Liquid crystal display device having two metastable states and driving method therefor
| Title: | Liquid crystal display device having two metastable states and driving method therefor |
|---|---|
| Patent Number: | 5,900,852 |
| Publication Date: | May 04, 1999 |
| Appl. No: | 08/665,168 |
| Application Filed: | June 14, 1996 |
| Abstract: | A high speed multiplex-driven liquid crystal display device is capable of utilizing a bistable nematic liquid crystal medium while maintaining high contrast, fast switching capability and a wide viewing angle. The chiral nematic liquid crystal medium has a twisted structure wherein a first voltage waveform applied in a first time period brings on Frederick's transition followed by a selected second voltage waveform applied in a second time period capable of generating one of two selectable metastable states representing switchable bistable states. |
| Inventors: | Tanaka, Takaaki (Suwa, JPX); Sato, Yuzuru (Suwa, JPX); Nomura, Hiroaki (Suwa, JPX) |
| Assignees: | Seiko Epson Corporation (Tokyo, JPX) |
| Claim: | What is claimed is |
| Claim: | 1. A liquid crystal display, comprising |
| Claim: | a nematic liquid crystal medium interposing a pair of substrates having a plurality of electrodes and alignment layers formed on respective opposing surfaces of said substrates, said nematic liquid crystal medium being in a preselected state comprising one of a first metastable state, a second metastable state, and an arbitrary state; and |
| Claim: | a drive circuit in communication with said nematic liquid crystal medium for |
| Claim: | (a) applying a reset voltage to induce a transition from the preselected state to a reset state in said nematic liquid crystal medium; and |
| Claim: | (b) after application of the reset voltage, applying one of |
| Claim: | (1) a first metastable activation voltage greater than a critical voltage and less than the reset voltage to place said nematic liquid crystal medium in the first metastable state; and |
| Claim: | (2) a second metastable activation voltage less than both the critical voltage and the reset voltage to place said nematic liquid crystal medium in the second metastable state. |
| Claim: | 2. The liquid crystal display of claim 1, wherein when the preselected state of said nematic liquid crystal medium is the first metastable state, said drive circuit applies the reset voltage and the second metastable activation voltage to induce the second metastable state therein. |
| Claim: | 3. The liquid crystal display of claim 1, wherein when the preselected state of said nematic liquid crystal medium is the second metastable state, said drive circuit applies the reset voltage and the first metastable activation voltage to induce the first metastable state therein. |
| Claim: | 4. The liquid crystal display of claim 1, wherein the first and second metastable states represent respective relaxation states in said nematic liquid crystal medium after inducing a Freedericksz transition therein. |
| Claim: | 5. The liquid crystal display of claim 1, wherein the drive circuit applies the reset voltage to induce a Freedericksz transition in said nematic liquid crystal medium. |
| Claim: | 6. The liquid crystal display of claim 1, wherein molecular directors of liquid crystal molecules in said nematic liquid crystal medium interposing said alignment layer planar surfaces are oriented at an angle .THETA. respectively therefrom. |
| Claim: | 7. The liquid crystal display of claim 1, wherein |
| Claim: | an orthogonally arranged scan electrode group and signal electrode group are respectively formed on said substrates, wherein cross points of said electrodes represent picture cells; and |
| Claim: | further comprising means coupled to said electrodes to provide time-shared addressing of said electrode groups. |
| Claim: | 8. The liquid crystal display of claim 7, wherein said drive circuit successively applies the reset voltage and one of said first and second metastable activation voltages to all said picture elements before said picture cells undergo time-shared addressing. |
| Claim: | 9. The liquid crystal display of claim 7, wherein said drive circuit applies the reset voltage and one of said first and second metastable activation voltages to said picture cells on a sequential scan electrode group basis. |
| Claim: | 10. The liquid crystal display of claim 7, wherein in said scan electrode group, n number of scan electrodes are divided into a plurality of electrode blocks comprising k number of scan electrodes, each of said blocks being sequentially line scanned such that each screen frame is time-shared n/k times. |
| Claim: | 11. The liquid crystal display of claim 1, wherein said drive circuit successively applies |
| Claim: | the reset voltage for a first time period; |
| Claim: | either metastable activation voltage for a second time period; and |
| Claim: | a third voltage less than a threshold voltage for a third time period. |
| Claim: | 12. The liquid crystal display of claim 1, wherein the arbitrary state includes an initial state having a twist angle .phi..sub.0. |
| Claim: | 13. The liquid crystal display of claim 12, wherein the first metastable state has a twist angle approaching .phi..sub.0 -180.degree.. |
| Claim: | 14. The liquid crystal display of claim 12, wherein the second metastable state has a twist angle approaching .phi..sub.0 +180.degree.. |
