Liquid crystal display device having two metastable states and driving method therefor

• US 5,594,464 A
• Filed: 05/07/1993
• Issued: 01/14/1997
• Est. Priority Date: 05/07/1992
• Status: Expired due to Term

• Current Assignee: Seiko Epson Corporation (Seiko Group)
• Original Assignee: Seiko Epson Corporation (Seiko Group)
• Inventors: Tanaka, Takaaki, Sato, Yuzuru, Nomura, Hiroaki
• Primary Examiner(s): Brier, Jeffery
• Assistant Examiner(s): Mengistu, Amare
• Application Number:
  US08/059,226
• Time in Patent Office: 1,348 Days
• Field of Search: 345/87, 345/89, 345/94, 345/95, 359/85, 359/91, 359/60, 359/56, 359/102, 359/63
• US Class Current: 345/94
• CPC Class Codes: G02F 1/1391 Bistable or multi-stable liquid crystal cells G02F1/141 takes precedence
• CPC Class Codes: G09G 2300/0486 Cholesteric liquid crystals, including chiral-nematic liquid crystals, with transitions between focal conic, planar, and homeotropic states
• CPC Class Codes: G09G 2310/06 Details of flat display driving waveforms
• CPC Class Codes: G09G 2310/061 for resetting or blanking
• CPC Class Codes: G09G 3/3629 using liquid crystals having memory effects, e.g. ferroelectric liquid crystals

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.

Claims

1. A liquid crystal display, comprising:

a chiral nematic liquid crystal medium interposing a pair of electrode substrates having alignment layers formed on their respective surfaces, said chiral nematic liquid crystal medium having a twisted structure with a twist angle φ

in a preselected state comprising one of a first metastable state, a second metastable state, a reset state, and an arbitrary state; and

a drive circuit in communication with said chiral nematic liquid crystal medium for

(a) applying a reset voltage across said liquid crystal display electrodes to induce a transition from the preselected state to the reset state in said chiral nematic liquid crystal medium; and

(b) after application of the reset voltage, applying one of;

(1) a first metastable activation voltage greater than a critical voltage and less than the reset voltage to place said chiral nematic liquid crystal medium in the first metastable state; and

(2) a second metastable activation voltage less than both the critical voltage and the reset voltage to place said chiral nematic liquid crystal medium in the second metastable state.

2. The liquid crystal display of claim 1, wherein when the preselected state of said chiral nematic liquid crystal medium is the first metastable state, said drive circuit applies the reset voltage and the second metastable activation voltage in succession to induce the second metastable state therein.

3. The liquid crystal display of claim 1, wherein when the preselected state of said chiral nematic liquid crystal medium is the second metastable state, said drive circuit applies the reset voltage and the first metastable activation voltage in succession to induce the first metastable state therein.

4. The liquid crystal display of claim 1, wherein the first and second metastable states represent respective relaxation states in said chiral nematic liquid crystal medium after inducing a Frederick'"'"'s transition therein.

5. The liquid crystal display of claim 1, wherein the drive circuit applies the reset voltage to induce a Frederick'"'"'s transition in said chiral nematic liquid crystal medium.

6. The liquid crystal display of claim 1, wherein molecular directors of liquid crystal molecules in said chiral nematic liquid crystal medium interposing said alignment layer planar surfaces are oriented at an angle Θ

therefrom.

7. The liquid crystal display of claim 1, wherein

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

further comprising means coupled to said electrodes to provide time-shared addressing of said electrode groups.

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.

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.

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 (k≦

n) number of scan electrodes, each of said blocks being sequentially line scanned such that each screen frame is time-shared n/k times.

11. The liquid crystal display of claim 1, wherein said drive circuit successively applies:

the reset voltage for a first time period;

either metastable activation voltage for a second time period; and

a third voltage less than either metastable activation voltage for a third time period.

12. The liquid crystal display of claim 11, wherein a selection period relative to said time-shared addressing occurs during the first and second time periods and a nonselection period occurs during the third time period.

13. The liquid crystal display of claim 11, wherein a selection period relative to said time-shared addressing occurs during the second time period and a nonselection period occurs during the first and third time periods.

14. The liquid crystal display of claim 11, wherein only scan electrodes including picture cells that must be rewritten with new data in a next succeeding screen frame are time-shared addressed during the first and second time periods.

