With a focus on automotive safety, passenger comfort and fuel economy, Sensata Technologies, formerly the Sensors & Controls division of Texas Instruments, is introducing a family of capacitive humidity / temperature sensors for climate control and windshield anti-fogging systems. Built on proven ASIC technology from Sensata and an advanced thin-film polymer capacitive sensing element, this new line of humidity / temperature sensor has a response time five times faster with twice the initial accuracy of traditional resistive sensor technologies.
Sensata Technology’s capacitive sensing element relies on a porous electrode, which allows water vapor to pass through to the thin-film dielectric as air is drawn over the element. The change in capacitance and the signal from an on-board temperature sensor are passed into the company’s proven ASIC signal conditioning process, which translates these readings into a relative humidity signal, which is far more accurate than other capacitive or resistive sensing systems. 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 require more water vapor to collect, slowing the response time.
This new sensor features an initial accuracy of +/- 4.0 percent, compared to the +/- 10 percent of competitive devices featuring capacitive or resistive sensor technology. Response time is less than 10 seconds over an operating range of 5 – 95 percent relative humidity, exceeding the industry norm. Over the useful life of the vehicle and after exposure to typical automotive environmental stress testing, the accuracy may shift +/- 4.0 percent, compared to industry average +/- 10 percent drift over life.
Climate Control Systems
The automotive industry is evolving beyond simple temperature controlled climate systems to include relative humidity to create a more comfortable cabin environment. Traditional zone-based or pre-set air-conditioning systems regulate air temperature regardless of the cabin’s relative humidity.
Sensata Technologies’ in-panel humidity / temperature sensor measures temperature and relative humidity within the cabin as air is drawn over the thin-film polymer capacitive element by a fan (aspirator, DC-motored or other). These signals are then sent to the air-conditioning control system, which will take any number of actions, including opening / closing the cabin recirculation door or turn on / off the AC compressor for optimal comfort.
Windshield Anti-Fogging Systems
Traditional windshield anti-fogging systems rely on the vehicle occupant to activate and adjust defogging functions via dashboard knobs and levers. Once set, the process often takes several minutes to defog each time condensation builds up on the windshield. More advanced systems remove the need for constant human attention from this equation, and reduce air-conditioning compressor stress by automatically monitoring humidity and temperature to seamlessly engage / adjust the system. This approach prevents fog buildup over the course of an entire trip.
Sensata Technologies’ windshield mounted humidity sensor attaches behind the rear-view mirror, or some other ‘out of sight’ location, and measures relative 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, allowing the occupants to focus on driving safely.
By enabling climate control and windshield anti-fogging systems to better optimize air-conditioning compressor usage, vehicle fuel economy is increased. According to a recent JD Power’s Survey, windshield defogging requires four times more energy than preventing fogging from occurring via a continuously running system.
“Our expertise with signal processing from the widest range of automotive sensors on the market allows us to vastly improve the passenger experience with this new line of humidity and temperature sensors,” said Abelardo Gonzalez, automotive HVAC marketing manager, Sensata 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.”