UNIT III_Smart Sensor.pptx

SMART SENSORS
by
Mr. G Sattibabu
Assistant Professor
Department of Electronics and Communication Engineering
Aditya College of Engineering & Technology
Aditya College of Engineering & Technology
UNIT - III

UNIT III: Smart Sensor (JNTUK R19 syllabus)
Occupancy and Motion Detectors: Ultrasonic, Microwave Motion, Capacitive
Occupancy, Visible and Near-Infrared Light, Far-Infrared Motion, PIR Motion,
Position, Displacement, and Level Sensors: Potentiometric, Gravitational,
Capacitive, Inductive and Magnetic, Optical, Ultrasonic, Radar
Velocity and Acceleration Sensors: Capacitive Accelerometers, Piezoresistive
Accelerometers, Piezoelectric Accelerometers, Thermal Accelerometers, Heated-
Plate Accelerometer, Heated- Gas Accelerometer, Gyroscopes, Piezoelectric Cables
Applications: Case studies in manufacturing industries, robotics

Part I: Occupancy and Motion Sensor/detectors

Occupancy sensor and motion sensor
• The Occupancy sensor detects presence of people or animals in the target
monitored area.
• The motion sensor responds to moving objects only.
 The difference between them is occupancy sensor produce signals whether an
object is stationary or not
 while motion sensor is sensitive to only moving objects.
• These types of sensors utilize some kind of a human body's property or body's
actions.
• For instance, a sensor may be sensitive to body weight, heat, sounds,
dielectric constant and so on.
• The sensors use infrared, ultrasonic, microwave, or other technology.

Working and Principles: All of the techniques below are used in the
design and development of occupancy sensor or motion sensor. These are
the basic principles in the design of such sensors.
1) Occupancy sensors are one kind of devices used for detecting whenever space is
empty then it is automatically deactivated the light so that the energy can be
conserved. This sensor may also activate the lights.
2) This device can also activate the lights routinely by detecting the occurrence of
people and provides security and convenience help.
3) Based on the laboratory like Lawrence Berkeley National, the strategies based on
occupancy can generate 24% of normal savings of lighting energy. Because of their
energy conservation and relative simplicity, these are united with energy code
permissions. These sensors are used in the latest construction and also it has a
general control feature used in retrofit projects.

4) Basic PIR (passive infrared) sensors detect movement and changes in their field of view.
These sensors are simple, providing basic occupied or un-occupied data. A common
example of a PIR sensor is a desk sensor that is typically placed on the underside of a
desk and is used to detect and report desk occupancy.
5) Ultrasonic sensors emit high-frequency sound waves, outside of human hearing range,
and use the doppler effect of returning sounds waves to detect people.
6) Sensors which detect changes in the air pressure due to opening of the doors and also
windows are referred as air pressure sensors.
7) The sensors which detect human body capacitance are referred as Capacitive Sensors.
8) Acoustic sensors utilize the sound produced by the people.
9) Photoelectric sensor works on the principle of interruption of light beams by the moving
objects

10) Optoelectrical sensor uses detection of variations in the illumination. It also uses
optical contrast in the region under target.
11) Pressure mat switches use the pressure sensitive long strips laid on the floors below
the carpets to detect weight of an intruder.
12) Stress detectors use strain gauges imbedded into floor beams, staircases, and other
structural components
13) Switch sensors utilizes electrical contacts connected to doors and windows.
14) Magnetic switches use a non-contact version of switch sensors.
15) Vibration detectors react to the vibration of walls or other building structures, also
may be attached to doors or windows to detect movements.
16) Glass breakage detectors are sensors reacting to specific vibrations produced by
shattered glass.
17) Infrared motion detectors are devices sensitive to heat waves emanated from warm or
cold moving objects.

18) Microwave detectors are active sensors responsive to microwave electromagnetic
signals reflected from objects.
19) Video motion detectors are video equipment which compares a stationary image
stored in memory with the current image from the protected area.
20) Video face recognition system uses image analysers that compare facial features with
a database.
21) Laser system detectors are similar to photoelectric detectors, except that they use
narrow light beams and combinations of reflectors.
22) Triboelectric detectors are sensors capable of detecting static electric charges carried
by moving objects

Ultrasonic Sensor:
Bats use a process
called “echolocation”
to locate prey or
other objects
Ultrasonic Sensor

Ultrasonic Sensor:
 An ultrasonic sensor is an electronic
device that measures the distance of a
target object by emitting ultrasonic
sound waves, and converts the
reflected sound into an electrical
signal.
 Ultrasonic waves travel faster than the
speed of audible sound (i.e. the sound
that humans can hear).
 Ultrasonic sensors have two main
components: the transmitter (which
emits the sound using piezoelectric
crystals) and the receiver (which
encounters the sound after it has
travelled to and from the target).

How Can We Measure Distance?
Bats use the same principle!
 In order to calculate the distance between the sensor
and the object, the sensor measures the time it takes
between the emission of the sound by the
transmitter to its contact with the receiver.
 The formula for this calculation is D = ½ T x C (where
D is the distance, T is the time, and C is the speed of
sound ~ 343 meters/second). For example, if a
scientist set up an ultrasonic sensor aimed at a box
and it took 0.025 seconds for the sound to bounce
back, the distance between the ultrasonic sensor and
the box would be:
D = 0.5 x 0.025 x 343
or about 4.2875 meters.

Microwave sensor
What is a Microwave Motion Sensor?
 A microwave motion sensor uses electro-magnetic radiation.
 It emits waves which are then reflected back to the receiver.
 The receiver analyzes the waves that are bounced back.
 If there is an object moving in the room, these waves are going to be altered. The
microwave detector is able to identify changes from moment to moment. Ideally, the
receiver should be receiving the same waves back again and again.
 Because of the way that microwave motion sensors work, they can be either more sensitive
or less sensitive. They can identify very minute changes (a totally empty house) or be
calibrated to require larger scale movement to avoid false positives.

Microwave sensor
What are the Capabilities of a Microwave Motion Sensor?
• Microwave sensors that are more advanced can also detect whether an individual is
moving towards or away from the sensor or moving randomly. These detectors are helpful
in sensing and differentiating between the ordinary movement and intruder movement.
This feature of these sensors makes them highly reliable.
• Microwave sensors are completely safe to use. They can be employed both inside and
outside a property and be placed across relatively large areas. They can also be configured
to detect different types of activity, such as ignoring certain areas of the home, perhaps
where pets or children might be active.

Microwave sensor
Benefits of a Microwave Motion Sensor
 Microwave motion detectors can be used in virtually any environment, including those that are not
otherwise hospitable to sensors, such as high heat environments that can set off photo-electric
sensors. This makes them one of the most versatile types of sensor system.
 Microwave detectors can go through walls and holes. This means they can cover a larger area of a
home or commercial property, including fairly large outdoor areas. Because of this, they’re usually
good for those who need to secure large areas of land.
 These detectors can also be programmed to reduce the amount of false alarms without having to
minimize the of correct positives, therefore improving accuracy and ease of use. Additionally,
microwave detectors are generally less expensive to purchase, even though they may be more
expensive to run.
 When shopping for sensors, it’s important to remember the everyday instances that could trigger a
false alarm, such as moving drapery or shifting sun patterns. Plus, the sensors require a continuous
power draw, so they may be expensive to run. They also only work at intervals rather than working
continuously, by sending out signals and then receiving them.

