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)翻譯譯文.docx

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January 1, 2013By: John Gammel, Silicon Laboratories Inc.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。SI7005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1 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?。服用前仔?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。2 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，人類感覺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90F人類感知如圖1。75F62，Extremely uncomfortable70F-74F52-60Quiteuncomfortable65F-69F44-50Somewhat uncomfortable60F-64F37-42，Comfortable but humid55F-59F1-35Comfortable50F-54F26-30Very comfortable49F75F62%Extremely uncomfortable70F74F52%60% Quiteuncomfortable65F69F44%50%Somewhat uncomfortable60F64F37%42%Comfortable but humid55F59F31%35%Comfortable50F54F26%30%Very comfortable49F25%A bit dryTraditionally, many environments have been controlled based on temperature. In recent years, the measurement of humidity has grown in importance, especially in living, storage, and manufacturing sites. Control of temperature and RH is also critical in the preservation of many materials including medications, foods, fabrics, and wood products5.Unacceptable humidity levels, especially when combined with temperature extremes, contribute significantly to the breakdown of materials. Heat accelerates deterioration, and high RH provides moisture, which promotes harmful chemical reactions. When combined, these factors can encourage insect activity and the growth of mold. Extremely low RH can also have damaging effects, desiccating sensitive materials and causing them to become brittle. Large fluctuations in temperature and RH also cause damage through expansion and contraction, accelerating deterioration.Accurate humidity measurement is a vital part of controlling humidity to prevent damage, discomfort, or to detect events that may have caused product damage during storage or transit. For widespread use, RH sensing must be available in a component form that enables easy, cost-effective integration with electronic controls6.3 Techniques for Measuring HumidityHumidity can be quantified in a number of ways, but the most important measurement for maintaining atmospheric quality is relative humidity (RH). This is the ratio of the actual water vapor present in air to the amount of water vapor present in saturated air, which cannot absorb any more moisture. Absolute humidity is defined as the mass of water vapor dissolved in a total volume of moist air at a given temperature and pressure7.The saturation level is generally called the dew point or frost point, depending on the temperature. The RH value can change significantly with even slight variations in temperature; a 1C change in temperature at 35C and 75% RH will introduce a 4% change in RH. A higher temperature increases the ability of air to absorb moisture and a lower temperature decreases its ability to absorb moisture. The RH of air decreases as the air is heated; when moist air is cooled, its capacity to absorb moisture decreases, causing the RH to increase. As a result, the amount of water vapor in air needed to reach the dew point increases with temperature. The dew point at 10C, for example, corresponds to a RH of 26% at 32C.The best-known instrument for humidity measurement is the psychrometer, which uses the wet-bulb/dry-bulb method. The device consists of two thermometers, one with an ordinary dry bulb and the other with a moist cloth covering the bulb (the wet bulb). As evaporation from the moist cloth occurs, the wet-bulb thermometer will show a lower temperature than the dry bulb as long as the air is not saturated with water vapor8. A look-up table is used to derive the RH from the two temperature readings. The disadvantages of a psychrometric sensor include slow response time, large physical size, and the maintenance issues of keeping one thermometer bulb wet and ensuring good airflow around it.The current most accurate method for measuring humidity is the chilled mirror hygrometer. This technique uses an optoelectronic mechanism to detect condensation that forms on a temperature-controlled mirror surface. The mirror is maintained at an accurately measured temperature and cooled until condensation forms. The condensation scatters the transmitting LEDs light, which results in a sudden drop in the output of the receiving phototransistor. The temperature at which condensation forms provides the dew point from which the humidity value can be calculated. Because of the mechanical systems they require, chilled-mirror hygrometer instruments are bulky, often expensive, and impractical for use in high-volume consumer, automotive, and residential applications.Mechanical hygrometers typically exhibit poor accuracyoften in the range of 10%. The most common example uses a piece of animal hair kept under tension. As humidity increases, the hair relaxes and stretches and this length change can be measured by a strain gauge.4 Electronic Humidity Sensing TechnologyElectronic humidity sensors overcome many of the size and cost problems that plague older techniques. The most commonly employed techniques rely either on a change in the resistance or capacitance of a hygroscopic material. A capacitive sensor consists of two electrodes separated by a dielectric material. Typically, as the water vapor content in the air increases, the sensors dielectric constant increases, changing the measured capacitance corresponding to the humidity level. A resistive sensor consists of two electrodes separated by a conductive layer. In this case, variations in humidity result in changes in the conductivity of the sensing layer9.New techniques for producing thin films have made these types of RH sensors accurate, stable and easy to manufacture in large quantities. The choice of hygroscopic material assures fast response times with little hysteresis. For instance, a polyimide film, which can be fabricated in thicknesses of 5 m, can respond to changes in humidity in 10 s while providing excellent stability. The accuracy of an electronic RH sensor is limited by its drift over time, generally caused by wide variations in temperature and humidity or the presence of pollutants.To enhance the accuracy of RH measurements, it is also helpful to measure the temperature to provide temperature compensation for the RH measurement of the device if necessary. To determine the dew point or absolute humidity, the ambient air temperature is also required. For instance, a 1C error in the measured temperature will produce approximately a 1C error in the dew-point calculation. For the best accuracy, humidity and temperature measurements should be taken as close as possible to each other, and ideally co-located on the same chip. Such proximity can be difficult to achieve with many traditional electronic sensor designsMany of todays electronic sensor designs use discrete resistive and capacitive sensors, hybrids, and multi-chip modules, as shown in Figure 3. These legacy approaches suffer from high bill of materials costs and component counts, large footprints, and the need for labor-intensive customer calibration10. A further problem is that discrete sensor solutions are often incompatible with standard surface mount technology assembly flows.5 A monolithic sensor A monolithic sensor still has to contend with the following manufacturing issues:The sensor must be kept clean and undamaged during manufacturing because the sensor element must be exposed to the environment to perform its functionThe extreme thermal cycle of solder reflow can shift the performance of humidity sensors, an effect that is not always included in manufacturers accuracy specificationsThe humidity sensor requires protection during the life of the product, requiring the use of some type of cover or filter, which can impede sensor responsiveness in some implementations.State-of-the-Art Sensor SolutionThe Si7005 temperature and humidity sensor from Silicon Labs addresses many of the design and manufacturing challenges posed by discrete, hybrid, and modular humidity sensor systems. The Si7005 sensor uses a hydrophobic cover material to provide lifelong protection for the sensor underneath. The cover, made from an expanded polytetrafluoroethylene hydrophobic filter material, protects against dust and most liquids, and its structure allows water vapor to pass through it, ensuring that the filter does not affect sensor response time. Since the optional cover on the Si7005 sensor (shown in Figure 4) is installed at the factory, no time or labor is spent adding and removing protective tape during PCB assembly, and the cover does not have to be engineered into the product design11.The Si7005 uses a polyimide film to detect changes in humidity. This thin, sensitive film is deposited over a metal finger capacitor. A precision bandgap-referenced circuit, located on the same die as the humidity sensor, provides temperature measurement. Co-location on the same die ensures that both temperature and humidity are measured in close proximity, providing exceptional measurement accuracy.The Si7005 uses a polyimide film to detect changes