Sensors in Different Applications Area.pdf

Sensor Technology
Sweta Barnwal Page 1
Sensors in Different Application Area
Topics Covered: Occupancy and Motion Detectors; Position, Displacement, and Level; Velocity and
Acceleration; Force, Strain, and Tactile Sensors; Pressure Sensors, Temperature Sensors
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 whenever 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.
• Sensors which detects changes in the air pressure due to opening of the doors and also
windows are referred as air pressure sensors.
• The sensors which detects human body capacitance are referred as Capacitive Sensors.
• Acoustic sensors utilizes the sound produced by the people.
• Photoelectric sensor works on the principle of interruption of light beams by the moving
objects
• Optoelectrical sensor uses detection of variations in the illumination. It also uses optical
contrast in the region under target.
• Pressure mat switches use the pressure sensitive long strips laid on the floors below the
carpets to detect weight of an intruder.
• Stress detectors use strain gauges imbedded into floor beams, staircases, and other structural
components
• Switch sensors utilizes electrical contacts connected to doors and windows.
• Magnetic switches use a non-contact version of switch sensors.
• Vibration detectors react to the vibration of walls or other building structures, also may be
attached to doors or windows to detect movements.
• Glass breakage detectors are sensors reacting to specific vibrations produced by shattered
glass.
• Infrared motion detectors are devices sensitive to heat waves emanated from warm or cold
moving objects.
• Microwave detectors are active sensors responsive to microwave electromagnetic signals
reflected from objects.
• Ultrasonic detectors are similar to microwaves except that ultrasonic waves are used instead
of EM waves.
• Video motion detectors are video equipment which compares a stationary image stored in
memory with the current image from the protected area.
• Video face recognition system uses image analyzers that compare facial features with a
database.

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• Laser system detectors are similar to photoelectric detectors, except that they use narrow
light beams and combinations of reflectors.
• Triboelectric detectors are sensors capable of detecting static electric charges carried by
moving objects
All of the above techniques are used in the design and development of occupancy sensor or
motion sensor. These are the basic principles in the design of such sensors.
Capacitive Occupancy Sensor
Figure 1 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. 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

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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 up to hundred
meters and used to find movement of people and animals.
The operating principle of the optical motion detectors is based on the detection of light
(either in the visible or nonvisible spectrum) emanated 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.
The optoelectronic motion sensors are very useful for indicating whether an object is moving
or stationary. But they cannot distinguish one moving object from 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.
The most important advantages of an optoelectronic motion sensor are simplicity and low
cost.

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Position, Displacement, and Level Sensor
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. Here term position refers to determination of object's co-ordinates (either
linear or angular) with respect to a selected reference. The term displacement refers to moving
from one position to another position for a specific distance or angle. A critical distance is
measured by proximity sensor.
A proximity sensor is a threshold version of a position detector. A position sensor is a linear
device whose output signal represents a distance to the object from a certain reference point.
A proximity sensor is a simpler device which generates the output signal when a certain
distance to the object becomes essential for an indication. A displacement sensor often is part
of a more complex sensor where the detection of movement is one of several steps in a signal
conversion. An example is a pressure sensor where pressure is translated into a displacement
of a diaphragm. Later the diaphragm displacement is converted into an electrical signal which
represents pressure. Hence the position sensors are essential for the design of many other
sensors.
Position and displacement sensors are static devices whose speed response usually is not
critical for the performance. Following specifications need to be considering while selecting
or designing the displacement or position sensors.
• How large is the displacement and of what type (linear, circular)?
• Amount of resolution and accuracy needed
• Material used to construct measured object (such as metal, plastic, fluid, ferromagnetic etc.)
• Space available to mount the detector
• Amount of play available in the moving assembly and required detection range
• environmental conditions available such as humidity, temperature, sources of interference,
vibration, corrosive materials etc.
• How much power is available for the sensor?
• Total mechanical wear expected over the entire lifetime of the machine
• What is the production quantity of the sensing assembly (limited number, medium volume,
mass production)?
• What is the target cost of the detecting assembly?