| Claim: | 15. The liquid crystal display of claim 13, wherein the second metastable state has a twist angle approaching .phi..sub.0 +180.degree.. |
| Claim: | 16. A method for driving a liquid crystal display incorporating a liquid crystal medium interposing a pair of substrates having a plurality of electrodes and alignment layers formed on respective surfaces of said substrates, the liquid crystal medium exhibiting a twisted structure in a preselected state comprising one of a first metastable state, a second metastable state, and an arbitrary state, said method comprising the steps of |
| Claim: | (A) applying a reset voltage to induce a transition from the preselected state to a reset state in the liquid crystal medium; and |
| Claim: | (B) applying a metastable voltage greater than a critical voltage and less than the reset voltage to place the nematic liquid crystal medium in the first metastable state. |
| Claim: | 17. The liquid crystal display driving method of claim 16, wherein the preselected state of the liquid crystal medium is the first metastable state. |
| Claim: | 18. The liquid crystal display driving method of claim 16, wherein the preselected state of the liquid crystal medium is the second metastable state. |
| Claim: | 19. The liquid crystal display driving method of claim 16, wherein the arbitrary state includes an initial state having a twist angle .phi..sub.0. |
| Claim: | 20. The liquid crystal display driving method of claim 19, wherein the first metastable state has a twist angle approaching .phi..sub.0 -180.degree.. |
| Claim: | 21. The liquid crystal display driving method of claim 20, wherein the second metastable state has a twist angle approaching .phi..sub.0 +180.degree.. |
| Claim: | 22. The liquid crystal display driving method of claim 19, wherein the second metastable state has a twist angle approaching .phi..sub.0 +180.degree.. |
| Claim: | 23. The liquid crystal display driving method of claim 16, further comprising |
| Claim: | providing an orthogonally arranged scan electrode group and signal electrode group respectively formed on the substrates, wherein cross points of the electrodes represent picture cells; and |
| Claim: | time-shared addressing the electrode groups. |
| Claim: | 24. The liquid crystal display driving method of claim 23, wherein said reset voltage applying step (A) comprises applying the reset voltage simultaneously to all the picture elements before said time-shared addressing step. |
| Claim: | 25. The liquid crystal display driving method of claim 24, wherein said metastable voltage applying step (B) comprises applying the metastable voltage to all the picture elements before said time-shared addressing step. |
| Claim: | 26. The liquid crystal display driving method of claim 23, wherein said reset voltage applying step (A) comprises applying the reset voltage to the picture elements on a sequential scan electrode group basis prior to said time-shared addressing step. |
| Claim: | 27. The liquid crystal display driving method of claim 26, wherein said metastable voltage applying step (B) comprises applying the metastable voltage to the picture elements on a sequential scan electrode group basis prior to said time-shared addressing step. |
| Claim: | 28. The liquid crystal display driving method of claim 16, wherein |
| Claim: | the first metastable state represents a relaxation state in the liquid crystal medium; |
| Claim: | wherein said reset voltage applying step (A) applies the reset voltage for a first time period to induce a Freedericksz transition in the liquid crystal medium; and |
| Claim: | wherein said metastable voltage applying step (B) comprises |
| Claim: | applying a metastable activation voltage less than the reset voltage and greater than the critical voltage for a second time period to place the liquid crystal medium in the first metastable state. |
| Claim: | 29. A method for driving a liquid crystal display incorporating a liquid crystal medium interposing a pair of substrates having a plurality of electrodes and alignment layers formed on respective surfaces of said substrates, the liquid crystal medium having a twisted structure in a preselected state comprising one of a first metastable state, a second metastable state, and an arbitrary state, said method comprising the steps of |
| Claim: | (B) applying a metastable voltage less than both a critical voltage and the reset voltage to place the nematic liquid crystal medium in the second metastable state, the critical voltage being less than the reset voltage. |
| Claim: | 30. The liquid crystal display driving method of claim 29, wherein the preselected state of the liquid crystal medium is the first metastable state. |
| Claim: | 31. The liquid crystal display driving method of claim 29, wherein the preselected state of the liquid crystal medium is the second metastable state. |
| Claim: | 32. The liquid crystal display driving method of claim 29, wherein the arbitrary state includes an initial state having a twist angle .phi..sub.0. |
| Claim: | 33. The liquid crystal display driving method of claim 32, wherein the first metastable state has a twist angle approaching .phi..sub.0 -180.degree.. |
| Claim: | 34. The liquid crystal display driving method of claim 33, wherein the second metastable state has a twist angle approaching .phi..sub.0 +180.degree.. |
