibility of the cabling connecting them to the signalprocessingunits. A possiblecompetingtechnologyto the use of oneofthe new pure optical sensors utilizes so-called ‘power-by-light’ or‘optically powered’ interfaces. Optical power is sent from the signalprocessinglocation to the sensinglocation via an optical fiber. At thesensinglocation, the optical power is convertedto electricalpower andstoredin the power-by-lightinterfaceunit. When a sensorinterrogationsignal is receivedby the interfaceunit, the storedoptical power is used

Optically poweredand interrogatedrotary position sensor 107

Optically PoweredInterfaceUnit

Fiber Optic

Optical Power C~=a~al

________ Sensors

Connections

Fig. 2. Schematicdiagramof a power-by-lightsensingsystem.

to interrogateaconventionalelectrical sensor,digitize its readoutandtransmit digital optical signals back to the signal processinglocation,either via the sameor a secondoptical fiber. A systemof this type isshownin Fig. 2. It doesnot requirethatelectricalpowerbe suppliedtothe sensinglocationandcombinesthe benefitsof fiber optic technology,digital datatransmissionandwell understoodandcharacterizedsensingtechnologyto providebenefitsover existing sensingsystems.

Optical powering and/or interrogation via optical fibers has beendemonstratedfor telephones,’°pneumaticcontrol valves” and forcesensors.’2A three-sensormultiplexedsystemusing power-by-light hasbeenreported,’3andat least one PhD thesis hasbeenwritten on thetopic.’4 In this paper, the design, fabrication and performanceof anoptically poweredrotary displacementsensorbasedupon a novel GaAstransmit/receivechip is described.The sensorwasdesignedto meettherequirementsfor a TLA sensorfor commercialaircraft andutilizes acommercially available capacitance based rotary displacementtransducer.

GENERAL DESIGN CONSIDERATIONS

In order to design and fabricate a successful electrically passiveoptically poweredTLA sensingsystem,a numberof factorsneedto betakeninto account.The transducermust be off-the-shelf, able to meetspecified accuracy and dynamic range requirements,and be able tofunction over specifiedaerospaceenvironmentalranges.The opticallypoweredunit shouldoperateover the sameenvironmentalranges,haveextremelylow powerconsumption,anduseasingleoptical fiber to bothsupplyoptical powerandto transmitdatabackto the signal processinglocation. The use of a single optical fiber mandateseither that sometype of optical coupler is used or that the optical power conversiondevice incorporatesadetectorandsourcethat are co-located.In order

10t~ W. B . .S’pillman, Jr. Ct a!.

to eliminate spurious signals that might occur due to reflections iii asingle fiber system, the optical powering should be interrupted whendataare being transmitted.

Design of a TLA sensingsystemis thereforea matter of straightfor-ward engineeringwith the exception of the optical/electrical powerconversionunit. The lack of availability of such a device has requiredthat it be developed and packagedbefore the TLA sensing systemcould be designed.

POWER CONVERSION UNIT DESIGN AND FABRICATION

Due to its stability overa wide temperaturerangeand the fact that bothopticalsourcesanddetectorscan he easily fabricatedfrom it (due to itsdirect bandgap nature), GaAs was chosen to he the basis of thesemiconductorpower conversionchip. In order that the chip could heusedto powerconventionalelectronicdevices,it was designedso as toproducea voltage output under nominal illumination of >5 volts. Inorder to promote efficient coupling to optical fibers, the optical toelectrical power conversionsegments(detectors)were arrangedsym-metrically aroundthe electrical to optical power conversionunit (LED)as shown in Fig. 3. This structure consists of an array of eightseries-connectedPIN photodiodecells integratedwith a light emittingdiode. The epitaxial layers of GaAs~’~ AlGaAs were grown bymolecularbeamepitaxy on a semi-insulatingsubstrate.Connectiontothe ‘back side’ of the detectorsegmentsand LED wasmadeby etchingportionsof the other layersaway to provide exposedareasof the GaAsn-layer for contacting by n-ohmic metallization. Isolation to enable

~gments(Detec,ors)

Semi-InsulatingSubstrate

Fig. 3. Schematicdiagramof GaAs optica!—electricalpowerconversionchip.

