Working and Concept of Modern sensors

8/10/2019 Working and Concept of Modern sensors

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Working and oncept of

Modern Sensors

-By

Raghavendra.S.Rao

TY ETC A, T3447

MIT T)


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INDEX

Touch sensors

Motion sensors

Environmental sensors Position sensors


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TOUCH SENSOR

How Does a Touchsc reen Work?

A basic touchscreen has three main components:

1 Touch sensor;

2 Controller;

3 Software driver.The touchscreen is an input device, so it needs to becombined with a display and a PC or other device tomake a complete touch input system.


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Touch Sensor

A touch screen sensor is a clear glass panel with a touchresponsive surface. The touch sensor/panel is placedover a display screen so that the responsive area of thepanel covers the viewable area of the video screen.

There are several different touch sensor technologies onthe market today, each using a different method to detecttouch input. The sensor generally has an electricalcurrent or signal going through it and touching thescreen can cause a voltage or signal change. This

change is used to determine the location of the touch tothe screen.


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Control ler

The controller connects between the touch sensor andthe PC. It takes information from the touch sensor andtranslates it into information that PC can understand.The controller determines what type of

interface/connection you will need on the PC. Controllersare available that can connect to a Serial/COM port (PC)or to a USB port. Specialized controllers are alsoavailable that work with DVD players and other devices.


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SoftwareDriver

The driver allows the touchscreen and computer to worktogether. It tells the computer's operating system how tointerpret the touch event information that is sent from thecontroller. Most touch screen drivers today are a mouse-

emulation type driver. This makes touching the screen assame as clicking your mouse at the same location on thescreen. This allows the touchscreen to work with existingsoftware and allows new applications to be developedwithout the need for touchscreen specific programming.


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Touchsc reen Techno logy

Resistive touchscreen

Capacitive touchscreen

Infrared touchscreen Surface acoustic wave (SAW) touchscreen

Strain gauge touchscreen

Optical imaging touchscreen Dispersive signal technology touchscreen


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Resist ive touchscreen

Structure

Resistive touch screensconsist of a glass or

acrylic panel that iscoated with electricallyconductive and resistivelayers made with indiumtin oxide (ITO) .The thinlayers are separated byinvisible spacers.


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4-w ire resist ive touchscreen


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5-w ire resis t ive touchsc reen

The construction of the panels are similarwith 4-wire technology, but for a 5-wiretouch screen all four bus bars are

connected to the lower, non-flexible layerof the screen. The flexible layer is alwaysused as a sense layer to read the voltage

connection point to the bottom layer.


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8-w ire resist ive touchscreen


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Resist ive touchscreen

Characters:1. Cost effective solutions

2. Activated by a stylus, a finger or gloved hand

3. Not affected by dirt, dust, water, or light4. 75%~85% clarify

5. resistive layers can be damaged by a very sharpobject


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Projected -capacit ive tou chsc reen

Structure

Projected capacitivetouchscreens have front and

back protective glassproviding optical and strengthenhancement options.

Its middle layer consists of a

laminated sensor grid ofmicro-fine wires, andoptical enhancement options.


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Surface-capacit ive touchsc reen

StructureSurface capacitive technology consists of auniform conductive coating on a glass panel.

Electrodes around the panels edge evenlydistribute a low voltage across theconductive layer, creating a uniform electricfield.


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Surface-capacit ive touchsc reen

Working principleA human body is an electric conductor, so when you touch the screenwith a finger, a slight amount of current is drawn, creating a voltage drop.The current respectively drifts to the electrodes on the four corners.Theoretically, the amount of current that drifts through the fourelectrodes should be proportional to the distance from the touch point tothe four corners. The controller precisely calculates the proportion of the

current passed through the four electrodes and figures out the X/Ycoordinate of a touch point.


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Capaci t ive touchscreen

Characters:

1. Durable and resistant to scratches fordemanding applications

2. Faster and more responsive

3. Immune to surface contaminants

4. Superior optical clarity, brighter display and

less surface reflection5. Must be touched by finger, will not work with

any non-conductive input


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Infrared touchsc reen

Infrared (IR) technology relies on the interruption of anIR light grid in front of the display screen. The touchframe contains a row of IR-light emitting diode (LEDs)and photo transistors, each mounted on two opposite

sides to create a grid of invisible infrared light.The IR controller sequentially pulses the LEDs to createa grid of IR light beams. When a stylus, such as a finger,enters the grid, it obstructs the beams. One or more

photo transistors from each axis detect the absence oflight and transmit signals that identifies the x and y

coordinates.


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Infrared touchsc reen


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SAW touchscreen

PrincipleSurface waves are readily absorbedwhen a soft object such as a fingertiptouches the substrate.SAW Touch Screenuse pure glasswith transmitting and receiving

piezoelectric transducers for both theX and Y axes.The touch screen controller sends anelectrical signal to the transmittingtransducer, which converts the signalinto ultrasonic waves within the glass.

