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CHAPTER 6
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SENSORS AND TRANDUCERS
6.1 INTRODUCTION
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OBJECTIVES
 To introduce the basic concepts in measurement
systems
 To define sensor terminology
 To identify sensor applications
 To present the need for microsensors
6.2 MEASUREMENT SYSTEMS
Input Signal
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=
Measurand
Sensor
(Input Tranducer)
Physical
Quantity
Chemical
Quantity
Eg. displacement,
presure
Eg. Gas concentration
Sensor : a device that converts a non-electrical physical
or chemical quantity into an electrical signal.
6.2 MEASUREMENT SYSTEMS
(cont……)
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System Boundary
Display
Input
Signal
Detect
Modify
Record
Output
Signal
Transmit
Fig. 6-1: Functional block diagram of a measurement
6.2 MEASUREMENT SYSTEMS (cont…)
System Boundary
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Input
Signal
Sensor
Processor
(input transducer)
Actuator
Output
Signal
(output transducer)
Fig. 6-2: Basic components of a measurement or
information-processing system
Processor : a device that modifies the electrical signal
coming from the sensor without changing the form of the
energy that describes the signal.
Actuator or output transducer : a device that converts
an electrical signal into a physical or chemical quantity.
6.3 CLASSIFICATION OF SENSING DEVICES
Table 6-1: Classification of sensors by signal form.
Form of Signal
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Measurands
Thermal
Temperature, heat, heat flow, entropy, heat capacity.
Radiation
Gamma rays, X-rays, ultra-violet, visible, infra red,
micro-waves, radio waves.
Mechanical
Displacement, velocity, acceleration, force, torque,
pressure, mass, flow, acoustic wavelength and
amplitude.
Magnetic
Magnetic field, flux, magnetic moment, magnetisation, magnetic permeability.
Chemical
Humidity, pH level and ions, concentration of gases,
vapours and odours, toxic and flammable materials,
pollutants.
6.3 CLASSIFICATION OF SENSING DEVICES
(cont……)
Table 6-1: Classification of sensors by signal form.
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Form of Signal
Measurands
Biological
Sugars, proteins, hormones, antigens.
Electrical
Charge, current, voltage, resistance, conductance,
capacitance, inductance, dielectric permittivity,
polarisation, frequency.
6.3 CLASSIFICATION OF SENSING DEVICES
(cont……)
Table 6-2: Classification of the human senses .
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Human
Sense
Sight
Hearing
Signal
Radiant
Mechanical
Measurand
Sensing
Device
Analogue Device
Intensity and
Rods and
Photographic
wavelength
cones in
film, photodiode,
of light
retina
Phototransistor
Intensity and
Cochlea in
Microphone
frequency of
inner ear
sound
Smell
Chemical
Odorants
Olfactory
receptor
cells in nose
Electronic nose
6.3 CLASSIFICATION OF SENSING DEVICES
(cont……)
Table 6-2: Classification of the human senses .
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Human
Sense
Touch
Signal
Mechanical
Measurand
Pressure,
Sensing
Device
Nerves
force
Analogue Device
Potentiometers
and LVDTs
(simple touch),
optical gauging
and tactical
arrays (complex
touch)
Taste
Biological
Proteins
Taste buds
in tongue
6.3 CLASSIFICATION OF SENSING DEVICES
(cont……)
Table 6-3: Classification of some common actuators.
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Function
Display
Actuator
Light emitting
Signal
Radiant
diode
Principle
Current generation of
photons
Visual display unit
Radiant
Fluorescent screen
Liquid crystal
Radiant
Transmittance of
display
polarised molecular
Crystals
Transmit
Loudspeaker
Mechanical
Generation of sound
Aerial
Radiant
Generation of radio
wave
Electric motor
Mechanical
Generation of motion
6.3 CLASSIFICATION OF SENSING DEVICES
(cont……)
Table 6-3: Classification of some common actuators.
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Function
Record
Actuator
Signal
Thermal printer
Thermal
Ink is melted
Magnetic recording
Magnetic
Magnetisation of thin
head
Principle
films on computer
disc
Laser
Radiant
Ablation of material
on optical disc
6.4 Ideal Sensor Characteristics
and Practical Limitations
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A sensor in its simplest form may be regarded as a system
with an input x (t) and output y (t).
(a) Self-exciting
(b) Modulating
External
xd
drive
System
Input
x (t)
SENSOR
Output
Input
y (t)
x (t)
SENSOR
Output
y (t) + yd
Fig. 6-3: Basic representation of self-exciting and
modulating sensor systems
6.4 Ideal Sensor Characteristics
and Practical Limitations
(cont……)
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A self-exciting sensor has its output energy supplied entirely
by the input signal x (t).
The general equation that describes a self-exciting sensor
system is
y (t )  F ( x(t ))
(6.1)
where F(x(t)) is the characteristic relationship that describes
the behavior of a self-exciting sensor.
6.4 Ideal Sensor Characteristics
and Practical Limitations
(cont……)
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Eg : A thermocouple → input signal = the difference in
junction temperatures ΔT(t) and the output = e.m.f Φ(t) in
volts.
(t )  F (T (t ))
In the case of the modulating sensor, the system equation
can be written more explicitly as
y(t )  F ( x(t )  xd )
where the external supply signal xd(t) should ideally be
stationary and noise free.
(6.2)
6.4 Ideal Sensor Characteristics
and Practical Limitations
(cont……)
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The ideal sensor not only
has a linear output signal
y(t) but it should
instantaneously follow
the input signal x(t),
whence
y (t )  S .x(t )
ym
Sensor
output
Slope, S
yd
(6.3)
xd = 0
0
The slope S is usually
referred to as the
sensitivity.
xd ≠ 0
Sensor input
xm-xd
Fig. 6-4: Ideal input-output
relationship of self-exciting
and modulating sensors.
General properties of a good sensor are:
1) Optimum measurement accuracy
2) Good durability
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3) Ease of calibration and reconditioning
4) High sensitivity
5) Good reproducibility
6) Long term stability
7) Fast response
8) Continuous operation
9) Insensitivity to electrical and other environmental
interference
10) Low fabrication, operation and maintenance cost