| Claim: | 35. The liquid crystal display driving method of claim 32, wherein the second metastable state has a twist angle approaching .phi..sub.0 +180.degree.. |
| Claim: | 36. The liquid crystal display driving method of claim 29, further comprising |
| Claim: | 37. The liquid crystal display driving method of claim 36, wherein said reset voltage applying step (A) comprises applying the reset voltage simultaneously to all the picture elements before said time-shared addressing step. |
| Claim: | 38. The liquid crystal display driving method of claim 37, wherein said metastable voltage applying step (B) comprises applying the metastable voltage to all the picture elements before said time-shared addressing step. |
| Claim: | 39. The liquid crystal display driving method of claim 36, wherein said reset voltage applying step (A) comprises applying the reset voltage to the picture elements on a sequential scan electrode group basis prior to said time-shared addressing step. |
| Claim: | 40. The liquid crystal display driving method of claim 39, wherein said metastable voltage applying step (B) comprises applying the metastable voltage to the picture elements on a sequential scan electrode group basis prior to said time-shared addressing step. |
| Claim: | 41. The liquid crystal display driving method of claim 29, wherein |
| Claim: | the second metastable state represents a relaxation state in the liquid crystal medium; |
| Claim: | applying a metastable activation voltage less than both the reset voltage and the critical voltage for a second time period to place the liquid crystal medium in the second metastable state. |
| Claim: | 42. A liquid crystal display device, comprising |
| Claim: | a pair of opposing substrates, each of said substrates having an electrode and a liquid crystal alignment layer disposed on an interior surface thereof; |
| Claim: | a nematic liquid crystal sandwiched between said substrates, said liquid crystal having a twisted structure of twist angle .phi. in an initial state; and |
| Claim: | driving means for applying across said electrodes |
| Claim: | a first voltage larger than a threshold value (V.sub.O) to induce a reset state of said liquid crystal; |
| Claim: | a second voltage below or above a critical value (Vth) and lower than the threshold value so as to cause said liquid crystal to select one of two metastable states different from said initial state; and |
| Claim: | a third voltage comprising a non-selection voltage. |
| Claim: | 43. The device of claim 42, wherein the twist angle of said liquid crystal is .phi.-180 degrees in a first one and .phi.+180 degrees in a second one of the two metastable states. |
| Claim: | 44. The device of claim 43, wherein |
| Claim: | the critical value includes plural critical values; and |
| Claim: | the first metastable state is selected when the absolute value of the second voltage is below a selected one of the critical values having the lowest absolute value. |
| Claim: | 45. The device of claim 43, wherein |
| Claim: | the second metastable state is selected when the absolute value of the second voltage falls between a first selected one of the critical values having the lowest absolute value and a second selected one of the critical values having the next lowest absolute value. |
| Claim: | 46. A liquid crystal display device, comprising |
| Claim: | a third voltage comprising a non-selection voltage |
| Claim: | wherein pretilt angles (.theta..sub.1, .theta..sub.2) formed by director vectors of said liquid crystal at interfaces between said liquid crystal and each of said liquid crystal alignment layers, said respective interior surfaces of said substrates having opposite signs in the initial state. |
| Claim: | 47. The device of claim 46, wherein the twist angle of said liquid crystal is .phi.-180 degrees in a first one and .phi.+180 degrees in a second one of the two metastable states. |
| Claim: | 48. A liquid crystal display device, comprising |
| Claim: | wherein a scanning electrode group (C.sub.1 -C.sub.2n) and a signal electrode group (S.sub.1 -S.sub.2m) are respectively disposed on said pair of substrates, said scanning and signal electrode groups together defining a matrix of picture elements; and |
| Claim: | wherein said driving means includes means for driving said picture elements by time-shared addressing. |
| Claim: | 49. The device of claim 48, wherein |
| Claim: | said driving means applies a periodic drive voltage including the first, second and third voltages to each picture element, the periodic drive voltage having a period including a selection period and a nonselection period; and |
| Claim: | wherein said driving means applies the first and second voltages during the selection period and the third voltage during the nonselection period respectively. |
| Claim: | 50. The device of claim 49, wherein, prior to applying said periodic drive voltage, said driving means initializes said liquid crystal to one of said metastable states by applying the first and second voltages to all picture elements. |
| Claim: | 51. The device of claim 50, wherein said driving means initializes the picture elements simultaneously. |
| Claim: | 52. The device of claim 50, wherein said driving means applies a plurality of first and second voltages to initialize said liquid crystal. |
| Claim: | 53. The device of claim 48, wherein |
