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Opportunities in Sensor Technology
INS RODUCTION
In automated control systems, sensors represent one of the critical
technologies that determine performance and the level of automation that is
achievable. Although often overlooked, sensors in many cases represent the
critical enabling technology. The computer processor, which usually appears to
be the heart of an automated system, in reality cannot provide performance
beyond that dictated by the sensor performance. Failures often are traceable
to the malfunctioning of some inexpensive sensor component. As the push
toward automation continues, the demands on the types, performance, and cost
of sensors will grow. Optical sensors appear to offer performance and cost
advantages that will enable many new applications to become possible.
While the sensor market itself is a modest $3 to 5 billion a year market in
the United States, it is the sensor performance and availability that enable many
applications to occur. The total market for automated controls is roughly $50
to 75 billion per year. Relatively inexpensive sensors are in many cases
embedded in expensive control systems, and it is the performance of the sensors
that determines the marketability of the total system. Sensor technology can be
highly leveraged and thus has significant economic impact. The difficulty in the
sensor marketplace, however, lies in the fact that the marketplace is highly
fragmented and diverse. Sensors that are useful in aircraft may have only
marginal utility in the chemical industry. Market development and penetration
of new products are hindered somewhat by an inhomogeneous marketplace. In
51
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52
PHO TONICS
spite of this, optical sensor technology is viewed as an enabling technology with
the ability to create new areas in automated control.
There are many different types of sensors in use today,~~3 and the
application in many cases determines the requirements of sensor performance.
In process control applications such as are found In chemical processing plants
or in power generation plants, it is desired to be able to determine fluid flow
rates, liquid levels, temperatures, pressure, rates of mung, status of various
components such as valves and switches, and electrical currents and voltages;
to inspect remotely various pieces of equipment; and to monitor personnel
status. Sensors to perform these functions must be tied together with a robust
telemetry system capable of providing the required bandwidth and, in some
cases, able to survive such adverse processes as electromagnetic interference
(EMI) and corrosion. Today conventional copper wire serves as the conduit in
a telemetry system and connects numerous sensors, many of which are incom-
patible with each other and therefore require special interfacing units.
In moveable platforms, such as automobiles, ships, and aircraft, sensors
are used for status monitoring as well as performance determination and
alarming. Stringent environmental constraints are placed on the sensors in
many of these applications. For jet engine monitoring, high temperatures as
well as severe EMI and space constraints are often encountered. Pressure,
temperature, and flow sensors, among others, are subjected to conditions that
accelerate degradation. Sensor telemetry also may dramatically affect the
performance and survivability of a sensor suite.4 For example, damage control
systems--which are commonly made up of smoke detectors and sensors for
liquid level, temperature, and rate of temperature rise--often fail in a fire not
because of damaged sensors but because of the rapid loss of the telemetry
system when it is exposed to severe heat conditions. Robotic devices, as they
become more autonomous, must be able to sense the presence of objects,
manipulate these objects, and place them in the proper location. As an
example, small, sensitive pressure sensors located remotely in robotic hands
are an important part of the control system for the arm. The ability to reliably
sense pressure at a remote location is currently inadequate, and device
improvements are desired.
In automotive applications, small inexpensive sensors must function in the
presence of high temperatures and EMI and in small spaces. Improvements
in pollution and engine monitoring devices are highly desirable. In exploration
for natural resources, highly sensitive sensing devices are often used. In oil
exploration, for example, long, sensitive arrays of acoustic sensors are used to
locate promising geological formations. Echoes or transmitted acoustic probe
signals yield valuable information about the composition and structure of
underlying strata. In well drilling, gyroscopes are used to determine the
direction of the drilling while other sensors attempt to determine the local
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OPPORTUNITIES IN SENSOR TECHNOLOGY
53
well-hole geology. In this application, high temperatures and corrosive gases
that affect device reliability are often encountered.
