簡(jiǎn)化了的濕度測(cè)量與單片機(jī)相對(duì)濕度傳感器

簡(jiǎn)化了的濕度測(cè)量與單片機(jī)相對(duì)濕度傳感器-外文文獻(xiàn)翻譯譯簡(jiǎn)化了的濕度測(cè)量與單片機(jī)相對(duì)濕度傳感器January1,2013By:JohnGammel,SiliconLaboratoriesInc.Sensors摘要：隨著工業(yè)科技日新月異，濕度傳感技術(shù)得到廣泛應(yīng)用。相較傳統(tǒng)的機(jī)械濕度計(jì)現(xiàn)代電子濕度傳感器通過(guò)吸濕材料的電容或者電阻監(jiān)測(cè)濕度等相關(guān)系數(shù)的變化。相對(duì)濕度傳感器實(shí)現(xiàn)了溫度和濕度的控制。SI70O5溫濕度傳感器采用疏水性覆蓋材料，使用聚酰亞胺莫變化檢測(cè)濕度變化。SI7005S集成了一個(gè)ADC,采用了非易失性存儲(chǔ)技術(shù)，具有一個(gè)12C接口，這種高集成度提高了耐用性和可靠性。關(guān)鍵詞：相對(duì)濕度；電子傳感器；濕度傳感器濕度測(cè)量濕度測(cè)量變得越來(lái)越重要。本文中我們討論單芯片的相對(duì)濕度（RH）傳感器，該單片機(jī)提供了強(qiáng)大的性能，便攜，低功耗，并且可以在各種各樣的應(yīng)用中實(shí)現(xiàn)精確的濕度檢測(cè)。濕度傳感技術(shù)被廣泛應(yīng)用于生活中：比如應(yīng)用于暖通空調(diào)和制冷技術(shù)中及醫(yī)療設(shè)備的CPAP呼吸機(jī)技術(shù)中;資產(chǎn)跟蹤和用于食品和醫(yī)藥等行業(yè)的存儲(chǔ)設(shè)備,工業(yè)控制系統(tǒng)，氣象儀器，汽車(chē)氣候控制和除霧，移動(dòng)計(jì)算設(shè)備。相對(duì)濕度（RH）的測(cè)量是環(huán)境檢測(cè)較困難的技術(shù)挑戰(zhàn)。濕度感測(cè)儀器通常依賴(lài)于溫度，壓力，質(zhì)量，一個(gè)機(jī)械，電的變化等物質(zhì)，當(dāng)水分被吸收，然后濕度感測(cè)儀器導(dǎo)出測(cè)量結(jié)果。機(jī)械濕度計(jì)在傳統(tǒng)上用人或動(dòng)物的毛發(fā)來(lái)測(cè)量濕度，因?yàn)轭^發(fā)的長(zhǎng)度隨濕度測(cè)量發(fā)生變化。如今，現(xiàn)代電子濕度傳感器通過(guò)測(cè)量電容或電阻或與濕氣相關(guān)的變化顯示濕度。相對(duì)濕度傳感器，特別是那些依賴(lài)于吸濕的聚合物電介體材料的電容變化進(jìn)行測(cè)量的，具有包括應(yīng)用程序和使用要求：需要電路板裝配過(guò)程中保護(hù)傳感器，特別是在焊料回流，并需要隨后補(bǔ)充水分傳感器需要保護(hù)傳感器不受損壞或污染。在產(chǎn)品的生命周期中，若長(zhǎng)時(shí)間暴露的溫度或濕度傳感器精度受影響，需要溫度校正和線(xiàn)性化適用于濕度讀數(shù)，其中的一些要求，源于電容式濕度傳感器使用的聚酰亞胺薄膜的自然特性。其它的是傳感器的開(kāi)放式腔體組件暴露的管芯和傳感器薄膜對(duì)環(huán)境的結(jié)果［1］。CMOS的制造技術(shù)使人們有可能建立一個(gè)國(guó)家的最先進(jìn)的電容式濕度傳感器，提供創(chuàng)新的，具有成本效益的封面?zhèn)鞲性７们白屑?xì)看看這些單芯片濕度傳感器，讓我們回顧一些相對(duì)濕度測(cè)量的基本原理。濕度基礎(chǔ)在空氣中發(fā)現(xiàn)的水蒸汽量相差很大，從接近零到飽和點(diǎn)。不足或過(guò)度的濕度，或在兩者之間擺動(dòng)時(shí)，可能會(huì)損壞敏感的材料和物體。人體使用蒸發(fā)冷卻作為其主要的溫度調(diào)節(jié)機(jī)制［2］。事實(shí)上，人類(lèi)感覺(jué)到從身體而不是溫度本身的熱傳遞的速率。圖1顯示了相對(duì)濕度如何影響我們的舒適性。當(dāng)濕度太高，汗液不易蒸發(fā)，人體可能會(huì)過(guò)熱，引起不適。高溫和低濕度的組合可以更有效的冷卻。露點(diǎn)溫度相對(duì)濕度在90°F人類(lèi)感知如圖1。>75°F>62%,Extremelyuncomfortable70。f-74°F52%-60%Quiteuncomfortable65°F-69。F44%-50%Somewhatuncomfortable60°F-64°F37%-42%,Comfortablebuthumid55°F-59°F1%-35%Comfortable50°F-54°F26%-30%Verycomfortable<49°F<25%Abitdry圖1RH水平的人類(lèi)感知傳統(tǒng)上，基于溫度的控制在許多環(huán)境中已被很好地應(yīng)用［3］。近年來(lái)，濕度的測(cè)量已經(jīng)越來(lái)越重要，特別是在居住，存儲(chǔ)和制造場(chǎng)所。溫度和相對(duì)濕度的控制對(duì)許多材料，包括藥物，食品，織物和木材產(chǎn)品的保存是至關(guān)重要的。不可接受的濕度水平，尤其是在與極端溫度相結(jié)合，顯著到材料的擊穿貢獻(xiàn)。熱加速惡化，以及高濕度提供水分，促進(jìn)有害的化學(xué)反應(yīng)。當(dāng)結(jié)合時(shí)，這些因素可以鼓勵(lì)昆蟲(chóng)活性和霉菌的生長(zhǎng)。