CA2079175A1 - Control system for a vehicle window defogger and de-icer - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A windshield heater control system employs a thermistor to obtain a measure of the temperature of window glass of a vehicle. This measured value is translated to digital form by the analog-to-digital converter in a microcontroller. The digital value is then compared with a sequence of threshold level values stored in the read only memory of the microcontroller in order retrieve a corresponding count value from which a duration count value indicative of a heating interval duration is derived. The duration count value is loaded into a programmable count-down timer in the microprocessor which times the desired interval. A power switch connects the vehicle's electrical supply to a resistive heating element formed as an integral part of the window for the heating interval. The functional relationship between the initial temperature sensed by the thermistor and the duration of the resulting heating interval is predetermined and is characterized by a marked increase in the heating interval duration when the sensed initial temperature is in the vicinity of zero degrees centigrade.

Description

2 ~ 7 ~ S

9 ~5 ~ 5 pA~rENT
CONTROI. 8YSq~E:M FOR A VEHICLE
WXNDOW DE~OG~ER AND DE-ICER

INTRODUCTION

This invention relates to automotive window defogging and de-icing systems and, more particularly, to control systems for use with electrically-operated windshield heaters.

~UMMARY OF T~E INV~NTION

Moisture may accumulate on an automobile's windows whenever the temperature of the glass sur~ace is low enough to cause condensation. on a warm and humid day, the outside surface of the windshield may fog whenever the automobile's air conditioner is turned on, cooling the glass. In colder weather, condensation may occur on the inside surface from the humidity added to the interior air by the breathing of passengers. Left outside overnight, a carls windows are frequently covered with dew or, if the temperature is below freeæing, with frost. Freezing rain or snow may coat the windows with a layer of ice. A de~ogging and de~icing system should be able to handle such varying conditions in order to restore good visibility.

Automotive forced-air heating systems typically include a dashboard-selected "defrost" mode in which warm 25 air is directed against the interior surface of the front windshield and the side door windows. Rear window glass, and less frequently the front windshield glasst may also include an electrical heating element in the form of a .rJrtj transparent resistive film or an array of thin wires imbedded in the glass. These electrical heating elements are typically activated by a manually-operated dashboard switch.

S In order to provide sufficient heat to ef~ectively defoy a~d de-ice the window glass, electrically energized heaters place a substantial load on the vehicle's electrical supply. Since the driver may easily farget to turn the heating element off once visibility i5 restored, it is desirable to provide a mechanism for turning off the heatiny element automatically. Prior automated mechanisms for controlling the ener~ization af a window heating element have included time~delay switches for turning off the heating element at the end o~ a heating interval of fixed duration; temperature-sensing mechanisms for turning off the heater after the window glass temperature has been elevated to a predetermined level; and moisture-sensing systems which automatically control the heater by detecting fogged or iced conditions.

The more complex defogging controls have not been widely accepted, and th~ simpler manually-operated dash~oard switch, sometimes augmented by a fixed-time-delay device for turnlng the heater off after a predetermined interval, continues to enjoy the most widespread use. In these popular systems, the heatex is activated only when the vehicle operator determines that fog or frost has accumulated to an undesirable level. A
panel light is frequently used to indicate that the heater has been activated (and that it should be deactivated after good visibility has been restored).

2 ~

The time delayed automatic turn~o~ Peature prevents unnecessary energy los~ when the driver fails to observe that the heate~ should be turned OFF. However, under some conditions, the fixed interval provided may be insufficient to clear the moisture from the glass, and the driver may not notice that the panel light has been extinguished to indicate that the heater needs to be restarted. Thus, the need to restart the heater when the timer has turned it off prematurely is not only annoying to the driver, it causes e!ven more energy COnSUTnptiOn because the window glass, which i~mediately begirls to cool after the heater is automatically turned off, must be reheated.

It is accordingly the primary object of the present invention to more efficiently defog and de-ice window glass by more effectively controlling the period o~
energiæation of an elactrically operated heating element.

It is a fuxther object of the invention to efficiently control an electrically energized window heating element by means of an inexpensive mechanism which uses readily av ilable components.

