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Sensors

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A Small review of sensors

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Sensors

  1. 1. SensorsEvolution in Measurements
  2. 2. Sensors:: A sensor (also called detector) is a converter that measures a physical quantityand converts it into a signal which can be read by an observer or by an (today mostlyelectronic) instrument. For example, a mercury-in-glass thermometer converts themeasured temperature into expansion and contraction of a liquid which can be read ona calibrated glass tube. A thermocouple converts temperature to an output voltagewhich can be read by a voltmeter. For accuracy, most sensors are calibrated againstknown standards. Sensors are used in everyday objects such as touch-sensitive elevator buttons(tactile sensor) and lamps which dim or brighten by touching the base. There are alsoinnumerable applications for sensors of which most people are never aware.Applications include cars, machines, aerospace, medicine, manufacturing androbotics. A sensor is a device which receives and responds to a signal when touched. Asensors sensitivity indicates how much the sensors output changes when themeasured quantity changes. For instance, if the mercury in a thermometer moves 1 cmwhen the temperature changes by 1 °C, the sensitivity is 1 cm/°C (it is basically theslope Dy/Dx assuming a linear characteristic). Sensors that measure very smallchanges must have very high sensitivities. Sensors also have an impact on what theymeasure; for instance, a room temperature thermometer inserted into a hot cup ofliquid cools the liquid while the liquid heats the thermometer. Sensors need to bedesigned to have a small effect on what is measured; making the sensor smaller oftenimproves this and may introduce other advantages. Technological progress allowsmore and more sensors to be manufactured on a microscopic scale as microsensorsusing MEMS technology. In most cases, a microsensor reaches a significantly higherspeed and sensitivity compared with macroscopic approaches.
  3. 3. Classification of measurement errorsSensor Principles:A good sensor obeys the following rules: Is sensitive to the measured property only Is insensitive to any other property likely to be encountered in its application Does not influence the measured propertyIdeal sensors are designed to be linear or linear to some simple mathematical functionof the measurement, typically logarithmic. The output signal of such a sensor islinearly proportional to the value or simple function of the measured property. Thesensitivity is then defined as the ratio between output signal and measured property.For example, if a sensor measures temperature and has a voltage output, the sensitivityis a constant with the unit [V/K]; this sensor is linear because the ratio is constant atall points of measurement.Resolution:The resolution of a sensor is the smallest change it can detect in the quantity that it ismeasuring. Often in a digital display, the least significant digit will fluctuate,indicating that changes of that magnitude are only just resolved. The resolution isrelated to the precision with which the measurement is made. For example, a scanningtunneling probe (a fine tip near a surface collects an electron tunneling current) canresolve atoms and molecules.
  4. 4. Sensor deviationsIf the sensor is not ideal, several types of deviations can be observed: The sensitivity may in practice differ from the value specified. This is called a sensitivity error, but the sensor is still linear. Since the range of the output signal is always limited, the output signal will eventually reach a minimum or maximum when the measured property exceeds the limits. The full scale range defines the maximum and minimum values of the measured property. If the output signal is not zero when the measured property is zero, the sensor has an offset or bias. This is defined as the output of the sensor at zero input. If the sensitivity is not constant over the range of the sensor, this is called non linearity. Usually this is defined by the amount the output differs from ideal behavior over the full range of the sensor, often noted as a percentage of the full range. If the deviation is caused by a rapid change of the measured property over time, there is a dynamic error. Often, this behavior is described with a bode plot showing sensitivity error and phase shift as function of the frequency of a periodic input signal. If the output signal slowly changes independent of the measured property, this is defined as drift (telecommunication). Long term drift usually indicates a slow degradation of sensor properties over a long period of time. Noise is a random deviation of the signal that varies in time. Hysteresis is an error caused by when the measured property reverses direction, but there is some finite lag in time for the sensor to respond, creating a different offset error in one direction than in the other. If the sensor has a digital output, the output is essentially an approximation of the measured property. The approximation error is also called digitization error. If the signal is monitored digitally, limitation of the sampling frequency also can cause a dynamic error, or if the variable or added noise changes periodically at a frequency near a multiple of the sampling rate may induce aliasing errors. The sensor may to some extent be sensitive to properties other than the property being measured. For example, most sensors are influenced by the temperature of their environment.
