Electro-optical systems have many advantages over conventional electrical systems. Among these are optics' insensitivity to electro-magnetic interference, good electrical isolation and the ability to make measurements in highly explosive areas without risk. These advantages promise to help improve the reliability of future aircraft engine control systems which will be entirely electronic digital. To improve the reliability of these systems, especially against lightning strikes, passive, optical, sensors and fiberoptic transmission lines are being considered for use in future engine systems. Also under consideration are actuators which receive their command signals over fiber optic cables. This paper reviews concepts used for optical instrumentation and actuation systems and discusses work being done by NASA Lewis Research Center in this area

Baumbick, R. J.

English

NASA Technical Reports Server

General Disclaimer 
One or more of the Following Statements may affect this Document 
 
 This document has been reproduced from the best copy furnished by the 
organizational source. It is being released in the interest of making available as 
much information as possible. 
 
 This document may contain data, which exceeds the sheet parameters. It was 
furnished in this condition by the organizational source and is the best copy 
available. 
 
 This document may contain tone-on-tone or color graphs, charts and/or pictures, 
which have been reproduced in black and white. 
 
 This document is paginated as submitted by the original source. 
 
 Portions of this document are not fully legible due to the historical nature of some 
of the material. However, it is the best reproduction available from the original 
submission. 
 
 
 
 
 
 
 
Produced by the NASA Center for Aerospace Information (CASI) 
https://ntrs.nasa.gov/search.jsp?R=19830007031 2020-03-21T05:01:05+00:00Z
M\ ---,
(11&SA-TA-83009) FIBEROPTICS TECHNOLOGY AND	
U83-15302
ITS APPLICATION TO PROPULSION CONTROL
SYSTEIIS (Mks&) 12 p HC A02/ff A01 CSCL 010	 unclas
G3/06 02433
r- W
0MGMAL PAGE tS
OF POOR QUALITY
FIBEROPTICS TECHNOLOGY AND ITS APPL ICATION TO
PROPULSION CONTROL SYSTEMS
Robert J. Baumbick
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio
Abstract
Electro-optical systems have many advantages over conventional electri-
cal systems. Among these are optics' insensitivity to electromagnetic in-
terference, good electrical isolation, and the ability to make measurements
in highly .explosive areas without risk. These advantages promise to help
improve the reliability of electronic digital engine control systems in fu-
ture aircraft. To improve the reliability of these systems, especially
-cr
	
