Abstract:
An LCD heater and method having temperature sensors made by electrically isolating portions of ITO across a display surface and measuring the resistance of such portions and developing an assessment of temperatures in the interior of the display surface based on sensor calibration data taken under known temperature conditions.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to liquid crystal displays (LCDs) and more particularly relates to LCD heaters and even more particularly relates to controls for LCD heaters. 
     In the past, LCD heater plates have been incorporated to provide a uniform temperature across the LCD surface. The heater has typically been deposited material such as indium tin oxide (ITO) applied to a glass cover placed adjacent to the liquid crystal layer within the display. Electrical current is then passed through the ITO coating across the display face to generate uniform heating. 
     Available temperature sensors, for providing feedback to thermal controls functions, are opaque and too large to be placed within the stacked layers comprising the display. Consequently, the thermal sensors have been placed about the periphery to provide information about the temperature of the LCD material. This method has limitations for determining the temperature of the central portion of the display and extent of thermal gradients present. 
     Thermal gradients form across the surface of the display due to non-uniform flow of heat from the liquid crystal material to the adjacent environment and display housing. 
     Consequently, there exists a need for improved thermal sensing across the LCD assembly, in order to provide better thermal management of the display. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an improved image for LCDs. 
     It is a feature of the present invention to include a thermal sensor disposed inwardly of the LCD periphery. 
     It is an advantage of the present invention to better assess the temperature gradient existing across an LCD viewing surface. 
     It is another object of the present invention to reduce visual discontinuities at interior portions of the LCD viewing surface. 
     It is another feature of the present invention to include a thermal sensor device made of transparent materials. 
     It is another advantage of the present invention to reduce visual discontinuities caused by placing opaque materials in the viewing surface of an LCD. 
     It is yet another object of the present invention to provide an LCD heater control device which is readily manufacturable. 
     It is yet another feature of the present invention to include a void in a heater surface across the viewing surface of an LCD. 
     The present invention is a method and apparatus for heating an LCD which is designed to satisfy the aforementioned needs, provide the previously stated objects, include the above-listed features and achieve the already articulated advantages. 
     Accordingly, the present invention is a method and apparatus providing a thermal sensor in an interior portion of an LCD viewing surface and controlling the LCD based upon an output of the sensor that is substantially invisible to the unaided eye. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may be more fully understood by reading the foregoing description of the preferred embodiments of the invention in conjunction with the appended drawings wherein: 
     FIG. 1 is a simplified diagram of an LCD heater and control of the present invention. 
     FIG. 2 is a schematic diagram of a portion of the control function of FIG.  1 . 
    
    
     DETAILED DESCRIPTION 
     Now referring to the drawings, wherein like numerals refer to like matter throughout, and more particularly to FIG. 1, there is shown a simplified representation of an LCD heater plate, of the present invention, generally designated  100 , having a viewable surface  102  with a non-viewable surface periphery  104  disposed therearound. Viewable surface  102  may have a transparent resistive heating element such as a film of ITO, or other transparent electrically conductive material, disposed thereon. Non-viewable surface periphery  104  is a common arrangement for LCDs where there may be drive electronics or periphery thermal sensors (not shown). A first thermal sensor  110  is shown having a first thermal sensor main body  112  with a first thermal sensor upper electrical isolation void  114  disposed adjacent thereto. First thermal sensor upper electrical isolation void  114  may be of various shapes and dimensions; however, it may be preferred that the dimensions of first thermal sensor upper electrical isolation void  114  be small enough to provide for electrical isolation of first thermal sensor main body  112  with the remainder of viewable surface  102 . First thermal sensor  110  may be a transparent conductor, such as ITO or other known similar material having otherwise suitable characteristics. Also shown adjacent to first thermal sensor main body  112  is first thermal sensor lower electrical isolation void  116 , which may be very similar to first thermal sensor upper electrical isolation void  114 . First thermal sensor  110  has a first thermal sensor sensing region  118  disposed therein which may be made of the same material as first thermal sensor main body  112 . The first thermal sensor sensing region  118  is disposed at a region on viewable surface  102  in which a temperature determination is desired. First thermal sensor sensing region  118  has different dimensional characteristics, with respect to first thermal sensor main body  112 , which will result in differing electrical resistance therethrough at differing temperatures. 
     Also shown is second thermal sensor  120 , which is similar in function to first thermal sensor  110 , but is disposed at a different location on said viewable surface  102 . Second thermal sensor  120  has a second thermal sensor main body  122 , a second thermal sensor upper electrical isolation void  124 , and a second thermal sensor lower electrical isolation void  126  associated therewith in a fashion similar to first thermal sensor  110 . Second thermal sensor  120  is shown having a second thermal sensor sensing region  128  therein, which has a differing shape characteristic from first thermal sensor sensing region  118 . Various shapes may be used for such sensing regions, and the precise shape or configurations are a matter of design detail which is capable of numerous variations depending upon particular requirements or desires of a particular LCD thermal sensor. The designs shown are merely representative of these countless possible variations. 