非常低的相對(duì)濕度也可有破壞作用，除濕敏感材料和使它們變脆。波動(dòng)較大的溫度和濕度也會(huì)造成損害，通過(guò)膨脹和收縮，加速惡化。準(zhǔn)確的濕度測(cè)量控制濕度，可以防止損壞和不適，或檢測(cè)到儲(chǔ)存或運(yùn)輸過(guò)程中造成的產(chǎn)品損壞事件的重要因素。相對(duì)濕度傳感必須是以可用的組件形式實(shí)現(xiàn)與電子控制結(jié)合及經(jīng)濟(jì)有效的集成［4］。測(cè)量濕度的技術(shù)濕度可以量化在許多方面，但對(duì)于維持大氣質(zhì)量的最重要的測(cè)量是相對(duì)濕度（RH）。這是水蒸汽存在于空氣中的實(shí)際量，這是不能吸收任何更多的水分的比率［5］。絕對(duì)濕度被定義為水蒸汽的質(zhì)量溶解在潮濕的空氣在給定溫度和壓力下的總體積。飽和電平通常被稱(chēng)為露點(diǎn)或霜點(diǎn)，這取決于相對(duì)濕度值是否可以隨溫度細(xì)微變化或者顯著變化，在溫度變化將引起相對(duì)濕度為的變化。較高的溫度增加了空氣中吸收水分的能力和較低的溫度降低其吸收水分的能力?？諝獾南鄬?duì)濕度降低的空氣被加熱，當(dāng)潮濕的空氣被冷卻時(shí)，其吸收水分的能力降低，從而導(dǎo)致相對(duì)濕度增加。其結(jié)果是，所需要的水蒸汽中空氣的量達(dá)到使溫度露點(diǎn)升高。10°C的露點(diǎn)，例如，對(duì)應(yīng)于26%相對(duì)濕度在32°C下。最知名的儀器用于濕度測(cè)量的是干濕計(jì)，它使用“wet-bulb/dry-bulb”的方法［6］。該裝置由兩個(gè)溫度計(jì)，一個(gè)是普通的干球（干球），另用濕布覆蓋在燈泡（濕球）。從濕布蒸發(fā)時(shí)，濕球溫度計(jì)將顯示的溫度低于干球只要空氣不飽和的水蒸汽。一個(gè)查找表被用來(lái)從兩個(gè)溫度讀數(shù)推導(dǎo)出相對(duì)濕度。濕度傳感器的缺點(diǎn)包括響應(yīng)時(shí)間慢，物理尺寸精度低，確保它周?chē)臍饬髁己玫木S護(hù)問(wèn)題。用于測(cè)量當(dāng)前濕度最準(zhǔn)確的方法是冷鏡濕度計(jì)。這種技術(shù)使用一個(gè)光電機(jī)制來(lái)檢測(cè)縮合形成的溫度控制鏡面［7］。鏡子維持在一個(gè)準(zhǔn)確的測(cè)量溫度，冷卻到凝結(jié)形式。縮合散射發(fā)射LED的光，這會(huì)導(dǎo)致在接收光電晶體管的輸出突然下降。在此凝結(jié)形成提供了可用于濕度值可以計(jì)算露點(diǎn)溫度。因?yàn)樗麄兯枰臋C(jī)械系統(tǒng)，冷鏡濕度計(jì)儀器體積大，往往價(jià)格昂貴，不切實(shí)際的大批量消費(fèi)電子、汽車(chē)和住宅應(yīng)用。機(jī)械濕度計(jì)通常具有較差的精度，通常在±10%的范圍內(nèi)。最常見(jiàn)的例子使用了一塊動(dòng)物毛發(fā)張力下保持。隨著濕度的增加，頭發(fā)松弛和伸展和該長(zhǎng)度變化可以由應(yīng)變儀來(lái)測(cè)量。電子濕度傳感技術(shù)電子濕度傳感器克服了許多困擾解決了舊技術(shù)的尺寸和成本的問(wèn)題。最常用的技術(shù)依賴(lài)于任何的吸濕性材料的電阻或電容的改變。一種電容傳感器包括由電介質(zhì)材料分隔開(kāi)的兩個(gè)電極。典型地，如在空氣中增大的水蒸汽含量，傳感器的介電常數(shù)增大，改變對(duì)應(yīng)于該濕度水平所測(cè)量的電容。一個(gè)電阻傳感器包括一個(gè)導(dǎo)電層隔開(kāi)的兩個(gè)電極［8］。在這種情況下，變化的濕度導(dǎo)致的變化的傳感層的導(dǎo)電性。新技術(shù)用于生產(chǎn)薄膜，這些類(lèi)型的濕度傳感器準(zhǔn)確，穩(wěn)定，易于制造大量。吸濕材料的選擇保證了快速的響應(yīng)時(shí)間有小的滯后。例如，聚酰亞胺膜，它可以在厚度為小于5微米來(lái)制造，可以在小于10秒濕度變化的響應(yīng)，同時(shí)提供良好的穩(wěn)定性。電子濕度傳感器的精度通常隨時(shí)間漂移，通常是由溫度和濕度或污染物的存在很大的差異的限制。以提高相對(duì)濕度測(cè)量的精度，這也是有效的測(cè)量溫度，以提供給設(shè)備在必要時(shí)的相對(duì)濕度測(cè)量的溫度補(bǔ)償。以確定的露點(diǎn)或絕對(duì)濕度，也需要環(huán)境空氣溫度。例如，在測(cè)量溫度1°C錯(cuò)誤將在露點(diǎn)計(jì)算生產(chǎn)約1°C錯(cuò)誤。為獲得最佳精度，濕度和溫度的測(cè)量，應(yīng)采取盡可能接近彼此，并且理想共同位于同一芯片上。這樣的接近度可以是難以實(shí)現(xiàn)與許多傳統(tǒng)的電子傳感器的設(shè)計(jì)［9］。許多當(dāng)今的電子傳感器設(shè)計(jì)中使用分立的電阻和電容式傳感器，混合，和多芯片模塊。這些傳統(tǒng)方法因材料清單（BOM）成本和元件數(shù)量大，以及需要?jiǎng)趧?dòng)密集，客戶(hù)校準(zhǔn)高的法案。進(jìn)一步的問(wèn)題是，離散傳感器解決方案通常是用標(biāo)準(zhǔn)不兼容的表面貼裝技術(shù)（SMT）裝配流程。