against lightning strikes, passive optical sensors and fiberoptic transmis-
sion lines are being considered for use in future engine systems. Also
W	 under consideration are actuators which receive their command signals over
fiber optic cables. This paper reviews concepts used for optical instru-
mentation and actuation systems and discusses work being done by NASA Lewis
Research Center in this area.
Introduction
The number of concepts for measuring physical parameters using optics
has increased dramatically in the past few years. This is due in large part
to the great strides made by the communications industry in component im-
provements, especially in the area of fiber optics and integrated optical
circuits. Optics has desirable advantages over electrical systems such as
immunity to electromagnetic interference, good electrical isolation, and no
fire risk. Optical measurements use phase, amplitude, wavelength, or polar-
ization modulation to measure physical parameters such as pressure, temper-
ature, position, magnetic field, and electrical current. Many optical sen-
sor schemes have been demonstrated in the laboratory and some prototypes
have been built and tested. Before widespread use of optical sensors
occurs, more work must be done on fiber coatings, sources, detectors, and
packaging.
Optical sensors for measuring acoustic pressure and magnetic fields
using phase modulation are well documented as are amplitude modulating sen-
sors for measuring temperature and rotation. Various i9strumentation
schemes are compared for sensitivity and performance.l,
Optical sensors with fiber optic transmission lines promise improved
reliability for propulsion control systems. Future engine control systems
are expected to be a"1 electronic, without hydromechanical backup. Special
precautions have to ^2 taken to guard the electronic control against light-
ing strikes. Elect-J cal wire systems must be heavily shielded and special-
circuitry is needed to protect the electronic control against damage from
lightning. Fiber optics promises a viable alternative to the conventional
wire systems since fiber optic cables are not electrical conductors and are
resistant to electromagnetic interference. In addition to sensing, optics
can be used to control actuators either as switching signals to control
4
locally generated electrical power or as a source of power to supply elec-
trical power to drive the actuators.
A conceptual fiber optic controlled engine (FOCE) is shovn^ in Fig. 1.
The electronic computer, optical sources, and detectors are located in a
controlled environment. Fiber optic cables connect the computer to optical
sensors and to optical-electrical converters at the actuators. The sensors
measure such engine parameters as pressure, temperature, speed, actuator
position, and may, in some cases, measure turbine tip clearance. The com-
puter accepts these measured signals, computes and outputs electrical sig-
nal: to the light sources which generate optical signals that are ui9d to
drive the actuators.
The environment in the nacelle is severe with temperatures ranging from
-55 * to 260 6 C. The fiber optic cables and connectors must operate reliably
under these conditions. This paper discusses work being done by NASA Lewis
Research Center on optical sensors and optically controlled actuators for
use in airbreathing engine control systems. The status of fiber optics
technology for use in this engine environment is reviewed.
Sensors
The first passive optical sensors to be built and tested at NASA Lewis
were a tachometer and rotary position encoder built by Spectronics, Inc.
Fig. 2 shows these instruments. The optical tachometer is a 9-pulse/
revolution sensor and the rotary encoder is a 9-bit word Gray code sensor
which measures 360 * of rotation. Both sensors were installed on an F100
engine with a 3.65-m cable connecting the sensors to the electronics box
that contained the sources and detectors. The sensors operated without
failure for over 100 cumulative hours of engine testing.
When airfoil tip clearances in compressors and turbines are excessive,
component performance and efficiency are adversely affected. To decrease
this loss closed-loop tip clearance control can be used. Optical sensing of
tip clearances is attractive for the reasons previously cited. An optical
tip clearance sensor, shown in Fig. 3, was built and tested in the labor-
atory by General Electric under contract to NASA Lewis. Fiber optic bundles
direct light to the sensor and carry the modulated signal to the detectors.
Light from the input bundle is directed across the gap between the engine
case and blade tips and is imaged onto a coherent output bundle which trans-
mits the signal to the detector array. As the blade intercepts part of the
light beam directed between the source and detector fibers, light that nor-
mally would fall on the detector bundle is blocked. Blade clearance is de-
termined by the position of the boundary between light and dark areas on the
detector array. The laboratory tests on the tip clearance sensor were con-
ducted with a compressor rotor stage driven by an electric motor. 3
Two concepts for measuring engine gas temperatures using optical tech-
niques are the Fabv Perot interferometer4 and the intensity modulated
rare-earth sensor. 	 In the Fabry Perot sensor shown in Fig. 4, a spectral
amplitude modulation is produced by multiple interference of a broad band
input spectrum. The modulation is related to the resonator thickness which
is a function of temperature. A metallic cylinder controls the spacing be-
tween two parallel mirrored surfaces whose faces are partially transmitting
and partially reflecting. The Fabry-Perot principle provides for multiple
reflections between mirrored surfaces. The maximum transmitted intensities
occur at wavelengths for which the resonator gap d - Na/2 where N is the
order of interference.
2
ORIGINAL PAGE IS
OF POOR QUALITY
The modulated spectrum produced by Fabry Perot is passed through a prism
which separates the spectral components according to their wavelength. The
dispersed spectrum is focused on the face of a 128-element linear CCD
array. The output from the CCD array is converted to 128 8-bit digital
words and fed into a microprocessor. Algorithms are then used to determine
the maximum intensity wavelengths, which are then correlated to resonator
gap thickness and hence temperature. This sensor is being built by OPCOA
Inc. (Anaheim, CA) under contract to NASA Lewis. This sensor is being de-