Microwave sensor
Microwave
Sensors
Active
Microwave
Sensors
Passive
Microwave
Sensors:
Active
Microwave
Sensors
Non-imaging
Radar Sensors
Imaging Radar
Sensors:
Active Microwave Sensors: The active sensors
usually transmit radio signals toward a target, which
spots the signal’s backscattered portion. Also, the
sensor measures the strength of the backscattered
signal by differentiating between the targets and the
time delay between the signals
Passive Microwave Sensors: Passive sensors have a
long wavelength with small energy radiated from a
body. Plus, the atmosphere can easily release them.
Hence, they are characterized by low spatial
resolution. Also, they are useful for applications like;
 Measuring atmospheric profiles
 Determining ozone content

Microwave sensor
Non-imaging Radar
 Non-imaging radars are profiling devices that measure in one linear direction.
Interestingly, these sensors include scatterometers and altimeters. Thus, the radar
altimeters send out short microwave pulses. Plus, it measures the distance from
sensors by gauging the round-trip time delay to targets.
 Indeed, the scatterometers are useful for making exact measurements (quantitative) of
backscattered energy from targets.
 Interestingly, scatterometry measurements can estimate wind speeds over ocean
surfaces. Also, it can measure the backscatter energy on land from various targets of
different surfaces and materials.
Imaging Radar
 Unlike non-imaging radar, imaging radar records the intensity of the signal reflection.
It also maps signal reflection in a two-dimensional image. Also, the radar images
have many dots that represent the backscatter for a specific area.

Microwave sensor
Advantages
 Microwave sensors can cut substantial amount of energy wastage by turning off lights automatically
when area is unoccupied. This saves 40% of electricity expenditure of the companies.
 Moreover, microwave sensors enable auto-dimming of lights which further optimizing the energy
charts.
 They are very sensitive and hence slightest movements are also being detected by them.
 They have wide coverage range which is about 120 meters.
 They can detect any motion even beyond the walls or behind the doors.
 They have benefits of high interference immunity, high precision and high reliability.
 They require one time servicing to provide lifespan operation.
 They can be used in harsh environments where heat cycles are irregular.
 Microwave sensors have wide variety of applications. They are used to monitor functions of bucket
elevators and belt conveyors.

Microwave sensor
Disadvantages
 Prone to false alarms due to blowing of objects due to wind, emission from fluorescent lights,
penetration through walls etc. For example, when they are used for lighting control in small
offices, the sensors switch ON lights even when someone is outside the office premises as they
can penetrate walls or glass.
 They have higher sensitivity and hence they can detect non-human presence such as animals,
fast moving objects in the air etc.
 Microwave frequency do not penetrate metal objects and hence microwave sensors can not
reach beyond any metal obstruction.
 Microwave radiation is hazardous for health and hence low power microwave sensors are
preferred.
 They operate at irregular intervals and hence intruders sometimes are left unnoticed.
 They are slightly costlier compare to PIR sensors.
 They consume more energy compare to PIR sensors.

Microwave sensor
Microwave Sensor Applications
The microwave sensors have a reliable performance in the following applications:
 Reverse car alarms
 Speed measure of vehicles
 Automated doors
 Respiratory monitors
 Liquid level measurement
 Home security systems
 They are also used for crane proximity detection.
 City municipalities also use them to monitor waste water and sewage levels.
 Microwave sensors are used in medical applications such as breast cancer treatment,
separation of red blood cells from white blood cells, liver tissue disease detection etc.

Capacitive Occupancy Sensor
Figure depicts basic circuit, capacitance between the test plate and earth is equal to value C1. In the
time when any person moves in the vicinity of the plate, it builds two additional capacitors;
 One between plate and body (Ca) and the other capacitor between body and earth (Cb). Hence the
resulting total capacitor between plate and earth will become larger by ΔC.
C = C1 + ΔC
 This type of sensor
is referred as
capacitive
occupancy sensor.
Being a conductive
medium with a high
dielectric constant, a
human body
develops a coupling
capacitance to its
surroundings.

Capacitive Occupancy Sensor
 These capacitances Ca and Cb greatly depends on factors such as human body size,
their clothing, carrying materials, type of surrounding objects, weather etc.
 The coupling capacitance will change due to movement of the persons in the target area.
 This will help the system discriminates static objects compare to the moving objects.
Here all the objects form some degree of a capacitive coupling with respect to one
another.

Optoelectronic Motion Sensor
• The most popular intrusion sensors are the optoelectronic motion sensors. This type of
motion sensor relies on EM radiation in the optical range. This Electromagnetic radiation
will have wavelengths range from 0.4 to 20 µm. The sensor will have distance ranges upto
hundred meters and used to find movement of people and animals.
Principle: The operating principle of the optical motion detectors is based on the detection
of light (either in the visible or nonvisible spectrum) radiated from the surface of a moving
object into the surrounding empty region. This radiation may be originated either by an
external light source and later got reflected by some object or it may be produced by the
object itself in the form of natural emission.
 The former sensor is referred as an active detector and the later one as a passive
detector.

 As mentioned, an active sensor
requires an additional light
source such as daylight, electric
lamp an infrared LED etc.
 The passive detectors perceive
mid and far infrared emission
from objects having
temperatures that are different
from the surroundings region.
 Both of these types of detectors
use an optical contrast as a
means of object recognition and
detection.

Advantages:
• The optoelectronic motion sensors are very useful for indicating whether an object is moving or
stationary.
• The most important advantages of an optoelectronic motion sensor are simplicity and low cost.
Disadvantages:
• But they cannot distinguish one moving object from the another.
• They cannot be utilized to accurately measure the distance to a moving object or its velocity.
The major application areas for the optoelectronic motion sensors are:
• security systems,
• energy management etc. In the energy management it is used to switch light on and off.
• It is also used for making "smart homes", in which we can control various appliances such as air
conditioners, cooling fans, stereo players and so on. This is also referred as home automation.

Visible and Near-Infrared Light Sensor
A Light Sensor generates an output signal indicating the intensity of light by measuring the
radiant energy that exists in a very narrow range of frequencies basically called “light”, and
which ranges in frequency from “Infra-red” to “Visible” up to “Ultraviolet” light spectrum.
ISO 20473 specifies the following scheme:
Designation Abbreviation Wavelength
Near-Infrared NIR 0.78–3 μm
Mid-Infrared MIR 3–50 μm
Far-Infrared FIR 50–1,000 μm

Principle of Light Sensor:
 The working principle of the light sensor is based on internal photoelectric effect, which
states that when light energy or photons are bombarded on a metal surface than it can
cause the free electrons from the metal to excite and jump out resulting in electron flow
or electric current.
 The amount of current produced depends on the energy of the photon (i.e. wavelength of
light). The emission of electrons from the metal surface occurs only after the light
reaches a certain threshold frequency that corresponds with the minimum energy required
by the electrons to break the metal bonds.
 Light sensors are more commonly known as “Photoelectric Devices” or “Photo Sensors”
because the convert light energy (photons) into electricity (electrons).

Infrared Sensors works on three fundamental Physics laws:
 Planck’s Radiation Law: Any object whose temperature is not equal to
absolute Zero (0 Kelvin) emits radiation.
 Stephan Boltzmann Law: The total energy emitted at all wavelengths by a
black body is related to the absolute temperature.
 Wein’s Displacement Law: Objects of different temperature emit spectra
that peak at different wavelengths that is inversely proportional to
Temperature.

