Chapter 1-Sensors and Conditioning Circuits

8/12/2019 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



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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



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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



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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



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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



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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


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:


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


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)


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


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


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


V

R

Rx

U0

Uout

0x

out

RU U

R


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Displacement sensor - inductive

LVDT (Linear Variable Differential Transformer)


Vo=k.Vin.x sec2 sec1Ferrite core

Primary Magnetic sheld

x0


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Mutual inductancedifferential transformer


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


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


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


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


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



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Strain gauge


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:


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:


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


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


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)
http://www.analog.com/http://www.analog.com/


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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)
http://www.analog.com/http://www.analog.com/


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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


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


Temperat re Thermoelectric effect


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TemperatureThermoelectric effect

Seebeck effecttemperature difference results in a potential

difference


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


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


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



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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



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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


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

Chapter 1-Sensors and Conditioning Circuits

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