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Prepared by: Roger Packham Dept. of Electrical and Computer Engineering Utah State University

ECE5320 Mechatronics Assignment#01: Literature Survey on Sensors and Actuators Topic: Humidity Sensor. Prepared by: Roger Packham Dept. of Electrical and Computer Engineering Utah State University E: rpackham@cc.usu.edu ; F: (435)797-3054 (ECE Dept.) W: http://cc.usu.edu/~rpackham.

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Prepared by: Roger Packham Dept. of Electrical and Computer Engineering Utah State University

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  1. ECE5320 MechatronicsAssignment#01: Literature Survey on Sensors and Actuators Topic: Humidity Sensor Prepared by: Roger Packham Dept. of Electrical and Computer Engineering Utah State University E: rpackham@cc.usu.edu ; F: (435)797-3054 (ECE Dept.) W: http://cc.usu.edu/~rpackham 3/11/2005

  2. Outline • Reference list • To probe further • About Humidity • Major applications • Basic working principle illustrated • A typical sample configuration in application (application notes) • Major specifications • Limitations • Relevant issues in applications (such as, how to choose, cost information, calibration.) ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

  3. Reference list • Smartec - http://www.smartec.nl/ • Inexpensive analog sensor, useful application tools and information. • Humirel - http://www.humirel.com/ • Variety of analog and digital output sensors. • Honeywell - http://content.honeywell.com/sensing/prodinfo/humiditymoisture/ • Good technical documentation and high quality sensors. • Sensirion - http://www.sensirion.com/en/sensors/humidity/default.htm • Compact digital output sensor packages. • Clark Solutions - http://www.clarksol.com/html/prodspecsFNA.htm • Handheld pshychrometric temperature and humidity units. • Sensor Magazine - http://www.sensorsmag.com/articles/0701/54/main.shtml Note: Pictures have origin noted in the properties window ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

  4. To Explore Further • Honeywell -provides many specific details and test data for their capacitive humidity sensors. http://content.honeywell.com/sensing/prodinfo/humiditymoisture/ • Sensor Magazinearticle discusses resistive and thermal sensors in depth. Useful article on choosing humidity sensor technology and devices.http://www.sensorsmag.com/articles/0701/54/main.shtml • Sensor Magazinearticle documents a one-wire miniature weather station. http://www.sensorsmag.com/articles/0800/62/index.htm ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

  5. About Humidity • RELATIVE HUMIDITY (%RH) is most commonly used. http://www.humirel.com/All_about_humidity.htm ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

  6. Calculating Humidity • The RELATIVE HUMIDITY is defined as the ratio of the partial pressure vapor to saturation vapor pressure at a given temperature. By measuring the relative humidity, we compare the actual quantity of water vapor to the maximum quantity of water vapor which can be found in these conditions of ambient temperature and pressure.DEW POINT is the temperature at which the wet gaseous atmosphere can not be held in equilibrium in its gaseous form only. It will characterized by the emergence of a liquid phase.ABSOLUTE HUMIDITY is the water vapor concentration in the air. This concentration value is either linked with the volume (in kg/m3, or g/m3 or grains/ft3) or linked with the mass (in kg/kg or g/kg or lb/lb). In the latter case, it will be named the MIXING RATIO. • Using the AMBIENT TEMPERATURE and one of the above three values, the other two can be calculated. http://www.humirel.com/All_about_humidity.htm ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

  7. Major applications • Office automation, room comfort control • Automotive cabin air control • Home appliances, Air conditioners • Industrial process control systems. Storage and warehouses • Climate control for green houses • Meteorological applications • Food processing • Medical applications • Chicken Coops, Pig Barns ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

  8. Measuring Methods • Capacitive - absorption of water into dielectric. • Small, dependable • Accurate over wide temperature range without compensation. • Resistive - measure the change in electrical impedance of a hygroscopic medium such as a conductive polymer, salt, or treated substrate. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

  9. Methods (cont. ) • Thermal Conductive - measure the absolute humidity by quantifying the difference between the thermal conductivity of dry air and that of air containing water vapor. • Psychrometric - cotton wicks water to make a wet bulb temperature sensor. An internal fan blows air across the wet bulb sensor, causing the moisture to evaporate, cooling the sensor in contrast to the dry one. • Handheld units. • Time tested, measure multiple ambient statistics at once. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

  10. Capacitive • Capacitance increases in value as water molecules are absorbed into its active polymer dielectric. The capacitor plates consists of a base plate and a water permeable platinum top plate. Polymers give protection to the base plate. • $10 to $68 • Substrate can include signal conditioning circuitry to produce a linear output, or a digital interface. • Thermoset-polymer (faster, more robust) or Thermoplastic ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

