Optical sensors and optically controlled actuators for use in airbreathing engine control systems are discussed. The environmental conditions in which the aircraft will operate require the fiberoptic cables and optical connectors to perform reliably at temperatures over the -55 C to 260 C range. The status of fiberoptics technology for operation in this environment is reviewed

Baumbick, R. J.

English

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https://ntrs.nasa.gov/search.jsp?R=19830009266 2020-03-21T05:31:35+00:00Z
Noi- 1 1537
Robert J. Baumbick
Lewis Research Center
Cleveland, Ohio
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1
NASA T.-chnica! Memorandum 82907
s
Status of Fiberoptics Technology for
Propulsion Control Systems
(14ASA-TM- 62907)
	STATUS OF f1FEFCE7ICS
IFCHhCLCGY FC6 FF,UFUISICN CCNIECL SYSTEMS
(NASA)	 12 p HC A32/Mr A01	 CSCL 2 IF
Uu Clas
G3/u7	 j1997
Prepared for the
1982 Aerospace Congress and Exposition
sponsored by the Society of Automotive Engineers
Anaheim, California, October 25-28, 1982
:d
t,
3
STATUS OF FIBEROPTICS TECHNOLOGY FOR PROPULSION CONTROL SYSTEMS
Robert J. Baumbick
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135
N	
Summary
w
Lightning and other sources of electromagnetic noise can be detrimental
to digital control systems. Heavy shielding of transmission lines from sen-
sors to computer is required to prevent electromagnetic pulses from making,
their way into the computer via the metallic transmission lines.
Fiberoptic transmission lines are inherently immune to EMI noise and
offer a potential alternative to heavy shielded metallic cables. Optics can
also be used to measure engine parameters such as temperature, pressure,
speed and actuator positions. The resulting optical signals can be trans-
mitted over fiberoptic cables to electronic computers located in a controlled
environment.
Optical signals can also be used to control actuator motion, either as
switching signals, which control power from local sources, or as a source of
Power to drive the actuator directly.
This paper discusses work being done by NASA Lewis Research Center in
the area of optical sensors and optically controlled actuators for use in
airbreathing engine control systems. The environmental conditions, in which
the aircraft will operate, require the fiberoptic cables and optical connec-
tors to perform reliably at temperatures over the -55' to 260 0 C range. The
status of fiberoptics technology for operation in this environment is re-
v;ewed.
Tntrndurtinn
The use of optical sensors and fiberoptic data transmission promises to
improve the reliability of propulsion control systems because of optics'
inherent immunity to electromagnetic interference. Lightning and other
sources of electromagnetic noise can be detrimental to digital electronic
control systems. Since future engine control systems are expected to be
entirely electronic, without hydromechanical backup, all metallic transmis-
sion lines between sensors, actuators, and the control computer will have to
be heavily shielded to prevent transmission, of lightning induced pulses to
the computer.
In an effort to improve control system isolation from electromagnetic
disturbances passive optical sensors together with fiberoptic data transmis-
sion are being considered for use on future digitally controlled propulsion
systems. These sensors would measure engine parameters such as speed,
actuator positions, temperature and pressure.
A fiberoptic controlled engine (FOCE) is shown conceptually in figure 1.
The electronic computer communicates with engine sensors and actuators via
optical signals transmitted along fiberoptic cables. Passive optical sen-
sors measure engine parameters and transmit these signals to the computer.
Optically controlled actuators respond to signals from the computer. The
actuator power can either be generated locally and controlled via optical
signals or the power can be supplied directly by the optical signal itself.
The environment in the nacelle will be severe with temperatures ranging
from -55 0 to 260° C. Optical fibers and connectors must operate reliably in
this environment. This paper discusses work being done by NASA Lewis Re-
search Center in the area of optical sensors and optically controlled
actuators for use in airbreathing engine control systems. The status of
fiberoptics technology for use in the engine environment is reviewed.
Sensors
In current electronically-controlled engine configurations all engine
parameter measurements, with the exception of pressure, are transmitted elec-
trically from engine mountea sensors to the computer. Pressure sensors are
usually located in the same box as the computer. Pneumatic signals from the
engine are directed to the pressure sensor via tubes which could be nonme-
tallic to prevent a conductive path for lightning to the computer box. Tem-
perature measurement in the turbine region is currently accomplished with
thermocouples. Likewise position and speed are measured electrically. These
signals are directed to the computer via metallic conductors. These conduc-
tors must oe heavily shielded to prevent aamage to the electronic control
from lightning. The use of fiberoptics along with optical sensors promises
to inprove the reliability of the control system by improving isolation from
electromagnetic disturbances.
The technology of optical encoders ana tachometers is relatively mature
compared to other types of optical sensors, although si g nificant development
effort is still needed to integrate these components into an engine control
system. Two passive optical sensors for measuring rotary position and speed
have been built and tested on an engine in an altitude facility. The rotary
encoder is 9 bit 360 0 encoder. The tachometer is a 9 pulse per revolution
encoder. More detailed information on these sensors is presented in
reference 1.
A review of work to aevelop optical pressure, temperature, and liquid
level sensors for aerospace applications is presented in reference 2. One
class of sensors c, -rently being worked on by NASA LeRC are optical sensors
to measure turbine gas temperatures. One concept discussed in reference 2
is beyond the proof of concept stage. This sensor uses rare-earth elements
whose optical transmission characteristics are a unique function of tempera-
ture. This sensor i; oeing developed for NASA by the United Technologies
Research Center ( 1:-,RC). In this scheme glass is doped with a rare-earth,
europium, an6 irawn into a fiber. Changes in transmission through the doped
fiber are measured. Rare-earth materials like europium have energy levels
located relatively close to the ground state. These states are optically
connected to higher excited states with energy differences corresponding to
wave lengths in the visible region. The absorption peaks correspond to
h
2
I..!^
OF P30R QUALITY
energy transitions between the ground states and the higher excited states.
The amount of bsorption is proportional to the population of ions at the
level where the absorbing transition originates. The population distribu-
tion of ions is a unique function of temperature. An experimental section
of euro ium doped glass was fabricated for a proof of concept demonstration
