JPS63155032A - Driving method for optical modulating element - Google Patents

Abstract

PURPOSE:To discriminate the contrast in accordance with an electric field by applying a voltage, which exceeds the threshold voltage of an optical modulating material and has the other polarity, to picture elements which are not selected out of picture elements on two scanning electrodes with the phase different between scanning electrodes. CONSTITUTION:For example, scanning electrodes S1 and S2 of a scanning electrode group 62 are simultaneously addressed, and a voltage which exceeds the threshold voltage of the optical modulating material and has one polarity is applied to selected picture elements out of picture elements (I1-S1) and (I1-S2) on scanning electrodes S1 and S2 with the phase different between scanning electrodes in the first step in an address period Tn. The voltage which exceeds the threshold voltage of the optical modulating material and has the other polarity is applied to picture elements which are not selected out of picture elements (I1-S1) and (I1-S2) on scanning electrodes S1 and S2 with the phase different between scanning electrodes in the second step of the address period Tn, and respective phases in the first step and those in the second step are made different from each other. Thus, the contrast is discriminated in accordance with the direction of the electric field.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for driving an optical modulation element, and more particularly to a method for driving an optical modulation element, such as a ferroelectric liquid crystal element, that discriminates contrast depending on an electric field.

Conventionally, a group of scanning electrodes and signal electrodes g are configured in a matrix, and a liquid crystal compound is filled between the electrodes to form a large number of pixels, and an address signal is selectively applied to the gate to display an image or information. Time-division driving is employed in which a predetermined information signal is selectively applied in parallel to the signal electrode group in synchronization with an address signal.

Most of these that were put to practical use were, for example, “Applied Physics Letters”.
sics Letter”) 1971, 18 (4
) No. 127-128, M, Shut (M, 5c)
hadt) and W, Helfrich (W, He1fric).
h) Co-authored “Voltage Dependent Optical Activity of a Twisted
Nematic Liquid Crystal” (“Voltag
e Dependent Optical Activ
ity of a Twisted Nema
ticLiquid Crystal”)
It was an N (twisted nematic) type liquid crystal.

In recent years, the use of bistable liquid crystal elements as an improved version of conventional liquid crystal elements has been proposed by both C1ark and Lagerwall in Japanese Patent Application Laid-open No. 107216/1982 and US Pat. No. 436792.
This is proposed in Specification No. 4, etc. As the bistable liquid crystal, a ferroelectric liquid crystal having a chiral smectic C phase (SmC") or H phase (SmH") is generally used.
In these conditions, the first
and a second optically stable state, and maintain that state when no electric field is applied, that is, have stability and respond quickly to changes in the electric field. Therefore, it is expected to be widely used in fields such as high-speed and memory-type display devices.

[Problem that the invention seeks to solve]

However, problems arise when the number of display pixels is extremely large and high-speed driving is required. That is, the threshold voltage for providing the first stable state in a ferroelectric liquid crystal cell having bistability for a predetermined voltage application time is -vth.
, and the threshold voltage for providing the second stable state is +
vth , , even if these threshold voltages are not exceeded, if a voltage continues to be applied for a long time, the display state written in the pixel (e.g., white state) will change to another display state (e.g., black state). state) may be reversed. FIG. 1 shows the threshold characteristics of a bistable ferroelectric liquid crystal cell.

Figure 7 shows DOBAMBC (7 in the figure) as a ferroelectric liquid crystal.
2) and the application time dependence of the threshold voltage (vth) required for switching when using HOBACPC (71 in the figure).

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. When applied to a driving element, for example, even if a certain pixel is switched to a bright state during scanning, if an information signal of less than vth is always applied from the next scanning onwards, it will be difficult to detect the problem while the scanning of one screen is completed. It can be seen that there is a risk that the pixel will be inverted to a dark state.

[Means for Solving the Problems] and [Operation] The purpose of the present invention is to provide a novel method for driving a liquid crystal element that solves the problems in conventional liquid crystal display elements or liquid crystal optical shutters as described above. In this method of driving an optical modulation material element, which forms a pixel having an optical modulation material and identifies contrast according to the direction of an electric field, at least two scan electrodes of the scan electrode group are simultaneously addressed, and the address In the first step within the period, one polarity voltage exceeding the threshold voltage of the optical modulation material is applied to a selected pixel among the pixels on at least two scanning electrodes at different phases for each scanning electrode, and the address In a second step within the period, the other polarity voltage exceeding the threshold voltage of the optical modulation material is applied to the unselected pixels among the pixels on at least two scanning electrodes at different phases for each of the scanning electrodes, and This is a method of driving an optical modulation element, particularly a ferroelectric liquid crystal element, in which each phase in the first step and each phase in the second step are different from each other.