  11. Application Notes • Special attention is required in order to minimize stray capacitance in the layout. The added capacitance will act as a parallel capacitance with the sensor and create a measurement error (+/- 6% with +25pF). • Capacitive sensors are limited by the distance the sensing element can be located from the signal conditioning circuitry, due to the capacitive effect of the connecting cable with respect to the relatively small capacitance changes of the sensor. A practical limit is <10 ft. • Laser trimmed sensors are interchangeable, not needing calibration. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

  12. Resistive • Many different kinds: • Acid or salt on electrodes • 10 s to 30 s response. • Field interchangeable. • Life >> 5 yrs. • Dunmore • Originally developed in 1940s • Can achieve 0.1%RH resolution over a narrow band. • Ceramic coating • New technology allows clear barrier for protection. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

  13. Thermal Conductive • Measure Absolute Humidity • When air or gas is dry, it has a greater capacity to“sink” heat. Gets reference value from a thermistor sealed in dry air • Can withstand Temperatures >200°F http://www.sensorsmag.com/articles/0701/54/main.shtml ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

  14. Thermal Sensor Properties • Resistant to chemical vapors by virtue of the inert materials used for their construction, i.e., glass, semiconductor material for the thermistors, high-temperature plastics, or aluminum. • Calibration is performed by placing the sensor in moisture-free air or nitrogen and adjusting the output to zero. • Absolute humidity sensors are very durable, operating at temperatures up to 575°F (300°C) – Dryers, cooking, dehydration. • Since one of the by-products of combustion and fuel cell operation is water vapor, particular interest has been shown in using absolute humidity sensors to monitor the efficiency of those reactions. http://www.sensorsmag.com/articles/0701/54/main.shtml ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

  15. Typical Configuration - The equivalent capacitor is charged through R2 and R4 to the threshold voltage (~ 0.67Vcc) and discharged through R2 only to the trigger level (~ 0.33Vcc), measuring oscillations are provided by the 555 timer. http://www.humirel.com/product/fichier/HS1101-HS1100.pdf ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

  16. Application Notes (cont.) • Compatible with automatized assembly processes, including wave soldering, reflow and water immersion. • Increased accuracy, precision available (i.e. high humidity accuracy), including fully linear or digital output. • Some sensors need to be shielded from bright light. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

  17. Specifications • Electrical: (Low end, High end) • Operating humidity range 0 - 100 %RH condensing/noncondensing • Operating temperature range -40 to 120 ºC-60 to 160 ºC • Capacitance value @ 25 ºC, 60 %RH, 100 KC 240 pF ±15 % • Capacitance range over 0-100 % RH 40 pF ±10 % • Temperature dependency -0.15% RH/ºC • Response time top 90% 60 s.5 s. • Linearity 2% RH ± .5% (no need to calibrate) • Stability over time(12 month) ± 3 % RH± .5 % RH • Storage: • Humidity range 0 – 100 %RH • Temperature -20 – 85 ºChttp://www.smartec.nl/humidity_sensor.htm ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

  18. Condensation • When an application calls for continuous monitoring of RH at humidity levels of 90% and above, take steps to avoid intermittent condensation. Some strategies are: 1. Maintain a good air mixing to minimize local temperature fluctuations. 2. The HIH-3602-A and -C use a sintered stainless steel filter to protect the sensor from splashing. A hydrophobic coating further suppresses condensation and wetting in rapidly saturating and de-saturating or splash prone environments. 3. Heat the RH sensor so that the active area is hotter than the local dew point. This can be done through an external heater or by self heating of the CMOS RH chip by operating it at a higher supply voltage. NOTE: Heating an RH sensor above ambient temperature changes its calibration and makes it sensitive to thermal disturbances such as air flow. When contemplating such an approach, MICRO SWITCH recommends selecting an HIH-3602 type sensor and getting application technical support. http://content.honeywell.com/sensing/prodinfo/humiditymoisture/ ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

  19. Method for Calibration • Climate Chamber • large, expensive, slow. • Salt Solutions: much better! • cheap, low cost, reasonably fast (30 min) data. • Special saturated salt solutions always generate predictable humidity levels in the air above the water. • This humidity measurement depends only on temperature. http://www.smartec.nl/pdf/apphs1002.pdf ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

  20. Salt Solutions and Method • Mix the solution into a glass container with a sealable top. A few cm of solution will effect the desired humidity, dependant only on temperature. • Place the sensor into the air above the solution and close the access hole carefully. Allow the sensor to stabilize for half an hour and record the value. (Smartec, 2001) - Be aware that the temperature of the container needs to be constant over the period, therefore it is recommended to use a well-isolated glass container. - Two point calibration - in general LiCl and NaCl solutions are used. - Three point calibration - LiCl, Mg(NO3) and K2SO4 solutions are recommended. http://www.smartec.nl/pdf/apphs1002.pdf ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

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