JPS63197920A - Driving method for optical modulation element - Google Patents

Abstract

PURPOSE:To enlarge a driving voltage margin by providing a period for applying voltage zero or a voltage the same in polarity as a write voltage, immediately before applying the write voltage to be applied to a picture element on a selected scanning electrode. CONSTITUTION:Between a scanning electrode group and an information electrode group which are being intersected, an optical modulating substance is provided, and in accordance with the electric field direction of a voltage applied to an intersection part which are used as a picture element, the contrast is identified. Immediately before applying a write voltage to be applied to the picture element on the selected scanning electrode, a period for applying voltage zero or a voltage the same in polarity as the write voltage is provided. For instance, by providing a prescribed pausing period T between an opposite polarity pre-pulse Vb and a write pulse Vw, the influence of the opposite polarity pre-pulse Vb at the time of writing can be reduced. In such a way, a normal display can be executed, and a driving voltage margin can be enlarged.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of the Invention] The present invention relates to a method for driving an optical modulation element capable of discriminating contrast depending on the direction of an electric field, and in particular to a method for driving an optical modulation element capable of discriminating contrast depending on the direction of an electric field.
This invention relates to a method for driving a ferroelectric liquid crystal device that produces two stable states.

[Prior art]

The use of liquid crystal elements with bistable properties was developed by Clark
k) and Lagerwall, published in Japanese Patent Application Laid-open No. 107216/1983 and U.S. Pat.
, 367.924, etc. Bistable liquid crystals are generally chiral smectic C phase (
A ferroelectric liquid crystal having SmC*) or H phase (SmH*) is used, and in these states, the transition between a first optically stable state and a second optically stable state occurs in response to an applied electric field. It has the property of taking either one of these and maintaining that state when no electric field is applied, that is, it has stability, and it also responds quickly to changes in the electric field, making it suitable for wide use in fields such as high-speed and memory-type display devices. It is expected.

In the case of a matrix display device in which a group of striped electrodes is provided on the inside of a pair of substrates used in this ferroelectric liquid crystal element, and the striped electrodes are wired so as to be orthogonal to each other, for example, No. 59-193426, No. 59-193427, No. 60-1560
The driving methods disclosed in Japanese Patent No. 46 and Japanese Patent No. 60-156047 can be applied.

Figure 8 is an example of this, and within the selection period on one scanning line, "white" is displayed.
This diagram illustrates a method of providing three types of periods: a clear period, (1) a selective "black" writing period, and (2) an auxiliary signal application period. In the clearing period (3), by applying a positive voltage to the scanning electrode and keeping the information electrode at Ov, all or a predetermined pixel on the scanning line is brought into a first stable state (hereinafter referred to as the "white" clear state). Make it. In the period (2), on the other hand, a negative voltage is applied to the scanning electrode, selectively applying only to the information electrode corresponding to the pixel (selected pixel) that is to be inverted to the second stable state (hereinafter referred to as the "black" state). A positive voltage is applied to the information electrodes corresponding to the other pixels (half-selected pixels), and a negative voltage is applied to the information electrodes corresponding to the other pixels (half-selected pixels).

As a result, an inverted electric field that is equal to or greater than the threshold value is generated in the selected pixel and equal to or less than the threshold value in the half-selected pixel, and "black" is written in the selected pixel, while the half-selected pixel maintains the "white" state. During the period (2), a voltage of opposite polarity to the period (2) is applied to the information electrode. This prevents voltages of the same polarity from being applied to pixels on other scanning lines for a long time, thereby preventing crosstalk.

[Problem that the invention seeks to solve]

According to the research of the present inventor, if a voltage with the opposite polarity to the write voltage is applied to the same pixel prior to the application of the write voltage, the write voltage will change depending on the amplitude of the reverse polarity voltage. It was found that the threshold voltage of the ferroelectric liquid crystal changes when the voltage is applied. A specific example of the dependence of this threshold voltage on the reverse polarity front pulse will be described later.

Therefore, for example, if the multiplexing drive shown in FIG.
It has been found that during the writing period, depending on the magnitude of the applied writing voltage (3V O), writing may not be possible. For this reason, the conventional driving method has a problem in that the driving voltage margin is extremely small.

