NASA Technical Memorandum 104454

Fiber-Optic Sensors for AerospaceElectrical Measurements:An Update

R.L. PattersonLewis Research CenterCleveland, Ohio

and

A.H. Rose, D. Tang, and G.W. DayNational Institute of Standards and TechnologyBoulder, Colorado

Prepared for the26th Intersociety Energy Conversion Engineering Conferencecosponsored by the ANS, SAE, ACS, AIAA, ASME, IEEE, and AIChEBoston, Massachusetts, August 4-9, 1991

NASA

https://ntrs.nasa.gov/search.jsp?R=19910016067 2020-06-06T06:20:10+00:00Z

FIBER-OPTIC SENSORS FOR AEROSPACEELECTRICAL MEASUREMENTS: AN UPDATE

Richard L. PattersonNASA Lewis Research Center

Cleveland, Ohio 44135

A. H. Rose, D. Tang and G. W. DayNational Institute of Standards and Technology

Boulder, Colorado 80303

ABSTRACT

Fiber-optic sensors are being developed for electricalcurrent, voltage, and power measurements in aerospaceapplications. These sensors are presently designed tocover ac frequencies from 60 Hz to 20 kHz. Thecurrent sensor, based on the Faraday effect in opticalfiber, is in advanced development after some initialtesting. Concentration is on packaging methods andways to maintain consistent sensitivity with changes intemperature. The voltage sensor, utilizing the Pockelseffect in a crystal, has excelled in temperature tests.This paper reports on the development of these sensors.It also relates the technology used in the sensors, theresults of evaluation, improvements now in progress, andthe future direction of the work.

in preparation. The advanced development of thissensor will concentrate on packaging methods to im-prove the temperature stability.

A prototype optical voltage sensor has been con-structed and tested. The sensor has excelled in thetemperature tests. Presently the voltage sensor is beingmodified to reduce sensitivity to vibration.

In this paper we will report the progress made onthe development of aerospace current and voltagesensors which use fiber-optic and optical sensing heads.We will describe the technology used in the sensors, theresults of evaluation, improvements now in progress, andthe future direction of the work.

2. ELECTRICAL CURRENT SENSOR1. INTRODUCTION

Fiber-optic sensors that measure electrical current,voltage, and power have many advantages over conven-tional sensors. They are relatively immune to EMI,have wide bandwidth, low mass, and excellent isolation.They also will not fail during over-voltages or current-surges that would normally damage a conventionalsensor.

Fiber-optic sensors developed for aerospace appli-cations are designed to be broadband and accurate forac frequencies as low as 60 Hz and as high as 20 kHz.This will allow use in 400 Hz aircraft systems, future 20kHz spacecraft systems (such as electro-mechanicalactuators or Advanced Launch Systems), and 60 Hzterrestrial systems. They are also designed to be stableover broad temperature ranges (-65°C to +125°C).

A prototype fiber-optic current sensor has under-gone significant testing and has operated successfully athigh vibration levels.[1] A second-generation device is

Technoloy

Figure 1 is a schematic diagram of the electricalcurrent sensor. The sensor uses the Faraday effect in anannealed coil of single-mode optical fiber through whichthe current carrying conductor passes.[2] The Faradayeffect is a rotation of the plane of polarization of lightas is propagates through a material in the direction of amagnetic field. The rotation of the plane of polarizationis proportional to the current flowing through theconductor. Multiple turns of fiber increase the sensi-tivity of the sensor.

A polarization maintaining (PM) fiber transportslinearly polarized light from the laser diode source tothe sensing coil. Another PM fiber transports the lightexiting the sensing coil to a polarizing beam-splitter andsensing photo-diodes. These optical elements convertthe rotation of the polarization state into a change intransmittance so that a direct measure of the current inthe conductor can be made.

Detectors LaserOpto-electrorespackage

i_'^Y ' - Single mode

`^ optical fibers''y - Polarizing ^/ MagneticbeamspGtter field

Conductor

Sensingcoi

Figure 1 Fiber-Optic Electrical Current Sensor

One of the prime areas of work during the past yearhas been on methods of packaging the current sensor,especially determining the best encapsulant for the fiber-optic sensing coil. The best stability achieved to date foran unpackaged coil is +8.4 x 10-5 /K.[31 When the fiberis encapsulated, the sensitivity to temperature increases.Changes in the state of encapsulating material, especiallyat low temperatures, lead to a variation in stress on theoptical fibers. The thermally induced stress placed onthe fibers charges the sensitivity of the sensor as afunction of temperature. Various encapsulants includinghigh viscosity Teflon and silicon lubricants have beeninvestigated; to date only a two-part silicon gel seemssuitable for temperatures as low as -65 °C.

A novel fiber-optic temperature sensor has been placedin the current-sensing head and is used for electronic

temperature compensation. The sensor is based on thetemperature dependence of the birefringence in bulkSi02 or MgF,.