Most of the sensors described above refer to devices that directly probe
physical phenomena. Another very important sensor class detects visible or
infrared radiation and is used for surveillance or position monitoring. In robotic
vision, solid-state cameras are used to obtain data on object location, size, and
orientation. The data are processed with various algorithms and are used to
control the movement of robotic devices. Solid-state cameras and processors
are widely used for monitoring security perimeters, for damage determination,
and for autonomous vehicle control. Focal plane arrays, which are microelec-
tronic chips with two-dimensional arrays of photodetectors, also are finding use
in passive optical radars such as IR search and track (IRST) equipment, for
satellite imaging applications, and potentially for autonomous equipment
control.
PHOTONIC SENSORS
Two of the principal classes of photonic sensors are fiber-optic sensors and
focal plane (FPA) array imaging sensors. Both are currently in development
and commercially available and promise significant advances in capabilities over
current approaches. Foreign competition in both areas has proven to be
substantial, and foreign manufacturers are in an excellent position to dominate
In the area of FPA in particular, since the operative technology involves micro-
electronic integrated-circuit approaches. If an edge exists for the United States
~ FPA and fiber sensor technology, it is due in large part to military investment
in these areas.
FIBER SENSORS
After 8 years of development, optical fiber sensors are begs ng to emerge
as competitive devices for performing sensing tasks such as those required for
aircraft engine and flight controls, for shipboard machinery and damage
controls, for medical probing, and for industrial process controls. Fiber sensors
operate by having the perturbation to be measured (e.g., temperature, pressure,
or displacement) modulate light propagating in a fiber. This can be ac-
complished by placing specially designed coatings on the fiber or by using the
fiber to conduct light to and from some material, placed in the fiber path, that
reacts to the perturbation to be sensed and modulates the fiber-guided light.
Fiber sensors have proved to be accurate and capable of operation in harsh
environments contaminated with high EMI, explosives, or corrosive gases.
Because of these attributes, fiber sensors offer unique opportunities to solve
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54
PHO TONICS
FIGURE 5.1 Examples of fiber-optic systems applications for marine use.
existing sensor problems encountered in many applications. Additionally, the
compatibility of optical sensors with optical telemetry makes possible the
development of all-optical, multielement sensor systems capable of supporting
large numbers of high-bandwidth sensor elements while at the same time
eliminating the requirement for transmitting electrical power to the sensor site
from the monitoring site.4 This combination of compatible fiber sensors and
fiber telemetry represents an intrinsic advantage over conventional electrical
technology. In Appendix E, the operation of fiber sensors is detailed, and the
state of the art outlined. An example of systems applications is shown in
Figure 5.1.
Fiber-optic hydrophores, gyros, and magnetometers make up several of
the high-performance sensor types. Performance parameters of these and
other fiber sensors are shown in Table 5.1. Fiber hydrophores constructed
with fiber interferometers have demonstrated detection sensitivities equal to
or better than conventional piezoelectric devices. One of the main advantages
of fiber-optic sensors is the spatial versatility of the sensing head. For an
element with 30 m of fiber, the element can be a 30-m-long straight, extended
element or a compact golf-ball-size omnidirectional element. The versatility
also allows for a planar type of configuration as well as shaded, extended
elements. A number of hydrophores for evaluation purposes have been
successfully tested at sea. Single-element phones display state-of-the-art
hydrophore performance. There has been considerable interest in both the
United States and Europe in hydrophore arrays. In this application, multi-
element sensors (driven by a single laser) with various types of multiplexing have
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OPPOR7UNITIESIN SENSOR TECHNOLOGY
55
been considered to reduce the cost of the array. As of yet, only small demon-
stration arrays with a few sensors have been built.
Two types of fiber-optic magnetic sensors have been demonstrated, the
Faraday effect sensors, which are useful for measurement of large magnetic
fields ~ ~ 1 Oe), and interferometric sensors utilizing magnetostrictive materials
for high-sensitivity devices (<1 mOe). In typical sensor designs, the sensor
fiber is bonded to a magnetostrictive material. As the magnetic field changes,
the strain in the material is transferred to a strain in the fiber core. Minimum
detectable DC fields of 10~ Oe and AC fields of 10~ Oe range have been
reported, which makes their performance equal to or better than existing,
competing room-temperature non6~ber magnetic sensors.