[Summary of the invention]

Therefore, it is an object of the present invention to solve the above-mentioned problems and, in particular, to provide a driving method that increases the driving voltage margin.

That is, the present invention provides a method for driving an optical modulation element that has an optical modulation substance between a scanning electrode group and an information electrode group that intersect, and identifies contrast according to the electric field direction of a voltage applied to the intersection that becomes a pixel. The present invention is characterized in that immediately before the application of the write voltage applied to the pixels on the selected scan electrode, there is a period during which a voltage of zero or a voltage having the same polarity as the write voltage is applied.

〔Example〕

FIG. 1 reveals the dependence of the reverse polarity pre-pulse on the threshold voltage vth of the ferroelectric liquid crystal. 11 in Figure 1
12 represents the threshold characteristic of the ferroelectric liquid crystal cell used in Example 1 described above, and 12 represents the threshold characteristic of the ferroelectric liquid crystal cell used in Example 2. In addition, vb shown in FIG. 1 is the amplitude of the reverse polarity pre-pulse (this voltage vb corresponds to the clear voltage), Vw is the amplitude of the write pulse, t, = t2 = 30μ
sec is each pulse application time.

According to FIG. 1, it can be seen that as the amplitude vb of the reverse polarity front pulse increases, the threshold voltage rapidly increases.

Therefore, the inventor conducted further experiments and found that the first
The reverse polarity pre-pulse (vb) and write pulse (Vw) shown in the figure
) has been found to be able to reduce the influence of the reverse polarity pre-pulse (vb) during writing.

FIG. 2 shows the threshold characteristics when a pause period T is provided between the reverse polarity pre-pulse vb and the write pulse Vw. The horizontal axis of FIG. 2 is the pause time Tμsec, and the vertical axis is the ratio (V th"/V th-) is plotted. Therefore, V th"/V
The closer the value of t h- is to 1, the smaller the influence of the reverse polarity front pulse vb is. In the figure, 21 is lVb/v
When w1=%, 22 is lVb/Vwl=1.23 is 1v
It represents the threshold characteristic when b/vWl=2.

According to FIG. 2, after the reverse polarity pre-pulse VC is applied,
By setting a predetermined pause period, the applied voltage Vw during writing can be reduced.
can be kept to low levels.

In a preferred embodiment of the present invention, the period t2 (=ΔT) during which the write voltage -Vw is applied and the period t2 (=ΔT) in which the write voltage -Vw is applied in the driving examples shown in FIGS.
It is preferable to set the relationship T≧t2 between the two. When setting the relationship T x t2, it may not be possible to sufficiently eliminate the influence of the aforementioned reverse polarity pre-pulse vb on the threshold voltage during writing.

Further, as shown in FIGS. 3 to 5, the amplitude of the information signal is set to be different from the amplitude of the write voltage, but as shown in FIG. According to the characteristic curve 21 when set to % with respect to Vw, when this reverse polarity front pulse exists, the threshold voltage increases by a little more than 1.5 times compared to when there is no reverse polarity front pulse vb. I can see that you are doing it.

For this reason, it becomes necessary to increase the write voltage, but
Corresponding to this increase, the voltage applied to the pixel when not selected (which includes the reverse polarity pre-pulse) also increases. In addition, the inventor's experiments have shown that multi-
When an AC voltage is applied to a pixel when not selected in plexing drive, the inversion threshold voltage is lowered (to about V2) compared to when the AC voltage is not present, causing crosstalk. It has been found.

Therefore, in a preferred embodiment of the present invention, when the write voltage amplitude is set to a level three times or more the information signal voltage amplitude, the application period of the write voltage is set to ΔT (μsec), and the amplitude of the write voltage is set to ΔT (μsec). V, and the period during which the voltage zero or the voltage with the same polarity as the write voltage is applied is T (μsec).
and before the period T, a period during which a voltage of opposite polarity to the write voltage is applied is ΔTb (μsec),
Further, when the amplitude of the voltage of the opposite polarity is Vb, the following relationship holds.

