JPS6246845B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for driving an optical modulation element such as a liquid crystal element, and more particularly to a time division driving method for a liquid crystal element used in an optical modulation element such as a display element or a shutter array.

Conventionally, liquid crystal display elements have been well known, which display images or information by configuring a scanning electrode group and a signal electrode group in a matrix, filling a liquid crystal compound between the electrodes, and forming a large number of pixels. There is. The driving method for this display element is a time-sharing method 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, most of them are used practically as display elements because of their relatively high response speed and low power consumption.
“Applied Letter Physics” Vol.18, No.4
(1971.2.15), P.127-128 “Voltage−
Dependent Optical Activity of a Twisted
TN shown in “Nematic Liquid Crystal”
This type of liquid crystal has a structure (helical structure) in which the molecules of the nematic liquid crystal, which have positive dielectric anisotropy in the absence of an electric field, are twisted in the thickness direction of the liquid crystal layer. ), and the liquid crystal molecules form a structure in which they are arranged parallel to each other on both electrode surfaces. 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, light does not pass through selected points, but light passes through non-selected points, making it possible to use it as an image element. becomes. However, when a matrix electrode structure is configured, there are areas where scanning electrodes are selected and signal electrodes are not selected, or areas where scanning electrodes are not selected and signal electrodes are selected (a "half-selected point"). The finite electric field is also strong. If the difference between the voltage applied to the selected point and the voltage applied to the half-selected point is sufficiently large, and the voltage threshold required to align liquid crystal molecules perpendicular to the electric field is set to a voltage value in between, the display element will function normally. This is why it works. However, in this method, when 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) is 1/N. will decrease at the rate of For this reason, the effective voltage difference between selected points and non-selected points when repeated scanning is performed becomes smaller as the number of scanning lines increases, and as a result, a decrease in image contrast and crosstalk can be avoided. This has become a serious drawback. This type of development requires 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 (that is, repeatedly scanning). In order to improve this point, voltage averaging method, dual frequency drive method, multiple matrix method, 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. An object of the present invention is to provide a novel optical modulation element, particularly a liquid crystal element driving method, which solves all the problems of conventional liquid crystal display elements as described above.

Another object of the present invention is to provide a method for driving a liquid crystal element having high-speed response.

Another object of the present invention is to provide a method for driving a liquid crystal device having high density pixels.

Furthermore, another object of the present invention is to provide a method for driving a liquid crystal element that does not generate crosstalk.

Furthermore, another object of the present invention is to provide a method for driving a liquid crystal element suitable for a partially rewritable display device.

Furthermore, another object of the present invention is to provide a novel method for driving a liquid crystal device using a liquid crystal that is bistable with respect to an electric field, particularly a chiral smect C-phase or H-phase liquid crystal that has ferroelectricity. There is a particular thing.

Furthermore, another object of the present invention is to provide a novel driving method suitable for a liquid crystal device having a high density of pixels and a large screen area.

Furthermore, a specific object of the present invention is to provide a stable driving method using a matrix electrode of a display element using ferroelectric liquid crystal.

The method for driving an optical modulation element of the present invention was developed to achieve the above-mentioned object, and more specifically includes a scanning electrode group and a signal electrode group, the scanning electrode group and the signal electrode group In a method for driving an optical modulation element having a structure in which an optical modulation material having bistable property with respect to an electric field is arranged between a selected scanning electrode of the scanning electrode group and a selected scanning electrode of the signal electrode group, A voltage V _ON1 that orients the bistable optical modulating material to a first stable state is applied between the signal electrode and the selected scanning electrode and the unselected signal electrode of the signal electrode group. and the bistable optical modulating material is in a second state.
At the same time, a voltage V _ON2 is applied to align the optical modulation substance in a stable state, and a threshold voltage −Vth ₂ (Vth 2 The voltage V _OFF is set to a value between Vth ₁ (which refers to the threshold voltage in the 2nd stable state) and Vth 1 (which refers to the threshold voltage in the 1st stable state).
is applied, and each of the voltages V _ON1 , V _ON2 , and V _OFF satisfies the following relationship: 2|V _OFF |<V _ON1 |, |V _ON2 |.