Evaluation

Figure 2 shows the uncompensated temperaturesensitivity of a current-sensing coil that has been encap-sulated with silicone gel. The greatest change in sensi-tivity is in the low temperature range below -30 °C.This low temperature zone has been the prime area ofdifficulty. After as many as 10 temperature cycles, thegel has not changed state and the temperature sensitivityhas remained consistent.

An early version of the electric current sensor wassubjected to swept vibrations between 5 Hz and 2 kHzat levels up to approximately 196 m/s2 (20g, where g isthe acceleration due to gravity) (typical of aircraft). Itwas also tested with random vibrations up to 182 m/s2(18.6g) (typical of spacecraft launch vehicles). Thesensor operated without failure under all of the vibration

100

r^ 90

^m 80O

z c 70

rn 60

50-80 -40 0 40 80 120

Temperature (°C)

Figure 2 Uncompensated Output of a Current SensorUsing a Fiber-Optic Pickup Coil Encapsulated withSilicone Gel.

testing. Vibrations did, however, induce noise into theoutput of the sensor, due perhaps to stress being deliv-ered to the sensing fiber coil. The highest noise outputoccurred when the vibration frequency was between 20and 700 Hz. At 90 Hz and approximately 49 m/s' (5g)the induced noise was equivalent to a current flow of4 A rms. An improved sensor package and a coilencapsulated in gel may reduce the induced noise.

3. VOLTAGE SENSOR

Technolol?y

Figure 3 shows a diagram of the fiber-optic voltagesensor which uses a technology very different from thecurrent sensor. The voltage sensor uses the Pockelseffect in a small piece of bulk bismuth germanate.Polarized light from a laser diode source is delivered tothe bulk material by optical fiber. The voltage to bemeasured is applied to two conductive plates on oppos-ing sides of the bulk material so that an electric field isset up transverse to the direction of propagation of thepolarized light. The electric field induces a linearbirefringence that changes the polarization state of thelight. The optics translate a change in polarization stateinto a change in intensity, and the intensity-modulatedlight returns to the sensing electronics via an opticalfiber [4].

A fiber-optic temperature sensor is used to compen-sate for the temperature dependence of the electro-opticeffect in bismuth germanate. The voltage sensor doesnot require an encapsulant, rather the optical compo-

PolarizingConductive plate I beam splitter

Multi- modeoptical fiber

Single mode GRIN lensoptical fiber

Bismuth Genmanate

Quarter-wave V2retarder

Figure 3 Diagram of the Fiber-Optic Voltage Sensor

nents are held together with UV-curing glue. An earlyversion of the voltage sensor used two optical fibers todeliver and return light. The noise floor was excessivebecause of the low light throughput. The latest versionemploys three fibers (one for input to the sensor headand two for return) and achieves a 10-fold decrease inthe noise floor to an acceptable 0.5 to 0.7 V/JMHz.

Evaluation

The voltage sensor has undergone 30 temperaturecycles over the range -65 0C to + 125 °C, and its temper-ature compensated output, as shown in Figure 4, fallswithin ± 1.3% over the temperature range of -70 °C to+130 °C. Output is about 1.2 mV rms per 1 V rms ap-plied to the input.

1.05 10 Vrms Input

aO

1.00 t1%N

NIST is presently redesigning the voltage sensor to usea single-mode optical fiber for input. After the voltagesensor is rebuilt, it will be submitted to vibration testsvery much like those described previously for the currentsensor.

4. RELATED WORK INOTHER ORGANIZATIONS

The US Navy has been supporting similar work atNIST Boulder on fiber-optic current sensors for ship-board applications. The Navy sensor has a differentconfiguration called the vertically annealed design(VAD) seen in Figure 5. In this design, the plane of thesensing coil was turned roughly perpendicular to thedirection of the PM input/output fibers. To date, thisdesign has been used successfully over the required 0°Cto +65°C range. Operation at lower temperatures hasnot been as successful, probably because of greaterstresses placed on the fiber.

0.95 1 I I I 1

-70 -40 0 40 70 100 130Temperature (°C)

Figure 5 Fiber-Optic Current Sensor Using the Verti-cally Annealed Design (VAD).

ME0Z

Figure 4 Output of the Voltage Sensor after 30 Tem-perature Cycles.

With the second version of the voltage sensor it wasdiscovered that movement of the fibers which connectthe sensing head to the electronics package causedexcessive noise. It is believed that this problem is dueto modal noise in the multi-mode fiber used for lightinput to the sensing head. To remedy the problem

Through a cooperative research and developmentagreement with NIST, a private U.S. corporation willsoon be producing fiber-optic current sensors. Proto-types have been shown to prospective customers, andcommercially available units will be offered in early1992.

5. FUTURE WORK

In the near term, NIST will finish building a second-

generation current sensor with a slightly larger sensorhead (for improved vibrational performance) containingthe new silicone gel as an encapsulant (for improvedtemperature stability). The new current sensor will alsoemploy improved electronics for lower noise levels.NASA Lewis will perform vibration tests on the latestcurrent sensor.