Components of IR Sensor
IR Transmitter: IR Transmitter acts as source for IR radiation. According to Plank’s
Radiation Law, every object is a source of IR radiation at temp T above 0 Kelvin. In most
cases black body radiators, tungsten lamps, silicon carbide, infrared lasers, LEDs of
infrared wavelength are used as sources.
Transmission Medium: As the name suggests, Transmission Medium provides passage
for the radiation to reach from IR Transmitter to IR Receiver. Vacuum, atmosphere and
optical fibers are used as medium.
IR receiver: Generally, IR receivers are photo diode and photo transistors. They are
capable of detecting infrared radiation. Hence IR receiver is also called as IR detector.
Variety of receivers are available based on wavelength, voltage and package.

Working of IR Sensor
An Infrared Sensor works in the following sequence:
 IR source (transmitter) is used to emit radiation of required wavelength.
 This radiation reaches the object and is reflected back.
 The reflected radiation is detected by the IR receiver.
 The IR Receiver detected radiation is then further processed based on its
intensity. Generally, IR Receiver output is small and amplifiers are used
to amplify the detected signal.

Applications
1) Consumer electronics: Ever wonder what’s behind your smartphone and tablets that allow for
auto screen brightness adjustments? Yes, it’s an ambient light sensor! It measures the ambient
light level of your surroundings and determines the suitable brightness of your screen!
2) Automobiles: Similarly, it is used in automobiles to support the drivers’ field of vision. The
present light sensor detects surrounding ambient light, and if it’s getting too dark, it’ll
automatically turn on light systems!
3) Agricultural Usages: We all know crops need mainly two things for growth; Sunlight and water.
This is where a light sensor comes to play, helping farmers keep their crops hydrated yet not over-
hydrating it. Here’s how: 1) A light sensor is connected to a sprinkler system, detecting levels of
sunlight and only activating it when the sun isn’t at its brightest. 2) It is used alongside other
temperature sensors to help gather informative data as well
4) Security applications: Commonly used in circuits for shipment cargos, light sensors are connected
to circuits and placed inside as it can detect whenever a container is open due to the change in
light exposure. This helps in better processing of lost goods and tracking of personnel.

Types of Infrared Sensor
IR sensors can be classified in two types based on presence of IR source:
1) Active Infrared Sensor
2) Passive Infrared Sensor
Active Infrared Sensor
Active Infrared Sensor contains both transmitter and receiver. Most of the cases LED or laser diode is
used as source. LED for non-imaging IR sensor and laser diode for imaging IR sensor are used.
Active IR Sensor works by radiating energy, received and detected by detector and further processed by
signal processor in order to fetch information required.
Examples of Active IR Sensor: Break Beam Sensor, Reflectance Sensor.
Passive Infrared Sensor
Passive Infrared Sensor contains detectors alone. There won’t be a transmitter component. These types of
sensors use object as IR source/ transmitter. Object radiates energy and it is detected by IR receivers. A
Signal processor is then used to interpret the signal to fetch information required.
Example of Passive IR Sensor: Thermocouple-Thermopile, Bolometer, Pyro-Electric Detector, etc.

The PIR Sensor
PIR sensor generates energy when exposed to heat. Human or animal body radiates energy in
the form of infrared radiation. Hence when human/animal come in the range of PIR motion
sensor, it receives thermal energy and hence motion is detected by the sensor.
The PIR sensor itself has two slots in it, each slot is made of a special
material that is sensitive to IR. The lens used here is not really doing much
and so we see that the two slots can 'see' out past some distance (basically
the sensitivity of the sensor).
 When the sensor is idle, both slots detect the same amount of IR, the
ambient amount radiated from the room or walls or outdoors.
 When a warm body like a human or animal passes by, it first intercepts
one half of the PIR sensor, which causes a positive differential change
between the two halves.
 When the warm body leaves the sensing area, the reverse happens,
whereby the sensor generates a negative differential change. These
change pulses are what is detected.
PIR sensor is used as occupancy sensor. PIR sensor is passive sensor as it
senses infrared signal emitted by various objects including human body. They
are used as alternative to microwave sensors.

The PIR Sensor
The PIR Sensor Construction
The IR sensor itself is housed in a hermetically sealed metal can to improve
noise/temperature/humidity immunity. There is a window made of IR-transmissive
material (typically coated silicon since that is very easy to come by) that protects
the sensing element. Behind the window are the two balanced sensors.
Advantages of PIR sensor
 Detects motion reliably in indoors as well as in day or dark.
 It consumes less energy (0.8W to 1.0W) compare to microwave sensor.
 They are cheaper compare to microwave sensors.
 They are good for electrical applications used in smaller and compact premises

The PIR Sensor
Disadvantages of PIR sensor
 They have lower sensitivity and less coverage compare to microwave sensors.
 It does not operate greater than 35-degree C.
 It works effectively in LOS (Line of Sight) and will have problems in the corner
regions.
 It is insensitive to very slow motion of the objects.
 Since PIR sensors sense heat signatures in room, they are not very sensitive if the
room itself is warm. Hence PIR sensors are not able to detect human beings in the
summer in some countries like INDIA.
 Snoozing is another problem with PIR sensors. PIR sensors may turn off even if there
is very little movement in occupied floors.
 Thieves may find it easy to fool PIR detection range as they have slotted detection
zone and not continuous one like microwave sensor.

Part II: Position, Displacement, and Level Sensors
The measurement of position, displacement or level is very essential for many vivid
applications such as process feedback control, transportation traffic control, robotics,
security systems and more.
Position Sensor: A position sensor is a sensor that detects an object's position. The term
position refers to determination of object's co-ordinates (either linear or angular) with
respect to a selected reference.
Displacement Sensor: A Displacement sensor is a device that measures the distance
between the sensor and an object by detecting the amount of displacement through a variety
of elements and converting it into a distance. The term displacement refers to moving from
one position to another position for a specific distance or angle.
Level Sensors: A level sensor is a device that is designed to monitor, maintain, and measure
liquid (and sometimes solid) levels.

Potentiometric Sensor
Potentiometric Position Sensor: The most commonly used of all the “Position
Sensors”, is the potentiometer because it is an inexpensive and easy to use position
sensor.
 It uses a wiper contact linked to a mechanical shaft that can be either angular
(rotational) or linear (slider type) in its movement along a track.
 This movement causes the resistance value between the wiper/slider and the two
end connections to change giving an electrical signal output that has a
proportional relationship between the actual wiper position on the resistive track
and its resistance value.
 In other words, resistance is proportional to physical position.

Potentiometer Construction
 Potentiometers come in a wide range of designs and sizes such as the commonly
available round rotational type or the longer and flat linear slider types. When used
as a position sensor the moveable object is connected directly to the rotational shaft
or slider of the potentiometer.
 A DC reference voltage is applied across the two outer fixed connections forming
the resistive element. The output voltage signal is taken from the wiper terminal of
the sliding contact as shown below.
 This configuration produces a potential or voltage divider type circuit output which
is proportional to the shaft position.

Potentiometer Construction
The output signal (Vout) from the
potentiometer is taken from the centre
wiper connection as it moves along
the resistive track, and is proportional
to the angular position of the shaft.
Example of a
simple Positional
Sensing Circuit

Advantages: Low cost, Low tech, easy to use
Disadvantages:
 wear due to moving parts, low accuracy, low repeatability, and limited frequency response.
 But there is one main disadvantage of using the potentiometer as a positional sensor. The range of
movement of its wiper or slider (and hence the output signal obtained) is limited to the physical size
of the potentiometer being used.
 Most types of potentiometers use carbon film for their resistive track, but these types are electrically
noisy (the crackle on a radio volume control), and also have a short mechanical life.
 Wire-wound pots also known as rheostats, in the form of either a straight wire or wound coil
resistive wire can also be used, but wire wound pots suffer from resolution problems as their wiper
jumps from one wire segment to the next producing a logarithmic (LOG) output resulting in errors
in the output signal. These too suffer from electrical noise.
Applications: for this type of high accuracy position sensor is in computer game joysticks, steering
wheels, industrial and robot applications.