Fiber-optic gyros fabricated with fiber Sagnac coils have provided
performance in the laboratory equaling the state of the art as achieved by the
best ring-laser gyros.s Nearly all gyro applications have relatively stringent size
requirements ranging from volumes of 1 ft3 in airplanes to volumes of a few
cubic inches in missiles. This makes packaging considerations important in any
engineered device. One of the leading approaches used today is the all-f~ber
technique. Since the light never leaves the fibers, discrete optical component-
alignment problems are avoided, minimizing gyro degradation due to vibration
or thermal cycling. Recently, a ruggedized packaged fiber gyro for oil well-
logging applications has been developed. This gyro is designed to operate to
depths in the earth of 2000 ft over temperatures ranging from 0 to 125° C and
to be able to withstand shocks to 100 g and vibrations of 40 g. This gyro can
detect 0.05 rotations in azimuth and 0.2 in inclination and illustrates the state
of the art in fiber gyro packaging.
There are many reasons why fiber-optic gyros are currently attracting
substantial interest. Fiber gyros are all solid-state and have no moving parts,
implying reduced maintenance compared to present-day spinning mass gyros.
Fiber gyros appear to have better potential sensitivity performance than the
corresponding theoretical limits for ring-laser gyros, as is indicated in Table 5.1.
They also do not have many of the problems that have plagued ring-laser
development, such as optical lock-in, which required mechanical dithering, and
precision block and high-quality mirror fabrication. Finally, they are con-
structed from inexpensive components and have the potential to be inexpensive
devices when compared with other technologies. Counterbalancing these
advantages is the fact that the fiber gyro is still in an earlier stage of develop-
ment than the technologies with which it is competing. Engineering issues of
dynamic range and drift remain to be satisfactorily resolved. Recalling that the
ring-laser gyro, which is now enjoying ~ successful introduction into commercial
applications, required a development period of two decades, one expects that
the much newer fiber-optic implementation will probably require another 5 to
7 years before it, too, successfully competes in a commercial marketplace.
OCR for page 56
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OCR for page 60
60
PHO TONICS
Fiber-optic sensors appear to be ideally suited for machinery and process
monitoring and control. In control applications, extreme sensitivities are not
generally required, whereas a premium is placed on cost, simplicity, and
reliability. The military is examining the use of f~ber-optic monitoring and
control sensors and has actively pursued the development of several sensor
types. Functions important for aircraft and ship controls are control of surface
displacement, rotation, torque, and speed. Although these functions are also
important in commercial processing and control, the high-EMI environment
of military platforms that results from extensive radar and radio communica-
tion operations provides an excellent incentive for the development of f~ber-
optic position sensors. At present both military and commercial companies
are developing fly-by-light flight control systems. Linear and rotary position as
well as differential hydraulic pressure sensors have been developed. Position
sensors operate as optical shutters, providing reflection, transmission, or
absorption of light supplied by a source fiber in accordance with a pattern or
mask inscribed in the moving member of the position sensor. These f~ber-optic
devices replace conventional resistive bridges or magnetic induction position
sensors. The first industrial application of these devices will undoubtedly be in
hazardous/explosive environments where the all-dielectric nature of fiber
sensors enhances safety. As indicated, improved fabrication, packaging, and
production techniques are required to produce cost-competitive fiber-optic
position sensors.
Oil-pollution-mon~toring sensors are important for shipboard as well as
industrial processing control. Fiber-optic-based sensors have demonstrated
substantial improvements in accuracy over conventional devices and have
demonstrated the ability to distinguish between oil and solid pollutants. Fiber
sensors have been installed on over 800 vessels.2
Fiber-optic control sensors appear ideally suited for machinery and process
monitoring. They generally possess adequate sensitivity for those applications.
Power plant equipment and heavy electrical machinery appear to be prime
candidates for early usage. The possibility of building distributed fiber sensors
and embedding fiber sensors in material promises to satisfy numerous long-
standing requirements. Fiber sensors embedded in composites, for example,
should provide strain-probing capabilities of parts in motion. Additional oppor-
tunities exist in aircraft and ship flight and machinery control systems. The
military has actively pursued the development of several sensor types, including
control and monitor sensors (e.g., for damage control and fuel status). These
are currently being evaluated and are expected to find application.
