Handbook of Modern Sensors

A sensor is often defined as a “device that receives and responds to a signal or ”. This definition is broad. In fact, it is so broad that it covers almost everything from a human eye to a trigger in a pistol. Consider the level-control system shown in Fig. 1.1 [1]. The operator adjusts the level of fluid in the tank by manipulating its valve. Variations in the inlet flow rate, temperature changes (these would alter the fluid’s viscosity and consequently the flow rate through the valve), and similar disturbances must be compensated for by the operator. Without control the tank is likely to flood, or run dry. To act appropriately, the operator must on a timely basis obtain information about the level of fluid in the tank. In this example, the information is generated by the sensor, which consists of two main parts: the sight tube on the tank and the operator’s eye, which produces an electric response in the optic nerve. The sight tube by itself is not a sensor, and in this particular control system, the eye is not a sensor either. Only the combination of these two components makes a narrow-purpose sensor (detector) that is selectively sensitive to the fluid level. If a sight tube is designed properly, it will very quickly reflect variations in the level, and it is said that the sensor has a fast speed response. If the internal diameter of the tube is too small for a given fluid viscosity, the level in the tube may lag behind the level in the tank. Then, we have to consider a phase characteristic of such a sensor. In some cases, the lag may be quite acceptable, while in other situations, a better sight tube design would be required. Hence, the sensor’s performance must be assessed only as part of a data acquisition system.

Since most of stimuli are not electrical, from its input to the output a sensor may perform several signal conversion steps before it produces and outputs an electrical signal. For example, pressure inflicted on a fiber optic pressure sensor, first, results in strain in the fiber, which, in turn, causes deflection in its refractive index, which, in turn, changes the optical transmission and modulates the photon density, and finally, the photon flux is detected by a photodiode and converted into electric current. Yet, in this chapter we will discuss the overall sensor characteristics, regardless of a physical nature or steps that are required to make signal conversions inside the sensor. Here, we will consider a sensor as a “black box” where we are concerned only with the relationship between its output electrical signal and input stimulus, regardless of what is going on inside. Also, we will discuss in detail the key goal of sensing: determination of the unknown input stimulus from the sensor’s electric output. To make that computation we shall find out how the input relates to the output and vice versa?

When selecting a sensor—the first thing to do is to outline requirements for the particular application. When knowing what is needed, one is ready to evaluate what is available. The evaluation starts by studying the sensor’s data sheet that specifies all its essential characteristics. The task then is to match the requirements to availability. It is tempting to get the best available sensor, yet selecting a too good sensor means that you will be paying for an overkill—not a good engineering practice. In this chapter we review the most typical sensor characteristics and requirements that are usually specified in data sheets, or at least should be specified.

Since a sensor is a converter of generally nonelectrical effects into electrical signals, one and often several transformation steps are required before the electric output signal can be generated. These steps involve changes of types of energy or physical properties of materials, wherein the final step shall produce electrical signal of a desirable format. As it was mentioned in Chap. 1, generally there are two types of sensors: direct and complex. A direct sensor is the one that can directly convert a nonelectrical stimulus into electric output signal. Many stimuli cannot be directly converted into electricity, thus multiple conversion steps would be required. If, for instance, one wants to detect displacement of an opaque object, a fiber optic sensor can be employed. A pilot light beam (excitation signal) is generated by the light emitting diode (LED). Then the light flux enters the optical fiber and propagates through it, then exits toward the object and is reflected from its surface. The reflected photon flux enters the receiving optical fiber and propagates toward a photodiode, where it is detected to produce electric current representing a distance from the fiber optic end to the object. We see that such a sensor involves transformation of electrical current into photons, propagation of photons through some refractive media (the fiber), reflection from the object, propagation again through the fiber, and conversion back into electric current. Therefore, such a sensing process includes two energy conversion steps and also manipulation of light.

Light is a very efficient form of energy for sensing a great variety of stimuli. Among many others, these include distance, motion, temperature, chemical composition, pressure, etc. Light has an electromagnetic nature. It may be considered as propagation of either quanta of energy or electromagnetic waves. This confusing duality nowadays is well explained by quantum electrodynamics [1] and both the quantum and wave properties are used for sensing.

Detection of humans embraces a very broad spectrum of applications, including security, surveillance, energy management (electric lights control), personal safety, man-machine interface, friendly home appliances, point-of-sale advertisements, robotics, automotive, interactive toys, novelty products, etc. Detectors of human bodies loosely can be subdivided into the following categories.

An object can be in either of two states—stationary or in motion. When we think of motion, we should consider a frame of reference, since an object may be moving with respect to one system of coordinates, yet be stationary with respect to the another system of coordinates, if that systems moves together with the object. A stationary object is described by its position within the selected coordinates—just like a chess figure position on a specific square has a coordinate notation, for example e2 (Fig. 9.1).

While the kinematics study positions of objects and their motions, the dynamics answers the question—what causes motion? Classical mechanics deal with moving objects whose velocities are substantially smaller than the speed of light. Moving particles, such as photons, atoms, and electrons, or, on the other side of the scale—planets and stars—are subjects of other branches of physics—quantum mechanics and the theory of relativity. A typical problem of classical mechanics is the question: “What is motion of an object that initially had a given mass, charge, dipole moment, position, etc. and was subjected to external objects having known mass, charge, velocity, etc.?” That is, classical mechanics deals with interactions of macro-objects. In a general form, this problem was solved by Sir Isaac Newton (1642–1727) who was born in the year when Galileo died. He brilliantly developed ideas of Galileo and other great mechanics. Newton stated his First Law as: “Every body persists in its state of rest or of uniform motion in a straight line unless it is compelled to change that state by forces impressed on it.” Sometimes, this is called a law of inertia. Another way to state the first law is to say that: “If no net force acts on a body, its acceleration is zero.”

