Abstract:
A system and method for adjusting a supply voltage provided by a power supply to a liquid crystal display includes a liquid crystal display having a glass panel, a power supply electrically connected to the liquid crystal display, the power supply configured to provide a supply voltage to the liquid crystal display, a temperature sensor configured to measure the temperature of the glass panel of the liquid crystal display and output a temperature output indicative of the temperature of the glass panel of the liquid crystal display, and a processor in communication with the power supply and the temperature sensor.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates to control systems for liquid crystal displays (“LCDs”). 
         [0003]    2. Description of the Known Art 
         [0004]    Passively driven Super Twisted Neumatic (“STN”) LCDs have competitive advantages over competing technologies such as Vacuum Fluorescent Displays (“VFDs”), Passive Organic Light Emitting Diode Displays (“OLEDs”) and a host of passively addressed Electrophoretic Electronic Paper type displays. STN LCDs generally are capable of operating at much higher multiplex ratios than VFDs and Passive OLEDs resulting in higher resolution capability. LCDs do not suffer from pixel aging image burn-in as experienced by emissive VFDs and OLEDs when the same image is displayed for long periods of times. Unlike most Electrophoeretic Displays, LCDs can be backlit which is an important attribute for night time operation such as in vehicular applications. 
         [0005]    However, STN LCDs require that the supply voltage of the STN LCD be adjusted relatively carefully. If the supply voltage is not carefully adjusted, the STN LCD may be difficult for a user to perceive. Unfortunately, setting the supply voltage once will not be suitable because the supply voltage required for a user to perceive the STN LCD changes as a function of temperature. Referring to  FIG. 1 , a graph generally representing the appropriate supply voltage (VLCD), as it relates to temperature, is shown. Generally, as the temperature increases, the supply voltage is generally decreased. 
         [0006]    STN LCDs used in environments where the temperature remains relatively stable, such as STN LCDs found in a home environment, generally do not need a sophisticated feedback system to adjust the supply voltage as the temperature changes. However, in environments where the there is a broad temperature range, such as the occupant compartment of an automobile, the STN LCD requires a sophisticated feedback system to properly adjust the supply voltage based upon a measured temperature reading. 
         [0007]    Prior art systems generally incorporate a temperature monitor located on or near the glass panel of the LCD, so as to measure the temperature of the glass panel of the LCD. The temperature monitor outputs a signal indicative of the temperature of the glass panel to a microprocessor. The microprocessor then sends an adjustment signal to a power supply that later adjusts the supply voltage accordingly. However, it has been discovered that this type of system does not always accurately adjust the supply voltage. Additionally, these systems may adjust the supply voltage in a linear fashion which, as shown in  FIG. 1 , is not suitable because the relationship between the supply voltage as it relates to temperature is not linear. Therefore, there is a need for an improved LCD control system and method. 
       BRIEF SUMMARY OF THE INVENTION 
       [0008]    A system for adjusting a supply voltage provided by a power supply to an LCD, according to the principles of the present invention includes an LCD having a glass panel, a power supply electrically connected to the LCD, the power supply configured to provide a supply voltage to the LCD, a temperature sensor configured to measure the temperature of the glass panel of the LCD and output a temperature output indicative of the temperature of the glass panel of the LCD, and a processor in communication with the power supply, the temperature sensor and a precise voltage reference. 
         [0009]    In its simplest form, the processor is configured to determine a desired voltage based on the temperature of the glass panel of the liquid crystal display. From there, the processor determines an error between the desired voltage and the supply voltage. Finally, the processor adjusts the supply voltage based on the error to output the desired voltage. 