As contemplated by the invention, the vehicle operator actuates a manually-operated switch whenever de~ogging or de-icing is desired. A sensor, such as a thermistor, provides an electric output signal indicative of the initial temperature of the window ~lass. An electronic timer responsive to this output signal turns off the heating element at the end of an interval whose duration is inversely related to the sensed initial window temperature.

2 ~ rj In accordance with a principal feature of the invention, a desired functional relationship between the sensed temperature and the duration o~ the resulting heating interval is permanently stored in an electronic memory, which may advantag~iously be implemented by the read only memory of a conventional microcontroller. Thls stored relationship operates as a lookup table and provides data for controlling the duration of the heating interval as an optimized function of the initial glass temperature~ The storecl relationship is further characterized in that the duration of the interval derived from the stored lookup table data increases substantially at a t~mperature near zero degrees centigrade, the freezing point of water. In this way, substantially longer heating intervals are provided when needed to melt, as well as to evaporat~, the accumulated moisture on an iced window.

In its preferred form, the heater control mechanism contemplated by the invention comprises an integrated circuit microcontroller which advantageously includes (1) an analog-to-digital converter for translating the electrical output signal from a temperature sensor into a digital value rela~ed to the sPnsed temperature, (2) a digital memory for storing lookup data which relates temperature values to counts, (3) a microprocessor for translating the digital value from the analog-to-digital converter into a count integer by consulting the stored lookup d~ta, (4) a count-down timer which is loaded with the count integer generated by the microprocessor, and (5) means for terminating the heating interval when the count-down timer is decremented to a final value. By implementing these functions with a microcontroller which 2~79~ 5 performs other functions within th~ vehicle, the incremental cost of the mechanism for con~rolling the gla5s heating element in accordance with the invention is quite low, since the only new components typically needed are the initial tamperature sensor and its associated wiring.

As contemplated by the invention, the temperature sensor may be further employed to sense the temperature at or near the conclusion of the timed heating interval to insure thak the glass temperature has been elevated above a predetexmi~ed minimum level. If the sensed glass temperature remains below that minimum level at the end of the first heating interval, the heater may again be turned on for a predetermined duration in attempt to automatically clear the remaining moisture from the windshield.

These and other objects, features and advantages of the present invention will be mor~ clearly understood by considering the following detailed description of a specific embodiment of the invenkion. In the course of this description, reference will frequently be made to the attached drawings.

BRIEF DESCRIP~IO~ OF TH~ DRA~IN~g Figure 1 is a schematic drawing of a preferred arrangement for controlling the operation of a window glass heating element in a defogging/de-icing system;

2 ~

Figure 2 i5 a graph showing the relationship between the reslstance of a temperature-sensing transducer and its ambi~nt temperature; and Figure 3 is a graph showing the relationship between the resistance presented by the temperature~sensing transducer and the optimized durat.ion of energization applied to the window glass heating olement as contemplated by the invention.

DETAILED OESC~IPTIO~

10As seen in Fig. 1, an automobile window includes a resistive heater seen at 12 formefl by a metal-based ~ilm sandwiched between two panes of glass. Suitable resistive ~ilms are well known in the art and are exemplified by the chromium-based coating techniques 15described in U.S. Patent 4,B44,985 issued on July 4, 1989 to Eugene P. Pharms, Charles J. Amberger and Ronald R.
Hymore. Alternatively, the resistive heater may be formed by thin resistive wires arranged in a an array or a sinuous path and imbedded or otherwise attached to the window glass.

As seen in Fig. 1, electrical terminals 11 connected to the resistive heating element provide a connection point to which a suitable electrical potential may be applied. An electrically operated power switch 16, which compri~es a suitable relay or a samiconductor switch, i5 used to energize the heating element 12 by connecting it to the power source 15 provided by the vehicle's battery/alternator system. The power switch 16 is turned on by the application of a control signal at control 2 ~ 7 ~

.input 1~ to energize the heating element 12, and i5 turned ~f by the application of a control signal at control input 19 to de-energize the heating element 12.

