Smith.rL."Sensors The Electrical Engineering Handbook Ed. Richard C. Dorf Boca Raton CRC Press llc. 2000
Smith, R.L. “Sensors” The Electrical Engineering Handbook Ed. Richard C. Dorf Boca Raton: CRC Press LLC, 2000
56 Sensors 56.2 Physical Sensors Temperature Sensors. Displacement and Force. Optical Radiation Sensors Ion-Selective electrode Gas chromatog 6.4 biosensors Rosemary L. Smith Immunosensor. Enzyme Sensor university of California, Davis 56.5 Micros 56.1 Introduction Sensors are critical components in all measurement and control systems. The need for computer-compatible sensors closely followed the advent of the microprocessor. Together with the always-present need for sensors in science and medicine, the demand for sensors in automated manufacturing and processing is rapidly growing n addition, small, inexpensive sensors are finding their way into all sorts of consumer products, from childrens toys to dishwashers to automobiles. Because of the vast variety of useful things to be sensed and sensor applications, sensor engineering is a multidisciplinary and interdisciplinary field of endeavor. This chapter introduces some basic definitions, concepts, and features of sensors and illustrates them with several examples. The reader is directed to the references and the sources listed under "Further Information" for more details and examples. There are many terms which are often used synonymously for sensor, including transducer, meter, detector, and gage. Defining the term sensor is not an easy task; however the most widely used definition is that which has been applied to electrical transducers by the Instrument Society of America(ANSI MC6. 1, 1975): Tran ducer-A device which provides a usable output in response to a specified measurand. A transducer is more generally defined as a device which converts energy from one form to another. a usable ouput refers to an optical, electrical, or mechanical signal. In the context of electrical engineering, however, a usable output refers to an electrical output signal. The measurand can be a physical, chemical, or biological property or condition to be measured Most, but not all, sensors are transducers, employing one or more transduction mechanisms to produce an electrical output signal. Sometimes sensors are classified as direct and indirect sensors according to how many transduction mechanisms are used. For example, a mercury thermometer produces a change in volume of mercury in response to a temperature change via thermal expansion, but the output is a mechanical displace- ment and not an electrical signal. Another transduction mechanism is required. a thermometer is still a useful sensor since humans can read the change in mercury height using their eyes as the second transducing element. However, in order to produce an electrical output for use in a control loop, the height of the mercury would ive to be converted to an electrical signal. This could be accomplished using capacitive effects. However, there are more direct temperature sensing methods, i. e, one where an electrical output is produced in response to a change in temperature. An example is given in the next section on physical sensors. Figure 56 1 depicts a c 2000 by CRC Press LLC
© 2000 by CRC Press LLC 56 Sensors 56.1 Introduction 56.2 Physical Sensors Temperature Sensors • Displacement and Force • Optical Radiation 56.3 Chemical Sensors Ion-Selective Electrode • Gas Chromatograph 56.4 Biosensors Immunosensor • Enzyme Sensor 56.5 Microsensors 56.1 Introduction Sensors are critical components in all measurement and control systems. The need for computer-compatible sensors closely followed the advent of the microprocessor. Together with the always-present need for sensors in science and medicine, the demand for sensors in automated manufacturing and processing is rapidly growing. In addition, small, inexpensive sensors are finding their way into all sorts of consumer products, from childrens’ toys to dishwashers to automobiles. Because of the vast variety of useful things to be sensed and sensor applications, sensor engineering is a multidisciplinary and interdisciplinary field of endeavor. This chapter introduces some basic definitions, concepts, and features of sensors and illustrates them with several examples. The reader is directed to the references and the sources listed under “Further Information” for more details and examples. There are many terms which are often used synonymously for sensor, including transducer, meter, detector, and gage. Defining the term sensor is not an easy task; however the most widely used definition is that which has been applied to electrical transducers by the Instrument Society of America (ANSI MC6.1, 1975): Transducer—A device which provides a usable output in response to a specified measurand. A transducer is more generally defined as a device which converts energy from one form to another. A usable ouput refers to an optical, electrical, or mechanical signal. In the context of electrical engineering, however, a usable output refers to an electrical output signal. The measurand can be a physical, chemical, or biological property or condition to be measured. Most, but not all, sensors are transducers, employing one or more transduction mechanisms