ELEC4623/ELEC9734: Semester 2 2009ELEC4623/ELEC9734 ... · Unbonded – free to stretch ... ohm/cm...

ELEC4623/ELEC9734: Semester 2 2009ELEC4623/ELEC9734: Semester 2, 2009

Dr Stephen RedmondfSchool of Electrical Engineering & Telecommunications

Email: [email protected]: 9385 6101Rm: 458, ELECENG (G17)

Medical Instrumentation (Webster): Chapter 2Medical Instrumentation (Webster): Chapter 2

Notes: https://subjects.ee.unsw.edu.au/elec4623/

Session 2, 2009 ELEC4623/ELEC9734 1

on http://vista.elearning.unsw.edu.au soon…

Biomedical Instrumentation, Measurement and DesignELEC4623/ELEC9734/

Lectures 5 & 6Principles and Operation of Basic Transducers and Sensors

Session 2, 2009 2ELEC4623/ELEC9734

Outline

TransducersPerformance characteristicsResistive transducers

P i

Photoelectric transducersPhotoemissive tubePhotovoltaic cell

PotentiometersStrain gauges

Differential transformers (LVDT)Inductive transducers

Junction cellPhotoconductive cellPhototransistorInductive transducers

Capacitance transducersPiezoelectric transducersBridge circuits

Temperature sensorsResistance basedThermoelectric –g

DC BridgesTypical bridge configurationAC Bridges

thermocouplesRadiation thermometryFiber optic sensor

Optical biosensorsPulse oximetry


Transducers

A transducer is a device which converts one form of energy to anotherA sensor is a type of transducer (with an electrical output)

Electrical/computing/biomedical engineers we are mainly interested in.

Electrical/computing/biomedical engineers we are mainly interested in electrical transducers (sensors) for physiological monitoring.

Transducer

ActuatorsSensors

Physical parameter Electrical Inpute.g. Piezoelectric:

D f ti > V ltPhysical parameter

Electrical Output

Electrical Input

Physical Output

Deformation -> Voltage

Voltage -> Deformation


p y p

Sensor performance characteristics

Transfer Function (TF)Relationship between physical input signal and electrical outputU ll thi l ti hi i t d h h i thUsually, this relationship is represented as a graph showing the relationship between the input and output signal as a function of frequency (frequency response)

For electrical systems frequency response is often dimensionlessy q y pFor transducers this is not true – the TF might be in V/m, or V/°C etc.

Alternatively the transfer function could be represented by a rational equation in s or jω (or even z for digital sensors)Y is usually the output and X the inputFor practical sensors, often n = 0,1 or 2 and m = 0

1( ) m mb s b s b s bY s −+ + + +

ELEC4623/ELEC9734

1 1 01

1 1 0

...( )( ) ...

m mm m

m m

b s b s b s bY sX s a s a s a s a

−−

−

+ + + +=

+ + + +Session 2, 2009 5


SensitivityDefined in terms of the relationship between input physical signal and output electrical signaland output electrical signalGenerally the ratio of a small change in electrical output signal to a small change in physical input signalThus it may be expressed as the derivative or slope (dy/dx) of theThus it may be expressed as the derivative or slope (dy/dx) of the input-output characteristic with respect to physical signal (if relationship is nonlinear) or by TF if it a linear systemExample: A Thermometer would have "high sensitivity" if a small temperature change resulted in a large voltage change. Typical units : Volts/Kelvin



Span or Dynamic RangeThe range of input physical signals which may be converted to electrical signals by the sensorg y

Signals outside of this range are expected to cause unacceptably large inaccuracy (perhaps due to nonlinearity)

This span or dynamic range is usually specified by the sensor supplier as the range over which other performance characteristics described in thethe range over which other performance characteristics described in the data sheets are expected to apply

AccuracyGenerally defined as the largest expected error between actual and ideal

i loutput signalsSometimes quoted as a fraction of the full scale outputExample: a thermometer might be guaranteed accurate to within 5% of FSO (Full Scale Output) ( p )

Typical Units: Kelvin (for thermometer)



