Chapter 1-Sensors and Conditioning Circuits
Transcript of Chapter 1-Sensors and Conditioning Circuits
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Measuring systems
Lecturer: Andras Kis
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In class demo: RTD and photodetector
1.2
USB connector
Arduino UNO board Conditioning circuit
PT 100 Sensor
Rload
U0
0V
Uout
Rsensor
Rload=1k
U0=5 V
Uout
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In class demo: RTD and photodetector
1.3Arduino UNO board Conditioning circuit
Sensor
Rload
U0
0V
Uout
Rsensor
U0=5 V
Uout
Noise reduction
Acquisition
(Analogdigital conversion)
Data analysis (recording, averaging, etc.)
Modeling
Conditioning
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Measurement chain
1.4
Modeling
Conditioning Acquisition
Data analysis
Comparison
Sensor
Noise
reduction
and signal
processing
Action
Chapter 1
Chapter 2
Chapter 3Chapter 4
Chapter 5
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Measuring systems
Sensors and their conditioning
Modeling sensors
Noise estimation and reduction
Data acquisition
Data analysis and treatment Comparison betweeen different measurement results
Measuring Systems 1.5
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References
Georges Asch, Acquisition de
donnes, Dunod, 2003
Ph. Robert, TE vol 17, Systmes de
mesure
Transparencies
Exercises + solutions
Measuring Systems 1.6
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Organisation
Room BC 01
Exercises
- 11 problem sets
- Discussions during the exercises
- Work at home Written mock exam (end November or early December )
bonus (max +1 on the final exam)
Written exam
Prerequisits: Electrotechnique 1 and 2
Needed for: TP Measuring systems
Measuring Systems 1.7
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Expected work load
1 credit = 30 work hours (source: EPFL, CRAFT)
3 credits x 30 = 90 hours total
- 10 h preparation for the exam
=80h
-3x14 lectures + exercises=38 h for individual work at home
= 2.5-3 h/week
Measuring Systems 1.8
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Chapter 1: Sensors and
conditioning circuits
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Sensors and conditioning circuits
Introduction
- Transducer: sensor, actuator
Passive sensors and their conditioning
- TemperatureRTD (resistance temperature detector)
- Displacementcapacitive sensors
- Displacementinductive sensors
- Light intensityphotoconductors
Active sensors and their conditioning
- Temperaturethermocouple (thermoelectric effect)
- Light intensityphotovoltaic cell photovoltaic effect
- Displacement - piezoelectric gauge
Measuring Systems 1.10
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Transducer
A transduceris an element that converts one physicalquantity into another physical quantity- Mercury thermometer (temperaturedisplacement)
- Accelerometer (accelerationvoltage)
- Electrode in a battery (ionelectrical charge)
- Motor (electrical currentmechanical moment)- LED (electrical currentlight)
TransducerPhysicalquantity
Physicalquantity
Measuring Systems 1.11
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Sensor - actuator
A sensor is a transducer that converts a physical quantity intoan electrical quantity:- Resistance thermometer (temperature - resistance)
- Photodetector (light - current)
An actuator is a transducer that converts an electrical quantityinto a non-electrical quantity- Piezo actuator (charge - displacement)
- Resistive heater (current - heat)- LED (current - light)
SensorMeasured
quantityElectrical quantity
(e, I, U, R, )
ActuatorElectrical quantity Physical quantity
1.12Measuring Systems
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Sensors
Sensitivity S: response in magnitude
Transfer function: frequency response
Noise: sensitivity to perturbations (internal and
external)
1.13
Sensor Electrical quantityMeasured
quantity
Noise
x y=f(x)
dySdx
Sensitivity
Measuring Systems
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Passive and active sensors
Passive sensors- require an external power source
Examples:
- Resistive thermometer
- Capacitive displacement sensor
Active sensors- generate the electrical signal from the
measured quantity
Examples:
- Thermocouplesthermoelectric effect- Accelerometerspiezoelectric effect
1.14
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Passive sensors
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Passive sensors
Measuring Systems 1.16
Measured quantity Sensitive
characteristic
Device
Temperature Resistance RTD (resistance temperature
detector)
Mechanical (Force,
pression, acceleration,
vibrations, sound,
displacement)
Resistance,
capacitance,
inductance
potentiometer, microphone
LVDT (linear variable differential
transformer), accelerometer,
strain gauge
Light intensity Resistance photoconductor
phototransistor
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Resistive temperature sensors (RTD)
Resistance of a metal as a function of
temperature:
Measuring Systems 1.17
0 0R R f T T
R Resistance at temperature T
R0 Resistance at temperatureT0
For platinum (PT100):
20 0 01R T R A T T B T T T temperature in C
T0 = 0CR0 = 100 W
A = 3.910-3C-1
B = -5.775 10-7C-2
200
160
120
80
Resistance
(W)
3002001000
Temperature (C)
Thin film
Wound wire Linear but low sensitivity
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Resistive temperature sensors (Thermistors)
Ceramics or polymers
Measuring Systems 1.18
Non-linear but high sensitivity
Generally described by Steinhart-
Hart equation:
31
ln( ) ln( )A B R C RT
RT=25C= 10000W
A = 1.03210-3C-1
B = 2.20810-4C-1
C = 1.27610-7C-1
Example: Omega 44006
10000
8000
6000
4000
2000
Resistance
(W)
1601208040
Temperature (C)
T Temperature in Kelvin
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Semiconducting diode thermometers
Si, Ge, etc.