  5. 5. List of sensors:: 1)Acoustic, sound, vibration: Geophone Hydrophone Lace Sensor a guitar pickup Microphone Seismometer 2)Automotive, transportation: Air-fuel ratio meter Blind spot monitor Crankshaft position sensor Curb feeler, used to warn driver of curbs Defect detector, used on railroads to detect axle and signal problems in passing trains Engine coolant temperature sensor, or ECT sensor, used to measure the engine temperature Hall effect sensor, used to time the speed of wheels and shafts MAP sensor, Manifold Absolute Pressure, used in regulating fuel metering. Mass flow sensor, or mass airflow (MAF) sensor, used to tell the ECU the mass of air entering the engine Oxygen sensor, used to monitor the amount of oxygen in the exhaust Parking sensors, used to alert the driver of unseen obstacles during parking manoeuvres Radar gun, used to detect the speed of other objects Speedometer, used measure the instantaneous speed of a land vehicle Speed sensor, used to detect the speed of an object Throttle position sensor, used to monitor the position of the throttle in an internal combustion engine Tire-pressure monitoring sensor, used to monitor the air pressure inside the tires Torque sensor, or torque transducer or torquemeter measures torque (twisting force) on a rotating system. Transmission fluid temperature sensor, used to measure the temperature of the transmission fluid Turbine speed sensor (TSS), or input speed sensor (ISS), used to measure the rotational speed of the input shaft or torque converter Variable reluctance sensor, used to measure position and speed of moving metal components Vehicle speed sensor (VSS), used to measure the speed of the vehicle Water sensor or water-in-fuel sensor, used to indicate the presence of water in fuel
  6. 6. Wheel speed sensor, used for reading the speed of a vehicles wheel rotation 3)ChemicalBreathalyzerCarbon dioxide sensorCarbon monoxide detectorCatalytic bead sensorChemical field-effect transistorElectrochemical gas sensorElectronic noseElectrolyte–insulator–semiconductor sensorFluorescent chloride sensorsHolographic sensorHydrocarbon dew point analyzerHydrogen sensorHydrogen sulfide sensorInfrared point sensorIon-selective electrodeNondispersive infrared sensorMicrowave chemistry sensorNitrogen oxide sensorOlfactometerOptodeOxygen sensorPellistorpH glass electrodePotentiometric sensorRedox electrodeSmoke detectorZinc oxide nanorod sensor 4)Electric current, electric potential, magnetic, radio:Current sensorElectroscopeGalvanometerHall effect sensorHall probeMagnetic anomaly detectorMagnetometerMEMS magnetic field sensorMetal detectorPlanar Hall sensorRadio direction finderVoltage detector
  7. 7. 5)Environment, weather, moisture, humidity:ActinometerBedwetting alarmCeilometerDew warningElectrochemical gas sensorFish counterFrequency domain sensorGas detectorHook gauge evaporimeterHumistorHygrometerLeaf sensorPyranometerPyrgeometerPsychrometerRain gaugeRain sensorSeismometersSNOTELSnow gaugeSoil moisture sensorStream gaugeTide gauge 6)Flow, fluid velocity:Air flow meterAnemometerFlow sensorGas meterMass flow sensorWater meter 7)Ionising radiation, subatomic particles:Bubble chamberCloud chamberGeiger counterNeutron detectionParticle detectorScintillation counterScintillatorWire chamber
  8. 8. 8)Navigation instruments: Air speed indicator Altimeter Attitude indicator Depth gauge Fluxgate compass Gyroscope Inertial navigation system Inertial reference unit Magnetic compass MHD sensor Ring laser gyroscope Turn coordinator Variometer Vibrating structure gyroscope Yaw rate sensor9)Position, angle, displacement, distance, speed, acceleration: Accelerometer Auxanometer Capacitive displacement sensor Capacitive sensing Free fall sensor Gravimeter Gyroscopic sensor Inclinometer Integrated circuit piezoelectric sensor Laser rangefinder Laser surface velocimeter LIDAR Linear encoder Linear variable differential transformer (LVDT) Liquid capacitive inclinometers Odometer Photoelectric sensor Piezoelectric accelerometer Position sensor Rate sensor Rotary encoder Rotary variable differential transformer Selsyn Sudden Motion Sensor Tilt sensor Tachometer
