Dynamic drive method and apparatus for a bistable liquid crystal display

• US 5,748,277 A
• Filed: 02/17/1995
• Issued: 05/05/1998
• Est. Priority Date: 02/17/1995
• Status: Expired due to Term

• Current Assignee: Kent State University
• Original Assignee: Kent State University
• Inventors: Huang, Xiao-Yang, Bos, Philip J., Yang, Deng-Ke
• Primary Examiner(s): Sikes, William L.
• Assistant Examiner(s): NGUYEN, TIEP HUU
• Application Number:
  US08/390,068
• Time in Patent Office: 1,173 Days
• Field of Search: 359/84, 359/55, 359/91, 345/210, 345/179, 345/169, 345/143, 345/95
• US Class Current: 349/169
• CPC Class Codes: G02F 1/13718 based on a change of the texture state of a cholesteric liquid crystal
• 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 2310/0205 Simultaneous scanning of several lines in flat panels
• CPC Class Codes: G09G 2310/06 Details of flat display driving waveforms
• CPC Class Codes: G09G 2320/0209 Crosstalk reduction, i.e. to reduce direct or indirect influences of signals directed to a certain pixel of the displayed image on other pixels of said image, inclusive of influences affecting pixels in different frames or fields or sub-images which constitute a same image, e.g. left and right images of a stereoscopic display
• CPC Class Codes: G09G 3/2011 by amplitude modulation
• CPC Class Codes: G09G 3/2014 by modulation of the duration of a single pulse during which the logic level remains constant
• CPC Class Codes: G09G 3/3629 using liquid crystals having memory effects, e.g. ferroelectric liquid crystals
• CPC Class Codes: G09G 3/3681 suitable for passive matrices only

Abstract

A flat-panel liquid crystal display. The display includes a flat sheet of bistable chiral nematic liquid crystal material activated by a drive circuit that individually controls the display state of multiple picture elements at a refresh rate 1000 scan lines per second. The driver circuitry activates the liquid crystal domains into homeotropic states over a relatively long activation period and then, during a short (˜1 msec.) selection period, either keeps the domains in a homeotropic state or initiate a transition to the transient twisted planar state. The drivers then activate the domains in an evolution phase to provide either focal conic or twisted planar end states across the two-dimensional array of picture elements.

Claims

1. A method of addressing a bistable chiral nematic liquid crystal material disposed between electrodes arranged on opposed sides of said liquid crystal and adapted to selectively apply an electric field through said liquid crystal material, said method comprising the steps of energizing said electrodes to establish a preparation voltage across said liquid crystal during a preparation phase, thereafter energizing said electrodes to establish a selection voltage across said liquid crystal during a selection phase for selecting a final display state for said liquid crystal, next energizing said electrodes to establish an evolution voltage across said liquid crystal during an evolution phase, and thereafter permitting said liquid crystal to exhibit the final display state.

2. The method according to claim 1 wherein after said evolution phase, the evolution voltage to said electrodes is reduced to allow said liquid crystal to exhibit one of its final display states.

3. The method according to claim 1 wherein after said evolution phase, the voltage to said electrodes is removed to allow said liquid crystal to exhibit one of its final display states.

4. The method according to claim 1 wherein said preparation voltage is of a magnitude, and said preparation phase is of a duration, sufficient to homeotropically align said liquid crystal material.

5. The method according to claim 4 wherein said preparation phase is up to about 40 ms in duration.

6. The method according to claim 1 wherein said liquid crystal is a chiral nematic liquid crystal and the magnitude of said selection voltage is selected to either maintain said liquid crystal in a homeotropic texture or to permit said liquid crystal to initiate a transition to a transient twisted planar texture.

7. The method according to claim 1 or 6 wherein said selection phase is less than about 2 ms in duration.

8. The method according to claim 1 wherein said liquid crystal is a chiral nematic liquid crystal and said selection voltage is of a magnitude sufficient to establish one of a focal conic or a twisted planar final display state.

9. The method according to claim 1 wherein said liquid crystal is a chiral nematic liquid crystal and said evolution voltage is of a magnitude, and said evolution phase is of a duration, effective to maintain liquid crystal that is in a homeotropic texture in said homeotropic texture, and to allow liquid crystal in a transient twisted planar texture to evolve to a focal conic texture.

10. The method according to claim 1 wherein said liquid crystal is a chiral nematic liquid crystal and said evolution voltage is of a magnitude, and said evolution phase is of a duration, effective to allow liquid crystal in a transient twisted planar texture to evolve to a reflecting twisted planar texture, and to allow liquid crystal in a homeotropic texture to evolve to a focal conic texture.

