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附录ATransdu__randsensorexcitationandmeasurementtechniquesManyoftodaysindustrialandinstrumentationapplicationsinvolvingsensor.Thefunctionofthesensorsystemistomonitorchangesandthenthisdatabacktothe__incontroller.Fora______voltageorcurrentmeasurementsensors__yberesistan__innature.Howeversomesensorsystem__ybeinductiveorcapacitiveinnaturethatistosaythefrequencyrangeofthesensorresistan__changeisnonlinear.Impedan__sensorssuchtypicalexampleistheproximitysensor-acampaignforthedetectionoftherelativedistan__ofobjects;Inadditioncapacitivesensorsorsensorsensibility-inthemedicaldevi__susedtomeasurebloodfloworbloodpressureorbloodqualitative____ysis.Inordertousethesecompleximpedan__sensorstherealizationofmeasurementtoprovideanexchangeofACexcitationsour__frequencyinthefrequencyrangeofthesensorforscanning.Thisarticleattemptstoexplainhowtheuseofsingle-chipdigitalwaveformgeneratortoeasilyachievethisupto10MHzfrequencyscanning.Alsointrodu__dakindofintegratedin__ntiveresponseanddigitalsignalpro__ssorDSPfeaturesacompletesingle-chipsensorsolutionthatsuitedtherequirementsofuptonearly50kHzfrequencyapplications.Sensors:workingprinciple.InspiredbythefrequencyofsensorsignalsbasedonsensorvaluesofLorCtoshowthecorrespondinginstantaneous__gnitudefrequencyorphasechanges.Forexampletheultrasoundwillshowaflowofphaseoffsetwhiletheproximitysensorwillcausetheratetochange.Trackingchangesinimpedan__thatisthemostcommonlyusedtomonitortheresonantfrequencycircuit.Capacitan__valueoftheresonantfrequencyisequaltothefrequencywheretheinductan__valuepoint.Thisisalsothelargestfrequencycurvefrequencyimpedan__valueofthecorrespondingpoint.Undernor__lcircumstan__sforexampleinstaticconditionsthesensorLRandChasauniquevalueintheresonantfrequencyimpedan__toDepartmentwiththegreatestvalue.WhenamovingobjectnearthesensorthensensorLandCvalueswillbechangedandanewresonantfrequency.Bymonitoringthechangesinresonantfrequencyandthusleadtochangesinimpedan__itispossibletospeculatethattherelativemovementofobjectsmovingawayfromthesensor.Calculatedresonantfrequency:calculationcircuitmeasuringtheresonantfrequencyoftheneedfortherelationshipbetweenfrequencyandimpedan__inparticularneeda__rtainfrequencyrangewiththeabilitytoscanthewaveformgenerator.A______low-costmethodisbasedontheAD5930waveformgenerator.AD5930withagroupofpre-setinthefrequencyrangeoftheabilitytoprovidealinearscan.On__theconditionsforsettingontheneedforfurthercontrolinadditiontoafrequencyscanforthestartofthetrigger.AD5930has__nyadvantages:theoutputfrequencyresolutionof28bitsoyoucanbelessthanthecontrolaccuracyof
0.1Hzoutputfrequency.Theoutputfrequencyrangeof0~10MHzthustheselectionofsensorswithalotofflexibility.Forexamplesomesensorsaverynarrowfrequencyrangebuttherequirementsinthisfrequencyrangewithhighresolution.Somesensors__yrequireawidefrequencyrangebutlowerresolutionrequirements.Thisapproachiseasytocalculatetheresonantfrequencyofthesensor.Systemblockdiagram:typicalblockdiagramofsuchasystemasshowninFigure