〔Example〕

As the optical modulating substance used in the driving method of the present invention, a small amount (
Both have two stable states, and take either a first optically stable state or a second optically stable state depending on the applied electric field. That is, a substance having a bistable state with respect to an electric field, particularly a liquid crystal having such a property, is used. Ferroelectric liquid crystal elements using this property are, for example,
U.S. Patent No. 4367924 and U.S. Patent No. 4563
It is disclosed in Publication No. 05.9.

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. ”) is suitable. Regarding this ferroelectric liquid crystal, “Le Jo
Urnal de Physioveletter
") Volume 36 (L-69), 1975 "Ferroelectric Liquid Crystals J
roelectricLiquid Crystal
sJ); “Applied Physics Letters” (
“Applied Physics Letter
s”) Volume 36 (No. 11) 1980 “Submicron Second Bistiple Electro-Optic Switching in Liquid Crystals J (
rSubmicr.

5econd B15table Electro
optic Switchingin Liqui
d CrystalsJ) ; “Solid State Physics” 16 (
141) 1981 r 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, desiloquinobenzylidene-P'-amino-2-methylbutylcinnamate (DOBAMBC
), hexyloxybenzylidene-P'-amino-2
-chloropropyl cinnamate (HOBACPC), p
-Desyloxybenzylidene-p' -amino-2methylbutyl-α-cyanocinnamate (DOB A M
B CC), p-tetradecyloxybenzylidene-p'
Amino-2-methylbutyl-d-cyanocinnamate (
TDOBAMBCC), p-octyloxybenzylidene-p'-amino-2-methylbutyl-α-chlorocinnamate (00BAMBCC), p-octyloxybenzylidene-p'-amino-2-methylbutyl-α-
Methyl cinnamate, 4゜4'-azoxycinnamic acid-bis(2-methylbutyl) ester, 4-o
-(2-methyl)butylresolcylidene-4'-octylalinine, 4-(2'-methylbutyl)phenyl-4
'-Octyloxyphenyl-4-carboxylate, 4-hexyloxyphenyl-4(2'-methylbutyl)biphenyl-4'-carboxylate, 4-octyloxyphenyl-4-(2'-methylbutyl)biphenyl- 4'-carboxylate, 4-hebutylphenyl-4-(4'-methylhexyl)biphenyl-4'-carboxylate, 4-(2'methylbutyl)phenyl-4-(4'methylhexyl)biphenyl-4' -carboxylate, which can be used alone or in combination of two or more, and can contain other cholesteric liquid crystals or smectic liquid crystals as long as they exhibit ferroelectricity.

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

Further, in the present invention, in addition to the above-mentioned SmC'' and SmH'', it is also possible to use ferroelectric liquid crystals that appear in chiral smect F phase, ■ phase, J phase, G phase, or other phases.

FIG. 8 schematically depicts an example of a ferroelectric liquid crystal cell. 81a and 81b are In2O3, 5no2 or ITO
It is a substrate (glass plate) coated with a transparent electrode such as (indium tin oxide), and an SmC'' phase liquid crystal with a liquid crystal molecular layer 82 oriented perpendicular to the glass surface is sealed between the substrates (glass plates). A thick line 83 represents a liquid crystal molecule, and this liquid crystal molecule 83 has a dipole moment (P±) 84 in a direction perpendicular to the molecule. When a voltage equal to or higher than a certain threshold is applied, the helical structure of the liquid crystal molecules 83 is unraveled, and the orientation direction of the liquid crystal molecules 83 can be changed so that all dipole moments (P±) 84 are oriented in the direction of the electric field.Liquid crystal molecules 83 has an elongated shape and exhibits refractive index anisotropy in its long axis direction and short axis direction. Therefore, for example, if polarizers are placed above and below the glass surface in a crossed nicol positional relationship, the voltage applied polarity can be adjusted. It is easily understood that the liquid crystal optical modulation element has optical characteristics that change depending on the thickness of the liquid crystal cell.Furthermore, if the thickness of the liquid crystal cell is sufficiently thin (for example, 1 μ
), the helical structure of the liquid crystal molecules unravels even when no electric field is applied, and the dipole moment Pa or pb is directed upward (94a) or downward (94b) as shown in FIG.
). In such a cell, as shown in FIG. 9, electric fields Ea or E with different polarities above a certain threshold value are applied.
When b is applied for a predetermined time, the dipole moment becomes electric field Ea
or upward 94a corresponding to the electric field vector of Eb, or
The direction is changed from downward 94b, and accordingly, the liquid crystal molecules are aligned in either the first stable state 93a or the second stable state 93b.