單片機(jī)傳感器技術(shù)傳感器必須保持清潔和完好的制造過(guò)程中，因?yàn)閭鞲衅髟仨毐槐┞对诃h(huán)境中，以執(zhí)行其功能回流焊的極端熱循環(huán)以及轉(zhuǎn)移濕度傳感器的性能，這并不總是包含在制造商的精度指標(biāo)，產(chǎn)品的生命過(guò)程中的濕度傳感器需要保護(hù)，需要使用某種類(lèi)型的覆蓋物或過(guò)濾器，它可以阻礙傳感器的響應(yīng)在一些實(shí)施方式中的先進(jìn)設(shè)備，在最先進(jìn)的傳感器解決方案［10］。該Si7005溫濕度傳感器由SiliconLabs公司解決了許多由離散，混合，模塊化和濕度傳感器系統(tǒng)所帶來(lái)的設(shè)計(jì)和制造方面的挑戰(zhàn)。該Si7005傳感器采用疏水性覆蓋材料，為傳感器的終身保障。該溫濕度傳感器有膨脹聚四氟乙烯（ePTFE）的疏水性過(guò)濾器的材料制成，可以防止灰塵和液體的大部分，并且其結(jié)構(gòu)允許水蒸汽通過(guò)它，以確保過(guò)濾器不會(huì)影響傳感器的響應(yīng)時(shí)間。因?yàn)樵赟i7005傳感器的可選的蓋被安裝在工廠(chǎng)，沒(méi)有時(shí)間或勞力花費(fèi)PCB組裝過(guò)程中添加和除去保護(hù)帶，蓋不必被設(shè)計(jì)在產(chǎn)品設(shè)計(jì)中。該Si7005使用的聚酰亞胺膜的變化來(lái)檢測(cè)濕度。此敏感薄膜被沉積在一個(gè)金屬手指的電容中。精密帶隙參考電路，位于同一芯片作為濕度傳感器，提供溫度測(cè)量。在同一裸片上檢驗(yàn)，確保溫度和濕度測(cè)量相似，提供出色的測(cè)量精度。該Si7005使用的聚酰亞胺膜的變化來(lái)檢測(cè)濕度［11。如果冷凝集上的傳感器，芯片上的加熱器可以被激活以干燥該傳感器和還原操作，一旦傳感器是在露點(diǎn)以上，管芯上的溫度傳感器確保微控制器（MCU）收集的濕度讀數(shù)可以采取這種加熱效應(yīng)。該器件的長(zhǎng)期測(cè)量漂移由于老化是不超過(guò)0.25％RH/年。，這是不到許多同類(lèi)競(jìng)爭(zhēng)器件漂流的一半。指定的精度包括津貼對(duì)回流焊的影響。作為一個(gè)整體的解決方案，Si7005也是出廠(chǎng)時(shí)校準(zhǔn)。該Si7005需要額外的單片集成來(lái)簡(jiǎn)化系統(tǒng)設(shè)計(jì)，并提供更大的模塊功能單一，緊湊的4毫米X4mmQFN封裝的優(yōu)勢(shì)。除了感測(cè)元件時(shí)，Si7005集成了一個(gè)ADC信號(hào)處理和非易失性存儲(chǔ)器來(lái)用于校準(zhǔn)的數(shù)據(jù)，以及一個(gè)I2C接口（圖2）。這種高集成度提高了耐用性和可靠性，降低了成本和開(kāi)發(fā)時(shí)間，并簡(jiǎn)化電路板設(shè)計(jì)，也有助于降低功耗。該Si7005平的平均值，1微安用于執(zhí)行一個(gè)溫度和每分鐘1濕度讀數(shù)。圖2Si7005濕度傳感器通過(guò)使用單片集成的創(chuàng)新設(shè)計(jì)，該Si7005傳感器帶來(lái)的成本效益和精確的相對(duì)濕度和溫度監(jiān)測(cè)，使其在濕度監(jiān)測(cè)和控制應(yīng)用范圍日益擴(kuò)大。參考文獻(xiàn)HUHongbin.Measuringfortemperaturecharacteristicoftemperaturerelay.ElectroMechanicalelement2003;6(9):46-48.SUNKai.Controllingtemperaturesystemofresistancestove.SensorTechnology2003;22(2):50-52.ZHONGGuomin,WANGGaiming.Asystemusedforcontrollingtemperature.Automatizationtransaction1993;19(2):223-226.CHENHua,FULihua.Applicationofcontrollingtemperatureusingfuzzycontroltheory.JournalofLiaoninguniversity1995;22(1):34-38.LIXianming.Researchforautomaticmeasuringsystemoftemperaturerelay.ElectronandAutomatization1997;(3):24-27.ShenJin,SongJingLing.AnAlldigitalTemperatureMeasuringSystemUsedinGrainBarns.TransactionofthechesesocietyforAgriculturalMachinery2001;(2):89-91.LiMinHui,JungDeqiong.ADeviceofTemperatureMeasuremenMadeupofDS1820andAT89C205.JournalofSichanNormalUniversity'1997;47(5):93-96?QiZhiCai'GaiShuang.EmbeddedControlSystemoftheCentralAirconditionerRoom,InstrumentTechniqueandSensor2002;25(5):25-26.ZhangPeirenZhouYanping.ALargeScaleTemperatureAlarmSystemBasedonWireBusandCANbus,Control&Automation簡(jiǎn)化了的濕度測(cè)