Chapter 3: Sensors and/or Transducers
Introduction to Different Types of
Sensors
1
Goals of the Chapter
 Define classification of sensors and some terminologies
 Introduce various types of sensors for measurement
purpose and their applications
 Example: Displacement, motion, level, pressure, temperature, …
2
Overview




Introduction
Classification of sensors
Passive sensors
Active sensors
3
Introduction
Signal
conditioning
element
Sensing
element
Non-electrical
quantity
Signal
conversion/
processing
element
Output
presentation
Electrical
signal
 Sensors
 Elements which generate variation of electrical quantities (EQ) in
response to variation of non-electrical quantities (NEQ)
 Examples of EQ
 Temperature, displacement, humidity, fluid flow, speed, pressure,…
 Sensor are sometimes called transducers
4
Introduction …
 Advantages of using sensors include
1. Mechanical effects such as friction is reduced to the minimum
possibility
2. Very small power is required for controlling the electrical system
3. The electrical output can be amplified to any desired level
4. The electrical output can be detected and recorded remotely at a
distance from the sensing medium and use modern digital
computers
5. etc …
5
Introduction - Use of Sensors
1. Information gathering: Provide data for display purpose
 This gives an understanding of the current status of the system
parameters
 Example: Car speed sensor and speedometer, which records the
speed of a car against time
2. System control: Signal from the sensor is an input to a
controller
Desired signal
Controller
System
under
control
Output signal
Sensor
6
Introduction – Sensor Requirements
 The main function of a sensor is to respond only for the
measurement under specified limits for which it is designed
 Sensors should meet the following basic requirements
1. Ruggedness: Capable of withstanding overload

Some safety arrangements should be provided for overload
protection
2. Linearity: Its input-output characteristics must be linear
3. Repeatability: It should reproduce the same output signal when
the same input is applied again and again
4. High output signal quality
5. High reliability and stability
6. Good dynamic response
7. No hysteresis, …
7
Overview




Introduction
Classification of sensors
Passive sensors
Active sensors
8
Classification of Sensors
 Sensors can be divided on the basis of




Method of applications
Method of energy conversion used
Nature of output signals
Electrical principle
 In general, the classification of sensors is given by
 Primary and secondary sensors
 Active and passive sensors
 Analog and Digital sensors
9
Primary and Secondary Sensors
 Classification is based on the method of application
 Primary sensor
 The input NEQ is directly sensed by the sensor
 The physical phenomenon is converted into another NEQ
 Secondary sensor
 The output of the primary sensor is fed to another (secondary)
sensor that converts the NEQ to EQ
NEQ
Straingauge
Load
cell
Primary
sensor
Weight
(Force F)
EQ
NEQ
Secondary
sensor
Displacement
d
Resistance
R
10
Active and Passive Sensor
 Classification based on the basis of energy conversion
 Active sensor
 Generates voltage/current in response to NEQ variation
 Are also called self-generating sensors
 Normally, the output of active sensors is in V or mV
 Examples
 Thermocouples: A change in temperature produces output voltage
 Photovoltaic cell: Change solar energy into voltage
 Hall-effect sensors, …
NEQ
Ex. Temperature
Active
sensors
EQ
Voltage or current
11
Active and Passive ….
 Passive sensors
 Sensors that does not generate voltage or current, but produce
element variation in R, L, or C
 Need an additional circuit to produce voltage or current variation
 Examples
 Thermistor: Change in temperature leads to change in resistance
 Photo resistor: Change in light leads to change in resistance
 Straingauge: Change in length or position into change in
resistance)
 LVDT, Mic
NEQ
Passive
sensors
R, L, C
12
Analog and Digital Sensors
 Classification based on the nature of the output signal
 Analog sensor
 Gives an output that varies continuously as the input changes
 Output can have infinite number of values within the sensor’s range
 Digital sensor
 Has an output that varies in discrete steps or pulses or sampled form
and so can have a finite number of values
 E.g., Revolution counter: A cam, attached to a revolving body whose
motion is being measured, opens and closes a switch
 The switching operations are counted by an electronic counter
13
Sensor Classification
 Sensors can also be classified according to the application
 Example
 In the measurements of :