HysteresisSome sensors do not return to the same output valueto the same output value when the input stimulus is cycled up or downStates depend on their pimmediate history (i.e. memory)The width of the expected

i t f therror in terms of the measured quantity is defined as the hysteresis



Nonlinearity (or linearity!): The maximum deviation from a linear response over the specified dynamic rangey g

NoiseAll sensors produce some output noise in addition to the output signalThe noise of the sensor limits the performance of the systemNoise is generally distributed across the frequency spectrum

White noise is evenly distributed (uniform noise spectral density)White noise is evenly distributed (uniform noise spectral density)Many common noise sources produce a white noise distribution

Coloured noise contributes differently at different frequencies


ResolutionSensor performance characteristics

ResolutionThe minimum detectable signal fluctuation (dependent on noise)

BandwidthAll sensors have finite response times to an instantaneous change in physical signal > low pass characteristicin physical signal -> low-pass characteristicDecay times represent the time after a step change in the input signal for the sensor output to return to its original value (only with AC coupled sensors) -> high-pass characteristicThe reciprocal of these times correspond to the upper and lower cutoff frequencies, respectively. The bandwidth of a sensor is the frequency range between these two frequencies


Resistive sensors - potentiometers

Potentiometers measure displacement either linear or rotationalCan excite with AC or DC voltagesgOutput is linear (± 0.01% of FSO) provided not electrically loadedFrictional and inertial components of such potentiometersFrictional and inertial components of such potentiometers should be low in comparison with activation force to minimise dynamic distortion


Resistive sensors - strain gauges

lR β=

Resistance R is related to length l and area of cross-section of the resistor A and resistivity β of the material as:

2

RA

dl ddR A ldA lA A

dR dl dA d

β ββ

β

−⇒ = − +

dR dl dA dR l A

ββ

⇒ = − +

Poisson’s ratio µ, relates the change in diameter D to the change in

( )1 2dR dl dR l

βμβ

= + +length l as so:dD dl

D lμ= −

2 2 24 4D DdD dA dDA dA

A Dπ π⎛ ⎞

= ⇒ = ⇒ =⎜ ⎟⎝ ⎠


Dimensional effect Piezoresistive effect


fractional change in resistancefractional change in strain

/ /

G

R R β β

=

Δ Δ

The Gage factor is

( )/ /1 2/ /

R RGL L L L

β βμΔ Δ⇒ = = + +

Δ Δ

G i f i i iG is a measure of sensitivityG primarily due to dimensional effects for metals

For semiconductors, the piezoresistive effect dominatesBut higher G for semiconductor devices is offset by their higherBut higher G for semiconductor devices is offset by their higher temperature coefficient of resistivity (see next slide)Applications of strain gauge: respiration detection and other plethysmography (volume change detection)


p et ys og ap y ( o u e c a ge detect o )

Properties of strain gauge materials

Material Composition Gage Factor Temp coefficient of resistivity ( C-1*10-5)

Constantan (advance) Ni45, Cu55 2.1 ±2( )

Isoelastic Ni36, Cr8, (Mn, Si, Mo)4 Fe32 3.52 to 3.6 +17

Karma Ni74, Cr20, Fe3 Cu3 2.1 +2

Manganin Cu84, Mn12, Ni4 0.3 to 0.47 ±2

All 479 Pt92 W8 3 6 t 4 4 ±24Alloy 479 Pt92, W8 3.6 to 4.4 ±24

Nickel Pure -12 to -20 670

Nichrome V Ni80, Cr20 2.1 to 2.63 10

Silicon (p type) 100 to 170 70 to 700

Silicon (n type) -100 to -140 70 to 700

Germanium (p type) 102

Germanium (n type) -150



Instruments that use strain gages of semiconductor materials often incorporatematerials often incorporate temperature compensation Strain gauges may be bonded or un-bondedbonded or un bonded

Bonded - to surrounding (backing) materialUnbonded – free to stretch

U b d d t iUnbonded strain gauge can be made of 4 sets of strain-sensitive wires connected to form a Wheatstone bridgeform a Wheatstone bridge