15. The liquid crystal display of claim 1, wherein the arbitrary state includes an initial state having a twist angle φ

₀.

16. The liquid crystal display of claim 15, wherein the first metastable state has a twist angle approaching φ

₀ -180°

.

17. The liquid crystal display of claim 16, wherein the second metastable state has a twist angle approaching φ

₀ +180°

.

18. The liquid crystal display of claim 17, wherein

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

further comprising means coupled to said electrodes to provide time-shared addressing of said electrode groups.

19. The liquid crystal display of claim 18 wherein in said scan electrode group, n number of scan electrodes are divided into a plurality of electrode blocks comprising k (k≦

n) number of scan electrodes, each of said blocks being sequentially line scanned such that each screen frame is time-shared n/k times.

20. The liquid crystal display of claim 15, wherein the second metastable state has a twist angle approaching φ

₀ +180°

.

21. A liquid crystal display device, comprising:

a chiral nematic liquid crystal medium interposing a pair of electrode substrates having alignment layers formed on their respective surfaces positioned in facing relation thereto, said chiral nematic liquid crystal medium having a twisted structure with a twist angle φ

₀ in an initial state and having first and second metastable states different from the initial state, the first and second metastable states representing respective relaxation states in said chiral nematic liquid crystal medium after inducing a Frederick'"'"'s transition therein; and

a drive circuit coupled to said chiral nematic liquid crystal display medium for

(a) applying a reset voltage to induce a Frederick'"'"'s transition in said chiral nematic liquid crystal medium from a current state, the current state being selected from the group consisting of the initial state, the first metastable state, the second metastable state, and an arbitrary state; and

(b) after application of the reset voltage, applying one of;

(1) a first metastable voltage greater than a critical voltage and less than the reset voltage to place said chiral nematic liquid crystal medium in the first metastable state; and

(2) a second metastable voltage less than both the critical voltage and the reset voltage to place said chiral nematic liquid crystal medium in the second metastable state.

22. The liquid crystal display of claim 21, wherein when the current state of said chiral nematic liquid crystal medium is the first metastable state, said drive circuit applies the reset voltage and the second metastable voltage in succession to induce the second metastable state therein.

23. The liquid crystal display of claim 21, wherein when the current state of said chiral nematic liquid crystal medium is the second metastable state, said drive circuit applies the reset voltage and the first metastable voltage in succession to induce the first metastable state therein.

24. The liquid crystal display of claim 21, wherein molecular directors of liquid crystal molecules in said chiral nematic liquid crystal medium interposing said alignment layer planar surfaces are oriented at an angle Θ

therefrom.

25. The liquid crystal display of claim 21, wherein the first metastable state has a twist angle approaching φ

₀ -180°

.

26. The liquid crystal display of claim 25, wherein the second metastable state has a twist angle approaching φ

₀ +180°

.

27. The liquid crystal display of claim 21, wherein the second metastable state has a twist angle approaching φ

₀ +180°

.

28. A method for driving a liquid crystal display incorporating a twisted nematic liquid crystal medium interposing a pair of electrode substrates having alignment layers formed on their respective surfaces, the nematic liquid crystal medium having a twisted structure with a twist angle φ

in a preselected state comprising one of a first metastable state, a second metastable state, a reset state, and an arbitrary state, said method comprising the steps of;

(A) applying a reset voltage across the liquid crystal display electrodes to induce a transition from the preselected state to the reset state in the nematic liquid crystal medium; and

(B) applying a first 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.

30. The liquid crystal display driving method of claim 28, wherein the preselected state of the nematic liquid crystal medium is the second metastable state.

31. The liquid crystal display driving method of claim 28, wherein the arbitrary state includes an initial state having a twist angle φ

₀.

32. The liquid crystal display driving method of claim 31, wherein the first metastable state has a twist angle approaching φ

₀ -180°

.

33. The liquid crystal display driving method of claim 32, wherein the second metastable state has a twist angle approaching φ

₀ +180°

.

34. The liquid crystal display driving method of claim 31, wherein the second metastable state has a twist angle approaching φ

₀ +180°

.

35. The liquid crystal display driving method of claim 28, further comprising:

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

time-shared addressing the electrode groups.

36. The liquid crystal display driving method of claim 35, wherein said reset voltage applying step (A) comprises applying the reset voltage simultaneously to all the picture elements before said time-shared addressing step.