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Fig-1Potentiometeraspositionsensor Fig-2Fluidlevelsensor
A position sensor can be built with a linear or rotary potentiometer or a pot for short.
R = δ (L/A)
Where R is the resistance of a conductor, A is the cross sectional area and L is the length of
the conductor.
As mentioned in the equation above, resistance linearly proportional to wire length. Hence
displacement measurement can be performed by making object to control the length of the
wire as carried out in potentiometer.
As shown in the figure-1 stimulus is coupled to the pot wire, whose movement will cause the
resistance to change accordingly. Most of the electronic circuits will use resistance
measurement as a function of voltage drop.
The voltage drop across the wiper of linear potentiometer V = E (d/D),
Where, D is full scale displacement, E is voltage across the pot , d is the displacement. Here
the output signal is proportional to the excitation voltage applied across the sensor.
Figure-2 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.
Displacement and position sensors
Displacement sensors are basically used for the measurement of movement of an object.
Position sensors are employed to determine the position of an object in relation to some
reference point.
Proximity sensors are a type of position sensor and are used to trace when an object has
moved with in particular critical distance of a transducer.
Displacement sensors
1. Potentiometer Sensors

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Schematic of a potentiometer sensor for measurement of linear displacement Potentiometer: electric circuit
Figure shows the construction of a rotary type potentiometer sensor employed to measure the
linear displacement. The potentiometer can be of linear or angular type. It works on the
principle of conversion of mechanical displacement into an electrical signal. The sensor has a
resistive element and a sliding contact (wiper). The slider moves along this conductive body,
acting as a movable electric contact.
The object of whose displacement is to be measured is connected to the slider by using
• A rotating shaft (for angular displacement)
• A moving rod (for linear displacement)
• A cable that is kept stretched during operation
The resistive element is a wire wound track or conductive plastic. The track comprises of
large number of closely packed turns of a resistive wire. Conductive plastic is made up of
plastic resin embedded with the carbon powder. Wire wound track has a resolution of the
order of ± 0.01 % while the conductive plastic may have the resolution of about 0.1 μm.
During the sensing operation, a voltage Vs is applied across the resistive element. A voltage
divider circuit is formed when slider comes into contact with the wire. The output voltage
(VA) is measured as shown in the figure. The output voltage is proportional to the
displacement of the slider over the wire. Then the output parameter displacement is calibrated
against the output voltage VA.
VA = I RA ………………………………………………………… (1)
But I = VS / (RA + RB)…………………………… (2)

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Therefore VA = VS RA / (RA +RB)……………….. (3)
As we know that R = ρ L / A, where ρ is electrical resistivity, L is length of resistor and A is
area of cross section
VA = VS LA / (LA + LB)…………………………………………… (4)
Applications of potentiometer
These sensors are primarily used in the control systems with a feedback loop to ensure that the
moving member or component reaches its commanded position.
These are typically used on machine-tool controls, elevators, liquid-level assemblies, forklift
trucks, automobile throttle controls. In manufacturing, these are used in control of injection
molding machines, woodworking machinery, printing, spraying, robotics, etc. These are also
used in computer-controlled monitoring of sports equipment.
2. Strain Gauges
The strain in an element is a ratio of change in length in the direction of applied load to the
original length of an element. The strain changes the resistance R of the element. Therefore,
we can say,
ΔR/R α ε;
ΔR/R = G ε………………………………………. (5)
Where G is the constant of proportionality and is called as gauge factor. In general, the value
of G is considered in between 2 to 4 and the resistances are taken of the order of 100 Ω.
Resistance strain gauge follows the principle of change in resistance as per the equation 5. It
comprises of a pattern of resistive foil arranged as shown in Figure 3. These foils are made of
Constantan alloy (copper-nickel 55-45% alloy) and are bonded to a backing material plastic
(ployimide), epoxy or glass fiber reinforced epoxy. The strain gauges are secured to the work
piece by using epoxy or Cyanoacrylate cement Eastman 910 SL. As the work piece undergoes
change in its shape due to external loading, the resistance of strain gauge element changes.
This change in resistance can be detected by a using a Wheatstone’s resistance bridge as
shown in Figure 4. In the balanced bridge we can have a relation,
R2/ R1 = Rx / R3 (2.2.6)
where Rx is resistance of strain gauge element, R2 is balancing/adjustable resistor, R1 and R3
are known constant value resistors. The measured deformation or displacement by the stain
gauge is calibrated against change in resistance of adjustable resistor R2 which makes the
voltage across nodes A and B equal to zero.
Applications of strain gauges
Strain gauges are widely used in experimental stress analysis and diagnosis on machines and
failure analysis. They are basically used for multi-axial stress fatigue testing, proof testing,
residual stress and vibration measurement, torque measurement, bending and deflection
measurement, compression and tension measurement and strain measurement.