         [0010]    Further objects, features and advantages of this invention will become readily apparent to persons skilled in the art after a review of the following description, with reference to the drawings and claims that are appended to and form a part of this specification. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a chart showing the relationship between the temperature of a glass panel of an LCD and an optimal supply voltage 
           [0012]      FIG. 2  is a perspective view of an LCD embodying the principles of the present invention; 
           [0013]      FIG. 3  is a block diagram of the LCD of  FIG. 2  and an LCD control system embodying the principles of the present invention; 
           [0014]      FIG. 4  is a flow chart illustrating a method for adjusting a supply voltage provided by a power supply to the LCD of  FIG. 2 ; 
           [0015]      FIG. 5  is a flow chart illustrating a method for determining a desired voltage for the LCD of  FIG. 2 ; 
           [0016]      FIG. 6  is a flow chart illustrating a method for determining an error between the desired voltage and the supply voltage for the LCD of  FIG. 2 ; 
           [0017]      FIG. 7  is a flow chart illustrating a method for adjusting the supply voltage based on the error to output the desired voltage; and 
           [0018]      FIG. 8  is a flow chart illustrating the methods of  FIGS. 5-7 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]    Referring to  FIG. 2 , an LCD  10  is shown. The LCD  10  generally includes a display area  12  whose periphery may be surrounded by a housing  14 . Of course, it should be understood that the display area  12  may incorporate into an instrument cluster of an automobile. The housing  14  functions to protect the LCD  10  from environmental hazards. A transparent panel  16 , generally made of glass or plastic, is retained by the housing  14  over the display area  12 . Similar to the housing  14 , the panel  16  protects the LCD  10  from environmental hazards as well as providing a transparent area for which the display area  12  can be perceived by a user. 
         [0020]    Referring to  FIG. 3 , a block diagram of the LCD  10  and a control system  18  is shown. The LCD  10  may include a heating element  22  for heating the LCD  10 . The heating element  22  is generally a material such as indium tin oxide (“ITO”) applied to the one of the glass layers of the LCD  10  of  FIG. 3 . Electrical current is then passed through the ITO coating to generate heat, so as to improve the response time of the LCD  10  at low temperatures. The LCD  10  also includes a temperature monitor  30  for measuring and outputting the temperature of the glass panel  16  of  FIG. 2 . 
         [0021]    The control system  18  includes a processor  32  configured by a set of instructions  34 . The control system  18  also includes a power supply  20  that provides a supply voltage to the LCD  10 . The processor  32  includes an analog to digital converter (“A/D”) with inputs. The inputs  36 ,  38 , and  40  are multiplexed to the A/D of the processor  32 . The input  36  is in communication with a temperature output  42  of the temperature monitor  30 . When the input  36  receives the temperature output  42  of the temperature monitor  30 , the input  36  will convert the temperature output  42  from an analog signal to a digital signal so that the processor  32  can interpret the data received from the temperature output  42 . 
         [0022]    The input  38  is in communication with a power supply output  48  via a voltage divider  49 . The input  38  converts the output  48  of the power supply  20  to a digital signal so that the processor  32  can properly interpret the output  48  of the power supply  20 . 
         [0023]    The input  40  is connected to a voltage point  50 . The voltage point  50  is located between a resistor  52  and a precision instrument  54 . The voltage point  50  represents a known predetermined voltage value. As will be described with more detail later in this description, the input  40  will measure the voltage point  50  so as to determine if the voltage point  50  is at the correct predetermined amount. If the voltage point  50  is not at the correct predetermined amount, the microprocessor  32  will correct the error of the supply feedback  38 . 
         [0024]    Finally, the processor  32  is connected to an input  56  of the power supply  20 . The input  56  of the power supply  20  allows the microprocessor  32  to control the power supply  20  based on the error value. This error value is essentially added the current control value. When the power supply  20  receives a new control value from the processor  32 , the power supply  20  will adjust the supply voltage to the LCD  10  to compensate for this error value. 
         [0025]    Referring to  FIG. 4 , a method  60  for adjusting the supply voltage of the LCD  10  is shown in its simplest form. As shown in step  62 , the processor  32  first determines a desired voltage based on the temperature of the glass panel  16 . Thereafter, as shown in step  64 , the processor  32  determines the error between the desired voltage and the actual supply voltage of the power supply  20 . Thereafter, the processor  32  adjusts the supply voltage outputted by the power supply  20  based on the error calculated in step  64 . 
         [0026]    As stated previously,  FIG. 4  illustrates the method  60  in its most simplest form.  FIGS. 5-7  illustrate the steps of method  60  of  FIG. 4  in more detail. As such, referring to  FIG. 5 , step  62  of method  60  is explained in more detail. First, as shown in step  68 , the processor  32  at input  36  receives a temperature output  42  from the temperature sensor  30 . Next, as illustrated in step  70 , the processor  32  determines the desired voltage based on the temperature output received in step  68 . 
