Basic Sensors and Sensing Principles
Transcript of 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
Basic sensors and principlesBasic sensors and principles
Prof. Kaj LindecrantzBIOMEDICAL ENGINEERINGBiomedical Instrumentation
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
Resistive sensorsResistive sensors
P t ti t+ uut -
Potentiometers
+ uin -
Prof. Kaj LindecrantzBIOMEDICAL ENGINEERINGBiomedical Instrumentation
Resistive sensorsResistive sensors
Strain gagesg g
ALR
Temperature effects have to
A
Temperature effects have to be considered
Prof. Kaj LindecrantzBIOMEDICAL ENGINEERINGBiomedical Instrumentation
Resistive sensorsResistive sensors
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
Prof. Kaj LindecrantzBIOMEDICAL ENGINEERINGBiomedical Instrumentation
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
Prof. Kaj LindecrantzBIOMEDICAL ENGINEERINGBiomedical Instrumentation
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
Prof. Kaj LindecrantzBIOMEDICAL ENGINEERINGBiomedical Instrumentation
V = voltage
Capacitive sensorsCapacitive sensors
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
Prof. Kaj LindecrantzBIOMEDICAL ENGINEERINGBiomedical Instrumentation
Capacitive sensorsCapacitive sensors
Si l f b i ti FSimple fabrication Accurate
F
Prof. Kaj LindecrantzBIOMEDICAL ENGINEERINGBiomedical Instrumentation
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
-
+
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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
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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
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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
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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
Prof. Kaj LindecrantzBIOMEDICAL ENGINEERINGBiomedical Instrumentation
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
Prof. Kaj LindecrantzBIOMEDICAL ENGINEERINGBiomedical Instrumentation
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
Prof. Kaj LindecrantzBIOMEDICAL ENGINEERINGBiomedical Instrumentation
Pulse OximetryPulse Oximetry
Prof. Kaj LindecrantzBIOMEDICAL ENGINEERINGBiomedical Instrumentation