Medical electronics part 10 Physiological transducers 10- Physiological...– Active transducers – Passive transducers • By the kind of principle used: – Physical – Chemical

Politechnika Łódzka, ul. Żeromskiego 116, 90-924 Łódź, tel. (042) 631 28 83

www.kapitalludzki.p.lodz.pl

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Prezentacja multimedialna współfinansowana przez

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Europejskiego Funduszu Społecznego w projekcie pt.

Medical electronics – part 10

Physiological transducers

Projekt współfinansowany przez Unię Europejską

w ramach Europejskiego Funduszu Społecznego

Physiological transducers 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 non-

electric phenomenon associated with the physiological events into electric

quantities:

• Voltage

• Current

• Resistance

• Capacitance

• Inductance




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




Performance characteristics of transducers

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





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





Dynamic chracteristics and parameters

For linear transducers all dynamic characteristics and parameters may be

calculated from transducer’s transmitance

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

)(

)()(

sX

sYsH




Example performance characteristics

Pressure sensor

Source: http://www.sensortechnics.com




Example performance characteristics

Pressure sensor

Source: http://www.sensortechnics.com




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





Potentiometric transducers

A B

C

Length (L)

Position (X)

VA=0 VB=Vsup

RAC = RAB* X / L

VAC = Vsup* RAC / RAB = X * Vsup / L

Output [V] Intput [mm] Sensitivity [V/mm]





Variable cpacitance transducers

Position (X)

C = ε * ε0 * S / X

Position (X)

H

d

C = (1 + ε * X / H) * ε0 * S / d





Variable inductance transducers

Position (X)

H

L = (1 + μ * X / H) * μ0 * G * n 2





Encoders

Position (X)

A = a0*20 + a1*2

1 + a2*22 + a3*2

3 = round(X / d)

a0…a3 = {0,1}

a3 a2 a1 a0

d




Transducers based on displacement

Force transducer

Force (F)

x = k * F

Displacement (x)

Example: hand dynamometer

Source:

http://www.rehmed.com.pl/index.php/product/image/

1006/baseline.png




Transducers based on displacement

Pressure transducer

Pressure (p)

x = k * F = k * p * S

Displacement (x)

Example: compression tester

Source:

http://www.narzedziaok.pl/files/81710[1].jpg




Strain gauge pressure transducers

The resistance of conductors and semiconductors depends on their

deformation. Relation of these two quantities for stretched wire

describes a gauge factor:

LL

RRg

/

/




Strain gauge pressure transducer

V

R

R R

R

R

R

V+

V- Source: http://www.sensortechnics.com




Piezo-electric transducers

Force (F)

Charge (Q)

Q = d * F

V = Q / C = d * F / C

Voltage (V)




Acoustic pressure transducers (microphones)

Magnet

Electro-dynamic

microphone

Electret microphone Piezo-electric

microphone

Crystal




Temperature transducers

• Thermocouple – two wires of different materials joined at either end

Vjunct ~T

• Electrical resistance transducer

R(t) = R0 * (1 + a * T)

• Thermistor

R(T) = a * eb * T

• Silicon diode

Vf ~T

• Infrared detectors




Flow transducers

Flow resistance (R)

p1 p2

Pressure

Flow (U)

U = R * (p1 – p2)

Rotaing speed (N)

Flow (U)

U = k * N




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

Politechnika Łódzka, ul. Żeromskiego 116, 90-924 Łódź, tel. (042) 631 28 83

www.kapitalludzki.p.lodz.pl

„Innowacyjna dydaktyka bez ograniczeń - zintegrowany

rozwój Politechniki Łódzkiej - zarządzanie Uczelnią,

nowoczesna oferta edukacyjna i wzmacniania zdolności

do zatrudniania osób niepełnosprawnych”

Prezentacja multimedialna współfinansowana przez

Unię Europejską w ramach

Europejskiego Funduszu Społecznego w projekcie pt.

Medical electronics – part 10

Physiological transducers

Documents

Medical electronics part 10 Physiological transducers 10- Physiological...– Active transducers – Passive transducers • By the kind of principle used: – Physical – Chemical