FIG. 3 shows a drive waveform with the above-mentioned rest period T, and (
a) is a scan selection signal applied to a selected scan electrode, (
b) is a scanning non-selection signal applied to unselected scanning electrodes; (c) is a black signal applied to selected information electrodes;
(d) represents a white signal applied to other information electrodes. (e) is a voltage waveform when a black signal is applied to the pixel on the above-mentioned scanning electrode, and (f) is a voltage waveform when a white signal is applied to other pixels.

The write voltage -Vw corresponds to the write voltage described above. In this driving example, l Vb/-Vwl = 1, and the clearing period j is equal to the writing period t2, so it is preferable to set the zero-voltage pause period T to 50 μsec or more.

Further, it is also possible to set the voltage during the pause period T used in this driving example to a voltage with the same polarity as the write voltage -Vw. The amplitude of the voltage applied during the pause period T at this time is set to be equal to or less than the threshold voltage. Further, the pause period T at this time may be shorter than the pause period when the voltage is zero. An example of its driving is shown in FIG. FIG. 4 (a) is the scanning selection signal, (b) is the scanning non-selection signal, (C) is the black signal, (d) is the white signal, (e
) is the voltage waveform applied to the selected pixel, and (f) is the voltage waveform applied to the non-selected pixel.

FIG. 5 represents another driving example of the present invention.

In this driving example, before sequentially applying a write voltage to the pixels on the scan electrode, a voltage of 3V is applied to all or a predetermined pixel at once.
Clear to white by applying o. The voltage 3vo for clearing this pixel corresponds to the above-mentioned reverse polarity pre-pulse Vc, and therefore the subsequent period T corresponds to the above-mentioned rest period. In this drive example, l Vb/-Vw l
=1 Te, 1. = 12, so the pause period is 75
It is preferable to set it to 0 μsec or more.

FIG. 6 is a block diagram showing a driving device for a ferroelectric liquid crystal element. That is, the driving section of the ferroelectric liquid crystal panel 61 is provided with a scanning side driving circuit 62 and a signal side driving circuit 63, and the scanning side driving circuit 62 receives scanning signals Sl+S2+...
The signal side 5 motion circuit 63 outputs the information signal ■1+I
2+... can be output.

The addresses of the scanning side drive circuit 62 and the signal side drive circuit 63 are determined by an address decoder 64, respectively. Further, the column data 66 is controlled by the CPU 65 and output to the signal side drive circuit 63.

FIG. 7 is a schematic diagram of a panel 71 having a matrix electrode structure in which a ferroelectric liquid crystal compound as an optical modulating substance is sandwiched between. 72 is a scanning electrode group with m numbers from S to Sm, and 73 is an information electrode group with n numbers from 11 to In.
It is a matrix of books. The scanning electrode group 72 is arranged from Sl to S2+ S3+ .

Sm is sequentially selected. Further, when each scanning signal is selected, a signal corresponding to the video information is output to I to In of the information electrode group 73.

As the liquid crystal having bistability that can be used in the driving method of the present invention, chiral smectic liquid crystal having ferroelectricity is most preferable, and among these, chiral smectic liquid crystal
Phase (SmC*) or H-phase (SmH*) liquid crystals are suitable. This ferroelectric liquid crystal is described in “Le Journard de Φfissic Letters”.
al de Physic 1etter”)
Volume 36 (L-69), 1975 [Ferroelectric Liquid Crystals J]
etricLiquid CrystalsJ)
; “Applied Physics Letters (“Appl
ied Physics Letters”) Volume 36 (
No. 11), 1980 [Submicron Second Bistiple Electro-Optic Switching System Liquid Crystal J (rSubmi
cr.

5econd B15table Electro
optic Switchingin Liqui
d CrystalsJ) ; “Solid State Physics 16 (
141) 1981 "Liquid Crystal", etc., and the ferroelectric liquid crystal 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-methylbutylcinnamate (DOBAMBC
), hexyloxybenzylidene-P'-amino-2
-Chloropropyl cinnamate (HOBACPC) and 4-o-(2-methyl)-butylresorsilitin-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 S m C * phase or S m H * phase, it is necessary to install a copper block with a heater embedded in the element. It can be supported by etc.