  9. 9. Ultrasonic thickness gaugeVariable reluctance sensorVelocity receiver 10)Optical, light, imaging, photon:Charge-coupled deviceColorimeterContact image sensorElectro-optical sensorFlame detectorInfra-red sensorKinetic inductance detectorLED as light sensorLight-addressable potentiometric sensorNichols radiometerFiber optic sensorsOptical position sensorPhotodetectorPhotodiodePhotomultiplier tubesPhototransistorPhotoelectric sensorPhotoionization detectorPhotomultiplierPhotoresistorPhotoswitchPhototubeScintillometerShack-HartmannSingle-photon avalanche diodeSuperconducting nanowire single-photon detectorTransition edge sensorVisible light photon counterWavefront sensor 11)Pressure:BarographBarometerBoost gaugeBourdon gaugeHot filament ionization gaugeIonization gaugeMcLeod gaugeOscillating U-tube
  10. 10. Permanent Downhole GaugePiezometerPirani gaugePressure sensorPressure gaugeTactile sensorTime pressure gauge 12)Force, density, level:BhangmeterHydrometerForce gaugeLevel sensorLoad cellMagnetic level gaugeNuclear density gaugePiezoelectric sensorStrain gaugeTorque sensorViscometer 13)Thermal, heat, temperature:BolometerBimetallic stripCalorimeterExhaust gas temperature gaugeFlame detectionGardon gaugeGolay cellHeat flux sensorInfrared thermometerMicrobolometerMicrowave radiometerNet radiometerQuartz thermometerResistance temperature detectorResistance thermometerSilicon bandgap temperature sensorSpecial sensor microwave/imagerTemperature gaugeThermistorThermocoupleThermometer
  11. 11. 14)Proximity, presence:Alarm sensorDoppler radarMotion detectorOccupancy sensorProximity sensorPassive infrared sensorReed switchStud finderTriangulation sensorTouch switchWired glove 15)Sensor technology:Active pixel sensorBack-illuminated sensorBiochipBiosensorCapacitance probeCatadioptric sensorCarbon paste electrodeDigital sensorsDisplacement receiverElectromechanical filmElectro-optical sensorFabry–Pérot interferometerFisheries acousticsImage sensorImage sensor formatInductive sensorIntelligent sensorLab-on-a-chipLeaf sensorMachine visionMicroelectromechanical systemsMicro-sensor arraysPhotoelasticityQuantum sensorRADAR o Ground-penetrating radar o Synthetic aperture radarRadar trackerSensor arraySensor fusion
  12. 12. Sensor gridSensor nodeSoft sensorSONARStaring arrayTransducerUltrasonic sensorVideo sensorVisual sensor networkWheatstone bridgeWireless sensor networkOther sensors and sensor related properties and conceptsActigraphyAnalog image processingAtomic force microscopyAtomic Gravitational Wave Interferometric SensorAttitude control (spacecraft), Horizon sensor, Earth sensor, Sun sensorCatadioptric sensorChemoreceptorCompressive sensingCryogenic particle detectorsDew warningDiffusion tensor imagingDigital holographyElectronic tongueFine Guidance SensorFlat panel detectorFunctional magnetic resonance imagingGlass break detectorHeartbeat sensorHyperspectral sensorsIRIS (Biosensor), Interferometric Reflectance Imaging SensorLaser beam profilerLittoral Airborne Sensor/HyperspectralLORROSMillimeter wave scannerMagnetic resonance imagingMoire deflectometryMolecular sensorNanosensorNano-tetherball SensorOmnidirectional cameraOptical coherence tomographyPhase unwrapping techniques
  13. 13. Positron emission tomographyPush broom scannersensitive air-conductivity sensorsQuantization (signal processing)Range imagingScanning SQUID microscopeSingle-Photon Emission Computed Tomography (SPECT)SmartdustSQUID, Superconducting quantum interference deviceSSIES, Special Sensors-Ions, Electrons, and Scintillation thermal plasmaanalysis packageSSMIS, Special Sensor Microwave Imager / SounderStructured-light 3D scannerSun sensor, Attitude control (spacecraft)Superconducting nanowire single-photon detectorThin-film thickness monitorTime-of-flight cameraTriDAR, Triangulation and LIDAR Automated Rendezvous and DockingUnattended Ground Sensors
  14. 14. Examples of each type of Sensors:Acoustic, sound, vibration: a)Hydrophone: A hydrophone (Greek "hydro" = "water" and "phone" = "sound") is a microphone designed to be used underwater for recording or listening to underwater sound. Most hydrophones are based on a piezoelectric transducer that generates electricity when subjected to a pressure change. Such piezoelectric materials, or transducers can convert a sound signal into an electrical signal since sound is a pressure wave. Some transducers can also serve as a projector, but not all have this capability, and may be destroyed if used in such a manner. A hydrophone can "listen" to sound in air, but will be less sensitive due to its design as having a good acoustic impedance match to water, which is a denser fluid than air. Likewise, a microphone can be buried in the ground, or immersed in water if it is put in a waterproof container, but will give similarly poor performance due to the similarly bad acoustic impedance match.