A temperature sen or 20 is attached in thermal contact with the glass 12. The sensor 20 adv~ntageously takes the form of thermilstor comprising a layer of resistive material h~ving a;high temperature coefficient.
The temperature-response characteristics of the thermistor are depicted in the graph of Fig. 2, in which the thermistor's varying rssistance is plotted on a logarithmic scalP. As shown in Fig. 2, when the temperature of the window glass is 0 decrees centigrade, the sensor 20 presents a resistance of approximately 250k ohms. When the temperature rises to 50 degrees centigrade, the resistance of the thermistor drops tenfold to approximately 25k ohms.

The manual control panel switch 25 is connected to control input 18 of the power switch 16 to energize the heating element 12 whenever the vehicle operator desire~
to initiate the defogging/de-icing function. The power switch 1~ then remains on and continu~s to energize the heating element 12 for a heating interval having an optimized duration as determined by a microcontroller indicated generally at 30 in Fig. 1.

25The microcontroller advantageously takes the form of a single integrated circuit which includes, among numerous subsystems, an analog-to-digital converter 32, a programmable memory unit 34 for storing optimized duration counts, and a digital count-down timer for 30generating an output signal at the conclusion of the 2 ~ 7 ~

optimized heatiny interval. Integrated circuit mi~rocontrollers suitable for implementing the present invention inc:Lude, for example, khe MC6805R3 8-bit microcontroller unit manufactured by Motorola Inc. The MC6805R3 includes an 8-bit microprocessor capable of addressing 4096 bytes of memory, memory mapped input/output I/0 ports~ a built-in 8-bit analog to-digital converter with provision for up to ~our analog inputs, a built-in 8-bit programmable timer with a 7-bit prescaler, and a 2048 byte programmable read-only memory unit (PROM) which may be used to store the optimized heating interval data as described below. The functions and capabilities of the MC6805R3 microcontroller unit, as well as other microcontrollers which could be employed to implement the invention, are described in detail in the Motorola Data Book entitled "Motorola's Microcontroller and Microprocessor Families", Volume 1, Motorola Inc.
(1988).

At the time the vehicle operator actuates the panel switch 25, the temperature of the window glass 10 is measured by the sensor 20 to form a potential which is applied to the input of the analog-to-digital conversion unit 32 within the microcontroller IC 30. A
substantially constant current from a high impedance source (formed by the resistance 21 connected between the supply terminal 22 and the sensor 20) is passed through the thermistor 20 to generate a potential substantially proportional to the thermistor resistance. This temperature-dependent signal is applied to the analog input of the analog-to-digital converter 32 via a resistance 23. A bipass capacitor 24 is connected in parallel with the sensor 20 to suppress noise signals.

2 ~ 7 5 When the panel switch 25 is actuat~d, it turns on the power switch 16 to beyin th~ heating inter~al, and simultaneously initiates khe calculation o~ the duration of this interval, a5 next describecl.

The microprocessor wlthin the ~icrocontroller 30 (not shown) searches the contents of the lookup table 34 formed by the sequences of threshold temperature levels and correspondirlg,duration counts. The microprocessor thus selects and retrieves 1:hat stored count value which corresponds to that stored range of temperature values which includes the sensed initial temperature.

As shown by the illustrative graphs seen in Figs. 2 and 3, for example, the lookup table data stored in memory establishes 4 threshold temperatures indicated by the dotted lines on the graphs. Whenever the sensed initial glass temperature is below 0 degr~es centigrade (32 degrees fahrenheit), the count delivered to the programmable ti~er 36 establishes a heating interval of 240 seconds (4 minutes). ~ith initial temperatures in the range from 0 to 10 degrees centigrade (32 to 50 degrees F), the count value from the table yields a heating interval of 120 seconds (2 minutes). When the temperature rises to the range between 10 and 21.1 degrees C ~0 - 70 degrees F), the heating interval is reduced to 60 seconds (1 minute)~ ~hen the initial glass temperature is sense to be within the range from 21.1 to 32.2 degrees C (70 to 90 degrees F), the interval is reduced to 40 seconds. Finally, if the gl~ss temperature exceeds 32.2 degrees C (90 degrees F), the heater i5 effectively disabled altogether. As seen in Figure 3, the duration of the interval increases, stairstep _g_ ~37,~"~7rj ~ashion, in a grad-lal fashion a~ the temperature drops through the range above freeæing, and then advantageously increases by a substanlial amount as the serlsed temperature crosses the freezing point.