When you touch the screen, youabsorb a portion of the wave travelingacross it. The received signal is thencompared to the stored digital map,the change recognized, and acoordinate calculate.


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SAW touchscreen


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SAW touchscreen

Characters:

1. Durable glass construction

2. High optical clarity3. Activated by a finger, gloved hand or soft

tip

4. Not completely sealable, can be affectedby large amounts of dirt, dust, and / orwater in the environment


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Technology Capacitive SAW Infrared Resistive

Transparence Very good>92%

Very good

>92%Very good

>92%75%~85%

Resolution Good Good Limited due tospacing of IR

sensors

good

Surface

Contaminants

/durability

Resistant to

moisture and

other surfacecontaminants

Adversely

affected by

moisture orSurface

contaminants

Potential for

False activation

or dead zonesFrom Surface

Contaminants

Unaffected by

Surface

contaminants.Polyester top

sheet is easily

scratched

Sensor substrate Glass with ITOcoating

Glass with ITO

coatingAny substrate Polyester top

sheet, glass

substrate withITO coating

Display size 8.4"-21" 10.4"-30" 10.4"-60" up to 19"

Touch method Human touch finger, gloved

hand or soft tip

Can use any

pointing deviceCan use any

pointing device


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Mult i-touch techno logy

Multi-touchdenotes a set of interactiontechniques which allow computer users tocontrol graphical applications with several

fingers. Products:

Apple iPhone, iPod touch, MacBook Air,

and MacBook Pro Microsoft Surface


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POSITION SENSORS

Position can be determined by a Hall Effect device (ordevices), embedded in the stator, which provide anelectrical signal representing the magnetic field strength.The amplitude of this signal changes as the magnetic

rotor poles pass over the sensor. Other sensing methods are shaft encoders and also

sensing the zero crossing points of currents generated inthe unenergised phase windings. The latter method is

known as "sensorless" position monitoring
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TYPES OF POSITION SENSORS

Optical position sensor

Hall effect position sensor

Electronic position sensors


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OPTICAL POSITION SENSOR

Mainly used in Unipolar BLDC machines

Rotor consists of optical sensors.

The optical sensor consists of

A light source three phototransistors P1, P2 and P3mounted on the

end plate of the motor, separated by 120o from eachother

A revolving shutter coupled to the shaft of the motor.


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When the shutter revolves, thephototransistors get exposed to the light in

the sequence of their numbers. In eachrevolution, the phototransistors generatethe pulses PI1, PI1and PI1which haveduration and phase displacement of 120o.


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WORKING

When light falls on the phototransistors P1, it generates a pulse andtransistor Q1gets turned on.

Current starts flowing through stator winding Ph1. This produces north poleat pole face of Ph1.

South pole gets attracted towards it and reaches the axis of pole face of Ph1.

Hence rotor revolves in anticlockwise direction. During the mean time, the light stops falling on P1 and starts falling on P2.

Hence pulse PI1 is generated which turns on the transistors Q2.

Current now starts flowing through the winding Ph2, producing a north pole.Hence rotor further rotates in anticlockwise direction so that rotor reachesthe axis of the pole face of .

In the meantime, starts falling on P3. This causes transistors Q3to turn onwhich produces north pole at the pole face of Ph3.

This rotates the rotor further in anticlockwise direction.

Switching sequence repeats and continuous rotation of the rotor is obtained.


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HALL EFFECT SENSORS

The Hall Effect uses three hall sensors within theBrushless DC Motor to help detect the position of therotor.

The magnetic field changes in response to the

transducer that varies its output voltage. Feedback iscreated by directly returning a voltage, because thesensor operates as an analogue transducer.

The distance between the Hall plate and a known

magnetic field can be determined with a group ofsensors, and the relative position of the magnet can bededuced.


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Sectional View of BLDC withHall sensor


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Electronic Sensor

In a brushless DC motor (BLDC), the rotor haspermanent magnets and the stator has anelectronically-controlled rotating field, using

sensors (rotary encoders or back-EMF) to detectrotor position.

As such they have no commutator, and tend tobe more efficient and more powerful than

commutated motors. They do require a morecomplicated motor controller


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TYPES OF SENSORS

Humidity sensor

Pressure Sensor

Temperature sensor


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HUMIDITY SENSOR

Measurement- Relative humidity (RH)is the amount ofwater vapor in the air compared with the amount ofvapor needed to make the air saturated at the air'scurrent temperature

Example:

At 3 p.m. the air has 9 grams of water vapor per cubicmeter of air. We divide 9 by 30 and multiply by 100 to get

a relative humidity of 30%

M f f H idi


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Manufacturers of HumiditySensor

Honeywell Sensirion

( HIH series ) ( SHT Series )


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Humidity Sensor from Honeywell( HIH series)

Packages are chemically resistant

operate in ranges of -40C to 85C ( -40Fto 185F)

Response time -15 sec in slowly moving air at25C

Low power design200 microA @ 5 Vdc

Operating volatge 45.8 v

On-chip signal processing ensures linear voltageoutput versus %RH.