signed to measure temperatures to 1000 C. A prototype model, Fig. 5, was
built and tested to temperatures of 575 C.
Another sensor measures temperature by measuring the change in light
intensity as it passes through a rare-earth material. This sensor is being
developed by United Technologies Research Center under contract to NASA
Lewis. Rare-earth materials were chosen for the sensor because they have
numerous absorption lines in the visible and near IR spectrum. Absorption
lines result from optical transitions originating in the electronic ground
state of the ion and from low lying excited states. The population distri-
bution of ions is a function of temperature. The strength of the absorption
is proportional to the populations of ions in the state from which the ab-
sorbing transition originates.
Figure 6 shows some of the energy levels of europium. Absorption peaks
arise from the transition from the low lying states to the 5D and
0
50 states. The expected distribution of ions in the lower states as a
1
function of temperature is shown in Fig. 7. 4
 The population of the ground
state No decreases with temperature. The 7F 
2 
state (N2) shows a
population that increases with temperature. Absorption lines that originate
in this sta;.e would increase in strength as temperature increases.
The physical configuration of this rare earth sensor is shown in Fig.
8. This configuration uses a YAG host doped with neodymium rather than
europium-doped glass because this sensor is designed for much higher temper-
atures (>1000 * C) than glass can stand. A sensor using this configuration
is currently being designed for testing in a turboshaft engine (Fig. 9) in
between the gas generator turbine and the power turbine. To measure temper-
ature by measuring the change in intensity of a light signal a separate
wavelength, at which there is no temperature dependent an absorption, is
used to factor out the losses in the cable and connectors.
Actuators
United Technologies under contract to NASA Lewis has fabricated a
high-temperature photoswitch and tested it over the temperature range -55^
to 260 * C. The photoswitch is used to turn on an electrical current in re-
sponse to an optical signal. The electronic components are made of gallium
arsenide r.-id are designed to switch up to 100 mA of current with an
off-state voltage of +20 V. The configuration is shown in Fig. 10. A power
supply is connected to a torque motor through a JFET power transistor. The
gate of the JFET is connected to the circuit containing the photo-
transistor. When an optical signal is applied to the photo transistor the
3
MET turns on allowing current to flow through the torque motor. The torque
motor drives a hydraulic servovalve which in turn supplies power to rn
actuator. Figure 11 shows the test phototransistor, diode, and JFET power
switch. These components have been tested over temperatures from -55 to
260 * C with satisfactory results.
An evolution of the system in the previous figure is shown in Fig. 12.
In this scheme no local power supply is required at the torque motor. Op-
tical power is transmitted to the actuator and converted to electrical
energy by a photovoltaic type device. Studies are now under way to evaluate
the optical power transmission and conversion scheme. A detailed evaluation
of light sources and detector-converters will define more precisely the
practical aspects of this approach.
Cables and Connectors
The development of rugged, reliable optical cables and connectors is
still needed before fiber optics can be seriously considered as a replace-
ment 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 0 C. There is in-house work going
on at NASA Lewis to develop connectors and cables capable of operating reli-
ably over the temperature range of -55 * to 260 0 C. Two types of connectors
are being used (see Fig. 13). The ITT connectors and Hewlett Packard con-
nectors shown are made of metal and are both butt type couplers. The ITT
connector has a jeweled ferrule into which the fiber fits. Both have aline-
ment sleeves that maintain the proper position between the two connectors.
The material used to hold the cable in the connectors is EPO-TEC 353ND which
has a low viscosity for easy flowing and high operating temperature capabil-
ity. The cables that are being tested are 100- and 200-um core cables.
Current research on buffer materials indicates that 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.
Discussion
The advantages of optics over conventional wire systems used for sensing
and actuator control have potential for improving the reliability of digital
electronic control systems. The immunity of fiber optics to EMI and its
good electrical isolation are especially important because the electronic
control system must be protected from damage by lightning. In conventional
wire systems heavy shielding and special pulse-arresting circuitry must be
used. With fiber optics and optical sensors this could be eliminated.
The fiber optic cables to be used in jet engine applications require
special high-temperature buffers. Fiber coatings currently available are
generally for low-temperature applications. Components for use on the air-
breathing engine must operate reliab','y over temperatures from -55 * to
260° C. Significant progress has been made in developing the various com-
ponents needed for the fiber optic 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, actuation, and
cabling technology.
4
ORIGINAL PAGE 15
OF POOR 011ALITY
References
1. Giallorenzi, T. G., Bucaro, J. A., Dandridge, A., Sig. 1, G. H., Cole,
J. H., Priest, R. G., and Rashleiqh, S. C., "Optical Fiber Sensor Tech-
nology." IEEE Journal of Quantum Electronics, Vol. QE-18, No. 4, Apr. 1982,
pp. 626-6 .
2. Harmer, A.L.; "Principles of Optical Fiber Sensors and Instrumentation";
Battelle Centres de recherche de Geneve, Geneva, Switzerland, 1982.
3. Poppel, G.L., Marple, D.T.F, and Raumbick, R.J. "Optical Tip Clearance
Sensor for Aircraft Engine Controls, r AIAA Paper 82-1131, June 1982.
4. Quick, W. H. and James, K. A., "Optical Temperature Sensors for Pro-
pulsion Control Systems (Fabry-Perot)," Rockwell Corp., Anaheim, CA, NASA
CR-165362, 1981.
5. Morey, W. W., Glenn, W. H., Decker, R. 0., and McClurg, W. C., "Design,
Fabrication and Testing of an Optical Temperature Sensor," United Tech-
nologies Research Center, East Hartford, CT, R8O-924624-11, July 1980.
5
U W W
- F
E
W Z ^	 ^
U
0. 
F a
a
8
u 0	 0
!- O	 v
^ W
	
