JPS6249608B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for driving a liquid crystal element, and more particularly to a method for displaying gradations in a ferroelectric liquid crystal element.

Conventionally, liquid crystal display elements are well known in which a scanning electrode group and a signal electrode group are configured in a matrix, and a liquid crystal compound is filled between the electrodes to form a large number of pixels to display images or information. . The driving method for this display element is time-division driving, in which an address signal is selectively and periodically applied to a group of scanning electrodes, and a predetermined information signal is selectively applied in parallel to a group of signal electrodes in synchronization with the address signal. However, this display element and its driving method had major and fatal drawbacks as described below.

That is, it is difficult to increase the pixel density or enlarge the screen. Among conventional liquid crystals, the response speed is relatively high and the power consumption is low, so most of the display elements used in practical use are, for example, TN type liquid crystals. This type of liquid crystal forms a structure (helical structure) in which nematic liquid crystal molecules with positive dielectric anisotropy are twisted in the liquid crystal layer thickness direction in the absence of an electric field, and these liquid crystal molecules are arranged in parallel on both electrode surfaces. forming a structure. On the other hand, when an electric field is applied, nematic liquid crystals with positive dielectric anisotropy are aligned in the direction of the electric field, resulting in optical modulation. When a display element is constructed using this type of liquid crystal with a matrix electrode structure, in the region where both the scanning electrode and the signal electrode are selected (selected point), there is a A voltage higher than the threshold is applied, and no voltage is applied to areas where neither the scanning electrode nor the signal electrode is selected (non-selected points), so the liquid crystal molecules maintain a stable alignment parallel to the electrode surface. . By arranging linear polarizers above and below such a liquid crystal cell in a cross nicol relationship with each other, light does not pass through selected points, but light passes through non-selected points, making it possible to use it as an image element. . However, when a matrix electrode structure is configured, there is a limited area in which scanning electrodes are selected and signal electrodes are not selected, or in areas where scanning electrodes are not selected and signal electrodes are selected (so-called "half-selected points"). The electric field becomes hot. The difference between the voltage applied to the selected point and the voltage applied to the half-selected point is sufficiently large,
If the voltage threshold required to align liquid crystal molecules perpendicular to the electric field is set to an intermediate voltage value, the display element will operate normally, but if the number of scanning lines (N) is increased, , the time during which an effective electric field is applied to one selected point while scanning the entire screen (one frame) (duty ratio) decreases at a rate of 1/N. For this reason, when repeated scanning is performed, the effective voltage difference between selected points and non-selected points becomes smaller as the number of scanning lines increases, and as a result, reduction in image contrast and crosstalk can be avoided. This has become a serious drawback. This phenomenon is caused by liquid crystals that do not have bistability (the stable state is when the liquid crystal molecules are aligned horizontally with respect to the electrode surface, and they are aligned vertically only while an electric field is effectively applied). ) is essentially an unavoidable problem that arises when driving using the temporal accumulation effect (ie, repeated scanning). In order to improve this point, voltage averaging methods, dual frequency driving methods, multiple matrix methods, etc. have already been proposed, but all of these methods are insufficient, and it is difficult to increase the screen size and density of display elements. Currently, the number of scanning lines has reached a plateau due to the inability to increase the number of scanning lines sufficiently. In particular, when the duty ratio becomes low, it becomes difficult to control the gradation of each pixel by voltage modulation, making it unsuitable for liquid crystal elements with a high density of wiring.

SUMMARY OF THE INVENTION An object of the present invention is to eliminate the above-mentioned drawbacks, and more specifically, to provide a driving method for grayscale display in a large-screen, high-density optical modulation element.

An object of the present invention is to provide a method for driving an optical modulation element in which pixels having an optical modulation substance exhibiting a first stable state and a second stable state with respect to an electric field are arranged, in which the applying a first signal to the pixel to orient the optical modulation material in a first stable state; b, in a second phase, the optical modulation material oriented in the first stable state and the optical modulation material oriented in a second stable state; This is achieved by a method of driving the optical modulation element that applies a second signal that causes pixels in which the optical modulation substance is mixed to be generated depending on the gradation.

The optical modulation material used in the driving method of the present invention has a bistable state consisting of a first optically stable state and a second optically stable state with respect to an electric field, and in particular has a bistable state with respect to an electric field as described above. A liquid crystal with bistability is used.

