Development of humidity sensors
In recent years, the country in the Humidity sensor gained rapiddevelopment of areas to improve. Humidity sensor humidity sensor isfrom the easy to the integrated, intelligent, multi-parameterdetection of the direction in which to carry out, for thedevelopment of a new generation of humidity / temperaturemeasurement and control bits required to create a favorable,humidity measurement techniques will also be advanced to a newlevel.
1, the characteristics of humidity sensor
Humidity sensor humidity sensor is theeasiest. Important resistance humidity sensor, capacitive twocategories.
1.1 humidity resistance
Humidity resistance is characterized by alayer on the substrate covered with a film being made of wetmaterials, when the adsorbed water vapor in the air when thehumidity sensitive film, the resistivity and the resistancecomponent value changes, the application of this feature canmeasure the humidity. Humidity resistance of many varieties, suchas metal oxide special humidity resistance, humidity resistance ofsilicon, ceramic humidity resistance and so on. Humidity resistanceof the strengths is the high sensitivity, linearity and importantflaw is the product of the interchangeability of difference.
1.2 Humidity Capacitance
Humidity Capacitance general is made ofpolymer film capacitors, polymer materials commonly used arepolystyrene, polyimide, cellulose acetate acid and so cool. Whenchanges occur when the ambient humidity, humidity capacitance ofthe dielectric constant change, it also changes the capacitance,the capacitance is inversely proportional to the change in absolutehumidity. Humidity Capacitance important strengths is the highsensitivity and product interchangeability, echo rate, and humidityhysteresis is small, easy to manufacture, easy to complete theminiaturization and integration, the accuracy of ordinary littlelower than the humidity resistance. The main foreign consumershumidity capacitor manufacturers have Humirel company, PhilipsCompany, Siemens Corporation. To Humirel company's consumer'sSH1100 -type humidity sensor capacitor, for example, themeasurement is ( 1% ~ 99% ) RH , at 55% RH when the capacitance of180pF (typical value). When the absolute humidity from 0 change to100% when the change in capacitance area is 163pF ~ 202pF .Temperature coefficient of 0.04pF / ℃ , humidity hysteresis is ±1.5% , echo time for 5S .
In addition to resistive, capacitive humiditysensor, there are ionic-type humidity sensor, humidity sensorcomponent type (application wet film a sense of the change ofweight change oscillation frequency), intensity-based humiditymillion pieces, sound appearance wave humidity sensor and so on.Humidity sensor linearity and purification of anti-poor, in thedetection of humidity, the humidity sensor to the provisionaldisclosure in the tested environment, it is easy to be purified toaffect the measurement accuracy and temporary stability of the.
2 features an integrated humidity sensor andthe product category
Currently, consumption abroad integratedhumidity sensor manufacturers and typical product of the importanceof identifying the Honeywell Corporation ( HIH-3602 , HIH-3605 ,HIH-3610 -type), Humirel Company ( HM1500 , HM1520 , HF3223 ,HTF3223 type), Sensiron Company ( SHT11 , SHT15 type). Theseproducts can be participating in three varieties of type:
2.1 The integrated linear voltage input typehumidity sensor
Typical products are HIH3605/3610 ,HM1500/1520 . The important feature is the use of constant voltagepower supply, built-in narrow circuit, the input and the absolutehumidity can be proportional to the V-level voltage signal, echoand rate, repeating well, anti-purification can be strong.
2.2 linear frequency input integratedhumidity sensor
Typical product of the HF3223 type. It usesmodular construction, is the frequency integrated humidity sensorinput type, in 55% RH when the input frequency of 8750Hz (typevalue), when the humidity from 10% of the change to 95% when theinput frequency from the 9560Hz is reduced to 8030Hz .
This sensor has good linearity with anti-disturb to strong, easywith digital circuit or microcontroller, the price of higherstrengths.
2.3 Frequency / temperature input typehumidity sensor integrated
Typical product of the HTF3223 type. Itaddition to HF3223 effect, it also increased the temperature signalinput, the application of negative temperature coefficient ( NTC )thermistor as a temperature sensor. When the ambient temperature,its resistance changes and the corresponding values from the NTCside leads, coupled with the second instrument can measure thetemperature.