g
N r	 o.,
a^a
ORIGINAL PA (,uE
BLACK AND WHITF PHOTOGRAPH
iO J
^- mQ w Q w\ \ = Q U
\
U Q O
\ Q i > > C^
\
\
a/
_C
C
a^
w ^
Q.'
J
mw
J ~ QU o^Q CL
C
JU
W
CO
CD
O S'
r/ ^
O:-)^QO p -V\wu
V\ w — V) a w
CL
V) V) CL d
OR!GINL%I_
BLACK AND WHITE PHOTOGRAPH
MOUNTING TUBE
APERTURE STOP;
r'	 - "RANDOM"
BUNDLE
^^ n BUNDLFS
LED	 (ONE FOk
SOURCE	 //	 EACH ROW)i	 IUI
7 Is 7
Figure 4. - Temperature sensitive gap
for use in Fabry-Perot temperature
sensor.
rO
^	 D1	 Q	 D3	 D.,1
ROTOR)	 Al g .^ g A 3
	An
n DETECTORS AND AMPLIFIERS
Figure 3. -Optical tip clearance sensor.
-ia
n
ROW S t^
_ 1^
^m FIBERS PER ROW
" LAYERWI SE-
COHERENT" BUNDLE,,
-SHROUDI	 ,
Figure 5. - Fabry-Perot temperature sensor.
ORIGINAL PAGE 19
OF POOR QUALITY
20403
50
1
18 0
,-: 16
^u
14
N
cu
E
12
10
3
8
7F
6
6
cp
`., 4 4
2 2
0	 0 1
Figure 6. - Energy level diagram for
europium in glass.
.7
a .6 "0
z
.5z
0
4
aoa
LA-
0
z .3
0
U
a N
P: 2
.2 N-
1 r	 t	 i	 I	 1	 1	 t
200	 300	 400	 500	 600	 700	 800
ICT (cm)-1
Figure 7. - Population of ground states of EU3+
LENS
APERTURE,	 r LENS	 3.2 mm 6.25 mm
It	 INPUT FIBER
	
REFLECTOR
OUTPUT FIBER
	
L RARE EARTH
DOPED CRYSTAL
Figure 8. - Schematic of (yag doped with neodymium) temperature sensor.
L
N
V
Z.
3 ^
V1 r
W
G. O
.r
N
Q I.L
O
COC
W
0
Of-
I
CL
r
L°
E
mr
A
oc
P
i
ORIGINAL PAGE IS
OF POOR QUALITY
0
N_NZ
3c 15
O
OFIGiNAL PAGC
BLACK AND WHITE PI-lGrOG RAP1!
Figure 1L - High tempera + ure photo-switch hardware.
TOROUE	 PHOTOVOLTAIC
MOTOR	 CELL
Figure 12. - Direct optical pmerconversion'hrough photo-
voltaic cell.
f r
I figure 13. - 100um Fiber-optic connectors.


Fiberoptics technology and its application to propulsion control systems

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server