Bistable liquid crystal display device

Computer graphics processing and selective visual display system – Display driving control circuitry

Reexamination Certificate

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Details

C345S205000, C345S210000, C345S211000, C345S087000, C345S088000, C345S092000, C345S095000, C345S097000

Reexamination Certificate

active

06320571

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates in general to liquid crystal display devices, and more particularly, to bistable twisted-nematic liquid crystal devices and a method of driving such display devices.
2. Discussion of the Background
A great deal of emphasis has been placed in recent years on liquid crystal display devices. With recognized advantages such as low electrical power consumption and small size, liquid crystal display devices are widely used for audio equipment, instrument panels, office automation equipment, and other uses.
Liquid crystals, which include ordered molecules or groups of molecules in a liquid state, are considerably useful for fabricating these devices for switching, modulating, and otherwise altering the characteristics of light beams. Both differences in transmittance and in the polarizing effect of such liquid crystals have been now utilized for fabricating these devices.
However, it would be more practical for a number of new applications to have a liquid crystal material which has two stable states, and which can be easily transformed from one stable state to the other, rapidly and with a minimum expenditure of energy.
To implement a high speed drive of liquid crystal devices, a variety of liquid crystal displays using bistable twisted-nematic (BTN) liquid crystals have been disclosed as exemplified in Japanese Published Patent Application No. 1-51818 and Japanese Laid-Open Patent Applications Nos. 6-230751, 8-101371 and 8-313878.
Bistable characteristics are shown for twisted-nematic liquid crystals in these disclosures, in which at least two pulse voltages are applied to produce an electric field across a liquid crystal cell. A first pulse is used to initiate the Freedricksz transition of the liquid crystal and a second pulse is used to subsequently relax the liquid crystal to either one of two metastable states, thereby modulating optical transmittance or reflectivity to be utilized as display devices.
Although principles for switching behavior of possible displays are presented in JPA 1-51818, no description is made of driving the displays. Also, JPA 6-230751, 8-101371 and 8-313878 propose basics of driving simple matrix type displays. However, no description is made of either the effects of temperature on display quality, or methods of compensating for the effects, which is deemed important for practical purposes for the display devices.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a novel liquid crystal display device and a method of driving the display device, which overcome the above-noted difficulties.
It is another object of the present invention to provide a novel liquid crystal display device of high display quality, which is capable of achieving a high speed driving over a wide range of temperatures for stable operations, and to provide a method of driving a liquid crystal display device.
To achieve the forgoing and other objects, and to overcome the drawbacks discussed above, a novel liquid crystal display device is provided, having a liquid crystal cell in the present invention. The liquid crystal cell includes two transparent insulating substrates arranged substantially in parallel, each with a confronting surface bearing at least one transparent electrode, an alignment film disposed over the transparent electrode, and a layer of liquid crystal materials contained between the insulating substrates. The liquid crystal material is chiral-nematic liquid crystal with a positive dielectric anisotropy. In addition, the surface of the alignment film is alignment treated with an anti-parallel alignment direction and pre-tilt angles formed on respective alignment film surfaces by a molecular axis of the liquid crystal material at an initial state, being equal to each other, and having a ratio of an unstrained pitch to a thickness of the layer of the liquid crystal material of approximately from 1 to 3.
The liquid crystal cell is capable of being switched by applying a plurality of voltages between first and second metastable states caused by relaxation from a state previously formed by the Freedricksz transition, and the first and second metastable states correspond to arrangements of the liquid crystal molecules gradually twisted between the substrates by 360° or 0°, respectively.
Potential voltages such as first, second, and third voltages are applied between the electrodes of the liquid crystal cell. The first voltage is used to initiate the Freedricksz transition of a layer of liquid crystal molecules, the second voltage is used to select either the first and second metastable states, and the third voltage is used to maintain the selected metastable state.
In the liquid crystal cell of the present invention, according to one aspect, a first voltage is a pulse voltage with a magnitude equal to or greater than a threshold voltage which is determined with respect to an initial state and the two metastable states, and at least either the amplitude or width of the pulse voltage may arbitrarily be adjusted. In addition, the third voltage used to maintain the selected metastable state is a pulse voltage with a magnitude smaller than a threshold voltage determined with respect to the two metastable states.
According to another aspect of the present invention, the liquid crystal cell has a plurality of delineated electrodes on each substrate to serve as scan electrodes or signal electrodes, and each of the electrodes is capable of being individually addressed in a multiplexed fashion by a device for applying voltages.
According to still another aspect of the present invention, the liquid crystal material contained between insulating substrates has a kinetic viscosity of at most 17 (mm
2
/sec) at 20° C., or of at most 40 (mm
2
/sec) at 0° C.
According to another aspect of the present invention, liquid crystal material contained between insulating substrates has an anisotropy of dielectric constant of at least 3.0.
According to another aspect of the present invention, the device for applying a voltage is further provided with a control or automatic control having a temperature sensor, to arbitrarily adjust at least either the amplitude or width of the first voltage applied to initiate the Freedricksz transition.
According to another aspect of the present invention, on one of the substrates of the liquid crystal display panel, red, green or blue color filters may further be provided in a matrix corresponding to individual pixels to thereby constitute a color display panel.
Methods are also disclosed for carrying out the driving of the liquid crystal cell by applying first, second, and third voltages between the electrodes, the first voltage being applied to initiate the Freedricksz transition of the layer of the liquid crystal molecules, the second voltage being applied to select either the first and second metastable states, and the third voltage being applied to maintain the selected metastable state. The first voltage is a pulse voltage with a magnitude of at least a threshold voltage determined with respect to an initial state and the two metastable states, and either the amplitude or width of the pulse voltage may arbitrarily be adjusted.


REFERENCES:
patent: 5742368 (1998-04-01), Chen
patent: 5748277 (1998-05-01), Huang et al.
patent: 5815132 (1998-09-01), Okada et al.
patent: 5840208 (1998-11-01), Yamaguchi et al.
patent: 5933203 (1999-08-01), Wu et al.
patent: 6111696 (2000-08-01), Allen et al.
Patent Abstracts of Japan, AN 89-062655/09, JP 01051818, Feb. 28, 1989.
Patent Abstracts of Japan, AN 5-121996, JP 6-230751, Aug. 19, 1994.
Patent Abstracts of Japan, AN 07199825, JP 08101371, Apr. 16, 1996.
Patent Abstracts of Japan, AN 07199824, JP 08313878, Nov. 29, 1996.

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