Abstract:
A circuit for controlling the temperature of a thin film conductive heater includes a control circuit for applying a voltage to the heater. The control circuit regulates the temperature of the heater element by using a modeling technique which assumes that the thin film coating functions as a single electrical resistor.

Description:
FIELD OF INVENTION 
     The present invention relates to a thin film resistive heater and more particularly to a circuit and method for controlling the temperature of a thin film resistive heater used to heat a liquid crystal display. 
     BACKGROUND OF INVENTION 
     FIG. 1 shows an exploded view of a conventional module  10  which is used for housing a Liquid Crystal Display (LCD)  14 . The module  10  includes a front frame  12 , an LCD  14 , a thin film resistive heater  16 , and a back frame  18 . The device  10  is held together by interlocking tabs and recesses. 
     As is known in the art, if the temperature of the LCD  14  becomes too cold, the liquid crystal material within the LCD  14  becomes increasingly viscous. If such a result occurs, the LCD  14  does not work properly. 
     In view of this problem, the thin film resistive heater  16  is provided to maintain the temperature of the LCD  14  within a certain temperature range. To perform its heating function, the thin film resistive heater  16  includes a glass substrate  20  which contains a thin film coating of, for example, indium tin oxide (ITO). The substrate  20  is attached to a plastic frame  22 . As is known in the art, when a current is passed through the thin film coating, heat is transferred to the glass substrate  20  thus creating a heater. 
     When the module  10  is assembled, glass substrate  20  of the thin film heater  16  is placed in thermal contact with the LCD  14 . This configuration allows for the transfer of heat from the heater  16  to the LCD  14 . 
     Typically, a thermistor  24  is placed on an outer edge of the glass substrate  20  to monitor the temperature of the thin film heater  16 . As is known in the art, the thermistor is a device whose resistance is a function of temperature. 
     Based on the design of the device shown in FIG. 1, the thermistor  24  is required to be placed on an outer edge of the thin film heater  16  to ensure that the thermistor  24  does not interrupt the viewing area of the LCD  14 . In particular, if the thermistor  24  is positioned at the center of the glass substrate  20 , a shadow will appear on the LCD  14 . This is obviously undesirable given that such a shadow would impact the usefulness and desirability of the LCD  14 . 
     FIG. 2 shows a conventional circuit that is used to control and measure the temperature of the thin film heater  16  shown in FIG.  1 . The circuit shown in FIG. 2 contains a power supply  26  which provides a supply voltage V SUPP  to bus bars  21  of the glass substrate  20  contained within the thin film heater  16 . As is known in the art, the application of the voltage V SUPP  creates a current I which in turn heats the substrate  20 . 
     The circuit shown in FIG. 2 also contains a temperature feedback circuit  28  which is connected to the power supply  26  and the thermistor  24 . The temperature feed back circuit  28  continuously measures the temperature of the thermistor  24 . 
     If the temperature of the thermistor  24  falls below a certain temperature To , the feedback circuit  28  instructs the power supply  26  to apply the voltage V SUPP  to the bus bars  21  to heat the glass substrate  20 . Conversely, if the temperature of the thermistor  24  reaches temperature To, the feedback circuit  24  then instructs the power supply  26  to remove the voltage V SUPP . 
     While the conventional device discussed above and shown in FIGS. 1 and 2 allows for the temperature of the thermistor  24  to be measured, the device still has significant drawbacks. In particular, the design of the device shown in FIGS. 1 and 2 results in a large thermal mass at the edge of the thin film heater  16  where the thermistor  24  is located. As is known in the art, large thermal masses resist changes in temperature. That is, large thermal masses either maintain temperature for prolonged periods of time or require an inordinately large amount of heat to achieve an increase in temperature. 
     Given that the edge of the thin film heater  16  is in contact with a large thermal mass and that the thermistor  24  is required to be positioned at this location, it is difficult to accurately monitor and maintain the temperature of thin film heater  16 . In view of this problem, there currently exists a need for a device which can accurately measure the temperature of the center of a thin film heater in a manner that is minimally impacted by the thermal mass proximate to the heater and does not affect, in any way, the viewing area of the heater. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the present invention to provide a device which can accurately measure the temperature of a thin film heater which is minimally impacted by a surrounding thermal mass. 
     It is another object of the invention to provide a device which can accurately measure the temperature of a thin film heater that does not, in any way, impact the viewing are of the thin film heater. 
     In accordance with the invention, a device and method are disclosed which regulate the temperature of a thin film heating element by using a modeling technique which assumes that the thin film coating of the heater functions as a single electrical resistor. 
     In accordance with one embodiment of the invention, a circuit for controlling the temperature of a heating device is disclosed where the circuit comprises: a heater containing a thin film coating; and a control circuit for applying a voltage to the heater; wherein the control circuit regulates the temperature of the heater by using a modeling technique which assumes that the thin film coating functions as a single resistor. 
     In accordance with another aspect of this embodiment of the invention, the control circuit includes: a power supply for applying a voltage to the heater; a resistor that is connected in series with the power supply and the heater; a voltage sensing device which measures voltage drops occurring across the resistor; and, a temperature control circuit which monitors the voltage sensing means and the power supply to regulate the temperature of the heater. 
     In accordance with another aspect of this embodiment of the invention, the temperature control circuit regulates the temperature of the heater by: (i) calculating the resistance of the thin film coating and,(ii) calculating the temperature of the thin film coating based on the calculated resistance. 
     In accordance with yet another aspect of this embodiment of the invention, the resistance of the thin film coating is calculated by using the equation R 2 =R 1 (V SUPP /V 1 −1). 
     In accordance with even yet another aspect of this embodiment of the invention, the temperature of the thin film coating is calculated by using the equation T 2 =T 0 +(R 2 −R 0 )/a. 
     In accordance with another embodiment of the invention, a method for calculating the temperature of a heater containing a thin film coating is disclosed, where the method comprises the steps of: (i) calculating a resistance of the thin film coating, and, (ii) calculating a temperature of the thin film coating based on the resistance calculated in step (i). 
     In accordance with another aspect of this embodiment of the invention, the resistance of the thin film coating is calculated by using the equation R 2 =R 1 (V SUPP /V 1 −1). 
     In accordance with still another aspect of this embodiment of the invention, the temperature of the thin film coating is calculated by using the equation T 2 =T 0 +(R 2 −R 0 )/a. dr 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings are included to provide an understanding of the invention and constitute a part of the specification. 
     FIG. 1 illustrates an exploded view of a conventional module which is used for housing an LCD; 
     FIG. 2 illustrates a conventional circuit which is used to control and measure the temperature of a thin film heater shown in FIG. 1; and, 
     FIG. 3 is a schematic representation of an illustrative embodiment of the present invention that is used to control and measure the temperature of a thin film heater. 
    
