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Chapter 1
Introduction to sensors
Bhagwati Charan Patel, G R Sinha and Naveen Goel
A sensor is a device that receives a signal or stimulus and responds to the stimulus in
the form of an electrical signal. The output signals correspond to some forms of
electrical signal, such as current or voltage. The sensor is a device that receives
different kinds of signal i.e. physical, chemical or biological signal and converts them
into an electric signal. The sensors are classied into different types based on the
applications, input signal, and conversion mechanism, material used in sensor
characteristics such as cost, accuracy or range. This chapter presents an overview
of sensors and their classications as thermal, magnetic optical, mechanical and
chemical. The transfer functions, characteristics and specications are also discussed
with introduction to basic forms of sensors.
1.1 Introduction
We can nd sensors everywhere, and the whole world is full of sensors and their
applications. There are many types of sensors available around us, in our ofces,
gardens, shopping malls, homes, cars, toys etc. These sensors make our lives so easy
and comfortable, starting from applications such as switching on the lights, fans,
television (TV), automatic adjustment of the room temperature by air conditioning
(AC), re alarm, detecting obstacles when the car is reversing, making a thumb
impression etc. A sensor is a device which receives signals as well as responding to a
signal or stimulus. The stimulus signals can be deed by the measure, property, or
state which is sensed. We also can say that a sensor is a translator that converts a
nonelectrical value to an electrical value [13]. The output signal of a sensor may be
in the form of voltage, current, or charge. A sensor has many forms of input
properties and electrical output properties. If there is small change in the sensed
quantity, it will cause a small change in the electrical output and the changes can be
detected with their measuring capabilities.
All the sensors are categorized on the basis of their uses, applications, material
used and some production technologies. Some sensors are classied also by their
doi:10.1088/978-0-7503-2707-7ch1 1-1 ªIOP Publishing Ltd 2020
characteristics such as cost, accuracy or range of sensor. There are two main types of
sensors: passive sensor and active sensor. A passive sensor does not require any extra
energy source and electric signal is produced directly in reply to stimulus of external
sources. This means that the sensor converts input energy to output signal energy
[1,4,5]. Examples of passive sensors include photographic, thermal, electric eld
sensing, chemical, infrared and seismic. The active sensors need external sources of
energy for their response, known as excitation signal. To produce the output signals,
sensors adopt necessary changes to these input signals. The active sensors are also
known as parametric sensors due to their own properties which can be modied in
response to an exterior effect and these properties can be afterward changed into
electric signals. Active sensors have a variety of applications related to meteorology
and observation of the Earths surface and atmosphere. Table 1.1 shows differences
between passive and active sensors.
The other types of sensors are based on their detection properties such as
variation mechanism, analog and digital. The detection properties of sensors include
electric, magnetic, physical, chemical etc, and variation mechanism includes con-
version of the input signal to output signal, whose examples are photoelectric,
thermoelectric, electrochemical, electromagnetic etc. Analog sensors produce an
analog output, i.e. continuous output signals are produced with respect to the
measured quantity, but a digital sensor is the opposite of analog sensors, with
discrete characteristics and digital output in nature. Sensors are also divided by their
detection properties, given in table 1.2
1.2 Sensor characteristics
Upon receiving the input stimuli, the sensor produces output which is obtained from
several conversion steps before it produces an electric signal [1,610]. The perform-
ance of sensors is described in terms of relationship between input and output
Table 1.1. Difference between passive and active lter.
Passive sensor Active sensor
Does not require external power It requires power
It can only be used to detect energy when the naturally
occurring energy is available
Provides its own energy source for
illumination
No interference problem in the environment Less interference problem
Can operate in the same environment condition Can operate in different
environment conditions
Sensitive to weather condition Not sensitive
Not well suited for darkness conditions Works well in darkness conditions
Difficulties in interpreting the output signals Easy to interpret
Less control of noise Better control of noise
Low price High price
Examples: camera, Sonar Examples: LASER, Radar etc
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signals. Sensors are characterized depending on the values of some of the important
parameters. The characteristics of sensors are described here in this section.
1.2.1 Transfer function
The transfer function shows the functional relationship between physical input
signal or stimulus (s) and electrical output signal (S), as
=
S
fs()
, where Sis
response to the stimuli. This function can be linear or non-linear depending on the
relation between input and output and nonlinearity may be in different forms such
as logarithmic, exponential, or power function [13]. In most of the cases, the
relationships are dened by unidimensional function which means that the relation
between the output and input is associated with one stimulus. This linear relation-
ship is described by:
=+Sabs() (1.1)
where
a
is the intercept used by the output signal at zero input signal and
b
is the
slope, also called sensitivity
S
. It is also known as the sensors output used by devices
to acquire data and depending on the property of sensors, this can be amplitude,
frequency, or phase. Other non-linear functions are given as:
=+SabsLogarithmic function: ln (1.2)
=SaeExponential function: (1.3)
ks
=Sk
s
a
1ln (1.4)
=+Sa asPower function: (1.5)
k
01
Table 1.2. Sensors based on their detection properties.
