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17.5 Complex Sensors 517 frequency (wavelength) of the acoustic sensor. Therefore, the oscillating sensor converts the mass value into a frequency shift. Because frequency and time are the easiest variables to measure by electronic circuits, the entire sensor’s accuracy is determined virtually by the ability to assure that the coefficient S m is known and does not change during the measurement (see Fig. 17.18 as an example of this type of a sensor). Molecules or larger particles of a chemical compound deposit on the surface of the crystal increasing its mass and, subsequently, lowering its resonant frequency. An electronic circuit measures the frequency shift, which is almost a linear measure of the chemical concentration in the sampled gas. Thus, this method is sometimes called a microgravimetric technique, as added mass is extremely small. The absolute accuracy of the method depends on such factors as the mechanical clamping of the crystal, temperature, and so forth; therefore, the over-the-range calibration is usually required. Oscillating sensors are extremely sensitive. For instance, a typical sensitivity is in the range of 5 MHz cm 2 /kg, which means that 1 Hz in frequency shift corresponds to about 17 ng/cm 2 added weight. The dynamic range is quite broad: up to 20 µg/cm 2 .To assure a selectivity, a crystal is coated with a chemical layer specific for the material of interest. Another type of a gravimetric detector is a surface-acoustic-wave (SAW) sensor. The SAW is a phenomenon of propagating mechanical waves along a solid surface which is in a contact with a medium of lower density, such as air [21]. These waves are sometimes called Reyleigh waves, after the man who predicted them in 1885. As with a flextural plate, the SAW sensor is a transmission line with three essential components: the piezoelectric transmitter, the transmission line with a chemically selective layer, and the piezoelectric receiver. An electrical oscillator causes the electrodes of the transmitter to flex the substrate, thus producing a mechanical wave. The wave propagates along the transmission surface toward the receiver. The substrate may be fabricated of LiNbO 3 with a high piezoelectric coefficient [22]. However, the transmission line does not have to be piezoelectric, which opens several possibilities of designing the sensor of different materials, like silicon. The transmission surface interacts with the sample according to the selectivity of the coating, thus modulating the propagating waves. The waves are received at the other end and converted back to an electric form. Often, there is another reference sensor whose signal is subtracted from the test sensor’s output. Typical designs of the acoustic sensors which can be adapted for measuring mass are covered in Section 12.6 of Chapter 12. Here, we briefly describe the gravimetric SAW sensor which is adapted for sensing gas concentrations (Fig. 17.14).The sensor is designed in the form of a flextural thin silicon plate with two pairs of the interdigitized electrodes deposited by use of the sputtering technology. A thin piezoelectric ZnO thin film is deposited beneath the electrodes, so that the plate can be mechanically excited by the external electronic circuit. The piezoelectric film is needed to give piezoelectric properties to the silicon substrate. The top surface of the sensing plate is coated with a thin layer of a chemically selective material (or glue, if the sensor is intended to detect air pollutants). The entire sensor is positioned inside a tube where the sampled gas is blown through. The left and right pairs of the electrodes are
518 17 Chemical Sensors Fig. 17.14. Flextural-plate SAW gas sensor; deflection of the membrane is exaggerated for clarity. connected to the oscillating circuit whose frequency f 0 is determined by the natural mechanical frequency of the sensor’s plate. The circuit contains an amplifier whose output drives the excitation electrode. Due to the piezoelectric effect, this results in flexing the membrane and propagation of the deflection wave from right to left. The wave velocity is determined by the state of the membrane and its coating. The change in the mechanical properties of the coating depends on its interaction with the sampled gas. Thus, the left electrodes will detect piezoelectric response either sooner or later, depending on how fast the wave goes through the membrane. The received signal is applied to the amplifier’s input as a feedback voltage and causes the circuit to oscillate. The output frequency is a measure of the sampled gas concentration. The reference frequency is usually determined before sampling the gas. One of the possible applications for the technique is the monitoring of heterogeneous samples, such as aerosols and suspensions. The mass increase due to impacting and sticking particles (liquid–aerosol or solid–suspension) produces a strong frequency shift; however, it is also sensitive to particle size, which means that it can be used either to detect the sizes of the particles or to monitor samples with constant particle dimensions. To improve the “stickiness” of the crystal, it can be treated chemically, or an electrostatic effect can be used. The theoretical sensitivity of the flextural plate sensor is given by S m =−1/2ρd, where ρ is the average density of the plate and d is its thickness [23]. At an operating frequency of 2.6 MHz, the sensor has sensitivity on the order of −900 cm 2 /g. So, for example, if the sensor having the area of 0.2 cm 2 captures 10 ng (10 −8 g) of material, the oscillating frequency is shifted by f =−(900)(2.6 × 10 6 )(10 −8 /0.2) = −117 Hz. The SAW sensors are quite versatile and can be adapted for measuring a variety of chemical compounds. The key to their efficiency is the selection of the coating. Table 17.1 gives examples of various SAW sensors.