Displacement
Motion
Temperature
Pressure
Light intensity
Level
Angular Speed
Acceleration
14
Overview
 Introduction
 Classification of sensors
 Passive sensors
 Resistive sensors
 Potentiometers, temperature dependent resistors, strain gauge,
photoconductors (photoresistors), Piezoresistive
 Capacitive sensors
 Inductive sensors
 Active sensors
15
Resistive Sensors - Potentiometer

Examples: Displacement, liquid level (in petrol-tank level
indicator) using potentiometer or rheostat

Convert s linear (translatory) or angular (rotary) displacement into
a change of resistance in the resistive element provided with a
movable contact
 Petrol-tank level indicator
 Change in petrol level moves a
potentiometer arm
 Output signal is proportion to the
external voltage source applied
across the potentiometer
 The energy in the output signal
comes from the external power
source
16
Resistive Sensors – Potentiometer …
 A linear or rotary movement of a moving contact on a slide
wire indicates the magnitude of the variable as a change in
resistance which can easily be converted by a proper
electrical circuit into measurements of volt or current
17
Resistive Sensors – Temperature Dependent Resistors
 Two classes of thermal resistors are
 Metallic element
 Semiconductor
 For most metals, the resistance increases with increase in
temperature
R(T )  R0[1  1T   2T 2  ...]  R0[1  T ]
 Where  is the temperature coefficient of resistance and given as
 
1 R
T R0
 Example: Platinum
 Has a linear temperature-resistance characteristics
 Reproducible over a wide range of temperature
 Platinum Thermometers are used for temperature measurement
18
Resistive Sensors – Temperature Dependent…
 Semiconductor based resistance thermometers elements
 The resistance of such elements decreases with increasing
temperature
 Example: Thermistor
 The resistance-temperature relationship is non-linear and
governed by
R (T )  R0 e
1 1
)
T T0
( 
;
T0  3000 K
 Where R0 is the resistance at absolute temp (in Kelvin) and  is
material constant expressed in degree Kelvin
 Most semiconductor materials used for thermometry
possess high resistivity and high negative temperature
coefficients
19
Resistive Sensors – Strain Gauges
 Is a secondary transducer that senses tensile or
compressive strain in a particular direction at a point on
the surface of a body or structure
 Used to measure force, pressure, displacement
R  R ( e)
 Where e=l/l is the strain
 The resistance of an unstrained conductor is given as
R
l
A
 Under strained condition, resistance of conductor changes
by R because of l, A, and/or 
20
Resistive Sensors – Strain Gauges …
 To find the change in resistance R,
R
R
R
R 
l 
A 

l
A


l
l
 l  2 A  
A
A
A
 Dividing both sides by R, we get the fractional change as
R l A 



R
l
A

 Let us define eL = l/l as the longitudinal stain and eT as
the transversal strain
 Also assume that eT = -eL ,where  is the Poisson’s Ratio
21
Resistive Sensors – Strain Gauges …
 Then, Gauge Factor, G is defined as
G
R/R R/R