- pn junctions
- Inexpensive and (mostly)
linear
- Limited temperature range
(-50150 C)
Measuring Systems 1.19
1.6
1.2
0.8
0.4Voltage
drop
(V)
4003002001000
Temperature (K)
I= 10 A
Lakeshore DT 400
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Conditioning circuits for resistive sensors
Measuring Systems 1.20
V
Rload
U0
UoutRsensor
Voltage divider
0sensor
out
load sensor
RU U
R R
ForRload=Rsensor=R:
0
2
out
UU
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Conditioning circuits for resistive sensors
Measuring Systems 1.21
R
RR
Rsensor=
=R+DR
V
U0
Uout
Wheatstone bridge
02
sensorout
sensor
R RU U
R R R
ForRsensor=R:
0outU
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Displacement sensor - resistive
Potentiometer
- Resistor with a sliding contact
- Acts as a voltage divider
Measuring Systems 1.22
V
R
Rx
U0
Uout
0x
out
RU U
R
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Displacement sensor - inductive
LVDT (Linear Variable Differential Transformer)
Measuring Systems 1.23
Vo=k.Vin.x sec2 sec1Ferrite core
Primary Magnetic sheld
x0
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Mutual inductancedifferential transformer
Measuring Systems 1.24
dt
diMMdt
diLiRu 211111 )'''(
dt
diMM
dt
diLLiRRu 12''2
'
22
''
2
'
22 )'''()()(
21111 )'''()( IMMjILjRU
1222222 )'''()''''''( IMMjILjLjRRU
For Rc>> i20
1
11
2
)(')(''U
LjR
xMxMjU
x
M
M
R2
u2
L1
L2
L2
R1
Pri
sec1
sec2
i1
i2
Rc
load''
2R
u1
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Mutual inductancedifferential transformer
Measuring Systems 1.25
x
M
M
R2
u2
L1
L2
L2
R1
Pri
sec1
sec2
i1
i2
Rc
load''
2R
u1
2 1
1 1
2
2
''( ) '( )
( ) (0) ... for 0
( ) (0) ... for 0
j M x M xU U
R j L
M x M ax bx x
M x M ax bx x
( ) ( ) 2M x M x ax
2ndorder approximation:
We get a linear relationship:
XLjR
UajU
11
12
2
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Capacitive displacement sensor
Capacitance
Measuring Systems 1.26
Microphone: sound (external pressure variations) cause the
membrane to vibrate (displacement dx)
d
AC
dx (sound)
dqi
dt
dqU R
dt
C x
UC U0
R
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Conditioning for capacitive sensors
Measuring Systems 1.27
Pressure sensor
UAC
C2
C1R
R
V
Uout
C2
C11
2
flexible metallic
membrane
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Strain gauge
Principle: change in resistance upon mechanical deformation
Measuring Systems 1.28
R resistance
r resistivity
l length
S cross-sectional
area
Sl l+Dl
initial
lR
Sr strained
l lR
S Sr r
D D
D
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Strain gauge
Measuring Systems 1.29
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Strain gauge
Measuring Systems 1.30
R lK
R l
D D
2 4K
Uout=f(U0, DR/R)?Source: Wikipedia
Rload
U0
0V
Uout
Rsensor
R
RR
V
U0
Uout
Rsensor=
=R+DR
S i
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Strain gauge
Let strain be the relative change in length and
stress sthe forceFper cross-sectional area S:
Measuring Systems 1.31
Strain and stress are related through the Youngs
modulus Yand Poisson ratio n
- In the direction parallel to the stress:
- Perpendicular to the stress:
l
l
D
F
Ss
Y
s
Y
s n n
S i
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Strain gauge
Surface change:
Resistance change:
Measuring Systems 1.32
2S l
S ln
D D
R l S
R l S
r
r
D D D D
1 2R lR l
rnr
D D D Dominant termsMetals: first term (geometry)Semiconductors: second term
F
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Force sensor
Based on a strain sensor attached to a test object
Measuring Systems 1.33
Force FDeformation of the
test object
Resistance change
of the gauge
Voltage drop
in the circuit
l
l=
F
A Y
R lK
R lD D
U RI
U RD D
gauge
extension
compression
/ 0l lD
/ 0l lD
S f f l i
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Sensors for force, pressure, acceleration
Measuring Systems 1.34
J1
J2
J4
J3
J'1J'4
J'3
J'2
ForceF PressureP= P1- P2 Acceleration a
F f P f k
a x fm
D
MEMS b d l t
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MEMS-based accelerometer
Micro-Electro-Mechanical systems: integration of electronics
and mechanical elements: sensors and actuators
www.analog.com
ADXL202 accelerometer
Analog Devices website
1.35
Mi l t h i l t (MEMS)
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Microelectromechanical systems (MEMS)
Micro-Electro-Mechanical systems: integration of electronics