In a preferred embodiment of the present invention, an electric field is formed between a scanning electrode group that is sequentially selected based on a scanning signal and a signal electrode group that faces the scanning electrode group and is selected based on a predetermined information signal. A selected scanning electrode of a liquid crystal element in which a bistable liquid crystal is sandwiched between the electrodes is supplied with an electric signal V ₁ (t) having different voltage polarity at phases t ₁ and t ₂ {the phases t ₁ and t ₂
The maximum value is V ₁ (t) _nax and the minimum value is V ₁ (t) _nio within a phase interval including Give electrical signals V ₂ and V ₂ ′ of different voltages. That is, in the portion of the selected scanning electrode line where the information signal is present, a unidirectional electric field V ₂ −V ₁ which provides the first stable state to the liquid crystal at phase t ₁ (or t ₂ ) is generated.
(t), and in the empty part, an electric field in the opposite direction that gives a second stable state at phase t ₂ (or t ₁ )
By providing V ₂ '−V ₁ (t) and satisfying the following relationship, the liquid crystal element can be driven particularly stably.

1<|V ₁ (t) _nax |/|V ₂ | 1<|V ₁ (t) _nio |/|V ₂ | 1<|V ₁ (t) _nax |/|V ₂ ′| and 1<| V ₁ (t) _nio |/|V ₂ ′| Details of the specific example will be explained later with reference to the drawings.

The optical modulation substance used in the driving method of the present invention takes either a first optically stable state or a second optically stable state depending on the applied electric field.
In other words, a substance that has a bistable state in response to an electric field,
In particular, liquid crystals having such properties are used.

As ^the 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.
Phase (SmH ^* ) liquid crystal is suitable. For more information on this ferroelectric liquid crystal, please refer to “LE JOURNAL DE
PHYSIQUE LETTERS” 36 (L-69) 1975),
“Ferroelectric Liquid Crystals”; “Applied
Physics Letters” 36 (11) 1980, “Submicro
Second Bi−stable Electrooptic Switching in
Liquid Crystals”; “Solid State Physics” 16 (141) 1981
The ferroelectric liquid crystals disclosed in these documents can be used in the present invention.

More specifically, examples of ferroelectric liquid crystal compounds used in the method of the present invention include decyloxybenzylidene-p'-amino-2-methylbutylcinnamate (DOBAMBC), hexyloxybenzylidene-p'-amino- 2-chloropropyl cinnamate (HOBACPC) and 4-o-(2-methyl)-
Examples include butyl resol cylidene-4'-octylaniline (MBRA8).

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

FIG. 1 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 transparent electrodes such as ITO (Indium-Tin Oxide), etc., and SmC ^* phase liquid crystal with liquid crystal molecular layer 22 oriented perpendicular to the glass surface is sealed between them. . A thick line 23 represents a liquid crystal molecule, and this liquid crystal molecule 23 has a dipole moment P⊥24 in a direction perpendicular to the molecule. Boards 21 and 2
When a voltage higher than a certain threshold is applied between the electrodes 1', the helical structure of the liquid crystal molecules 23 is unraveled, and the orientation direction of the liquid crystal molecules 23 can be changed so that the dipole moment P⊥24 is all oriented in the direction of the electric field. can. The liquid crystal molecules 23 have an elongated shape and exhibit refractive index anisotropy in the long axis direction and the short axis direction. Therefore, for example, polarizers arranged in a crossed nicol position can be placed above and below the glass surface. For example, it is easily understood that the liquid crystal optical modulation element is a liquid crystal optical modulation element whose optical characteristics change depending on the polarity of applied voltage. Furthermore, when the thickness of the liquid crystal cell is made sufficiently thin (for example, 1μ), the helical structure of the liquid crystal molecules unravels even when no electric field is applied, and its dipole moment P or P ' is upward 34 or downward 3
4'. When an electric field E or E' with a different polarity above a certain threshold is applied to such a cell as shown in Fig. 2, the dipole moment electric field E or E' will move upward 34 or downward depending on the electric field vector. 34', and accordingly the liquid crystal molecules are oriented either in the first stable state 33 or in the second stable state 33'.