(fig. 2^. Two wavelengths of light are transmitted via the optical cable to
the sensors. One wavelength passes through the rare-earth. It's transmis-
sion characteristic is a function of temperature. The other wavelength is
not affected by the rare-earth material and thus represents a reference
wavelength to factor out the cable and connector losses. Laboratory tests
of this sensor included operation up to 400 * C. Accuracy and resolution of
this laboratory model were better than 1%.
A rare-earth temperature sensor is being developed by UTRC for testing
in a turboshaft helicopter engine.	 The engine, figure 3, 4s a GE-T700 tur-
boshaft engine with a 5 stage axial and a single stage centrifugal compres-
sor. The compressors are driven by a two stage air cooled gas generator
turbine. A tandem two stage, uncooled power turbine drives the helicopter
rotor. The optical temperature probe will be installed in an existing ther-
mocouple hole at a location in between the two turbines as shown in figure
4. The maximum temperature in this region is expected to be about 850 C.
The sensor being developed for engine testing is designed to measure
temperatures up to 850 ` C. The configuration chosen for this sensor is
shown in figure 5. The sensing e l ement is neodymium embedded in a YAG
(Yttrium Aluminum Garnet) host material, instead of glass, because YAG has a
higher melting point. Sapphire lenses are used to focus the light into the
sensing element ano back into the receiving cable. The probe designed to
hold the rare-earth sensor is shown in figure 6. The fiberoptic cables carry
the signal to the sensor and return the modulated signal to the detector.
The fibers are quartz and have 200 pw cores.
Reference 3 discusses in more detail the rare-earth temperature sensor
developed by United Technologies Research Center (UTRC).
Actuators
The illustration shown in figure 1 implies the FOIE uses optically acti-
vated actuators. The actuators considered here are two stage electro hydrau-
lic servovalves that drive hydraulic pistons. Minimum power levels required
to operate servovalve torque motors is 60 pW. This power requirement can be
higher depending on the valve size. However, °ven the minimum power require-
ment is considerable for optical transmission and conversion. Two confi-
gurations for optically controlled actuators are shown in figure 7. One
configuration, (1a) requires a local source of electric power. The optical
signal is used to operate a photoswitch which controls the flow of electrical
energy to the torque motor. The second configuration (7b) implies that suf-
ficient optical power can be transferred to and efficiently converted by an
opto-electric converter to drive the torque motor of the servo a3lve. This
latter configuration does not require a local power supply.
1
3
4UTRC has built a model of a high temperature photoswitch which controls
power to a servovalve. This system, similar to the ,:onfiguration shown in
figure 7(a), consists of components shown schematically in figure 8.
This system consists of a diode to protect the electronics from the in-
ductive load, a JFET power switch and phototransistor. All these components
are made of gallium arsenide because specifications for this system require
operation at high temperature (260 * C) for extended periods of time. A pho-
tograph of the high temperature photoswitch is shown in figure 9. This pho-
toswitch has the capability to switch up to 100ma of current with off state
voltage of 20 V.
The second configuration (optical power transmission) is more difficult
to realize because the level of technology is not sufficient to efficiently
couple high power light sources into optical fibers and to efficiently con-
vert the transmittea optical power into electrical power. Further research
in these component areas is required. Higher output light sources, low loss
couplers and more efficient optical to electrical converters are required
before this type of system becomes practical.
Cables and Connectors
The development of rugged, reliable optical cables and connectors is
still needea before fiber optics can be seriously considered as a replacement
for electric cables. An optical cable made of glass core and glass cladding
usually has a buffer material applied to the cladding to give the optical
fiber protection. Most available cables come with buffer material that
breaks down at temperatures above 125 ` C. There is in-house work going on
at NASA Lewis to develop connectors and cables capable of operating reliably
over the temperature range of -55 ` C to 260 0 C. Two types of connectors are
being used. These are shown in figure 10. The ITT connectors and Hewlett
Packard connectors shown are made of metal. Both are butt type couplers.
The ITT connector has a jeweled ferrule into which the fiber fits. Both
have alignment sleeves that maintain the proper position between the two
connectors. The material used to hold the cable in the connectors is
EPO-TEC 353N0 which has a low viscosity for easy flowing and high operating
temperature capability. The cables that are being tested are 100 and 200 um
core cables. Current research on buffer materials indicates polyimides or
silcone rubber with teflon jackets have the most potential for success. The
high temperature testing of cables and materials is being done in-house at
NASA Lewis.
DitcIlSSlnn
Optical sensors, optically controlled actuators, and fiberoptic transmis-
sion lines offer advantages over electrical systems because of optics' inhe-
rent immunity to EMI and lightning induced currents. This is important when
digital control systems without hydromechanical backup are used. Work spon-
sored by NASA in optical temperature sensors and optically controlled actua-
tors has been presented. Further work areas are identified in order to
realize power transmission and conversion without the need for local power
supplies.
The fiberoptic cables to be usea in jet engine applications require
special high temperature buffers. Fiber coatings currently available are
generally for low temperature applications. Components for use on air-
breathiny engines must operate reliably over temperatures from -50" to 260 0
C. Significant progress has been wade in developing the various components
needed for fiberoptic engine control system. More engine test experience
and system integration effort is needed before fiber optics can be considered
a practical alternative to electric sensing, actiation, and cabling
technology.
REFERENCES
1. Baumbick, R. J.; "Fiberoptic Sensors for Measuring Angular Position and
Rotational Speed," NASA TM 81454, National Aeronautics and Space Adminis-
tration (Cleveland, Ohio), March 1980.
2. Baumbick, R. J., Alexander, J., Katz, R., and Terry, J.; "Optical Sen-
sors for Aeronautics and Space," NASA TM 81407, National Aeronautics
and Space Administration (Cleveland, Ohio), January 1980.
3. Morey , W.W., Glenn, W. H., Decker, R. 0., McClurg, W. C.; "Design
Fabrication and Testing of an Optical Temperature Sensor," NASA CR
165125, Unitea Technologies Research Center, East Hartfora, CT (1980).
5
ORIUMAL P;, t3
OF POOR QUALITY
	DIGITAL	 I
COMPUTER
OPTICAL CABLE
SENSORS
	