Furthermore, in the present invention, the above-mentioned S m C*, S m H*
In addition, it is also possible to use ferroelectric liquid crystals that appear in chiral smectic F phase, (2) phase, J phase, G phase, or other phases.

FIG. 9 schematically depicts an example of a ferroelectric liquid crystal cell. 91a and 91b are In2O3, SnO2 and I
A substrate (glass plate) coated with a transparent electrode such as TO (indium tin oxide), between which a liquid crystal molecular layer 92 is oriented perpendicular to the glass surface.
C* phase liquid crystal is sealed. Thick line 93
represents a liquid crystal molecule, and this liquid crystal molecule 93 has a dipole moment (on P) 94 in a direction perpendicular to the molecule. When a voltage equal to or higher than a certain threshold is applied between the electrodes on the substrates 91a and 91b, the helical structure of the liquid crystal molecules 93 is unraveled, and the orientation direction of the liquid crystal molecules 93 is changed so that all dipole moments (P) 94 are oriented in the direction of the electric field. can be changed.

The liquid crystal molecules 93 have an elongated shape 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 this is a liquid crystal optical modulation element whose optical characteristics change depending on the polarity of applied voltage. Furthermore, if the thickness of the liquid crystal cell is made sufficiently thin (for example, 1μ
), the helical structure of the liquid crystal molecules unwinds even when no electric field is applied, as shown in Figure 1O, and the dipole moment Pa or pb is directed upward (104a) or downward (104a).
104b). As shown in FIG. 10, when an electric field Ea or Eb with a different polarity above a certain threshold value is applied to such a cell for a predetermined period of time, the dipole moment increases by 1 in an upward direction with respect to the electric field vector of the electric field Ea or Eb.
04a or downward 104b, and accordingly the liquid crystal molecules are aligned in either the first stable state 103a or the second stable state 103b.

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. 10, for example, when the electric field Ea is applied, the liquid crystal molecules enter the first stable state 103a.
This state is stable even when the electric field is turned off.

Furthermore, when an electric field Eb in the opposite direction is applied, the liquid crystal molecules are oriented to a second stable state 103b 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 Ea 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 from 0.5μ to
20μ, especially 1μ to 5μ is suitable.

Hereinafter, the present invention will be explained according to examples.

[Example 1] Striped I with a pitch of 100 μm and a width of 62.5 μm
Two square glass substrates each having a TO film as an electrode were prepared, and a 1000% insulating film was placed on each glass substrate.
A 500-year-old SiO2 film and a 500-year-old polyvinyl alcohol film were provided thereon as an alignment control film.

Next, after rubbing the surface of the polyvinyl alcohol film provided on each substrate, silica beads with an average particle size of 1.5 μm were sprinkled on one substrate, and epoxy adhesive was applied as a sealant around the other substrate. The agent was applied. After that, a cell was created by stacking the two substrates so that the upper and lower ITO films were perpendicular and the upper and lower rubbing directions were parallel, and inside this cell was Chisso's rC3-, which had been heated to the Tokichi phase. After filling with 1014J (trade name), S m C* exhibiting ferroelectricity was revealed by slow cooling.

An alternating current pulse shown in FIG. 1 was applied to this ferroelectric liquid crystal cell. The characteristic curve plotting the reversal voltage at that time is the first
This is characteristic curve 11 in the figure. However, the phases t1 and t2 of the above-mentioned AC pulses were each 30 μsec to 30 μsec.

Next, when the AC pulse shown in FIG. 2 was applied to the above-mentioned ferroelectric liquid crystal cell, V thT/V th'''' was measured, and the characteristic curve shown in FIG. 2 was obtained. However, (7) 21 in Fig. 2 is lVb/-Vwl = 'A, 2
2 is ・l Vb/-Vw l =1.23 is l Vb
/ - Vw l = 2, and the phases t1 and t2 of the above AC pulse are each 30a%.
It was sec.

Next, multiplexing driving was performed by applying a driving waveform voltage shown in FIG. 5 to the above-described ferroelectric liquid crystal cell. At this time, the voltage was 1±3v.