  15. 15. Automotive, transportation: a) Air–fuel ratio meter An air–fuel ratio meter monitors the air–fuel ratio of an internal combustionengine. Also called air–fuel ratio gauge, air–fuel meter, or air–fuel gauge. It readsthe voltage output of an oxygen sensor, sometimes also called lambda sensor, whetherit be from a narrow band or wide band oxygen sensor. The original narrow-band oxygen sensors became factory installed standard inthe late 1970s and early 80s. In recent years, a newer and much more accurate wide-band sensor, though more expensive, has become available. Most stand-alone narrow-band meters have 10 LEDs and some have more.Also common, narrow band meters in round housings with the standard mounting 21/16" and 2 5/8" diameters, as other types of car gauges. These usually have 10 or 20LEDs. Analogue needle style gauges are also available. As stated above, there are wide-band meters that stand alone or are mounted inhousings. Nearly all of these show the air–fuel ratio on a numeric display, since thewide-band sensors provide a much more accurate reading. And since they use moreaccurate electronics, these meters are more expensive.
  16. 16. b)Curb feelers Curb feelers or curb finders are springs or wires installed on a vehiclewhich act as "whiskers" to warn drivers that they are too close to the curb or otherobstruction. The devices are fitted low on the body, close to the wheels. As the vehicleapproaches the curb, the protruding feelers act as whiskers and scrape against thecurb, making a noise and alerting the driver in time to avoid damaging the wheels orhubcaps. The feelers are manufactured to be flexible and do not easily break.Chemical: Electrochemical gas sensorThe sensors contain two or three electrodes, occasionally four, in contact with anelectrolyte. The gas diffuses into the sensor, through the back of the porous membraneto the working electrode where it is oxidized or reduced. This electrochemical reactionresults in an electric current that passes through the external circuit. In addition tomeasuring, amplifying and performing other signal processing functions, the externalcircuit maintains the voltage across the sensor between the working and counterelectrodes for a two electrode sensor or between the working and reference electrodesfor a three electrode cell. At the counter electrode an equal and opposite reaction
  17. 17. occurs, such that if the working electrode is an oxidation, then the counter electrode isa reduction. Hydrocarbon dew pointThe hydrocarbon dew point is the temperature (at a given pressure) at which thehydrocarbon components of any hydrocarbon-rich gas mixture, such as natural gas,will start to condense out of the gaseous phase. It is often also referred to as the HDPor the HCDP. The maximum temperature at which such condensation takes place iscalled the cricondentherm.[1] The hydrocarbon dew point is a function of the gascomposition as well as the pressure. The hydrocarbon dew point is universally used in the natural gas industry as animportant quality parameter, stipulated in contractual specifications and enforcedthroughout the natural gas supply chain, from producers through processing,transmission and distribution companies to final end users.The hydrocarbon dew point of a gas is a different concept from the water dew point,the latter being the temperature (at a given pressure) at which water vapor present in agas mixture will condense out of the gas.In the United States, the hydrocarbon dew point of processed, pipelined natural gas isrelated to and characterized by the term GPM which is the gallons of liquifiablehydrocarbons contained in 1,000 cubic feet (28 m3) of natural gas at a statedtemperature and pressure. When the liquifiable hydrocarbons are characterized asbeing hexane or higher molecular weight components, they are reported as GPM(C6+).[2][3] However, the quality of raw produced natural gas is also often characterized by theterm GPM meaning the gallons of liquifiable hydrocarbons contained in 1,000 cubicfeet (28 m3) of the raw natural gas. In such cases, when the liquifiable hydrocarbons inthe raw natural gas are characterized as being ethane or higher molecular weightcomponents, they are reported as GPM (C2+). Similarly, when characterized as beingpropane or higher molecular weight components, they are reported as GPM (C3+).[4] Care must be taken not to confuse the two different definitions of the term GPM.
