OVERVIEW MATERIALS FOR SENSORS—IDENTIFYING NEEDS
When you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meager and unsatisfactory kind…
Sensors are widely used in many different applications, and sensor technology has become a basic enabling technology in many instances. The rapid increase in the interest in sensors has been driven by numerous applications, such as intelligent manufacturing processing, 1 in which sensors can provide a large benefit. In addition, sensors are of great importance in safety-related areas, with applications ranging from assessing the integrity of aircraft to monitoring the environment for hazardous chemicals.
Selected examples of sensor applications and types of sensors were chosen to illustrate the different driving forces and considerations discussed above: technology push, commercial market pull, military applications, life-cycle management, and regulatory demands. These illustrations are not intended to be comprehensive. Rather, they are relevant examples from which the committee identified general conclusions concerning research needs in sensor materials and the rapid development of sensor technologies for high-payoff applications. Each example describes an application, discusses the key technical issues pertaining to the use of sensors, and identifies key sensor material needs. Each sensor-type example describes the physical phenomena being sensed, a taxonomy of the different sensor types, and sensor materials issues related to the application. Of necessity, these cases are simplifications of reality. For instance, sensor needs are introduced as if each sensor were an independent entity, although in reality many applications require arrays of sensors or fusion of information obtained from many different types of sensors. Also, the level of technical detail for each example differs according to the domain being discussed.
New types of sensors are made possible with new materials that are produced using advanced processing technologies. To a greater extent than this technology push, market pull is driving increased activity in sensors. The primary market needs can be categorized as economic, regulatory, and unique government requirements.2 Economic motivations for improved sensor materials and technology include reducing the cost of product manufacture, increasing a product's functionality at low additional cost, and improving the quality of the product. These motivations also improve product competitiveness. For example, the quality, safety, and comfort of automobiles have been greatly enhanced by the many sensors incorporated into the operation of modern vehicles (Shepard, 1992). Similarly, the cost of manufacturing and the frequency of defects in automobiles have been dramatically reduced by the increased use of sensors during manufacture. An equally important economic driver is the development and incorporation of sensors into products that aid in extending usable life. Examples include sensors for engine oil that monitor the integrity of motor oil in an engine, allowing a user to change oil only when it is necessary due to lubricant degradation, and sensors that can detect corrosion or metal fatigue in older aircraft
in lieu of more expensive externally applied inspection procedures.
Sensors have been essential in satisfying a profusion of government-mandated regulatory requirements which include such applications as measuring chemical effluent from factories and exhaust gases from automobiles. These sensors can also have significant economic impact and effect on the quality of life.
Government agencies have many unique, wide-ranging sensor needs. The military has been on the leading edge of applying sensor technologies to improve its operational capability. For instance, because of extensive media coverage, the general public is now well aware of "smart" weapons used during the recent Persian Gulf conflict. Sensors were used to develop necessary information about the target, and once the weapons were launched they were guided to the target in real time by other types of on-board sensors. A very demanding need for new sensors is represented by National Aeronautics and Space Administration's Earth Orbiting Satellite program, which has the goal of monitoring changes in chemical composition and temperature of the earth's atmosphere (Zorpette, 1993). In this case, new materials and technologies will be required to provide sensors that possess the needed sensitivity in the spectral regions of importance. The reduction in the size of the military forces and the resulting closing of military bases have led to a requirement for sensors capable of monitoring the clean-up and disposal of numerous toxic organic compounds, chemical warfare agents, and obsolete munitions. Sensors will be required for on-line control to manufacture low-volume specialty components or ultra-high-performance military aircraft. Without sensor-based control for these specialized needs, the cost per unit would very likely be prohibitive.
Because of the diversity of sensor technologies and applications and the resulting diversity of materials needs for sensors, it is frequently possible to satisfy a given need with more than one type of sensor. A key finding of this committee is that an "ideal" sensor material does not exist apart from the context of a specific application. This fact has a significant effect on planning R&D of sensor materials and systems. To accelerate sensor development, an R&D strategy that maintains a broad applications-driven research base is necessary. This requires the identification and support of critical core competencies. As will be seen in the following chapters that contain examples of sensor applications and sensor materials, sensor development draws on a wide variety of technical disciplines—from physics to engineering and from chemistry to materials science to process engineering. The diverse nature of sensor technology development requires an interdisciplinary culture. It further requires an applications focus on selected sensor technologies and materials. Risk, scientific and technological impact, and advancement of a knowledge base for sensors must be considered in order to identify the most promising opportunity areas that can have a major impact and lead to a large return on investment.
The first two chapters of this part contain illustrations of applications that dictate sensor requirements as well as the resultant impact on sensor materials. Chapter 3, "Selected Sensor Applications in Manufacturing," first discusses the role of sensors in dynamically tailoring the curing cycle of advanced polymeric composites to achieve superior properties at reduced cost. The chapter concludes with several examples of leading edge applications of sensors in the production of micro-electronic components. Chapter 4, "Selected Sensor Applications for Structural Monitoring and Control," moves the application domain to the service environment. It discusses selected examples of the emerging use of sensors to make structural components more "intelligent.''
The last two chapters of this section address two quite different classes of sensors. Chapter 5, "Long Wavelength Infrared Sensors," discusses sensors that provide "night" vision in the long-wavelength infrared (LWIR) regime. The advantages and limitations of three materials options are considered in the context of fundamental considerations that derive from an understanding of basic physics, manufacturability, and the application domain. The chapter explains how some of the ideas presented in Chapter 3 could be applied to produce these sensors at much lower cost. The next chapter, Chapter 6, "Chemical Sensors," develops a taxonomy for this broad sensor class, describes some promising applications areas (e.g., the detection of
toxic chemicals in the environment), and highlights the key materials challenges.
Each of these chapters concludes with a wrap-up of sensor material needs. These recommendations are presented within the context of the information discussed in the chapter. They are illustrative of the analytical approach recommended by the committee. They are not intended to be considered as the most important material needs in the entire universe of sensor technology. Chapter 7 in Part III contains a summary of these illustrative sensor material needs.
Defense Base Forecast. 1993. Long ride of growth predicted for sensor market worldwide. National Defense 73(Oct):4.
Shepard, L.M. 1992. Automotive sensors improve driving performance. Ceramic Bulletin 71(6):905–913.
Zorpette, G. 1993. Sensing climate change. IEEE Spectrum 30(7):20–27.