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Humidity sensor suits windshield anti-fogging apps

Posted: 30 Oct 2006 ?? ?Print Version ?Bookmark and Share

Keywords:Sensata? sensors? humidity? capacitive sensing? automotive?

Sensata's humidity sensor

Sensata Technologies announced that its capacitive-based humidity sensors with integrated temperature function, for determining relative humidity in climate control and windshield anti-fogging applications, responds five times faster with twice the initial accuracy of traditional resistive-sensor technologies.

"The value of a more accurate sensor with a significantly faster response time will be apparent to anyone who has struggled with windshield defogging while continuously fiddling with the thermostat controls to get comfortable," said Abelardo Gonzalez, automotive HVAC marketing manager.

The capacitive sensing element relies on a porous electrode, which allows water vapor to pass through to a thin-film dielectric as air is drawn over the element. The change in capacitance and the signal from the module's on-board temperature sensor pass through a signal conditioning stage, which determines relative humidity. The more common resistive sensing elements, by contrast, measure voltage change between two resistive materials as water vapor collects on the surface of the sensing element. The larger resistive elements also require more water vapor, slowing the response time.

The product also features an initial accuracy for relative humidity of 4 percent, compared to 10 percent for competing sensors, either capacitive or resistive. Response time is less than 10s over an operating range of 5 percent to 95 percent relative humidity. Variation in accuracy over the useful life of the vehicle (10 years) is 4 percent, versus &plusmn10 percent drift for the average industry sensor.

Beating the zone
In typical operation, the Sensata sensor measures temperature and humidity within the cabin as air is drawn over the thin-film polymer capacitive element by a fan (aspirator, DC-motored or other), in contrast to traditional systems which use zone-based or preset air-conditioning systems to regulate air temperature regardless of the cabin's relative humidity. These signals are transmitted to the air-conditioning control system, which opens or shuts the cabin recirculation door or turns on or off the AC compressor for optimal passenger comfort.

Similarly, traditional windshield anti-fogging systems rely on the vehicle occupant to activate and adjust defogging functions via simple dashboard control. Once set, the process often takes several minutes to defog each time condensation builds up on the windshield. The Sensata system removes the need for constant human attention by automatically monitoring humidity and temperature to seamlessly engage/adjust the system. This approach prevents fog buildup over the course of an entire trip.

More specifically, the sensor attaches behind the rear-view mirror and measures humidity, while other on-board sensors monitor the temperature of the glass and ambient air closest to the windshield. The control unit combines and processes these three signals to determine if the AC compressor should be used to dry the heated or cooled air for defogging. This technique touts a fourfold savings in energy; citing a JD Power Survey, Sensata says that traditional windshield defogging requires four times more energy than preventing fogging from occurring via a continuously running system.

Pricing information for the product family is pending.

- Vince Biancomano

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