Types Properties
Thermal sensor Temperature, heat, flow of heat etc
Electrical sensor Resistance, current, voltage, inductance, etc
Magnetic sensor Magnetic flux density, magnetic moment, etc
Optical sensor Intensity of light, wavelength, polarization, etc
Chemical sensor Composition, pH, concentration, etc
Pressure sensor Pressure, force etc
Vibration sensor Displacement, acceleration, velocity, etc
Rain/moisture sensor Water, moisture, etc
Tilt sensors Angle of inclination, etc
Speed sensor Velocity, distance etc
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=
Ssa
b(1.6)
k
where constant numbers are given by k.
There are some sensors that may not full the above properties and in such cases,
higher-order polynomial approximation is required.
1.2.2 Full-scale input (FSI)
This is dened as the difference between the maximum and minimum values of input
stimulus which can be represented in decibels (dB). This is also a logarithmic
measurement in terms of ratios of power or force and voltage. Decibels are
calculated as equal to 20 times the log of the force, current, or voltage:
=s
s
1dB 20log (1.7)
2
1
where
s
2and
s
1
are the maximum and minimum values of input, respectively.
1.2.3 Full-scale output (FSO)
Full-scale output indicates the changes between the maximum and minimum values
of electrical output signals when maximum and minimum input stimulus is applied.
The FSO also includes all the deviations from the ideal transfer function.
1.2.4 Accuracy
Accuracy is an important characteristic in sensors which is calculated in terms of
error in measurement and dened as the difference between measured value and true
value. It is represented in terms of % of full scale or % of reading.
=−=AMTAbsoulute error Measured value True value (1.8)
evv
where
T
v
is calculated by taking the mean of an innite number of measurements and
relative error can also be calculated as
==
RMT
T
Relative error Absolute error
True value (1.9)
e
vv
v
The accuracy rating indicates a collective effect of variation, linearity, calibration,
repeatability errors, dead band etc, in measurements used in sensors.
1.2.5 Calibration
There are many sensors available but to get the best possible sensor with optimal
value of accuracy, the sensor needs to be calibrated in the device where it will be
used. It is an adjustment or set of adjustments made on a sensor or device to make
that device function accurately and error free. For instance, we have to measure the
pressure with an accuracy
±
5 pa, and a given sensor is rated with an accuracy of
±
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10 pa. Can we use this pressure sensor? Yes, we can, but the given sensor needs to be
calibrated and we have to nd out its initial transfer function during calibration.
In the calibration method, we have to nd out its particular variables. These
variables describe the complete transfer function and should be identied before
calibration. Calibration of linear devices is calculated by equation (1.1) and variable
aand bshould be determined accurately.
In order to get constant values in the equation with good accuracy, the linear
transfer function is calculated as
=+
abp(
.Tond constants aand b, a sensor
can be exposed with two pressure values (
p
1
and
p
2
) with respect to their
corresponding output voltages (
v
1
and
v
2
), then we get
=+
v
abp()
11and
=+
v
abp()
22
, and the constants are calculated as
=
=−bvv
pp av bpand (1.10)
12
12
11
and pressure of calibration can be computed as
=
pva
b(1.11)
The calibration error is actually a type of inaccuracy which is accepted by
manufacturers during the time when the devices or sensors are calibrated in the
factory. This obtained error is not uniform and can change during the process of
calibration.
1.2.6 Hysteresis
Hysteresis is a common phenomenon or characteristic which is caused by changing
properties of a material such as frictional and structural changes. Hysteresis error is
the difference between two output values that correspond to the same input
depending on direction followed by the sensor, as shown in gure 1.1.
The value of hysteresis error is represented by a positive or negative percentage of
the given pressure range. We can identify this hysteresis error for half of the given
range of pressure reference point.
Decreasing
pressure
Increasing
pressure
Pressure
Sensor
Output
Hysteresis
Reference point pressure
Figure 1.1. Hysteresis.
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1.2.7 Non-linearity
The linearity function in a sensor is calculated by the maximum deviation by straight
line given in equation (1.1), over the specied dynamic range. The non-linearity of
the sensor is calculated by the measurement of the difference in Y-axis of two lines of
equal slope, one passes through the minimum points and the other passes through
the maximum points of the output curve. The total non-linearity is the difference
between intercept value of Y-axis and a parallel line which goes through the
maximum deviation point. The total non-linearity is approximated by determining
the maximum deviation at the midpoint of the X-axis and the largest error is
observed between the actual and calculated values.
