JPS6033535A - Driving method of optical modulating element - Google Patents

Abstract

PURPOSE:To enable a bistable state by applying selectively information signals to a signal electrode group in synchronization with a sacnning electrode then having a signal application period when the signals different from the previously applied information signals are applied prior to application of the information signals. CONSTITUTION:A liquid crystal molecular layer 12 is vertically oriented to the plane of glass and is sealed between glass base plates 11 and 11' which are coated with transparent electrodes. When a voltage of a specified threshold or above is impressed between the electrodes on the plates 11 and 11', the spiral structure of liquid crystal molecules 13 are unraveled and the orienting direction of the molecules 13 can be changed in such a way that all dipole moments 14 are directed toward the electric field. The molecules 13 have a slender shape and exhibit refractive anisotropy in the long axis direction and short axis direction thereof. Therefore if polarizers disposed in the positional relation of crossed nicols with each other are placed on the top and bottom of the glass surfaces, the liquid crystal optical modulating element of which the optical characteristic changes with the impressed polarity of the voltage. The optical modulating element has the advantages that the response speed is fast and that the orientation of the liquid crystal molecules has a bistable state.

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 that display images or information by configuring a scanning electrode group and a signal electrode group in a matrix shape and filling a liquid crystal compound between the electrodes to form a large number of pixels are often used. Are known.

As a method of driving this display element, the scanning electrode group is sequentially
Time-division driving is adopted in which an address signal is selectively applied periodically and a predetermined information signal is selectively applied in parallel to a group of signal electrodes in synchronization with the address signal, but this display element and its driving method are However, it had major and fatal flaws 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 practically used as display elements because they have relatively high response speed and low power consumption, for example, M, 5ch.
adt and W, He1frich, ApPlied P
hysics Letters, Vol. 18. No
, 4 (1971, 2, 15), P, 127-128 Voltage-Dependent 0ptical
Activity of a TwistedNema
This type of liquid crystal uses a TN (twisted nematic) type liquid crystal shown in ``Tic Liquid Crystal.'' In this type of liquid crystal, molecules of nematic liquid crystal, which has positive dielectric anisotropy in the absence of an electric field, form a liquid crystal layer. It forms a twisted structure (helical structure) in the thickness direction, and the molecules of this liquid crystal are arranged parallel to each other on both electrode surfaces.On the other hand, when an electric field is applied, it has a positive dielectric anisotropy. Nematic liquid crystals are aligned in the direction of the electric field, and as a result light modulation can occur.When a display element is constructed using this type of liquid crystal with a matrix electrode structure, the area where both the scanning electrode and the signal electrode are selected ( A voltage higher than the threshold required to align the liquid crystal molecules perpendicularly to the electrode surface is applied to the selected points (selected points), and no voltage is applied to areas where neither the scanning electrode nor the signal electrode is selected (non-selected points). Therefore, the liquid crystal molecules maintain a stable alignment parallel to the electrode plane.By arranging linear polarizers in a cross-Nicol relationship above and below such a liquid crystal cell, it is possible to prevent light from passing through at selected points. First, since light passes through non-selected points, it can be used as an image element.However, when a matrix electrode structure is configured, scanning electrodes are selected and signal electrodes are not selected in areas or scanning A finite electric field is also present in a region where no electrode is selected and a signal electrode is selected (so-called "half-selected point"). The voltage applied to the selected point and
If the difference in voltage applied to the half-selection point is sufficiently large and 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. However, in this method, the number of scanning lines (N
), the time during which an effective electric field is applied to one selected point (duty ratio) while scanning the entire screen (one frame) 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, resulting in a decrease in image contrast and crosstalk. This is a drawback that is difficult to avoid. This type of development requires liquid crystals that do not have a bistable state (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. At present, the number of scanning lines has reached a plateau due to the inability to increase the number of scanning lines sufficiently.The object of the present invention is to provide a novel optical modulation element that solves all the problems of conventional liquid crystal display elements as described above. In particular, it is an object of the present invention to provide a method for driving a liquid crystal element.

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 cause 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, it has an IJ electrode structure having a scanning electrode group and a signal electrode group, By selectively and sequentially applying a scanning signal to the scanning electrode group periodically and selectively applying an information signal to the signal electrode group in synchronization with the scanning signal, a gap is created between the scanning electrode group and the signal electrode group. In a method for driving an optical modulation element that performs optical modulation of an optical modulation substance having a bistable state with respect to an arranged electric field, the signal electrode is synchronized with a scanning signal applied to a scanning electrode selected from the scanning electrode group. After selectively applying an information signal to the signal electrode group and before selectively applying the next information signal to the signal electrode group in synchronization with the scanning signal applied to the next selected scan electrode, It is characterized in that it has an auxiliary signal application period in which a signal different from the applied information signal is applied.