量與單片機(jī)相對(duì)濕度傳感器2003;16(2):25-26..ChangFeireining-inC8051seriesmicrocontrollerdevelopmentandtheCprogramminglanguage[M].beijing:Beijinguniversityofaeronauticsandpress.2005;25:45-90..ZhaoWenBo,LiuWenTaomicrocontrollerC51programminglanguage[M].johnwiley&sonspress.2005.180~190.SimplifyingHumidityMeasurementwithSingle-ChipRelativeHumiditySensorsJanuary1,2013By:JohnGammel,SiliconLaboratoriesInc.Sensors:WithchangingindustrialscienceandAbstracttechnology,thehumiditysensingtechnologyiswidelyused.Comparedwithtraditionalmechanicalhygrometermodernelectronichumiditysensorbyhygroscopicmaterialcapacityorresistancemonitorchangesinhumidity,suchascorrelationcoefficient.Relativehumiditysensorfortemperatureandhumiditycontrol.SI7005temperatureandhumiditysensorishydrophobiccovermaterial,theuseofpolyimidetestinghumiditychanges.SI7005SintegratesaADC,usedthenonvolatilestoragetechnology,witha12cinterface,thehighlevelofintegrationtoimprovethedurabilityandreliability.Keywords：Relativehumidity;electronicsensors;humiditysensorHumiditymeasurementHumiditymeasurementisincreasinglyimportantinanexpandingnumberofindustries.Inthisarticlewediscussasingle-chiprelativehumidity(RH)sensorthatoffersrobustperformance,asmallsize,andlowpowerconsumptionandcanenableaccuratehumiditysensinginawidevarietyofapplications.Humiditysensingtechnologyiscriticaltoawiderangeofapplications,includingHVACandrefrigeration;healthcareequipment,suchasCPAPmachinesandventilators;assettrackingandstoragedevicesforthefoodandpharmaceuticalindustries;industrialcontrolsystems;meteorologicalinstruments;automotiveclimatecontrolanddefogging;andmobilecomputingdevices.Despitethenumberofindustriesandapplicationsthatrequireit,relativehumidity(RH)measurementisamongthemoredifficulttechnicalchallengesinenvironmentalsensing.Humiditysensinginstrumentstypicallyrelyonmeasurementsoftemperature,pressure,mass,oramechanicalorelectricalchangeinasubstancewhenmoistureisabsorbedfromwhichthehumiditycanthenbederived.Mechanicalhygrometers(Figure1)havetraditionallyusedhumanoranimalhairtomeasurehumiditybecausethelengthofthehairchangesmeasurablywithincreasinghumidity.Nowadays,modernelectronichumiditysensorsoperatebymeasuringthemoisture-relatedchangesinelectricalcapacitanceorresistanceorthetemperatureatwhichcondensationappears[1].RHsensors,inparticularthosethatrelyonmeasurementsofthecapacitancechangesofhygroscopicpolymericdielectricmaterials,haveapplicationandusagerequirementsthatinclude:Theneedtoprotectthesensorduringboardassembly,especiallyduringsolderreflow,andtheneedtosubsequentlyrehydratethesensor.