$35 - $40 per unit price


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Humidity Sensor from Sensirion( SHT Series )

2 sensors for relative humidity & temperature

Very easy-to-use due to calibration & digital 2-wire interface

Calibrated & digital output (2-wire interface) Fast response time < 4 sec.

Temp. accuracy: +/- 0.5C @ 25C

Low power consumption (typ. 30 W)

Operating Voltage 2.8 - 5.5v

Cost $18 - $24 per unit price


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COMPARISON

2 sensors for relative humidity &temperature

Very easy-to-use due to calibration& digital 2-wire interface

Calibrated & digital output (2-wire

interface) Fast response time < 4 sec.

Temp. accuracy: +/- 0.5C @ 25C

Low power consumption (typ. 30

W) Operating Voltage 2.8 - 5.5v

Cost $18 - $24 per unit price

a. 2 sensors for relative humidity &temperature

b. Very easy-to-use due to calibration& digital 2-wire interface

c. Calibrated & digital output (2-wire

interface)d. Fast response time < 4 sec.e. Temp. accuracy: +/- 0.5C @ 25C

f. Low power consumption (typ. 30W)

g. Operating Voltage 2.8 - 5.5vh. Cost $18 - $24 per unit price


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PRESSURE SENSOR

AbsoluteA Sensor That Measures InputPressure in Relation to a Zero Pressure. WeWill Use the Absolute Pressure Sensor to

Calculate Altitude. DifferentialA Sensor That Is Designed to

Accept Simultaneously Two IndependentPressure Sources. The Output Is Proportional to

the Difference Between the Two Sources. WeWill Use the Differential Pressure Sensor toCalculate Indicated Airspeed.


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BASICS

Cl f M t l P


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Classes of Motorola PressureSensors

Uncompensated Pressure SensorsThese standard, low cost,sensors permit manufacturers to design and add their own externaltemperature compensation and signal conditioning networks.

Compensated Pressure SensorsThese sensors have built in

temperature compensation and signal conditioning. Integrated Pressure SensorsThese integrated sensors have built

in temperature compensation and signal conditioning just like theCompensated Pressure Sensors. The Integrated Pressure Sensorsare designed specifically to use with microcontroller applications and

have an easy one wire interface to hook to the A/D port on amicrocontroller.

For each three classes there are a variety of (A)bsolute and(D)ifferential pressure sensors.

M i & Mi i P


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Maximum & Minimum PressureParameters

For the Absolute Pressure Sensor it is obvious that the pressure rangeneeds to be greater than 115 kPa in order to exceed the Zero PressureLevel of anywhere in the U.S. and smaller than 1kPa in order to exceed120,000ft. In order to keep the maximum sensitivity an Absolute PressureSensor should have a pressure range of 0-115 kPa which makes theMotorola MPX4115A an obvious first choice.

For the Differential Pressure Sensor the three Motorola Differential PressureSensors with the highest sensitivity have Maximum Pressure Ratings of 10,50, and 100 kPa which would have the ability to measure MaximumIndicated Airspeeds of 280, 640, and 900 MPH respectively. Assuming theIndicated Airspeed will not exceed 600 MPH this makes the 50 kPa sensor

the obvious choice because it can measure Indicated Airspeeds in excessof 600 MPH and has twice the sensitivity of the 100kPa sensor. This sensoris the Motorola MPX5050D.

Th M t l MPX5050D


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The Motorola MPX5050DDifferential Pressure Sensor

Features

Pressure Range From 0 to 50 kPa

2.5% Maximum Error over 0 to 85 C

Ideally suited for Microprocessor orMicrocontroller-Based Systems

Temperature Compensated Over40 to+125 C

Maximum Power Rating of 50 mW with a

Typical Power Rating of 35 mW

High Sensitivity of 90 mv/kPa that wouldbe able to measure velocity from 0-640MPH


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OTHER MANUFACTURERS


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TEMPERATURE SENSORS

Following are the Types of temperaturesensors:

Thermocouples

Resistance Temperature Detectors (RTDs)

Thermistors

Infrared Sensors

Semiconductors


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Thermocouples

Two wires of different metalalloys.

Converts thermal energy intoelectrical energy.

Requires a temperaturedifference between measuring

junction and reference junction.

Easy to use and obtain.

Thermocouples

Resistance Temperature


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Resistance TemperatureDetectors (RTDs)

Wire wound and thin filmdevices.

Nearly linear over a widerange of temperatures.