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Strain gauges are primarily used as sensors for machine tools and safety in automotives. In
particular, they are employed for force measurement in machine tools, hydraulic or pneumatic
press and as impact sensors in aerospace vehicles.
A pattern of resistive foils Wheatstone’s bridge
3. Linear variable differential transformer (LVDT)
Construction of a LVDT sensor
It is a primary transducer used for measurement of linear displacement with an input range of
about ± 2 to ± 400 mm in general. It has non-linearity error ± 0.25% of full range. Figure
shows the construction of a LVDT sensor. It has three coils symmetrically spaced along an
insulated tube. The central coil is primary coil and the other two are secondary coils.
Secondary coils are connected in series in such a way that their outputs oppose each other. A

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magnetic core attached to the element of which displacement is to be monitored is placed
inside the insulated tube.
Working of LVDT sensor
Due to an alternating voltage input to the primary coil, alternating electro-magnetic forces
(emfs) are generated in secondary coils. When the magnetic core is centrally placed with its
half portion in each of the secondary coil regions then the resultant voltage is zero. If the core
is displaced from the central position as shown in Figure, say, more in secondary coil 1 than in
coil 2, then more emf is generated in one coil i.e. coil 1 than the other, and there is a resultant
voltage from the coils. If the magnetic core is further displaced, then the value of resultant
voltage increases in proportion with the displacement. With the help of signal processing
devices such as low pass filters and demodulators, precise displacement can be measured by
using LVDT sensors.
LVDT exhibits good repeatability and reproducibility. It is generally used as an absolute
position sensor. Since there is no contact or sliding between the constituent elements of the
sensor, it is highly reliable. These sensors are completely sealed and are widely used in
Servomechanisms, automated measurement in machine tools.
A rotary variable differential transformer (RVDT) can be used for the measurement of
rotation. Readers are suggested to prepare a report on principle of working and construction of
RVDT sensor.
Applications of LVDT sensors
• Measurement of spool position in a wide range of servo valve applications
• To provide displacement feedback for hydraulic cylinders
• To control weight and thickness of medicinal products viz. tablets or pills
• For automatic inspection of final dimensions of products being packed for dispatch
• To measure distance between the approaching metals during Friction welding process
• To continuously monitor fluid level as part of leak detection system
• To detect the number of currency bills dispensed by an ATM

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Tactile sensors
In general, tactile sensors are used to sense the contact of fingertips of a robot with an object.
They are also used in manufacturing of ‘touch display’ screens of visual display units
(VDUs) of CNC machine tools. Figure 2.4.9 shows the construction of piezo-electric
polyvinylidene fluoride (PVDF) based tactile sensor. It has two PVDF layers separated by a
soft film which transmits the vibrations. An alternating current is applied to lower PVDF
layer which generates vibrations due to reverse piezoelectric effect. These vibrations are
transmitted to the upper PVDF layer via soft film. These vibrations cause alternating voltage
across the upper PVDF layer. When some pressure is applied on the upper PVDF layer the
vibrations gets affected and the output voltage changes. This triggers a switch or an action in
robots or touch displays.
Schematic of a tactile sensor

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Force Sensor
Sir Franklin Event off, in the 1970s, found some materials, when subjected to force, can
change their resistance values. These materials were known as Force-Sensing Resistors. These
materials are used to produce a sensor that can measure the Force. A Force Sensor is a sensor
that helps in measuring the amount of force applied to an object. By observing the amount of
change in the resistance values of force-sensing resistors, the applied force can be calculated.
Working Principle
The general working principle of Force Sensors is that they respond to the applied force and
convert the value into a measurable quantity. There are various types of Force Sensors
available in the market based on various sensing elements. Most of the Force Sensors are
designed using Force-Sensing Resistors. These sensors consist of a sensing film and
electrodes.
The working principle of a Force-sensing resistor is based on the property of ‘Contact
Resistance’. Force-sensing resistors contain a conductive polymer film that changes its
resistance in a predictable manner when force is applied on its surface. This film consists of,
sub-micrometres sized, electrically conducting and non-conducting particles arranged in a
matrix. When force is applied to the surface of this film, the microsized particle touches the
sensor electrodes, changing the resistance of the film. The amount of change caused to the
resistance values gives the measure of the amount of force applied.
To improve the performance of the Force-Sensing resistors various efforts are being made
with multiple different approaches such as, to minimize the drift of polymer various electrode