Potentiometer displacement sensors
Potentiometer displacement sensor is a primary sensor which converts the linear motion or the angular
motion of a shaft into changes in resistance. It is a type of resistive displacement sensor.
 Linear potentiometers are sensors that produce a resistance output proportional to the linear
displacement or position.
 Linear potentiometers are essentially variable resistors whose resistance is varied by the movement
of a slide over a resistance element.
 Rotary potentiometer are sensors that produce resistance output proportional to the angular
displacement or position. They can be either wire wound or conductive plastic, and either
rectangular or cylindrical.

Potentiometer displacement sensors
Principles and working
 The Figure illustrates the basic principle of a linear potentiometer. The linear potentiometer employs an
electrically conductive linear slide member (also called wiper) connected to a variable wire wound resistor
(winding) that changes resistance to be equated to the linear position of the device that is monitored.
 As the sliding contact moves along the winding, the resistance changes in linear relationship with the
distance from one end of the potentiometer.
 To measure displacement, a potentiometer is typically wired as a ‘voltage divider’ so that the output
voltage is proportional to the distance travelled by the wiper. A known voltage is applied to the resistor
ends.
 The contact is attached to the moving object of interest. The output voltage at the contact is proportional to
the displacement.
 The resolution is defined by the number of turns per unit distance, and loading effects of the voltage
divider circuit should be considered.
 A rotary potentiometer employs a rotary slide member connected to a variable wire wound resistor that
changes resistance to be equated to the angular position of the device that is monitored.
 Other principles of operations are same as that of linear potentiometer.

Potentiometer level sensors
 Figure shown below depicts
gravitational fluid level sensor using
a float.
 As the liquid level changes either on
upward direction or downward
direction than float position changes.
 This results into variation in the
wiper arm across the resistance. This
results into measurement of level
position.

Gravitational Sensor:
Gravitational Sensor:
A gravity sensor measures the direction and intensity of gravity.
Using such data, we can check the relative direction of a device
within a space.
A linear accelerometer provides data on acceleration, excluding
gravity.
In other words, a linear accelerometer measures the acceleration,
excluding the impact of gravity on a certain object. Using this
sensor, we can find out how fast a car is driving.
With a gravity sensor and a linear accelerometer, a navigation
application allows you to track the direction of a car.

Capacitive Sensor:
A capacitive sensor is a passive sensor that works on the principle of variable capacitances. It
is used to measure physical quantities such as displacement, pressure, etc.
Construction of capacitive sensors: A capacitive sensors contains two conducting parallel metal plates
separated by a dielectric medium.
Working Principle of capacitive sensors: The capacitance between these two plates
can be expressed as
Where ϵ is the permittivity of the medium, A is the area of the plates and d is the distance
between two plates.
The capacitance of the sensors is measured using the bridge circuit. The output impedance of
the sensors is given by
Where C is the capacitance and f is the frequency of excitation. So a capacitive sensors can
be used to measure the mechanical vibrations

Capacitive Sensor:
The capacitance between two plates can be varied by any of
the following methods.
 By changing the distance between two plates (d)
 By changing the permittivity of the dielectric medium (ϵ)
 By changing the area of overlapping of plates (A)

Capacitive Sensor:
By changing the distance between two plates: The capacitance can be varied by changing the distance
between two plates. From the equation for C, we can observe that C and d are inversely proportional to
each other. That is, the capacitance value will decrease with increasing distance and vice-versa. This
principle can be used in a sensor by making the left plate fixed and the right plate movable by the
displacement that is to be measured as shown in the figure.
 The change in distance between two plates will vary the capacitance of the sensors. Change in
capacitance can be calibrated in terms of the measurand. These types of sensors are used to measure
extremely small displacements. The distance capacitance curve is shown in the figure.

Capacitive Sensor:
By changing the permittivity of the dielectric medium: Another method to change the capacitance
value is by changing the permittivity of the dielectric material (ϵ). The permittivity and capacitance
value are directly proportional to each other.
 In this arrangement, a dielectric
material is filled into the space
between the two fixed plates. It can
be moved using the arm. This
causes a variation in dielectric
constant in the region. The change
in dielectric constant will vary the
capacitance of the sensors.

Capacitive Sensor:
By changing the area of overlapping of plates: The capacitance can also be changed by
varying the area of overlapping of plates.
 As shown in the figure, one plate is kept fixed and the other movable. When the plate
is moved, the area of overlapping of plates changes, and the capacitance also changes.
The capacitance value and area are directly proportional to each other. These types of
sensors are used to measure relatively large displacements. The distance-capacitance
curve is shown in the figure

Capacitive Sensor:
Capacitive level measurement
 There is a probe inserted at the middle of the
tank which is a dielectric material.
 The probe form one electrode of the capacitor
and the metal sheet at the walls form the other.
 Capacitance is measured across the probe and
the metal plate on the wall.
 The liquid inside the tank forms the dielectric
material of the capacitor if the liquid is non-
conductive.
 If the liquid is conductive then we have to cover
the probe with a dielectric sheath to deliver a
capacitance function.

Capacitive Sensor:
Advantages of capacitive sensors
 Sensitivity is high.
 Requires small power to operate.
 Loading effect is low because of high input impedance.
 Good frequency response.
Disadvantages
 Limited in its application for products of changing electrical properties
(especially moisture content)

Inductive Sensor:
An inductive sensor is a device that uses the principle of electromagnetic induction to
detect or measure objects. An inductor develops a magnetic field when a current flows
through it; alternatively, a current will flow through a circuit containing an inductor when
the magnetic field through it changes. This effect can be used to detect metallic objects that
interact with a magnetic field.
The inductive sensor is based on Faraday's law of induction. The temporal variations of the Magnetic Flux Φ
through a N turns circuit will induce a voltage e which follows:
by assuming that the induced magnetic field B is homogeneous over a section S (the Magnetic flux will be
expressed Φ = B X S

Inductive Sensor:
 Designed for non-contact measurement
of displacement, distance, position,
oscillation and vibrations. They are
particularly suitable when high
precision is required in harsh industrial
environments (pressure, dirt,
temperature)
 Inductive Proximity Sensor: An
inductive sensor is a non-contact type
of sensor, helpful in the detection of
metallic objects. It can sense ferrous as
well as non-ferrous materials. The
sensing range is up to 100 mm.
However, the level of sensitivity defers
while sensing non-ferrous material.
Look at the table below.

Inductive Sensor:
Sensitivity when different
objects are present, Sn =
Operating distance
Fe37 (Iron) 1 x Sn
Stainless Steel 0.9 x Sn
Brass bronze 0.5 x Sn
Copper 0.4 x Sn
Aluminum 0.4 x Sn
An inductive proximity sensor consists of four elements – the coil,
the oscillator, the trigger circuit, and an output.

Inductive Sensor:
Coil: The coil generates the necessary electromagnetic field.
Cup-shaped ferrite magnetic core holds the coil inside. The cup-
shaped core is necessary to concentrate the coil magnetic field on
the front area of the sensor.
Oscillator: The oscillator is generally an LC oscillator. It
produces radio frequency (100 kHz to 1 MHz) which helps to
generate an electromagnetic field.
Trigger Circuit: The trigger circuit senses the change in
amplitude of oscillation and gives the signal to solid-state output.
Output Circuit: The output circuit has a transistor NPN or PNP.
After receiving the gate signal, the transistor switches ON and
gives an output.

