© From J. G. Webster (ed.), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, Ch 02 Basic Sensors and Principles

© From J. G. Webster (ed.), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, 1998.

2.1 Displacement measurements 2.2 Resistive (2.2) 2.4 Inductive (2.4) 2.5 Capacitive (2.5) 2.6 piezoelectric (2.6) 2.7 Temperature measurements * 2.8 Thermocouples * 2.9 Thermistors * 2.10 Radiation thermometry * 2.11 Fiber-optic temperature

© From J. G. Webster (ed.), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, Displacement Measurements

© From J. G. Webster (ed.), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, Resistive Sensors

© From J. G. Webster (ed.), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, Potentiometric devices for measuring displacements Translational displacement measurement Rotational displacement measurement

© From J. G. Webster (ed.), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, Strain Gages Nanometer

© From J. G. Webster (ed.), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, Gage factor: G Piezoresistive effect (due to strain-induced change in the lattice structure of the material) Dimensional effect

© From J. G. Webster (ed.), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, Figure 2.2 (a) Unbonded strain-gage pressure sensor. The diaphragm is directly coupled by an armature to an unbonded strain-gage system. With increasing pressure, the strain on gage pair B and C is increased, while that on gage pair A and D is decreased. (b) Wheatstone bridge with four active elements. R 1 = A, R 2 = B, R 3 = D, and R 4 = C when the unbonded strain gage is connected for translation motion. Resistor R y and potentiometer R x are used to initially balance the bridge. v i is the applied voltage and  v 0 is the output voltage on a voltmeter or similar device with an internal resistance of R i. Armature Strain-gage wires A D (a) C B Diaphragm (b) c b d a R4R4  o o ii R3R3 R2R2 RyRy RiRi RxRx R1R1

© From J. G. Webster (ed.), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, Figure 2.3 Typical bonded strain-gage units (a) Resistance-wire type. (b) Foil type. (c) Helical-wire type. Arrows above units show direction of maximal sensitivity to strain.[Parts (a) and (b) are modified from Instrumentation in Scientific Research, by K. S. Lion. Copyright  1959 by McGraw-Hill, Inc. Used with permission of McGraw-Hill Book Co.]

© From J. G. Webster (ed.), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, Semiconductor strain gages: Higher gage factor Greater resistivity-temperature coefficient Higher nonlinearity

© From J. G. Webster (ed.), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, Semiconductor strain-gage units (d) Integrated cantilever-beam force sensor. (a) Unbonded, uniformly doped. (b) Diffused p-type gage. c) Integrated pressure sensor. Pressure 1Pressure 2

© From J. G. Webster (ed.), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, Semiconductor strain-gage units (cont.) c) Integrated pressure sensor. Pressure 1Pressure 2 High sensitivity Good temperature compensation

© From J. G. Webster (ed.), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, 1998.

Figure 2.5 Mercury-in-rubber strain-gage plethysmography (a) Four-lead gage applied to human calf. (b) Bridge output for venous-occlusion plethysmography. (c) Bridge output for arterial-pulse plethysmography. [Part (a) is based on D. E. Hokanson, D. S. Sumner, and D. E. Strandness, Jr., "An electrically calibrated plethysmograph for direct measurement of limb blood flow." 1975, BME-22, 25-29; used with permission of IEEE Trans. Biomed. Eng., 1975, New York.]

© From J. G. Webster (ed.), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, Bridge Circuits

© From J. G. Webster (ed.), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, p

2.4 Inductive Displacement Sensors Inductive Displacement Sensors Advantage:not affected by the dielectric properties of the environment Disadvantage:affected by external magnetic field Fig. 2.7(a) PrincipleChanging the geometric form factor or moving a magnetic core within the coil  alteration in self-inductance Property Change of inductance vs displacement ： not linear Advantage:1. Low power requirement 2. produces large variations in inductance

© From J. G. Webster (ed.), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, Fig. 2.7(b) Principle ： Induced voltage in the secondary coil is a function of ： coil geometry (seperation and axial alignment), numbers of primary and secondary turns, frequency and amplitude of the excitation voltage, Property ： Induced voltage in the 2 nd coil vs coil seperation: nonlinear Trick ： To maximize the output signal ： choose the resonance frequency of the secondary coil

© From J. G. Webster (ed.), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, Fig. 2.7(C) LVDT To measure:Pressure, displacement, force Principle ： Motion of a high permeability alloy slug between the two secondary coils  Change the coupling between them Trick ： To widen the region of linearity ： connect the two coils in opposition Property ： Linearity over a large range, a change of phase by 180 (when the core passes through the central position), saturation on the end How good ？ (0.5 to 2 mV)/(0.01 mm/V), displacement 0.1 to 250 mm,  0.25% linearity Advantage: Disadvantage:More complex signal processing

© From J. G. Webster (ed.), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, Figure 2.6 Inductive displacement sensors (a) Self-inductance. (b) Mutual inductance. (c) Differential transformer. c d c c d (a)(b)(c) c d aa a b e d c b b d

© From J. G. Webster (ed.), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, Figure 2.7 (a) As x moves through the null position, the phase changes 180 , while the magnitude of v o is proportional to the magnitude of x. (b) An ordinary rectifier-demodulator cannot distinguish between (a) and (b), so a phase-sensitive demodulator is required. a b e d c a b e d c

© From J. G. Webster (ed.), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, Capacitive Sensors

© From J. G. Webster (ed.), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, Sensitivity K =

© From J. G. Webster (ed.), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, Figure 2.8 Capacitance sensor for measuring dynamic displacement changes

© From J. G. Webster (ed.), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, Piezoelectric Sensors Distortion of asymmetrical crystal lattice  charge reorientation  relative displacement of + & - charges  surface charges of opposite polarity on opposite sides

© From J. G. Webster (ed.), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, q = k f (k: piezoelectric constant, C/N) (2.13)  Cv = k f  v = k f / C = k f / (  0  r A / x) = k f x / (  0  r A) (2.14)

© From J. G. Webster (ed.), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, (b) R = R a R s /(R a + R s ) » R a C = C s + C c + C a Charge generator i s = Kdx/dt isis C R iCiC iRiR + - i a = 0 oo Amplifier (a) Charge generator q = Kx RsRs CsCs CcCc CaCa + i Amplifier = 0 oo - Amplifier Cable e Crystal x

© From J. G. Webster (ed.), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, 1998.

Figure 2.9 (a) Equivalent circuit of piezoelectric sensor, where R s = sensor leakage resistance, C s = sensor capacitance, C c = cable capacitance, C a = amplifier input capacitance, R a = amplifier input resistance, and q = charge generator. (b) Modified equivalent circuit with current generator replacing charge generator. (From Measurement Systems: Application and Design, by E. O. Doebelin. Copyright  1990 by McGraw-Hill, Inc. Used with permission of McGraw-Hill Book Co.) (b) R = R a R s /(R a + R s ) » R a C = C s + C c + C a Charge generator i s = Kdx/dt isis C R iCiC iRiR + - i a = 0 oo Amplifier (a) Charge generator q = Kx RsRs CsCs CcCc CaCa + i Amplifier = 0 oo - Amplifier Cable e Crystal x

© From J. G. Webster (ed.), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, Figure 2.10 Sensor response to a step displacement (From Measurement Systems: Application and Design, by E. O. Doebelin. Copyright  1990 by McGraw-Hill, Inc. Used with permission of McGraw-Hill Book Co.)