3.ThroughtheBF-535DSPpro__ssorAD5930digitalwaveformgeneratorset.AD5930needsarisingfromthesinusoidaloutputvoltagewaveformforlow-passfilteringandamplificationinordertoeliminatethe__sterclockMCLKmirroringthefrequencyandhighfrequencynoisegeneratedbyfeedthrough.Afterfilteringthesensorsignalcanbeusedasasour__ofexcitationfrequency.Accordingtotheimpedan__ofthesensorresponsesignalamplification__ybeneededinordertoentertheADCADCdynamicrange.Sensoroutputandthefrequencyofthesour__ofbothin__ntivesintotheAD726612bit2MSPSdualsimultaneoussamplingADC.ADCoutputdatawillbestoredinmemoryinordertodofurther____ysistocalculatethephaseandamplitudeofthesensoroffset.Completeintegratedsensorsolutions:separationdescribedaboveisacommonsolutionforimpedan__measurementofthesensorsolution.Theprogram__yrequire__nydiscretecomponentssothesensorisacost-____ysissolution.Theseseparatecomponentswillincreasetheirownsour__soferror.Thedesignofactivecomponentswillincreasethenumberofphaseerrorwhichistheneedforcorrection.InadditiontheDSPalsoneedtodealwithsomecomplex__the__ticalcalculationsthis__yrequireexternalmemorytostoretheoriginaldataoftheADCwhichwouldfurtherincreasecosts.Addresstheabove-mentioned____ysisoftheissueoflow-frequencysensorsolutionsAD5933/4devi__itwillbythe__inpro__ssingmoduleareintegratedintoonechip.Thecoreofthechipincludingthree__inmodules:thefrequencyofscanningforthedirectdigitalsynthesizerDDSwaveformgenerator;usedtomeasurethesensorsresponsetothe12bit1MSPSADC;andfinallytotheADCdatafor1024pointDiscreteFourierTransformDFTcalculationsoftheDSPengine.TheresultsofDFTcalculationstoprovidearealpartRandani__ginarypartIdatawhichcaneasilycalculatetheimpedan__.Usingthefollowingformulaiseasytocalculatetheimpedan__amplitudeandphase:Inordertodeterminetheactualvalueoftherealimpedan__Zωistypicallyrequiredtoperformfrequencyscanning.Cancalculatetheimpedan__ofeachfrequencypointwhichcandrawarelationshipbetweenfrequencyandamplitudecurves.Soitiseasytomeasure100Ω~20MΩresistan__withinthescope.Thesystemallowsuserstosetupa2Vpeak-to-peakPK-PKofthesinusoidalsignalasanexternalfrequencysour__excitationload.Outputrangecanbesetto1V500mVand200mV.Frequencyresolutioncanbe27bit
0.1Hz.Therealizationofthefrequencyofscanning:Inordertoachievethefrequencyofscanningtheusermustfirstsetuptherequiredfrequencyofscanningconditions:theneedforastartfrequencyfrequencyintervalandsweeppoints.Thentheneedforastartcom__ndtostartscanning.FrequencypointsineachscanADCcompletedthefirst1024samplesandthencalculatingtheDFTinordertoprovidethewaveformoftherealandi__ginarypartsofthedata.Therealandi__ginarypartsofthedatathroughtheI2Cinte_____intheformoftwo16bitwordsavailabletotheuser.DSP-chippro__ssingunituserdoesnothavetheadvantageofcomplex__the__ticalcalculationsandneednotstoreADCrawdataonlytwo16bitdata.ThereforeitallowstheDSPtochoosecheapersolutionsasgreatlyredu__thepro__ssingpowerofthefinalrequirements.Withoutcalibrationofthesystemasaresultcanonlyusethetypicalvalueofsensitivityandoffsettheoutputvoltageisconvertedtopressurethepressuremeasuredwillhavea__rginoferrorasshowninFigure.Thisinitialerrorwithoutcalibrationbythefollowingcomponents:
1.offseterror:Asthepressureintheentirerangeofverticalshiftto__intainaconstantsotheproliferationandlaserconditioningconverterchangestheamendmentwouldhaveoffseterror.
2.Thesensitivityoferrorresultinginerrorsindirectproportiontothesizeandpressure:Ifthedevi__ishigherthanthetypicalvalueofthesensitivitythesensitivityoftheerrorwillbeincrementalpressurefunctionseeFigure
1.Ifthesensitivityislowerthanthetypicalvaluethenthesensitivityoftheerrorwillbedecreasingfunctionofpressure.Thecauseoftheerrordiffusionpro__ssistochange.
3.LinearityError:Thisisaninitialerrorfactorlessaffectedtheerroristhecauseofthephysicalnon-linearsiliconbutwiththesensoramplifiershouldalsoincludenon-linearamplifier.Linearerrorcurvecanbeconcavecurveitcouldbeaconvexcurve.