Inductive Sensor:
Working Principle of Inductive Proximity Sensor:
 When a metal target enters the magnetic field created by coil, eddy current
circulates within the target. This causes load on the sensor which decreases the
oscillator’s amplitude. As the target reaches close to the sensor further the
oscillator’s amplitude decreases.
 The trigger circuit is normally a Schmitt trigger. It monitors the amplitude of an
oscillator. If the oscillator’s amplitude reaches a predetermined level, the
trigger circuit gives the signal to the output circuit to switch-ON the output.

Inductive Sensor:
Application of Inductive Proximity Sensors
 You can use an Inductive proximity sensor to count the
metal cans. Inductive Sensor counting tins application
 Can be used to monitor the rotational speed of the
machine.
 In conveyor application, you can use it to monitor the
position.
 In a pipe manufacturing plant, the sensors are best for
metal pipe detection for further processing of pipe.
 Robotic arm control is possible with the help of inductive
sensors.
 The monitoring and counting can be done without
actually touching the target. That is the biggest
advantage.

Inductive Sensor:
Advantages of Inductive Proximity Sensors
 Contactless sensing.
 High switching rate.
 Long-life as no moving parts are there.
 Easy installation.
 It can withstand harsh environmental conditions.
 It has very predictable results and performance.
Disadvantages of Inductive Proximity Sensors
 Can sense only metal.
 The sensing range of an inductive sensor dependents on the type of metal being detected, its
shape, its size and also coil size used in the design. Due to above reason, inductive sensor has
distance limitations for sensing
 Range detection limitation. The maximum detection range is 100mm.

Magnetic Sensor:
A magnetic sensor is a sensor that detects the magnitude of magnetism and
geomagnetism generated by a magnet or current. There are many different types of
magnetic sensors. This section explains the typical sensor types and their features.
1) Coils:

Magnetic Sensor:
1)Coils:
 Coils are the simplest magnetic sensors that can detect changes of the magnetic flux density. As
shown in Figure 1, when a magnet is brought close to the coil, the magnetic flux density in the coil
increases by ΔB. Then, an induced electromotive force/induced current that generates a magnetic
flux in a direction that hinders an increase in magnetic flux density is generated in the coil.
Conversely, moving the magnet away from the coil reduces the magnetic flux density in the coil, so
induced electromotive force and induced current will be generated in the coil to increase the
magnetic flux density.
 Also, since there is no change in the magnetic flux density when the magnet is not moved, no
induced electromotive force or induced current will be generated. By measuring the direction and
magnitude of this induced electromotive force, it is possible to detect the change in magnetic flux
density.
 Because of its simple structure, a coil is not easily damaged. However, the output voltage depends
on the rate of change of the magnetic flux. It may not be possible to use a coil to detect a fixed
magnet or magnetic flux that changes very slowly.

Magnetic Sensor:
2) Reed Switch
A reed switch is a sensor in which metal pieces (reed) extending from both the left and
right sides are enclosed in a glass tube with a gap at the overlapping position of the
reeds. When a magnetic field is applied externally, these reeds are magnetized. When
the reeds are magnetized, the overlapping parts attract each other and come into contact,
then the switch turns on.

Magnetic Sensor:
3) Hall Effect Sensor: Hall effect sensor is a magnetic sensor
Hall Effect Sensor is the solid-state
device which switches to active state
when it is introduced in magnetic
field. The output voltage of hall effect
sensor is dependent on magnetic field
around it. When the magnetic field
across the semiconductor slab changes
the magnetic flux density also changes
due to which the output voltage of hall
effect sensor varies.
Principle of Hall Effect Sensor: The
hall effect sensor works on the
principle of hall effect.

Magnetic Sensor:
Principle of Hall Effect Sensor: The hall effect sensor works on the principle of hall effect.
 According to hall effect when a semiconductor slab is placed in magnetic field provided that
magnetic field lines are perpendicular to the axis of semiconductor specimen and current is
allowed to pass along the axis of semiconductor specimen then the charges carriers of the
semiconductor device experiences magnetic force.
 Due to this magnetic force they are pushed sidewards i.e towards the edges of the slab. As a
consequence of this the electric field is created due to accumulation of charge carriers across the
edges. Thus, the output voltage varies with the variation in the magnetic field. Hall effect is based
on the Lorentz principle.
 Hall Effect sensors uses this phenomenon of Hall effect for sensing fundamental quantities such
as position, velocity, polarity etc. The two crucial term associated with magnetic field are
magnetic flux density and polarity (North Pole and South Pole). The hall effect sensors uses these
terms for sensing.
 The output voltage generated by the sensor is directly dependent on magnetic flux density. Thus,
if magnetic field across the sensor changes the output from hall effect also changes. In this way it
provides sensing operation.

Magnetic Sensor:
Applications of Hall Effect Sensor
 Hall Effect Sensors are used for sensing positions thus, they are often used as
proximity sensors.
 They can also be used in the application in which we use optical and light sensors.
 Hall effect sensors are better to use because optical and light sensors are likely to get
affected by environmental conditions while Hall Effect sensors can also work
efficiently in the dust, air or other external environmental factors.

Optical Sensor:
Optical Position Sensors
Optical position sensors operate using one of two principles.
 In the first type, light is transmitted from an emitter and sent
over to a receiver at the other end of the sensor.
 In the second type, the emitted light signal is reflected from the
object being monitored returned towards the light source. A
change in the light characteristics (e.g. wavelength, intensity,
phase, polarization) is used to establish information about the
object’s position.

Optical Sensor:
Optical Displacement Sensor
Principle: Light is sent through a transmitting fiber
and is made to fall on a moving target. The
reflected light from the target is sensed by a
detector. With respect to intensity of light reflected
from its displacement of the target is measured.
Description: It consists of a bundle of transmitting
fibers coupled to the laser source and a bundle of
receiving fibers coupled to the detector as shown
in the figure. The axis of the transmitting fiber and
the receiving fiber with respect to the moving
target can be adjusted to increase the sensitivity of
the sensor

Optical Sensor:
Working:
 Light from the source is transmitted through the transmitting fiber and is made to fall on the
moving target. The light reflected from the target is made to pas through the receiving fiber and
the same is detected by the detector.
 Based on the intensity of the light received, the displacement of the target can be measured, (i.e.)
if the received intensity is more than we can say that the target is moving towards the sensor and if
the intensity is less, we can say that the target is moving away from the sensor.
Application as MEDICAL ENDOSCOPE
 Optical fibers are very much useful in medical field. Using low quality, large diameter and short
length silica fibers we can design a fiber optic endoscope or fibroscope
 A medical endoscope is a tubular optical instrument, used to inspect or view the internal parts of
human body which are not visible to the naked eye. The photograph of the internal parts can also
be taken using this endoscope.

Optical Sensor:
Optical level Sensor

Optical Sensor:
Optical level Sensor
 The working principle of the optical water level sensor, the product contains a infrared light-
emitting diode and a photosensitive receiver.
 The light emitted by the LED is directed into the lens at the top of the sensor.
 When the liquid is immersed in the lens of the photoelectric level switch, the light is refracted
into the liquid, so that the receiver does not receive or can only receive a small amount of
light.
 The tank infrared level sensor in operating conditions, and the receiver can drive an internal
electrical switch to activate an external alarm or control circuit. If there is no liquid, the light
from the LED is reflected directly from the lens back to the receiver.
 The optical level sensor will output a high voltage value or a low vlotage value according to
water state or waterless state.
 When the prism of level sensor is in liquid, the level sensor will output a low voltage; When
the prism of level sensor is in air, the level sensor will output a high voltage.