A lookup table of the kind described, which e~tablishes only a limited number of discrete heating intervals associated with~ each oP a correspondingly limited nur~er of pred~termined temperature ranges, has been found to be highly effective, yet places the minimum demand on microcontroller's limited memory CapaG.ity~ an important quality since the microcontroller may typically be used for other monitoring and control functions, such as controlling the alternator, etc., and these functions compete for available program and data storag~ space.

Greater accuracy can he achieved by a simple routine which delivers an output count interpolated from adjacent t~mperature and count values in the table~ As an illustration, an interpolation routine expressed as a Pascal function is set ~orth below:

INTERPOLATION ROUTINE
type lookup=arrayEI..4] of integer;
const tabletemp: lookup = (32,50,70,90~;
tablevalue: lookup = (240,120,60,40);
var intemp, outcount: integer;
Function DelayCount(temperature: integer): integer;
var index, temp_delta, count_delta, temp_increment: integer;
begin 7 ~

if temperaturc >= 90 thcn begitl if tcmperaturc > 300 then { the thcrmistor circuit is presunnably shortesl. ) delaycount:=120 else delaycount:=0;
exit;
end elsc if temperature <3 32 ihen begin ;f temperature ~ -50 then ( the thermistor circuit is presumably open. }
delaycount:=120 else delaycount:-240;
exit;
end;
inde~:= I;
while tabletemplindex~ < temperature do inc(index);
temp_delta:=tabletemp[index~- tabletemp[pred(index)~;
2 O coun~_delta:=tableval ue[index~ - tablevaluelpred(index)];
temp_increment:=temperature - tabletemp[pred(index));
delaycount:=tablevalue[pred(index)] +
trunc(temp_increment ~ (count_delta / temp_delta));
end;
* * *

In the illustrative routine above, the ~unction ~elayCount receives an integer "temperature" expressing the initial qlass temperatllre in fahrenheit and retuxns a integer equal to the number o~ seconds oP desired delay. The effect of the interpolation is to smooth the stairstep curve shown in Fiq. 3 into a sequence of linear line segments joining the four discrete points established by the stored lookup table values. It may also be noted that, in the routine above, unexpectedly high temperature indicationls may reasonably be asæumed to result from a the~mistor circuit failure, in which case the DelayCount function returns a default heating interval o~ 120 seconds.

In the preferred embodiment which has been described, the heating element is simply turned off at the conclusion o~ the programmed heating interval, and does not restart unless the vehicle operator again actuates the panel switch 25. A paoel light, which is typically part of, or adjacent to, the dashboard switch 25, is illuminated whenever the heating element is energi~ed, and turns off at the end o~ the heating interval to inform th~ vehicle driver that the heating cycle is terminated. Because the duration of the heating interval has been optimized based on the initial temperature reading as determined by the lookup table values, the driver will not normally need to take any further action after the heatinq cycle terminates.

However, under severe icing conditions, it may be necessary to restart the heating element. As contemplated by a further feature of the invention, this restart cycle may also be readily automated by 2 ~ r~

progra~ming microcontroller 30 to sense the glass temperature at the conclusion of the heating interval and initiate a ~ontinuation heating interval of predetermined duration, such as an a~ditional 60 seconds, ~henever the post-heating cycle glass temperature is ~ound to be below a predetermined minimum value, pref~rably about 0 degrees centigrade. For example, the microcontroller 30 may be programmed such that, if the glass temperature ~ollowinq a given heatiny interval is below 2 deyrees ~ (36 deyrees F~, an additional heating interval will be initiated.

The number of additional heating intervals which can be automatically initiated in this ~ashion may advantag~ously be limited. If the glass temperature remains below the restart level after a predetermined number of additional heating intervals have been applied, the microcontroller then terminates further automatic heating interval~, and the driver must a~firmatively request additional heating intervals by again actuating the panel switch 25.