Details of the specific example will be explained later with reference to the drawings.

The optical modulation material used in the driving method of the present invention is a material that takes either a first optically stable state or a second optically stable state depending on the applied electric field, that is, a material that has a bistable state with respect to the 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, and among these, a chiral smectic liquid crystal having a chiral smectic C phase (SmO*) or an H phase (SmH ) is suitable. Regarding this ferroelectric liquid crystal, please refer to "LE JOU"
RNAL DEPHYSIQUE LETTER 8″ average (L-69) 1975°rFerroelectri
c Liquid Crystals J'.

rSubmicro 5econd B1-5table
eElectrool)tieSwitching
In Liquid Crystals J; ``Solid State Physics Mikawa (141) 1981 F Liquid Crystals'', etc., and the ferroelectric liquid crystals disclosed therein can be used in the present invention.

More specifically, as an example of the ferroelectric liquid crystal compound used in the method of the present invention, decyloxybenzylidene-P-amino-2-methylbutylcinname) (DOBAMBC
), hexyloxybenzylidene-P-amino-2-chloropropylcinnamate (HOBACPC) and 4
-o-(2-methyl)-butyl resol cylidene-4-octylaniline (MBRA8) and the like.

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 SmH* phase, the element may be placed in a copper block with a heater embedded, etc., as necessary. can be supported.

Figure 1 schematically depicts an example of a ferroelectric liquid crystal cell.
It is a curtain plate (glass plate) coated with a transparent electrode such as T O (Inrtium-Tin Oxide), and an SmC* phase liquid crystal in which the liquid crystal molecular layer 12 is oriented perpendicular to the glass surface is sealed in between. There is.

A thick line 13 represents a liquid crystal molecule, and this liquid crystal molecule 13 has a dipole moment (Pl) 14 in a direction perpendicular to the molecule.

When a voltage higher than a certain threshold is applied between the electrodes on the fishing plates 11 and 11, the helical structure of the liquid crystal molecules 13 is unraveled, and the liquid crystal molecules 13 are adjusted so that all the dipole moments (P, 1) 14 are directed in the direction of the electric field. The orientation direction can be changed. The liquid crystal molecules 13 have an elongated shape; 7, and exhibit refractive index anisotropy in the long axis direction and the short axis direction. Therefore, for example, if polarizers are placed above and below the glass surface in a crossed nicol positional relationship, 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)
As shown in Figure 2, even when no electric field is applied, the helical structure of the liquid crystal molecules is released, and its dipole moment) P or P' is directed upward (24) or downward (24').
take either of the following states. When an electric field E or E' with a different polarity above a certain threshold value is applied to such a cell for a predetermined time as shown in FIG.
′ upward 24 or downward 2
4', and accordingly the liquid crystal molecules are aligned in either the first stable state 23 or the second stable state 23'.

There are two advantages to using such a ferroelectric liquid crystal as an optical modulation element. Firstly, the response speed is extremely fast, and secondly, the alignment of liquid crystal molecules has a bistable state. To explain the second point 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 23, and this state remains stable even when the electric field is turned off.

Moreover, when an electric field E in the opposite direction is applied, the liquid crystal molecules are oriented to the second stable state 23' and the orientation of the molecules is changed.
Even after breaking out of the Shimi world, it remains in this state. 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, the thickness is 0.5μ.
-20μ, especially 1μ-5μ are suitable.

A liquid crystal-electro-optical device using this type of ferroelectric liquid crystal and having an IJ electrode structure has been proposed, for example, by Clark and Lagabar, US Pat. No. 4,367,924.

Next, a specific example of a method for driving a ferroelectric liquid crystal will be explained using FIGS. 3 to 5.

FIG. 3 is a schematic diagram of a cell 31 having a matrix electrode structure in which a ferroelectric liquid crystal compound (not shown) is sandwiched between. 32 is a scanning electrode group, and 33 is a signal electrode group. First, the case where scan electrode SI is selected will be described. FIG. 4(a) and FIG. 4(b) are scanning signals, which are electrical signals applied to the selected scanning electrode S and the other scanning electrodes (unselected scanning type '@) S, respectively.
t, Ss, 84... are shown. FIG. 4(c) and FIG. 4(d) show information signals from selected signal electrodes 1. , I9.1.
, and electrical signals given to unselected signal electrodes I, , , T, are shown.