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configurations are being tested, testing with sensor by replacing the polymer with new
materials such as carbon nanotubes, etc….
Applications of Force Sensor
The main usage of the Force sensor is to measure the amount of force applied. There are
various types and sizes of force sensors available for different types of applications. Some of
the applications of Force sensor that uses force-sensing resistors includes pressure-sensing
buttons, in musical instruments, as car-occupancy sensors, in artificial limbs, in foot-pronation
systems, augmented reality,etc….
Examples of Force Sensors
There are many types of force sensors available for different types of applications. Some of
the examples of force sensors are Load cells, pneumatic load cells, Capacitive Load cells,
Strain gauge load cells, hydraulic load cells, etc…
Besides force sensors, there is also a category of Force transducers. The main difference
between a force sensor and force transducer is that the transducer converts the amount of force
measured or applied into a measurable small electrical voltage output signal. Whereas the
output of a Force sensor is not an electrical voltage.
Velocity & Acceleration Sensor
A velocity receiver (velocity sensor) is a sensor that responds to velocity rather than absolute
position. For example, dynamic microphones are velocity receivers. Likewise,
many electronic keyboards used for music are velocity sensitive, and may be said to possess a
velocity receiver in each key. Most of these function by measuring the time difference
between switch closures at two different positions along the travel of each key.
There are two types of velocity receivers, moving coil and piezoelectric. The former contains
a coil supported by springs and a permanently fixed magnet and require no output signal
amplifiers. Movement causes the coil to move relative to the magnet, which in turn generates
a voltage that is proportional to the velocity of that movement.
Piezoelectric sensor velocity receivers are similar to a piezoelectric accelerometer, except that
the output of the device is proportional to the velocity of the transducer. Unlike the moving
coil variety, piezoelectric sensors will likely require an amplifier due to the small generated
signal.
Acceleration Sensor: Physically, acceleration is a vector quantity having both direction and
magnitude that is defined as the rate at which an object changes its velocity with respect to
time. It is a measure of how fast speed changes. An object is accelerating when its velocity is
changing. [1. ] In order to measure acceleration, an acceleration sensor called accelerometer is

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used. Accelerometer measures in units of g. A g is the acceleration measurement for gravity
which is equal to 9.81 m/s². However, depending on altitude, this measurement can be 10
m/s² in some place.
Working principle of an accelerometer
The design of an accelerometer is based on the application of physics phenomenon. In
aviation, accelerometers are based on the properties of rotating masses. In the world of
industry, however, the design is based on a combination of Newton's law of mass acceleration
and Hooke's law of spring action. This is the most common design applied to the making of
accelerometers, and therefore, in this wiki page I will focus on explaining the accelerometer's
working principle based on this combination of Newton's law and Hooke's law. Figure 1
shows a simplified spring-mass system. In figure 1a, the mass of mass m is attached to a
spring at equilibrium position x0 which in turn is attached to the base. The mass can slide
freely on the base. Suppose that the base friction is negligible. Figure 1b shows the mass is
moving to the right by a displacement of Δx = x - x0. Since the mass is slowing down, the
direction of acceleration vector is to the left. In this case, the mass is subject to the force
according Newton's second law and Hooke's law.
A simplified spring-mass system accelerometer
According to Newton's second law, if a mass, m, is undergoing an acceleration, a, then there
must be a force, F, acting on the mass with a magnitude of F = ma
Like any other sensors, a typical accelerometer possesses several characteristics which are
needed to understand. Those characteristics are explained as follows:
• Sensitivity: sensitivity is the output voltage produced by a certain force measured in g's. It is
usually expressed in terms of volts per unit of acceleration under the specified conditions. The
sensitivity of accelerometers is typically measured at a single reference frequency of a sine-
wave shape. In the United States of America, it is 100 Hz while in most European countries it