Lower-sensitivity fiber sensors have been incorporated into numerous
control system demonstrations, and many control-type fiber sensors are
commercially available. Included in this group are temperature, pressure, flow,
torque, current, liquid-level, and several other types of fiber sensors. These are
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OPPORTUNITIES IN SENSOR TECHNOLOGY
61
described in greater detail in Appendix E along with other fiber sensors used in
medical applications.
SOLID-STATE IMAGING DEVICES
Since 1970, extensive research and development have been devoted to
metal-oxide (MOS) semiconductor technology utilizing charge-coupled device
(CCD) and charge-injection device (CID) concepts, leading to a revolution in
data handling and processing of radiation-induced signals. Significant advances
in multiplexing and amplifying signals from optical and infrared detector arrays
have resulted. Devices resulting from these advances are finding application in
strategic, tactical, and ecological reconnaissance and surveillance, both from the
ground and from the air, as well as in consumer products such as miniature
video cameras for various uses. Silicon technology was the early basic
technology used in charge-coupled devices, and the first imaging CCDs were
sensitive in the spectral region of 0.4 microns to 1.0 microns. Other windows at
3 to 5 microns and 8 to 12 microns have subsequently become accessible using
indium antimonide (InSb) and mercury-cadmium telluride (HgCdTe) material
systems. Imaging for regions below about 2 microns can be carried out with
ambient or active illumination, whereas for longer wavelengths, the objects' own
thermal radiation provides the signal to be detected. Research with InAsSb
strained-layer superlattices, gallium arsenide/aluminum gallium arsenide
(GaAs/AlGaAs), and other materials for use in IR devices also shows promise.
Charge-coupled and/or charge-injection concepts have yielded, and promise to
continue to yield, significant improvements in the performance of large-scale
integrated detector arrays. Substantial increases in sensitivity with savings in
weight, size, power dissipation, and cooling requirements have been realized
with these new arrays. Increased scene information, coupled with evolving
improved signal/clutter-processing technologies, offers capabilities for autono-
mous detection and classification that were heretofore unattainable. Area
arrays of 256 x 256 element complexity are becoming available in all spectral
regions, with larger visible arrays having already been demonstrated. These will
replace linear, mechanically scanned arrays in many applications, thus
eliminating several moving parts, reducing size, and increasing reliability.
In the visible and near infrared, large, buried-channel silicon CCD arrays
have been used in the space telescope. Silicon imagers now can incorporate
both storage and time-delay-integration (TDI) modes, which increases
sensitivity significantly. Monolithic platinum silicide Schottky barrier IR-CCDs
have demonstrated important improvement in scene uniformity, with good near-
IR sensitivities. Many of these devices have already found their ways into both
military and commercial cameras. Because of these detector improvements,
video cameras have shrunken in size during the last decade by over an order
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62
PHO TONICS
of magnitude, a development that has opened up a vast consumer market to
these products.
In the infrared, scanned linear detector array technology continues to
dominate. However, with continued progress in IR area staring arrays, it is
expected that these new arrays will replace many of the scanning systems, thus
eliminating the need for a conventional scanning mechanism and simplifying
focusing optics. However, to compensate for array nonuniformities, correction
on a pixel-by-pixel basis must be incorporated. Additionally, detector
responsivity and readout nonlinearities require increased computations to
correct for these flaws in an array output. These arrays, therefore, need to be
closely coupled with a solid-state processor in order to perform properly.
It is becoming clear that, as CCD/CID technology evolves, sophisticated,
autonomously operated devices become possible, which in the future will affect
many systems. For the first time, focal plane arrays permit image generation
in a small, efficient sensor head. These imagers, when coupled with the
appropriate processors and algorithms, permit target detection in clutter and
object identification. These capabilities are becoming pacing requirements for
advances in robotics, autonomous vehicles, surveillance, and data collection.
Small, inexpensive imagers/processors would permit widespread use in
applications where the size, orientation, and position of various objects are
desired. Passive IR search and track radars are required for ship and aircraft
navigation and protection, whereas small, smart seeker heads are required for
guided munitions, as well as for a variety of consumer products, such as
collision-avoidance devices and home protective systems.
