Abstract:
An LCD heater having independent closed loop control over heating for a plurality of zones on the display, wherein each zone has a relatively homogeneous heat dissipation or heat sinking properties and one of the zones is a central region of the display which is heated by an ITO film and has its temperature sensed, by a transparent sensor, at a central point within the viewable section of the LCD.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application relates to a U.S. Patent application by Briley and Davis, entitled PATTERNED THERMAL SENSOR, filed on May 3,1999 and having Ser. No. 09/303,729, which application is incorporated herein in its entirety by this reference. 
    
    
     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 systems and controls for LCD heaters. 
     In the past, LCD heater plates have been incorporated to increase the temperature of 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 was then passed through the ITO coating across the display face to generate heat. 
     While these LCD heaters have enjoyed considerable use in the past, they have several serious drawbacks, especially when used with the new optical compensators with thermal sensitivity. LCDs typically have varied heat dissipation characteristics. The differing rates of cooling of various portions of an LCD display typically result in differing temperatures and a concomitant optical non-uniformity. 
     Consequently, there exists a need for improved thermal management across the LCD assembly, in order to provide better optical performance 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 closed loop LCD heating system. 
     It is an advantage of the present invention to better manage the temperature gradient existing across an LCD viewing surface. 
     It is another feature of the present invention to include a plurality of closed loop heating systems, where each closed loop controls the temperature of a display region having a relatively isothermal dissipation characteristic. 
     It is another advantage of the present invention to provide for more precision in the heating of LCD displays. 
     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 plurality of independent closed loop LCD heating systems where each closed loop has associated with it a display region with generally an isothermal heat dissipation characteristic. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may be more fully understood by reading the following 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 of the present invention. 
     FIG. 2 is a schematic diagram of the heater of FIG. 1, where the dashed lines represent heat flow from the heaters to the thermal sensors. 
    
    
     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 panel (either inclusive of or exclusive of associated optical components, such as polarizers, optical compensators, glass etc.), 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 connections (not shown). A top thermal sensor  112  is shown disposed in a centrally located position along the top edge of the viewing area. Top thermal sensor  112  is preferably a patterned transparent sensor made of ITO or other substance and in accordance with the above cross referenced patent application. 
     A middle thermal sensor  114  is disposed below the top thermal sensor  112  and preferably in a similar location so as to sense the temperature at a laterally central point in the middle of the viewable surface  102 . Similarly bottom thermal sensor  118  is shown disposed at a laterally central position at the bottom of the viewable surface  102 . Sensors  112 ,  114 , and  118  are preferably similar in design and operation. A reference thermistor  116  is shown disposed at the bottom of the viewable surface  102  in close proximity and at a known distance from bottom thermal sensor  118 . The reference thermistor  116  is not essential, but may be preferred because the thermistor  116  would be a well-known thermal sensor which has well-known and predictably acceptable operational characteristics including, but not limited to, mean time before failure characteristics. Reference thermistor  116  is used to provide a quality check on the operation of bottom thermal sensor  118 , and presumably the other similar constructed sensors  112  and  114  as well. LCD panel  100  is also shown having a left side edge heater  120 , a bottom edge heater  122 , a right side edge heater  124  and a top edge heater  126 . Heaters  120 ,  122 ,  124  and  126  are preferably well-known foil heaters, such as those available from manufacturers, such as Minco Products, Inc. and having model numbers HFK22074. These foil heaters are preferably placed between the LCD optical stack and its mounting hardware. 
     The present invention divides display  100  into three independent regions, each with an associative heating control loop. The first loop comprises the top thermal sensor  112  and the top edge heater  126 . The top edge of the display has a generally isothermal heat dissipation characteristic and consequently is provided with the top edge heater  126 , which preferably provides generally uniform heating along the top edge of the viewable surface  102 . 
     The middle portions of the viewable surface  102  typically have dramatically different heat dissipation characteristics from any of the edges. Additionally, the middle portions of the viewable surface  102  have a generally isothermal heat dissipation characteristic, at least when compared to the periphery of the viewing surface  102 . The middle portions of the viewing surface  102 , which are covered with a sheet of ITO  103 , form, in combination with the middle thermal sensor  114 , the second independent heating control loop. In the presently described embodiment of the present invention, the third heating control loop is comprised of the bottom thermal sensor  118 , the left side edge heater  120 , the bottom edge heater  122 , and the right side edge heater  124 . In this embodiment, the left, bottom, and side edges of the viewable surface  102  have generally similar heat dissipation characteristics, while the top edge and the middle of the viewable surface  102  each have distinctly different heat dissipation characteristics. 
     Now referring to FIG. 2, there is shown a heater system of FIG. 1, generally designated  200 , which clearly shows three independent control loops  222 ,  224 , and  228 , outlined by dotted lines. The bottom independent heater control loop  228  includes bottom thermal sensor  118 , which senses the temperature at the bottom of the viewable surface  102  (FIG. 1) and generates a bottom thermal sensor signal which may or may not need to be amplified by amp  208 . Bottom thermal sensor signal is compared, via comparator  210 , with the reference thermal signal generated by reference thermistor  116 . A correction signal, representative of the difference between these signals, is generated and provided to each of the control loops  222 ,  224 , and  228 , where it is combined with the amplified thermal sensor signals from thermal sensors  112 ,  114 , and  118  respectively. The bottom thermal sensor signal, after any adjustments owing to the correction signal have been made, is compared with a bottom target temperature  238 . When the corrected bottom temperature signal is lower than prescribed by the bottom target temperature  238 , then the bottom heater driver  248  activates the edge heaters  120 , 122 , and  124 . The top heating control loop  222  is operated similarly to bottom control loop  228 , except that the top thermal sensor  112  does not provide input to generate a correction signal as does bottom control loop  228 . Instead, the correction signal generated using bottom thermal sensor signals is used as a proxy correction signal. Top control loop  222  includes amp  202 , top target temperature  232 , and top heater drive  242  in method analogous to that described above for bottom control loop  228 . Similarly, middle control loop  224  is similar to top control loop  222 . 
     The term “isothermal”, as used herein with respect to heat dissipation and heat sinking properties, is intended to generally convey a state of similar heat dissipation qualities and characteristics and not necessarily exact equality. A relatively isothermal relationship is truly intended. The level of homogeneity of heat dissipation characteristics to be considered “isothermal” is a matter of designer&#39;s choice. 
     It should be understood that the number of control loops is also a matter of designer&#39;s choice, but it is generally preferred that the number of control loops be minimized. To accomplish this, it may be desirable to provide for thermal isolation control mechanisms at various points around the display to increase the uniformity in heat dissipation. In a simplified embodiment, all of the edges would be part of one control loop, and the middle of the viewing surface would be a second control loop. 
     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 arrangement 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.