Abstract:
A drive scheme is described for a display capable of gray scale. Multiple two-level pulses are used in conjunction with dynamic relaxation techniques to write pixels ON or OFF and to adjust the gray level of pixels. Only two voltage levels are used, a maximum level U, and a minimum level 0. This reduces the complexity of the electronics so that the only one voltage generator is needed for the display.

Description:
FIELD OF THE INVENTION  
       [0001]     Electrical drive schemes which enable high-speed gray scale writing of cholesteric (chiral nematic) liquid crystal displays are provided.  
       BACKGROUND OF THE INVENTION  
       [0002]     Information can be displayed on electronically modulated surfaces such as liquid crystal displays (LCDs). Such displays can be used for signage, shelf labels, or large scale displays such as billboards.  
         [0003]     Various types of LCDs are known in the art. Flat panel LCDs can use two transparent glass plates as substrates, as described in U.S. Pat. No. 5,503,952. Such displays are expensive and bulky. Flexible, electronically-written display sheets using nematic liquid crystals materials are disclosed in U.S. Pat. No. 4,435,047. The sheets can be thin glass, or a polymer, for example, Mylar polyester. The nematic liquid crystals require continuous electrical drive to remain transparent. U.S. Pat. No. 5,437,811 discloses a light-modulating cell having a chiral nematic liquid crystal (cholesteric liquid crystal, or ChLC) in polymeric domains contained by patterned glass substrates. The chiral nematic liquid crystal has the property of being driven between a planar state reflecting a specific visible wavelength of light, and a light scattering focal conic state. These two states are stable and can be maintained in the absence of an electric field. This enables larger displays.  
         [0004]     Various drive schemes are known for use with liquid crystal displays. For example, U.S. Pat. Nos. 5,251,048 and 5,644,330 disclose driving methods to switch chiral nematic materials between stable states. However, the update rate of these displays is about 10-40 milliseconds per line of the display, which is too slow for most practical applications. For example, it would take 10-40 seconds to update a 1000 line display. U.S. Pat. Nos. 5,748,277 and 6,154,190 disclose fast driving schemes, called dynamic driving schemes, for chiral nematic displays. The dynamic driving schemes described include a preparation step  1 , selection step  2 , and evolution step  3 , as shown in  FIG. 1A , and optionally further include a pre-holding step  4  and a post-holding step  5  as shown in  FIG. 1B . The driving schemes require complicated electronic driving circuitry. For example, all column and row drivers for a display must output bi-polar and multiple level voltages. The driving scheme results in the appearance of an undesirable black bar shifting across the display during image writing. U.S. Pat. No. 6,268,840 discloses a unipolar waveform drive method to implement the above-mentioned dynamic driving schemes. However, because the preparation step, the selection step, and the evolution step each require distinct voltage amplitudes, both column and row drivers are required to generate multilevel unipolar voltages.  
         [0005]     Rybalochka et al. describes U/{square root}2 dynamic driving schemes in Simple Drive Scheme for Bistable Cholesteric LCDs, SID 2001, pp. 882-885, and in Dynamic Drive Scheme for Fast Addressing of Cholesteric Displays, SID 2000, pp. 818-821. The U/{square root}2 dynamic driving scheme requires a two-level column driver and a two-level row driver, which output either U or 0 voltage, as shown in  FIGS. 1C and 1D . These drive schemes for producing focal conic or planar states do not produce undesirable black shifting bars during writing, but cause the entire frame to go black during the writing.  
         [0006]     U.S. Patent Application Publication No. 2002/0109661 A1 discloses a drive scheme for a gray scale bistable cholesteric reflective display utilizing variable frequency pulses. The addressing method includes applying a predetermined number of pulses to a first plurality of electrodes, and applying a like number of the predetermined number of pulses to a second plurality of electrodes. Each of the predetermined number of pulses has a different frequency, wherein the predetermined number of pulses are applied within a set time period. This disclosure utilizes multiple voltage sources as well as multilevel display drivers, which adds cost and complexity to the power supply and display drivers.  
         [0007]     U.S. Patent Application Publication No. 2003/0085863 A1 discloses a dynamic drive scheme wherein multiple voltages are used to supply a pulse to the liquid crystal between the transient planar state and the stable planar state to drive the display to the focal conic state. More than two voltages are used to derive the appropriate waveforms for the drive scheme. The use of the drive scheme as applied to gray scale displays is disclosed.  
         [0008]     There is a need for a simple, low cost, and fast drive scheme for cholesteric liquid crystal displays that is capable of achieving a gray scale of multiple gray levels using a two-level voltage driving method.  
       SUMMARY OF THE INVENTION  
       [0009]     A method of forming a gray scale on a bistable liquid crystal material display is presented, wherein the method includes applying a first number of pulses to a first plurality of electrodes, and applying a second number of pulses to a second plurality of electrodes, wherein each pulse has the same voltage.  
       ADVANTAGES  
       [0010]     A low cost, effective, and fast gray scale dynamic driving scheme is presented wherein both row and column drivers require only two voltage outputs: U or 0. This reduces power supply and complexity requirements. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     The invention can be understood with reference to the following exemplary drawings:  