[2].TheneedtoprotectthesensorfromdamageorcontaminationduringtheproductlifecycleThepotentialimpactonsensoraccuracyofprolongedexposuretoextremesoftemperatureand/orhumidity.Theneedtoapplytemperaturecorrectionandlinearizationtothehumidityreadings.SomeoftheserequirementsstemfromthenaturalcharacteristicsofthepolyimidefilmsusedincapacitiveRHsensors.Othersarearesultofthesensor'sopen-cavitypackagethatexposesthedieandthesensorfilmtotheenvironment.CMOSmanufacturingtechniqueshavemadeitpossibletobuildstate-of-the-artcapacitivehumiditysensorsthatprovideinnovative,cost-effectivecoversforthesensingelement.Beforetakingacloserlookatthesesingle-chiphumiditysensors,let'sreviewsomeofthebasicprinciplesofRHmeasurement[4].FundamentalsofHumidityTheamountofwatervaporfoundinaircanvarydramatically,fromclosetozerotothepointofsaturation.Insufficientorexcessivehumidity,orswingsbetweenthetwo,candamagesensitivematerialsandobjects.Thehumanbodyusesevaporativecoolingasitsprimarytemperature-regulationmechanism.Inactualfact,humansfeeltherateofheattransferfromthebodyratherthantemperatureitself.Figure2showshowtherelativehumidityaffectsourcomfort.Whenhumidityissohighthatperspirationcannoteasilyevaporate,thebodymayoverheat,causingdiscomfort.AcombinationofhightemperatureandlowRHallowsmoreeffectivecooling.Figure1.HumanperceptionofRHlevelsDewPointTemperatureRHat90°FHumanPerception>75°F>62%Extremelyuncomfortable70°F-74°F52%-60%Quiteuncomfortable65°F-69°F44%-50%Somewhatuncomfortable60°F-64°F37%-42%Comfortablebuthumid55°F-59°F31%-35%Comfortable50°F-54°F26%-30%Verycomfortable<49°F<25%AbitdryTraditionally,manyenvironmentshavebeencontrolledbasedontemperature.Inrecentyears,themeasurementofhumidityhasgrowninimportance,especiallyinliving,storage,andmanufacturingsites.ControloftemperatureandRHisalsocriticalinthepreservationofmanymaterialsincludingmedications,foods,fabrics,andwoodproducts[5.]Unacceptablehumiditylevels,especiallywhencombinedwithtemperatureextremes,contributesignificantlytothebreakdownofmaterials.Heatacceleratesdeterioration,andhighRHprovidesmoisture,whichpromotesharmfulchemicalreactions.Whencombined,thesefactorscanencourageinsectactivityandthegrowthofmold.ExtremelylowRHcanalsohavedamagingeffects,desiccatingsensitivematerialsandcausingthemtobecomebrittle.LargefluctuationsintemperatureandRHalsocausedamagethroughexpansionandcontraction,acceleratingdeterioration.Accuratehumiditymeasurementisavitalpartofcontrollinghumiditytopreventdamage,discomfort,ortodetecteventsthatmayhavecausedproductdamageduringstorageortransit.Forwidespreaduse,RHsensingmustbeavailableinacomponentformthatenableseasy,cost-effectiveintegrationwithelectroniccontrols[6].TechniquesforMeasuringHumidityHumiditycanbequantifiedinanumberofways,butthemostimportantmeasurementformaintainingatmosphericqualityisrelativehumidity(RH).Thisistheratiooftheactualwatervaporpresentinairtotheamountofwatervaporpresentinsaturatedair,whichcannotabsorbanymoremoisture.Absolutehumidityisdefinedasthemassofwatervapordissolvedinatotalvolumeofmoistairatagiventemperatureandpressure[7].Thesaturationlevelisgenerallycalledthedewpointorfrostpoint,dependingonthetemperature.TheRHvaluecanchangesignificantlywithevenslightvariationsintemperature;a1°Cchangeintemperatureat35°Cand75%RHwillintroducea4%changeinRH.Ahighertemperatureincreasestheabilityofairtoabsorbmoistureandalowertemperaturedecreasesitsabilitytoabsorbmoisture.TheRHofairdecreasesastheairisheated;whenmoistairiscooled,itscapacitytoabsorbmoisturedecreases,causingtheRHtoincrease.Asaresult,theamountofwatervaporinairneededtoreachthedewpointincreaseswithtemperature.Thedewpointat10°C,forexample,correspondstoaRHof26%at32°C.Thebest-knowninstrumentforhumiditymeasurementisthepsychrometer,whichusesthe"wet-bulb/dry-bulb"method.Thedeviceconsistsoftwothermometers,onewithanordinarydrybulbandtheotherwithamoistclothcoveringthebulb(thewetbulb).Asevaporationfromthemoistclothoccurs,thewet-bulbthermometerwillshowalowertemperaturethanthedrybulbaslongastheairisnotsaturatedwithwatervapor[8.]Alook-uptableisusedtoderivetheRHfromthetwotemperaturereadings.Thedisadvantagesofapsychrometricsensorincludeslowresponsetime,largephysicalsize,andthemaintenanceissuesofkeepingonethermometerbulbwetandensuringgoodairflowaroundit.Thecurrentmostaccuratemethodformeasuringhumidityisthechilledmirrorhygrometer.Thistechniqueusesanoptoelectronicmechanismtodetectcondensationthatformsonatemperature-controlledmirrorsurface.Themirrorismaintainedatanaccuratelymeasuredtemperatureandcooleduntilcondensationforms.ThecondensationscattersthetransmittingLED'slight,whichresultsinasuddendropintheoutputofthereceivingphototransistor.Thetemperatureatwhichcondensationformsprovidesthedewpointfromwhichthehumidityvaluecanbecalculated.Becauseofthemechanicalsystemstheyrequire,chilled-mirrorhygrometerinstrumentsarebulky,oftenexpensive,andimpracticalforuseinhigh-volumeconsumer,