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is 160 Hz. This is because they are removed from the power line frequencies and their
harmonics. The LIS3L02AL accelerometer which is used in the Nintendo Wii Nunchuk has a
typical sensivity of 0.66 V/g at Vdd = 3.3V and
temperature = 25 degrees as can be seen from figure 3.
• Frequency response: is the output signal over a range of frequency where the sensor should
operate. Unfortunately, STMicroelectronics does not give any information about the
frequency response in their datasheet for the LIS3L02AL accelerometer.
• Acceleration rage: is the level of acceleration supported by the sensor's output signal
specification, typically specified in ±g. From figure 3, we can see that the acceleration range
for the LIS3L02AL accelerometer is typically ±2g.
• Sensitivity change due to temperature: is specified as a % change per ºC. The value of this
parameter for LIS3L02AL accelerometer is ±0.01.
• Nonlinearity: is a measurement of the deviation of an accelerometer response from a perfectly
linear response. It is specified as a percentage with respect to either full-scale range (%FSR)
or ± full scale (%FS).
• Zero-g level: this is measured in V, and it specifies the range of voltages that may be expected
at the output under 0g of acceleration
• Cross-axis sensitivity: is a measure of how much output is seen on one axis when acceleration
is imposed on a different axis, typically specified as a percentage. The coupling between two
axes results from a combination of alignment errors, etching inaccuracies, and circuit
crosstalk.
• Noise density: Noise Density, in ug/rt(Hz) RMS, is the square root of the power spectral
density of the noise output. Total noise is determined by the equation:
Noise = Noise Density * sqrt(BW * 1.6)
where BW is the accelerometer bandwidth, set by capacitors on the accelerometer outputs.
Until now, all the basic characteristics of an accelerometer have been explained. One
important characteristic is the sensitivity and the question is how to improve the sensitivity of
the accelerometer. For analog-output sensors, sensitivity is ratiometric to supply voltage.
Therefore, the sensitivity can be improved by increasing the supply voltage for the sensor as
long as it does not exceed the maximum rating power supply specified by the datasheet. So
this means that doubling the supply, for example, doubles the sensitivity

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Level Sensor
A level sensor is a device for determining the level or amount of fluids, liquids or other
substances that flow in an open or closed system. There are two types of level measurements,
namely, continuous and point level measurements.
Continuous level sensors are used for measuring levels to a specific limit, but they provide
accurate results. Point level sensors, on the other hand, only determine if the liquid level is
high or low.
The level sensors are usually connected to an output unit for transmitting the results to a
monitoring system. Current technologies employ wireless transmission of data to the
monitoring system, which isuseful in elevated and dangerous locations that cannot be easily
accessed by common workers.
Types of Level Sensors
The following are the major types of level sensors:
Ultrasonic Level Sensors
Ultrasonic level sensors are used for detecting the levels of viscous liquid substances and bulk
materials as well. They are operated by emitting acoustic waves at frequency range of 20 to
200 kHz. The sound waves are then reflected back to a transducer.
The response of ultrasonic sensors is influenced by pressure, turbulence, moisture and
temperature. In addition, the transducer is required to be mounted appropriately to obtain
better response.
Capacitance Level Sensors
Capacitance level sensors are used for detecting the levels of aqueous liquids and
slurries. They are operated by employing a probe for monitoring level changes. These changes
are converted into analog signals.
The probes are usually made of conducting wire with PTFE insulation. However, stainless
steel probes are highly sensitive and hence they are suitable for measuring granular, non-
conductive substance or materials with low dielectric constant.
Capacitance sensors are easy to use and clean as they do not have any moving components.
They are commonly employed in applications involving high temperature and pressure.
Optical Level Sensors
Optical level sensors detect liquids containing suspended materials, interface between two
immiscible liquids and the presence of sediments. They are operated based on the
transmission changes of infrared light emitted from an infrared LED. The interference from
the emitted light can be eliminated by employing a high energy infrared diode and pulse
modulation methods.

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Continuous optical level sensors, on the other hand, employ highly concentrated laser light
that can permeate dusty environments and detect liquid substances. However, its application is
limited to industries due to maintenance and high cost.
Microwave Optical Sensors
Microwave level sensors are used for applications involving varying temperature and
pressure, and dusty and moist environments, as microwaves can easily penetrate under these
conditions without requiring air molecules for energy transmission. These sensors can detect
conductive water and metallic substances. The measurements are carried using time domain or
pulse reflectometry.
However, complex arrangement and high cost are the major disadvantages of microwave
sensors.
Applications
Some of the major applications of level sensors include the following:
• Pharmaceutical applications
• Detecting ink level in printers
• Hopper bins
• Waste water treatment plants
• Food and beverage applications

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