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TherelationshipbetweenthemeasuredandthesensoroutputsignalisusuallyobtainedbycalibrationtestsandisreferredtoasthesensorsensitivityK1=output-signalincrement/measuredincrement.Inpractice,thesensorsensitivityisusuallyknown,and,bymeasuringtheoutputsignal,theinputquantityisdeterminedfrominput=output-signalincrement/K1.
3CharacteristicsofanIdealSensor
Thehighsensorshouldexhibitthefollowingcharacteristics.
(a)highfidelity-thesensoroutputwaveformshapebeafaithfulreproductionofthemeasured;
thereshouldbeminimumdistortion.
(b)Thereshouldbeminimuminterferencewiththequantitybeingmeasured;
thepresenceofthesensorshouldnotalterthemeasuredinanyway.
(c)Size.Thesensormustbecapableofbeingplacedexactlywhereitisneeded.
(d)Thereshouldbealinearrelationshipbetweenthemeasuredandthesensorsignal.
(e)Thesensorshouldhaveminimumsensitivitytoexternaleffects,pressuresensor,forexample,areoftensubjectedtoexternaleffectssuchvibrationandtemperature.
(f)Thenaturalfrequencyofthesensorshouldbewellseparatedfromthefrequencyandharmonicsofthemeasurand.
Sensorscanbedividedintothefollowingcategories:
1ElectricalSensor
Electricalsensorexhibitmanyoftheidealcharacteristics.Inadditiontheyofferhighsensitivityaswellaspromotingthepossibleofremoteindicationormesdurement.
Electricalsensorcanbedividedintotwodistinctgroups:
(a)variable-control-parametertypes,whichinclude:
(i)resistance
(ii)capacitance
(iii)inductance
(iv)mutual-inductancetypes
Thesesensorallrelyonexternalexcitationvoltagefortheiroperation.
(b)self-generatingtypes,whichinclude
(i)electromagnetic
(ii)thermoelectric
(iii)photoemissive
(iv)piezo-electrictypes
Theseallthemselvesproduceanoutputvoltageinresponsetothemeasurandinputandtheireffectsarereversible.Forexample,apiezo-electricsensornormallyproducesanoutputvoltageinresponsetothedeformationofacrystallinematerial;
however,ifanalternatingvoltageisappliedacrossthematerial,thesensorexhibitsthereversibleeffectbydeformingorvibratingatthefrequencyofthealternatingvoltage.
2ResistanceSensor
Resistancesensormaybedividedintotwogroups,asfollows:
(i)Thosewhichexperiencealargeresistancechange,measuredbyusingpotential-dividermethods.Potentiometersareinthisgroup.
(ii)Thosewhichexperienceasmallresistancechange,measuredbybridge-circuitmethods.Examplesofthisgroupincludestraingaugesandresistancethermometers.
3CapacitiveSensor
Thecapacitancecanthusmadetovarybychangingeithertherelativepermittivity,theeffectivearea,orthedistanceseparatingtheplates.Thecharacteristiccurvesindicatethatvariationsofareaandrelativepermittivitygivealinearrelationshiponlyoverasmallrangeofspacings.Thusthesensitivityishighforsmallvaluesofd.
Unlikethepotentionmeter,thevariable-distancecapacitivesensorhasaninfiniteresolutionmakingitmostsuitableformeasuringsmallincrementsofdisplacementorquantitieswhichmaybechangedtoproduceadisplacement.
4InductiveSensor
Theinductancecanthusbemadetovarybychangingthereluctanceoftheinductivecircuit.
Measuringtechniquesusedwithcapacitiveandinductivesensor:
(a)A.C.excitedbridgesusingdifferentialcapacitorsinductors.
(b)A.C.potentiometercircuitsfordynamicmeasurements.
(c)D.C.circuitstogiveavoltageproportionaltovelocityforacapacitor.
(d)Frequency-modulationmethods,wherethechangeofCorLvariesthefrequencyofanoscillationcircuit.
Importantfeaturesofcapacitiveandinductivesensorareasfollows:
(i)resolutioninfinite
(ii)accuracy±
0.1%offullscaleisquoted
(iii)displacementranges25*10-6mto10-3m
(iv)risetimelessthan50uspossible
Typicalmeasurandsaredisplacement,pressure,vibration,sound,andliquidlevel.