3 single-chip intelligent temperature /optical fiber temperature sensor
2002 in Sensiron company successfullydeveloped the world's first SHT11 , SHT15 type of intelligenttemperature / temperature sensor, its dimensions of only 7.6 ( mm )× 5 ( mm ) × 2.5 ( mm ), volume and Huo Chaitou similar. Factory,each sensor has done fine in the room temperature specifications,standardized coefficients are compiled into the program stored inthe corresponding calibration memory , in the measurement processcan take the initiative to implement the calibration of absolutehumidity. They are not only accurate measurement of absolutetemperature, but also to measure the temperature and dew point. Thearea is the absolute temperature measurement 0 ~ 100% , resolutionup to 0.03% RH , maximum precision of ± 2% RH . Measuring thetemperature field is -40 ℃ ~ +123.8 ℃ , resolution is 0.01 ℃ . Dewpoint measurement accuracy of <± 1 ℃ . In the measurement ofhumidity, temperature and degree A / D converter bits to identifyup to 12 bits, 14 bits. Application approach can reduce theresolution rate of progress in measuring, reducing chip powerconsumption.SHT11/15 product interchangeability, echo rate, andstrong anti-disturb can not demand for internal components, fit allkinds of microcontroller, commonly used in medical devices andtemperature / humidity conditioning in piecemeal.
Chip, including the absolute humidityexternal sensors, temperature sensors, reducing device, 14 -bit A /D converter, the calibration memory ( E2PROM ), volatile memory (RAM ) is shaped state storage device, cyclic redundancy check code( CRC ) storage devices, wire serial interface, control unit,heater and low voltage detection circuit. The measurement principleis the first application of two sensors identify the occurrence ofabsolute humidity, temperature, signal, and then through narrow,identify sent to A / D converter implemented modules / digitalconversion, calibration and error correction, and ultimatelythrough the absolute humidity-wire serial interface and temperaturedata to the μC . Given SHT11/15 enter the absolute humidityabsolute humidity readings and the measured non-linear relationshipbetween the absolute humidity to obtain accurate data, thenecessary application μC practice of reading the value ofnon-linear compensation. In addition, when the ambient temperatureTA ≠ +25 ℃ , they also demand the implementation of the absolutehumidity sensor temperature compensation.
A heater outside the chip. Will form thefirst storage device 2 position "1" when the heater power, thesensor can reduce the temperature of about 5 ℃ , the supply currentalso increased 8mA (with +5 V power supply). Heater can be used tocomplete the following three effects: ① measured before and afterheating through the analogy of absolute humidity and temperaturecan be determined sensing device can normal tasks; ② use heaters inwet conditions to prevent condensation sensor; ③ demand for usewhen measuring the dew point heater.
Humidity, dew point measurement is a criticalparameter, which indicates that the cooling process, water vaporcondensation occurs in the final temperature. In order to calculatethe dew point, Sensirion company also provided to the players for ameasurement of the dew point of the program "SHTxdp.bsx" .Applications can control the program through an external heater,off, and then follow the measured values of temperature andabsolute humidity, dew point is calculated. Echoed in the commandinterface of this program running on the computer screen displaysto remind the operator to ">" . First, the output from thekeyboard player on the letter "S" , then output the correspondingnumber, you can obtain the following consequences:
Output number "1" when the Celsiustemperature measurements and display the DGC = XX.X ; output number"2" , measure and demonstrate the absolute humidity % RH = XX.X ;output number "3" , open the heater, the sensor temperaturedecreases 5 ℃ ; output digital "4" when closed heater, the sensortemperature; output figures "5" , the display dew point temperaturedpc = XX.X .
4, a typical product of an integrated humidity sensor targetskills
Integrated humidity sensor measurement can bereached by ordinary 0 ~ 100% . However, some manufacturers will beto ensure the accuracy of measurement field goals tied for the 10%~ 95% . Let total +3.3 V high voltage for electrical humidity /temperature test bits, the choice of SHT11 , SHT15 sensors. Thesensors measure the phase of the mission in the current of 550μA ,even the task current 28μA ( 12 bit) or 2μA ( 8 bits). Acquiescedto sleep when the power is in the form ( SleepMode ), supplycurrent of only 0.3μA (typical). Just do not measure the end of thenew command to take the initiative to go to sleep in the form, makethe chip power consumption to a minimum. In addition, they alsohave low voltage detection effect. When the supply voltage is lowerthan +2.45 V ± 0.1V , the form of storage device in the first 6-bit immediate update, the chip is not the task, and thus play asecurity role in protection.