Sketch equivalent circuit…


Resistive sensors - strain gages

A bonded strain-gauge element consisting of a metallic wireA bonded strain-gauge element, consisting of a metallic wire, etched foil, vacuum deposited film, or semiconductor bar as shown below, is cemented to the strained surfaceDeviation from linearity of these bonded gauges is typically 1%Deviation from linearity of these bonded gauges is typically 1% and compensation for temperature variations is made by using a second identical strain-gauge, which is not under strainIt is common to use bonded strain-gauges in four-arm bridgesIt is common to use bonded strain-gauges in four-arm bridges as this provides for some inherent temperature compensation as well as yielding up to four times larger output signal

Session 2, 2009 ELEC4623/ELEC9734


Strain gages are generally mounted on cantilevers andStrain gages are generally mounted on cantilevers and diaphragms and measure the deflection of these materials



Elastic-resistance strain gauges are also used extensively in biomedical applications, especially in cardiovascular and respiratory dimensional or plethysmographic (volume change) measurementmeasurement

These simple systems normally consist of silicone rubber tubing filled with mercury or with an electrolyte or conducting paste (NOTE: Mercury is being phased out! Google: “As madpaste. (NOTE: Mercury is being phased out! Google: As mad as a hatter”)

The ends are sealed with metal electrodes and as the tube stretches the diameter decreases and the length increasesstretches, the diameter decreases and the length increases thus causing an increase in resistance

The resistance/unit length of typical gauges is 0.02-2 ohm/cm The elastic strain gauge is linear within 1% forohm/cm. The elastic strain gauge is linear within 1% for maximum extensions of 10% of their length but for larger extensions the linearity rapidly falls


Inductive displacement transducers

Self-Inductance (L) is the property of a circuit element to produce a back-emf to resist a change in magetic fluxFor a coil with n turns, enclosing a total flux of Φ=BA (flux , g (density x area of loop) the inductance is:

nL Φ=

Faraday’s law of induction states:

i

dtdnv Φ

=

vdi

dt

=

⇒

i.e. for constant rate of change we get a constant voltageSession 2, 2009 ELEC4623/ELEC9734 19

Ldt=


Mutual-Inductance is the property of a circuit element to produce a back-emf to resist a change in magetic flux in another circuit elementTotal flux enclosed by a loop is the sum of the fluxes from the loop and its neighbouring loopsConsider two loops:p

L12 = L21, are the mutual inductances between the coilsAny change in either current induces a voltage



(a) Self inductance (b) Mutual inductance (c) Differential transformer

Any inductor L can be used to measure displacement by varying any of the coil parameters:Can be affected by external magnetic fields Differential transformer: Output Vcd = Vce – Vde (since one coil is reversed)e e sed)


Linear variable differential transformers (LVDT)

Probably the most widely used displacement transducersVery robust, can detect displacements in the nanometre (nm) rangeVery robust, can detect displacements in the nanometre (nm) range Formed from a transformer in which the coupling between the primary and a split secondary depends on the position of a ferromagnetic plunger as shown belowThe two halves of the secondary winding are connected in series


The two halves of the secondary winding are connected in series opposition. Thus the output is zero when the plunger is in the centre position and increases on either side but with opposite phase

Linear variable differential transformers (LVDT)

Oscillator frequency is usually between 60 Hz and 20 Khz.

Commercial units have typical responsivity (sensitivity) in theCommercial units have typical responsivity (sensitivity) in the range of 5 - 200 mV/volt excitation/millimetre displacement, full scale displacement of 0.1 – 250 mm and a linearity of ±0.25%.