The midpoint (X
m
) is calculated as:
=+XXX X
2(1.12)
m
max min
min
⎜⎟
and the line connecting end points represents the total amount of non-linearity,
which is calculated by % of linearity as:
=
+−
+
×
()
()
YY
Y
% Linearity 100% (1.13)
YY
YY
2
2
max s
max
min min
min min
where
Y
s
is the actual Yvalue at this Xvalue and if the actual measured Yvalues pass
through zero, then it is simplied to be:
=
×
()
()
Y
% Linearity 100% (1.14)
Y
Y
2
2
s
max
max
1.2.8 Resolution
The resolution of a sensor is dened as minimum detectable signal uctuation while
reading or measuring some quantity using a suitable sensor. This is also an ability of
the measurement to obtain and notice minor changes in the characteristic of the
measurement result.
1.2.9 Saturation
Every sensor has its certain operation limit, which is the state where the output
signal of the sensor will no longer to respond at some level despite increasing the
input stimuli values. This characteristic is referred as saturation of the sensor. There
is a point where output of the sensor does not respond as required to an increase in
the input stimuli, and that particular point is known as saturation point or threshold
point.
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1.2.10 Repeatability
Repeatability means occurrence of a value again and again when the system returns
to the same position multiple times to measure the range of output signals. Direction
of approach is an important issue to measure the position in repeatability, which
can be represented as the maximum change among readings of output as notied by
two repeated cycles unless otherwise specied. It is generally denoted as % of full
scale (FS):
δ=Δ×
FS 100% (1.15)
r
1.2.11 Dead band
Dead band is a region where the sensitivity of a sensor does not have any effect. In
this range, the output can remain nearly zero over the entire dead band ranges
without any change in measurement.
1.2.12 Reliability
Reliability of the sensor node is the ability to perform the desired function under any
given circumstances a for denite period. This can be represented in terms of
statistics as a probability that the sensor will operate without failure in the stated
time interval. When the performance of the sensor is exceeded under given circum-
stances, it causes a failure of the sensor and it can be temporary or permanent. So
before manufacturing or designing the sensor, it should be properly checked,
considering the various worst circumstances and conditions.
1.2.13 Output characteristics
Just like the input characteristics, this depends upon the type of input which the
sensor is measuring, output characteristics depend upon the type of electrical output
we are getting from the sensors. The output of the sensor can be voltage, current,
impedance or it can be a function of any other quantities. So, the output quantity
should be acceptable by the other stages of the instrumentation system, because as
mentioned for measurement or in an instrumentation system, we not only have a
sensor but there are also other components present in the overall system. These
components should be compatible with the output of the sensor. The electrical
output of the sensor should be acceptable by the other stages of the measurement
system, so that the components and their evaluations can work over it.
1.2.14 Impedance
Impedance of a circuit is the total effective resistance or the measure of opposition to
the passes of current when an AC voltage source is applied. It is represented as Zand
its unit is ohm (Ω). This is calculated by applying a voltage source to the sensor with
a resistor and measuring the changed voltage across the resistor and sensor device.
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Input impedance is the impedance that is observed by the voltage source between the
two terminals of the circuit. Now, we are assuming that the applied voltage source is
the idle source but in an actual case this voltage source will also have some nite
resistance. In the case of output impedance, in the circuit we have connected some
load to the output terminal and the circuit is giving some voltage; ideally this output
voltage should also appear across the two terminals of this load. From the load
perspective, it will also have some nite resistance with this output voltage and this
nite resistance are known as the output impedance of the circuit or device.
1.2.15 Excitation
Excitation is the appropriate electrical signal required to operate the active sensor. It
is given by the range of voltage and/or current. The frequency of the excitation signal
must also be specied in some sensors. Transfer function of the sensor may be
altered by changing this excitation signal or it would cause output error.
1.2.16 Dynamic characteristics
Dynamic characteristics of a sensor are determined by analyzing the response of
transfer function, span, calibration and input (steps, impulse, ramp, sinusoidal) of
the sensor. This is when input response varies and the response does not follow it due
to coupling characteristics and this is determined with a time dependent behavior,
also known as dynamic characteristics. When a sensor does not respond immedi-
ately, the given value of input is not a properly received value and is different from
the real time value, then dynamic error will occur.
1.2.17 Precision
It can be described as the nearest among a set of values and it is dissimilar from
accuracy. Let
p
tbe the true value of the variable
p
and a random experiment
measure
p
1
,
p
2
,,
p
i
as the value of
p
. Then our measurements
p
1
,
p
2
,,
p
i
are
precise when they are very close to each other but not necessarily close to true value
p
t. However, if we say
p
1
,
p
2
,,
p
i
are accurate, it means that they are close to true
value
p
tand hence they are also close to each other. Hence accurate measurements
are always precise.
