EPANDING THE CONTENT OF AN OUTLINE using notes.pptx
cn3-Sensors and Transducers-12.ppt
1. Chapter 3: Sensors and/or Transducers
1
Introduction to Different Types of
Sensors
2. 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
4. Introduction
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
Sensing
element
Signal
conditioning
element
Signal
conversion/
processing
element
Output
presentation
Non-electrical
quantity
Electrical
signal
5. 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
6. 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
6
Controller
System
under
control
Output signal
Desired signal
Sensor
7. 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
9. 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
10. 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
10
Load
cell
Strain-
gauge
NEQ
Secondary
sensor
Weight
(Force F)
Primary
sensor
NEQ EQ
Displacement
d
Resistance
R
11. 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, …
11
Active
sensors
NEQ
Ex. Temperature
EQ
Voltage or current
12. 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
12
Passive
sensors
NEQ R, L, C
13. 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
14. 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
16. 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
16
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
17. 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
18. Resistive Sensors – Temperature Dependent Resistors
Two classes of thermal resistors are
Metallic element
Semiconductor
For most metals, the resistance increases with increase in
temperature
Where is the temperature coefficient of resistance and given as
Example: Platinum
Has a linear temperature-resistance characteristics
Reproducible over a wide range of temperature
Platinum Thermometers are used for temperature measurement
18
]
1
[
...]
1
[
)
(
R 0
2
2
1
0 T
R
T
T
R
T
0
1
R
R
T
19. 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
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
K
T
e
R
T T
T 0
0
)
1
1
(
0 300
;
)
(
R 0
20. 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
Where e=l/l is the strain
The resistance of an unstrained conductor is given as
Under strained condition, resistance of conductor changes
by R because of l, A, and/or
20
)
(
R e
R
A
l
R
21. Resistive Sensors – Strain Gauges …
To find the change in resistance R,
Dividing both sides by R, we get the fractional change as
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
A
l
A
A
l
l
A
R
A
A
R
l
l
R
2
R
A
A
l
l
R
R
22. Resistive Sensors – Strain Gauges …
Then, Gauge Factor, G is defined as
G is also known as Strain-Sensitivity factor; rearranging
terms, we get
Where 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
L
e
G
/
)
2
1
(
L
e
/
L
e
l
l
G
R/R
/
R/R
23. 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
24. 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
25. 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
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
E
De
R
R
26. 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
28. Capacitive Transducers
The parallel plate capacitance is given by
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)
d
A
C r
0
Displacement
measurement can be
achieved by varying d,
overlapping area A
and the dielectric
constant
Schematic of a capacitive transducer.
28
29. 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
30. 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
31. 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
32. Capacitive Sensor – Proximity Switch
32
• This arrangement completes the circuit and provides the necessary
feedback path for the output circuit to evaluate.
• 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.
C1
C3
C2
P
34. Capacitive Transducers - Linear Displacement
Variable area capacitance displacement transducer
Where:
ra=inner cylinder radius
rb=outer cylinder radius
L=length of the cylinder
34
35. Overview
Introduction
Classification of sensors
Passive sensors
Resistive sensors
Capacitive sensors
Inductive sensors
Active sensors
35
Introduction to Instrumntaion Eng‘g - Ch. 3Sensors and Transducers
36. Inductive Sensors
For a coil of n turns, the inductance L is given by
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
R
n
l
n
A
L
2
2
37. 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
Linear Variable Differential Transformer (LVDT)
Schematic diagram of a differential transformer
The output voltage is
a function of the
core’s displacement
Widely used for
translating linear
motion into an
electrical signal
37
38. LVDT - Output Characteristics
Output characteristics of an LVDT
38
39. 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
40. 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
41. LVDT and Bellow Combination
Bellows produce small displacement
Amplified by LVDT and potentiometer
41
42. 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.
Inductive proximity sensor
42
S
43. 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
45. 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
45
)
(
....
)
(
)
( 2
1
2
2
2
1
2
1 T
T
T
T
T
T
E
emf
Induced
46. Active Sensors – Thermocouple …
Typical material combinations used as thermocouples
To get higher output emf
Connect two or more Thermocouples in series
For measurement of average temperature
Connect Thermocouples in parallel
46
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
47. 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
48. 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
heat generated ( )
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.
2
rms
I
48
50. 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
51. 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
52. 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
55. 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
56. 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
The piezoelectric effect
With certain
crystals, this
deformation will
produce a potential
difference at the
surface of the
crystal
This effect is called
piezoelectric effect
56
57. Piezoelelectric Transducers …
Induced charge is proportional to the impressed force
Q = d F
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
58. Piezoelelectric Transducers ….
Piezoelelectric sensors have good frequency response
Example: Accelerometer
58
Piezoelectric accelerometer
59. 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
61. 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
Where
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
t
I
K
V C
H
H
62. 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
63. 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
65. 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
66. Tachometric Generators
Voltage generated is proportional to rotation of the shaft
66
D.C. tachometric generator A.C. tachometric generator