One of the fundamentals of physics is that mass is a conserved quantity. It cannot be created or destroyed. In the absence of sources or sinks of mass, its quantity remains constant regardless of boundaries. However, if there is influx or outflow of mass through the boundaries, the sum of influx and efflux must be zero. Whatever mass comes in and not stored, it must go out. When both are measured over the same interval of time, mass entering the system (M _in) is equal to mass leaving the system (M _out) [1]. Therefore,

The fundamentals of acoustics are given in Sect. 4.10 and the reader is encouraged to familiarize herself with materials of that section. Here we will discuss the acoustic sensors for various frequency ranges. The audible range sensors are generally called the microphones, however, the name is often used even for the ultrasonic and infrasonic waves. In essence, a microphone is a pressure transducer adapted for transduction of sound waves over a broad spectral range that generally excludes very low frequencies below few Hz. The microphones differ by their sensitivity, directional characteristics, frequency bandwidth, dynamic range, sizes, etc. Also, their designs are quite different depending on the media from which sound waves are sensed. For example, for perception airwaves or vibrations in solids, the sensor is called a microphone, while for operation in liquids, it is called a hydrophone (even if liquid is not water) from the Greek name of the mythological water serpent Hydra. The main difference between a pressure sensor and acoustic sensor is that the latter does not need to measure constant or very slow changing pressures. Its operating frequency range usually starts at several hertz (or as low as tens of millihertz for some special applications), while the upper operating frequency limit is quite high—up to several megahertz for the ultrasonic applications and even gigahertz in a surface acoustic wave device. The operating frequency range of a microphone or hydrophone depends on the particular application. Figure 13.1 illustrates the spectral ranges of a human and bat ears—they differ quite dramatically and practically do not overlap. Thus, a microphone for the human use and for detecting sounds of bats shall have different frequency characteristics.

The water content in surrounding air is an important factor for the well-being of humans and animals. The level of comfort is determined by a combination of two factors: relative humidity and ambient temperature. You may be quite comfortable at −30 °C (−22 °F) in Siberia, where the air is usually very dry in winter, and feel quite miserable in Cleveland near lake Erie at 0 °C (+32 °F), where air may contain substantial amount of moisture. Humidity is an important factor for operating certain equipment, for instance, high impedance electronic circuits, electrostatic sensitive components, high voltage devices, fine mechanisms, etc. A rule of thumb is to assure a relative humidity near 50 % at normal room temperature (20–25 °C). This may vary from as low as 38 % for the Class-10 clean rooms to 60 % in hospital operating rooms. Moisture is the ingredient common to most manufactured goods and processed materials. It can be said that a significant portion of the U.S. GNP (Gross National Product) is moisture [1].

Figure 3.41 shows a spectrum of the electromagnetic waves. On its left-hand side, there is a region of the γ-radiation. Then, there are the X-rays that, depending on the wavelengths, are divided into hard, soft, and ultrasoft rays. However, a spontaneous radiation from the matter not necessarily should be electromagnetic: there is the so-called nuclear radiation, which is emission of particles from the atomic nuclei. A spontaneous atomic decay can be of two types: the charged particles (α and β particles, and protons), and uncharged particles, which are the neutrons. Some particles are complex like the α-particles, which are the nuclei of helium atoms consisting of two neutrons, while other particles are generally simpler, like the β-particles which are either electrons or positrons. Ionizing radiations are given that name because as they pass through various media that absorbs their energy, additional ions, photons, or free radicals are created.

From prehistoric times people are aware of heat and trying to assess its intensity by measuring temperature. Perhaps the simplest, and certainly the most widely used, physical phenomenon for temperature sensing is thermal expansion. This forms the basis of the liquid-in-glass thermometers. For the electrical transduction, different methods of sensing are employed. Among them are: the resistive, thermoelectric, semiconductive, optical, acoustic, and piezoelectric detectors. For measuring temperature, the sensor shall be thermally coupled to the object. The coupling may be physical (contact) or remote (non-contact), but a thermal coupling always must be established for the sensor to produce a measurable electrical response.

Sensors for measuring and detecting chemical and biological substances are pervasively employed yet are, for the most part, unobtrusive. They are used to help run our cars more efficiently, track down criminals, and monitor our environment and health. Examples of uses include monitoring of oxygen in automobile exhaust systems, glucose levels in samples from diabetics and carbon dioxide in the environment. In the laboratory, chemical detectors are the heart of key pieces of analytical equipment employed in the development of new chemicals and drugs and to monitor industrial processes. Progress has been impressive and the literature is full of interesting developments. Recent developments include a broad spectrum of technologies, such as improved screening systems for security applications [1] and miniaturization of systems once only used in laboratories [2]. Chemical sensors respond to stimuli produced by various chemicals or chemical reactions. These sensors are intended for identification and quantification of chemical species (including both liquid and gaseous phases). Chemical sensors can be stand-alone devices, or part of larger, more complex systems that include instrumentation for chemical reactions, separations, or other processes.

Methods of sensor fabrication are numerous and specific for each particular design. They comprise processing of semiconductors, optical components, metals, ceramics, and plastics. Here, we briefly overview some materials and the most popular processing techniques.