4.LagError:Inmostcasesthelagerrorcanbeignoredcompletelybecausesiliconhasahigherdegreeofmechanicalstiffness.Changesingeneraljustalotofpressuretoconsiderthecaseofhysteresiserror.Calibrationcaneliminateorgreatlyredu__theseerrorsandcompensationtechniqueisusuallyrequiredtoidentifytheparametersoftheactualtransferfunctionratherthansimplytheuseoftypicalvalues.Potentiometeradjustableresistan__andotherhardwarecanbeusedinthecompensationpro__sswhilethesoftwareisabletoachievemoreflexibilityintheworkofthiserrorcompensation.CalibrationmethodthatcaneliminatethetransferfunctionagainsttheAgencytocompensatetheoffsetdrifterrorsuchastheauto-zerocalibrationmethod.Offsetzerocalibrationisusuallycarriedoutunderpressureespeciallyinthedifferentialsensorbecauseundertheconditionsofthenominaldifferentialpressureisusually
0.Forpuresensoroffsetcalibrationwillbedifficultbecauseiteitherneedstoreadapressuresystemtomeasuretheatmosphericpressureintheenviro__entundertheconditionsofthecalibrationofpressureorneedtoobtainthepressureofexpectationsofthepressurecontroller.Zerodifferentialpressuresensorisveryaccuratecalibrationbecausethepressureofstrictde__rcationis
0.Ontheotherhandthepressureof0:00isnottheaccuracyofthecalibrationdependsonthepressurecontrollerortheperfor__n__measurementsystem. Calibrationisveryimportantselectionpressurewhichdeterminestheaccuracytoobtainthebestpressurerange.Infactaftercalibrationoffsetactualstandardfixed-pointerrorintheDepartmentandhasbeento__intainas__llerminimumvalue.Thereforethereferen__pointsmustbeinaccordan__withthescopeofthetargetselectionpressureandpressurerangecannotbeconsistentwiththescopeofwork. Inordertoconvertthepressureoftheoutputvoltagevalueusuallyasaresultoftheactualsensitivityisunknownandthereforethe__the__ticalmodelusedforatypicalsingle-pointcalibrationsensitivity. SaidthattheredcurvecalibrationoffsetPCAL=0aftertheerrorcurvetheerrorcanbefoundthatcalibrationcurverelativetotheblackbeforetheerrorhadaverticaloffsetcurve.Thiscalibrationmethodandcalibrationmethodthatismorestringentrequirementstoachieveahighercost.Howevercomparedwiththecalibrationpointthemethodcansignificantlyimprovetheaccuracyofthesystembecausethemethodisnotonlyanoffsetcalibrationthecalibrationofthesensorsensitivity.Thereforethecalculationerrorcanbeusedintheactualvalueofsensitivityandtheatypicalvalues. Thatimprovetheaccuracyofthegreencurve.Herecalibrationistrillion0-500barfullscaleconditions.Asthe__rkedpointontheerrorcloseto0sothepressureofexpectationstobethes__llestrangeofmeasurementerrorthecorrectsetofthesepointsitisparticularlyimportant. Someapplicationsrequirethepressureinthewholerangeofhighaccuracy.Intheseapplicationscanbeusedmulti-pointcalibrationmethodtogetthebestresults.Inmulti-pointcalibrationmethodnotonlyconsideredtheerrorofoffsetandsensitivitybutalsotakesintoaccountmostofthelinearerrorcurveshowninpurple.The__the__ticalmodelusedherewitheachcalibrationintervalbetweenthetworeferen__pointsexactlythesameasatwo-tiercalibration. Asmentionedearlierthelinearformofaconsistenterrorandtheerrorcurveinlinewiththequadraticequationofthecurvewithapredictablesizeandshape.Didnotusetheamplifierforthesensorespeciallybecauseofthenonlinearsensorisbasedonthenatureofmechanicalreasonsthisiscausedbythepressureofsiliconthin-film.Lineardescriptionoftheerrorcharacteristicsofatypicalexamplecanbecalculatedtheaveragelinearerrortodeterminethepolynomialfunctiona×2+bx+cbetheparameter.Determinetheabandcofthemodelafterthesametypeofsensorisvalid.Thismethodcanbenofixedpoints__rkedthefirstthreecasesoflinearerrorcompensationeffectively.ExamplesofcompensationMPX2300MotorolaMPX2300isabloodpressuremeasurementis__inlyusedinthetemperaturecompensationsensor.Polynomialmodelcanbeanaverageof10sensorstobelinearerrorcompensationoftheerroraftertheinitial__ximumlinearityerrorofabouttentoone-twentiethasshownindottedlineinFigure
3.Theerrorcompensationmethodcanbeonlytwopointscalibrationforhigh-perfor__n__low-costsensorstoimprovethedevi__fullscaleerroroflessthan
0.05%.Ofcoursedesignengineerspracticalapplicationinaccordan__withtheaccuracyrequirementschoosingthemostappropriatecalibrationmethodinadditiontotheneedtoconsidersystemcost.Asaresultofavarietyofintegrationoptionsandcompensationtechnologydesignengineerscandesignrequirementsaccordingtodifferentmethodsofselectingappropriate.附录B传感器和传感器激励和测量技术当今的许多工业和仪器仪表应用都涉及传感器测量传感器的功能就是监视系统中的变化,然后将此数据反馈给主控制器用于简单的电压或电流测量的传感器可能是电阻性的但是,有些传感器系统可能是电感性或电容性的,就是说在传感器频率范围内阻抗变化是非线性的这类复阻抗传感器的典型例子就是接近传感器——用于检测一个运动物体的相对距离;另外,容性传感器或感性传感器——在医用设备中用于测量血流或者分析血压或血质为了用这些“复阻抗传感器”实现测量,必须提供一种交流(AC)激励频率源在传感器的频率范围内进行扫描本文试图说明如何采用单芯片数字波形发生器轻松实现这种高达10MHz的频率扫描还介绍了一种带集成激励、响应和数字__处理器(DSP)功能完整的单芯片传感器解决方案,它适合要求高达近50kHz激励频率的应用传感器工作原理通过传感器的激励频率__会根据传感器的L或C瞬时值表现出相应的幅度、频率或者相位的改变例如,超声波液流计会表现出相位偏移,而接近传感器会引起幅度改变跟踪这种变化阻抗的最常用方法就是监视电路的谐振频率谐振频率就是电容值等于电感值所在的频率点这也是频率曲线上最大阻抗值对应的频率点在正常情况下,例如在静态条件下,传感器的L,R和C都具有一个唯一值,在谐振频率Fo处具有最大阻抗值当一个运动物体接近传感器时,那么传感器的L和C值就会改变,并且产生一个新的谐振频率通过监测谐振频率的变化(从而导致阻抗的变化),就有可能推测出运动物体相对传感器的__距离计算谐振频率计算电路的谐振频率需要测量频率和阻抗的关系,尤其是需要一个能够在一定频率范围内具有扫描能力的波形发生器一种简单、低成本的实现方法就是采用AD5930波形发生器AD5930具有在一组预设置的频率范围内提供线性扫描的能力一旦条件设定,就无需进一步的控制,除了一个用于启动频率扫描的触发器AD5930具有许多优点输出频率的分辨率为28bit,所以用户能以小于