11. The method according to claim 1 wherein said liquid crystal is a chiral nematic liquid crystal and said preparation phase includes a pre-aligning sequence comprising applying a voltage effective to homeotropically align said liquid crystal, followed by a time interval of from about 0.5 to about 5 ms wherein the voltage is removed or reduced to a magnitude effective to permit said liquid crystal to commence a transition to a transient twisted planar texture, and then applying a voltage effective to homeotropically align the liquid crystal for the balance of said preparation phase.

12. The method according to claim 1 wherein said liquid crystal is a chiral nematic liquid crystal having a pitch length in said twisted planar texture effective to reflect light in the visible spectrum.

13. A method for displaying information comprising the steps of:

a) providing a layer of bistable liquid crystal material and arranging electrodes on opposed sides of the liquid crystal material to form an array of display elements;

b) coupling the electrodes to a drive circuit for energizing the electrodes to selectively activate the display elements of the array to one of a first and a second final display; and

c) addressing selected display elements of the array by sequentially energizing the liquid crystal material at a selected display element location with a preparation signal during a preparation phase, reducing the preparation signal to a selection signal for pre-disposing the liquid crystal material to later attain a desired final display state, changing the signal across the display element to an evolution signal during an evolution phase, and then allowing the display element to exhibit the desired final display state which is different than the state of the liquid crystal material during application of said selection signal.

14. The method of claim 13 wherein the first final display state is a twisted planar texture of the liquid crystal material and the second final display state is a focal conic texture of the liquid crystal material.

15. The method of claim 13 comprising providing a layer of chiral nematic liquid crystal material having a pitch length effective to reflect light in the visible spectrum.

16. The method of claim 13 comprising arranging said electrodes on one side of the liquid crystal layer in generally parallel rows and electrodes on an opposite side of the liquid crystal layer in generally parallel columns substantially orthogonal to said rows, whereby said display elements are defined by the points on said rows and columns at which said rows and columns would intersect when in the same plane, and further wherein a plurality of rows of display elements are simultaneously in said preparation phase.

17. The method according to claim 13 comprising arranging said electrodes on one side of the liquid crystal layer in generally parallel rows and electrodes on an opposite side of the liquid crystal layer in generally parallel columns substantially orthogonal to said rows, whereby said display elements are defined by the points on said rows and columns at which said rows and columns would intersect when in the same plane, and further wherein a plurality of rows of display elements are simultaneously in said evolution phase.

18. A display apparatus comprising:

a) a layer of bistable chiral nematic liquid crystal material;

b) multiple electrodes spaced on opposite sides of the liquid crystal layer for applying selected activation voltages across multiple picture elements of the liquid crystal layer; and

c) control electronics for sequentially setting a display state of multiple picture elements of the liquid crystal layer comprising circuitry for;

i) applying a preparation signal across a selected picture element during a preparation phase;

ii) applying a selection signal across said selected picture element during a selection phase to select a predetermined final display state; and

iii) applying an evolution signal across said selected picture element during an evolution phase.

19. The apparatus of claim 18 wherein the control electronics comprises circuitry for applying bipolar preparation, selection and evolution signals to the multiple picture elements.

20. The apparatus of claim 18 wherein electrodes on one side of the liquid crystal layer are arranged in generally parallel rows and electrodes on an opposite side of the liquid crystal layer are arranged in generally parallel columns substantially orthogonal to said rows and said picture elements are defined by the points on said rows and columns at which said rows and columns would intersect when in the same plane, wherein when the control electronics applies a voltage signal to a row electrode, all picture elements in that row are electrically connected to the same voltage signal and when the control electronics applies a voltage signal to a column electrode, all picture elements in that column are connected to the same voltage signal.

21. The apparatus of claim 18 wherein the control electronics includes a timer to control the timer durations of the preparation, selection and evolution signals.

22. The apparatus of claim 20 wherein the control electronics includes circuitry for applying said preparation signal to a plurality of rows of said picture elements.

23. The apparatus of claim 20 wherein said control electronics includes circuitry for applying said evolution signal to a plurality of rows of said picture elements.

24. The apparatus of claim 20 wherein said control electronics includes circuitry for simultaneously applying said selection signal to a plurality of rows of said picture elements.

25. The apparatus of claim 18, wherein said control electronics circuitry predisposes said layer of bistable chiral nematic liquid crystal material to a state different than said predetermined final display state during said selection phase.