1 to 21 volts, and the rest period T to 50 μs e
c.

When the phase was set to t1=j2=30μ trace sec, a normal still image was formed.

On the other hand, as a comparative example, multiplexing driving was performed in exactly the same manner as described above except that the use of the pause period T was omitted.
Writing to the pixel above 3 could not be performed.

[Example 2] rC3-101 in the ferroelectric liquid crystal cell used in Example 1
44 (product name) to Chisso rC5-1011J (
A cell was prepared in the same manner as in Example 1, except that the ferroelectric liquid crystal (trade name) was used instead. When the alternating current pulse shown in FIG. 1 was applied to this ferroelectric liquid crystal cell and the reversal voltage at this time was measured, a threshold characteristic shown by characteristic curve 12 in FIG. 1 was obtained.

Next, by applying a driving waveform voltage shown in FIG. 5 to this ferroelectric liquid crystal cell, multi-bracing driving was performed, and a normal still image was formed. However, in this case, the voltage is 1±3v0I=21 volts, and the rest period is T.
=50μsecV1t, =t2=30μsec
And so.

On the other hand, as a comparative example, when the pause period T=OμSeC, the scanning electrodes S, , S
No writing was done to the pixels above 2 and 83.

〔Effect of the invention〕

As described above, according to the present invention, it is possible to eliminate the influence of reverse polarity pulses during writing, and thereby normal display can be performed.

[Brief explanation of the drawing]

FIGS. 1 and 2 are characteristic diagrams showing the influence of a reverse polarity pre-pulse on the threshold voltage of a ferroelectric liquid crystal. Figures 3, 4, and 5 show the multi-function device used in the present invention.
FIG. 3 is a waveform diagram of a drive voltage used for plexing drive. FIG. 6 is a block diagram of a display panel using the driving method of the present invention. FIG. 7 is a plan view of the matrix electrode used in the present invention. FIG. 8 is a waveform diagram of the driving voltage used in the conventional driving method. 9 and 10 are perspective views schematically showing the ferroelectric liquid crystal element used in the present invention. ￥3 figure (C) -needle 1-l'' Cd) 'j-1'] t'tl-4 (b) O a (d) Q-''--] ttl-1

Claims (8)

[Claims] (1) In a method of driving an optical modulation element, which has an optical modulation substance between a crossed scanning electrode group and an information electrode group, and identifies contrast according to the electric field direction of a voltage applied to the intersection that becomes a pixel, A method for driving an optical modulation element, which includes a period of applying a voltage of zero or a voltage having the same polarity as the write voltage immediately before applying a write voltage to a pixel on a selected scan electrode. (2) Claims that have a relationship of T≧△T, where △T is the application period of the write voltage, and T is a period during which the voltage of zero or the same polarity as the write voltage is applied. The driving method according to item 1. (3) The application period of the write voltage is ΔT (μsec),
The amplitude of the write voltage is V, and the period during which the voltage zero or the voltage with the same polarity as the write voltage is applied is T (μsec
), before the period T, the period during which a voltage with the opposite polarity to the write voltage is applied is △Tb (μsec), and the amplitude of the voltage with the opposite polarity is Vb, (△T・V/△Tb・Vb)・T≧50 (μsec) The driving method according to claim 1, wherein the driving method has the following relationship. (4) The driving method according to claim 1, wherein a voltage for clearing the contrast of the pixels is applied to all or a predetermined number of pixels before applying the write voltage to the scanning electrode. (5) Driving according to claim 1, wherein a voltage for clearing the contrast of the pixel is applied to the pixel on the scan electrode during the selection period of the scan electrode, prior to application of the write voltage. Law. (6) The driving method according to claim 1, wherein the optical modulating substance is a ferroelectric liquid crystal. (7) The driving method according to claim 6, wherein the ferroelectric liquid crystal is a chiral smectic liquid crystal. (8) The driving method according to claim 7, wherein the thickness of the chiral smectic liquid crystal is set to be thin enough to eliminate the helical structure inherent in the chiral smectic liquid crystal in the absence of an electric field.