As the core moves through the null position, the phase changes by 180° whilst the magnitude of the output voltage Vo is proportional to the displacement x

An ordinary rectifier-demodulator cannot distinguish between directions and hence a phase sensitive demodulator is required


Capacitive transducers

The capacitance C of a parallel plate capacitor of area A and separation d, containing a dielectric of relative permittivity εris given by A

Th it th b difi d b h i A d

ε0 is dielectric permittivity of free spacedAC r 0εε=

The capacitance may thus be modified by changing ε, A or d, giving rise to the three basic types of capacitive displacement transducers (see next slide)

(a) Variable permittivity(b) Variable area(c) Variable separation

In practice three plate capacitive transducers (d) are used asIn practice three plate capacitive transducers (d) are used as these relate displacement linearly to the differential change in capacitance


Capacitive transducers

xdAC

xdAC rr

+==

εεεε 02

01 ,Q: For (d), given:

xdxd +−

Show that: 21 CCx −=


21 CCd +=

Piezoelectric sensorsWhat is piezoelectricity ?p y

Strain/stress causes a redistribution of charges and results in a net electric dipole p(a dipole is kind of a battery!)

A piezoelectric material pproduces voltage by redistributing charge under mechanical strain/stressBiomedical applications:

⇒ Accelerometer (motion, posture)

⇒ Microphone (heart sound, BP)

⇒ Ultrasound (imaging blood flow)


⇒ Ultrasound (imaging, blood flow)

Piezoelectric sensors

Generate an electrical potential when we apply mechanical strain (sensor)

An applied electrical potential results in a physical deformation of the material (actuator)

Quartz is the best known naturally occurring piezoelectric material

Flexible piezoelectric plastic polymer film (polyvinylidine fluoride - PVDF) is a recent development


Piezoelectric transducersFor piezoelectric material charge density q is directly proportional toFor piezoelectric material, charge density q is directly proportional to applied force f :

k thepiezoelectric strain coefficient is typically 2x10-12 C/N for

kfq =

k = thepiezoelectric strain coefficient is typically 2x10 12 C/N for quartz

Since the opposite surfaces are metallised, a capacitor is formedTh l di h h d d i iThe voltage corresponding to the charge produced is given as:

εεAkfx

Ckfv ==

A better equation describing the physical processes involved is simply (K = proportionality constant in C/m, and x = deflection in m)

0εε rAC

( p p y / , )

q Kx=Session 2, 2009 28ELEC4623/ELEC9734

(a) equivalent circuit of piezoelectric sensor where CS

Piezoelectric transducerspiezoelectric sensor where CSand RS = sensor capacitance and leakage resistance, RA andCA = amplifier input resistance and capacitance, Cc = cable capacitance, q = charge p , q ggenerator

(b) modified equivalent circuit with current generator replacing charge generator

RsCs

Cc RaCa

replacing charge generator

Note that for voltage amplifier as shown, amplifier and cable input characteristics are about

dsame magnitudes as sensor so affect circuit

Require a charge amplifier (or integrator)

RCintegrator)


Piezoelectric transducers

Typical frequency response curve of piezoelectric transducerExhibits mechanical resonance at high frequency (modeled by R L andExhibits mechanical resonance at high frequency (modeled by R, L and C in series)

useful for clock frequency in oscillatorscan be suppressed by low-pass (LP) filtercan be suppressed by low-pass (LP) filter

Input impedance of amplifier and capacitance of transducer + cable govern high-pass (HP) cutoff


Charge amplifier

This is basically an integratorCreate a short circuit to virtual ground of opampU f db k it t t tUse feedback capacitor to capture current

This converts charge to voltage


Wheatstone bridge

Ideal for measuring small changes in resistance

h h b dOutput VG is zero when the bridge is balanced and R1/R2 = R3/Rx

Resistance type transducers may be d fconnected to one or more arms of

the bridge circuit and the variation detected either by null-balance or deflection-balance circuits

The first method uses a variable resistance in another branch to balance out the output and the other uses the actual amount ofother uses the actual amount of bridge unbalance to determine the change in transducer resistance


Wheatstone bridge

Linear!