© From J. G. Webster (ed.), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, Figure 2.11 (a) High-frequency circuit model for piezoelectric senor. R s is the sensor leakage resistance and C s the capacitance. L m, C m, and R m represent the mechanical system. (b) Piezoelectric sensor frequency response. (From Transducers for Biomedical Measurements: Application and Design, by R. S. C. Cobbold. Copyright  1974, John Wiley and Sons, Inc. Used by permission of John Wiley and Sons, Inc.) Output voltage Input force (b) Mechanical resonance Frequency (a) RtRt CsCs CmCm LmLm RmRm f c Usable range

© From J. G. Webster (ed.), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, Temperature Measurements 2.7 Temperature measurements * 2.8 Thermocouples * 2.9 Thermistors * 2.10 Radiation thermometry * 2.11 Fiber-optic temperature

© From J. G. Webster (ed.), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, Thermocouples Thermocouple: Advantages: Fast response time (Tc: 1 ms) Small size (Diameter: 12 um) Long-term stability Ease of fabrication Disadvantages: Small output voltage low sensitivity Need for a reference temperature

© From J. G. Webster (ed.), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, Peltier emg: an emf due solely to the contact of two unlike metals and the junction temperature Net Peltier emf  (T1 – T2) Thomson (Lor Kelvin) emf: an emf due to the temperature gradient along each single conductor Net Thomson emf  (T1 – T2)^2 Seebeck (1821) discovered an emf (electromotive force) across a junction of two dissililar metals Thermoelectric thermometry Seebeck voltage: (By empirical calibration)) E = a T + (1/2) b T^2 + …

© From J. G. Webster (ed.), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, Empirical thermocouple laws (1)Homogeneous circuits (2)Intermediate metals (3)Intermediate (or successive) temperature In a circuit composed of a single homogeneous metal, one cannot maintain an electric current by the application of heat alone Same emf The net emf in a circuit consisting of an interconnection of a number of unlike metals, maintained at the same temperature, is zero. Lead wires may be attached to the thermocouple without affecting the accuracy of the measured emf Calibration curves derived for a given reference-junction temperature can be used to determine the calibration curves for another reference temperature.

© From J. G. Webster (ed.), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, Cold Junction Source: ails.aspx?cid=221&id=2709 Source: measurements/thermal/temperature/introduction/thermocouples

© From J. G. Webster (ed.), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, Thermocouple Cold Junction Compensator An electrically simulated cold junction

© From J. G. Webster (ed.), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, Thermistors Thermocouple: Advantages: Disadvantages:

© From J. G. Webster (ed.), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, 1998.

Figure 2.13 (a) Typical thermistor zero-power resistance ratio-temperature characteristics for various materials. (b) Thermistor voltage-versus-current characteristic for a thermistor in air and water. The diagonal lines with a positive slope give linear resistance values and show the degree of thermistor linearity at low cerrents. The intersection of the thermistor curves and the diagonal lines with the negative slope give the device power dissipation. Point A is the maximal current value for no appreciable self-heat. Point B is the peak voltage. Point C is the maximal safe continuous current in air. [Part (b) is from Thermistor Manual, EMC-6,  1974, Fenwal Electronics, Framinham, MA; used by permission.]  Temperature, ° C Water Air 0.1 mW 1 mW 10 mW 100 mW 1 W 100  1 k  10 k  100 k  1 M  A C B Current, mA (a) (b) Resistance ratio, R/R 25º C

© From J. G. Webster (ed.), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, Figure 2.14 (a) Spectral radiant emittance versus wavelength for a blackbody at 300 K on the left vertical axis; percentage of total energy on the right vertical axis. (b) Spectral transmission for a number of optical materials. (c) Spectral sensitivity of photon and thermal detectors. [Part (a) is from Transducers for Biomedical Measurements: Principles and Applications, by R. S. C. Cobbold. Copyright  1974, John Wiley and Sons, Inc. Reprinted by permission of John Wiley and Sons, Inc. Parts (b) and (c) are from Measurement Systems: Application and Design, by E. O. Doebelin. Copyright  1990 by McGraw-Hill, Inc. Used with permission of McGraw-Hill Book Co.] 123 Wavelength,  m (a) (b) (c) Wavelength,  m 1520 T = 300 K m = 9.66  m % Fused silica Sapphire Arsenic trisulfide Thallium bromide iodine Indium antimonide (InSb) (photovoltaic) Lead sulfide (PbS) All thermal detectors Wavelength,  m % Total power Spectral radient emittance, W-cm -2 ·mm -1