and mechanical elements: sensors and actuators
www.analog.com
ADXL202 accelerometer
Analog Devices website
Movement of the beam
controlled by springs with
spring constant k
1.36Measuring Systems
Mi l t h i l t (MEMS)
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Microelectromechanical systems (MEMS)
Force on a mass msubject to
acceleration a:
Restoring force from the spring:
So the deflection is:
It is read out by measuring the electrical
capacitace between the fingers
F ma
F k x Dm
x ak
D
C C x
1.37
Li ht i t it t
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Light intensity measurements
Photoconductor
- Highly resistive semiconductor (for
example CdS)
- Under illumination, electron-hole
pairs are excited and the resistance
decreases
- Requires a voltage source to operate
in a similar way to RTDs
Measuring Systems 1.38
I
Vdark
Increasing
light
intensity
Li ht i t it t
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Ic
Vce
Light intensity measurements
Phototransistor
- npn or pnp junction- Light absorbed in the base-collector junction
generates electrons that are injected into the
base and amplified by the transistors current
gain
- Higher responsivity (A/W) but longerresponse time and higher dark currents than
photodiodes
1.39
dark
Increasing
light
intensity
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Active sensors
Measuring Systems 1.40
Temperat re Thermoelectric effect
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TemperatureThermoelectric effect
Seebeck effecttemperature difference results in a potential
difference
Measuring Systems 1.41
Thomson effectheat transport due to electrical current
TA TB
conductor
TA 0
e-move from B to A energy loss
temperature increase in the middle ofthe conductor
e-move from A to B energy is
absorbed temperature decrease in
the middle
TA
TB
A B
TA
TB
current: I
Temperature Thermoelectric effect
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TemperatureThermoelectric effect
Peltier effect
Measuring Systems 1.42
Thermoelectric effect - common name for these three effects Sensor: thermocouple
Actuator: Peltier element
current I
Conductor 1Conductor 2
- The energy of an electron depends on the temperature, work function (type of the
conductor) and local EM field
- By passing from 1 to 2, the energy of an electron is modified, resulting in heat
being absorbed (cooling) or generated (heating)
Thermocouple
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Thermocouple
Measuring Systems 1.43
S - sensitivity
- characteristic of the AB metal pair- typically 10-100 mV/K
Most commontype K:
- chromel (90% Ni, 10% Cr) / alumel
(95% Ni, 2% Mn, 2% Al, 1% Si)- S = 41mV/K at room temperature
Hot junction
Cold junction
h cV S T T
Metal A
Metal B
Metal C
Practical devices have built-in cold junction compensation
Displacement Piezoelectric effect
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DisplacementPiezoelectric effect
1.44
Before polarisation After polarisationq ds
q induced charge
d piezoelectric coefficient
s mechanical stress
contacts
Occurs in materials with no inversion symmetry
Displacement Piezoelectric effect
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DisplacementPiezoelectric effect
Sources of mechanical stress
- Force, deformation, vibration,
sound
Materials
- Quartz, ceramics (PZT), PVDF
Applications
- Force and pressure sensors- Accelerometers
- Microphones
Measuring Systems Chapter 1- 45
Displacement Piezoelectric effect
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DisplacementPiezoelectric effect
Measuring Systems 1.46
AccelerometerForce sensor
Light intensity measurements Photodiode
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I
V
Light intensity measurements - Photodiode
Light is absorbed in a pn junction
Photoexcited charge carriers are
separated in the internal electric field
Voltage is generated
Non-linear response
Measuring Systems 1.47
dark
Increasing
light
intensity
Key Points
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Key Points
There is a large number of sensors and measurement principles
Passive sensors - based on measurements ofR,L, C; require apower supply
Active sensorsdirectly use the measured quantity for generating
the signal
The signal is obtained with the use of a conditioning circuit
When choosing an appropriate sensor, keep in mind the operating
principle, the measurement range, possible sources of errors