It is to be understood that the specific mechanisms and techniques which have been described are merely illustrative application~ of the principles of the invention~ Numerous modifications may be made to these arrangements without departing from the true spirit and scope of the invention.

Claims (10)

1. A vehicle window defogging and de-icing system comprising, in combination, an electrically operated heating element formed as an integral part of said vehicle window, switching means for connecting said heating element to a source of electrical power for a heating interval, a temperature sensing device in thermal contact with said window for measuring the initial temperature of said window at the beginning of said heating interval, and control means responsive to said temperature sensing device for varying the duration of said heating interval as an inverse function of said initial temperature.

2. A system as set forth in claim 1 wherein said control means comprises and integrated circuit microcontroller having including a preset read-only memory and wherein said inverse function is established by data stored in said read-only memory.

3. An arrangement for defogging and de-icing a glass vehicle window comprising, in combination, a electrically operated heating element for heating said window, a temperature-responsive sensor in thermal contact with said window for developing an electrical signal having a magnitude related to the temperature of said window, an electronic memory for storing a plurality of values specifying heating interval durations, a manually operated switch accessible to the operator of the vehicle, means responsive to the actuation of said switch for electrically energizing said heating element, means jointly responsive to the magnitude of said electrical signal and to the actuation of said switch for generating an digital output value based upon at least a selected one of said values stored in said memory, and a digital timer for de-energizing said heating element when said count-down time is decremented at the conclusion of an interval having a duration established by said digital value.

4. The defogging and de-icing arrangement set forth in claim 3 wherein said plurality of values establish increasing heating interval durations for increasing temperatures detected by said sensor.

5. The arrangement set forth in claim 4 wherein said plurality of values establish a substantially longer heating duration for temperatures detected by said sensor which are below zero degrees centigrade.

6. The arrangement set forth in claim 4 further comprising, in combination, means for re-energizing said heating element for a continuation interval of predetermined duration when said sensor detects a window temperature below a preset restart level at the conclusion of said heating interval.

7. The method of defogging and de-icing a vehicle window glass which comprises, in combination, the steps of:

storing a series of preset time duration values and corresponding temperature range values in an electronic memory, manually actuating an electrical switch, sensing the temperature of said window glass when said switch is actuated to form an initial temperature signal, retrieving a selected one of said stored duration values in response to and based upon said measured temperature signal, energizing a resistive heating element to heat said window glass in response to the actuation of said switch, and terminating the energization of said resistive heating element at the conclusion of a heating interval having a duration established by said selected one of said stored duration values.

8. The method as set forth in claim 7 further comprising, in combination, the steps of:
sensing the final temperature of said window glass at the conclusion of said heating interval, and maintaining the energization of said heating element for a predetermined continuation interval in the event said final temperature is below a predetermined restart level.

9. A defogging and de-icing control system for controlling the energization of an electrical heating element which is an integral part of a window glass panel in a vehicle, said system comprising, in combination, a manually-operated switch positioned to be accessible to the driver of said vehicle, thermistor in thermal contact with said glass panel, said thermistor exhibiting a resistance which is inversely related to the temperature of said panel, an integrated circuit microcontroller comprising, in combination, an analog-to-digital converter having an input connected to said thermistor for generating a digital output signal having a magnitude indicative of the temperature sensed by said thermistor, a read-only memory for storing a sequence of duration values corresponding to a sequence of threshold temperature values, programmed processing means for comparing said digital output signal with said sequence of threshold temperature values to retrieve at least a selected one of said duration values from said memory, and digital timing means for generating a termination signal at the conclusion of a heating interval having a duration corresponding to the selected one of said duration values, and switching means connected between a source of electrical energy and said heating clement for energizing said heating element in response to the actuation of said manually-operated switch and de-energizing said heating element in response to said termination signal.

10. The system set forth in claim 9 wherein said microcontroller compares the temperature of said glass panel as sensed by said thermistor at the conclusion of said heating interval with a predetermined restart threshold temperature and maintains the energization of said heating element for a continuation interval of predetermined duration if the sensed temperature is below said restart threshold level.