Window fog detector

A vehicle (10) includes a windshield presenting an inner surface (14) including a test section (16) therein. A cooling device (24) is attached to the inner surface (14) for cooling the section (16) to a temperature a few degrees below the temperature of the inner surface (14) adjacent the test section (16). A sensor (26) is disposed adjacent the inner surface (14) for sensing condensation on the test section (16) and for providing a heating signal to a defogging control (28) in response to condensation on the test section (16) for initiating the heating of the surface surrounding the test section (16) by the vehicle heater (18) to prevent condensation from forming on the surface surrounding the test section (16).

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method and apparatus for the detection 
of incipient fog on surfaces of various types. This invention is 
particularly useful for detecting incipient fog on windows or windshields 
of vehicles. 
2. Description of the Prior Art 
The personal experience of many drivers suggests that under certain 
conditions sudden fogging of the interior of the windshield can constitute 
a critical safety hazard. Such "white out" conditions are created when 
moist ambient air is allowed to contact the windshield surface which is at 
a temperature either at or below the dew point for the existing relative 
humidity. Naturally, if the development of dense fog is particularly 
sudden, the driver's vision may be severely impaired, endangering both the 
driver and other motorists. 
Several fog detection systems have been tested and deployed in the past, 
but these approaches all suffer from the same disadvantage. These prior 
art approaches develop a fog presence signal only after fog has begun to 
form on the monitored surface. However, under certain conditions, dense 
fog can form very suddenly and the only means to clear the obscuring fog 
may be too slow to provide the driver with unimpaired vision at all times. 
This fogging problem is particularly dangerous when driving into the sun 
which may be at a low elevation. A low sun angle, coupled with optical 
scattering or diffusion from fog on the inside surface of the windshield 
can temporality blind the driver and put many people at risk. 
Traditional defogging systems rely on either air blown at or over the 
fogged surface, or electric heating of the glass surface to dissipate the 
fog. In most vehicular applications the standard defogger for the 
windshield is an electric blower designed to blow warm air on the glass 
that is within the driver's field of view. Embedded or surface heating 
elements are commonly used for rear glass surfaces to accomplish defogging 
or de-icing. Regulatory considerations restrict the use of such electric 
heating grids to the rear glass, and reduce the available options for 
windshield defogging to either blown air or transparent electric heating 
films, which are very expensive and relatively fragile. 
A well known problem with a blower defogger is that it is not fast acting, 
and they are even less efficient if warm air is not available to 
accelerate the removal of the condensate, i.e., fog. Such a situation is 
common on first startup, when the engine is cold and the heater core is 
not able to add the necessary heat to the defogger air stream. In fact, 
under some situations operating the blower can exacerbate the situation by 
blowing moisture laden air onto the windshield inner surface, which may be 
far below the dew point in temperature. 
This blower induced fogging is not as common now as it once was because 
automotive vehicle manufacturers now partly dry the air that is blown 
across the windshield. Such drying is accomplished by operating the air 
conditioning compressor whenever the defogger is engaged. Passing ambient 
air over the substantially cooled evaporator coil of the A/C system 
"pre-condenses" much of the humidity, thereby preventing its deposition on 
the moderately cool glass surface. After the air has been dehumidified, it 
is often passed through a heat exchanger in order to improve defogging 
efficacy. 
In essence, the problem in achieving efficient defogging in an automotive 
environment is the delay in activating the defogging means. That is, once 
fog is detected there is an essentially unavoidable delay in clearing the 
glass. Increasing the air flow rate over the glass and/or adding more heat 
to the air stream is only partly feasible because of space and power 
limitations. A more practical and desirable approach to the problem would 
be to use a predictive fog sensing method wherein the conditions leading 
to fog formation would be monitored and evaluated by an intelligent 
controller which could begin the defogging process before the onset of 
actual, observable fog. Such a preemptive defogging system would, in fact 
not be a defogger at all because with proper inputs and controls fog could 
never actually form. The necessity for a rapid response to air and glass 
temperature in addition to accurate monitoring of relative humidity makes 
such an approach technically difficult. Implementing such a predictive 
system would be difficult in an automotive setting where environmental 
changes are rapid and occur over a very wide temperature range. 
SUMMARY OF THE INVENTION AND ADVANTAGES 
The invention provides an assembly for preventing the formation of 
condensation on a surface. The assembly comprises a cooling device 24 for 
cooling a test section 16 of the surface to a temperature below the 
temperature of the surface surrounding the test section 16, and a sensor 
26 for sensing condensation on the test section 16 and for providing a 
signal in response to condensation on the test section 16 for heating the 
surface surrounding the test section 16 to prevent condensation from 
forming on the surface surrounding the test section 16. 
Accordingly, the invention includes a method of preventing the formation of 
condensation on a surface by cooling a test section 16 of the surface to a 
temperature below the temperature of the surface surrounding the test 
section 16, sensing condensation on the test section 16, and providing a 
signal in response to condensation on the test section 16 for heating the 
surface surrounding the test section 16 to prevent condensation from 
forming on the surface surrounding the test section 16. 
The present invention describes a novel approach to an incipient fog sensor 
and control system which does not require the accurate monitoring of 
environmental variables. The invention is unique and uncomplicated for 
preventing fog from suddenly appearing on the windshield of an automotive 
vehicle.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to the Figures, wherein like numerals indicate like or 
corresponding parts throughout the several views, a vehicle is generally 
shown at 10 in FIG. 1. The vehicle 10 comprises a window 12, specifically 
a windshield, presenting an inner surface 14 including a test section 16 
therein adjacent the remainder of the inner surface 14. 
