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Medical Instrumentation Application and Design, 4th Edition

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Presentation on theme: "Medical Instrumentation Application and Design, 4th Edition"— Presentation transcript:

1 Medical Instrumentation Application and Design, 4th Edition
Chapter 2. Basic Sensors and Principles Robert A. Peura and John G. Webster Medical Instrumentation Application and Design, 4th Edition John G. Webster, Univ. of Wisconsin, Madison ISBN:

2 fig_02_01 Figure 2.1 Three types of potentiometric devices for measuring displacements (a) Translational. (b) Single-turn, (c) Multiturn. (From Measurement Systems: Application and Design, by E. O. Doebelin. Copyright © 1990 by McGraw-Hill, Inc. Used with permission of McGraw-Hill Book Co.)

3 fig_02_02 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. R1 = B, R2 = A, R3 = D, and R4 = C when the unbonded strain gage is connected for translational motion. Resistor Ry and potentiometer Rx are used to initially balance the bridge, vi is the applied voltage and Dvo is the output voltage on a voltmeter or similar device with an internal resistance of Ri. fig_02_02

4 eq_02_01 eq_02_01

5 fig_02_03 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 (c) 1959 by McGraw-Hill, Inc. Used with permission of McGraw-Hill Book Co.]

6 table_02_01 table_02_01

7 fig_02_04 Figure 2.4 Typical semiconductor strain-gage units (a) Unbonded, uniformly doped, (b) Diffused p-type gage, (c) Integrated pressure sensor, (d) Integrated cantilever-beam force sensor. (From Transducers for Medical Measurements: Application and Design, by R. S. C. Cobbold. Copyright © 1974, John Wiley and Sons, Inc. Reprinted by permission of John Wiley and Sons, Inc.)

8 fig_02_05 Figure 2.5 Isolation in a disposable blood-pressure sensor. Disposable blood pressure sensors are made of clear plastic so air bubbles are easily seen. Saline flows from an intravenous (IV) bag through the clear IV tubing and the sensor to the patient. This flushes blood out of the tip of the indwelling catheter to prevent clotting. A lever can open or close the flush valve. The silicon chip has a silicon diaphragm with a four-resistor Wheatstone bridge diffused into it. Its electrical connections are protected from the saline by a compliant silicone elastomer gel, which also provides electrical isolation. This prevents electric shock from the sensor to the patient and prevents destructive currents during defibrillation from the patient to the silicon chip.

9 fig_02_06 Figure 2.6 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.]

10 fig_02_07 Figure 2.7 Inductive displacement sensors (a) Self-inductance, (b) Mutual inductance, (c) Differential transformer.

11 fig_02_08 Figure 2.8 (a) As x moves through the null position, the phase changes 180°, while the magnitude of vo 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.

12 fig_02_09 Figure 2.9 Capacitance sensor for measuring dynamic displacement changes.

13 eq_02_08

14 fig_02_10 Figure 2.10 (a) Equivalent circuit of piezoelectric sensor, where Rs = sensor leakage resistance, Cs = sensor capacitance, Cc = cable capacitance, Ca = amplifier input capacitance, Ra = 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.)

15 eq_02_13 eq_02_13

16 fig_02_11 Figure 2.11 Sensor response to a step displacement (From Doebelin, E. O Measurement Systems: Application and Design, New York: McGraw-Hill.)

17 eq_02_17

18 fig_02_12 Figure 2.12 (a) High-frequency circuit model for piezoelectric sensor. Rs is the sensor leakage resistance and Cs the capacitance. Lm, Cm, and Rm represent the mechanical system, (b) Piezoelectric sensor frequency response. (From Transducers for Biomedical Measurements: Principles and Applications, by R. S. C. Cobbold. Copyright (c) 1974, John Wiley and Sons, Inc. Reprinted by permission of John Wiley and Sons, Inc.)

19 fig_02_13 Figure 2.13 Thermocouple circuits (a) Peltier emf. (b) Law of homogeneous circuits, (c) Law of intermediate metals, (d) Law of intermediate temperatures.

20 eq_02_21 eq_02_21

21 fig_02_14 Figure 2.14 The LT1025 electronic cold junction and the hot junction of the thermocouple yield a voltage that is amplified by an inverting amplifier.

22 fig_02_15 Figure 2.15 (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 currents. The intersection of the thermistor curves and the diagonal lines with 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, Framingham, MA; used by permission.]

23 Figure 2.16 (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.] fig_02_16

24 eq_02_25 eq_02_25

25 fig_02_17 Figure 2.17 Stationary chopped-beam radiation thermometer (From Transducers for Biomedical Measurements: Principles and Applications, by R. S. C. Cobbold. Copyright (c) 1974, John Wiley and Sons, Inc. Reprinted by permission of John Wiley and Sons, Inc.)

26 fig_02_18 Figure 2.18 The infrared thermometer opens a shutter to expose the sensor to radiation from the tympanic membrane. From J. G. Webster, Ed., Bioinstrumentation, New York: John Wiley & Sons, 2004.

27 fig_02_19 Figure 2.19 Details of the fiber/sensor arrangement for the GaAs semiconductor temperature probe.

28 fig_02_20 Figure 2.20 (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.

29 Figure 2.21 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; CO2 (not shown), 10,600 nm. (b) Filters. A Corning 5-56 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. fig_02_21

30 fig_02_22 Figure 2.22 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. fig_02_22

31 fig_02_23 Figure 2.23 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.

32 eq_02_28

33 fig_02_24 Figure 2.24 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 mA flow through RL.

34 fig_02_25 Figure 2.25 Voltage–current characteristics of irradiated silicon p-n junction. For 0 irradiance, both forward and reverse characteristics are normal. For 1 mW/cm2, open-circuit voltage is 500 mV and short-circuit current is 0.8 mA. For 10 mW/cm2, open-circuit voltage is 600 mV and short-circuit current is 8 mA.


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