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HANDBOOK OF MODERN SENSORS PHYSICS,
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HANDBOOK OF MODERN SENSORS PHYSICS,
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To the memory of my father
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Preface Seven years have passed sin
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Contents Preface ..................
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Contents XI 4.5 Lenses ............
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Contents XIII 7.7.1 Micropower Impu
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Contents XV 15 Radiation Detectors
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Contents XVII Table A.9 Physical Pr
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1 Data Acquisition “It’s as lar
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1.1 Sensors, Signals, and Systems 3
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1.1 Sensors, Signals, and Systems 5
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1.2 Sensor Classification 7 oscillo
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1.3 Units of Measurements 9 Table 1
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Table 1.7. SI Basic Units Reference
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2 Sensor Characteristics “O, what
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2.2 Span (Full-Scale Input) 15 Fig.
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2.4 Accuracy 17 2.4 Accuracy Avery
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2.6 Calibration Error 19 To compute
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2.8 Nonlinearity 21 Fig. 2.4. Trans
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2.12 Resolution 23 (A) (B) Fig. 2.7
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2.16 Dynamic Characteristics 25 2.1
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2.16 Dynamic Characteristics 27 Sub
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2.17 Environmental Factors 29 2.17
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2.18 Reliability 31 guide. For inst
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2.20 Uncertainty 33 • Thermal sho
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Table 2.2. Uncertainty Budget for T
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3 Physical Principles of Sensing
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3.1 Electric Charges, Fields, and P
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3.2 Capacitance 45 The capacitor ma
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3.2 Capacitance 47 (A) (B) Fig. 3.6
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3.2 Capacitance 49 h 0 (A) (B) Fig.
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3.3 Magnetism 51 N S S N (A) (B) Fi
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3.3 Magnetism 53 electric charge ca
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3.3 Magnetism 55 3.3.3 Toroid Anoth
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3.4 Induction 57 • Changing the o
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3.5 Resistance 59 Fig. 3.16. Voltag
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3.5 Resistance 61 For pure resistan
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3.5 Resistance 63 resistor) and the
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3.5 Resistance 65 Fig. 3.19. Strain
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3.6 Piezoelectric Effect 67 (A) (B)
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3.6 Piezoelectric Effect 69 Another
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3.6 Piezoelectric Effect 71 Another
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3.6 Piezoelectric Effect 73 absorpt
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3.6 Piezoelectric Effect 75 that en
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3.7 Pyroelectric Effect 77 circuit
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3.7 Pyroelectric Effect 79 through
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3.7 Pyroelectric Effect 81 Fig. 3.2
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3.8 Hall Effect 83 Fig. 3.30. Hall
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Table 3.2. Typical Characteristics
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3.9 Seebeck and Peltier Effects 87
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3.10 Sound Waves 93 If we consider
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3.11 Temperature and Thermal Proper
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3.11 Temperature and Thermal Proper
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3.12 Heat Transfer 99 to about 35
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3.12 Heat Transfer 101 (A) (B) Fig.
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3.12 Heat Transfer 103 Fig. 3.41. S
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3.12 Heat Transfer 105 Fig. 3.42. S
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3.12 Heat Transfer 107 Fig. 3.44. W
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3.12 Heat Transfer 109 (A) (B) Fig.