Assume that all resistance values in the bridge are initially equal to R0and that R0 << Ri (the input resistance of the meter). If R1 and R3increase by ΔR and R and R decrease by ΔR thenincrease by ΔR, and R2 and R4 decrease by ΔR, then

00

. iRVVR

ΔΔ =

If the meter’s internal resistance Ri is now included in the calculationThis reduces to the previous expression if R0 << RiUse Thevenin equivalent to calculate:

( / )R R VΔ


2

( / )1 ( / )[1 ( / ) ]

o io

o i o

R R VVR R R R

ΔΔ =

+ + Δ−

Wheatstone bridge

It is common practice to incorporate a balancing scheme in bridge circuit as shown by introducing resistor RY and potentiometer RXas shownT i i i l di ff R i i l 10 i hTo minimize loading effects RX is approximately 10 times the resistance of the bridge leg and RY is selected to limit the maximum adjustmentStrain gauge applications normally use a value of R ~ 25 timesStrain gauge applications normally use a value of RY ~ 25 times the resistance of the bridge leg. AC balancing circuits are more complicated because a reactive as well as resistive imbalance must be compensated


Equivalent

circuits

Wheatstone bridge

Typical bridge configurationsI)Double push-pull: all four arms are identical transducers, with

R R R R R andR2 = R3 = R1 = R4 = R andΔR2= ΔR4 = - ΔR3 = - ΔR1 = ΔR

ii) Single push-pull: R2 and R3 are transducers withtransducers with

R2 = R3 = R andΔR2= - ΔR3 = ΔRR1 and R4 are fixed and equal usually to R.

iii) Simple transducer: R2 is the only transducer

R1 = R2 = R3 = R4 = Rll l f lwill cause a non-linear output for large

changes in R2


All for small changes in ΔR

AC BridgesR

R/2

AC bridges are usually formed, by replacing the resistances in R2 and R3 in the DC bridge by a

f

SV

2VS

VS

RR

2VS δ

transformerThis gives better isolation between the oscillator and the bridge and permits the two

2 R

g psides of the bridge to be exactly equal and opposite in excitationAnother advantage is that the bridge requires only two

AC resistive bridge and its equivalent circuit

bridge requires only two resistors, both of which are usually transducers A single ended output is easily obtained relative to the centreobtained relative to the centre-tap of the transformer


Capacitive and inductive bridges and equivalent circuitsL/2

Inductive and capacitive bridges can be similarly obtained L

L2

VS δSV

1L 1R

obtained

Push-pull types are preferred and result in the

2R2L

R/2

LLLRRR,LLL 2121

δ=δ=δ+===

preferred and result in the configurations and equivalent circuits shown right

LLL 21 δ=δ−=δ

21

21:1 +

C12Cright

SV CC

2VS δ

C2


Photoelectric Transducers

Employed in two ways in the measurement of physiological events in living subjects

Photosensor functions as a detector of the changes in intensityPhotosensor functions as a detector of the changes in intensity of light at a given wavelength.Photosensor detects changes in light intensity in which wavelength is relatively unimportant.

These functions are achieved by five different types of photoelectric transducers:

photoemissive tubephotoemissive tubephotovoltaic cellphotodiode (junction cell)photoresistor (photoconductive cell) p (p )phototransistor


Photoemissive Tube

An evacuated or gas filled bulb with two electrodes (the cathode and anode) O th th d th iOn the cathode there is a coating of a specially prepared material that releases electrons when illuminatedFor electron emission to occur, there are certain restrictions on the type of surface and the wavelength of the impinging g p g glight.

Electron emission is only possible if the wavelength is shorter (energy greater) than a certain threshold value (energy required to release an electron from the metal - work function)


Photoemissive Tube

A relatively high voltage (10-200V) must be applied between the two electrodesThe electrons released by light quanta are attracted to the y g qanodeThe electron flow constitutes a current that is linearly proportional to the intensity of the incident light

Both gas filled and vacuum photoemissive detectors respond quickly to changes in light intensityGas filled response times are approximately 10 msec and for a p pp yvacuum approximately 10 nsec

Note also that both exhibit a small current flow with no light (dark current). Typically 10-7 to 10-9 A


Photovoltaic Cells

Develops a voltage that can drive a substantial current through a galvanometer or other low impedance circuit.Consist of a thin coating of some non-metal like selenium orConsist of a thin coating of some non metal like selenium or silicon on an iron or steel backing. Above this is a thin transparent film of metal. The film and selenium are insulated from each other, and this region constitutes the barrier layer. When the barrier layer is illuminated, light quanta are absorbed, electrons are released, and a potential difference appears across the barrier. The transparent metal film becomes negative and seleniumThe transparent metal film becomes negative and selenium positive. Completion of the circuit between the two electrodes causes a current to flowThe resistance between the electrodes decreases withThe resistance between the electrodes decreases with illumination