         [0012]      FIG. 1A  is 3-phase dynamic drive scheme (prior art);  
         [0013]      FIG. 1B  is 5-phase dynamic drive scheme (prior art);  
         [0014]      FIG. 1C  is a phase diagram of a U/{square root}2 dynamic driving scheme (prior art);  
         [0015]      FIG. 1D  illustrates a U/{square root}2 dynamic driving scheme (prior art);  
         [0016]      FIG. 2  is a perspective view of a cholesteric liquid crystal display;  
         [0017]      FIG. 3A  is a schematic of a cholesteric liquid crystal material in a planar state reflecting light;  
         [0018]      FIG. 3B  is a schematic of a cholesteric liquid crystal material in a focal conic state forward scattering light;  
         [0019]      FIG. 3C  is a schematic of a cholesteric liquid crystal material in a homeotropic state transmitting light;  
         [0020]      FIG. 3D  is a plot of the response of reflectance of a cholesteric liquid crystal material to a pulsed voltage (prior art);  
         [0021]      FIG. 4  is a system diagram with separate row and column drivers;  
         [0022]      FIG. 5  is a system diagram with a common row and column driver;  
         [0023]      FIG. 6  is a phase diagram of a driving scheme with an exploded view of the selection phase;  
         [0024]      FIG. 7   a  is an illustration of the row, column, and resultant pixel waveforms during the preparation phase of the invention;  
         [0025]      FIG. 7   b  is an illustration of the row, column, and resultant pixel waveforms during the evolution phase of the invention;  
         [0026]      FIG. 7   c  is an illustration of the row, column, and resultant pixel waveforms during the pre-selection, selection, and post-selection phases of the invention;  
         [0027]      FIG. 8  is a graph of 16 various reflectance levels with equal sub-selection pulse widths; and  
         [0028]      FIG. 9  is a graph of 16 various reflectance levels with unequal sub-selection pulse widths. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0029]     A low cost, effective, and fast gray scale dynamic driving scheme for a liquid crystal display is described. In the driving scheme, both row and column drivers require only two voltage outputs, U or 0, reducing power supply and complexity requirements.  
         [0030]     As used herein throughout, row voltage is referred to as Urow, and column voltage is referred to as Ucolumn. Urow and Ucolumn can be the same or different. However, both Urow and Ucolumn can be referenced herein as U.  
         [0031]      FIG. 2  depicts an exemplary structure for a cholesteric liquid crystal display  10 . Display  10  can include a substrate  15 , which can be glass, a polymer, or other suitable material. For example, the substrate  15  can be a thin, transparent, polymeric material, for example, a polyester plastic such as Kodak Estar film base, or a polycarbonate. The substrate can be transparent. The substrate can have a thickness of between 20 and 200 microns, for example, about 125 microns.  
         [0032]     Electrodes in the form of first patterned conductors  20  can be formed over substrate  15 . First patterned conductors  20  can be any electrically conductive material, for example, copper, aluminum, or nickel. If first patterned conductors  20  are opaque material, the material can be a metal oxide so that the first patterned conductors  20  are light absorbing. First patterned conductors  20  can be tin-oxide or indium-tin-oxide (ITO). The material of first patterned conductors  20  can be formed as a layer over substrate  15  by any suitable technique, for example, coating, printing, vapor or thin film deposition, or sputtering. The layer can be patterned to form first patterned conductors  20  in any known manner, for example, by photolithography, skiving, laser etching, or chemical etching. The first patterned conductors  20  can have a resistance of less than 250 ohms per square.  
         [0033]     A light modulating material  30  such as a polymer dispersed cholesteric layer can overlay first patterned conductors  20 . The polymer dispersed cholesteric layer  30  can include a polymeric host material with dispersed cholesteric liquid crystal materials, such as Merck BL112, BL118, or BL126, available from E.M. Industries of Hawthorne, N.Y., or those disclosed in U.S. Pat. No. 5,695,682. Application of electrical fields of various amplitude and duration can drive a chiral nematic material into a reflective state, a transmissive state, or an intermediate state. These cholesteric materials have the advantage of maintaining a given state indefinitely after the field is removed.  
         [0034]     The polymeric host material can be deionized photographic gelatin or another organic binder such as polyvinyl alcohol (PVA) or polyethylene oxide (PEO). The liquid crystal material can be dispersed in the deionized gelatin. For example, an 8% concentration of liquid crystal material such as BLI 18, can be dispersed in a 5% deionized gelatin aqueous solution to create domains of the liquid crystal in an aqueous suspension. The dispersion  30  can be coated over the patterned first conductors  20 . The dispersion can be coated on the patterned first conductors 20 by known methods, including equipment associated with photographic films.  
         [0035]     Electrodes in the form of second patterned conductors  40  can overlay polymer dispersed cholesteric layer  30 . In use, second patterned conductors  40  can be supplied with sufficient conductivity to establish an electric field across polymer dispersed cholesteric layer  30 . Second patterned conductors  40  can be formed using materials such as aluminum, silver, platinum, carbon, tungsten, molybdenum, tin, indium, or combinations thereof, by means known in the art, such as vacuum deposition. Oxides of the metals can be used to form darkened second patterned conductors  40  to absorb light. Tin-oxide or indium-tin-oxide coatings permit second patterned conductors  40  to be transparent. Electrodes  20  and  40  on opposite sides of the layer  30  and can form rows and columns, respectively. The intersection of a row and column defines a pixel for applying an electric field.  