l / l
eL
 G is also known as Strain-Sensitivity factor; rearranging
terms, we get
G  (1  2 ) 
 Where
/
eL
/
eL
is the Piezoresistive term
 For most metals, the Piezoresistive term is about 0.4 and
0.2 <  < 0.5
 Thus, Gauge factor for metallic stain gauges is in the range 2.0–2.5
(not sensitive)
22
Resistive Sensors – Strain Gauges …
 Sensitive measurements require very high Gauge factors in
the range of 100-300
 Such factor can be obtained from semiconductor strain
gauges
 Due to the significant contribution from the Piezoresistive term
23
Piezoresistive Pressure Sensor
 Piezoresistivity is a strain dependent resistivity in a single
crystal semiconductor
 When pressure is applied to the diaphragm, it causes a
strain in the resistor
 Resistance change is proportional to this strain, and hence change
in pressure
24
Resistive Sensors – Photoconductor
 Are light sensitive resistors with non-linear negative
temperature coefficient
 Are resistive optical radiation transducers
 Photoconductors have resistance variation that depends
on illumination
 The resistance illumination characteristics is given by
R  RD e E
 Where RD is Dark Resistance and E is illumination level in Lux
 Photoconductors are used in
 Cameras, light sensors in spectrophotometer
 Counting systems where an object interrupts a light beam hitting
the photoconductor, etc.
25
Photoconductive Transducers
 A voltage is impressed on the semiconductor material
 When light strikes the semiconductor material, there is a
decrease in the resistance resulting in an increase in the current
indicated by the meter
 They enjoy a wide range of applications and are useful for
measurement of radiation at all levels
 The schematic diagram of this device is shown below
26
Overview
 Introduction
 Classification of sensors
 Passive sensors
 Resistive sensors
 Capacitive sensors
 Inductive sensors
 Active sensors
27
Capacitive Transducers
 The parallel plate capacitance is given by
A
C   0 r
d
 d= distance between plates
 A=overlapping area
 0 = 8.85x10-12 F/m is the absolute permittivity, r =dielectric
constant (r =1 for air and r =3 for plastics)
 Displacement
measurement can be
achieved by varying d,
overlapping area A
and the dielectric
constant
Schematic of a capacitive transducer.
28
Capacitive Transducers – Liquid Level Measurement
 A simple application of such
a transducer is for liquid
measurement
 The
dielectric
constant
changes
between
the
electrodes as long as there is
a change in the level of the
liquid
Capacitive transducer for liquid level
measurement.
29
Capacitive sensors – Pressure Sensor
 Use electrical property of a capacitor to measure the
displacement
 Diaphragm: elastic pressure senor displaced in proportion
to change in pressure
 Acts as a plate of a capacitor
30
Capacitive Sensor – Proximity Switch
 A capacitive sensor functions like a typical capacitor.
 The metal plate in the end of the sensor electrically
connects to the oscillator, and the object to be sensed acts
as the second plate.
 When this sensor receives power, the oscillator detects
the external capacitance between the target and the
internal sensor plate.
31
Capacitive Sensor – Proximity Switch
• This arrangement completes the circuit and provides the necessary
feedback path for the output circuit to evaluate.
C2
C1
P
C3
• Capacitance increases as any object (P) gets closer because
additional capacitance paths C2 & C3 are added and increase in value
as the separation reduces. C1 is always present.
32
Capacitive Sensor – Proximity Switch
33
Capacitive Transducers - Linear Displacement
 Variable area capacitance displacement transducer
Where:
 ra=inner cylinder radius
 rb=outer cylinder radius
 L=length of the cylinder
34
Overview
 Introduction
 Classification of sensors
 Passive sensors
 Resistive sensors
 Capacitive sensors
 Inductive sensors
 Active sensors
Introduction to Instrumntaion Eng‘g - Ch. 3Sensors and Transducers
35
Inductive Sensors
 For a coil of n turns, the inductance L is given by
n2 n2
L  A 
l
R
 Where





n: Number of turns of the coil
l: Mean length of the magnetic path
A: Area of the magnetic path
: Permeability of the magnetic material
R: Magnetic reluctance of the circuit
 Application of inductive sensors
 Force, displacement, pressure, …
 Inductance variation can be in the form of



Self inductance or
Mutual inductance: e.g., differential transformer
Eddy current
36
Linear Variable Differential Transformer (LVDT)
 Input voltage (alternating current): One primary coil
 There will be a magnetic coupling between the core and the coils
 Output voltage: Two secondary coils connected in series
 Operates using the principle of variation of mutual
inductance
 The output voltage is
a function of the
core’s displacement
 Widely used for
translating linear
motion into an
electrical signal
Schematic diagram of a differential transformer
37
LVDT - Output Characteristics
Output characteristics of an LVDT
38
LVDT – Applications
 Measure linear mechanical displacement
 Provides resolution about 0.05mm, operating range from  0.1mm
to  300 mm, accuracy of  0.5% of full-scale reading
 The input ac excitation of LVDT can range in frequency from 50 Hz
to 20kHz
 Used to measure position in control systems and precision
manufacturing
 Can also be used to measure force, pressure, acceleration,
etc..
39
LVDT – Bourdon Tube Pressure Gauge
 LVDT can be combined
with a Bourdon tube
 LVDT converts
displacement into an
electrical signal
 The signal can be
displayed on an electrical
device calibrated in
terms of pressure
40
LVDT and Bellow Combination
 Bellows produce small displacement
 Amplified by LVDT and potentiometer
41
Inductive Sensors –proximity switch
 Coil inductance increases as iron / steel object (S ) gets
closer, because lines of magnetic flux can flow through the
iron, making the effective path shorter.
S
Inductive proximity sensor
42
Inductive sensor-proximity switch
 An inductive proximity sensor has four components;
 The coil, oscillator, detection circuit and output circuit.
 The oscillator generates a fluctuating magnetic field the
shape of a doughnut around the winding of the coil that
locates in the device’s sensing face.
 When a metal object moves into the inductive proximity
sensor’s field of detection, Eddy circuits build up in the
metallic object, magnetically push back, and finally reduce
the Inductive sensor’s own oscillation field.
43
Overview