ENABLING TECHNIQUES
Fiber-optic sensors have been in development for several years, and
adequate performance has been demonstrated. One of the barriers to
commercialization has been the diversity of the sensor market. While several
fiber sensors are commercially available, compatibility among the sensor types
has not been realized, and serious packaging of laboratory devices has occurred
in only a few cases. Most of the fiber sensors listed in previous sections perform
adequately in laboratory environments; however, drifts in calibration, due to
environmental changes and other problems, have been encountered when poorly
engineered or packaged devices have been fielded. It appears that all the
components needed to fabricate fiber sensors are available, so that future
development efforts can concentrate on making commercially viable sensors.
Compatibility of packaging of several types of fiber sensor fill lead to the ability
to multiplex multiple sensors on a common optical fiber. This in turn will lead
to cost and performance advantages that will make fiber sensors the technology
of choice. The strength of optical fiber sensors lies in the ability to use a
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OPPORTUNITIES IN SENSOR TECHNOLOGY
63
common technology base to sense numerous physical parameters. Automated
manufacture of fiber sensor arrays and associated telemetry promises significant
cost advantages over current practices. This fact, coupled with resistance to
EMI and corrosive environments and competitive performance, should ensure
the widespread use of this technology.
As for the many other technologies emerging from U.S. laboratories, the
difficulty in commercializing the technology rapidly relative to foreign
competition is a major concern in the optical fiber sensor area. Projects to
standardize fiber sensors, so that compatibility issues are resolved, and to
integrate fiber telemetry are desirable. The military ~11, in all probability,
perfect the sensitive surveillance types of fiber sensors, and these will diffuse
into the commercial marketplace with time.
Focal plane array development, on the other hand, requires more research
to address fabrication problems even though many commercial FPAs are
currently available. The FPA technology is still evolving, and continued
research is required to develop better materials-growth approaches, interface
improvements, fabrication yield improvements, and signal processing to support
image acquisition and processing. Nonuniformity of detector response from
pixel to pixel requires considerable post-processing for IR-FPAs and reflects the
difficulties in materials growth and fabrication of the array. Array yields remain
low, which reflects the evolving nature of designs and of processing approaches.
Considerably better control over device parameters has been realized with
visible FPA structures versus IR-FPAs. This is traceable to the use of highly
developed silicon technology in the visible region of the spectrum--whereas InSb
and HgCdTe material systems and processing techniques for IR-FPAs are less
developed. Funding and incentives are needed to make focal plane arrays
cheaper and more reliable.
Of equal importance is the development of custom processing hardware
and software to support focal plane array data acquisition and processing. The
lack of very efficient image processing algorithms limits the ability to perform
real-time feature extraction, clutter suppression, and related data manipulation
and determines the size of the processor needed to perform a function. In many
cases, a small sensor head with a volume of a few cubic inches creates sufficient
data to require a VAX-size or larger computer for processing. The long-term
goal, which is supported by the potential of the technology, is to develop
algorithms and processors efficient enough to permit the processor to also fit
into a volume comparable to that occupied by the sensor. This will open the
door to many autonomous vehicle, robotic, and military applications.
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PHO TONICS
REFERENCES
1. Giallorenzi, T. G., J. A. Bucaro, A. Dandridge, G. H. Sigel, J. H. Cole, S.
C. Rashleigh, and R. G. Priest. 1982. Optical fiber sensor technology.
IEEE J. Quant. Elec. QE-18~4~:626-665.
Pitt, G. D., et al. 1985. Optical-fibre sensors. IRE Proc. 132~4~:214-248.
Jackson, D. A., and J. D. C. Jones. 1986. Fibre optic sensors. Optica Acta
33~12~:1469-1503.
4. Dakin, J. P. 1987. Multiplexed and distributed optical fire sensor systems.
J. Phys. Eng. 20:954-967.
5. Bergh, R. A., H. C. Lefevre, and H. J. Shawl 1984. An overview of 6~ber-
optic gyroscopes. J. Lightwave Technol. LT-2~2~:91-107.
Representative terms from entire chapter:
sensor technology