  18. 18. Although GPM is an additional parameter of some value, most pipelineoperators and others who process, transport, distribute or use natural gas are primarilyinterested in the actual HCDP, rather than GPM. Furthermore, GPM and HCDP arenot interchangeable and one should be careful not to confuse what each one exactlymeans.Electric current, electric potential, magnetic, radio a)Magnetic anomaly detector A magnetic anomaly detector (MAD) is an instrument used to detect minutevariations in the Earths magnetic field. The term refers specifically to magnetometersused by military forces to detect submarines (a mass of ferromagnetic material createsa detectable disturbance in the magnetic field); the military MAD gear is a descendentof geomagnetic survey instruments used to search for minerals by the disturbance ofthe normal earth-field. To reduce interference from electrical equipment or metal in the fuselage ofthe aircraft, the MAD sensor is placed at the end of a boom or a towed aerodynamicdevice. Even so, the submarine must be very near the aircrafts position and close tothe sea surface for detection of the change or anomaly. The size of the submarine andits hull composition determine the detection range. MAD devices are usually mountedon aircraft.
  19. 19. There is some misunderstanding of the mechanism of detection of submarines inwater using the MAD boom system. Magnetic moment displacement is ostensibly themain disturbance, yet submarines are detectable even when oriented parallel to theEarths magnetic field, despite construction with non-ferromagnetic hulls. Forexample, the Soviet-Russian Alfa class submarine, whose hull is constructed out oftitanium to give dramatic submerged performance and protection from detection byMAD sensors, is still detectable This is due in part to the fact that even submarines with titanium hull will stillhave a substantial content of ferromagnetic materials as the nuclear reactor, steamturbines, auxiliary diesel engines and numerous other systems will be manufacturedfrom steel and nickel alloys. b)Radio direction finder A radio direction finder (RDF) is a device for finding the direction to a radiosource. Due to low frequency propagation characteristic to travel very long distancesand "over the horizon", it makes a particularly good navigation system for ships, smallboats, and aircraft that might be some distance from their destination (see Radionavigation). The distinct technology Range and Direction Finding was theabbreviation used to describe the predecessor to radar. In use, the RDF operator would first tune the receiver to the correctfrequency, then manually turn the loop, either listening or watching an S meter todetermine the direction of the null (the direction at which a given signal is weakest) ofa long wave (LW) or medium wave (AM) broadcast beacon or station (listening forthe null is easier than listening for a peak signal, and normally produces a moreaccurate result). This null was symmetrical, and thus identified both the correct degreeheading marked on the radios compass rose as well as its 180-degree opposite. Whilethis information provided a baseline from the station to the ship or aircraft, thenavigator still needed to know beforehand if he was to the east or west of the station inorder to avoid plotting a course 180-degrees in the wrong direction. By takingbearings to two or more broadcast stations and plotting the intersecting bearings, thenavigator could locate the relative position of his ship or aircraft. Later, RDF sets wereequipped with rotatable ferrite loopstick antennas, which made the sets more portable
  20. 20. and less bulky. Some were later partially automated by means of a motorized antenna(ADF). A key breakthrough was the introduction of a secondary vertical whip orsense antenna that substantiated the correct bearing and allowed the navigator toavoid plotting a bearings 180 degrees opposite the actual heading. After World War II,there many small and large firms making direction finding equipment for mariners,including Apelco, Aqua Guide, Bendix, Gladding (and its marine division, Pearce-Simpson), Ray Jefferson, Raytheon, and Sperry. By the 1960s, many of these radioswere actually made by Japanese electronics manufacturers, such as Panasonic, FujiOnkyo, and Koden Electronics Co., Ltd. In aircraft equipment, Bendix and Sperry-Rand were two of the larger manufacturers of RDF radios and navigation