1.2.18 Environmental factors
Environmental conditions are major factors which affect input and output stimuli of
the sensor. There are mainly three types of factors namely air, soil and water. Each
sensor is designed to work only with certain conditions. Noise can also affect the
output signal of sensor and aging can degrade the performance of the sensor. There
are some other factors that reduce the performance of sensors, such as electrical,
mechanical, chemical and thermal etc. Therefore, performance and long-term
stability of the sensor can be improved by designing the components for extreme
conditions, so that all these factors do not create an adverse effect during the
operation of the sensor.
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1.2.19 Uncertainty
Every measurement has some uncertainty. Uncertainty in the data contains some
variable values that make them deviate from the correct or original values and is
measured by the amount of error as mean or average value of a data set. Error is the
difference between the true value and the measured value. Uncertainty is the range
of values within which the true value lies in some levels of condence.
1.2.20 Application characteristics
Sensors are not generally designed for general purpose and are application oriented.
Sensors are required according to applications of different types of sensors such as:
speed sensor for synchronizing the speed of multiple motors; temperature sensor
used for controlling the temperature; ultrasonic sensor for measuring the distance,
etc.
1.3 Types of sensors
There are many sensors commonly used in various applications [1,1121]. All these
sensors are categorized as per their physical properties like temperature, resistance,
pressure, heat ow etc. The following is a brief discussion on different types of
sensors.
1.3.1 Temperature sensors
A temperature sensor is used to measure the amount of energy in the form of heat
and cold produced by an object and system. It allows one to sense or detect any
physical change to that energy and gives the output as analog or digital.
Temperature sensors are used in various applications such as notication of
environmental temperature, medical instruments, automobiles etc. According to
application and its characteristics, many different types of temperature sensors are
available. There are basically two types of temperature sensors, contact temperature
sensor and non-contact temperature sensor. In contact temperature sensor, there is
physical contact with the object being sensed and to monitor the change in
temperature, conduction is used. It is used to sense solids, liquids or gases over a
wide range of temperatures. In a non-contact temperature sensor, we use convection
and radiation properties to measure the changes in temperature. It uses radiant
energy in the form of heat and cold.
Thermostat: The thermostat is a kind of contact temperature sensor employ-
ing an electro-mechanical component and using two thermally different kinds
of metals, nickel, copper, tungsten or aluminium etc, which are stuck together
to form a Bi-metallic strip. When it is cold, one of the strips is contracted and
its contacts are closed and current passes through the thermostat. When it is
hot, one metal strip is expanded and opens the contacts to stop the ow of
current.
Thermistor: The thermistor is another type of temperature sensitive device or
resistance whose electrical resistance changes as the object temperature
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changes. This is made up of semiconductor materials. When temperature of
the object or surroundings increases or decreases, resistance will also increase
or decrease. How much the resistance will increase or decrease depends on the
properties of the semiconductor material. The thermistor is of two types:
positive temperature coefcient, (PTC) and negative temperature coefcient,
(NTC). In PTC, resistance value increases with an increase in the temperature
and in NTC, its resistance value goes down with an increase in the temper-
ature. Thermistors are used for precise temperature measurement, control
and compensation. Thermistors are highly sensitive and exhibit non-linear
characteristics of resistance versus temperature. Generally, these are made up
of manganese, nickel, cobalt, copper and iron.
Resistive temperature detector: The resistive temperature detector (RTD) is
also known as resistance thermometer, and used for measurement of temper-
ature. It is based on the temperature coefcient of sensors and generally
composed of high-purity conducting metals like platinum, copper or nickel.
These materials are looped into a coil whose changes of electrical resistance
depend on a temperature function. The working principle of an RTD is very
similar to that of the thermistor.
Thermocouple: The thermocouple is a device which is used for the measurement
of the temperature variation in a measurement of sensors. The thermocouples
are coupled with two metals joined together forming a junction. Thus,
there are two junctions in the metals, one is called hot junction and other is
called cold junction, also referred as measuring junction and reference
junction, respectively. These junctions are kept at different temperatures
due to the change of EMF (electromotive force) induced in a thermocouple
and output voltage obtained with the help of the relationship between the
voltage and temperature. When the two junctions are at different temper-
atures, a voltage is developed across the junction which is used to measure
the temperature sensor. The thermocouple is based on three main effects:
Thomson effect, Seebeck and Peltier effect. It has broadest range of
temperatures of all the temperature sensors, covering from 200 °Cto
2000 °C.