         [0036]     Second patterned conductors  40  can be formed by printing conductive ink, for example, Electrodag 423SS screen printable electrical conductive material from Acheson Corporation. Such printed materials are finely divided graphite particles in a thermoplastic resin. The second patterned conductors  40  can be formed using the printed inks to reduce display cost. Forming the display  10  by using a flexible support for substrate  15 , laser etching material to form first patterned conductors  20 , machine coating polymer dispersed cholesteric layer  30 , and printing second patterned conductors  40  results in very low display fabrication costs.  
         [0037]      FIG. 3A  and  FIG. 3B  show two stable states of cholesteric liquid crystals. In  FIG. 3A , a high voltage field has been applied and quickly switched to zero potential, which converts the cholesteric liquid crystals to a planar state  22 , regardless of initial state. Incident light  26  having a specified wavelength and polarization striking cholesteric liquid crystals in planar state  22  is reflected as reflected light  28  to create a bright image. In  FIG. 3B , application of a lower voltage field leaves the cholesteric liquid crystals in a transparent focal conic state  24 , regardless of initial state. Incident light  26  striking cholesteric liquid crystals in focal conic state  24  is forward scattered. Second patterned conductors  40  can be black to absorb forward scattered light  27  and create a dark image when the liquid crystal material is in focal conic state  24 . As a result, a viewer perceives a bright or dark image depending on whether the cholesteric material is in planar state  22  or focal conic state  24 , respectively. The cholesteric liquid crystal material also displays a plurality of reflective states or textures when a part of the cholesteric material is in planar state  22  and the rest is in focal conic state  24 . A viewer can perceive gray level images when closely spaced areas (“domains”) of the liquid crystal material are manipulated to display different reflective states. Multiple domains form a pixel. In  FIG. 3C , cholesteric liquid crystals are in a homeotropic state  25  when a high voltage is applied. Incident light  26  illuminating cholesteric liquid crystals in homeotropic state  25  is transmitted as transmitted light  29 .  
         [0038]      FIG. 3D  illustrates the state of the liquid crystal material after the application of various driving voltages thereto. The liquid crystal material can begin in a first state, either the reflecting planar state  22  shown in  FIG. 3A  or the non-reflecting focal conic state  24  shown in  FIG. 3B , and can be driven with an AC voltage having a root mean square (RMS) amplitude above V 4 , as shown in  FIG. 3D . When the voltage is removed quickly, the liquid crystal material switches to the reflecting state and will remain reflecting. If driven with an AC voltage between V 2  and V 3 , the material will switch into the non-reflecting state and remain so until the application of a second driving voltage. If no voltage is applied, or the voltage is well below V 1 , then the material will not change state, regardless of the initial state. The application of voltages below V 1  will create optical effects but will not cause a switch in the state of the material.  
         [0039]     The driving scheme can use pulse trains with only two voltage levels: U or 0. One voltage source at one voltage can be used for the drive scheme. Circuits and systems for generating pulse trains with different voltage levels for the rows or columns to drive cholesteric liquid crystal displays depending upon the driving scheme are well known. Examples include those described in U.S. Pat. Nos. 6,154,190 and 6,268,840. These circuits and systems can be adapted for use with a single voltage source supplying two voltage levels.  
         [0040]      FIG. 4  shows a display system having a power source  190  providing power to a voltage generator  195 . Examples of power sources can include batteries, solar cells, ac power or other means. The voltage generator  195  is responsible for converting the power from the power source into useable voltages, such as U and other voltages needed to power various system circuits. Data interface  150  can receive information from an external source, wherein the information can contain details of what the display  10  will show after the writing process. Controller  155  can receive data from the data interface  150  to determine the correct voltage waveforms to be presented to the first patterned conductors  20  and the second patterned conductors  40  to drive the display. Row driver  160  and column driver  185  can be supplied with voltage U from the voltage generator  195 . The voltages U or 0V can be selectable on each output of the row driver  160  and column driver  185  by the controller  155 . The controller  155  can be responsible for supplying the correct control signals to the row driver  160  and the column driver  185 , as well as the correct timings of the desired waveforms. Outputs of the row driver  160  and column driver  185  are routed to the first patterned conductors  20  and second patterned conductors  40 , respectively, through interconnects  167 .  
         [0041]      FIG. 5  depicts a display system similar to that of  FIG. 4 , with the exception that the first patterned conductors  20  and second patterned conductors  40  can be driven from a common driver. Because both sets of patterned conducts  20  and  40  can be driven with two voltages, 0V and U, a common driver can be utilized. This can reduce the cost of the system.  