| Claim: | wherein said driving means applies the second voltage during the selection period and the third and first voltages during the nonselection period respectively. |
| Claim: | 54. The device of claim 53, wherein, prior to applying said periodic drive voltage, said driving means initializes said liquid crystal to one of said metastable states by applying the first and second voltages to all picture elements. |
| Claim: | 55. The device of claim 54, wherein said driving means initializes the picture elements simultaneously. |
| Claim: | 56. The device of claim 54, wherein said driving means applies a plurality of first and second voltages to initialize said liquid crystal. |
| Claim: | 57. The device of claim 48, wherein |
| Claim: | said scanning electrode group (C.sub.1 -C.sub.2n) is divided into n blocks of k scanning electrodes in which k is less than n; and |
| Claim: | wherein each block is line-sequentially scanned such that said driving means time-shares each screen n/k times. |
| Claim: | 58. The device of claim 48, wherein said driving means applies a drive voltage including said first and second voltages once to each picture element and subsequently applies the drive voltage only to each line of picture elements defined by a scanning electrode (C.sub.1 -C.sub.2n) and including a picture element for which the metastable state of said liquid crystal is desired to be changed. |
| Claim: | 59. Driving method for a liquid crystal display device that sandwiches nematic liquid crystal between a pair of substrates equipped with a plurality of electrodes and liquid crystal alignment layers, the liquid crystal having a twisted structure in its initial state and having two metastable states different from said initial state as relaxation states of the liquid crystal, the method comprising the steps of |
| Claim: | inducing a reset state in the liquid crystal; |
| Claim: | applying a selection voltage for generating one of the metastable states in the liquid crystal; and |
| Claim: | applying a non-selection voltage below a threshold value specified for the two metastable states for maintaining the selected one of the metastable states in the liquid crystal. |
| Claim: | 60. The driving method of claim 59, wherein |
| Claim: | the liquid crystal display device further comprises a scanning electrode group and a signal electrode group disposed on the pair of substrates, respectively, the scanning and signal electrode groups together defining a matrix of picture elements; and |
| Claim: | wherein said inducing, selection voltage applying and nonselection voltage applying steps each comprise driving the picture elements using time-shared addressing techniques. |
| Claim: | 61. The driving method of claim 60, wherein |
| Claim: | said inducing step and selection voltage applying step are performed during a selection period of time-shared addressing; and |
| Claim: | wherein the non-selection voltage applying step is performed during a non-selection period of the time-shared addressing. |
| Claim: | 62. The driving method of claim 60, wherein |
| Claim: | said selection voltage applying step is performed during a selection period of time-shared addressing; and |
| Claim: | wherein said non-selection voltage applying and inducing steps are performed during a non-selection period of the time-shared addressing. |
| Claim: | 63. The driving method of claim 60, wherein |
| Claim: | said inducing step comprises inducing the Freedericksz transition for all picture elements at least once prior to time-shared addressing; and |
| Claim: | wherein said selection voltage applying step comprising applying the selection voltage at least once prior to time-shared addressing for selecting one of the metastable states. |
| Current U.S. Class: | 345/87; 345/94; 345/95; 349/128; 349/179 |
| Current International Class: | G09G 336 |
| Patent References Cited: | 3837730 September 1974 Hatfield et al. 4239345 December 1980 Berreman et al. 4333708 June 1982 Boyd et al. 4411496 October 1983 Nonomura et al. 4505548 March 1985 Berreman et al. 4529271 July 1985 Berreman et al. 4566758 January 1986 Bos 4582396 April 1986 Bos et al. 4601542 July 1986 Meyer 4601543 July 1986 Boyd et al. 4601544 July 1986 Cheng et al. 4717243 January 1988 Boyd et al. 4770500 September 1988 Kalmanash et al. 5095376 March 1992 Umeda et al. 5189535 February 1993 Mochizuki et al. 5287205 February 1994 Yamazaki et al. 5488499 January 1996 Tanaka et al. |
| Other References: | M. Schadt and W. Helfrich entitled, "Voltage-Dependent Optical Activity of A Twisted Nematic Liquid Crystal", Applied Physics Letters, vol. 18(4), pp. 127-128 (1971). T.J. Scheffer and J. Nehring "A New, Highly Multiplexable Liquid Crystal Display", Applied Physics Letters, vol. 45(10), pp. 1021-1023 (Nov. 15, 1984). Robert B. Meyer and R. N. Thurston, "Discovery of DC Switching of a Bistable Boundary layer Liquid Crystal Display", Applied Physics Letters, vol. 43(4), pp. 342-344, (Aug. 15, 1983). "Numerical modelling of twisted nematic devices"; D.W. Berreman, Bell Laboratories, New Jersey; Phil. Trans. R. Soc. Lond. A, pp. 203-216, 1983. |
| Primary Examiner: | Saras, Steven J. |
| Assistant Examiner: | Vail, Seth D. |
| Attorney, Agent or Firm: | Gabrik, Michael T. |
| Accession Number: | edspgr.05900852 |
| Database: | USPTO Patent Grants |
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