In FIGS. 4 and 5, the horizontal axis represents time and the vertical axis represents voltage, respectively. For example, when displaying a moving image, the scanning electrode groups 32 are sequentially and periodically selected.

Now, for a predetermined voltage application time 1+ or t, the threshold chamber for providing the first stable state of a liquid crystal cell having bistability is set to pressure , -V thl, and the second stable state is provided. Assuming that the threshold voltage of the terminal is −1−V th, the electrode signal given to the selected scanning electrode 32 (s, ) is the fourth
As shown in figure (a), at phase (time) 1+, 2V
is an alternating voltage that becomes -2V at phase (time) 11. When electrical signals having multiple phase intervals with mutually different voltages are applied to the scanning electrodes selected in this way, the difference between the first or second stable state of the liquid crystal corresponding to the optical "dark" or "bright" state is generated. The important effect is that a change in state can be caused quickly. - On the other hand, the other scanning electrodes S2 to S, . . . are in a grounded state as shown in FIG. Further, the electric signal given to the selected signal electrodes 11°T, , I, as shown in FIG.
and unselected signal electrode ■2. (2) The electrical signal applied to 4 is <-V as shown in FIG. 4(d). In the above, each voltage value is set to a desired value that satisfies the following relationship.

V < V th* < 3 V -3v<, -vth, <-v Among each pixel when such an electric signal is applied, for example, the voltage applied to each pixel and B in FIG. The voltage waveforms are shown in FIGS. 5(a) and 6).

That is, as is clear from FIG. 5) and (b), a voltage of 3 V exceeding the threshold value V th2 is applied to the pixel A on the selected scanning line at phase t. In addition, for pixel B existing on the same scanning line, the threshold value -v at phase t1
A voltage of -3V exceeding th is applied. Therefore, depending on whether or not 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 the stable state of 2.

On the other hand, as shown in FIGS. 5(C) and (d), on unselected scanning lines, the voltage applied to all pixels is
or -■, neither of which exceeds the threshold voltage. Therefore, the liquid crystal molecules in each pixel other than on the selected scanning line 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 the 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 does not change and the contrast does not deteriorate at all.

Next, let's consider the problems that may actually occur when driving a display device. In Figure 3, on the scanning electrodes S, -Sl... and on the signal electrodes, ~I,...
Among the pixels formed at the intersections of . Now, if we pay attention to the display on the signal electrode ■1 in FIG. 3, we can see that the pixel (4) corresponding to the scanning electrode
All other pixels (B) are in the "dark" state.

As an example of the driving method in this case, FIG. 6(a) shows a time-series representation of the scanning signal, the information signal applied to the signal electrode I, and the voltage applied to the pixel.

For example, when driven as shown in FIG. 6(a), when the scanning signal S1 is scanned, a voltage 3■ exceeding the threshold value v tht is applied to the pixel A at time t.
Regardless of prior history, the pixel person transitions (switches) to a stable state in one direction, the "bright" state. After that, s, ~ss
. . is scanned, as shown in FIG. 6(a).
A voltage of V continues to be applied, which is the threshold value −v th,
, the pixel can maintain a "bright" state. However, when displaying information in which one signal (corresponding to "dark" in this case) is continuously given on one signal electrode, the number of scanning lines is extremely large, and high-speed driving may be required. The following data shows that problems can arise when

Figure 7 shows DOBAMBC (7th grade) as a ferroelectric liquid crystal material.
Voltage threshold (vth) required for switching when using 72) in the figure and HOBACPC (71 in Figure 7)
It is a four-dimensional plot of the application time dependence of In both cases, the liquid crystal thickness is 1.6μ, and the temperature is controlled at 70°C. In the case of this experiment, the substrates on both sides in which the liquid crystal is to be sealed are glass plates on which ITO is vapor-deposited, and the V
thl~, Vtht (miV1:h).

As is clear from FIG. 7, the threshold value vth is dependent on the application time, and it is understood that the shorter the application time, the steeper the slope becomes. From this, if we take the driving method as shown in FIG. 6(a) and apply it to an element that has an extremely large number of scanning lines and is driven at high speed, S, even if it is switched to the "bright" state during scanning, the voltage of -V is always applied after scanning, so the pixel person is "
It can be seen that there is a danger of reversing to a "dark" state.