There are two advantages to using such a ferroelectric liquid crystal as an optical modulation element. First, the response speed is extremely fast, and the second liquid crystal molecule orientation has bistability. The second point will be explained with reference to FIG. 2, 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 change their orientation, 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 a fast response speed and bistability, it is preferable that the cell be as thin as possible, and generally 0.5 μ to 20 μ, particularly 1 μ to 5 μ is suitable. A liquid crystal-electro-optical device having a matrix electrode structure using a ferroelectric liquid crystal of this kind has been proposed by Clark and Ragaval in US Pat. No. 4,367,924, for example.

A preferred specific example of the driving method of the present invention is shown in FIG.
-Explain using B.

FIG. 3A-a is a schematic layout diagram of the electrodes of a cell 11 having a matrix electrode structure in which a ferroelectric liquid crystal compound (not shown) is sandwiched between. 12 is a scanning electrode group, and 13 is a signal electrode group. 3A-b and A-c respectively show the electric signal given to the selected scan electrode 12s and the electric signal given to the other scan electrodes (unselected scan electrodes) 12n, and FIG. 3A-d and Ae represent the electric signal applied to the selected signal electrode 13s and the electric signal applied to the unselected signal electrode 13n, respectively. In FIGS. 3A-b to 3-e, 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,
Assuming that the threshold voltage for providing the first stable state of a liquid crystal cell having bistable property is Vth ₁ , and the threshold voltage for providing the second stable state is -Vth ₂ , the voltage applied to the selected scan electrode 12s is The applied electrode signal is an alternating voltage that is V ₁ at phase (time) t ₁ and -V ₁ at phase (time) t ₂ as shown in FIG. 3A-B. When electrical signals having a plurality of phase intervals with mutually different voltages are applied to the scanning electrodes selected in this way, the first phase of the liquid crystal corresponds to the optical "bright" or "dark" state.
Alternatively, an important effect can be obtained in that a state change between the second stable states can be quickly caused.

On the other hand, the other scanning electrodes 12n are as shown in FIG.
- As shown in c, it is in a grounded state and the electrical signal is 0. Further, the electric signal applied to the selected signal electrode 13s is _V2 as shown in FIG. 3A-d, and the electric signal applied to the unselected signal electrode 13s is
The electrical signal applied to n is -V ₂ as shown in FIG. 3A-e. In the above, each voltage value is set to a desired value that satisfies the following relationship.

V ₂ , (V ₁ − V ₂ ) < Vth ₁ < V ₁ + V ₂ and − (V ₁ +
V ₂ )＜−Vth ₂ ＜−V ₂ ,−(V ₁ −V ₂ ).

Each pixel when such an electric signal is applied, that is, pixel A shown in FIG.
The voltage waveforms applied to B, C, and D, respectively, are shown in a, b, c, and d of FIG. 3B. That is, as is clear from FIGS. 3B-a to 3-d, a voltage V ₁ +V ₂ exceeding the threshold value Vth ₁ is applied to the pixel A on the selected scanning line at phase t ₂ . Also, in pixel B existing on the same scanning line, the phase
At t ₁ , a voltage exceeding the threshold value −Vth ₂ −(V ₁ +V ₂ ) is applied. Therefore, depending on whether a signal electrode is selected on the selected scanning electrode line, if the signal electrode is selected, the liquid crystal molecules are aligned in the first stable state, and if not selected, the liquid crystal molecules are aligned in the first stable state. Align the orientation to a second stable state. In any case, it has nothing to do with the previous history of each pixel.

On the other hand, as shown in FIG. 3B, c and d, the voltages applied to all pixels C and D are V ₂ or -V on unselected scan lines. ₂ , and neither 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, a signal for one line is written, and that signal state can be maintained until the next selection after one frame is completed. Therefore, even if the number of scanning electrodes increases, the actual duty ratio remains unchanged and contrast does not decrease at all. At this time, the values of V ₁ and V ₂ and the phase (t ₁ + t ₂ ) = T
Although it depends on the liquid crystal material used and the thickness of the cell, the value of is usually 3 volts to 70 volts and 0.1
A range of μsec to 2 msec is used. Here, in the method of the present invention, the electrical signal applied to the selected scanning electrode is changed from a first stable state (assumed to be a "bright" state when converted into an optical signal) to a second stable state (assumed to be a "bright" state when converted into an optical signal). Selected scan electrodes include:
An important feature is that it provides an alternating voltage signal, for example from +V ₁ to -V ₁ . Further, the voltages applied to the signal electrodes are set to be different voltages to specify a bright or dark state.

