Abstract:
An apparatus and method for temperature sensing and for detecting cracks or breakage of a windshield. A resistance element is connected to the windshield and a resistance monitoring circuit is connected to the resistance element for monitoring an electrical property of the resistance element. A physical integrity monitoring device can be connected to the resistance monitoring circuit for determining the physical integrity of the windshield based upon the monitored electrical property of the resistance element. Also, a temperature calculator can be used to determine a temperature based upon the monitored electrical property associated with the resistance element and upon a relationship between resistance and temperature that has been stored in a memory device.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     This invention relates generally to automotive windshields, and in particular to a temperature sensing and windshield crack detection device. 
     2. Discussion 
     The proper functioning of a vehicle&#39;s heated backlight of a rear windshield is an important aspect for the daily operation of a vehicle. This proper functioning of the heated backlight is made even more critical during the colder months of the year, when fogging and ice formation on the rear windshield is more likely to occur. 
     Malfunctions of the heated backlight can occur due to several reasons. A proper and expedient identification of the root cause for the malfunction is very useful for diagnosis of the malfunction. However, previous rear windshield configurations do not allow for early detection of cracks which would be severe enough to render the heated backlight inoperational. Moreover, past configurations do not provide for temperature monitoring of the vehicle&#39;s outside environment that would enable the vehicle to perform such functions as automatically turning on the heated backlight whenever the temperature falls below a predetermined set point. 
     SUMMARY OF THE INVENTION 
     In accordance with the teachings of the present invention, the present invention is an apparatus and method for temperature sensing and/or for crack detection of a windshield. A resistance element is connected to the windshield. A resistance monitoring circuit which is connected to the resistance element monitors an electrical property of the resistance element. In one embodiment, a physical integrity monitoring device can be connected to the resistance monitoring circuit for determining the physical integrity of the windshield based upon the monitored electrical property of the resistance element. In a second embodiment, a temperature calculator can be used to determine a temperature based upon the monitored electrical property associated with the resistance element and upon a relationship between resistance and temperature that has been stored in a memory device. 
    
    
     Additional advantages and features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1a is a frontal view of an exemplary configuration for a rear windshield with a heated backlight; 
     FIG. 1b is a frontal view of an exemplary configuration for a rear windshield with an antenna; 
     FIG. 2 is a flowchart showing the temperature calibration steps of the present invention; 
     FIG. 3 is a schematic block diagram of the electronic components of the preferred embodiment of the present invention; and 
     FIG. 4 is a flowchart showing the operational steps for detecting cracks and temperature sensing. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1a shows an exemplary rear windshield 10 of a vehicle which has a heated backlight 12 in order to heat the glass 11 of the rear windshield 10. A rear wiper motor 14 cleans the rear windshield 10 by powering a wiper blade so as to produce wiper fan pattern 16. The heated backlight 12 includes a first heated backlight node 18 and a second heated backlight node 20 in order to monitor the voltage drop between these two points that are located on the heated backlight 12. 
     While the preferred embodiment of the present invention depicts the nodes (18 and 20) being located at the extreme points of the heated backlight 12, it is to be understood, however, that the present invention is not limited to such a placement of the nodes. Other locations are contemplated within the scope of the present invention that enable the voltage drop of the heated backlight 12 to be ascertained. 
     A thermocouple 30 is provided during installation of the rear windshield 10 onto a vehicle in order to calibrate the resistance of the heated backlight 12 with temperature measurements. The temperature measurements are used to correlate the resistance of the heated backlight 12 to the temperatures measured by thermocouple 30. A micro-controller (not shown) stores the temperature values from thermocouple 30. The present invention allows for temperature sensing by comparing the resistance of the heated backlight 12 with the temperature values acquired during rear windshield factory installation. The micro-controller is preferably stored in the gear housing 32 of the rear wiper motor 14. The preferred embodiment includes using a micro-controller that is a part of a rear node assembly that controls other aspects of the rear portion of a vehicle. 
     Once the calibration has been completed, thermocouple 30 is removed from the rear windshield 10. Also with respect to thermocouple 30, locations other than the center of the windshield can be used so as to be able to sense the ambient temperature. 
     Additionally, the present invention includes using such other devices besides a thermocouple for calibration purposes. For example, an infrared thermometer can be used which can be remotely mounted and focused upon the windshield in order to perform temperature measurements. 
     FIG. 1b shows another embodiment wherein an exemplary rear windshield 10 of a vehicle has an antenna 33 that receives radio waves. Antenna 33 includes a first node 35 and a second node 37 in order to monitor the voltage drop between these two points that are located on the heated backlight 12. This voltage drop can be used to determine whether the rear windshield 10 has a crack or has been shattered (such as, in a car theft situation). 
     FIG. 2 depicts the processing steps for calibrating the present invention in order to do temperature sensing. Start indication block 60 indicates that block 64 is to be performed. Block 64 indicates that a thermocouple is to be placed on the rear windshield at some point during the installation of the rear windshield upon the vehicle. At block 68, the temperature is recorded via the thermocouple. Block 72 records the resistance of the heated backlight while the rear windshield is at the temperature that was recorded at block 68. 
     While blowing hot air upon the rear windshield, block 76 records the temperature of the rear windshield via the thermocouple. Substantially concurrently, block 80 records the resistance of the heated backlight. 
     These two points of temperature versus resistance determine the relationship between temperature and resistance. The preferred embodiment uses a linear relationship of resistance versus temperature and stores the slope and the y-intercept of the linear relationship in a micro-controller (at block 84) that is to be placed in the gear housing of the rear wiper motor. 
     While the present invention preferably uses a linear relationship to express the temperature versus resistance relationship, it is to be understood that the present invention is not limited to only a linear relationship, but includes other mathematical relationships that indicate the temperature versus resistance relationship. For example, the present invention contemplates acquiring additional temperatures versus resistance points in order to use a parabolic relationship. 
     FIG. 3 shows the preferred electronic circuitry for performing the temperature sensing and rear windshield crack detection for the present invention. The voltage of the vehicle&#39;s battery is indicated at reference numeral 100 while ground is represented at reference numeral 102. The vehicle&#39;s wire harness connects the battery to the circuitry of the present invention. The resistance of the wire harness is shown at reference numeral 104. The voltage drop of current shunt 106 is monitored by a first A/D converter 110. This voltage drop is provided to the micro-controller 112. 
     A parallel circuit shown generally at 114 uses a first switch 116 and a second switch 118 in order to control the current flow to the rear wiper motor 14 and to the heated backlight (which is shown as heated backlight resistance 120). When first switch 116 is open and second switch 118 is closed, a second A/D converter 124 is used to monitor the voltage drop of the heated backlight resistance 120 at first heated backlight node 18 and heated backlight node 20. This voltage drop is then sent from the second A/D converter 124 to micro-controller 112. 
     If micro-controller 112 determines that a crack is present on the rear windshield based upon the information from the AND converters (110 and 124), then micro-controller 112 sends a signal to crack detection indicator 130 so as to provide indication that a crack is present. Crack detection indicator 130 preferably provides this indication to a person while diagnostics are being performed upon the vehicle (such as when a vehicle is brought to a garage to repair the vehicle&#39;s heated backlight). Moreover, crack detection indicator 130 also may include being connected to a vehicle&#39;s anti-theft device and activating it when indicator 130 detects a crack or breakage of the windshield. 
     Moreover, if the micro-controller 112 determines that the temperature as calculated by the voltage drop values from the first A/D converter 110 and the second A/D converter 124 are below a predetermined threshold value, then micro-controller 112 automatically turns on rear defrost 132. FIG. 4 depicts the processing steps preferably performed internally by micro-controller 112 in order to perform the crack detection and temperature sensing functions of the present invention. 
     FIG. 4 depicts the steps for temperature sensing and crack detection which are performed while first switch 116 is open and second switch 118 is closed. Moreover, temperature sensing is preferably performed only when the heated backlight is not activated. With reference to FIG. 4, start indication block 150 indicates that block 154 is to be performed. At block 154, the micro-controller obtains the voltage drop of the current shunt and at block 158 determines the current at the current shunt using the following equation: 
     