As a drive form to prevent such a phenomenon, for example, Fig. 6 (
The method shown in b) can be used.

This method does not send the scanning signal and the information signal continuously, but instead uses a predetermined time interval Δt as the auxiliary signal application period.
This represents a mode in which an auxiliary signal is provided to ground the signal electrode during this period. During this auxiliary signal application period, the scanning electrode is also grounded, so the voltage applied between the scanning electrode and the signal electrode is 0 volts. Dependency can be virtually eliminated. Therefore, it is possible to prevent the "bright" state generated in the pixel from being reversed to the "dark" state.

Moreover, the same thing can be said about other pixels.

As described above, the present invention has the advantage that although the ferroelectric liquid crystal has the characteristics shown in FIG. 7, once written information is maintained over a period until the next writing is performed. It is characterized by the ability to

A particularly preferred embodiment of the present invention can be implemented by applying signals represented by the time chart of FIG. 8 to the scanning electrodes and the signal electrode group.

8 in FIG. 8 is expressed as a predetermined voltage value appropriately determined depending on the liquid crystal material, liquid thickness, set temperature, substrate surface treatment conditions, etc., and the scanning signal is an alternating pulse of ±2. An information signal is sent to the signal electrode group in synchronization with the pulse, and this is a voltage of +V or -■ corresponding to "bright" or "dark" information, respectively. Now, looking at the scanning signal in time series, 5n (nth scanning electrode) and Sn++ (n
A time interval Δt is provided as an auxiliary signal application period while the first scanning electrode) is selected. During this time, when an auxiliary signal with a polarity opposite to that of the signal during Sn scanning is sent to the signal electrode group, the time-series signal given to each signal electrode is, for example, as shown in Figure 8, (1) and (1). Become. That is,
The auxiliary signals 1, 2', 3', 4' and 5' in FIG. 8 have polarities opposite to those of the information signals 1, 2, 3 and 4.5, respectively. For this reason, for example, if we look at the voltage applied to pixel A in Figure 8 in chronological order, even if the same information signal is continuously applied to one signal electrode, the voltage actually applied to the pixel is Since the voltage below vth is alternating, the dependence of the voltage application time on the threshold voltage in the ferroelectric liquid crystal is eliminated, and the desired (in this case "bright") information formed during scanning is There is no need to worry about reversal during the writing process. Figure 9(a) shows a simplified electrical system diagram for driving a ferroelectric liquid crystal cell using the driving configuration shown in Figure 8. This is what is shown. The signal given to the scan electrode group is formed by sending a clock signal (C8) generated by a clock generator to a scan electrode selector that selects a scan electrode, and sending this to a scan electrode driver.

On the other hand, the signal (DM) applied to the signal electrode group is sent from the output signal (DS) of the data generator and the clock signal (O8) to a data modulator that can form an information signal and an auxiliary signal, and further Supplied through the signal electrode driver.

FIG. 9(b) is an example of a signal output by the data modulator, and corresponds to the signal I in FIG. 8 in the above embodiment.

Moreover, FIG. 9(C) schematically shows a data modulator for outputting the signal shown in FIG. 9(b)@ above, and includes two inverters 91 and 92 and two AN
It is composed of D circuits 93 and 94 and one OR circuit 95.

Further, FIG. 10 shows another embodiment according to the gist of the present invention. That is, an example is shown in which a ±3V pulse is applied instead of the ±2V pulse applied to the selected scanning electrode used in the embodiment of FIG.

In order to effectively achieve the driving method of the present invention, the electrical signal applied to the scanning electrode or the signal electrode does not necessarily have to be a simple symmetrical square wave signal as explained in the above specific example. is self-evident. For example, it is also possible to drive with a sine wave or a triangular wave as long as an effective time width is given. Furthermore, since vth can have different values depending on the surface treatment conditions of the two substrates sandwiching the liquid crystal, if the surface treatment conditions of the two substrates are different,
Correspondingly, it is also possible to provide an asymmetrical signal with respect to Ov (ground). Moreover, in this embodiment,
Although only the auxiliary signal that inverts the polarity of the previous information signal is used, it may also be a signal that inverts the polarity of the next information signal, or a signal whose voltage absolute value is different from the voltage absolute value of the information signal. It's okay. Further, as an auxiliary signal, it is also possible to provide a desired signal obtained by statistically processing not only the contents of the previous information signal but also a plurality of previous information signals.

