Biomedical Instrumentation

Transcription

1 Biomedical Instrumentation Winter 393 Bonab University

2 Transducer,, and Actuator Transducer: a device that converts energy from one form to another : converts a physical parameter to an electrical output (a type of transducer, e.g. a microphone) Actuator: converts an electrical signal to a physical output (opposite of a sensor, e.g. a speaker) 2 Type of s Displacement s: resistance, inductance, capacitance, piezoelectric Temperature s: Thermistors, thermocouples Electromagnetic radiation s: Thermal and photon detectors

3 Displacement Measurements Used to measure directly and indirectly the size, shape, and position of the organs. Displacement measurements can be made using sensors designed to exhibit a resistive, inductive, capacitive or piezoelectric change as a function of changes in position. Directly: Change in the diameter of blood vessels / Cardiac chambers Indirectly: Change in displacement Quantify movement of liquids through heart valves Movement of microphone diaphragm movements and murmurs of heart 3

4 esistive sensors - potentiometers Measure linear and angular position esolution a function of the wire construction Measure velocity and acceleration Wire-wound Carbon-film Metal-film Conducting plastic Ceramic 2 to 500mm 0 o and more (50 o ) 4

5 esistive sensors strain gages Devices designed to exhibit a change in resistance as a result of experiencing strain to measure displacement in the order of nanometer. For a simple wire: L A A change in will result from a change in (resistively), or a change in L or A (dimension). The gage factor, G, is used to compare various strain-gage materials G L / / L 2 / L / L 5 Is Poisson s ratio D L / / D L Diameter Dimensional Semiconductor has larger G but more sensitive to temperature Piezoresistive

6 Strain-gage For semiconductors: times metals 6

7 Wheatstone Bridge v o is zero when the bridge is balanced- that is when: Proof? 2 4 / / 3 If all resistor has initial value 0 then if and 3 increase by, and 2 and 4 decreases by, then v 0 Proof? 0 v i 7

8 Unbonded strain gage: 8 Error in Fig. 2.2 legend: = A, 2 = B, 3 = D, 4 = C With increasing pressure, the strain on gage pair B and C is increased, while that on gage pair A and D is decreased. ) )( ( ) ( ) ( V V V V V V V V V V i o i b a o i b i a Initially before any pressure = 4 and 3 = 2 A B C D Wheatstone Bridge Say, blood Pressure

9 Bonded strain gage: - Metallic wire, etched foil, vacuum-deposited film or semiconductor is cemented to the strained surface - Advantages: ugged, cheap, low mass, available in many configurations and sizes - To offset temperature use dummy gage wire that is exposed to temperature but not to strain 4-bounded gages on a cantilever beam measures bite force in dental research 9

10 Bonded strain gage terminology: Carrier (substrate + cover) 0

11 Strain gages variety Available in many: Size Configurations Dimensions of stress

12 Semiconductor Integrated Strain Gages (Si, or Ge) Pressure strain gages sensor with high sensitivity But, more temperature sensitive And non-linear N, P-type substrate gage factor sign Opposite material doped in the substrate Integrated cantilever-beam force sensor 2

13 Blood-pressure sensor Clear plastic 4 cm To patient Saline Flush valve Gel IV tubing Silicon chip Electrical cable Figure 4.5 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. 3

14 Elastic-esistance Strain Gages Extensively used in Cardiovascular and respiratory dimensional and volume determinations. As the tube stretches, the diameter decreases and the length increases, causing the resistance to increase b) venous-occlusion plethysmography c) arterial-pulse plethysmography Filled with a conductive fluid (mercury, conductive paste, electrolyte solution. esistance = /cm, linear within % for 0% of maximal extension 4

15 Inductive s Ampere s Law: flow of electric current will create a magnetic field Faraday s Law: a magnetic field passing through an electric circuit will create a voltage + - v i v N d dt + v v 2 - N N v N v2 N2 5

16 Inductive s Ampere s Law: flow of electric current will create a magnetic field 6 Self-inductance Mutual inductance Differential 2 transformer L n di v L dt G n = number of turns of coil G = geometric form factor = effective magnetic permeability of the medium Faraday s Law: a magnetic field passing through an electric circuit will create a voltage

17 LVDT v o v cd v ce v de + _ LVDT : Linear variable differential transformer - full-scale displacement of 0. to 250 mm mv for a displacement of 0.0mm - sensitivity is much higher than that for strain gages Disadvantage: requires more complex signal processing 7 (a) As x moves through the null position, the phase changes 80, 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.