As a liquid crystal having bistability that can be used in the driving method of the present invention, a chiral smectic liquid crystal having ferroelectricity is most preferable, and chiral smectic liquid crystals having chiral smectic C phase (SmC ^* ) or H phase (SmH ^* ) LCDs are suitable. Regarding this ferroelectric liquid crystal, “LE JOURNAL DE
PHYSIQUE LETTRE”) Volume 36 (L-69) 1975
``Ferroelectric Liguid Crystals'' ``Applied Physics Letters''
(“Applied Physics Letters”) Volume 36, No. 11
No., 1980 "Submicro Second Bistable Electroobject Switching in Liquid Crystals"
(“Submicro Second Bistable Electrooptic
"Switching in Liquid Crystals");"Solid State Physics" 16 (141) 1981 "Liquid Crystals", etc., and the ferroelectric liquid crystals disclosed therein can be used in the present invention.

FIG. 2 schematically depicts an example of a ferroelectric liquid crystal cell. 21 and 21' are In ₂ O ₃ , SnO ₂
It is a substrate (glass plate) coated with a transparent electrode such as ITO (Indium-Tin Oxide), between which a layer 22 is oriented perpendicular to the glass surface.
Enclosed is SmC ^* phase or SmH ^* phase liquid crystal. A thick line 23 represents a liquid crystal molecule, and this liquid crystal molecule 23 has a dipole moment 24 (P⊥) in a direction perpendicular to the molecule.
When a voltage higher than a certain threshold is applied between the electrodes on the substrates 21 and 21', the helical structure of the liquid crystal molecules 23 is unraveled, and the liquid crystal molecules 23 change their alignment direction so that all the dipole moments 24 are oriented in the direction of the electric field. Can be done. The liquid crystal molecules 23 have an elongated shape and exhibit refractive index anisotropy in the major and minor axis directions. Therefore, for example, if crossed Nicol polarizers are placed above and below the glass surface, voltage can be applied. It is easily understood that the liquid crystal modulation element has optical characteristics that change depending on the polarity. Furthermore, when the thickness of the liquid crystal cell is made sufficiently thin (for example, 1μ), the helical structure of the liquid crystal molecules is unraveled even when no electric field is applied, as shown in Figure 3, and the dipole moment P or P' is either upward 34 or downward 34'. In such a cell, as shown in Fig. 3, an electric field E of different polarity above a certain threshold or
When E′ is given, the dipole moment becomes the electric field E
Alternatively, the direction is changed to upward direction 34 or downward direction 34' in accordance with the electric field vector of E', and accordingly, the liquid crystal molecules are aligned in either the first stable state 33 or the second stable state 33'. There are two advantages to using such a ferroelectric liquid crystal as a light modulation element. Firstly, the response speed is extremely fast, and secondly, the alignment of liquid crystal molecules has bistability. The second point will be explained with reference to FIG. 3, for example. When the electric field E is applied, the liquid crystal molecules are oriented in a first stable state 33, and this state remains stable even when the electric field is turned off. When an electric field E' in the opposite direction is applied, the liquid crystal molecules are oriented to a second stable state 33' and the orientation of the molecules is changed, but they remain in this state even after the electric field is turned off. Further, as long as the applied electric field E does not exceed a certain threshold value, each orientation state is maintained. In order to effectively realize such fast response speed and bistability, it is preferable for the cell to be as thin as possible, and generally
0.5μ to 20μ, especially 1μ to 5μ are suitable. A liquid crystal electro-optical device having a matrix electrode structure using this type of ferroelectric liquid crystal has been proposed by Clark and Ragabal in US Pat. No. 4,367,924, for example.

A preferred example of the driving method of the present invention is shown in FIG.