to produce an electrical output signal. Sometimes sensors are classified as direct and indirect sensors according to how many transduction mechanisms are used. For example, a mercury thermometer produces a change in volume of mercury in response to a temperature change via thermal expansion, but the output is a mechanical displacement and not an electrical signal. Another transduction mechanism is required. A thermometer is still a useful sensor since humans can read the change in mercury height using their eyes as the second transducing element. However, in order to produce an electrical output for use in a control loop, the height of the mercury would have to be converted to an electrical signal. This could be accomplished using capacitive effects. However, there are more direct temperature sensing methods, i.e., one where an electrical output is produced in response to a change in temperature. An example is given in the next section on physical sensors. Figure 56.1 depicts a Rosemary L. Smith University of California, Davis
TABLE 56.1 Physical and Chemical Transduction Principles Secondary Signal Primary Signal Thermal Electrical Magnetic Radiant Chemical Mechanical(Fluid) mechanical and Friction effects(e.g-, Magneto-mechanical Photoelastic systems acoustic effects(e.g-, friction calorimeter) effects(e.g, Pi diaphragm, gravity Cooling effects(e.g Resistive, capacitive, and magnetic effect birefringence) balance, echo sounder) thermal flow meters) inductive effects terferometers Sagnac effect Doppler effect Thermal Thermal expansion Seebeck effect Thermooptical effects Reaction activation (bimetal strip, liquid-in-glass Thermoresistance (e.g, in liquid crystals) (e.g. thermal and gas thermometers, Pyroelectricity Radiant emission resonant frequency Thermal (ohnson) Radiometer effect Electrical Electrokinetic and electro- (resistive) Charge collectors Biot- Savart's law Electrooptical effects Electrolysis mechanical effects(e.g probe (e.g,Kerr piezoelectricity, electro- effect Pockels effect meter, Amperes law) Electroluminescence Magnetic magnetomechanical effects Thermomagnetic effects Thermomagnetic effects Magnetooptical effects (eg-, magnetostriction (e.g, Righi-Leduc effect) (e. (e. g, Faraday effect) Nernst effect Cotton. Mouton effect (e.g, Ettingshausen Galvanomagnetic effec (e.g-, Hall effect, magnetoresistance Radiant Radiation pressure Bolometer thermopile hotoelectric effects (e.g, photovoltaic effect, h photoconductive effect) Hygrometer Calorimeter Nuclear magnetic (Emission and Electrodeposition cell Thermal col Conductimetry resonance absorption)spectroscopy amperometry Flame ionization Volta effect Source: T. Grandke and J. Hesse, Introduction, Vol. 1: Fundamentals and General Aspects, Sensors: A Comprehensive Survey w. Gopel, J. Hesse, and J. H. Zemel, Eds, Weinheim, Ger VCH, 1989. with permission. c2000 by CRC Press LLC
© 2000 by CRC Press LLC TABLE 56.1 Physical and Chemical Transduction Principles Secondary Signal Primary Signal Mechanical Thermal Electrical Magnetic Radiant Chemical Mechanical (Fluid) mechanical and Friction effects (e.g., Piezoelectricity Magneto-mechanical Photoelastic systems acoustic effects (e.g., friction calorimeter) Piezoresistivity effects (e.g., piezo- (stress-induced diaphragm, gravity Cooling effects (e.g., Resistive, capacitive, and magnetic effect) birefringence) balance, echo sounder) thermal flow meters) inductive effects Interferometers Sagnac effect Doppler effect Thermal Thermal expansion (bimetal strip, liquid-in-glass and gas thermometers, resonant frequency) Seebeck effect Thermooptical effects Reaction activation Thermoresistance (e.g., in liquid crystals) (e.g., thermal Pyroelectricity Radiant emission dissociation) Thermal (Johnson) noise Radiometer effect (light mill) Electrical Electrokinetic and electro- Joule (resistive) Charge collectors Biot-Savart’s law Electrooptical effects Electrolysis mechanical effects (e.g., heating Langmuir probe (e.g., Kerr effect) Electromigration piezoelectricity, electro- Peltier effect Pockel’s effect meter, Ampere’s law) Electroluminescence Magnetic Magnetomechanical effects Thermomagnetic effects Thermomagnetic effects Magnetooptical effects (e.g., magnetorestriction, (e.g., Righi-Leduc effect) (e.g., Ettingshausen- (e.g., Faraday effect) magnetometer) Galvanomagnetic effects Nernst effect) Cotton-Mouton effect (e.g., Ettingshausen Galvanomagnetic effects effect) (e.g., Hall effect, magnetoresistance) Radiant Radiation pressure Bolometer thermopile Photoelectric effects Photorefractive effects Photosynthesis, (e.g., photovoltaic effect, Optical bistability -dissociation photoconductive effect) Chemical Hygrometer Calorimeter Potentiometry Nuclear magnetic (Emission and Electrodeposition cell Thermal conductivity cell Conductimetry resonance absorption) spectroscopy Photoacoustic effect Amperometry Chemiluminiscence Flame ionization Volta effect Gas-sensitive field effect Source: T. Grandke and J. Hesse, Introduction, Vol. 1: Fundamentals and General Aspects, Sensors: A Comprehensive Survey, W. Gopel, J. Hesse, and J. H. Zemel, Eds., Weinheim, Germany: VCH, 1989. With permission