Radar Sensor:
The sensor which is used to measure the distance, velocity and movements of objects above wide
distances is known as a radar sensor and also measures the relative speed of the noticed object. This
sensor uses wireless detecting technology like FMCW (Frequency Modulated Continuous Wave) to
detect the motion by figuring out the object’s shape, position, motion trajectory & motion
characteristics.  As compared to other types of sensors,
these sensors are not affected by
darkness & light. These sensors can
detect longer distances & it is secure
for people & animals. Here the carrier
frequency is modulated constantly in a
small range of bandwidth. Once the
signal from an object is reflected back,
then it is feasible to determine the
distance & also the object speed by
comparing frequency.
 This sensor uses an extremely high
carrier frequency to produce a very thin
beam cone and also notices even small
objects without interference from
adjacent objects above large distances.

Radar Sensor:
Radar Sensor Working Principle
 The working principle of a radar sensor is to compute the speed of an object
along with its direction by detecting the change in frequency wave which is
known as Doppler Effect.
 A radar sensor includes an antenna that emits a high-frequency (62 GHz)
transmitted signal. This transmitted signal also includes a modulated signal
with a lower frequency (10 MHz). This sensor gets the signal once it is
returned back from an object. So this sensor evaluates the phase shift between
the two frequencies. Here, the difference in transmitting time & receiving time
will determine the distance between the sensor & an object.

Radar Sensor:
Automotive Radar Sensor Block Diagram
 The block diagram of the 24 GHz wideband & short-range automotive radar sensor is
shown below. This block diagram includes a VCO, PRF (pulse repetition frequency),
LNA (low noise amplifier), DSP (digital signal processing) & two antennas.

Radar Sensor:
VCO: The term VCO stands for voltage-controlled oscillator which is used to generate an o/p signal whose
frequency changes with the amplitude of voltage for an input signal above a reasonable frequencies range.
Power Splitter: A power splitter or power divider is used to divide a single RF line into above one line & split the
power.
Power Amplifier: A power amplifier is used to change a signal from a low-power to a higher power.
SP (Signal Processing): Signal processing focuses on modifying, synthesizing & analyzing signals like images,
sound, & scientific measurements.
PRF (Pulse Repetition Frequency): The pulse repetition frequency is the number of pulses of a repeating signal
within a specific unit time, usually measured in pulses for each second.
Mixer: The mixer is used to generate both the frequencies sum & difference which are applied to it. So the
frequencies difference will be of IF (Intermediate Frequency) type.
LNA (Low Noise Amplifier): It is used to amplify the weak RF signal and this signal is received by using an
Antenna. This amplifier’s output can be connected to Mixer.
Antennas: This system includes transmit & receive channels where the transmit channels are mainly used to drive
different antennas & also provide beam steering capabilities. Multiple receive channels provide the angular data
regarding the target because there is a phase difference between received signals by dissimilar receive antennas.

Radar Sensor:
 The concept used by the 24 GHz SRR (Short Range Radar) sensors is pulsed radar. This sensor
includes the transmitting & receiving path, the control & DSP (digital signal processing) circuits.
 The target at range ‘R’ can be detected by measuring the elapsed time in between a transmitter
signal & a correlated received signal.
Application:
 The main aim of this radar sensor is to decrease potential danger & traffic accidents faced by the
vehicle driver. In this system, different sensors are located in different places of the car so that the
exact measurement of object distance & speed of objects in front, behind, or beside.
 Every sensor in this system transmits the signals to calculate, if there is anybody in the region of
the car then informs the driver regarding it. These signals cover upto 30 m distance but, if the
distance in between the target & car was less than two meters, then the car generates an alarm
sound to give an alert to the driver so that the car driver can take the appropriate action to avoid a
collision.

Radar Sensor:
Radar Sensor Types
There are different types of radar sensors which include the following

Radar Sensor:
Millimeter-Wave Radar Sensor
 The sensor which uses millimeter waves is known as a millimeter-wave radar sensor.
Generally, millimeter waves have a 30 to 300 GHz frequency domain. Among them,
77Ghz & 24Ghz radar sensors are used in automobiles for collision avoidance. The
millimeter-wave wavelength ranges in between centimeter wave & lightwave. The
advantages of millimeter-wave are photoelectric guidance and microwave guidance.
 Millimeter-wave radar has many characteristics as compared to centimeter wave radar-
like spatial resolution is high, simple integration, and small size. As compared to optical
sensors like lasers, infrared, cameras, this sensor has a strong capacity to penetrate
smoke, dust, fog & anti-interference capacity. These radar sensors are used in security,
automotive electrons, intelligent transportation, and drones.

Radar Sensor:
CW Doppler Radar Sensor
 A CW Doppler radar sensor or continuous wave Doppler radar operates at 915
MHz frequency. This radar sensor works with Doppler Effect for measuring the
object’s speed at various distances. This sensor transmits a microwave signal to a
target & analyzes the change in frequency in the reflected signal, the difference
between the reflected & transmitted frequencies, and also measures the target
speed precisely which is relative to the radar.

Radar Sensor:
FMCW Radar Sensor
 The term “FMCW” stands for frequency modulated continuous wave radar.
This sensor frequency will be changed with the time based on the triangle
wave’s law. The echo signal frequency which is received by the radar is
similar to the emission frequency. They both are triangular waves but there is
a tiny difference in time. So this tiny difference is used to calculate the target
distance.

Radar Sensor:
Advantages
 The radar sensor is independent of different weather conditions
 Bears excessive cold & heat
 It works in bad lighting conditions
 It works in the dark
 Its maintenance is free
 It provides a great range of functions
 This sensor is used for indoor & outdoor purposes
 This sensor has many features as compared to other sensors

Radar Sensor:
Disadvantages
The disadvantages of radar sensors include the following.
 It cannot differentiate & resolve numerous targets which are extremely close like
our eye.
 It cannot identify the color of the objects.
 It cannot observe objects which are too deep and in the water.

Radar Sensor:
Applications
The applications of radar sensors include the following.
 Radar sensors are used where vehicle detection is required or avoiding a collision when equipment is moving.
Vehicle detection mainly includes trucks, trains, cars, toll booths, shipping canals, railroads, etc. Collision
avoidance includes ports, manufacturing, low-visibility factory environments & onboard mobile equipment.
 Military
 Security system
 Automotive electrons
 Intelligent traffic radar
 UAV radar
 Intelligent lighting
 Industrial control
 Medical treatment
 Sports

Radar Sensor Vs Ultrasonic Sensor
Radar Sensor Ultrasonic Sensor
• The radar sensor is used to change the
signals from microwave echo to
electrical.
• An ultrasonic sensor is used to measure
the distance to an object with ultrasonic
sound waves.
• These sensors work with
electromagnetic waves.
• These sensors work by producing sound
waves.
• Similar to ultrasonic, the waves from
this sensor will reflect the target &
travel at a known speed very fast.
• The sound waves travel at the speed of
sound to the target where they reflect the
target & come back to the sensor.
• The electromagnetic waves of this
sensor will respond in a different way
to particular materials because they
are reflected off the exterior.
• The sound waves of this sensor will not
respond to particular materials.
• These sensors are affected through
different variables
• These sensors are affected by temperature.
• These sensors are used in oil & gas,
pulp & paper, clarifiers, granular solids,
plastic pellets, pharmaceuticals, etc.
• These sensors are used for measuring the
flow of liquid, solids level, open-channel
flow, object profiling & presence detection.