18 Capacitive s Capacitive sensors For a parallel plate capacitor: 0 = dielectric constant of free space r = relative dielectric constant of the insulator A = area of each plate x = distance between plates C 0 r A x Change output by changing r (substance flowing between plates), A (slide plates relative to each other), or x. 8

19 Capacitive s Sensitivity of capacitor sensor, K C x Sensitivity increases with increasing plate size and decreasing distance When the capacitor is stationary x o the voltage v =E. A change in position : x = x -x o produces a voltage o X ( j) Characteristics of capacitive sensors: High resolution (<0. nm) Dynamic ranges up to 300 µm (reduced accuracy at higher displacements) High long term stability (<0. nm / 3 hours) Bandwidth: 20 to 3 khz v o = v E. V ( j) E / x 0 r A 2 x i + o j j + i dv c dt c

20 Piezoelectric s Measure physiological displacement and record heart sounds. Certain materials generate a voltage when subjected to a mechanical strain, or undergo a change in physical dimensions under an applied voltage. Uses of Piezoelectric External (body surface) and internal (intracardiac) phonocardiography Detection of Korotkoff sounds in blood-pressure measurements Measurements of physiological accelerations Provide an estimate of energy expenditure by measuring acceleration due to human movement. 20

21 Piezoelectric s V o q k kf piezoelectric constant, C/N To find V o, assume system acts like a capacitor (with infinite leak resistance): V o q C kf C k for Quartz = 2.3 pc/n k for barium titanate = 40 pc/n kfx 0 A r Capacitor: C 0 r A x (typically pc/n, a material property) 2 For piezoelectric sensor of -cm 2 area and -mm thickness with an applied force due to a 0-g weight, the output voltage v is 0.23 mv for quartz crystal 4 mv for barium titanate crystal.

22 Models of Piezoelectric s Piezoelectric polymeric films, such as polyvinylidene fluoride (PVDF). Used for uneven surface and for microphone and loudspeakers. 22

23 Transfer Function of Piezoelectric s View piezoelectric crystal as a charge generator: q Kx K proportionality constant x deflection s : sensor leakage resistance C s : sensor capacitance C c : cable capacitance C a : amplifier input capacitance a : amplifier input resistance a a 23

24 Transfer Function of Piezoelectric s Convert charge generator to current generator: i i s c q Kx i i c s dv C dt i i o K i s dx dt dq dt Vo K dx dt a a Vo X j Ks j j j Current a a 24 K s = K/C, sensitivity, V/m = C, time constant

25 Transfer Function of Piezoelectric s - 25

26 Example 2.2 C = 500 pf leak = 0 G a = 5 M What is f c,low? Current f c, low for f c, low 2C a 500M 2 ( (5000 )( )( Hz ) 0.64 Hz ) 26

27 Transfer Function of Piezoelectric s Voltage-output response of a piezoelectric sensor to a step displacement x. Decay due to the finite internal resistance of the PZT q VC V 0 Kx C Kx The decay and undershoot can be minimized by increasing the time constant =C. 27

28 Transfer Function of Piezoelectric s 28

29 High Frequency Equivalent Circuit V o j Ks j X j j s 29

30 Temperature Measurement The human body temperature is a good indicator of the health and physiological performance of different parts of the human body. Temperature indicates: -Shock (low blood perfusion to tissue) by measuring the big-toe temperature -Infection by measuring skin temperature -Arthritis by measuring temperature at the joint -Body temperature during surgery -Infant body temperature inside incubators 30 Temperature sensors type -Thermocouples -Thermistors -adiation and fiber-optic detectors -p-n junction semiconductor (2 mv/ o C)

31 Thermocouple Electromotive force (emf) exists across a junction of two dissimilar metals. Two independent effects cause this phenomena: - Contact of two unlike metals and the junction temperature (Peltier) T B A B T 2 T E = f(t T 2 ) 2- Temperature gradients along each single conductor (Lord Kelvin) E = f (T 2 - T 22 ) 3 Advantages of Thermocouple: fast response (=ms), small size (2 μm diameter), ease of fabrication and long-term stability, high range Disadvantages: Small output voltage, low sensitivity, need for a reference temperature

32 Thermocouple Empirical calibration data are usually curve-fitted with a power series expansion that yield the Seebeck voltage. T B A B T 2 T E = f(t T 2 ) E at bt T: Temperature in Celsius eference junction is at 0 o C 32

33 Thermocouple Laws - Homogeneous Circuit law: with a circuit composed of a single homogeneous metal, one cannot maintain an electric current by the application of heat alone. See Fig. 2.3b 2- Intermediate Metal Law: The net emf in a circuit consisting of an interconnection of a number of unlike metals, maintained at the same temperature, is zero. See Fig. 2.3c -Second law makes it possible for lead wire connections 3- Successive or Intermediate Temperatures Law: See Fig. 2.3d The third law makes it possible for calibration curves derived for a given reference-junction temperature to be used to determine the calibration curves for another reference temperature. 33 E 23 E 3 E 2 a T 3 2 bt 3 a T 2 2 bt 2 T T 2 T 3

