测厚仪基本原理.ppt

TheoryofTransmissionGaugingforThicknessMeasurementofMetals BasicPrinciple Detector Source BasicPrinciples Trans missionDetector Source BackscatterDetector PropertiesofRadiation AllradiationhastwodistinctpropertiesEnergySimilartotheCOLORofvisiblelight usuallyexpressedinkeVormeVIntensitySimilartoBRIGHTNESSofvisiblelight Light SoundX rays gamma Brightness Intensity Color Frequency Volume Pitch Frequency NumberofPhotons Energy keV ThreeWaystouseRadiationforMeasurement ThreemodesofmeasurementmaybeusedaccordingtotheapplicationTransmissionmode usedfordirectmeasurementofstripthicknessFluorescencemode usedtomeasurethicknessofmetalliccoatingsBackscattermode usedtomeasurethicknessofpaintcoatings MeasurementofThickness DetectedBeamIntensity DetectorIntensityID Thickness 0 q 2q 6q q thicknessatwhichI 0 37 I0 2q thicknessforbestpercent noisetosignalratio 6q maximummeasurable thickness GenerationofX rays Gamma rays NaturalDecayofIsotope Am241 Cs137aremostcommonlyusedSingleFixedEnergy Mono chromatic ReliableLowerCostLowPowerCannotbeturnedoff X rayTube HighvoltageaccelerateselectronstoatargetEnergySpectrumSelectableEnergySafer TurnoffHVtostopradiationoutputGreaterphotonintensityLowerNoiseHigherCost RadiationSourcesforMeasuringSteel ThicknessRanges SteelSr90 Betaray 0 05 1 00mmAm241 GammaRay 0 15 6 25mmCs137 GammaRay 6 0 125mm100kV X ray 0 1 10 0mm160kV X ray 0 2 25 0mm240kV X ray 1 0 60 0mm Radio IsotopeSourceOutput Radiationisemittedinalldirections non preferentiallySelf absorptionlimitsradiationavailableformeasurementRadioactivesourceoutputisnotalinearfunctionofsourceactivity Half LivesofRadiationSources Isotopeshave HalfLives thetimeforactivitytodecaytohalfoftheoriginalvalueAm241433yearsCs13730yearsCm24417 8yearsSr9029 6yearsKr8510 7yearsPm1372 6years DetectionofX raysandGamma rays IonChamber ScintillationDetector HV HV X rayknocksfreeelectronsfromatomsofgastomakeions e current X rayexcitescrystaltogeneratelight LightDetector ScintillationCrystal StatisticalNoise Noiseiscausedbystatisticsofradiationgeneration absorption anddetectionNoiseismeasuredas1sigma 68 3 oftime2sigma 95 5 oftime3sigma 99 7 oftime1 65sigma 90 oftime Detector Source Strip VD Noiseband VD Time Noisevs t q Minatt 2qforIsotopeMinatt qforX ray MeasurementResponseTime Thicknesst Time Actualthicknessofproduct Thicknesschangeasmeasured bygauge T0 TR 63 ResponseTimeTR Timetoreach63 offinalreading 99 3 offinalreadingisreachedafter5 TR MeasurementUpdateTime Thickness Time Actualthicknessofproduct T0 63 Updatetime Analogoutputofgauge WhydoX raysgiveaBetterMeasurement X raysourceswillgivebettermeasurementresultsbecauseMorephotonsgivelowernoiseSourceenergycanbeadjustedtoobtainoptimummeasurementenergyMorephotonsallowfasterresponse shorterintegrationtimes withacceptablenoisevaluesMorephotonspermituseoflargerairgaps EffectofPassAngle AdvantagesIsotope X ray IsotopeLowercostHigherreliabilityLowermaintenanceEasyalloycorrectionSlowermills linesMeasurementonlyapplications X rayHighradiationoutputFasterresponseBestsignal noiseratioNo optimum thicknessLargerairgaps DisadvantagesIsotope X ray IsotopeFixedoptimumthicknessSlowerPoorsignal noiseratioSmallairgapsLimitedcontrolapplicationsSpeciallicensing X rayHighercostMoremaintenanceComplexalloycorrection SuperCompPlus AlloyCorrectionofTransmissionMeasurement Detector RadiationSource Intensity I0 Intensity ID