FIG. 1A-a is a schematic diagram of a cell 11 having a matrix electrode structure in which a ferroelectric liquid crystal compound is sandwiched between. 12 is a scanning electrode group, and 13 is a signal electrode group. FIGS. 1A-b and 1A-c respectively show an electric signal applied to the selected scan electrode 12(s) and an electric signal applied to the other scan electrodes (unselected scan electrodes) 12(n), FIGS. 1A-d and A-e show the electric signal applied to the selected (information-bearing) signal electrode 13(s) and the unselected (information-free) signal electrode 13(s), respectively.
(n) represents an electrical signal given to Figure 1A
-b to Ae, the horizontal axis represents time and the vertical axis represents voltage. For example, when displaying a moving image, the scanning electrode groups 12 are sequentially and periodically selected.
Now, if the threshold voltage for providing the first stable state of a liquid crystal cell having bistability is V _th1 and the threshold voltage for providing the second stable state is −V _th2 , then the selected scanning electrode 12 As shown in FIG. 1A-B, the electrical signal applied to (s) is an alternating voltage such that at phase (time) _t1 it is 2V and at phase (time) _t2 it is -V. Further, the other scanning electrodes 12(n) are in a grounded state as shown in FIG. 1A-c, and receive an electrical signal O. On the other hand, the electrical signal applied to the selected signal electrode 13(s) is O at phase _t1 and V at phase _t2 , as shown in FIG. ) is the first electric signal given to
O as shown in Figure A-e. In the above,
The voltage value V is V<V _th1 <2V and -V>-V _th2 >-2V
is set to a desired value that satisfies FIG. 1B shows the voltage waveform applied to each pixel when such an electric signal is applied. B-a in Figure 1B,
B-b, B-c and B-d correspond to pixels A, B, C and D in FIG. 1A, respectively. That is,
As is clear from FIG. 1B, all pixels on the selected scanning line are applied with a voltage of -2V exceeding the threshold voltage _-Vth2 in the first phase _t1 .
First, one beam is brought into an optically stable state (second stable state). Among these, the pixel A corresponding to the information signal presence transitions to the other optically stable state (first stable state) because a voltage of 2V exceeding the threshold voltage _Vth1 is applied at the second phase _t2 . . In addition, in the pixel B existing on the same scanning line and corresponding to no information signal, the applied voltage in the second phase _t2 is a voltage V that does not exceed the threshold voltage _Vth1 , so one optical stable state remains in place.

On the other hand, on unselected scanning lines as shown in pixels C and D, the voltages applied to all pixels C and D are +V or O, neither of which exceeds the threshold voltage. Therefore, the liquid crystal molecules in each pixel C and D maintain the orientation corresponding to the signal state when scanned last time without changing the orientation state. That is, when a scanning electrode is selected, first, in a first phase _t1 , one of the electrodes is brought into an optically stable state, and in a second phase _t2 , a signal for one line is written, The signal state can be maintained until the next frame is selected. Therefore, even if the number of scanning electrodes increases, the actual duty ratio does not change, and contrast reduction and crosstalk do not occur at all.

At this time, the value of the voltage value V and the value of the phase (t ₁ + t ₂ )=T depend on the liquid crystal material used and the thickness of the cell, but are usually 3 volts to 70 volts and 0.1
It is used in the range of μsec to 2 msec.

In order to effectively achieve the driving method of the present invention, the electrical signals applied to the scanning electrodes or signal electrodes need not necessarily be simple rectangular wave signals as explained in FIGS. 1b to 1e. It is obvious that it is good. For example, it is also possible to drive with a sine wave or a triangular wave.

FIG. 4 shows a schematic diagram of a matrix electrode structure when applied to a liquid crystal-optical shutter. At this time, 41 is a pixel, and the electrodes on both sides of this portion are made of transparent material. 42 represents a scanning electrode group, and 43 represents a signal electrode group.

FIG. 5 shows another modified embodiment. The difference from the embodiment shown in FIG. 1 is that the voltage at phase t ₁ of the scanning signal 12 (s) shown in FIGS _. Te-V
is being applied. The advantage of this method is that the maximum voltage value of the signal applied to each electrode is only half that of the embodiment shown in FIG.

At this time, FIG. 5A-a shows the waveform of the voltage applied to the selected scanning electrode 12(s);
The unselected scanning electrode 12(n) is shown in FIG.
It is grounded as shown in b, and the electrical signal is Obold. 5A-c shows the waveform of the voltage applied to the selected signal electrode 13(s), and FIG. 5A-d shows the waveform of the voltage applied to the unselected signal electrode 13(s).
The voltage waveform applied to (n) is shown. FIG. 5B shows the waveforms of the voltages applied to each pixel A, B, C and D. That is, B-a in FIG. 5B,
B-b, B-c and B-d correspond to pixels A, B, C and D in FIG. 1A, respectively.

In the explanation of the present invention so far, it is assumed that the liquid crystal compound layer corresponding to one pixel is uniform and the orientation is aligned in one of the stable states over the entire area of one pixel. Ta. However, since the alignment state of the ferroelectric liquid crystal is extremely delicately affected by interaction with the surface of the substrate, if the difference between the applied voltage and the threshold voltage V _th1 or -V _th2 is small,
Due to slight differences in the local substrate surface, a situation may occur in which stable alignment states in mutually opposite directions coexist within one pixel. Utilizing this, it is possible to add a signal that gives gradation to the second phase of the information signal. For example, the driving method and scanning signal described in FIG. 1 are exactly the same, and the pulse at phase t ₂ of the information signal applied to the signal electrode corresponds to the gradation shown in FIG. 6 a to d. By changing the number, it is possible to obtain a gradation image.