As is well known, the vehicle includes a heater 18 for heating the inner 
surface 14 by forcing heated air through a defroster and/or heater duct 20 
which extends through the instrument panel 22. 
A cooling device 24 is attached to or supported near the inner surface 14 
for cooling the section 16 to a temperature a few degrees below the 
temperature of the inner surface 14 adjacent the test section 16. Examples 
of such cooling devices 24 are shown in FIGS. 4 and 5. 
A sensor 26 is disposed adjacent the inner surface 14 for sensing 
condensation on the test section 16 and for providing a heating signal in 
response to condensation on the test section 16 for initiating the heating 
of the surface surrounding the test section 16 to prevent condensation 
from forming on the surface surrounding the test section 16. More 
specifically, the heating signal is sent to the defogger control 28 which 
controls the cooling at the cooling device 24, the sensing at the sensor 
26 and the operation of the heater 18. The defogger control 28 may be 
incorporated into the on board computer. 
As noted above, the conditions that lead to fog, i.e., condensate, 
formation on the inner surface 14 of automotive windshields are well 
known, and are related to the temperature and humidity of the air in 
contact with the inner surface 14 of the glass and the glass surface 
temperature. Whenever the glass surface is below the dew point for the 
instant relative humidity, moisture will condense onto the cool glass 
surface, forming fog. Rather than accurately monitoring all the variables 
necessary to predict the incipient formation of fog on the windshield (or 
any other surface), the present invention relies on selectively cooling a 
representative or test section 16 of the glass 12 to a temperature a few 
degrees cooler than the surrounding glass area. In principle, the 
mechanism employed by the instant invention is that water will condense on 
the coolest surface first, thereby providing an early warning signal for 
the defogger control 28 to activate the heater 18, i.e., the defogging 
system. That is, a sample surface or test section 16, representative of 
the entire glass surface 14, but much smaller in area, is maintained at a 
temperature lower than the entire surface 14, and that sample surface or 
test section 16 is monitored for the formation of first condensation or 
fog. This test method is related to the "mirror" dew point measurement 
technique wherein a cooled mirror is used to accurately determine the 
actual dew point. 
Several systems may be used to perform the function of the sensor 26 to 
detect the formation of fog or condensation on the cooled sample surface 
or test section 16. The embodiment of FIG. 2 discloses a dual channel, 
differential, reflection photometer where a first photo detector 30 
monitors the sample surface or test section 16 and a second photo detector 
32 monitors the surface 14 surrounding the test section 16. Any condensate 
fog that would form on the cooled test section 16 would scatter and 
reflect more illumination energy back to the corresponding detector 30, 
compared to the energy reflected back from the uncooled section 14 of the 
glass 12 which would not have any condensate fog present initially. Of 
course, if there is no intervention by a defogging mechanism, i.e., the 
heater 18 fog may also form on the uncooled surface 14 after a given 
period of time. A dual channel, differential system as shown in FIG. 2 
compensates for ambient light interference, temperature effects and 
changes in the illumination source. 
As shown in FIG. 3, a simpler incipient fog sensor could be configured with 
a single photo sensor 34 where predetermined diffuse reflection levels 
could be used to trigger the defogger control 28. The use of a pulsed 
infrared LED 36 as the illumination light source would provide significant 
sunlight immunity for either the dual channel or single channel 
configuration. The photo detector 34 would view through a filter 38 and an 
optical lens 40. The signal processing circuit 42 could include an ambient 
light suppression function, reference and comparator functions, a 
synchronous detection function, andl/or a detector and oscillator, and an 
LED driver. Accordingly, the sensor 26 may include a photo detector for 
sensing light from condensation on the test section 16. 
The assembly 10 also includes a thermo-sensor 44 for sensing the 
temperature of the surface 14 surrounding the test section 16 and cooling 
the test section 16 to a temperature at least one degree below the 
temperature of the surface surrounding the test section 16. The first 
photo detector 30 senses light from the test section 16 to provide a test 
signal and the second photo detector 32 senses light from the surface 14 
adjacent the test section 16 to provide an actual signal, and a comparator 
46 provides the heating signal in response to a predetermined 
differentiation between the test and actual signals for initiating the 
heater 18 to heat the inner surface 14. 
The test section 16 may be cooled by the thermoelectric cooler module, 
generally shown at 48 in FIG. 4. The cooler module 48 includes a fan 50 
for driving ambient air through the cool side 52 and onto the test section 
16. A heat sink 56 extracts heat from the heated side of the module 48 and 
a deflector 58 directs warm air away from the test section 16. As shown in 
FIG. 5, a switch mode controller 60 controls the operation of the cooler 
module 48 in response to the temperature of the window. More specifically, 
a first temperature sensor 62 sensing the temperature of the test section 
16 while a second temperature sensor senses the temperature of the ambient 
air. Of course, a wide variety of devices may be utilized for cooling the 
test section and sensing the temperatures. 
The invention has been described in an illustrative manner, and it is to be 
understood that the terminology which has been used is intended to be in 
the nature of words of description rather than of limitation. 
Obviously, many modifications and variations of the present invention are 
possible in light of the above teachings. It is, therefore, to be 
understood that within the scope of the appended claims, wherein that 
which is prior art is antecedent to the characterized novelty and 
reference numerals are merely for convenience and are not to be in any way 
limiting, the invention may be practiced otherwise than as specifically 
described.