Can be made small enoughto have response times ofa fraction of a second.

Require an electrical

current to produce avoltage drop across thesensor


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Thermistors A semiconductor used as a temperature sensor. Mixture of metal oxides pressed into a bead, wafer or other

shape. Beads can be very small, less than 1 mm in some cases. The resistance decreases as temperature increases,

negative temperature coefficient (NTC) thermistor.


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MOTION SENSORS

Types of motion sensors:

Accelerometers

Gyroscopes

Gravity sensors


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Accelerometer Types

Common Accelerometer Types Resistive

Strain Gauge

Piezoresistive

Micromachined Thin-Film

Capacitive

Piezoelectric

RESISTIVE OPERATING


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RESISTIVE OPERATINGPRINCIPLE

+ Power - Signal+ Signal - Power

Mass

Sensing Resistor#1

Flexure

Sensing Resistor#2

FixedResistors

CAPACITIVE OPERATING


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CAPACITIVE OPERATINGPRINCIPLE

Power SignalGround

Mass

Sensing Capacitor#1

Built-In ElectronicsFixed Capacitors

Sensing Capacitor#2

Flexure

Insulator

Insulator


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Resistive / CapacitiveTypicalCharacteristics

Measure down to 0 Hz (DC response)Limited dynamic range (


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PIEZOELECTRIC

Piezoelectric Effect Word origin comes from the greek work piezen which translates

to squeeze.

The generation of an electrical signal by a dielectric material as itis subjected to a mechanical stress.

F

F

+

-

Piezoelectric

Material+ + + + + +

- - - - - -


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Piezoelectric MaterialsNaturally Piezoelectric

Rochelle SaltOne of first materials used to make sensors

TourmalineSensitive to hydrostatic pressure

Exotic, Man-Made MaterialsLangasiteLithium Niobate

Cultured Quartz


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CALIBRATION METHODS

Absolute Method Single channel test where the sensor is subjected to a known,

accurate and reliable measure of a

Drop Test

Gravity Inversion Test Handheld Shaker

Test

Sensor

Amplifier,

Attenuator,

Filter, Etc...

Voltmeter,

Analyzer,

Scope,

Etc...

Known

Measure of

a


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Drop Test Accelerometer is allowed to free-fall in Earths gravity

which varies by less than +/-0.5% around the globe

Signal Out

Accelerometer

Mounting

Mass

Flexible

Monofilament Line

ElasticSuspension Cords

Impact ForceFixed

Supports

Earths Gravity0 Deg Latitude: 9.78 m/s2

90 Deg Latitude: 9.32 m/s2

Altitude Correction: -3 mm/s2 per 1000 m above sea level


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Relative Method

Dual channel test where the test sensorand calibrated reference are subjected tothe identical input acceleration. The ratio ofthe output signals provides the calibrationfactor.

Laser Fringe Counting (Primary Method)

Back-to-Back Calibration (Secondary Method)

Test

Sensor

Amplifier,

Attenuator,

Filter, Etc... Voltmeter,

Analyzer,

Scope,

Etc...

Input

Signal

ReferenceSensor

Amplifier,

Attenuator,Filter, Etc...


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Laser CalibrationNon-contacting measurement principle

Structure not affected by measurement deviceUtilizes fringe counting of laser light

This method provides primary calibration as it isbased on a constant on naturethe wavelength

of light

ExpensiveRequires relatively large accelerations at high

frequencies25 gs at 5 kHz; 50 gs at 10 kHz; 100 gs at 20

kHz

Procedure and set-up is documented in approved ISOStandard ISO 5347-1


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GYROSCOPES

Types:

Laser ring Gyroscopes

Draper tuning fork gyroscope

Piezoelectric Gyroscopes


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Laser Ring Gyroscopes

Two signals sent around ring

Different path lengths create abeat frequency.

Aarea of ringPperimeter of ring


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Dead Band

Dead Band -No changein beat frequency forsmall rotation rates

Due to frequency lock-

in

r- backscatteringamplitude


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Scaling Difficulties

Derived Equation for Laser Gyroscope

Beat Freq = (M) Angular Velocity - 1/MDead Band = 1/M^2

M = Scaling Factor


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M = 10-4

-Dead Band = 108timesbigger

-Time varying term larger

-Slope of response lower

Change Bandwidth

To lower Dead Band, wavelengthcould be decreased.

Lower slopeDecreasedSensitivity

A

crL

2


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Draper Tuning Fork Gyro

The rotation of tinescauses the CoriolisForce

Forces detected

through eitherelectrostatic,electromagnetic orpiezoelectric.

Displacements aremeasured in theComb drive


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Advantages

Lower input voltage than vibrating mass Measures rotation in two directions with a

single device

Adjusting orientation electronically is possible Disadvantages

Less sensitive

Output is large when = 0


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END

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Working and Concept of Modern sensors