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. 3, for example, when the electric field Ea is applied, the liquid crystal molecules are oriented in a first stable state 93a, and this state remains stable even when the electric field is turned off. Moreover, when an electric field Eb in the opposite direction is applied, the liquid crystal molecules are oriented to the second stable state 93b and the orientation of the molecules is changed.
It remains in this state even if the electric field is turned off. Further, each orientation state is maintained as long as the applied electric field Ea does not exceed a certain threshold value. In order to effectively realize such fast response speed and bistability, it is preferable that the cell be as thin as possible, and generally, the thickness is 0.5μ to 20μ.
, especially 1μ to 5μ is suitable.

Preferred specific examples of the driving method of the present invention will be explained with reference to FIGS. 1 to 6.

In FIG. 6, a ferroelectric liquid crystal compound is sandwiched in between.

Now, to simplify the explanation, an example will be shown in which a black and white binary signal is displayed. In FIG. 6, the pixels indicated by diagonal lines are "black", and the others are connected to a scanning side drive voltage generation circuit 64 that generates a non-selection voltage. The scanning circuit 65 generates a scanning signal that sequentially scans the scanning electrodes. On the other hand, on the signal side, there is a serial-to-parallel conversion circuit 66 that converts a display signal input in the form of a serial signal from an external circuit into a parallel signal for -lines, a line memory 67 that holds this signal for the scanning time, and a line memory 67 that holds this signal for the scanning time. It is comprised of a signal side drive voltage generation circuit 68 that generates information signals (white signal, black signal) according to the output state of the memory 9.

1-5 depict preferred embodiments of the invention. Ss (N) of the LID (A) is the address period T n (n = 1, 2, 3, ・-n
) is the scan selection signal Ss (N+1) applied to the Nth (=2n-1) scan electrode addressed by N+1 (=
The scan selection signal SN applied to the 2n)th scan electrode corresponds to the scan non-selection signal applied to the unaddressed scan electrodes, a white signal and a black signal, respectively. Further, IN is an information non-selection signal that is applied to the signal electrode when information is not selected within the address period. Note that, in the present invention, the voltage of each signal is a voltage set with reference to the potential of an unaddressed scanning electrode.

FIG. 1(B) is a time-series representation of the voltage waveform when the driving voltage shown in FIG. Intersection with (pixel)
It is based on the display state of.

According to FIG. 1(B), the first scan electrode S1 and the second scan electrode S2 are simultaneously addressed during the address period T, and the scan electrodes S1 and 82 are selected to perform the scan selection shown in FIG. 1(A), respectively. Signals Ss (N) and Ss (N+1) are applied. Within the address period Tn, the phases t1 and t3 correspond to the phases that cause the pixels on the scanning electrode S (N) to display white and black (contrast) states, respectively, and the phases t2 and t4 correspond to the phases on the scanning electrode S (N+1). This corresponds to the phase that causes each pixel to display white and black (contrast) states. Therefore, in this example, the pixel (r+
s+) is in a black display state, the black signal Is (B) shown in FIG. 1(A) is applied to the signal electrode 11 at phase t3, while the pixel (II- 5
2) is in a white display state, so a white signal 1s (W) is applied to the signal electrode 11 at phase t2. Furthermore, since neither the white signal l5 (W) nor the black signal Is (B) is selected in phases t1 and t4, which are phases other than phases t2 and t3, the information non-selection signal ■ shown in FIG. is applied.

Also, since the pixel (I2 Sl) on the scanning electrode S1 is in a white display state, the signal electrode I2 is in a white display state at the phase t1.
Since the white signal 1s (W) shown in FIG. 1(A) is applied to the pixel (12 S2) on the scanning electrode S2, the white signal 1s (W) shown in FIG. A signal l5(W) is applied. Therefore, in phases t3 and t4, the white signal l5 (W) and the black signal Is (
An information non-selection signal IN that does not select any of B) is applied.

As a result, for example, in the pixel (r+s+), 2Vo at phase t1 and V at phase t2. , -3V o in phase t3.

At phase t4, a voltage of O is applied, and at this time the voltage is ■. The threshold voltage of one of the ferroelectric liquid crystals (threshold voltage that inverts the display state to black) is −V th l , and the threshold voltage of the other ferroelectric liquid crystal (threshold voltage that inverts the display state to white) vth
When set to 2, the voltage is V. Threshold voltage vth.

and Vth2, -3V　o＜-Vth　,　＜-2V　.

2vo<Vth2ku3v.