    
     DESCRIPTION OF THE INVENTION 
     FIG. 3 shows a circuit  40  in accordance with the present invention. The circuit  40  is used to control and measure the temperature of a thin film heater containing a glass substrate with a thin film coating. 
     The circuit  40  contains a power supply  42  and resistor R 1 . Both devices are connected in series to bus bars  21  of the glass substrate  20 . Power supply  42  provides a supply voltage V SUPP  to bus bars  21  of the substrate  20  to produce a current I though the thin film coating. The current I in turn heats the substrate  20 . 
     The circuit  40  also includes a voltmeter  44  and a temperature control circuit  46 . The voltmeter  44  is used to measure voltage drops which occur across resistor R 1 . The temperature control circuit  46  is used to control both the power supply  42  and voltmeter  44 . 
     The circuit  40 , unlike the conventional device described above, does not require the use of a thermistor to measure the temperature of the edge of the substrate  20 . The circuit  40  developed in accordance with the present invention overcomes this problem by assuming, that the thin film coating on the lass substrate  20  constitutes a resistor R 2 . Based on this assumption, the circuit  40  uses a mathematical modeling, technique to calculate the temperature T 2  of the thin film coating, and substrate  20 . 
     It is important to point out that the resistor R 2  shown of FIG. 3 is provided for illustrative purposes only. The thin film coating, on the glass substrate  20  shown in FIG. 3 does not include a resistor R 2 . The circuit  40  only assumes that the thin film coating constitutes a resistor R 2  for modeling purposes. This assumption is valid given that the resistance of the material used for the thin film coating, such as indium tin oxide, is generally found to be a function of the material&#39;s temperature. 
     Based on this assumption, the circuit  40  calculates the temperature T 2  of the thin film coating on the glass substrate  20  using a mathematical modeling technique which involves a two step calculation. This two step calculation is described in detail below with reference to Equations 1 through 4. 
     First, the circuit  40  calculates the assumed resistance R 2  of the thin film coating. This value is calculated on a realtime basis, usually once every second, by the temperature control circuit  46 . 
     The temperature control circuit  46  performs this first calculation by using, Equation 1 below. 
     
       
           R   2   =R   1 ( V   SUPP   /V   1 −1)  Equation 1  
       
     
     Referring to Equation 1, R 1  is a known resistance value, V 1  is a known value based on measurements obtained by voltmeter  44 , and V SUPP  is a known value. With these known variables, R 2  is calculated by the temperature control circuit  46 . 
     Once R 2  is computed, the second calculation is performed by the circuit  46 . The second calculation computes the temperature T 2  of the thin film coating on the substrate  20  by using Equations 2 through 4 below. 
     As indicated above, the resistance R 2  of the thin film coating on glass substrate  20  can be expressed as a function of the coating&#39;s temperature. This mathematical temperature is shown as Equation 2 below. 
     
       
           R   2   =R   0   +a ( T   2   −T   0 )+ b ( T   2   −T   0 ) 2 +  Equation 2  
       
     
     By truncating Equation 2, the resistance R 2  of the coating can be approximated by Equation 3 below. 
     
       
           R   2   =R   0   +a ( T   2   −T   0 )  Equation 3  
       
     
     Equation 3 can then be solved for T 2  and represented as shown in Equation 4 below. 
     
       
           T   2   =T   0 +( R   2   −R   0 )/ a   Equation 4  
       
     
     Referring to Equation 4, T 0  is a reference temperature of the thin film coating contained on glass substrate  20  at which the coating&#39;s resistance R 0  is known, R 0  is a resistance of the coating at the temperature T 0 , and the coefficient “a” is a constant that is unique to the particular materials of the coating. With these variables being known and R 2  being known from the first calculation, T 2  is then calculated by the temperature control circuit  46 . This calculation is also performed on a realtime basis which is usually once/second. 
     By assuming that the thin film coating contained on the glass substrate  20  constitutes a resistor and using the mathematical models described above, the circuit  40  developed in accordance with the present invention calculates the temperature T 2  of the coating on the glass substrate contained within a thin film heater. Moreover, circuit  40  is able to calculate the temperature T 2  in a manner that is minimally impacted by the thermal mass proximate to the thin film heater and does not impact the viewing are of the substrate  20 . 
     The present invention is not to be considered limited in scope by the preferred embodiments described in the specification. For example, while the invention described herein is used as a device for heating for LCD&#39;s, the invention can be used in any type of thin film heating device. Additional advantages and modifications, which will readily occur to those skilled in the art from consideration of the specification and practice of the invention, are intended to be within the scope and spirit of the following claims.