Spectral characteristics of photovoltaic cells

As we can see from the graph, selenium (Se) cells have a spectrum that covershave a spectrum that covers the visible spectrum

These devices with a filterThese devices, with a filter, have a spectral curve closely resembling that of the human eyehuman eye


Photovoltaic Cells

The relationship between light intensity and the voltage developed by the photovoltaic sensor is not linear if the device is operated without a resistive load

If a resistive load is placed across the device, the current flow becomes more linearly related to light intensity as the resistance is decreasedHence, it is necessary to employ a low resistance galvanometer or measuring circuit to indicate light intensity if a linear scale is to be b i dobtained

The most undesirable feature of the photovoltaic cell in biomedical applications is its sensitivity to temperature changes as the load pp y p gresistance is varied

Many cells have an optimum value for the resistance that can be connected across the device to minimise the sensitivity change with temperature, but not necessarily the one that yields the maximum power transfer or linearity of current with light intensity


Photodiodes (Junction Cells)

Consist of a P-N junction and produce a potential across the junction due to the creation of hole-electron pairs, which are direct result of the absorption of light quanta

Usually P-I-N used to spread depletion electric field across wider distancePhotoconductive (reverse bias) mode give linear response to intensity

Many different types of junction exist, each designed to have a desired spectral sensitivityThe response of junction photocells is short enough (ms to ns) to be p j p gused as light detectors in which there is a rapid modulation of illumination (e.g. pulse oximetry)Because of their small size, high sensitivity, and short response time, junction photocells see a wide variety of uses as light detectors. One

th t i i t t i bi di l i i i i ti lluse, that is very important in biomedical engineering, is in optically coupled isolators

In these devices a photo-junction cell is used to detect the light produced by an LED, mounted a few mm away. Thus, the desired signal modulates the light produced by LED, and the light is detected by the junctionthe light produced by LED, and the light is detected by the junction photocellIn this way, the signal is coupled from one circuit to another without any electrical interconnection, thereby providing excellent electrical isolation


Phototransistors

A considerable increase in current sensitivity but also increase in response timeThe incident radiation is caused to strike the base region which is intentionallythe base region, which is intentionally left unconnectedHoles generated in the base region by the radiation cause the base potential to rise, forward biasing the base-emitter junctionjunctionElectrons flow into the base from the emitter to neutralise the extra holes. Because of the close proximity of the collector junction, the probability of an electron combining with a hole is small, and most of the electrons areand most of the electrons are immediately drawn into the collector region, which is at a high positive potentialAs a result, the total collector current is much larger than the photo-generatedmuch larger than the photo generated current (β times as large)


Photoconductive cells (photoresistors)

Consist of a thin film of a material (selenium, germanium, silicon, or a metal halide or sulphide)

100K 10K

ResistanceConductance

When exposed to certain types of radiant energy, exhibits the photoconductive phenomenon (decrease in resistance with light)

10K 1K

stan

ce,

ce (

mho

)

R

( g )In most photoconductive cells the conductance is nearly linear with high intensitySuch devices are extremely 100

1K 100Res

is R

Con

duct

anc

Such devices are extremely sensitive photo-detectors and are often employed as light-controlled switches

10010

1 10 100 1000

Illumination (ft c)


Comparison of Photoelectric TransducersMost important characteristicsMost important characteristics when evaluating various photoelectric sensors:

spectral sensitivity (wavelength)response time type of output (voltage or current)linearity with illumination i iintensity

Spectral response of the human eye for night and day vision, along with the spectral gsensitivities of photocells, photoconductors, and photoemissive tubes

The numbers in brackets identify the spectral peaks for the S curves: P-T denotes a phototransistor


Photoelectric sensor comparison

Type Advantages DisadvantagesPhotoemissive Wide spectrum,

hi h iti itRequires high voltage.O l d llhigh sensitivity,

fast response,linear response.