Since voltage 3Vo is applied to the element (II 32) at phase t2, writing is performed to a white display state within the address period.

In addition, in this example, the application time of the same polarity voltage applied to the pixels on the scanning electrode when not addressing is the phase t1.
When the period of Δt1 is set as Δt1, it can be set to 2Δt1 at maximum, so the above-mentioned reversal phenomenon can be effectively prevented. Further, in a preferred embodiment of this example, the phases t1 to t
4 period is Δt1 to Δt4, Δ1. =Δt2=
It is preferable that Δt3=Δt4=O, 1 m5ec to 2m5ec. Furthermore, in a preferred embodiment, the voltage V. can be set from 3 volts to 70 volts.

In this example, the voltages applied to the pixels on the scan electrode when not addressed are voltages ■, 0 and - as shown in the figure.
V. Therefore, the display state of the pixel written within the address period is not inverted by the voltage applied when it is not being addressed. That is, when the scanning electrode is addressed, writing for two lines is performed, and the display state is maintained until the next address after one frame ends.

In this driving example, for example, by canceling the writing operation after one frame, it not only reproduces the problem of displaying a still image, but also sequentially moves the scanning electrodes so that frames are periodically repeated one after another. Still images and moving images can also be displayed by addressing the refresh drive.

FIG. 2 shows another preferred driving example of the present invention.

In the driving example shown in FIG. 2, the waveform of the scanning selection signal Ss (N+1) used in the driving example shown in FIG.
The same driving voltage as in the driving example shown in FIG. 1 was used.

That is, in the driving example of FIG. 1, phases t2 and t4 correspond to the white writing phase and the black writing phase, respectively, for the pixel on the scanning electrode (N+1), whereas in the driving example of FIG. In the example, phases t2 and t4 correspond to the black and white writing phases, respectively. FIG. 2(B) shows time-series waveforms when driving using the driving waveform shown in FIG. 2(A).

FIG. 3 shows a driving example in which three scanning electrodes are simultaneously addressed within an address period in another specific example of the present invention. Ss (N) in FIG. 3(A) is the address period T m
The scan selection signal Ss (N+1) applied to the N (-3m-2)th scan electrode addressed by (m = 1, 2, 3, -m) is N+1 (=3m-1)
The scan selection signal applied to the ゛th scan electrode, Ss (N
+2) represents a scan selection signal applied to the N+2 (=3m)th scan electrode. At this time, the information signals applied to the signal electrodes include the information selection signals 1s (W) and Is (B) used in the driving example of FIG.
is used. A phase t1 within the address period Tm causes a pixel on the Nth scanning electrode to display white, a phase t2 causes a pixel on the N+1th scanning electrode to display black;
Phase t3 is the phase that makes the pixel on the N+2th scanning electrode display white, phase t4 is the phase that makes the pixel on the Nth scanning electrode display black, and phase t5 is the phase that makes the pixel on the N+1th scanning electrode display black. The phase t6, which is the phase for displaying white, corresponds to the phase for displaying black for the pixel on the N+2 scanning electrode.

Furthermore, with the driving method of the present invention, four or more scan electrodes can be addressed simultaneously within the address period. At this time, the number of scanning electrodes to be addressed simultaneously is 2 to 1.
Approximately 0 pieces is preferable.

The driving example in FIG. 4 represents a driving example when the information selection signals l5 (W) and Is (B) used in the driving example in FIG. 1 are each made into alternating waveforms. In this example, the voltage applied to the pixels on the scan electrode when not being addressed can have an average value of 0. In this example, voltages 3Vo and -3Vo are selectively applied in one of phases t and t5, and voltage 3Vo and 3VO are applied in one of phases t3 and t7.
vo and 13V.

is selectively applied. On the other hand, the phase t2+t4+t
6 and t8, a voltage is applied so that the average voltage of the pixel when not addressed is 0.

FIG. 5 represents another preferred driving example of the present invention. Ss (N) in FIG. 5(A) is the address period Tn (
n=1.2.3. ---n) addressed by N (=
2 The scan selection signal applied to the n=1)th scan electrode, 5s(N+1) is the scan selection signal applied to the N+1 (=2n)th scan electrode, and SN is the scan non-selection signal applied to the unaddressed scan electrode. This is a selection signal. I (N (
W)→N+1 (W)), I (N (W)→N+l
(B)), I(N(B)→N+1 (W)) and I(N
(B)→N+1 (B)) are information signals selectively applied to the signal electrodes. Information signal 1 (N (
W)→NN+1W)) is the information signal when pixels on the Nth and N+1st scanning electrodes are respectively set to white, and the information signal I (N (W)→N+l (B)) is the information signal when the pixels on the Nth and N+1st scanning electrodes are set to white.
This is an information signal when the pixels on the first scanning electrode are white and black, respectively, and the information signal 1 (N (B) → N+1 (W
)) is the information signal when the pixels on the Nth and N111th scanning electrodes are black and white, respectively, and the information signal I (N
(B)→N+1 (B)) is an information signal when pixels on the Nth and N+1st scanning electrodes are respectively set to black. In this example, the voltage applied to the pixels on the scan electrode when not being addressed can have an average value of 0.