         [0042]      FIG. 6  is a phase diagram of a drive scheme. The drive scheme first applies a preparation waveform to the entire display during the preparation phase  60 . This resets the cholesteric liquid crystals, ChLC, of display  10  into the homeotropic state regardless of the previously written state. After a period of preparation time, Tprep, the drive scheme implements the pre-selection  70 , selection  80  and post-selection  90  phases simultaneously.  
         [0043]     First patterned conductors  20  not yet addressed are said to be in pre-selection  70 , whereby voltage waveforms are applied with a 50% duty cycle with a resultant RMS voltage of U/{square root}2. Due to the hysterysis of the ChLC, this voltage holds the ChLC of display  10  in the homeotropic state until they are ready to be addressed.  
         [0044]     The selection phase  80  can be accomplished by addressing one group of pixels associated with an intersection of one first patterned conductor and all second patterned conductors for a selection time, Ts. After the selection time Ts has passed, another group of pixels associated with an intersection of a different first patterned conductor and all second patterned conductors can be addressed for a second selection time Ts. It is during the selection phase  80  that the final state of the pixels of display  10  are determined. The selection phase occurs individually on all first patterned conductors  20  until all have been addressed. After the selection phase  80 , the post-selection phase  90  begins.  
         [0045]     In the post-selection phase, the ChLC of display  10  can be held in the homeotropic state if, during the selection phase  80  ChLC of display  10  were held in the homeotropic state. If the CHLC of display  10  were allowed to relax out of the homeotropic state into the transient planar state during the selection phase  80 , the post-selection phase  90  can allow the ChLC of display  10  to evolve into the focal conic state. The voltage waveform of the post-selection phase has a RMS voltage of U/{square root}2.  
         [0046]     After the post-selection phase  90 , the evolution phase  100  enables any cholesteric liquid crystals of display  10  that may be in transient planar state to evolve into a focal conic state over the period of evolution, Tev. After the evolution phase  100  for the final first conductor is completed, all power can be removed from the display  10 . Any ChLC of display  10  held in the homeotropic state throughout the writing process can relax through the transient planar state to the stable planar state.  
         [0047]     Gray scale can be achieved in the selection phase  80 , where the selection time, Ts, is divided into sub-selections  110 , as shown in  FIG. 6 . The selection phase  80  can be sub-divided by any number n, where n must be at least 1. During any sub-selection  110 , a voltage waveform for any pixel of the patterned conductor can have a voltage of either 0V or U. There may be as many as 2 n  different combinations of 0 or U voltages during the entire selection time Ts. The sub-selection pulse widths can have an equal time subdivision, or they can be of different time subdivisions. The different patterns of pulses of 0V and U applied to the ChLC can alter the intrinsic relaxation of the ChLC from the hometropic state to the transient planar state. For example, a voltage of U can be periodically applied while the ChLC is relaxing to the transient planar state to drive the cholesteric liquid crystals toward a hometropic state during selection phase  80 . At the end of the selection phase  80 , domains of the ChLC of display  10  can be in a transient planar state due to the combinations of 0V and U applied. The domains in transient planar state can evolve into a focal conic state during post-selection phase  90  or evolution phase  100 . The gray scale level is perceived by the number of closely spaced domains of ChLC that are in the stable planar state or the focal conic state.  
         [0048]      FIG. 7   a  is a diagram of exemplary applied waveforms for the preparation phase  60  that can be applied to first patterned conductors  20  and second patterned conductors  40 . The resultant pixel waveforms illustrate that the pixel voltages alternate between +U and −U, however the RMS voltage is U.  FIG. 7   b  is a diagram of exemplary applied waveforms for the evolution phase  100  that could be applied to first patterned conductors  20  and second patterned conductors  40 . The resultant pixel waveforms illustrate that the pixel voltage is U/{square root}2.  FIG. 7   c  is a diagram of exemplary applied waveforms that could be applied to the first patterned conductors  20  and second pattern conductors  40  for the phases of pre-selection  70 , selection  80 , and post-selection  90 . In this example, the selection phase  80  has been divided into two sub-selection phases  110 .  
         [0049]     The voltages used in the described drive scheme can be formed using a unipolar driver or a bi-polar driver. According to certain embodiments, a unipolar driver is used to reduce cost and complexity of the driver.  
         [0050]     The methods described herein have been reduced to practice. Shown below are values obtained for equally and unequally spaced sub-selection pulses using the 2 n  method exemplified in  FIG. 6 , wherein n is 5. Sixteen gray scale levels were achieved. The results are shown graphically in  FIG. 8  and  FIG. 9 , respectively. The value represents the binary representation of the sub-selections, where a “1” indicates a waveform of U, and a “0” represents a waveform of 0V. For example, the value of “15” in binary is “01111.” 
                                                           Equal Sub-selections vs. Reflectance                    Normalized       Value   Reflectance   Reflectivity                    31   2.69%   0.00       15   2.92%   0.02       30   3.37%   0.05       23   4.28%   0.12       7   4.37%   0.12       29   7.89%   0.38       14   9.20%   0.47       27   11.97%   0.68       19   12.39%   0.71       11   12.50%   0.72       3   12.68%   0.73       13   15.89%   0.96       12   16.00%   0.97       6   16.11%   0.98       21   16.22%   0.99       0   16.41%   1.00                  
 