© From J. G. Webster (ed.), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, Figure 2.15 Stationary chopped-beam radiation thermometer (From Transducers for Biomedical Measurements: Principles and Applications, by R. S. C. Cobbold. Copyright  1974, John Wiley and Sons, Inc. Reprinted by permission of John Wiley and sons. Inc.)

© From J. G. Webster (ed.), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, Figure 2.16 Details of the fiber/sensor arrangement for the GaAs semiconductor temperature probe.

© From J. G. Webster (ed.), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, Figure 2.17 (a) General block diagram of an optical instrument. (b) Highest efficiency is obtained by using an intense lamp, lenses to gather and focus the light on the sample in the cuvette, and a sensitive detector. (c) Solid-state lamps and detectors may simplify the system.

© From J. G. Webster (ed.), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, Figure 2.18 Spectral characteristics of sources, filters, detectors, and combinations thereof (a) Light sources, Tungsten (W) at 3000 K has a broad spectral output. At 2000 K, output is lower at all wavelengths and peak output shifts to longer wavelengths. Light-emitting diodes yield a narrow spectral output with GaAs in the infrared, GaP in the red, and GaAsP in the green. Monochromatic outputs from common lasers are shown by dashed lines: Ar, 515 nm; HeNe, 633 nm; ruby, 693 nm; Nd, 1064 nm; CO 2 (notshown), nm. (b) Filters. A Corning 5-65 glass filter passes a blue wavelength band. A Kodak 87 gelatin filter passes infrared and blocks visible wavelengths. Germanium lenses pass long wavelengths that cannot be passed by glass. Hemoglobin Hb and oxyhemoglobin HbO pass equally at 805 nm and have maximal difference at 660 nm. (c) Detectors. The S4 response is a typical phototube response. The eye has a relatively narrow response, with colors indicated by VBGYOR. CdS plus a filter has a response that closely matches that of the eye. Si p-n junctions are widely used. PbS is a sensitive infrared detector. InSb is useful in far infrared. Note: These are only relative responses. Peak responses of different detectors differ by 107. (d) Combination. Indicated curves from (a), (b), and (c) are multiplied at each wavelength to yield (d), which shows how well source, filter, and detector are matched. (e) Photon energy: If it is less than 1 eV, it is too weak to cause current flow in Si p-n junctions.

© From J. G. Webster (ed.), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, Figure 2.19 Forward characteristics for p-n junctions. Ordinary silicon diodes have a band gap of 1.1 eV and are inefficient radiators in the near-infrared. GaAs has a band gap of 1.44 eV and radiates at 900 nm. GaP has a band gap of 2.26 eV and radiates at 700 nm.

© From J. G. Webster (ed.), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, Figure 2.20 Fiber optics. The solid line shows refraction of rays that escape through the wall of the fiber. The dashed line shows total internal reflection within a fiber. Air n = 1.0 Coating Fiber n1n1 33 44 11 22  ic n2n2

© From J. G. Webster (ed.), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, Figure 2.21 Photomultiplier An incoming photon strikes the photocathode and liberates an electron. This electron is accelerated toward the first dynode, which is 100 V more positive than the cathode. The impact liberates several electrons by secondary emission. They are accelerated toward the second dynode, which is 100 V more positive than the first dynode, This electron multiplication continues until it reaches the anode, where currents of about 1  A flow through R L.

© From J. G. Webster (ed.), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, Figure 2.22 Voltage-current characteristics of irradiated silicon p-n junction. For 0 irradiance, both forward and reverse characteristics are normal. For 1 mW/cm 2, open-circuit voltage is 600  V and short-circuit current is 8  A.