37. The liquid crystal display driving method of claim 36, wherein said first metastable voltage applying step (B) comprises applying the first metastable voltage to all the picture elements before said time-shared addressing step.

38. The liquid crystal display driving method of claim 35, 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.

39. The liquid crystal display driving method of claim 38, wherein said first metastable voltage applying step (B) comprises applying the first metastable voltage to the picture elements on a sequential scan electrode group basis prior to said time-shared addressing step.

40. The liquid crystal display driving method of claim 28, wherein

the first metastable state represents a relaxation state in the nematic liquid crystal medium;

wherein said reset voltage applying step (A) applies the reset voltage for a first time period to induce a Frederick'"'"'s transition in the nematic liquid crystal medium; and

wherein said first metastable voltage applying step (B) comprises;

applying a first metastable activation voltage less than the reset voltage and greater than the critical voltage for a second time period to place the nematic liquid crystal medium in the first metastable state; and

thereafter applying a third voltage less than the critical voltage for a third time period to maintain the nematic liquid crystal medium in the first metastable state.

41. The liquid crystal display driving method of claim 40, wherein a selection period relative to said time-shared addressing step occurs during the first and second time periods and a nonselection period occurs during the third time period.

42. The liquid crystal display driving method of claim 40, wherein a selection period relative to said time-shared addressing step occurs during the second time period and a nonselection period occurs during the first and third time periods.

29. The liquid crystal display driving method of claimer 28, wherein the preselected state of the nematic liquid crystal medium is the first metastable state.

43. A method for driving a liquid crystal display incorporating a twisted nematic liquid crystal medium interposing a pair of electrode substrates having alignment layers formed on their respective surfaces, the nematic liquid crystal medium having a twisted structure with a twist angle φ

in a preselected state comprising one of a first metastable state, a second metastable state, a reset state, and an arbitrary state, said method comprising the steps of;

(A) applying a reset voltage across the liquid crystal display electrodes to induce a transition from the preselected state to the reset state in the nematic liquid crystal medium; and

(B) applying a second metastable voltage less than both a critical voltage and the reset voltage to place the nematic liquid crystal medium in the second metastable state.

44. The liquid crystal display driving method of claim 43, wherein the preselected state of the nematic liquid crystal medium is the first metastable state.

45. The liquid crystal display driving method of claim 43, wherein the preselected state of the nematic liquid crystal medium is the second metastable state.

46. The liquid crystal display driving method of claim 43, wherein the arbitrary state includes an initial state having a twist angle φ

₀.

47. The liquid crystal display driving method of claim 46, wherein the first metastable state has a twist angle approaching φ

₀ -180°

.

48. The liquid crystal display driving method of claim 47, wherein the second metastable state has a twist angle approaching φ

₀ +180°

.

49. The liquid crystal display driving method of claim 46, wherein the second metastable state has a twist angle approaching φ

₀ +180°

.

50. The liquid crystal display driving method of claim 43, further comprising:

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

time-shared addressing the electrode groups.

51. The liquid crystal display driving method of claim 50, wherein said reset voltage applying step (A) comprises applying the reset voltage simultaneously to all the picture elements before said time-shared addressing step.

52. The liquid crystal display driving method of claim 51, wherein said second metastable voltage applying step (B) comprises applying the second metastable voltage to all the picture elements before said time-shared addressing step.

53. The liquid crystal display driving method of claim 50, 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.

54. The liquid crystal display driving method of claim 53, wherein said second metastable voltage applying step (B) comprises applying the second metastable voltage to the picture elements on a sequential scan electrode group basis prior to said time-shared addressing step.

55. The liquid crystal display driving method of claim 50, wherein

the second metastable state represents a relaxation state in the nematic liquid crystal medium;

wherein said reset voltage applying step (A) applies the reset voltage for a first time period to induce a Frederick'"'"'s transition in the nematic liquid crystal medium; and

wherein said second metastable voltage applying step (B) comprises;

applying a second metastable activation voltage less than both the reset voltage and the critical voltage for a second time period to place the nematic liquid crystal medium in the second metastable state; and

thereafter applying a third voltage less than the second metastable state activation voltage for a third time period to maintain the nematic liquid crystal medium in the second metastable state.

56. The liquid crystal display driving method of claim 55, wherein a selection period relative to said time-shared addressing step occurs during the first and second time periods and a nonselection period occurs during the third time period.