1.3.2 Position sensors
The position sensor detects the position of an object either linearly or in rotation
with respect to some xed point or position. Position can be determined by the
distance between two points moving away from some xed points. We can measure
the displacement of position in a straight line by linear sensor and angular
displacement using rotational sensors. Position sensors are also known as potenti-
ometers and used to measure the displacement of the object. A potentiometer can be
an electrical or resistive type of sensor, because its working principle is based on
change in resistance of wire with its length. This converts rotary or linear displace-
ment to electrical voltage. The resistance of wire is directly proportional to length of
wire. If the length of wire changes then the resistance of wire also changes.
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Potentiometers are available as rotary and linear potentiometers in the market, and
can be used to measure the angular position and linear position, respectively;
through voltage division the changes in resistance can be used to create an output
voltage that is directly proportional to the input displacement.
The sensors have three terminals, where the one in the middle is known as the
wiper, and the other two are known as the ends. The wiper is a movable contact
where resistance is measured with respect to it and either one of the end terminals.
The displacement of the moving object is measured with the help of the sliding
element of the potentiometer. When position of the moving body changes then its
resistance between two xed points also changes. The result is obtained in the form
of differential output voltage which varies linearly with the movement of core
position. The resulting output signal has both the amplitude and polarity.
Amplitude is calculated as linear function of the displacement and polarity gives
the direction of movement. Major advantages of the potentiometer include user
friendly operation, low cost, high amplitude output and the sensors are used for
measuring even large displacement, but its operating cycles are limited.
1.3.3 Light sensors
A light sensor is a photoelectric passive sensor which changes the light energy into an
electrical signal output. It measures the ambient light which is surrounding light,
room light and reected light. The major component of a light sensor is the light
dependent resister (LDR) or photoresistor. It is a resistor that depends on the light
which changes its resistance depending on the amount of light incident on it. The
sensors are made up of semiconductor materials and therefore when light is incident
on semiconductor material it becomes low conductive and therefore has less
resistance. When we increase the light intensity, its resistance decreases and vice
versa which is shown in gure 1.2. Intensity of light falling on an LDR is measured
in lux.
There are different kinds of light sensors such as photoresistors, photodiodes,
photovoltaic cells, phototubes, photomultiplier tubes, phototransistors, charge
coupled devices (CCDs) etc.
Resistance
Light Intensity
Resistance decreasing
with light intensity
Figure 1.2. Resistance decreases with light intensity.
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1.3.4 Sound sensor
A sound sensor is also known as auditory and used to detect the intensity of sound.
It converts the acoustic wave into an electrical signal output. These sensors can also
detect sound pressure waves which are not within the audible range, making them
suitable for a wide range of tasks. Sound sensors are mostly used for security
purposes.
1.3.5 Proximity sensor
A proximity sensor can be used for detecting the presence of a nearby object without
any physical contact. It emits an electromagnetic eld for a beam of electromagnetic
radiation as infrared instances and changes in the eld returning a signal. The object
being sensed is often referred to as the proximity sensors target. Depending on
different types of proximity sensors, different targets are used. For example, an
inductive proximity sensor needs a metal object, whereas a capacitive photoelectric
sensor is suitable for a plastic target. A proximity sensor has high reliability due to
the absence of mechanical parts and lack of physical contact between the sensor and
target. It has very short range when used as a touch switch. It is commonly used in
industrial applications, manufacturing of food production, mobile phones etc.
1.3.6 Accelerometer
This sensor is used to detect the acceleration of an object, and operates by sensing
the acceleration of gravity, and the direction of the object is calculated. This sensor is
a kind of microelectromechanical system (MEMS), which uses a silicon integrated
circuit. These sensors convert the mechanical motion caused in an accelerometer
into an electrical signal by using the piezoelectric, piezo-resistive and capacitive
components.
1.3.7 Infrared sensor
An infrared (IR) sensor consists of two packs, one is Rx (receiver) and the other is Tx
(transmitter). Transmitters are used in transmitting the rays in the infrared spectrum
and the receiver receives the IR spectrum range. In the IR spectrum, the voltage is
given between its terminals and then it emits rays. The main principle of working of
an IR sensor is reectivity by an object. When an object is placed in front of the
transmitter it tends to reect the rays that are coming from the IR sensor back to the
IR sensor. Whenever a ray that is reected by an object is received by the receiver it
generates a voltage level across the terminal. This voltage level depends upon the
intensity of light that is reected by the object. Transmitter and receiver are placed
side by side, and the IR transmitter transmits a signal within a limited range and
going to a certain distance. When IR rays hit the surface, some rays are reected
depending upon the colour of the surface. The brighter the colour the more IR rays
are reected; similarly, the darker the surface the more IR rays are absorbed by the
surface and fewer IR rays are reected back.