instruments.Environment, weather, moisture, humidity: Humistor A humidity sensor has a sensing portion which usually comprises ahumidity-sensitive resistor composed of an organic polymer, such as a polyamideresin, polyvinyl chloride or polyethylene, or a metal oxide. A capacitive humiditysensor detects humidity based on a change of capacitance between two detectionelectrodes provided on a semiconductor substrate. The capacitance type humiditysensor detects humidity by measuring the change in the electrostatic capacity of anelement corresponding to the ambient humidity. A resistive humidity sensor detectsrelative humidity by measuring the change in the resistance of an elementcorresponding to the ambient humidity. Most of the resistance type humidity sensorsinclude an electrolytic, polymeric, or metallic oxide sensor element. An impedancehumidity sensor changes its electrical impedance as the humidity of the surroundingenvironment changes, and the measured impedance is converted into humidityreadings. A humidity sensor measures the humidity level by measuring the change inthe resistance of an element or the change in the electrostatic capacity of that elementas it absorbs or releases moisture. Humidity sensors can be used not only to measure
  21. 21. the humidity in an atmosphere but also to automatically control humidifiers,dehumidifiers, and air conditioners for humidity adjustment.Flow, fluid velocity Mass flow sensor A mass air flow sensor is used to find out the mass flowrate of air entering afuel-injected internal combustion engine. The air mass information is necessary for theengine control unit (ECU) to balance and deliver the correct fuel mass to the engine.Air changes its density as it expands and contracts with temperature and pressure. Inautomotive applications, air density varies with the ambient temperature, altitude andthe use of forced induction, which means that mass flow sensors are more appropriatethan volumetric flow sensors for determining the quantity of intake air in each pistonstroke.Ionising radiation, subatomic particles: Geiger counter Geiger counter instruments consist of two main elements; the Geiger-Mullertube, and the processing and display electronics. The radiation sensing element is an
  22. 22. inert gas-filled Geiger-Muller tube (usually containing helium, neon or argon withhalogens added) which briefly conducts electrical charge when a particle or photon ofradiation makes the gas conductive by ionization. The tube has the property of beingable to amplify each ionization event by means of the Townsend avalanche effect andproduces an easily measured current pulse which is passed to the processingelectronics.Navigation instruments a) Yaw-rate sensor:: A yaw-rate sensor is a gyroscopic device that measures a vehicle’s angularvelocity around its vertical axis. . The angle between the vehicles heading and vehicleactual movement direction is called slip angle, which is related to the yaw rate. Themeasurement of plane to the earth from the ground my home. Yaw rate sensors are used in aircraft and in the electronic stability control systemsof cars.b)A ring laser gyroscope (RLG):: A ring laser gyroscope (RLG) consists of a ring laser having two counter-propagating modes over the same path in order to detect rotation. It operates on theprinciple of the Sagnac effect which shifts the nulls of the internal standing wavepattern in response to angular rotation. Interference between the counter-propagating
  23. 23. beams, observed externally, reflects shifts in that standing wave pattern, and thusrotation. A certain rate of rotation induces a small difference between the time it takeslight to traverse the ring in the two directions according to the Sagnac effect. Thisintroduces a tiny separation between the frequencies of the counter-propagatingbeams, a motion of the standing wave pattern within the ring, and thus a beat patternwhen those two beams are interfered outside the ring. Therefore the net shift of thatinterference pattern follows the rotation of the unit in the plane of the ring. Examples of RLG applications:  Airbus A320[3]  Agni III  ASM-135 US Anti-satellite missile.  Boeing 757-200.  Boeing 777  MK39 Ships Internal Navigation System used in NATO surface ships and submarines  P3 OrionOptical, light, imaging, photon: Electro-optical sensors:Electro-optical sensors are electronic detectors that convert light, or a change inlight, into an electronic signal. They are used in many industrial and consumerapplications, for example: Lamps that turn on automatically in response to darkness Position sensors that activate when an object interrupts a light beam Flash detection, to synchronize one photographic flash to another