34 Thermocouple circuits 34

35 Thermoelectric Sensitivity α For small changes in temperature: T E T A B T 2 E at de dt 2 a bt 2 bt E = f(t T 2 ) Differentiate above equation to find, the Seebeck coefficient, or thermoelectric sensitivity. Generally in the range of V/ o C at 20 o C. 35

36 Thermistors Thermistors are semiconductors made of ceramic materials whose resistance decreases as temperature increases. Advantages: -Small in size (0.5 mm in diameter) -Large sensitivity to temperature changes (-3 to -5% / o C) -Blood velocity -Temperature differences in the same organ -Excellent long-term stability characteristics (=0.2% /year) Disadvantages: -Nonlinear -Self heating -Limited range 36

37 Circuit Connections of Thermistors Bridge Connection to measure voltage 3 V v a v b 2 t Amplifier Connection to measure currents 37

38 esistance ratio, / 25º C Thermistors esistance elationship between esistance and Temperature at zero-power resistance of thermistor. t 0 e [ ( T T )/ 0 TT 0 = material constant for thermistor, K (2500 to 5000 K) T o = standard reference temperature, K T o = K = 20C = 68F ] Temperature coefficient t dt dt (% / K) 2 T is a nonlinear function of temperature Figure 2.3 Typical thermistor zero-power resistance ratio-temperature characteristics for various materials Temperature, C

39 Voltage, V Voltage-Versus-Current Characteristics The temperature of the thermistor is that of its surroundings. However, above specific current, current flow generates heat that make the temperature of the thermistor above the ambient temperature A B C Air Water Current, ma (b) 39 Figure 2.3 (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 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.

40 adiation Thermometry The higher the temperature of a body the higher is the electromagnetic radiation (EM). Electromagnetic adiation Transducers - Convert energy in the form of EM radiation into an electrical current or potential, or modify an electrical current or potential. Medical thermometry maps the surface temperature of a body with a sensitivity of a few tenths of a Kelvin. Application Breast cancer, determining location and extent of arthritic disturbances, measure the depth of tissue destruction from frostbite and burns, detecting various peripheral circulatory disorders (venous thrombosis, carotid artery occlusions) 40

41 Examples (adiation Thermometry) 4

42 adiation Thermometry Sources of EM radiation: Acceleration of charges can arise from thermal energy. Charges movement cause the radiation of EM waves. The amount of energy in a photon is inversely related to the wavelength: E ev J Thermal sources approximate ideal blackbody radiators: Blackbody radiator: an object which absorbs all incident radiation, and emits the maximum possible thermal radiation (0.7 m to mm). 42

43 Spectral radiant emittance, W-cm % Total power Power Emitted by a Blackbody Stefan-Boltzman law Power emitted at a specific wavelength: W C C2 5 e T Unit : W/cm 2. m C = 3.74x0 4 (W. m 4 /cm 2 ) C 2 =.44x0 4 (m. K) T = blackbody temperature, K = emissivity (ideal blackbody = ) -2 mm m = 9.66 m T = 300 K 00% Wavelength for which W is maximum: 43 m 2898 T m m varies inversely with T - Wien s displacement law 5 0 Wavelength, m 5 20 (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. 25

44 Spectral radiant emittance, W-cm % Total power Power Emitted by a Blackbody Stefan-Boltzman law Power emitted at a specific wavelength: W C C2 5 e T Total radiant power: -2 mm m = 9.66 m 00% W t 2 W d T Unit : W/cm 2. m Wavelength, m T = 300 K Stefan' s constant ( W / cm 2 ) K 4 80% of the total radiant power is found in the wavelength band from 4 to 25 m 44

45 Thermal Detector Specifications Infrared Instrument Lens Properties: -pass wavelength > m -high sensitivity to the weak radiated signal -Short response -espond to large bandwidth Fused silica Sapphire Arsenic trisulfide Thallium bromide iodine Thermal Detectors -Low sensitivity -espond to all wavelength Photon (Quantum) Detector -higher sensitivity -espond to a limited wavelength Wavelength, m All thermal detectors Lead sulfide (PbS) Fig. a 00 Indium antimonide (InSb) (photovoltaic) 45 Fig. a) Spectral transmission for a number of optical materials. (b) Spectral sensitivity of photon and thermal detectors Wavelength, m Fig. b

46 adiation Thermometer System Suitable instrument to: Amplify Process Display The weak signals from detectors 46 Figure 2.5 Stationary chopped-beam radiation thermometer - Mirror focuses the beam on the detector - Chopped beam high gain AC amplifier - Mean value (subject to drift) is blocked - Filter: a dc signal proportional to temperature

47 adiation Thermometer System, Example esponse time: 0. second Accuracy : 0. o C 47

48 Fiber-Optic Temperature s (self reading) -Small and compatible with biological implantation. -Nonmetallic sensor so it is suitable for temperature measurements in a strong electromagnetic heating field. Gallium Arsenide (GaAs) semiconductor temperature probe. The amount of power absorbed increases with temperature 48