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1.3.8 Pressure sensor
Pressure is an external force exerted on a surface in unidirectional areas. We
commonly measure the pressure of liquid, air and other gases. A pressure sensor
monitors this pressure and is sometimes called a pressure transmitter as it converts
pressure into an electrical signal. The most common type of pressure sensor is the
strain gauge-based pressure sensor. Conversion of pressure into electrical signal is
achieved through the physical deformation of strain gauge which is bound into the
diaphragm of the pressure sensor. The strain will produce a change in electrical
resistance which is proportional to the pressure. Change in voltage is the result of
ambient pressure. A pressure sensor can also be used to measure other variables such
as uid or gas ow, speed, water level, and altitude.
1.3.9 Ultrasonic sensors
An ultrasonic sensor uses ultrasonic waves for the purpose of sensing and measuring
the distance of a particular object. Ultrasonic waves are very high frequency waves.
The sensors have two main transducers, namely transmitter and receiver. A trans-
mitter uses 40 KHz of frequency wave transmitted in the air and when it is blocked
by an object then its gets reected and bounced back to the sensor. These reected
waves are absorbed by the receiver of the sensor. So, the total time taken by the
ultrasonic waves to travel from the transmitter to the object and again from the
object to the receiver of the sensor is given by the output of the sensor. Ultrasonic
sensors are used in many applications such as robotics, driverless cars, for measuring
distance, and also in radar systems etc.
1.3.10 Touch sensor
Touch sensors are sensitive to touch, pressure and force. The sensors operate as
switches and when the surface of the sensor is touched the current starts to ow in
the circuit just like current owing in a closed circuit. When there is no contact, it
performs like an open circuit and no ow of current is reported. There are two types
of touch sensors, capacitive and resistive. The touch sensors are used popularly in
modern gadgets such as smartphones, and other handy devices.
Capacitive sensor: The capacitive sensor has an important element as a
capacitor. Parallel capacitors are generally placed like top and bottom plates
at some certain distance and between these parallel capacitor plates there is a
dielectric medium. The main principle of change in capacitance is used such
that it may be caused by change in overlapping area, change in distance
between two plates and change in dielectric constant. Changes of these
parameters can be made by the physical variables like displacement, force,
pressure and ow of liquid. Capacitance and output impedance are measured
with a bridge circuit. An extremely small force is needed to operate them and
hence they are very useful for a small system. The sensors are highly sensitive
with good frequency response and high output. As force requirement is small,
thus the power requirement is also less to operate the sensors. The metallic
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parts of the sensor must be insulated from each other in order to reduce the
effect of stray capacitance.
Resistive sensor: The resistive sensor is based on the change in resistance of
the material and is used to measure temperature, displacement, moisture etc.
A slider is free to move between two points and at a certain point we get the
zero output and at some other point we get the maximum output. The output
voltage is obtained between these two points and it is directly proportional to
displacement. So, the change in length of the wire causes the change in the
value of the resistance. This property is utilized to measure the changes in
displacement using resistivity and resistance. When the xed voltage is
applied across end terminals of the sensor, a proportional voltage is generated
across the slider and this voltage can be calculated using voltage divider rule.
As distance increases, the output voltage will also increase. The resistive
technique used in this sensor can be used to sense or measure linear
displacement.
1.3.11 Humidity sensor
Humidity is the amount of water present in the surrounding air and a hygrometer is
the device which measures humidity directly. Humidity is a non-electrical quantity
that is converted into electrical quantity by using resistance, capacitance and
impedance properties. There are various parameters that change due to humidity.
There are ve basic types of humidity sensor: resistive hygrometer, capacitive
hygrometer, microwave refractometer, aluminium oxide hygrometer and crystal
hygrometer.
Resistive hygrometer: In a resistive hygrometer, the main element is a material
whose resistance changes with the change in humidity or relative humidity. A
wire or electrode coated with hydroscopic salt (lithium chloride) can be used
for measurement of the humidity. Resistance of salt changes with humidity
because hydroscopic salt absorbs moisture and its resistance decreases.
Capacitive hygrometer: In a capacitive hygrometer, the changes in humidity
are caused by the changes in the capacitance. Dielectric medium is used in the
capacitor and the capacitor consists of two electrodes or plates and a
dielectric medium is there between the plates. There is also some hydroscopic
material which exhibits the change in dielectric constant with the change in
the humidity. Therefore, such hydroscopic material or salt can also be used
for construction of a capacitive hygrometer. If the change is very small, then
the capacitor includes a frequency determining element in the oscillator and
another frequency is produced by the beat frequency oscillator. This
frequency is heterodyned and the difference in frequency is a measure of
relative humidity.
Microwave refractometer: A microwave refractometer consists of two cav-
ities, each coupled with Klystron. Klystron is a material which produces
microwaves in which one cavity is lled with dry air and another cavity is
lled with a mixture whose humidity is measured. In the mixture, water
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vapour will be present and due to the presence of water vapour, there will be a
change in dielectric constant, and frequency of one of the oscillators changes
consequently. If there is no change in dielectric constant, its frequency is
going to be constant, whereas in the mixture water vapours are present, and
there is change in dielectric constant which results in change in its frequency.