Optically poweredand interrogatedrotary position sensor 109

~u~rlatnceB~

Semi-InsulatingSubstrate

p-ohmic material ____________

n-ohmic material I____________

polyimide

Fig. 4. Detail of serieselectricalconnectionsbetweenphotodiodearray elements.

seriesconnectionof the detectorstageswas accomplishedas shown in

Fig. 4. The designwas implementedin such a way as to maximize thesheet conductivity. This was done by controlling conductivity whilemaintaining a thickness to the limit imposed by the need for litho-graphic patterningof contactsover the isolation grooves. Maximizingthe sheetconductivity was particularly important for the LED whichhas to carry a relatively high current density. The fabrication of theintegrateddeviceswas accomplishedusinga five masklevel process.Inorderof their application,thesefive levelswere: (1) n-ohmic mesaetch,

Fig. 5. Centralareaof power conversionchip.

110 W. B. Spiliman,Jr. et al.

(2) isolation mesaetch, (3) n-ohmic metallizationliftoff, (4) polyimideinsulation etch, and (5) p-ohmicmetallizationliftoff. The centralregionof the device showing the mesastructureof the LED anddetectorsispictured in Fig. 5. The substrate used allowed fabrication of >50devices.The substratewasdicedand the individual devicespackagedinTO-18 standard electroniccomponenthousings to allow them to hecompatiblewith conventionalelectronicfabricationtechniques.

TLA SENSOR SYSTEM DESIGN

The primary concern in designing the TLA system is the need toconserve power provided to the sensor electronics by the opticalinterface device. Thus, design of the sensor electronics involvednumerousconstraints,while the systemcontroller andsignal processingdesign were quite straightforward. The system controller providesoptical power to the sensorand receivesdigitally encodedinformationfrom the sensorovera single optical fiber. As shown in Fig. 6. opticalpowerto the sensoris periodically interrupted.This power interruptionsignals the sensorto measure,encodeand transmit the current thrustlever angle. A 1 mW, 780nm semiconductorlaser was chosenas theoptical source.This providesoptical power near the responsivitypeakof the power conversionunit. Optical power to the sensorwas limitedto 1 mW to minimize the visual exposurehazard in the event of abrokenfiber or removal of a connector.Figure 7 depictsthe essentialelementsof the TLA sensor. A detection circuit within the sensordetermineswhen optical power from the system controller has beenremoved.An FET switch then enablesthe shaft position measurementandencodingcircuitry.

The shaft position transduceris a rotatingdifferential capacitor.Thisdevicewaschosenfor its simplicity, inherentaccuracyandcompatibility

Tpower off ~Po~veroff

~Cyclcoil

Optical Powerfrom Controller

off

~: T ‘Data ~

- oniLA Opticall)ata From Sensor

off

Fig. 6. Poweranddata timing.

Optically poweredand interrogatedrotary position sensor 111

Energy PowerStorage Switch ~

Electro ducer

Fiberoptic jn~e Encoder

Fig. 7. TLA sensorblock diagram.

with ultra low power measurementtechniques.The capacitorconsistsof a circular common plate, two semicircularplates and a grounded,semicircularcopper shield rotating in the air gap between. This isshownschematicallyin Fig. 8. Capacitancebetweenthe commonplateand either of the split plates is dependentupon the unshieldedarea.The sum of the capacitanceof both plates is a constant,since totalunshieldedareais invariant. Angular position information is obtainedby measuringthe capacitanceof both plates,C, and C2

U (degrees)=90 ± (1)

Figure 9 depicts the transducermeasurementcircuitry. The commonplate of the differential capacitor is excited with a voltage ramp(constant current source). Both split plates are incorporated intoidentical differentiator circuits. During a measurementcycle, eachdifferentiator circuit reachesaDC level proportional to the unshaded

MovableGroundedSemi-circularShield

0

nspu,~th FixedCommon

FixedSplit Plate withCapacitanceC2

Fig. 8. Capacitiverotary position transducer.