Now, in the above, the operation and driving methods of bistable, ferroelectric liquid crystals have been explained based on mostly ideal conditions. For example, a bistable liquid crystal does not actually remain in one stable state for an infinitely long time in the absence of an electric field. To give a more specific example, a ferroelectric liquid crystal with a thickness of about 3 μm or more at a temperature of 70°C
In the case of the DOBAMBC layer, some helical structure initially remains in the SmC ^* phase. When a unidirectional electric field (for example, +30 V/3 μm) is applied to this layer in the layer thickness direction, the helical structure is completely unwound and the liquid crystal molecules are transferred to a uniform in-plane alignment state. After this, when the state is returned to a no-electric field state, the liquid crystal molecules gradually return to a helical structure, so if the liquid crystal cell is sandwiched between a pair of upper and lower crossed Nicols and the transmitted light is observed,
The contrast of the display gradually decreases. The speed at which this unidirectional stable state is released strongly depends on the surface condition (surface material, surface treatment, etc.) of the pair of substrates sandwiching the liquid crystal material and the thickness of the liquid crystal layer.
In addition, in the previous explanation, the threshold voltages Vth ₁ and Vth ₂ necessary for transition to a stable state in one direction were defined as unique values, but these values also strongly depend on factors such as the surface condition of the substrate, so they cannot be determined individually. There are large variations depending on the cell. Furthermore, the threshold voltage also depends on the voltage application time, and as the voltage application time increases, the threshold voltage tends to decrease. There is a risk of transfer between states and crosstalk.

From the above, when trying to actually create and drive an optical modulation element stably, it is necessary to apply voltages V _ON1 and V _ON2 that are oriented to the first and second stable states at the selected point and to the non-selected points. It is preferable that the values of the applied voltages V _OFF are set as far away from the average threshold voltages V _{th 1} and V _{th 2} as possible. When considering variations between elements or within elements,
For the value of |V _OFF |, |V _ON1 | and |V
It was confirmed that it is preferable for the value of _ON2 | to be twice or more in order to obtain stability. In addition, when realizing such voltage application conditions using the driving method described in FIG. 3, which can quickly change the state between two stable states, it is necessary to According to the voltage applied to the corresponding pixel without information (Fig. 3B)
- b), the voltage value |V ₁ −V ₂ | at phase t ₂ is also sufficiently spaced from the value of V ON1, similar to the voltage V _OFF applied to the non-selected pixel, and especially _{V ON1} It is preferable to set it to 1/1.2 or less. Therefore, corresponding to the example of FIG. 3, the condition is 1<|V ₁ (t)|/|V ₂ |<10. More generally speaking, the voltage applied to each pixel and the electrical signal applied to each electrode need not be symmetrical or stepped. In order to express it generally, including this case, the electrical signal (voltage due to the difference from ground potential) applied to the scanning electrode within phase t ₁ + t ₂
Let V ₁ (t) _nax be the maximum value of V ₁ (t) _nax and let V ₁ (t) nio be the minimum value of When the electric signal corresponding to the absence of information (both are voltages due to the difference from the ground potential) is V ₂ ', it is preferable for stable driving that the following conditions be satisfied.

1<|V ₁ (t) _nax |/|V ₂ |<10 1<|V ₁ (t) _nio |/|V ₂ |<10 1<|V ₁ (t) _nax |/|V ₂ ′| <10 and 1<|V ₁ (t) _nio |/|V ₂ '|<10 Based on the embodiment described in FIG. 3, the electrical signal V ₁ applied to the scanning electrode and the electrical signal applied to the signal electrode When the ratio of ±V ₂ is changed, the maximum voltage |V ₁ +V ₂ | applied to the selected point (between the selected signal electrode and the selected or unselected scanning electrode) and the unselected point (unselected signal electrode) and the selected or unselected scan electrode), and at the selected point, the phase t ₁ of FIG. _3B -a, for example.
(or the phase t ₂ in FIG. 3 B-b) and the applied voltage |V ₂ −V ₁ | (both are expressed as absolute values) are graphed in FIG. 4. As can be seen from this graph, the value of the ratio k=|V ₁ /V ₂ | is preferably in the range of 1<k, particularly 1<k<10.