5LinearVariable-differentialTtransformer
6Piezo-electricSensor
7ElectromagneticSensor
8ThermoelectricSensor
9PhotoelectricCells
10MechanicalSensorandSensingElements
Ininformationage,theinformationindustryincludesinformationgathering,transmission,processthreeparts,namelysensortechnology,communication,computertechnology.Becauseofultralargescaleintegratedcircuit’srapiddevelopmentafterhavingbeendevelopedModerncomputertechnologyandcommunication,notonlyrequestssensorprecisionreliability,speedofresponseandgaininformationcontentrequestmoreandmorehighbutalsorequestsitscosttobeinexpensive.Theobvioustraditionalsensoriseliminatedgraduallybecauseofthefunction,thecharacteristic,thevolume,thecostandsoon.Asworlddevelopmanycountriesarespeedinguptothesensornewtechnology’sresearchandthedevelopment,andallhasobtainedtheenormousbreakthrough.Nowthesensornewtechnologydevelopmentmainlyhasfollowingseveralaspects:
Usingthephysicalphenomenon,thechemicalreaction,thebiologicaleffectasthesensorprinciplethereforetheresearcheswhichdiscoveredthenewphenomenonandtheneweffectarethesensortechnologicalimprovingways.itisimportantstudiestodevelopednewsensor’sthefoundation.JapaneseSharpCorporationusesthesuperconductivitytechnologytodevelopsuccessfullythehightemperaturesuperconductivitymagneticsensorandgetthesensortechnologysignificantbreakthrough.Itssensitivityissohighandonlyinferiorinthesuperconductivityquantuminterferencecomponent.Itsmanufacturecraftisfarsimplerthanthesuperconductivityquantuminterferencecomponent.Mayuseinmagnetismimageformationtechnology.Soithasthewidespreadpromotedvalue.
Usingtheimmunebodyandtheantigenmeetsoneanothercompoundwhentheelectrodesurface.Itcancausetheelectrodepotentialchangeandusethisphenomenontobepossibletogeneratetheimmunitysensor.Theimmunitysensormakeswiththiskindofimmunebodymaytosomeorganisminwhetherhasthiskindofantoriginalworkinspection.Likemayinspectsomebodywiththehepatitisvirusimmunebodywhethercontractsthehepatitis,playstoisfast,theaccuraterole.TheUSUCsixthbranchhasdevelopedthiskindofsensor.
Thesensormaterialistheimportantfoundationforsensortechnology,becausethematerialsscienceisprogressiveandthepeoplemaymakeeachkindofnewsensorForexamplemakingthetemperaturesensorwiththehighpolymerthinfilm;
Theopticalfibercanmakethepressure,thecurrentcapacity,thetemperature,thedisplacementandsoonthemanykindsofsensors;
Makingthepressuretransmitterwiththeceramics.Thehighpolymercanbecometheproportionadsorptionandthereleasehydrogenalongwiththeenvironmentrelativehumiditysize.Thehighpolymerelectricityliesbetweentheconstanttobesmall,thehydrogencanenhancethepolymerthecoefficientofdialecticalloss.Makingthecapacitorthehighpolymerdielectricmedium,determinestheelectriccapacitycapecitythechange,thenobtainstherelativehumidity.Makingtheplasmausingthisprincipletogatherthelegitimatepolystyrenefilmtemperaturesensorbelow,ithasthecharacteristic.
Measuredthewetscopeiswide;
Thetemperaturerangeiswide,mayreach-400℃~+1,500℃;
Thespeedofresponseisquick,issmallerthan1S;
Thesizeissmall,mayuseinthesmallspacemeasuringwet;
Thetemperaturecoefficientissmall.
Theceramicelectriccapacitytypepressuretransmitterisonekinddoesnothavetheintermediaryfluidthedrytypepressuretransmitter.Usestheadvancedceramictechnology,theheavyfilmelectronictechnology,itstechnicalperformanceisstable,theyeardriftingquantityissmallerthan0.1%F.S,warmfloatsissmallerthan±
0.15%/10K,anti-overloadsstrongly,mayreachthemeasuringrangeseveralhundredtimes.Thesurveyscopemayfrom0to60mpa.GermanE+HCorporationandtheAmericanKahloCorporationproductisattheleadingposition.
Theopticalfiberapplicationissendthematerialsignificantbreakthrough,itsusesinmostearlytheopticalcommunicationtechniques.Intheopticalcommunicationusediscoveredworksasenvironmentalconditionchangeandsoonthetemperature,pres-sure,electricfield,magneticfield,causesthefiberoptictransmissionlightwaveintensity,thephase,thefrequency,changeandsoonthepolarizationcondition,thesurveylightwavequ