Velocity and Acceleration Sensors
What is a Velocity Sensor?
A velocity sensor is a device used to measure the change in distance over time.
• As the vibration amplitude increases, the output of the sensor increases.
• Velocity sensors are also available in a variety of shapes, sizes, sensitivity levels,
and technologies.
• They are commonly used for measuring the speed of moving objects, such as
vehicles or machinery vibration.

What is an Accelerometer?
An accelerometer is a device that measures the change in velocity over time of a
reference mass.
Using Newton’s Law, mass times acceleration equals force (F = m x a).
Accelerometers can measure both the magnitude and direction of this force.
• They come in various shapes, sizes, sensitivity levels, and technologies.
• Some are small and lightweight, while others are large and robust.
• They can be used in a variety of applications, including automotive, aerospace,
military, and industrial to measure vibration.

Accelerometer works based on Newton's second
law of motion i.e. F = m*a, where 'a' is
acceleration, 'F' is applied force to the mass 'm'
attached to the wall through spring (having
coefficient 'k').
➨F = m*a = Fs = K*x, here x is displacement of
the body from initial rest position.
➨m*a = k*x
➨a = f(x), acceleration is the function of
displacement
➨Hence if x is known, acceleration ('a') can be
found out easily. There are various techniques of
finding displacement 'x' viz. resistive techniques,
capacitive techniques and inductive techniques.

There are two types of acceleration forces: static forces and dynamic forces.
• Static forces are forces that are constantly being applied to the object (such as
friction or gravity).
• Dynamic forces are “moving” forces applied to the object at various rates (such
as vibration, or the force exerted on a cue ball in a game of pool).
 This is why accelerometers are used in automobile collision safety systems, for
example. When a car is acted on by a powerful dynamic force, the accelerometer
(sensing a rapid deceleration) sends an electronic signal to an embedded
computer, which in turn deploys the airbags.

Capacitive Accelerometers
Capacitive Accelerometers: Capacitive accelerometers are similar in operation to
piezoresistive accelerometers, in that they measure a change across a bridge; however,
instead of measuring a change in resistance, they measure a change in capacitance.
• The sensing element consists of two parallel plate capacitors acting in a differential mode.
• These capacitors operate in a bridge configuration and are dependent on a carrier
demodulator circuit or its equivalent to produce an electrical output proportional to
acceleration.

• Several different types of capacitive
elements exist. One type, which utilizes
a metal sensing diaphragm and alumina
capacitor plates, can be found in Figure
5.2.12. Two fixed plates sandwich the
diaphragm, creating two capacitors, each
with an individual fixed plate and each
sharing the diaphragm as a movable
plate.
• When this element is placed in the
Earth’s gravitational field or is
accelerated due to vibration on a test
structure, the spring mass experiences a
force. This force is proportional to the
mass of the spring-mass and is based on
Newton’s Second Law of Motion.

F = ma, where F = inertial force acting on spring-mass
m = distributed mass of spring-mass
a = acceleration experienced by sensing element
Consequently, the spring-mass deflects linearly according to the Spring Equation.
X = F/k where X = deflection of spring-mass
k = stiffness of spring-mass
The resulting deflection of the spring-mass causes the distance between the electrodes and the spring-mass to vary.
These variations have a direct effect on each of the opposing capacitor gaps according to the following equation.
C2 = AE [ε / (d + X)] and,
C2 = AE [ε / (d – X)] where C = element capacitance
AE = surface area of electrode
ε = permittivity of air
d = distance between spring-mass and electrode

Working:
 Most of the accelerator use capacitive technique as explained below.
 ➨C = f (A/d), where 'A' is the area of plate and 'd' is the distance between the plates. If distance is
known, capacitance (C) can be found out.
 To have the similar functionality to initial spring mass system, a modified version is employed using
two plates, one fixed and the other movable as shown in figure. Due to gravitational force, distance
between plates 'd' varies which depends on acceleration 'a' of the body. This results into change in the
capacitance between plates.
 Capacitance measurement helps in determining value of 'd' (from equation C= f(d/x) which helps us
calculate value of 'd' (from equation, d = f(x) ).
 Capacitive accelerometers, also known as vibration sensors, rely on a change in electrical capacitance
in response to acceleration.
 Accelerometers utilize the properties of an opposed plate capacitor for which the distance between
the plates varies proportionally to applied acceleration, thus altering capacitance. This variable is
used in a circuit to ultimately deliver a voltage signal that is proportional to acceleration.
 Capacitive accelerometers are capable of measuring constant as well as slow transient and periodic
acceleration.

 Capacitive-acceleration sensors fundamentally contain at least two components; the
primary is a ‘stationary’ plate (i.e., connected to the housing) and the secondary plate is
attached to the inertial mass, which is free to move inside the housing. These plates form
a capacitor whose value is a function of a distance d between the plates (Figure).
 The sensing material is either a flat plate of nickel or electronic chip supported above
the substrate surface by two torsion bars attached to a central pedestal.
 A capacitive accelerometer rarely exceeds a maximum displacement of 20 μm.
Therefore, such a small displacement requires a reliable measurement of drifts and
various interferences.
 When subject to a fixed or constant acceleration, the capacitance value is also a constant,
resulting in a measurement signal proportional to uniform acceleration, also referred to
as DC or static acceleration. Figure shows an example of a capacitive accelerometer

Piezoresistive Accelerometers
 A piezoresistive accelerometer produces resistance changes in strain gauges that are part of the
accelerometer’s seismic system.
 Piezoresistive accelerometers have a very wide bandwidth which allows these to be used for
measuring short duration (high frequency) shock events such as crash testing.
 Piezoresistive accelerometers can be gas or fluid damped which protects the accelerometer; but
also further widens the dynamic range by preventing the accelerometer from reaching its
internal resonant frequency.
 Piezoresistive accelerometers measure down to zero hertz so they can also be used to
accurately calculate velocity or displacement information.
 Piezoresistive accelerometers typically have a very low sensitivity which makes them less
useful for accurate vibration testing.
 Piezoresistive accelerometers are also sensitive to temperature variation so a temperature
compensation will be required but many now include this compensation internally.

Piezoresistive Accelerometers
 Piezoresistive accelerometers are much more expensive than the capacitive MEMS
accelerometers so they’re generally not used for lower frequency and amplitude testing.
 Piezoresistive accelerometers are by far the best type for impulse/impact measurements
where the frequency range and amplitude are typically high; examples include
automotive crash testing, and weapons testing.
 Piezoresistance accelerometers are much less sensitive than piezoelectric accelerometers,
and they are better suited to vehicle crash testing.

Piezoelectric Accelerometers:
A piezoelectric accelerometer utilizes the piezoelectric effect (piezoelectric materials
produce electricity when put under physical stress) to sense change in acceleration.
Piezoelectric accelerometers are most commonly used in vibration and shock
measurement.
The sensing element of a piezoelectric accelerometer consists of two basic
components:
• Piezoceramic material
• Seismic mass

 One side of the piezoelectric material is connected to a rigid post at the sensor base. A so-called
seismic mass is attached to the other side. When the accelerometer is subjected to vibration, an
inertial force is generated which acts on the piezoelectric element (compare Figure 2). According
to Newton’s Law this force is equal to the product of the acceleration and the seismic mass. By
the piezoelectric effect a charge output proportional to the applied force is generated. Since the
seismic mass is constant the charge output signal is proportional to the acceleration of the mass
 When the accelerometer is subjected to vibration, a force is generated and a small millivolt
change is measured. This voltage is proportional to the acceleration of the mas

 Over a wide frequency range both sensor base and seismic mass are exposed to the same
acceleration magnitude. Hence, the sensor measures the acceleration of the test object.
 Within the usable operating frequency range, the sensitivity is independent of frequency.
 A piezoelectric accelerometer can be regarded as a mechanical low-pass with resonance peak.
 It shows the typical resonance behavior and defines the upper frequency limit of an accelerometer.