57. The liquid crystal display driving method of claim 55, wherein a selection period relative to said time-shared addressing stop occurs during the second time period and a nonselection period occurs during the first and third time periods.

58. A liquid crystal display, comprising:

a chiral nematic liquid crystal medium interposing a pair of electrode substrates having alignment layers formed on their respective surfaces, said chiral nematic liquid crystal medium having a twisted structure with a twist angle φ

₀ in an initial state and having first and second metastable states which are different from the initial state and represent respective relaxation states in said liquid crystal medium; and

a drive circuit in communication with said chiral nematic liquid crystal display for

(a) applying a reset voltage across said chiral liquid crystal display electrodes to induce a transition from a preselected state, comprising one of the initial state, the first metastable state, and the second metastable state, to a reset state in said chiral nematic liquid crystal medium; and

(b) after application of the reset voltage, applying one of;

(1) a first metastable activation voltage greater than a critical voltage and less than the reset voltage to place said chiral nematic liquid crystal medium in the first metastable state; and

(2) a second metastable activation voltage less than both the critical voltage and the reset voltage to place said chiral nematic liquid crystal medium in the second metastable state.

59. The liquid crystal display of claim 58, wherein when the preselected state of said chiral nematic liquid crystal medium is the first metastable state, said drive circuit applies the reset voltage and the second metastable activation voltage in succession to induce the second metastable state therein.

60. The liquid crystal display of claim 58, wherein when the preselected state of said chiral nematic liquid crystal medium is the second metastable state, said drive circuit applies the reset voltage and the first metastable activation voltage in succession to induce the first metastable state therein.

61. The liquid crystal display of claim 58, wherein the first and second metastable states represent respective relaxation states in said chiral nematic liquid crystal medium after inducing a Frederick'"'"'s transition therein.

62. The liquid crystal display of claim 58, wherein the drive circuit applies the reset voltage to induce a Frederick'"'"'s transition in said chiral nematic liquid crystal medium.

63. The liquid crystal display of claim 58, wherein molecular directors of liquid crystal molecules in said chiral nematic liquid crystal medium interposing said alignment layer planar surfaces are oriented at an angle Θ

therefrom.

64. The liquid crystal display of claim 58, wherein

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

further comprising means coupled to said electrodes to provide time-shared addressing of said electrode groups.

65. The liquid crystal display of claim 64, 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.

66. The liquid crystal display of claim 64, 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.

67. The liquid crystal display of claim 64, wherein in said scan electrode group, n number of scan electrodes are divided into a plurality of electrode blocks comprising k (k≦

n) number of scan electrodes, each of said blocks being sequentially line scanned such that each screen frame is time-shared n/k times.

68. The liquid crystal display of claim 58, wherein said drive circuit successively applies:

the reset voltage for a first time period;

either metastable activation voltage for a second time period; and

a third voltage less than either metastable activation voltage for a third time period.

69. The liquid crystal display of claim 68, wherein a selection period relative to said time-shared addressing occurs during the first and second time periods and a nonselection period occurs during the third time period.

70. The liquid crystal display of claim 68, wherein a selection period relative to said time-shared addressing occurs during the second time period and a nonselection period occurs during the first and third time periods.

71. The liquid crystal display of claim 68, wherein only scan electrodes including picture cells that must be rewritten with new data in a next succeeding screen frame are time-shared during the first and second time periods.

72. The liquid crystal display of claim 58, wherein the first metastable state has a twist angle approaching the φ

₀ -180°

.

73. The liquid crystal display of claim 72, wherein the second metastable state has a twist angle approaching φ

₀ +180°

.

74. The liquid crystal display of claim 58, wherein the second metastable state has a twist angle approaching φ

₀ +180°

.

75. A method for driving a liquid crystal display incorporating a twisted nematic liquid crystal medium interposing a pair of electrode substrates having alignment layers formed on their respective surfaces, the nematic liquid crystal medium having a twisted structure with a twist angle φ

in an initial state and having first and second metastable states which are different from the initial state and represent respective relaxation states, said method comprising the steps of;

(A) applying a reset voltage across the liquid crystal display electrodes to induce a transition from the preselected state, comprising one of said initial state, said first metastable state, and said second metastable state, to a reset state in the nematic liquid crystal medium; and

(B) applying a first 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.