In addition, NIST will finish its improvements toreduce vibration sensitivity in the voltage sensor. NASALewis will do vibration testing on the voltage sensoralso.

In the fall of 1991, NIST will begin design of a fiber-optic electric power sensor which will contain currentand voltage sensing devices in the same sensing head.Only optical fibers will connect the sensing head to apackage of processing electronics.

6. SUMMARY

The first generation fiber-optic current sensorshowed excessive changes in sensitivity with temperaturedue to stresses impressed on the fibers by encapsulants,especially at low temperatures.[ 11 The first currentsensor was given severe vibration testing and alwaysoperated without failure. It did, however, exhibit someundesirable response (noise in the output) to mechanicalvibration. Development of the second-generationcurrent sensor is near completion. There are strongindications that the temperature problem has beensolved, and that accuracy will be about ± 1% full scaleover the temperature range of -65 °C to +125 °C. Thenew current sensor will be vibration tested.

Very desirably, the output of the voltage sensorchanges very little with temperature. The optics havebeen improved to obtain a noise floor of 0.7V/3 MHz.The sensor design is being modified to reduce vibrationsensitivity. The voltage sensor will be vibration tested inthe summer of 1991.

Design of the fiber-optic power sensor will begin inthe fall of 1991.

7. ACKNOWLEDGEMENTS

This work was supported by the NASA Office of Aero-nautics and Exploration Technology through the HighCapacity Power element of the CMI Space TechnologyInitiative. This manuscript represents U.S. Governmentwork and is not subject to copyright.

S. REFERENCES

[1] Richard L. Patterson, A. H. Rose, D. Tang, and G.W. Day, "A Fiber-Optic Current Sensor for AerospaceApplications," Proceedings of the 251h Intersociety EnergyConversion Engineering Conference, Reno, Aug 12-17,1990, Vol I, pp. 500-504.

[21 D. Tang, A. H. Rose, G. W. Day, and S. M. Etzel,"Annealing of Linear Birefringence in Single Mode FiberCoils: Application to Optical Fiber Current Sensors,"accepted for publication in the Journal of LightwaveTechnology, 1991.

[3] D. Tang, A. H. Rose, and G. W. Day, "Optical FiberCurrent Sensors with Temperature Stabilities near theMaterial Limit," Proceedings of the 71h OFS Conference,Sydney, NSW, December, 2-6, 1990, pp. 77-80.

[4] A. H. Rose and G. W. Day, "Optical Fiber VoltageSensors for Broad Temperature Ranges," to be pub-lished in the SPIE Proceedings of OE/FIBERS'91,September, 3-6, 1991.

4

National and Report Documentation PageSpace Administration

1. Report No. 2. Government Accession No. 3. Recipients Catalog No.

NASA TM -104454

4. Title and Subtitle 5. Report Date

Fiber-Optic Sensors for Aerospace Electrical Measurements:An Update

6. Performing Organization Code

7. Author(s) 8. Performing Organization Report No.R.L. Patterson, A.H. Rose, D. Tang, and G.W. Day E-6295

10. Work Unit No.

590-13-419. Performing Organization Name and Address

11. Contract or Grant No.National Aeronautics and Space AdministrationLewis Research CenterCleveland, Ohio 44135 - 3191

13. Type of Report and Period Covered

Technical Memorandum12. Sponsoring Agency Name and Address

National Aeronautics and Space Administration14. Sponsoring Agency CodeWashington, D.C. 20546-0001

15. Supplementary NotesPrepared for the 26th Intersociety Energy Conversion Engineering Conference cosponsored by ANS, SAE, ACS, AIAA,ASME, IEEE, and AIChE, Boston, Massachusetts, August 4-9, 1991. R.L. Patterson, NASA Lewis Research Center;A.H. Rose, D. Tang, and G.W. Day, National Institute of Standards and Technology, Boulder, Colorado 80303.Responsible person, R.L. Patterson, (216) 433-8166.

16. AbstractFiber-optic sensors are being developed for electrical current, voltage, and power measurements in aerospace applica-tions. These sensors are presently designed to cover ac frequencies from 60 Hz to 20 kHz. The current sensor, basedon the Faraday effect in optical fiber, is in advanced development after some initial testing. Concentration is onpackaging methods and ways to maintain consistent sensitivity with changes in temperature. The voltage sensor,utilizing the Pockels effect in a crystal, has excelled in temperature tests. This paper reports on the development ofthese sensors. It also relates the technology used in the sensors, the results of evaluation, improvements now inprogress, and the future direction of the work.

17. Key Words (Suggested by Author(s)) 18. Distribution StatementSensors Unclassified - UnlimitedOptical fibers Subject Category 35

19. Security Classif. (of the report) 20. Security Classif. (of this page) 21. No. of pages 22. Price'

Unclassified Unclassified 6 A02

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