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3.13 Light 111 [38] whose emissivit
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3.14 Dynamic Models of Sensor Eleme
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References 119 specified and three
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References 121 34. Thomson, W. On t
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124 4 Optical Components of Sensors
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150 Optical Components of Sensors 1
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5 Interface Electronic Circuits 5.1
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5.1 Input Characteristics of Interf
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5.1 Input Characteristics of Interf
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5.2 Amplifiers 157 (A) (B) Fig. 5.5
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5.2 Amplifiers 159 Fig. 5.7. Voltag
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5.2 Amplifiers 161 (A) (B) Fig. 5.9
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5.2 Amplifiers 163 Fig. 5.11. An eq
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5.3 Excitation Circuits 165 5.3.1 C
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5.3 Excitation Circuits 167 (A) (B)
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5.3 Excitation Circuits 169 Fig. 5.
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5.3 Excitation Circuits 171 atures.
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5.3 Excitation Circuits 173 (A) (B)
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5.4 Analog-to-Digital Converters 17
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Table 5.2. Binary Bit Weights and R
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5.5 Direct Digitization and Process
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5.5 Direct Digitization and Process
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5.6 Ratiometric Circuits 191 (A) (B
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5.7 Bridge Circuits 193 determine t
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5.7 Bridge Circuits 195 Fig. 5.38.
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5.7 Bridge Circuits 197 Fig. 5.39.
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It can be solved for the temperatur
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5.8 Data Transmission 201 (A) (B) (
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5.8 Data Transmission 203 wire meth
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5.9 Noise in Sensors and Circuits 2
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5.10 Batteries for Low Power Sensor
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References 225 nickel-metal hydrate
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6 Occupancy and Motion Detectors Se
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6.2 Microwave Motion Detectors 229
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6.2 Microwave Motion Detectors 231
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6.3 Capacitive Occupancy Detectors
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6.3 Capacitive Occupancy Detectors
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6.4 Triboelectric Detectors 237 6.4
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6.5 Optoelectronic Motion Detectors
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and the facet pitch is 6.5 Optoelec
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References 251 Fig. 6.17. Calculate
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7 Position, Displacement, and Level
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7.1 Potentiometric Sensors 255 (A)
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7.2 Gravitational Sensors 257 (A) (
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7.3 Capacitive Sensors 259 (A) (B)
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7.3 Capacitive Sensors 261 Fig. 7.7
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7.4 Inductive and Magnetic Sensors
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7.5 Optical Sensors 275 Therefore,
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7.5 Optical Sensors 277 (A) (B) (C)
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7.5 Optical Sensors 279 (A) (B) Fig
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7.5 Optical Sensors 281 Fig. 7.32.
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7.5 Optical Sensors 283 (A) (B) (C)
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7.5 Optical Sensors 285 is proporti
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7.6 Ultrasonic Sensors 287 (A) (B)
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7.7 Radar Sensors 289 (A) (B) Fig.
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7.7 Radar Sensors 291 (A) (B) Fig.
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7.8 Thickness and Level Sensors 293
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References 299 8. Dakin, J. P., Wad
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8 Velocity and Acceleration Acceler
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8.1 Accelerometer Characteristics 3
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8.2 Capacitive Accelerometers 305 a
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8.3 Piezoresistive Accelerometers 3
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8.5 Thermal Accelerometers 309 Fig.
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8.5 Thermal Accelerometers 311 forc
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8.6 Gyroscopes 313 8.6 Gyroscopes N
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8.6 Gyroscopes 315 (A) (B) Fig. 8.1
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8.6 Gyroscopes 317 Products Company
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8.7 Piezoelectric Cables 319 (A) (B
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References 321 (A) (B) Fig. 8.16.Ap
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9 Force, Strain, and Tactile Sensor
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9.1 Strain Gauges 325 (A) (B) Fig.
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9.2 Tactile Sensors 327 9.2 Tactile
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9.2 Tactile Sensors 329 (A) (B) Fig
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9.2 Tactile Sensors 331 Fig. 9.7. P
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9.2 Tactile Sensors 333 Fig. 9.9. T
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9.3 Piezoelectric Force Sensors 335
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References 337 14. Karrer, E. and L
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10 Pressure Sensors “To learn som
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10.3 Mercury Pressure Sensor 341 1
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10.4 Bellows, Membranes, and Thin P
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10.5 Piezoresistive Sensors 345 Fig
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10.5 Piezoresistive Sensors 347 bri
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10.6 Capacitive Sensors 349 (A) (B)
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10.7 VRP Sensors 351 (A) (B) Fig. 1
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10.8 Optoelectronic Sensors 353 Fig
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10.9 Vacuum Sensors 355 (A) (B) Fig
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References 357 (A) (B) (C) Fig. 10.