Frequency changes are measured as the measure of humidity.
Aluminium oxide hygrometer: In an aluminium oxide hygrometer, aluminium
oxide is coated on anodized aluminium and this aluminium oxide exhibits a
change in the dielectric constant with respect to changes in humidity. There
are two electrodes in which one is the inner electrode and the other is the
outer electrode made from a very thin layer of material like gold. Some pores
are presents in the inner layer. Due to the change in humidity, dielectric
constant changes and this change can be measured to measure the humidity
by bridge or electric method. The errors are much reduced and the response
time is small and therefore the response is very fast.
Crystal Hygrometer: In a crystal hygrometer, crystals are coated with
hydroscopic materials (hydroscopic polymers). These crystals are used as
frequency determination elements in the oscillator, and therefore just like
with the capacitive hygrometer, if there is change in humidity then frequency
also changes. Frequency changes due to the humidity as the mass of the
crystal changes with amount of water absorbed by the coating. This change in
frequency is measured. Humidity sensors are used in industry, agriculture, the
medical eld, environment monitoring etc.
1.3.12 Colour sensor
A colour sensor is used to detect and identify various colour patterns and convert
them into desired frequency as output. It consists of four photodiodes of red, green,
blue and clear (no colour). All these photodiodes are connected in parallel and work
as lters. For example, if we have to detect red colour, we use red colour lter for
this purpose. Colour light signals are sensed by the photodiodes and we get the
square wave signals with the frequency directly proportional to light intensity and
that is transferred to the microcontroller and we get the result of colour.
1.3.13 Chemical sensor
A chemical sensor is a device which transmits chemical information from a chemical
reaction. The chemical information may be of composition, concentration and
chemical activity which originates from a chemical reaction or from physical
activities. It has different applications such as for home appliances and the chemical
industries. The chemical sensor usually contains two basic components, which are a
chemical resonance system known as the receptor and a physical chemical trans-
ducer. The receptor interacts with analytic molecules and the transducer sends the
electric signal. A test sample is given to the receptor which checks composition
connected with the transducer. The transducer collects the information from the
receptor and sends it to the signal amplier. This amplies the signal from the
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transducer and sends it as output signals. There are two types of chemical sensors
used to detect the composition: optical sensor and electro chemical sensor.
Optical sensor: In the optical sensor, there are an emitter and a detector as the
main elements. The emitter senses the light to the optical sensor and the light rays
fall on the analyte and these rays may be reected or refracted. These reected
or refracted lights are passed through the detector. Now the detector receives
these lights and according to their intensity, the chemical compound present is
analysed. Operation of an optical sensor is very simple and it uses absorption
coefcient characteristics of the medium and path length travelled by the rays.
Electrochemical sensor: The electrochemical sensor operates by acting on gas
molecules of interest and produces an electric signal proportional to the
compound present in the gas. It consists of sensing modules and electrodes,
separated by a thin layer of electrolyte. There are two plates and the centre is
lled by electrolytes. One plate is the cathode and the other plate is the anode.
An external membrane is introduced in solution and it is absorbed by certain
ions from the solution. Therefore, chemical properties of the solution change and
the electromagnetic eldwillalsochange;andconsequentlychangeinthe
electromagnetic eld ensures that the chemical composition is present in the gas.
1.3.14 Seismic sensor
A seismic sensor measures small movements of the ground and also amplies and
records these small movements. It is also known as a seismometer, and is mostly
used in measuring the details of earthquakes, volcanic eruptions and other
vibrations. There are two types of seismic sensor, inertial seismometer and strain
meter or extensometer seismic sensor.
Inertial Seismometer: The inertial seismometer consists of a weight suspended
from a frame by a spring. The frame moves due to the vibration being
measured but the mass is held stationary due to the spring. It is used to
measure a large-scale vibration such as an earthquake. Now the movement of
mass is converted for output as a digital electric signal. Since both types of
seismic sensors most commonly output an electric signal, calibration is
necessary to derive a relationship between the input and output.
Strain meter or extensometer seismic sensor: In a strain meter seismic sensor, a
strain gauge is used to measure the motion relative to the various points. It
generally is used for smaller scale measurement and movement of mass is
converted for output as a digital electric signal.