         I.sub.CURRENT SHUNT =(V.sub.CURRENT SHUNT)/(R.sub.CURRENT SHUNT) 
    
     Since the resistance of current shunt had been previously stored in the micro-controller (such as during installation of the rear windshield in the factory) and the voltage drop of the current shunt had been furnished by the first A/D converter, the current level at the current shunt can be determined by the micro-controller at block 158. 
     Block 162 obtains the voltage drop of the heated backlight via the second A/D converter. Block 166 calculates the resistance of the heated backlight using the following equation: 
     
         R.sub.HEATED BACKLIGHT =(Voltage.sub.HEATED BACKLIGHT)/(I.sub.CURRENT SHUNT) 
    
     Since the voltage drop of the heated backlight had been obtained at block 162 and the current of the current shunt had been determined at block 158, the resistance of the heated backlight can be calculated at block 166. If the calculated resistance of the heated backlight corresponds to that of an open circuit (i.e., infinity or a lack of continuity), the crack detection indicator is activated at block 170. 
     Block 174 calculates the present temperature based upon the calculated resistance of block 166 and upon the micro-controller stored values that determine the relationship between temperature and resistance. Block 174 preferably uses the following equation: 
     
         Temperature.sub.PRESENT =(stored slope value)×(R.sub.HEATED BACKLIGHT)+(stored y-intercept) 
    
     The micro-controller can perform a number of operations based upon the calculated temperature of block 174. One such operation includes the operation of block 178 wherein an automatic rear defrost is activated if the calculated temperature is below a predetermined threshold value. An optical device can be used in conjunction with the present invention in order to detect that ice is on the window. For example, the defrost device would be activated if: the temperature as determined by the present invention is below a certain threshold (such as below 32 degrees Fahrenheit); and the optical device determines that the visibility of the windshield is below a certain threshold (such as less than 75% visibility). Termination block 182 indicates that processing ends. 
     The embodiments which have been set forth were for the purpose of illustration and were not intended to limit the invention and will be appreciated by those skilled in the art that various changes and modifications may be made to the embodiments discussed in this specification without departing from the spirit and scope of the invention as defined by the appended claims.