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11 Flow Sensors It’s a simple tas
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11.2 Pressure Gradient Technique 36
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11.3 Thermal Transport Sensors 363
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11.3 Thermal Transport Sensors 365
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11.4 Ultrasonic Sensors 367 A senso
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11.4 Ultrasonic Sensors 369 Fig. 11
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induced in the liquid. The magnitud
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11.6 Microflow Sensors 373 (A) (B)
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11.7 Breeze Sensor 375 (A) (B) Fig.
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11.9 Drag Force Flow Sensors 377 Wi
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References 379 11. Philip-Chandy, R
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12 Acoustic Sensors “Your ears wi
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12.3 Fiber-Optic Microphone 383 (A)
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12.4 Piezoelectric Microphones 385
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12.5 Electret Microphones 387 Fig.
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12.6 Solid-State Acoustic Detectors
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References 391 References 1. Hohm,
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13 Humidity and Moisture Sensors 13
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13.1 Concept of Humidity 395 Table
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13.2 Capacitive Sensors 397 Fig. 13
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13.3 Electrical Conductivity Sensor
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13.4 Thermal Conductivity Sensor 40
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13.6 Oscillating Hygrometer 403 chi
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References 405 9. Jachowicz, R. S.
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14 Light Detectors “There is noth
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14.1 Introduction 409 Table 14.1. B
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14.2 Photodiodes 411 Maximum revers
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14.2 Photodiodes 413 when i = 0), w
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14.2 Photodiodes 415 (A) (B) (C) Fi
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14.2 Photodiodes 417 Fig. 14.9. Res
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14.3 Phototransistor 419 Fig. 14.12
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14.4 Photoresistors 421 (A) (B) Fig
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14.5 Cooled Detectors 423 (A) (B) F
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14.6 Thermal Detectors 425 Table 14
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14.6 Thermal Detectors 427 Fig. 14.
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Table 14.3. Typical Specifications
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14.6 Thermal Detectors 431 A dual e
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14.6 Thermal Detectors 433 Fig. 14.
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14.6 Thermal Detectors 435 (A) (B)
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14.6 Thermal Detectors 437 Section
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14.7 Gas Flame Detectors 439 P = V
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References 441 housing assures wide
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15 Radiation Detectors Figure 3.41
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15.1 Scintillating Detectors 445 Fi
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15.2 Ionization Detectors 447 emitt
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15.2 Ionization Detectors 449 Fig.
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15.2 Ionization Detectors 451 parti
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15.2 Ionization Detectors 453 There
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References 455 the pure intrinsic t
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16 Temperature Sensors When a scien
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16 Temperature Sensors 459 from whi
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16.1 Thermoresistive Sensors 461 2.
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16.1 Thermoresistive Sensors 463 Ta
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16.1 Thermoresistive Sensors 465 Fi
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Page 534 and 535: 17.5 Complex Sensors 515 Fig. 17.12
Page 538 and 539: Table 17.1. SAW Chemical Sensors 17
Page 540 and 541: 17.6 Chemical Sensors Versus Instru
Page 550 and 551: References 531 13. LaCourse, W.R. P
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Page 554 and 555: 18.1 Materials 535 etching is a key
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Page 562 and 563: 18.2 Surface Processing 543 sensor
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Page 574 and 575: References 555 Fig. 18.17. Bonding
Page 576 and 577: Appendix Table A.1. Chemical Symbol
Page 578 and 579: Table A.4. SI Conversion Multiples
Page 580 and 581: Table A.4 Continued Light cd/in. 2
Page 582 and 583: Table A.4 Continued Cup 2.36588 ×
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Appendix 567 Table A.11. Thermoelec
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Table A.14. Mechanical Properties o
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Appendix 571 Table A.18. Typical Em
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Table A.20. Characteristics of C-Zn
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Table A.23 Continued Manufacturer P
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Table A.25. Properties of Glasses S
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Index α-particles, 443 A/D, 175, 1
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Index 581 Coulomb’s law, 40 cross
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Index 583 H 2 O, 108 Hall, 82 Hall
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Index 585 occupancy sensors, 227 od
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Index 587 SAW, 75, 388, 404, 496, 5
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Index 589 velocity sensor, 302 vert
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