112 W. B. Spiliman,Jr. et al

P’ate ..J3,,~ ~ (~dV ~ 1‘\~ ~

~~ilage Ramp [ ____ IGenerator ~ ~lo Irarlrmlttor

vi S

Fig. 9. TLA sensormeasurementcircuitry.

capacitanceof thatplate. By taking the sum,difference andnegativeofthe sum of thesetwo voltagesandcomparingthem to the voltagerampdriving the transducer, three timing pulses are generated.This isdepictedin Fig. 10. The periodbetweenthe first (t1) and last (t~)pulsesis proportional to the measurementrangeof the sensor.The timing ofthe middle (t2) pulse is dependentupon shaftposition. Shaft position isthencalculatedfrom

2t~— t —

O (degrees)= 90 - (2)

V( C- -~C - i Riii1i Input

controller

Time

Fig. 10. TLA sensorposition encoding.

Optically poweredand interrogatedrotary positionsensor 113

Since the measurementcircuitry relies on an unregulatedDC powersupply (a bank of capacitors), the measurementtechnique must beindependentof power supply voltage variation betweensensorinter-rogation cycles and the decreasein power supply voltage that occursduring eachsensorinterrogation.The sensingtechniqueusedtoleratessuch changesby allowing the slope of the voltage ramp driving thedifferential capacitorto vary as a function of power supply voltage atthe beginning of an interrogation cycle. Slope linearity, however, iscritical to sensoraccuracyanddesignefforts concentratedon this issue.

ELECTRO-OPTICINTERFACE PERFORMANCE

Evaluation of the photodiodeportion of the electro-optic interfacechips consistedof two separatetests. First, open-circuit voltage andshort-circuitcurrent were measuredwith respectto increasingincidentoptical power. Second,voltage andcurrentwere measuredfor increas-ing load resistancewhile holding the optical input power fixed. Figure11 shows the open-circuitvoltageandshort-circuitcurrent plotted as afunction of incident optical power. Figure 12 is a plot of photocellvoltage versus current for an increasingresistive load under constantpower optical illumination. The input optical power was changedindiscretestepsto generatethe family of curves.Figure 13 presentsthesedata in a slightly different manner. Electrical power out is plottedversus terminal voltage for an increasing load. This indicates thedesiredoperatingpoint to achievemaximum photoelectricconversionefficiency at a given optical power input. The maximum powerconversionefficiencyfor a typical chip is about8%.

I sho~~rcuit::~c~u1t IShonCircuit : v ~~)Ci~uit

0 200 400 600 800 1000 1200Incident Optical Power

(‘aW)

Fig. 11. Short-circuit current and open-circuit voltage versusincident optical power.

114 W. B. Spiliman, Jr. et al.

30

25 IncidentOptical Power

1 20lILA) iS

V (Volts I

Fig. 12. Current versusvoltagefor increasingincidentoptical power.

Of the 60 chips taken from the first wafer, 33 were judged usable.Acceptancecriteria were an open-circuitvoltagegreaterthan 5 V andshort-circuitcurrent greaterthan 20 1uA when the device was illumin-atedwith 1 mW of optical power from a200/240~m optical fiber. Forthe usable devices,the mean open-circuit voltage was 5~97V with avarianceof 023 V. The meanshort circuit current was 24~7/AA with avarianceof 2~43ptA.

The LED portion of the power conversion chip was tested byapplyinga 5 V. 1% duty cycle pulse train acrossthe deviceandaseriesconnected50-Q resistor. Emittedoptical power was measuredusing acalibratedphotodetector.Typical optical power from the LED portionof the device was 80 ~iW. Peak wavelengthof emissionwas 890 nm.Mean responsivity of the devices tested was 5~8/1W/mA with avarianceof 0~8~.tW/mA.

120

IncidentOptical Power100 - ~~l200pW

(10 - 7 ~ 600 iW300~W

Electrical Power e ~ ; .

OpenCircuit Voltage (Volts)

Fig. 13. Electricalpower versusvoltagefor increasingincidentoptical power.

Optically poweredand interrogatedrotary position sensor 115

TLA SYSTEM PERFORMANCE

Figure 14 shows a benchtest prototypeTLA sensorsystemcurrentlybeing evaluated by an airframe manufacturer in preparation foreventual flight testing. This system consists of a controller/signalprocessor,a TLA sensor,andseveralmetersof 200/240micron, stepindex fiber. The system was characterizedwith respect to accuracy,performanceat temperatureextremes,andsensorpowerconsumption.

