Medical electronics part 10 Physiological transducers

Transkrypt

1 Medical electronics part 10 Prezentacja multimedialna współfinansowana przez Unię Europejską w ramach Europejskiego Funduszu Społecznego w projekcie pt. Innowacyjna dydaktyka bez ograniczeń - zintegrowany rozwój Politechniki Łódzkiej - zarządzanie Uczelnią, nowoczesna oferta edukacyjna i wzmacniania zdolności do zatrudniania osób niepełnosprawnych Politechnika Łódzka, ul. Żeromskiego 116, Łódź, tel. (042)

2 2 Introduction to transducers Transducers are devices which convert one form of energy into another. In designing of elctronic devices it is usual practice to convert all nonelectric phenomenon associated with the physiological events into electric quantities: Voltage Current Resistance Capacitance Inductance

3 3 Classification of transducers There are three main ways of transducers classification: By the process used to convert the signal energy into an electrical signal: Active transducers Passive transducers By the kind of principle used: Physical Chemical By application for measuring a specific physiological variable, e.g.: Flow transducers Pressure transducers Temperature transducers

4 Performance characteristics of transducers 4 Static chracteristics and parameters Accuracy algebraic difference between the indicated value and the true or theoretical value of the measurand Precision degree of repeatability of a measurement Resolution the smallest measureable input increment Sensitivity transfer ratio of output to input Drift change of baseline or sensitivity with time, temperature etc. Linearity closeness of a transducer s calibration curve to a specified straight line

5 Performance characteristics of transducers 5 Static chracteristics and parameters Treshold the smallest change in the measurand that will result in a measurable change in the transducer output Noise unwanted signal at the output due either to internal source or to interference Hysteresis describes change of output with the same value of input but with different history of input variation Span total operating range of the transducer Saturation input value above which output value remains unchanged Conformance - closeness of a transducer s calibration curve to a specified curve fon an inhently non-linear transducer

6 Performance characteristics of transducers 6 Dynamic chracteristics and parameters For linear transducers all dynamic characteristics and parameters may be calculated from transducer s transmitance H( s) Y( s) X ( s) Examples: magnitude and phase characteristics, frequency range, response time, phase shift, group delay etc. For non-linear transducers all dynamic characteristics and parameters have to be determined independently

7 Example performance characteristics 7 Pressure sensor Source:

8 Example performance characteristics 8 Pressure sensor Source:

9 9 Position, displacement and movement transducers Displacement measurements are of great interest because they form the basis of many transducers for measuring pressure, force, acceleration, temperature, etc. The following types of transducers are generally used for position, displacement and motion measurements: Potentiometric transducers Variable capacitance transducers Variable inductance transducers Encoders

10 10 Position, displacement and movement transducers Potentiometric transducers Length (L) A V A =0 Position (X) B V B =V sup R AC = R AB * X / L C V AC = V sup * R AC / R AB = X * V sup / L Output [V] Intput [mm] Sensitivity [V/mm]

11 11 Position, displacement and movement transducers Variable cpacitance transducers d H Position (X) Position (X) C = ε * ε 0 * S / X C = (1 + ε * X / H) * ε 0 * S / d

12 12 Position, displacement and movement transducers Variable inductance transducers H Position (X) L = (1 + μ * X / H) * μ 0 * G * n 2

13 13 Position, displacement and movement transducers Encoders Position (X) d a 3 a 2 a 1 a 0 A = a 0 *2 0 + a 1 *2 1 + a 2 *2 2 + a 3 *2 3 = round(x / d) a 0 a 3 = {0,1}

14 Transducers based on displacement 14 Force transducer Example: hand dynamometer Displacement (x) x = k * F Force (F) Source: /baseline.png

15 Transducers based on displacement 15 Pressure transducer Example: compression tester Displacement (x) Pressure (p) x = k * F = k * p * S Source:

16 Strain gauge pressure transducers 16 The resistance of conductors and semiconductors depends on their deformation. Relation of these two quantities for stretched wire describes a gauge factor: g R L / / R L

17 Strain gauge pressure transducer 17 V+ R R R V R R R V- Source:

18 18 Piezo-electric transducers Force (F) Voltage (V) Charge (Q) Q = d * F V = Q / C = d * F / C

19 Acoustic pressure transducers (microphones) 19 Electro-dynamic microphone Electret microphone Piezo-electric microphone Crystal Magnet

20 Temperature transducers 20 Thermocouple two wires of different materials joined at either end V junct ~T Electrical resistance transducer R(t) = R 0 * (1 + a * T) Thermistor R(T) = a * e b * T Silicon diode V f ~T Infrared detectors

21 Flow transducers 21 Pressure Flow (U) p 1 p 2 U = R * (p 1 p 2 ) Flow resistance (R) Flow (U) U = k * N Rotaing speed (N)

22 22 Summary Classification of transducers Performance characteristics of transducers Position, displacement and movement transducers Transducers based on displacement Strain gauge pressure transducers Piezo-electric transducers Acoustic pressure transducers (microphones) Temperature transducers Flow transducers

23 Medical electronics part 10 Prezentacja multimedialna współfinansowana przez Unię Europejską w ramach Europejskiego Funduszu Społecznego w projekcie pt. Innowacyjna dydaktyka bez ograniczeń - zintegrowany rozwój Politechniki Łódzkiej - zarządzanie Uczelnią, nowoczesna oferta edukacyjna i wzmacniania zdolności do zatrudniania osób niepełnosprawnych Politechnika Łódzka, ul. Żeromskiego 116, Łódź, tel. (042)