- _ =TORS
S PEED	 FUEL
POSITION	 ENGINE	 VALVE
PRESSURE
TEMPERATURE	 VARIABLE
(OPTICAL)	 GEOM.
Figure 1. - Fiberoptic config.ired engine (FOCE).
a
C 30- 54'15
Figure 2. - Optical temperature sensor using europium-doped fiber.
OPTICAL TEMPERATURE
SENSOR
9-ar()hjGj ri k-	. L pA- 13
OF POOR QUALITY
Figure 3. - T700-GE-700 turboshaft engine.
L-11
HOt WTION MODULI	 POWIN 1U261NI MODULI
Figure 4. - View showing inter turbine location of optical sensor.
.m
10
ORIGINAL PAGE tS
OF POOR QUALITY
r LENS
APERTURE,	 rLENS 12 mm 425 mm
i f	 INPUT FIBER	 ,'	 REFLECTOR
OUTPUT ABER	 L RARE EARTH
DOPED CRYSTAL
Figure 5. - Schematic of Iyag doped with neodym`.um) temperature sensor.
Figure 6. - Rare-earth temperature probe.
HYDRAULIC
POWERCOMPUTER
ORIGINAL PALM tS
Of POOR QUALITY
OPTICAL TO
ELECTRICAL
LOCALLY
GENERATED
ELECTRICAL
POWER
(a)
COMPUTER
SERVOVALVE
ACTUATOR
HYDRAULIC
POWER
aI
IF
OPTICAL TO
ELECTRICAL
	 SERVOVALVE
OPTICAL
CABLE'
ACTUATOR
Ibl
(a) Optically controlled switch with locally generated power o.
driving actuator.
(bl Optical power transmission  and conversion at actuator.
Figure 7. - Configurations for driving actuators with optical sig-
nals.
DIODE
Gal_xAlxAs
PHOTOTRANSISTOR
GaAs
JFET
POWER
SWITCH -'
Fioure8. - Configurations for optical switching of
actuators.
OFEGiN iL PAGa_ Ij
OF POOR QUAL;TY
Fiqure 9. - High temperature photo-switch hardware.
I figure 10. - 1001im Fiber-op'.ic connectors.


Status of fiberoptics technology for propulsion control systems

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