FIG. 11 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, 111 is a pixel, and the electrodes on both sides of this part are made of transparent material. The matrix electrode is composed of a scanning electrode group 112 and a signal electrode group 113.

[Brief explanation of the drawing]

1 and 2 are perspective views schematically showing an optical modulation element used in the method of the present invention. FIG. 2 is an explanatory diagram showing the waveform of FIG. 6(a) is an explanatory diagram of a time chart used in a driving method other than the present invention. FIGS. 8 and 10 are time charts used in a driving method of the present invention. It is an explanatory diagram of a chart. Figure 9(a)
is an explanatory diagram of a flowchart for driving according to the time chart shown in FIG. 8, and FIG. (c) is a circuit diagram for forming the data modulator output signal (DM) in FIG. 9(g)). Figure 7 shows
FIG. 2 is an explanatory diagram showing the dependence of voltage application time on dark value voltage in a ferroelectric liquid crystal. FIG. 11 is a schematic plan view of a liquid crystal-optical shutter using the method of the present invention. 11.11; Substrate 12 coated with transparent electrode; Liquid crystal molecule layer 13; Liquid crystal molecule 14; Dipole moment (P↓) 24; Upward dipole moment 24'; Downward dipole moment 23; First stable state 23 ;Second stable state 32 (SI, St,...);Scanning electrode group (scanning electrode)
33 (■1.■2....); Signal electrode group (signal electrode)
Patent applicant Canon Co., Ltd. 1 11 Sun 1-0. Procedural amendment (voluntary) Commissioner of the Japan Patent Office Kazuo Wakasugi 1 Indication of the case 1988 Patent application No. 142954'j2 Name of the invention Method for driving an optical modulation element 3 Relationship with the case by the person making the amendment Patent applicant's office 3-30-2 Shimomaruko, Ota-ku, Tokyo Name (10
0) Canon Co., Ltd. Canon Co., Ltd. (Telephone: 758-2111) 5. Drawings subject to correction Contents of surface finish correction

Claims (1)

[Scope of Claims] (1) It has a matrix electrode structure having a scanning electrode group and a signal electrode group, a scanning signal is selectively and sequentially applied periodically to the scanning electrode group, and the scanning signal is applied to the signal electrode group in sequence. A method for driving an optical modulation element that optically modulates an optical modulation substance having a bistable state with respect to an electric field arranged between the scanning electrode group and the signal electrode group by selectively applying an information signal in synchronization with the signal. , after the information signal is selectively applied to the signal electrode group in synchronization with the scan signal applied to the scan electrode selected from the scan electrode group, and the scan signal applied to the next selected scan electrode. Optical modulation characterized by having an auxiliary signal application period for applying a signal different from the information signal selectively applied to the signal electrode group before selectively applying the next information signal to the signal electrode group in synchronization. Driving method of element. (2) The method for driving an optical modulation element according to claim 1, wherein the electrical signals applied to the scanning electrodes selected from the scanning electrode group have different voltage phases. (8) The optical system according to claim 1 or 2, wherein the electrical signal applied to a selected signal electrode of the signal electrode group and the electrical signal applied to an unselected signal electrode are at different voltages. Driving method of modulation element. (4) The method of driving an optical modulation element according to claim 2 or 3, wherein the electrical signals applied to the scanning electrodes selected from the scanning electrode group have phases with different voltage polarities. (5) The electric signal applied to the selected signal electrode of the signal electrode group and the electric signal applied to the unselected signal electrodes have different voltage polarities. Driving method of optical modulation element. (6) The electrical signal applied during the auxiliary signal application period has a voltage polarity different from that of the information signal applied to the signal electrode group in synchronization with the scanning signal applied to a selected one of the scanning electrodes. A method for driving an optical modulation element according to claim 1. (7) The method for driving an optical modulation element according to claim 1, wherein the optical modulation substance having bistability is a ferroelectric liquid crystal. (8) The method of driving an optical modulation element according to claim 7, wherein the ferroelectric liquid crystal is a liquid crystal having a chiral smectoid phase. (9) The method for driving an optical modulation element according to claim 8, wherein the liquid crystal having a chiral smectic phase is a liquid crystal phase that does not form a helical structure. (Conversion) The method for driving an optical modulation element according to claim 8 or 9, wherein the liquid crystal having a chiral smectoid phase is a liquid crystal having a C phase or an H phase.