No Fog in Here

With the exception of teenagers out on a hot date, most people find steamed up car windows pretty annoying. "Fogged windows consistently rank among the top ten consumer complaints with new cars," notes Tom Urbank, an advanced development engineer at Delphi Automotive Systems. But fog problems no longer steam Urbank. Working with Delphi's Electronics Integration Team, he and Sensor Development Engineer Tim King have developed a system to automatically counteract fogging.

It's not the only automatic fog fighting system on the market. European cars have had them for a couple of years, and Delphi itself has systems in production for Renault. But there's still plenty of room for innovation. Previous systems either detect fog with optical sensors or use environmental measurements to infer fogging conditions. Urbank faults both methods. "Optical sensors only detect fog once it's begun to form," he says. "And inference methods have too many variables to accurately predict fogging."

Delphi's new automatic fog control system differs from these approaches in some important ways. For one, it cleverly integrates a custom dew point and glass temperature sensor into existing automotive interiors in a manner that balances the need to sense interior conditions against the obtrusiveness of the sensor itself. For another, its patented control methods allow the system to make pre-emptive strikes against fog.

Fog watcher

Delphi's system begins with a custom two-sided sensor designed by King. Outwardly, it looks like a tiny (20 mm across) hockey puck mounted behind the rear view mirror. Inside, it contains a flex circuit board and three sensors. One measures the glass temperature, and another reads the inside cabin temperature. The third picks up humidity inside the cabin. "We scoured the market for the right sensors, and did a lot of testing," King recalls. Some of the problems included finding a humidity sensor whose response time matched those of the thermistors. Many of the low-end humidity sensors he evaluated took minutes to work rather than seconds required for fog control.

The two thermistors also had to be thermally isolated from one another inside the sensor package—something King accomplished by sandwiching an air gap and flex circuit board between the two temperature sensors. Finally, the sensors had to be chemically resistant to everything from take-out food to suntan lotion and cleaning fluids.

To determine the best location for the sensor, Urbank spent a lot of time in an Oldsmobile "so heavily instrumented that it was ugly," he says. The car allowed him to simulate a variety of fogging conditions—right down to the contribution of the occupants' breath. From that work, Urbank and King identified an optimum sensing location right behind the rear view mirror. There, the sensor can take glass temperature readings directly—avoiding the problem of inferring temperatures from locations further away from the glass. And as an added bonus, many mirrors already have a wiring harness to support sensor power and communications.

Patented algorithm

Once the dew point and glass temperature sensor collects the raw fogging data, Delphi's patented control algorithms take over. "We control fog based on the differential between dew point and glass temperature at all times," Urbank says. To do that, the system first sends the sensor outputs through a serial connection into the existing climate-control microprocessor, which converts the ambient temperature and humidity data into dew point equations.

With these equations in mind and test data from the instrumented car in hand, Urbank ultimately developed linear interpolation tables that reduce windshield temperature and dew point relationships to a single-number that captures the probability of fog formation. This value automatically triggers various fog-fighting countermeasures—like turning on the air conditioner compressor, adjusting blower speeds, and regulating the recirculation vents. "There's a progression of countermeasures," Urbank says.

Contact Tom Urbank, Delphi Harrison Thermal Systems, 200 Upper Mountain Rd., Bldg. 6, Lockport, NY 14094; Tel: (716) 439-2293; or Enter 550; www.delphiauto.com.