         [0027]    Referring to  FIG. 6 , the more detailed explanation of step  64  of  FIG. 4  is shown. As a reminder, step  64  stated that the processor  32  determines the error between the desired voltage and the actual supply voltage. First, as illustrated in step  72 , the processor  32  at input  40  measures the voltage point  50 . Next, at step  74 , the processor  32  then determines the difference between a preset value and the voltage point  50  to determine a voltage point error. Generally, the preset value and the voltage point  50  should be the same value. Thereafter, a processor  32  at input  38  measures the feedback value  49 . 
         [0028]    It has been discovered that the power supply  20  does not always output a supply voltage that matches previously determined desired voltage due to A/D converter errors. In an effort to make up for this deficiency, the feedback value  49  is corrected as a result of the error measured at the voltage point  50 . In step  78 , the supply feedback voltage  49  is modified based on the voltage point error. Thereafter, in step  80 , the error between the desired voltage and the supply feedback voltage is corrected for A/D converter errors by using the voltage point error. 
         [0029]      FIG. 7  explains in more detail step  66  of method  60 . As a reminder, step  66  was the step of adjusting the supply voltage to obtain the desired voltage based on the calculated error between the voltage point corrected supply voltage feedback and the desired value. In step  82 , a determination is made by the processor  32  if the error is less than a preset hysteresis value. 
         [0030]    As shown in step  84 , if the error is not less than a hysteresis value, the processor  32  adjusts the power supply voltage to obtain the desired voltage by adding (or subtracting depending on the loop polarity configuration) a proportion of the error to the current power supply control value. This allows the feedback loop to quickly get to within the hysteresis value (conventional PID control loop). However, if the error is less than the hysteresis value, the error is filtered as shown in step  86 . 
         [0031]    While the error is being filtered, a determination is made, as shown in step  88 , if the filter timer has been exceeded. If the filter timer has not been exceeded, the method returns to step  82 . Otherwise, the method continues to step  90  where a determination is made if the filtered error is greater than a trip value or less than a negative trip value. If the filtered error is greater than the trip value, the power supply is instructed to increase the voltage to the LCD by a small amount, usually around 20 millivolts as shown in step  92 . If the filtered error is less than the negative trip value, the voltage to the LCD from the power supply is decreased by a small amount usually 20 millivolts as shown in step  94 . If no condition is true step  90 , the method returns all the way to step  68  of  FIG. 5 . 
         [0032]    It is important to recognize is that if the error is greater than the hysteresis value, then the supply voltage is at least 8 counts away from the desired value and PID loop, as disused in step  84 , is used to quickly adjust the supply voltage by adding a proportion of the error to the current control value. In this manner, when the supply voltage has a large variation from the desired voltage, it is quickly adjusted to within the hysteresis value via  84 . When the error is less than the noise floor (&lt;hysteresis value), then the error is filtered and the loop is controlled in a very slow fashion to get to exactly the desired supply voltage. By so doing, a quick response for large errors and a slow filtered response for errors that are in the noise floor can be achieved. 
         [0033]    Noise is a fairly large problem in feedback systems. Here, noise can be present at inputs  36 ,  38 , and  40 . By filtering the error, all of the noise sources are filtered to obtain the correct average value (i.e. the average noise is zero and so when you filter the signal you are left with the signal component). This approach avoids filtering each of the input components and allows the loop to respond quickly for large errors such as is seen during power up and during fast temperature changes. The slow nature of the filtered loop does not allow your eye to see any flicker component as a result of only allowing small adjustments to be made periodically. 
         [0034]    In an effort to better show how the flow chart is shown in  FIGS. 5-7  interact,  FIG. 8  is provided. In  FIG. 8 , like reference numerals are used to illustrate like steps shown in  FIGS. 5-7 . 
         [0035]    As a person skilled in the art will readily appreciate, the above description is meant as an illustration of implementation of the principles this invention. This description is not intended to limit the scope or application of this invention in that the invention is susceptible to modification, variation and change, without departing from the spirit of this invention, as defined in the following claims.