Intermediat Electronic Measurand Transduction Signal Mechanism FIGURE 56.1 Sensor block diagram. Active sensors require input power to accomplish transduction. Many sensors employ multiple transduction mechanisms in order to produce an electronic output in response to the measurand. sensor block diagram identifying the measurand and associated input signal, the primary and intermediate transduction mechanisms, and the electronic output signal. Active sensors require an external power source in order to produce a usable output signal, e.g., the piezoresistor. Table 56. 1 is a 6x 6 matrix of the more commonly employed physical and chemical transduction mechanisms. Many of the effects listed are described in more detail in this handbook(see Chapters 53-58) In choosing a particular sensor for a given application, there are many factors to be considered These deciding factors or specifications can be divided into three major categories: environmental factors, economic factors, and the sensor characteristics. The most commonly encountered factors are listed in Table 56.2, although not all of these factors may be pertinent to a particular application. Most of the environmental factors determine the packaging of the sensor, with packaging meaning the encapsulation or insulation which provides protection and isolation and the input/output leads or connections and cabling. The economic factors determine the type of manufacturing and materials used in the sensor and to some extent the quality of the materials(with respect to lifetime). For example, a very expensive sensor may be cost effective if it is used repeatedly or for very long periods of time. On the other hand, a disposable sensor, such as is desired in many medical applications, should be inexpensive. The sensor characteristics of the sensor are usually the specifications of primary concern. The most important parameters are sensitivity, stability, and repeatability. Normally, a sensor is only useful if all three of these parameters are tightly specified for a given range of measurand and time of operation. For example, a highly sensitive device is not useful if its output signal drifts greatly during the measurement time and the data obtained is not reliable if the measurement is not repeatable. Other output characteristics, such as selectivity and linearity, can often be compensated for by using additional, independent sensor input or with ignal conditioning circuits. In fact, most sensors have a response to temperature, since most tranducing effects are temperature dependent. Sensors are most often classified by the type of measurand, i.e., physical, chemical, or biological. This is a much simpler means of classification than by transduction mechanism or output signal (e.g, digital or analog) since many sensors use multiple transduction mechanisms and the output signal can always be processed, conditioned, or converted by a circuit so as to cloud the definition of output. a description of each class and examples are given in the following sections. The last section introduces microsensors and gives some examples TABLE 56.2 Environmental Factors Economic Factors Sensor Characteristics Humidity effects Availability Susceptibil y to Em interferences Power consumption Frequency response c 2000 by CRC Press LLC
© 2000 by CRC Press LLC sensor block diagram identifying the measurand and associated input signal, the primary and intermediate transduction mechanisms, and the electronic output signal. Active sensors require an external power source in order to produce a usable output signal, e.g., the piezoresistor. Table 56.1 is a 6 ¥ 6 matrix of the more commonly employed physical and chemical transduction mechanisms. Many of the effects listed are described in more detail in this handbook (see Chapters 53–58). In choosing a particular sensor for a given application, there are many factors to be considered. These deciding factors or specifications can be divided into three major categories: environmental factors, economic factors, and the sensor characteristics. The most commonly encountered factors are listed in Table 56.2, although not all of these factors may be pertinent to a particular application. Most of the environmental factors determine the packaging of the sensor, with packaging meaning the encapsulation or insulation which provides protection and isolation and the input/output leads or connections and cabling. The economic factors determine the type of manufacturing and materials used in the sensor and to some extent the quality of the materials (with respect to lifetime). For example, a very expensive sensor may be cost effective if it is used repeatedly or for very long periods of time. On the other hand, a disposable sensor, such as is desired in many medical applications, should be inexpensive. The sensor characteristics of the sensor are usually the specifications of primary concern. The most important parameters are sensitivity, stability, and repeatability. Normally, a sensor is only useful if all three of these parameters are tightly specified for a given range of measurand and time of operation. For example, a highly sensitive device is not useful if its output signal drifts greatly during the measurement time and the data obtained is not reliable if the measurement is not repeatable. Other output characteristics, such as selectivity and