自动化专业外文文献文档格式.docx
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1SensorElements
Althoughthereareexception,mostsensorconsistofasensingelementandaconversionorcontrolelement.Forexample,diaphragms,bellows,straintubesandrings,bourdontubes,andcantileversaresensingelementswhichrespondtochangesinpressureorforceandconvertthesephysicalquantitiesintoadisplacement.Thisdisplacementmaythenbeusedtochangeanelectricalparametersuchasvoltage,resistance,capacitance,orinductance.Suchcombinationofmechanicalandelectricalelementsformelectromechanicaltransducingdevicesorsensor.Similarcombinationcanbemadeforotherenergyinputsuchasthermal.Photo,magneticandchemical,givingthermoelectric,photoelectric,electromaanetic,andelectrochemicalsensorrespectively.
2SensorSensitivity
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.circuitstogiveavoltageproportionaltovelocityforacapacito