In addition to utilizing variations in the substrate surface condition that naturally occur during substrate processing, it is also possible to artificially utilize, for example, a substrate surface condition having a minute mosaic pattern.

Examples of ferroelectric liquid crystal compounds include desyloxybenzolidene-P'-amino-2-methylbutylcinnamate (DOBAMBC) and hexyloxybenzylidene-P'-amino-2-chloropropylcinnamate (HOBACPC). and 4-O-(2
-methyl)-butyl-resolcylidene-4'-octylaniline (MBRA8) and the like.

When constructing an element using these materials, the element is supported by a copper block with a heater embedded, etc., as necessary, in order to maintain the temperature state such that the liquid crystal compound becomes the SmC ^* phase or SmH ^* phase. be able to.

The method of the present invention can be widely applied to the field of optical shutters or displays such as liquid crystal optical shutters and liquid crystal televisions.

[Brief explanation of the drawing]

FIGS. 1A and 1A are plan views schematically showing a liquid crystal element used in the driving method of the present invention. Figure 1 A, b
FIG. 2 is an explanatory diagram showing signals of selected scanning electrodes. FIGS. 1A and 1C are explanatory diagrams showing signals of unselected scanning electrodes. FIGS. 1A and 1D are explanatory diagrams showing information signals of selected signal electrodes. FIGS. 1A and 1e are explanatory diagrams showing information signals of unselected signal electrodes. FIGS. 1B and 1A are waveform diagrams of voltages applied to the liquid crystal of pixel A. FIGS. 1B and 1B are waveform diagrams of voltages applied to the liquid crystal of pixel B. FIGS. 1B and 1C are waveform diagrams of voltages applied to the liquid crystal of pixel C. FIGS. 1B and 1D are waveform diagrams of voltages applied to the liquid crystal of pixel D. FIG. 2 is a perspective view schematically showing a liquid crystal element having a chiral smectoid phase liquid crystal. FIG. 3 is a perspective view schematically showing a liquid crystal element used in the present invention. FIG. 4 is a plan view of a liquid crystal-light shutter using the driving method of the present invention. FIGS. 5A and 5A are explanatory diagrams showing signals of selected scanning electrodes in another specific example. FIGS. 5A and 5B are explanatory diagrams showing signals of unselected scanning electrodes in another specific example. FIGS. 5A and 5C are explanatory diagrams showing information signals of selected signal electrodes in another specific example. FIGS. 5A and 5D are explanatory diagrams showing information signals of unselected signal electrodes in another specific example. FIGS. 5B and 5A are waveform diagrams of voltages applied to the liquid crystal of pixel A in another specific example. FIGS. 5B and 5B are waveform diagrams of voltages applied to the liquid crystal of pixel B in another specific example. FIGS. 5B and 5C are waveform diagrams of voltages applied to the liquid crystal of pixel C in another specific example. FIGS. 5B and 5D are waveform diagrams of voltages applied to the liquid crystal of pixel D in another specific example. FIGS. 6a, 6b, 6c, and 6d are explanatory diagrams showing examples of waveforms of voltages applied to signal electrodes. 11...Liquid crystal element, 12...Scanning electrode group, 12
(s)...Selected scanning electrode, 12(n)...
Unselected scanning electrode, 13...Signal electrode group, 1
3(s)...selected signal electrode, 13(n)...
...Signal electrode not selected, 33...Liquid crystal oriented in the first stable state, 33'...Liquid crystal oriented in the second stable state, 34...Upward dipole moment P, 34'...Downward dipole moment P′.

Claims (1)

[Claims] 1. In a liquid crystal device in which a pixel is formed by arranging a ferroelectric liquid crystal at the intersection of a scanning electrode group and a signal electrode group, by applying a one-polarity voltage signal to the pixel, , by forming a first alignment state of the ferroelectric liquid crystal and then applying a voltage signal of the other polarity, a first alignment state and a second alignment state are generated in the pixel, and the voltage signal of the other polarity is applied. A liquid crystal device characterized in that the area ratio between a first alignment state and a second alignment state is varied according to a voltage waveform.