In order to effectively achieve the driving method of the present invention,
The electrical signals given to the scanning electrodes or signal electrodes are not necessarily the same as those shown in FIGS.
It is obvious that the signal does not need to be a simple rectangular wave signal as explained in . For example, it is also possible to drive with a sine wave or a triangular wave, as long as an effective time span is provided.

FIG. 5 is a schematic plan view of a liquid crystal-light shutter as an example of a preferable application of the driving method of the present invention. Here, 41 is a pixel, and the electrodes on both sides of this portion are made of transparent material.
The matrix electrode is composed of a scanning electrode group 42 and a signal electrode group 43.

[Brief explanation of the drawing]

FIG. 1 is a perspective view schematically showing a liquid crystal element having a chiral smectoid phase liquid crystal. FIG. 2 is a perspective view schematically showing the bistability of the liquid crystal element used in the method of the present invention. FIGS. 3A and 3A are plan views schematically showing the arrangement of electrodes of a liquid crystal element used in the driving method of the present invention. FIGS. 3A and 3B are electrical signal waveform diagrams applied to selected scanning electrodes. FIGS. 3A and 3C are signal waveform diagrams applied to unselected scanning electrodes. 3A and 3d are information signal waveform diagrams applied to selected signal electrodes. FIGS. 3A and 3e are information signal waveform diagrams applied to unselected signal electrodes. Figure 3 B, a
is a waveform diagram of the voltage applied to the liquid crystal of pixel A. FIGS. 3B and 3B are waveform diagrams of voltages applied to the liquid crystal of pixel B. FIG. FIGS. 3B and 3C are waveform diagrams of voltages applied to the liquid crystal of pixel c. FIGS. 3B and 3D are waveform diagrams of voltages applied to the liquid crystal of pixel D. FIG. 4 is a graph showing changes in drive stability due to changes in the absolute value k of the ratio of the electric signal V ₁ applied to the scanning electrode and the electric signal ±V ₂ applied to the signal electrode. FIG. 5 is a schematic plan view of a liquid crystal-light shutter as an example of a preferable application of the driving method of the present invention. 11...Liquid crystal element, 12...Scanning electrode group, 12
a...Selected scan electrode, 12n...Unselected scan electrode, 13...Signal electrode group, 13s...
Selected signal electrode, 13n... unselected signal electrode, 33... liquid crystal aligned in the first stable state, 33'... liquid crystal aligned in the second stable state,
34...Upward dipole moment P, 34'...
Downward dipole moment P', 41...pixel, 42
...Scanning electrode group, 43...Signal electrode group.

Claims (1)

[Claims] 1. A device comprising a scanning electrode group and a signal electrode group, a ferroelectric liquid crystal is arranged between the scanning electrode group and the signal electrode group, and an intersection between the scanning electrode group and the signal electrode group. A scanning selection signal is applied to the scanning electrode, and the scanning selection signal has a first phase and a second phase, each having one polarity voltage with respect to the voltage applied to the scanning non-selection electrode. and the other polarity voltage, for the pixel on the selected scan electrode,
In the first phase, a voltage exceeding the threshold voltage of one of the ferroelectric liquid crystals is applied to form a first alignment state, and in the second phase, a voltage exceeding the threshold voltage of the other ferroelectric liquid crystal is applied to form a second alignment state. A liquid crystal device that forms an image by pixels forming a state, wherein a voltage amplitude is selected to be applied to an intersection between the selected scanning electrode and a selected signal electrode of the signal electrode group. 2 of the voltage amplitude applied to the intersection of the selected signal electrode and the selected signal electrode.
A liquid crystal device characterized by more than double the 2. Furthermore, the voltage amplitude applied to the intersection between the selected scan electrode and a signal electrode not selected from the signal electrode group is changed to the voltage amplitude applied to the intersection between the selected scan electrode and the selected signal electrode. The liquid crystal device according to claim 1, wherein the voltage amplitude is set to 1/1.2 or less of the voltage amplitude applied to the liquid crystal device.