         [0051]    
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                   
               
               
                   
               
               
                 Unequal Sub-selections vs. Reflectance 
               
             
          
           
               
                   
                   
                 Normalized 
               
               
                 Value 
                 Reflectance 
                 Reflectivity 
               
               
                   
               
             
          
           
               
                 31 
                 3.00% 
                 0.00 
               
               
                 29 
                 4.05% 
                 0.08 
               
               
                 27 
                 4.76% 
                 0.14 
               
               
                 15 
                 5.32% 
                 0.18 
               
               
                 23 
                 6.95% 
                 0.31 
               
               
                 25 
                 7.09% 
                 0.32 
               
               
                 7 
                 7.64% 
                 0.36 
               
               
                 19 
                 9.16% 
                 0.48 
               
               
                 3 
                 9.82% 
                 0.53 
               
               
                 13 
                 10.50% 
                 0.59 
               
               
                 21 
                 11.22% 
                 0.64 
               
               
                 9 
                 11.75% 
                 0.68 
               
               
                 1 
                 12.08% 
                 0.71 
               
               
                 30 
                 14.79% 
                 0.92 
               
               
                 28 
                 15.21% 
                 0.95 
               
               
                 0 
                 15.81% 
                 1.00 
               
               
                   
               
             
          
         
       
     
         [0052]     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.  
       Parts List  
       [0000]    
       
           1  Preparation step  
           2  Selection step  
           3  Evolution step  
           4  Pre-holding step  
           5  Post-holding step  
           10  Display  
           15  Substrate  
           20  First patterned conductors  
           22  Planar state  
           24  Focal conic state  
           25  Homeotropic state  
           26  Incident light  
           27  Forward scattered light  
           28  Reflected light  
           29  Transmitted light  
           30  Polymer dispersed cholesteric layer  
           40  Second patterned conductors  
           60  Preparation Phase  
           70  Pre-selection Phase  
           80  Selection Phase  
           90  Post-selection Phase  
           100  Evolution  
           110  Sub-selection pulse  
           150  Data interface  
           155  Controller  
           160  Row driver  
           167  Interconnects  
           185  Column driver  
           190  Power source  
           195  Voltage generator  
           200  Common driver