Introduction
Classification of sensors
Passive sensors
Active sensors
 Thermoelectric transducers
 Photoelectric transducers
 Piezoelectric transducers
 Hall-effect transudes
 Tachometric generators
44
Active Sensors - Thermocouple
 Thermoelectric transducers provide electrical signal in
response to change in temperature
 Example: Thermocouple
 Thermocouple: Converts thermal energy into electrical
energy
 Application: To measure temperature
 Contains a pair of dissimilar metal wires joined together at
one end (sensing or hot junction) and terminated at the
other end (reference or cold junction)
 When a temperature difference exists b/n the sensing
junction and the reference, an emf is produced
Induced emf  E   (T1  T2 )   (T12  T22 )  ....   (T1  T2 )
45
Active Sensors – Thermocouple …
 Typical material combinations used as thermocouples
Type
Materials
Temp. Range
Output voltage
(mV)
T
Copper-Constantan
-2000C to 3500C
-5.6 to 17.82
J
Iron-Constantan
0 to 7500C
0 to 42.28
E
Chromel-Constantan
-200 to 9000C
-8.82 to 68.78
K
Chromel-Alumel
-200 to 12500C
-5.97 to 50.63
R
Platinum = 13%
Rhodium = 87%
0 to 14500C
0 to 16.74
 To get higher output emf
 Connect two or more Thermocouples in series
 For measurement of average temperature
 Connect Thermocouples in parallel
46
Active Sensors – Thermocouple …

Applications


Temperature measurement
Voltage measurement


Rectifier based rms indications are waveform dependent
They are normally designed for sinusoidal signals


Hence, error for non-sinusoidal signals
Use thermocouple based voltmeters

Here, temperature of a hot junction is proportional to the true rms
value of the current
47
Active Sensors – Thermocouple Meter
 The measured a.c. voltage signal is
applied to a heater element
 A thermocouple senses the
temperature of the heater due to
2
heat generated (I rms
)
 The d.c. voltage generated in the
thermocouple is applied to a
moving-coil meter
 The thermocouple will be calibrated to read
current (Irms)
 AC with frequencies up to 100 MHz may
be measured with thermocouple meters
 One may also measure high frequency
current by first rectifying the signal to DC
and then measuring the DC
Schematic of a
thermocouple meter.
48
Overview




Introduction
Classification of sensors
Passive sensors
Active sensors
 Thermoelectric transducers
 Photoelectric transducers
 Piezoelectric transducers
 Hall-effect transudes
 Tachometric generators
49
Photoelectric Transducers
 Versatile tools for detecting radiant energy or light
 Are extensively used in instrumentation
 Most known photosensitive devices include
1. Photovoltaic cells
 Semiconductor junction devices used to convert radiation energy
into electrical energy
50
Photovoltaic Cells
 When light strikes the barrier between the transparent metal layer
and the semiconductor material, a voltage is generated
 The output of the device is strongly dependent on the load
resistance R
 The most widely used applications is the light exposure meter in
photographic work
Schematic of a photovoltaic cell.
51
Photoelectric Transducers …
2. Photo diode
 A diode that is normally reverse-biased=> Current is very low
 When a photon is absorbed, electrons are freed so current starts to
flow, i.e., the diode is forward biased
 Has an opening in its case containing a lens which focuses
incident light on the PN junction
3. Photo transistor
 Also operate in reverse-biased
 Responds to light intensity on its lens instead of base current
52
Photo transistor
53
Overview