     Also shown disposed on viewable surface  102  is third thermal sensor  170 , which has a third thermal sensor main body  172 , third thermal sensor upper electrical isolation void  174 , and third thermal sensor lower electrical isolation void  176  in a fashion similar to first thermal sensor  110 , first thermal sensor main body  112 , first thermal sensor upper electrical isolation void  114 , and first thermal sensor lower electrical isolation void  116  respectively. Third thermal sensor  170  is shown disposed at a predetermined distance from first thermal sensor  110 . Third thermal sensor  170  is shown as a uniform design across the surface of heater plate  100  and may be used as a reference for better assessing the temperature at first thermal sensor sensing region  118 . 
     First thermal sensor  110 , second thermal sensor  120 , and third thermal sensor  170  are coupled to control device  160  through first thermal sensor first line  132  and first thermal sensor second line  134 ; second thermal sensor first line  142  and second thermal sensor second line  144 ; and third thermal sensor first line  152  and third thermal sensor second line  154 , respectively. 
     Also shown coupled to viewable surface  102  is first heating strip  180  having first heating strip right side contact  186  and first heating strip left side contact  188  coupled thereto is first heater drive line  182  and second heater drive line  184 , which represent potentially numerous other drive lines coupled to other heating portions of viewable surface  102  to produce heat in a well known manner. 
     The LCD heater plate  100  of the present invention may be constructed using known manufacturing techniques such as etching and masking. Etching the previously deposited ITO from surfaces  102  and  104  or masking surfaces  102  and  104  during the ITO deposition phase, can be used to arrive at the first thermal sensor  110 , second thermal sensor  120 , and third thermal sensor  170 . 
     Another point about sensor placement and shape upon the LCD heater plate  100  is that placement and shaping of the sensor element governs the size of the sensing area, as to whether it is localized ( 118 ,  128 ) or averaging ( 172 ) across the display surface. 
     Another consideration is that the LCD heater plate  100  can be comprised of many thermal sensors embedded between thermal heat strips, an example of which is first heating strip  180 . The quantity of sensors applied to the LCD heater plate  100  depends upon the display application and the designer&#39;s need for thermal management to meet specified requirements. 
     In operation, viewable surface  102  can be heated by applying a voltage differential across points around non-viewable surface periphery  104 , thereby causing current to flow through the heat producing resistive film, all in a well-known manner. Temperature sensors (not shown) disposed adjacent to non-viewable surface periphery  104 , can be used to sense the temperature around non-viewable surface periphery  104  in a traditional and well-known manner. Temperature assessments can be made at interior portions of viewable surface  102  by using first thermal sensor  110 , second thermal sensor  120 , and third thermal sensor  170 . Due to the differing electrical and mechanical characteristics of ITO or other similar materials, depending upon its temperature, an assessment of a temperature at a particular region on viewable surface  102  can be made. The LCD heater plate  100  is calibrated by making numerous resistance measurements across first thermal sensor  110 , second thermal sensor  120 , and third thermal sensor  170  at various known temperatures. Later during operation of LCD heater plate  100  resistance measurements are made at the various sensing regions and compared to resistance levels measured during calibration. An assessment of the present temperature is then readily calculated using known techniques. 
     Now referring to FIG. 2, there is shown a representative circuit which may be used in assessing the temperature gradient from a central portion of viewable surface  102  and various points around non-viewable surface periphery  104 . 
     The patterned thermal sensor  208 , which could be first thermal sensor  110 , second thermal sensor  120  or third thermal sensor  170 , becomes one leg of a four element resistor bridge that has a voltage applied at circuit point  204  relative to circuit point  220 . The other three resistors,  210 ,  212 , and  214 , are selected to provide a minimal electrical difference between points  222  and  224  at a specified reference temperature for the thermal sensor  208 . As the sensed temperature migrates away from the reference temperature, a voltage differential of positive or negative polarity develops across circuit points  222  and  224 . The amplifier  216  buffers this differential, provides gain correction as required by the application, and presents the resultant signal to its output  206 . 
     The output  206  is provided to the thermal control circuits for use in determining how much heater current to provide to LCD heater plate  100  heater elements adjacent to the thermal sensor  208 . The thermal control circuits will seek to apply heater power to maintain an application specific voltage differential across circuit points  222 . 
     The circuit  200  is provided for each thermal sensor placed on LCD heater plate  100 . 
     It is thought that the method and apparatus of the present invention will be understood from the foregoing description and that it will be apparent that various changes may be made in the form, construction, steps and arrangements of the parts and steps thereof, without departing from the spirit and scope of the invention or sacrificing all of their material advantages. The form herein described is merely a preferred or exemplary embodiment thereof.