 In order to achieve a wider operating frequency range the resonance frequency must be increased.
This is usually done by reducing the seismic mass.
 However, the lower the seismic mass, the lower the sensitivity.
 Therefore, an accelerometer with high resonance frequency, for example a shock accelerometer, will
be less sensitive whereas a seismic accelerometer with high sensitivity has a low resonance
frequency. Figure shows a typical frequency response curve of an accelerometer when it is excited
by a constant acceleration.

Benefits or advantages of Accelerometer sensor
➨It is simple to interface and rugged in design.
➨It has high impedance.
➨It offers higher sensitivity.
➨It has high frequency response.
➨It is available at lower cost due to advancement in MEMS
technology
➨It uses built-in signal conditioning circuit to measure
capacitance.

Drawbacks or disadvantages of Accelerometer sensor
➨An Accelerometer measures chance in velocity only. It does not measure a
constant velocity.
➨An Accelerometer can not measure rotation around its own axis of movement.
Due to this, it is used in conjunction with gyroscope to measure angular
velocity.
➨It is sensitive to temperature and operates over limited temperature range.
➨Its efficiency degrades over time.
➨It requires external power for its operation.
➨The other disadvantages are less longevity and hysteresis error.

Heated- Gas Accelerometer:
 A Heated- Gas Accelerometer device consisting
of a chamber of gas with a heating element in
the center, four temperature sensors around its
edge.
 Hold accelerometer level→hot gas pocket rises
to the top-center of the accelerometer’s
chamber→all sensors measure same temperature
 Tilt the accelerometer→hot gas pocket collects
closer to one or two temperature
sensors→sensors closer to gas pocket measure
higher temperature
 electronics compares temperature measurements
and outputs pulses (pulse duration encodes
sensor o/p)

Gyroscope:
What is a Gyroscope?
A gyroscope is defined as The device has a spinning disc that is mounted on the base such that it can
move freely in more than one direction so that the orientation is maintained irrespective of the
movement in the base. Accelerometers measure linear acceleration (specified in mV/g) along one or
several axis. A gyroscope measures angular velocity (specified in mV/deg/s)

Design of Gyroscope
A gyroscope can be considered as a massive rotor that is fixed on the
supporting rings known as the gimbals. The central rotor is isolated from the
external torques with the help of frictionless bearings that are present in the
gimbals. The spin axis is defined by the axle of the spinning wheel.
The rotor has exceptional stability at high speeds as it maintains the high-
speed rotation axis at the central rotor. The rotor has three degrees of
rotational freedom.

Gyroscope Working Principle
The working principle of gyroscope is based on gravity and is explained as the
product of angular momentum which is experienced by the torque on a disc to
produce a gyroscopic precession in the spinning wheel.
• This process is termed gyroscopic motion or gyroscopic force and is defined as
the tendency of a rotating object to maintain the orientation of its rotation.
• We know that the rotating object possesses angular momentum and this needs
to be conserved.
• This is done because when there is any change in the axis of rotation, there will
be a change in the orientation which changes the angular momentum.
Therefore, it can be said the working principle of gyroscope is based on the
conservation of angular momentum.

Types of Gyroscopes
The following are the three types of gyroscopes:
1. Mechanical gyroscope
2. Optical gyroscope
3. Gas-bearing gyroscope

Mechanical Gyroscope
• The working principle of the mechanical gyroscope is based on the
conservation of angular momentum. This is also one of the most commonly
known gyroscopes.
• The mechanical gyroscope is dependent on the ball bearing to spin.
• These gyroscopes are replaced with modern forms of gyroscopes as they are
noisier.
• They find applications in the navigation of large aircraft and missile
guidance.

Optical Gyroscopes
• These gyroscopes are dependent on the ball
bearing or the rotating wheel.
• They are also not based on the conservation of
angular momentum rather on interference of
light.
• Optical gyroscopes use two coils of optic fibre
that are spun in different orientations. Since
there is no movement in the optical gyroscopes,
these are considered to be durable and find
applications in modern spacecraft and rockets.

Optical Gyroscopes
• Optical gyroscopes, with virtually no moving parts, are used in commercial
jetliners, booster rockets, and orbiting satellites.
• Such devices are based on the Sagnac effect, first demonstrated by French
scientist Georges Sagnac in 1913.
• In Sagnac’s demonstration, a beam of light was split such that part traveled
clockwise and part counterclockwise around a rotating platform.
• Although both beams traveled within a closed loop, the beam traveling in the
direction of rotation of the platform returned to the point of origin slightly after
the beam traveling opposite to the rotation.
• As a result, a “fringe interference” pattern (alternate bands of light and dark) was
detected that depended on the precise rate of rotation of the turntable.

Gas-Bearing Gyroscopes
• In a gas-bearing gyroscope, the amount of friction between the
moving parts is reduced by suspending the rotor with the help of
pressurized gas.
• NASA used a gas-bearing gyroscope in the development of the
Hubble telescope.
• When compared to the other types of gyroscopes, gas-bearing is
quieter and more accurate.

Applications of Gyroscope
 Gyroscopes find applications in the compasses of boats, spacecraft, and
aeroplanes. The orientation and the pitch of the aeroplane are determined
against the steady spin of the gyroscope.
 In spacecraft, the navigation of the desired target is done with the help of a
gyroscope. The spinning centre of the gyroscope is used as the orientation point.
 The stabilization of the large boats and satellites is done with the help of
massive gyroscopes.
 Gyroscopes are used in gyrotheodolites for the maintenance of the direction in
tunnel mining.
 Gyroscopes along with accelerometers are used in the design of smartphones
providing excellent motion sensing.

What is the difference between Accelerometer and Gyroscope?
Accelerometer Gyroscope
It is used for measuring the linear
movement and for the detection of
tilt
It is used for the measurement of all
types of rotation but fails in the
identification of movement
The signal-to-noise ratio is lower The signal-to-noise ratio is higher
This cannot be used for the
measurement of angular velocity
This can be used for the
measurement of angular velocity
It is used for sensing axis orientation It is used for sensing angular
orientation

Piezoelectric Cables: Piezo cable is another form of
Piezo polymer sensors, designed as a coaxial cable,
the Piezo polymer is the “dielectric” between the
center core and the outer braid. When the cable is
compressed or stretched, a charge or voltage is
generated proportional to the stress.
The Piezo Cable is constructed with a piezoelectric
polymer (PVDF) insulation layer between the copper
braided inner conductor and the outer shield.
Protected by a rugged polyethylene jacket, the cable
has provided excellent service in buried and fence-
mounted sensors for airports and other installation
perimeter security applications
Piezoelectric Cables:

Piezoelectric Cables:
FEATURES
 Passive, Long Length Sensor
 Very Tough, Water Resistant and
 Flexible
 Temperature Stability to 85EC
 Self-Shielded Coaxial Construction
 High Voltage Response
 Low Impedance Per Unit Length
 Simplified Interconnections
 Field Repairable

APPLICATIONS
 Perimeter Intrusion Detection
 Safety and Security Fencing
 Door Edge/Vehicle Bumper Switch
 Cable Tampering Detector
 Remote Impact/Detonation Vibration
 Sensing
 Large Area Switch Mats
 Patient Mattress Monitor
 Sports Scoring
 Weather Sensing/Rain/Hail
 Geophones

UNIT III_Smart Sensor.pptx

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