automotive,andresidentialapplications.Mechanicalhygrometerstypicallyexhibitpooraccuracy—oftenintherangeof±10%.Themostcommonexampleusesapieceofanimalhairkeptundertension.Ashumidityincreases,thehairrelaxesandstretchesandthislengthchangecanbemeasuredbyastraingauge.ElectronicHumiditySensingTechnologyElectronichumiditysensorsovercomemanyofthesizeandcostproblemsthatplagueoldertechniques.Themostcommonlyemployedtechniquesrelyeitheronachangeintheresistanceorcapacitanceofahygroscopicmaterial.Acapacitivesensorconsistsoftwoelectrodesseparatedbyadielectricmaterial.Typically,asthewatervaporcontentintheairincreases,thesensor'sdielectricconstantincreases,changingthemeasuredcapacitancecorrespondingtothehumiditylevel.Aresistivesensorconsistsoftwoelectrodesseparatedbyaconductivelayer.Inthiscase,variationsinhumidityresultinchangesintheconductivityofthesensinglayer[9.]NewtechniquesforproducingthinfilmshavemadethesetypesofRHsensorsaccurate,stableandeasytomanufactureinlargequantities.Thechoiceofhygroscopicmaterialassuresfastresponsetimeswithlittlehysteresis.Forinstance,apolyimidefilm,whichcanbefabricatedinthicknessesof<5pm,canrespondtochangesinhumidityin<10swhileprovidingexcellentstability.TheaccuracyofanelectronicRHsensorislimitedbyitsdriftovertime,generallycausedbywidevariationsintemperatureandhumidityorthepresenceofpollutants.ToenhancetheaccuracyofRHmeasurements,itisalsohelpfultomeasurethetemperaturetoprovidetemperaturecompensationfortheRHmeasurementofthedeviceifnecessary.Todeterminethedewpointorabsolutehumidity,theambientairtemperatureisalsorequired.Forinstance,a1°Cerrorinthemeasuredtemperaturewillproduceapproximatelya1°Cerrorinthedew-pointcalculation.Forthebestaccuracy,humidityandtemperaturemeasurementsshouldbetakenascloseaspossibletoeachother,andideallyco-locatedonthesamechip.SuchproximitycanbedifficulttoachievewithmanytraditionalelectronicsensordesignsManyoftoday'selectronicsensordesignsusediscreteresistiveandcapacitivesensors,hybrids,andmulti-chipmodules,asshowninFigure3.Theselegacyapproachessufferfromhighbillofmaterialscostsandcomponentcounts,largefootprints,andtheneedforlabor-intensivecustomercalibration[10].Afurtherproblemisthatdiscretesensorsolutionsareoftenincompatiblewithstandardsurfacemounttechnologyassemblyflows.AmonolithicsensorAmonolithicsensorstillhastocontendwiththefollowingmanufacturingissues:ThesensormustbekeptcleanandundamagedduringmanufacturingbecausethesensorelementmustbeexposedtotheenvironmenttoperformitsfunctionTheextremethermalcycleofsolderreflowcanshifttheperformanceofhumiditysensors,aneffectthatisnotalwaysincludedinmanufacturers'accuracyspecificationsThehumiditysensorrequiresprotectionduringthelifeoftheproduct,requiringtheuseofsometypeofcoverorfilter,whichcanimpedesensorresponsivenessinsomeimplementations.State-of-the-ArtSensorSolutionTheSi7005temperatureandhumiditysensorfromSiliconLabsaddressesmanyofthedesignandmanufacturingchallengesposedbydiscrete,hybrid,andmodularhumiditysensorsystems.TheSi7005sensorusesahydrophobiccovermaterialtoprovidelifelongprotectionforthesensorunderneath.Thecover,madefromanexpandedpolytetrafluoroethylenehydrophobicfiltermaterial,protectsagainstdustandmostliquids,anditsstructureallowswatervaportopassthroughit,ensuringthatthefilterdoesnotaffectsensorresponsetime.SincetheoptionalcoverontheSi7005sensor(showninFigure4)isinstalledatthefactory,notimeorlaborisspentaddingandremovingprotectivetapeduringPCBassembly,andthecoverdoesnothavetobeengineeredintotheproductdesign[11].TheSi7005usesapolyimidefilmtodetectchangesinhumidity.Thisthin,sensitivefilmisdepositedoverametalfingercapacitor.Aprecisionbandgap-referencedcircuit,locatedonthesamedieasthehumiditysensor,providestemperaturemeasurement.Co-locationonthesamedieensuresthatbothtemperatureandhumidityaremeasuredincloseproximity,providingexceptionalmeasurementaccuracy.TheSi7005usesapolyimidefilmtodetectchangesinhumidity.Thisthin,sensitivefilmisdepositedoverametalfingercapacitor.Aprecisionbandgap-referencedcircuit,locatedonthesamedieasthehumiditysensor,providestemperaturemeasurement.Co-locationonthesamedieensuresthatbothtemperatureandhumidityaremeasuredincloseproximity,providingexceptionalmeasurementaccuracy.Ifcondensationgathersonthesensor,anon-chipheatercanbeactivatedtodrythesensorandrestoreoperationoncethesensorisabovethedewpoint.Theon-dietemperaturesensoralsoensuresthatthemicrocontroller(MCU)thatcollectsthehumidityreadingcantakethisheatingeffectintoaccount.Thedevice'slong-termmeasurementdriftduetoagingisnomorethan0.25%ofRH/yr.,whichislessthanhalfthedriftofmanycompetingdevices.Thespecifiedaccuracyincludesanallowancefortheeffectsofsolderreflow.Asamonolithicsolution,theSi7005isalsofactorycalibrated.TheSi7005takesadvantageofadditionalmonolithicintegrationtoeasesystemdesignandtoprovidethefunctionsofmuchlargermodulesinasingle,compact4mmx4mmQFNpackage.Inadditiontothesensingelements,theSi7005integratesanADC,signalprocessing,nonvolatilememoryforcalibrationdata,andanI2Cin