The phases t1 and t2 in this example are Nth and N+1, respectively.
The phases t5 and t6, which are the phases for making the pixel on the Nth scanning electrode white, are the phases for making the pixel on the Nth and N+1th scanning electrode black, respectively.

On the other hand, phases t3 and t4 are phases in which a voltage is applied so that the voltage applied to the pixel when not being addressed has an average value of O.

〔Effect of the invention〕

The mechanism of switching of a ferroelectric liquid crystal by an electric field in a bistable state is not necessarily clear microscopically, but in general, after switching to a predetermined (first) stable state with a strong electric field for a predetermined time, If the electric field is left in a state where no electric field is applied, it is possible to maintain that state almost permanently, but if the electric field is so weak that it does not switch for a certain period of time (in the example explained earlier, vth
If an electric field of opposite polarity is applied for a long time even if the voltage is below (corresponding to a voltage of However, in the present invention, when the phase period is Δt, the application time of the voltage of the same polarity applied to the pixel can be set to 2Δt at maximum.
The above-mentioned inversion does not occur. Moreover, in the present invention, since no DC component is applied to pixels when they are not being addressed, the durability of the liquid crystal element can be extended.

[Brief explanation of the drawing]

Figure 1 (A), Figure 1 (B), Figure 2 (A), Figure 2 (
B), Figure 3 (A), Figure 3 (B), Figure 4 (A). 4(B), FIG. 5(A), and FIG. 5(B) are voltage waveform diagrams of driving examples used in the driving method of the present invention. FIG. 6 is a block diagram of a liquid crystal device used in the present invention. FIG. 7 is an explanatory diagram showing the threshold characteristics of the ferroelectric liquid crystal element used in the present invention. 8 and 9 are schematic perspective views of the ferroelectric liquid crystal element used in the present invention. Fuji 10 (c) Seven pipes non-1112 signal SN ' □+fP 柘いξ田R13 [NO−1／lスφい− 富 Z times (A) L back office box R Shinno 5N O□ i@kiflE' (gA Es(βn)
'-, 7, u Information board non-I key angle LJ O- Mizuguchi (A) Medical information board non-I signal 'A 5N' □ PfilflltTe(fA I5(w) o Oral information board non-&Fe signal IN O-1+
N N' φV
I Vl I---〇-N Police jiri ■i Oo 3 mogi sweep Lily sweet ga Vl
”, j: ; 2 27
#mouth (ha) 11 last year F Shinkarasu 5No □ 1 draw Fi non-ru T Touka 11%I O-Choriki (A
) L Gennon error signal <;No □
l/'I small ψ J −N small jψ
φ φ phi

Claims (6)

[Claims] (1) In a method of driving an optical modulation element, in which a pixel having an optical modulation substance is formed at an intersection between a scanning electrode group and a signal electrode group, and contrast is identified according to the direction of an electric field, one of the scanning electrode groups At least two scan electrodes are addressed simultaneously, and in a first step within the addressing period, one polarity voltage is applied to a selected pixel among the pixels on the at least two scan electrodes, exceeding the threshold voltage of the optical modulation material. is applied at different phases to each of the scanning electrodes, and in a second step within the addressing period, the other polarity exceeding the threshold voltage of the optical modulation substance is applied to unselected pixels among the pixels on at least two scanning electrodes. A method for driving an optical modulation element, characterized in that voltages are applied at different phases to each of the scanning electrodes, and each phase in the first step and each phase in the second step are different from each other. (2) The driving method according to claim 1, wherein two scanning electrodes are addressed simultaneously within the addressing period. (3) The driving method according to claim 1, wherein three scanning electrodes are addressed simultaneously within the addressing period. (4) The driving method according to claim 1, wherein the optical modulating substance is a ferroelectric liquid crystal. (5) The driving method according to claim 4, wherein the ferroelectric liquid crystal is a chiral smectic liquid crystal. (6) The thickness of the chiral smectic liquid crystal is set to be sufficiently thin to eliminate the helical structure of the chiral smectic liquid crystal in the absence of an electric field.
Driving method described in section.