linearity, can often be compensated for by using additional, independent sensor input or with signal conditioning circuits. In fact, most sensors have a response to temperature, since most tranducing effects are temperature dependent. Sensors are most often classified by the type of measurand, i.e., physical, chemical, or biological. This is a much simpler means of classification than by transduction mechanism or output signal (e.g., digital or analog), since many sensors use multiple transduction mechanisms and the output signal can always be processed, conditioned, or converted by a circuit so as to cloud the definition of output. A description of each class and examples are given in the following sections. The last section introduces microsensors and gives some examples. FIGURE 56.1 Sensor block diagram. Active sensors require input power to accomplish transduction. Many sensors employ multiple transduction mechanisms in order to produce an electronic output in response to the measurand. TABLE 56.2 Environmental Factors Economic Factors Sensor Characteristics Temperature range Cost Sensitivity Humidity effects Availability Range Corrosion Lifetime Stability Size Repeatability Overrange protection Linearity Susceptibility to EM interferences Error Ruggedness Response time Power consumption Frequency response Self-test capability
56.2 Physical Sensors Physical measurands include temperature, strain, force, pressure, displacement, position, velocity, acceleration optical radiation, sound, flow rate, viscosity, and electromagnetic fields. Referring to Table 56.1, all but those transduction mechanisms listed in the chemical column are used in the design of physical sensors. Clearly, they comprise a very large proportion of all sensors. It is impossible to illustrate all of them, but three measurands stand out in terms of their widespread application: temperature, displacement(or associated force), and optical radiation Temperature Sensors Temperature is an important parameter in many control systems, most familiarly in environmental control stems. Several distinctly different transduction mechanisms have been employed. The mercury thermometer was mentioned in the Introduction as a nonelectrical sensor. The most commonly used electrical temperature sensors are thermocouples, thermistors, and resistance thermometers. Thermocouples employ the Seebeck effect, which occurs at the junction of two dissimilar metal wires. A voltage difference is generated at the hot junction due to the difference in the energy distribution of thermally energized electrons in each metal. This voltage is measured across the cool ends of the two wires and changes linearly with temperature over a given range, depending on the choice of metals. To minimize measurement error the cool end of the couple must be kept at a constant temperature, and the voltmeter must have a high input impedance The resistance thermometer relies on the increase in resistance of a metal wire with increasing temperature As the electrons in the metal gain thermal energy, they move about more rapidly and undergo more frequent collisions with each other and the atomic nuclei. These scattering events reduce the mobility of the electrons, and since resistance is inversely proportional to mobility, the resistance increases. Resistance thermometers onsist of a coil of fine metal wire. Platinum wire gives the largest linear range of operation. To determine the resistance indirectly, a constant current is supplied and the voltage is measured. a direct measurement can be made by placing the resistor in the sensing arm of a Wheatstone bridge and adjusting the opposing resistor to"balance"the bridge, which produces a null output. A measure of the sensitivity of a resistance thermometer is its temperature coefficient of resistance: TCR=(AR/R)(1/AT) in units of resistance per degree of temperature Thermistors are resistive elements made of semiconductor materials and have a negative coefficient of resistance. The mechanism governing the resistance change of a thermistor is the increase in the number of conducting electrons with an increase in temperature due to thermal generation, i.e., the electrons which are the least tightly bound to the nucleus (valence electrons) gain sufficient thermal energy to break away and become influenced by external fields. Thermistors can be measured in the same manner as resistance thermon eters, but thermistors have up to 100 times higher TCR value Displacement and Force Many types of forces are sensed by the displacements they create. For example, the force due to acceleration of a mass at the end of a spring will cause the spring to stretch and the mass to move. Its displacement from the zero acceleration position is governed by the force generated by the acceleration(F= m a)and the restoring force of the spring. Another example is the displacement of the center of a deformable membrane due to a difference in pressure across it. Both of these examples use multiple transduction mechanisms to produce an electronic output: a primary mechanism which converts force to displacement(mechanical to mechanical)and then an intermediate mechanism to convert displacement to an electrical signal (mechanical to electrical). Displacement can be measured by an associated capacitance. For example, the capacitance associated with gap which is changing in length is given by C=area x dielectric constant/gap length. The gap must be very small compared to the surface area of the capacitor, since most dielectric constants are of the order of 1 x 10-1 farads/cm and with present methods, capacitance is readily resolvable to only about 10-l farads. This is because measurement leads and contacts create parasitic capacitances the same order of magnitude. If the capacites 8 is measured at the generated site by an integrated circuit(see Section III), capacitances as small as 10- far c 2000 by CRC Press LLC