0.1Hz的控制精度输出频率其输出频率范围为0~10MHz,从而对选择传感器具有很大的灵活性例如,有些传感器的频率范围很窄,但是要求在此频率范围内具有很高的分辨率还有些传感器可能需要很宽的调频范围,但是分辨率要求较低采用这种方法很容易计算出传感器的谐振频率系统框图通过BF-535DSP处理器设置AD5930数字波形发生器需要对从AD5930产生的正弦波输出电压波形进行低通滤波和放大以便消除主时钟(MCLK)、镜像频率和高频噪声产生的馈通经过滤波的__可用作传感器的激励频率源根据传感器的阻抗响应__可能需要进行放大以便使其进入模数转换器(ADC)的动态范围内传感器的输出和激励频率源都输入到AD7266一种12bit、2MSPS的同步采样双ADC将ADC输出的数据保存在存储器中以便做进一步的分析以计算出传感器的相位和幅度偏移完整的集成传感器解决方案上面介绍的分立解决方案是一种常用的传感器阻抗测量解决方案该方案可能需要许多分立元件,所以是一种高成本的传感器分析解决方案这些单独的元件还会增加自身的误差源设计中的有源元件还会增加相位误差,这也是需要校正另外,还需要DSP处理一些复杂的数学计算,这样可能需要外部存储器来存储原始的ADC数据,从而会进一步增加成本解决上述低频率传感器分析问题的解决方案是AD5933/4器件,它将上述主要处理模块都集成到一颗芯片中该芯片的内核包括3个主要单元用于提供频率扫描的直接数字频率合成器(DDS)波形发生器;用于测量传感器的响应的12bit、1MSPSADC;以及最后能够对ADC测量数据进行1024点离散傅立叶变换(DFT)运算的DSP引擎DFT运算结果提供一个实部(R)和一个虚部(I)数据,从而可以方便地计算出阻抗采用下面的公式很容易计算出阻抗的幅度和相位为了确定实际的实数阻抗值Zω,通常需要进行频率扫描可以计算出每个频率点的阻抗,从而可以得出一条频率与幅度的关系曲线这样就很容易测量出100Ω~20MΩ范围内的阻抗该系统允许用户设置一个2V峰峰值(PK-PK)的正弦__作为外部负载的激励频率源输出范围还可以设置为1V,500mV和200mV频率分辨率可以达到27bit(
0.1Hz)实现频率扫描为了实现频率扫描,用户必须首先设置频率扫描所需要的条件需要一个起始频率、频率间隔和扫频点数然后需要一个启动命令开始扫描在每个扫描频点,ADC先完成1024个采样,然后进行DFT计算以便提供该波形的实部和虚部数据此实部和虚部数据通过I2C接口以两个16bit字形式提供给用户片内DSP处理单元的优点是用户不必进行复杂的数学计算,也无需存储ADC原始数据,只需提供两个16bit的数据因此,它还允许选择更便宜的DSP解决方案,因为大大降低了对最终处理能力的要求由于未经标定的系统只能使用典型的灵敏度和偏移值将输出电压转换为压力,测得的压力将产生如图1所示的误差这种未经标定的初始误差由以下几个部分组成
1.偏移量误差由于在整个压力范围内垂直偏移保持恒定,因此变换器扩散和激光调节修正的变化将产生偏移量误差
2.灵敏度误差产生误差大小与压力成正比如果设备的灵敏度高于典型值,灵敏度误差将是压力的递增函数如果灵敏度低于典型值,那么灵敏度误差将是压力的递减函数该误差的产生原因在于扩散过程的变化
3.线性误差这是一个对初始误差影响较小的因素,该误差的产生原因在于硅片的物理非线性,但对于带放大器的传感器,还应包括放大器的非线性线性误差曲线可以是凹形曲线,也可以是凸形曲线
4.滞后误差在大多数情形中,滞后误差完全可以忽略不计,因为硅片具有很高的机械刚度一般只需在压力变化很大的情形中考虑滞后误差标定可消除或极大地减小这些误差,而补偿技术通常要求确定系统实际传递函数的参数,而不是简单的使用典型值电位计、可调电阻以及其他硬件均可在补偿过程中采用,而软件则能更灵活地实现这种误差补偿工作一点标定法可通过消除传递函数零点处的漂移来补偿偏移量误差,这类标定方法称为自动归零偏移量标定通常在零压力下进行,特别是在差动传感器中,因为在标称条件下差动压力通常为0对于纯传感器,偏移量标定则要困难一些,因为它要么需要一个压力读取系统,用以测量其在环境大气压力条件下的标定压力值,要么需要获取期望压力的压力控制器差动传感器的零压力标定非常精确,因为标定压力严格为0另一方面,压力不为0时的标定精确度取决于压力控制器或测量系统的性能选择标定压力 标定压力的选取非常重要,因其决定了获取最佳精度的压力范围实际上,经过标定后实际的偏移量误差在标定点处最小并一直保持较小的值因此,标定点必须根据目标压力范围加以选择,而压力范围可以不与工作范围相一致 为了将输出电压转换为压力值,由于实际的灵敏度往往是未知,因此在数学模型中通常采用典型灵敏度进行单点标定 红色曲线表示进行偏移量标定(PCAL=0)后的误差曲线,可以发现误差曲线相对于表示标定前误差的黑色曲线产生了垂直偏移 这种标定方法与一点标定法相比要求更为严格,实现成本也更高然而与一点标定法相比,该方法可显著提高系统的精度,因为该方法不仅标定了偏移量,还标定了传感器的灵敏度因此在误差计算中可以使用灵敏度实际值,而__型值 绿色曲线表示精度提高在这里,标定是在0至500兆巴(满标度)条件下进行由于在标定点上误差接近于0,因此为了在期望的压力范围内得到最小的测量误差,正确地设定这些点就显得尤为重要 某些应用中要求在整个压力范围内保持较高的精确度在这些应用中,可以采用多点标定法来得到最理想的结果在多点标定法中,不仅考虑了偏移量和灵敏度误差,还考虑了大部分的线性误差,紫红色曲线所示这儿用的数学模型与每个标定间距(在两个标定点之间)的两级标定完全一样 如前所述,线性误差具有一致的形式,且误差曲线符合二次方程的曲线,具有可预测的大小和形状对于未采用放大器的传感器更是如此,因为传感器的非线性从本质上是基于机械原因(这是由硅片的薄膜压力引起) 线性误差特性的描述可以通过计算典型实例的平均线性误差,确定多项式函数(a×2+bx+c)的参数而得到确定了a、b和c后得到的模型对于相同类型的传感器都是有效的该方法能在无需第3个标定点的情况下有效地补偿线性误差 摩托罗拉MPX2300的补偿实例,MPX2300是一种主要应用于血压测量的温度补偿传感器多项式模型可由10个传感器的平均线性误差得到,补偿后的误差约为最大初始线性误差的十至__分之一,如图3虚线所示该误差补偿方法只需两点标定即可将低成本传感器改进为高性能器件(误差小于满标度的
0.05%)当然设计工程师要根据实际应用的精确度要求,选择最适合的标定方法,此外还需要考虑系统成本由于有多种集成度和补偿技术可供选择,设计工程师可根据不同的设计要求选择适当的方法 。