  24. 24. Light-addressable potentiometric sensor: A light-addressable potentiometric sensor (LAPS) is a sensor that uses light (e.g.LEDs) to select what will be measured. Light can activate carriers in semiconductorsand example is the pH-sensitive LAPS (range pH4 to pH10) that uses LEDs incombination with (semi-conducting) silicon and pH-sensitive Ta2O5 (SiO2; Si3N4)insulator. The LAPS has several advantages over other types of chemical sensors. Thesensor surface is completely flat, no structures, wiring or passivation are required. Atthe same time, the "light-addressability" of the LAPS makes it possible to obtain aspatially resolved map of the distribution of the ion concentration in the specimen.The spatial resolution of the LAPS is an important factor and is determined by thebeam size and the lateral diffusion of photocarries in the semiconductor substrate. Byilluminating parts of the semiconductor surface, elctron-hole pairs are generated and aphotocurrent flows. The LAPS is a semiconductor based chemical sensor with anelectrolyte-insulator-semiconductor (EIS) structure. Under a fixed bias voltage, theAC (kHz range) photocurrent signal varies depending of the solution. A two-dimensional mapping of the surface from the LAPS is possible by using a scanninglaser beam.Pressure: a) Pressure sensor A pressure sensor measures pressure, typically of gases or liquids. Pressure is anexpression of the force required to stop a fluid from expanding, and is usually stated interms of force per unit area. A pressure sensor usually acts as a transducer; it generatesa signal as a function of the pressure imposed.Pressure sensors are used for control and monitoring in thousands of everydayapplications. Pressure sensors can also be used to indirectly measure other variablessuch as fluid/gas flow, speed, water level, and altitude. Pressure sensors canalternatively be called pressure transducers, pressure transmitters, pressure senders,pressure indicators and piezometers, manometers, among other names.
  25. 25. b) ProximityA proximity sensor is a sensor able to detect the presence of nearby objects withoutany physical contact.A proximity sensor often emits an electromagnetic field or a beam of electromagneticradiation (infrared, for instance), and looks for changes in the field or return signal.The object being sensed is often referred to as the proximity sensors target. Differentproximity sensor targets demand different sensors. For example, a capacitivephotoelectric sensor might be suitable for a plastic target; an inductive proximitysensor always requires a metal target.The maximum distance that this sensor can detect is defined "nominal range". Somesensors have adjustments of the nominal range or means to report a graduateddetection distance.Proximity sensors can have a high reliability and long functional life because of theabsence of mechanical parts and lack of physical contact between sensor and thesensed object.Proximity sensors are also used in machine vibration monitoring to measure thevariation in distance between a shaft and its support bearing. This is common in largesteam turbines, compressors, and motors that use sleeve-type bearings.International Electrotechnical Commission (IEC) 60947-5-2 defines the technicaldetails of proximity sensors.A proximity sensor adjusted to a very short range is often used as a touch switch.
  26. 26. Applications Parktronic, car bumpers that sense distance to nearby cars for parking Ground proximity warning system for aviation safety Vibration measurements of rotating shafts in machinery [1] Top dead centre (TDC)/camshaft sensor in reciprocating engines. Sheet break sensing in paper machine. Anti-aircraft warfare Roller coasters Conveyor systems Touch screens on mobile devices that come in close proximity with the face All these deviations can be classified as systematic errors or random errors.Systematic errors can sometimes be compensated for by means of some kind ofcalibration strategy. Noise is a random error that can be reduced by signal processing,such as filtering, usually at the expense of the dynamic behavior of the sensor.

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