Introduction
Classification of sensors
Passive sensors
Active sensors
 Thermoelectric sensors
 Photoelectric sensors
 Piezoelectric sensors
 Hall-effect sensors
 Tachometric sensors
54
Piezoelelectric Transducers
 Convert mechanical energy into electrical energy
 If any crystal is subject to an external force F, there will be
an atomic displacement, x
 The displacement is related to the applied force in exactly the
same way as elastic sensor such as spring
 Asymmetric crystalline material such as Quartz, Rochelle
Salt and Barium Tantalite produce an emf when they are
placed under stress
 An externally force, entering the sensor through its
pressure port, applies pressure to the top of a crystal
 This produces an emf across the crystal proportional to the
magnitude of the applied pressure
55
Piezoelelectric Transducers
 A piezoelectric crystal is placed between two plate
electrodes
 Application of force on such a plate will develop a stress
and a corresponding deformation
 With certain
crystals, this
deformation will
produce a potential
difference at the
surface of the
crystal
 This effect is called
piezoelectric effect
The piezoelectric effect
56
Piezoelelectric Transducers …
 Induced charge is proportional to the impressed force
Q=dF
 d= charge sensitivity (C/m2)/(N/m2) = proportionality constant
 Output voltage E= g t P
 t= crystal thickness
 P = impressed pressure
 g=voltage sensitivity (V/m)/(N/m2)
 Shear stress can also produce piezoelectric effect
 Widely used as inexpensive pressure transducers for
dynamic measurements
57
Piezoelelectric Transducers ….
 Piezoelelectric sensors have good frequency response
 Example: Accelerometer
Piezoelectric accelerometer
58
Piezoelelectric Transducers …
 Example: Pressure Sensors
 Detect pressure changes by
the displacement of a thin
metal
or
semiconductor
diaphragm
 A pressure applied on the
diaphragm causes a strain on
the piezoelectric crystal
 The crystal generates voltage
at the output
 This voltage is proportional to
the applied pressure
59
Overview




Introduction
Classification of sensors
Passive sensors
Active sensors
 Thermoelectric transducers
 Photoelectric transducers
 Piezoelectric transducers
 Hall-effect transudes
 Tachometric generators
60
Hall-effect Transducers
 Hall voltage is produced when a material is
 Kept perpendicular to the magnetic field and
 A direct current is passed through it
 The Hall-voltage is expressed as
VH  K H
 Where




IC 
t
Ic: Control current flowing through the Hall-effect sensor, in Amps
: Flux density of the magnetic field applied, in Wb/m2
t: Thickness of the Hall-effect sensor, in meters
KH : Hall-effect coefficient
 Hall-effect sensors are used to measure flux density
 Can detect very week magnetic fields or small change in magnetic
flux density
61
Hall-effect Transducers …
 Like active sensors, it generates
voltage VH
 It also need an external control
current IC like passive sensors
 The sensor can be used for
measurement of
 Magnetic quantities (B, )
 Mobility of carriers
 Very small amount of power
62
Hall-effect Transducers …
 Magnetic field forces
electrons to concentrate
on one side of the
conductor (mainly uses
semiconductor)
 This accumulation
creates emf, which is
proportional to the
magnetic field strength
 Used in proximity
sensors
63
Overview




Introduction
Classification of sensors
Passive sensors
Active sensors
 Thermoelectric transducers
 Photoelectric transducers
 Piezoelectric transducers
 Hall-effect transudes
 Tachometric generators
64
Tachometric Generators
 Tachometer – any device used to measure shaft’s rotation
 Tachometric generator
 A machine, when driven by a rotating mechanical force, produces
an electric output proportional to the speed of rotation
 Essentially a small generators
 Tachometric generators connect to the rotating shaft,
whose displacement is to be measured, by, e.g.,
 Direct coupling or
 Means of belts or gears
 They produce an output which primarily relates to speed
 Displacement can be obtained by integrating speed
 Types of Tachometric generators: Generally a.c. or d.c.
65
Tachometric Generators
 Voltage generated is proportional to rotation of the shaft
D.C. tachometric generator
A.C. tachometric generator
66
Application of sensors
67
Application of sensors
MAGNETIC LEVITATION
68
Application of sensor
Infrared reflection sensor
69
Application of sensors
Robots
70
Application of sensors
71