© 2000 by CRC Press LLC 56.2 Physical Sensors Physical measurands include temperature, strain, force, pressure, displacement, position, velocity, acceleration, optical radiation, sound, flow rate, viscosity, and electromagnetic fields. Referring to Table 56.1, all but those transduction mechanisms listed in the chemical column are used in the design of physical sensors. Clearly, they comprise a very large proportion of all sensors. It is impossible to illustrate all of them, but three measurands stand out in terms of their widespread application: temperature, displacement (or associated force), and optical radiation. Temperature Sensors Temperature is an important parameter in many control systems, most familiarly in environmental control systems. Several distinctly different transduction mechanisms have been employed. The mercury thermometer was mentioned in the Introduction as a nonelectrical sensor. The most commonly used electrical temperature sensors are thermocouples, thermistors, and resistance thermometers. Thermocouples employ the Seebeck effect, which occurs at the junction of two dissimilar metal wires. A voltage difference is generated at the hot junction due to the difference in the energy distribution of thermally energized electrons in each metal. This voltage is measured across the cool ends of the two wires and changes linearly with temperature over a given range, depending on the choice of metals. To minimize measurement error the cool end of the couple must be kept at a constant temperature, and the voltmeter must have a high input impedance. The resistance thermometer relies on the increase in resistance of a metal wire with increasing temperature. As the electrons in the metal gain thermal energy, they move about more rapidly and undergo more frequent collisions with each other and the atomic nuclei. These scattering events reduce the mobility of the electrons, and since resistance is inversely proportional to mobility, the resistance increases. Resistance thermometers consist of a coil of fine metal wire. Platinum wire gives the largest linear range of operation. To determine the resistance indirectly, a constant current is supplied and the voltage is measured. A direct measurement can be made by placing the resistor in the sensing arm of a Wheatstone bridge and adjusting the opposing resistor to “balance” the bridge, which produces a null output. A measure of the sensitivity of a resistance thermometer is its temperature coefficient of resistance: TCR = (DR/R)(1/DT) in units of % resistance per degree of temperature. Thermistors are resistive elements made of semiconductor materials and have a negative coefficient of resistance. The mechanism governing the resistance change of a thermistor is the increase in the number of conducting electrons with an increase in temperature due to thermal generation, i.e., the electrons which are the least tightly bound to the nucleus (valence electrons) gain sufficient thermal energy to break away and become influenced by external fields. Thermistors can be measured in the same manner as resistance thermometers, but thermistors have up to 100 times higher TCR values. Displacement and Force Many types of forces are sensed by the displacements they create. For example, the force due to acceleration of a mass at the end of a spring will cause the spring to stretch and the mass to move. Its displacement from the zero acceleration position is governed by the force generated by the acceleration (F = m · a) and the restoring force of the spring. Another example is the displacement of the center of a deformable membrane due to a difference in pressure across it. Both of these examples use multiple transduction mechanisms to produce an electronic output: a primary mechanism which converts force to displacement (mechanical to mechanical) and then an intermediate mechanism to convert displacement to an electrical signal (mechanical to electrical). Displacement can be measured by an associated capacitance. For example, the capacitance associated with a gap which is changing in length is given by C = area ¥ dielectric constant/gap length. The gap must be very small compared to the surface area of the capacitor, since most dielectric constants are of the order of 1 ¥ 10–13 farads/cm and with present methods, capacitance is readily resolvable to only about 10–12 farads. This is because measurement leads and contacts create parasitic capacitances the same order of magnitude. If the capacitance is measured at the generated site by an integrated circuit (see Section III), capacitances as small as 10–15 farads