basic sensors and sensing principles

Basic sensors and principlesBasic sensors and principles

Session 1: Displacement Session 3: Optics and RadiationSession 1: Displacement Resistive sensors

Inductive sensors

Session 3: Optics and Radiation Radiation sources

Geometrical and fiber optics Inductive sensors

Capacitive sensors

Geometrical and fiber optics

Optical filters

Piezoelectric sensors

Session 2: Temperature Radiation sensors

Thermocouples

Thermistors

Radiation

Fiber opticsProf. Kaj LindecrantzBIOMEDICAL ENGINEERING

Biomedical Instrumentation

Fiber optics


Prof. Kaj LindecrantzBIOMEDICAL ENGINEERINGBiomedical Instrumentation


Session 1: Displacement Session 3: Optics and RadiationSession 1: Displacement Resistive sensors

Inductive sensors

Session 3: Optics and Radiation Radiation sources

Geometrical and fiber optics Inductive sensors

Capacitive sensors

Geometrical and fiber optics

Optical filters

Piezoelectric sensors

Session 2: Temperature Radiation sensors

Thermocouples

Thermistors

Radiation

Fiber opticsProf. Kaj LindecrantzBIOMEDICAL ENGINEERING

Biomedical Instrumentation

Fiber optics

Resistive sensorsResistive sensors

P t ti t+ uut -

Potentiometers

+ uin -



Strain gagesg g

ALR

Temperature effects have to

A

Temperature effects have to be considered



Bridge circuitsgSuitable when small changes in resistanceCompensation for temperature effectsCompensation for temperature effects

Wheatstone bridge

112 EREREV X

1//1 213213 RRRR

ERR

ERR

EVXX

G

- +

E


Inductive sensorsInductive sensors

L 2G NonlinearL = n2Gμ L = inductance b f t f il

- Nonlinear

+ Can be made extremely sensitive

n = number of turns of coil G = geometric form factor μ = effective permeability of medium μ effective permeability of medium


Capacitive sensorsCapacitive sensors

AQCxV

QC r0

For small x

ε0 = dielectric constant of free spaceε = relative dielectric constant of the insulator

AC 0εr = relative dielectric constant of the insulatorA = area of the platesx = distance between plates

xC r0

x distance between platesThe distance is modified to alter the capacitance

Q = chargeV = voltage


V = voltage


How do we measure theHow do we measure the displacement?

Let us use the well known negativeLet us use the well known negative feedback of the op-amp: Inverting amplifier x

AC r0C CjZV

1p x

Ci

Cx

i

x

i

x

i Cj

CjZZ

VV

1

0

CCj C-

Ci

Vi Vx

i

x

i

CC

CjCj

KxxA

C

r

i 0

~ +i Vo

io KxVV



Si l f b i ti FSimple fabrication Accurate

F


Piezoelectric sensorsPiezoelectric sensors

Pi l t i t i l t l t i t ti l hPiezoelectric materials generate electric potential when strained1 Force1. Force 2. Crystal lattice disordered 3 Displacement of electrical charges3. Displacement of electrical charges

The induced charge q is proportional to the force f appliedappliedq = kf f

-

+


ThermocouplesThermocouples

A l t ti f i t j ti fAn electromotive force exists across a junction of dissimilar metals

T1 T2

+ -+ -

E=f(T1-T2)

Figure 2 14Figure 2.14


ThermistorsThermistorsSemiconductors made of ceramic materialsIncreasing resistance with decreasing temperature (opposite to metals) 0

TTTTt

00

TTt

teRR β t i l k t t (K)β = material konstant (K)

T0 = reference temperature at which the resistance is R00

Rt = resistance at temperature Tt


RadiationRadiation

Every body above 0 K transmits electromagnetic powerEvery body above 0 K transmits electromagnetic powerThis power is detected e.g. by an infrared sensor

Radiant energy

IR-detector

energy


Fiber opticsFiber optics

S f th ti l i b b d b th iSome of the optical power is absorbed by the semi-conducting sensor

Th t f b b d i ithThe amount of absorbed power increases with temperature

Transmitting fibre

Receiving fibre Gallium arsenide Receiving fibre (GaAs) sensor


Optical measurementsOptical measurementsSource Optics Filter Test object Detector

Tungsten lamps

Light

Lens

Fiber optics

Amplitude reduction: partly silvered glass,

Finger

Earlobe

Thermal sensors: thermistorLight

emitting diodes

gpolaroid filters

Color filters: Organic dye in

Quantum sensors: photo diodeLasers Organic dye in

gelatinphoto diode, photo transistor


Optical measurementsOptical measurements

P l i t t ti i t i l bl dPulse oximeter: oxygen saturation in arterial bloodTwo wavelengths: red and infraredMake use of varying part to ensure measuring arterial

blood

Arterial bloodArterial bloodon

Venous blood

Other tissueAmbient lightA

bsor

btio

Ambient lightA

Time


Pulse OximetryPulse Oximetry


basic sensors and sensing principles

Documents

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principles basic sensors

t ti f i t j ti f

fingerearlobe thermal

t ttte r r

organic dye inquantum

radiation session