Sensor power budget calculations indicate that 15 1IJ (electricalenergy) is consumedduring an interrogation cycle at 125°C. Thisagreesclosely with observedsystemperformance.A systemsamplingrate of 5 Hz was thus establishedto provide adequateelectricalpowerto the sensorelectronicsunder worst casetest conditions.The systemhas been operated at a 20 Hz rate at room temperaturewith nodegradationin accuracy.

Systemaccuracywas determinedby coupling the sensorshaft to acalibratedBE! rotary encoderaccurateto 0~01°.Angular positionwasincrementedin 5° steps over the range —60° to +60°.An angularposition display built into the system controller was monitored andcomparedto the displayedoutput from the BE! encoder. Figure 15indicates that TLA system accuracyat room temperaturewas betterthan±0~2°over the measurementrangeof the sensor.

In order to determinethe performanceof the sensorover tempera-ture, the input shaft was locked in position at 0°and the sensorwasplaced in an environmentalchamber. Displayed angularposition was

Fig. 14. BenchtestTLA system.

116 W. B Spit/man.Jr. et a!.

A~lar ~ ~EIIIIEII~IIIIIIIIIIIIIIII~2 Lrror

degreesi

- 6(1 4(1 -2(1 (I 2(1 4(1 (11 -

MeasuredPosition(deprces(

Fig. 15. InferredpositIon versusmeasuredposition -

monitored while the temperatureof the chamberwas adjustedfrom—55°Cto + 125°Cin 10°Csteps. Figure 16 indicates that displayedangularposition varied by ±0~05°.

DISCUSSION AND SUMMARY

Once the integratedpower conversion unit had beendesigned.labri-cated and packaged,the designof the optically poweredTLA sensingsystemwas reasonablystraightforward.The optical to electrical powerconversionefficiency of —8%. however,is abouta factorof threelowerthancan he obtainedusingGaAs.5 It is presentlyfelt that the reducedpower conversionefficiency of our devicesis due to two factors. First.the devices as fabricatedhaveno antireflectioncoating. In addition, asignificant fraction of the chip is taken up by mesaetchingregionsand

((.2 —

11.4

• Shalt Angle

0.2

AngularPosition 0.0 • • • •(degrees) — -

-((.2

-((-3

-0.4((5 .

-60 -40 -20 (1 211 40 6(1 8(1 l(Xl 120Temperature(°C

Fig. 16. Changein measuredposition with temperature.

Optically poweredand interrogatedrotary position sensor 117

metallic contacts,thereby reducingusable detectorarea. Future ver-sionsof the chip will be fabricatedso as to increasepower conversionefficiency. Sensingsystemswith satisfactoryperformance,however,canbe designedand built without higher efficiency devices, as we havedemonstrated.To improve upon the demonstrationsystem, the nextstepsshouldbe (1) to producea higherefficiencyof optical to electricalpowerconversion,and (2) to implementthe remainderof the electronicdesignwith a single ASIC (applicationspecific integratedcircuit) chipplus a few discrete components.The size of the optically poweredinterfaceunit could thenbe greatlyreducedandits reliability enhanced.

To summarize,we havedesigned,fabricatedand testedan opticallypowered and interrogatedangular displacementsensing system thatutilizes a unique opto-electricalpower conversion chip. The systemdemonstratedadynamic range of 120°and an accuracyof —0•2% fullscale. No significant measurementerror was observed when thetemperaturewas varied over the range —55°Cto + 125 °C.Althoughthe target application in this case was throttle lever angle positionmeasurement,in principle this technology could be applied to anyelectrically based sensor. It then offers an attractive method forobtainingthe benefitsof EMI immunity and weight reduction offeredby fiber opticswhile continuingto utilize reliableandwell characterizedexisting sensortechnology.

ACKNOWLEDGEMENT

MBE growth of wafers for this project was done by W. Schaff ofCornell University. GaAs device fabricationwas accomplishedin theNational NanofabricationFacility at Cornell.

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118 W. B. Spiliman, Jr. et al.

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