Only produces a small current when illuminated.

Photovoltaic Low voltage for appreciable Slow response affected byPhotovoltaic Low voltage for appreciable current.

Slow response affected by temperature changes

Photodiode (Junction cell)

Small size, fast response,high sensitivity.

Thermal noise voltage( ) g y

Photoconductive cell(Photoresistor)

Large resistance change with illumination, good sensitivity.

Problems in relating signal and light intensity linearly

Phototransistor High sensitivity,high efficiency.

Slow response


Biomedical application of photosensors: Absorption spectroscopy

Many molecules in the human body absorb/emit light at various wavelengths

In some interesting compounds the absorption/emission changesIn some interesting compounds the absorption/emission changes based on an altered property of the moleculeHaemoglobin is one interesting example (bottom right figure)


Pulse OximetryLED

IR

Detected light reflects concentration of absorbing substance, based on Beer-Lambert Law:

Fingerlight

I0 = Incident light intensityI = Transmitted light intensityc = Concentration of absorber (include arterial and venous blood +

Photodetector

tissue)ε = Extinction coefficient of absorber (function of wavelength)d = Path length of absorber

dceII ε−= 0

Typical sites: finger, ear, foreheadSignal consists of pulsatile (AC) and non-pulsatile (DC) componentsnon pulsatile (DC) components


Often quoted as SpO2: refers to the pulse oximeters

Pulse Oximetryto the pulse oximetersestimate of SaO2 (arterial oxygen saturation)

[ ]

This is usually empirically

[ ][ ] [ ]2

22 (%)

HbOHbHbOSaO+

=

This is usually empirically determined (linear fit) from the ratio of red to IR fractional variations

With good reason, it is a good approximation to theory

DCAC /


irir

redred

DCACDCACR

//

=

Future trends: Wearable pulse oximetersMade possible by miniature p y

LED and photosensors!


Temperature Sensors

1. Resistance based

a. Resistance temperature detectors (RTDs)

b. Thermistors

2. Thermoelectric – thermocouples

3. Fiber optic sensor


Resistance Temperature Detector (RTD)

RTDs are made of materials whose resistance changes in accordance with temperatureMetals such as platinum nickel and copper are commonly usedMetals such as platinum, nickel and copper are commonly usedThey exhibit a positive temperature coefficient


ThermistorsOver a small dynamic range a thermistor can b li i d

Thermistors are made from polymer/ceramic materialGenerally they have a negative

be linearised

Generally, they have a negative temperature coefficient (NTC)Ro is the resistance at a reference point (in the limit absolute 0)point (in the limit, absolute 0)

Self heating is a problemFor a low currents, voltage-current relationship is linearFor high currents it heats itself, reduces its resistance and hence the voltage gdrops again


Electronic interface (for thermistors and RTDs)

OR RTD


Thermocouples

Seebeck EffectWhen a pair of dissimilar metals are joined at one end, and there is a temperature difference between the joined ends and the openis a temperature difference between the joined ends and the open ends, thermal emf is generated, which can be measured in the open ends

This forms the basis of thermocoupless o s t e bas s o t e ocoup es

21V aT bT= +


2OUTV aT bT= +

Fiber optic temperature sensors

Nortech's fiber-optic temperature sensor probe consists of a gallium arsenide crystal and a dielectric mirror on one end ofgallium arsenide crystal and a dielectric mirror on one end of an optical fiber and a stainless steel connector at the other end

Light produced by LED is transmitted along the optical fiber via total internal reflectiontotal internal reflection

Amount of power absorbed by the semiconductor crystal increased with temperature


ReferencesMost of this material is directly taken from the following:

Webster: chapters 1 and 2Principles of Applied Biomedical Instrumentation by L.A. Geddes and L.E. Baker, published by John Wiley & Sons, 1975Piezoelectric transducer:Piezoelectric transducer: http://www.designinfo.com/kistler/ref/tech_theory_text.htm


ELEC4623/ELEC9734: Semester 2 2009ELEC4623/ELEC9734 ... · Unbonded – free to stretch ... ohm/cm...

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