76. The liquid crystal display driving method of claim 75, wherein the preselected state of the nematic liquid crystal medium is the first metastable state.

77. The liquid crystal display driving method of claim 75, wherein the preselected state of the nematic liquid crystal medium is the second metastable state.

78. The liquid crystal display driving method of claim 75, further comprising:

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

time-shared addressing the electrode groups.

79. The liquid crystal display driving method of claim 78, wherein said reset voltage applying step (A) comprises applying the reset voltage simultaneously to all the picture elements before said time-shared addressing step.

80. The liquid crystal display driving method of claim 79, wherein said first metastable voltage applying step (B) comprises applying the first metastable voltage to all the picture elements before said time-shared addressing step.

81. The liquid crystal display driving method of claim 78, 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.

82. The liquid crystal display driving method of claim 81, wherein said first metastable voltage applying step (B) comprises applying the first metastable voltage to the picture elements on a sequential scan electrode group basis prior to said time-shared addressing step.

83. The liquid crystal display driving method of claim 75, wherein

said reset voltage applying step (A) applies the reset voltage for a first time period to induce a Frederick'"'"'s transition in the nematic liquid crystal medium;

and

wherein said first metastable voltage applying step (B) comprises;

applying a first metastable activation voltage less than the reset voltage and greater than the critical voltage for a second time period to place the nematic liquid crystal medium in the first metastable state; and

thereafter applying a third voltage less than the critical voltage for a third time period to maintain the nematic liquid crystal medium in the first metastable state.

84. The liquid crystal display driving method of claim 83, wherein selection period relative to said time-shared addressing step occurs during the first and second time periods and a nonselection period occurs during the third time period.

85. The liquid crystal display driving method of claim 83, wherein a selection period relative to said time-shared addressing step occurs during the second time period and a nonselection period occurs during the first and third time periods.

86. A method for driving a liquid crystal display incorporating a twisted nematic liquid crystal medium interposing a pair of electrode substrates having alignment layers formed on their respective surfaces, the nematic liquid crystal medium having a twisted structure with a twist angle φ

in an initial state and having first and second metastable states representing respective relaxation states, said method comprising the steps of;

(A) applying a reset voltage across the liquid crystal display electrodes to induce a transition from a preselected state comprising one of said initial state, said first metastable state, and said second metastable state, to the reset state in the nematic liquid crystal medium; and

(B) applying a second metastable voltage less than both a critical voltage and the reset voltage to place the nematic liquid crystal medium in the second metastable state.

87. The liquid crystal display driving method of claim 86, wherein the preselected state of the nematic liquid crystal medium is the first metastable state.

88. The liquid crystal display driving method of claim 86, wherein the preselected state of the nematic liquid crystal medium is the second metastable state.

89. The liquid crystal display driving method of claim 86, further comprising:

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

time-shared addressing the electrode groups.

90. The liquid crystal display driving method of claim 89, wherein said reset voltage applying step (A) comprises applying the reset voltage simultaneously to all the picture elements before said time-shared addressing step.

91. The liquid crystal display driving method of claim 90, wherein said second metastable voltage applying step (B) comprises applying the second metastable voltage to all the picture elements before said time-shared addressing step.

92. The liquid crystal display driving method of claim 89, 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.

93. The liquid crystal display driving method of claim 92, wherein said second metastable voltage applying step (B) comprises applying the second metastable voltage to the picture elements on a sequential scan electrode group basis prior to said time-shared addressing step.

94. The liquid crystal display driving method of claim 89, wherein

wherein said reset voltage applying step (A) applies the reset voltage for a first time period to induce a Frederick'"'"'s transition in the nematic liquid crystal medium;

and

wherein said second metastable voltage applying step (B) comprises;

applying a second metastable activation voltage less than both the reset voltage and the critical voltage for a second time period to place the nematic liquid crystal medium in the second metastable state; and

thereafter applying a third voltage less than the second metastable state activation voltage for a third time period to maintain the nematic liquid crystal medium in the second metastable state.

95. The liquid crystal display driving method of claim 94, wherein a selection period relative to said time-shared addressing step occurs during the first and second time periods and a nonselection period occurs during the third time period.

96. The liquid crystal display driving method of claim 94, wherein a selection period relative to said time-shared addressing step occurs during the second time period and a nonselection period occurs during the first and third time periods.