1.3.15 Magnetic sensor
Magnetics sensors respond to the presence or interruption of a magnetic eld like ux,
strength and direction by producing a proportional output. It converts magnetic
information into an electrical signal for processing by the electronic circuit. A magnetic
sensor is used in different types of application such as sensing position, velocity and
movement of an object. There are different kinds of technology used to design a
magnetic sensor. Fluxgate, Hall effect, resistive, inductive, proton processing etc, have
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a dissimilar approach of using magnetic sensors. A resistive magnetic sensor keeps the
electrical resistance of the magnetic eld and an inductive magnetic sensor uses coils
surrounding its magnetic material, which have the ability to detect changes within the
Earthsmagneticeld. A uxgate magnetic sensor uses the approach of changing ux
parameters. Each type of technology focuses on a specic area for identifying
measurements to be detected. Sensitivity of the magnet is increased by combining
layers of magnetic alloys and the magnetic eld is surrounded by an electric current,
and variation within the eld is detected. The output of a magnetic sensor increases
with a strong magnetic eld and decreases with a weak magnetic eld.
1.4 Comparison of different sensors
This section presents a tabular comparison of different types of sensors [1,10,1830]
in terms of advantages and disadvantages, as shown in table 1.3.
Table 1.3. Comparison of different sensors.
Sensors Advantages Disadvantages
Temperature
sensors
Reference temperature not required
Large response time
Easy display
Durable
Self-heating error from applied
power
Difcult to calibrate
Position sensor Accurate, reliable, and predictable
measurement
Higher switching rate
High susceptibility to noise
Sensing range depends on the
type of metal of the target object
Sound sensor Used in speech recognition software
Easy to manipulate sound in real time
Does not require cabling compare
to wired mic
Sound les require more
memory size
Interference cancelation is
required
Limited coverage area
Light sensor Requires very small power and
voltage
Available in different shapes and
sizes
Easy to integrate with a lighting
system
Quick response time and low cost
Nonlinear characteristics
If applied voltage exceeded, it
will cause irreversible damage to
the photo resistor
Temperature sensitive
Vulnerable to surges and spikes
Accelerometer Good response at higher frequencies
Withstands high temperature
Small size
Sensitive to high frequency
Requires external power
(Continued)
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Table 1.3. (Continued )
Sensors Advantages Disadvantages
Infrared sensor Operates with low power
Capable of detecting presence or
absence of light
Does not require contact with object
Not affected by corrosion or oxidation
Strong noise immunity
Requires line-of-sight
deployment
Gets blocked by common
objects
Limited range
Affected by environmental
conditions
Transmission data rate is slow
Pressure sensor High output signal level
Low cost
Technological robustness
High hysteresis
Sensitive to vibrations
Movable contacts
Ultrasonic
sensor
Sensing capability to all the materials
Not affected by dust, rain, snow etc
Works in any adverse conditions
Higher sensing distance
Not affected by colour or
transparency of objects
Can be used in dark environments
Sensitive to variation in the
temperature
Difculties in reading reections
from soft, curved, thin and
small objects
Cannot work in a vacuum
Sensing accuracy is affected by
soft materials
Smoke and
gas sensor
Simple and low-cost technology
Measures ammability of gases
Linear output and low power
requirements
Wide measurement range
Higher sensitivity, resolution and
reliability
Requires air or oxygen to work
Narrow or limited temperature
range
Humidity
sensor
Does not require much maintenance
Flexibility to use
No ageing effects
Sensitive to dewing and
substances
Limited accuracy and
measurement range
Color sensor Easy to change or modify setups
without even re-programing the
sensor device
Easy to implement
Common color space, used in a wide
range of devices
Lens subject to contamination
Sensing range affected by colour
And reectivity of target
Chemical
sensor
Linear output, low power
requirements and good resolution
Narrow or limited temperature
range
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1.5 Modern sensors
The sensor technologies have changed a lot in the last decade in terms of compact-
ness, smartness and sensitivity. The traditional sensors such as photosensors, optical
sensors, capacitive sensors and almost all sensors have been replaced by their
integrated circuit forms such as MEMS (microelectromechanical system). The
sensors are embedded in all modern computing and navigation devices in
compact forms and this is why an ordinary smartphone carries around 22 sensors
for various purposes. The technologies of sensors have further advanced and
become intelligent as smart sensors and available in wearable forms. This may be
seen in smart watches, smart gadgets or a large application such as self-driving
cars where hundreds of smart sensors are involved for seamless and smooth
driving without assistance of a driver. The same can also be seen in robotics,
medical diagnosis, braincomputer interface (BCI) and many more, where AI
(articial intelligence) has empowered the sensors with intelligence and smartness
for emerging and modern applications such as industry, healthcare and sophis-
ticated automation.
1.6 Conclusions
This chapter presented an overview of sensors with basics, characteristics and
different types of sensors. A sensor is a device that receives a signal and converts it
into an electrical signal. These sensors are classied on the basis of their applications,
cost, accuracy and range. Sensors are classied also into different categories like
thermal, electrical, magnetic optical, mechanical and chemical sensors. The sensor
technologies have become advanced now and cognitive and smart sensors are being
used in all modern applications.
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