JPS6249607B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for driving an optical modulation element, and more particularly to a method for time-divisional driving of an optical modulation element such as a display element or an optical shutter array.

2. Description of the Related Art Liquid crystal display elements have been well known in the art, which display images or information by configuring scanning lines and data lines in a matrix, filling a liquid crystal compound between the electrodes, and forming a large number of pixels. As a driving method for this display element, time-division driving is adopted, in which address signals are selectively and periodically applied to the scanning lines, and predetermined information signals are selectively applied in parallel to the data lines in synchronization with the address signals. 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, for example, M. M.Schadt and Tabrieux. “Applied Physics Letters” by W. Heltrich, Volume 18, Number 4 (February 15, 1971), Page 127
~128 (“Applied Physics Letters” Vo18, No.4
(1971.2.15), P127-128) “Voltage Day Pendant Optical Activity.
“Voltage-Dependent of a Twisted Nematic Liquid Crystal”
Optical Activity of a Twisted Nematic
TN (Twisted Nematic) shown in “Liquid Crystal”
This type of liquid crystal uses a nematic liquid crystal that has a positive dielectric anisotropy in the absence of an electric field and forms a structure (helical structure) in which the molecules of the nematic liquid crystal are twisted in the thickness direction of the liquid crystal layer. , the liquid crystal molecules form a structure arranged in parallel 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 area where both the scanning line and the data line are selected (selected point), there is a A voltage above the threshold is applied, and no voltage is applied to areas where both the scanning line and the data line are not selected (non-selected points), so the liquid crystal molecules maintain a stable alignment parallel to the electrode plane. .
By arranging linear polarizers above and below such a liquid crystal cell in a cross-Nicol relationship with each other, light does not pass through selected points, but light passes through non-selected points, making it possible to use it as a picture line element. Become. However, when a matrix electrode structure is configured, there is a finite area in which scanning lines are selected and data lines are not selected, or in areas where scanning lines are not selected and data lines are selected (so-called "half-selected points"). The electric field becomes hot. 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 the liquid crystal molecules perpendicular to the electric field is set to a voltage value in between, the display element will function normally. However, when increasing 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) increases. It decreases at a rate of 1/N.
For this reason, when repeated scanning is performed, the effective voltage difference between selected points and non-selected points becomes smaller as the number of scanning lines increases, and as a result, reduction in image contrast and crosstalk can be avoided. This has become a serious drawback. This phenomenon is caused by liquid crystals that do not have bistability (the stable state is when the liquid crystal molecules are aligned horizontally with respect to the electrode surface, and they are aligned vertically only while an electric field is effectively applied). ) is essentially an unavoidable problem that arises when driving using the temporal accumulation effect (ie, repeated scanning). In order to improve this point, voltage averaging methods, dual-frequency driving methods, multiple matrix methods, etc. have already been proposed, but all of these methods are insufficient, and the need for larger screens and higher density display elements Currently, the number of scanning lines has reached a plateau due to the inability to increase the number of scanning lines sufficiently.

SUMMARY OF THE INVENTION An object of the present invention is to provide a novel method for driving a liquid crystal element that solves all the problems in conventional liquid crystal display elements or liquid crystal light shutters 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.

That is, an object of the present invention is to drive an optical modulation element having a structure including a scanning line and a data line, and an optical modulation material having bistability with respect to an electric field arranged between the scanning line and the data line. In the method, the polarity and the opposite polarity of the scan selection signal and the information selection signal at the time of writing the new image information to the selected scan line and the data line to be rewritten, respectively. By applying the electrical signal, the optical modulating material corresponding to the part where new image information is to be rewritten is oriented in a first stable state, and the image written in the previous writing step is partially rewritten. an erasing step for erasing; and an information selection signal for applying a scanning selection signal to the scanning line of a portion where new image information is to be rewritten, and orienting the optical modulation material to a second stable state in accordance with the rewritten image information to the data line. This is achieved by a method of driving an optical modulation element having a partial writing step in which a partial write step is applied in synchronization with the scanning selection signal.

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

As a liquid crystal having bistability that can be used in the driving method of the present invention, a chiral smectic C-phase (SmC ^* ) or H-phase (SmH ^* ) liquid crystal having ferroelectricity is suitable. Regarding this ferroelectric liquid crystal, “LE JOURNAL DE
PHYSIQUE LETTERS”) 36 (L-69) 1975,
“Ferroelectic Liquid Crystals”; “Applied Physics Letters”
Physics Letters”) 36 (11) 1980 “Submicro
``Submicro Second Bistable Electrooptic Switching in Liquid Crystals''
"Switching in Liquid Crystals");"Solid State Physics" 16 (141) 1981 "Liquid Crystals", etc., and the ferroelectric liquid crystals disclosed therein can be used in the present invention.

Furthermore, in the present invention, in addition to the above-mentioned SmC ^* and SmH ^* , chiral smectate I phase (SmI ^* ), J
Phase (SmJ ^* ), G phase (SmG ^* ), F phase (SmF ^* )
or K phase (SmK ^* ) can be used.

FIG. 1 schematically depicts an example of a ferroelectric liquid crystal cell. 11 and 11' are In ₂ O ₃ , SnO ₂
It is a substrate (glass plate) coated with a transparent electrode such as ITO (Indium-Tin Oxide), and the layer 12 is oriented perpendicular to the glass surface.
Enclosed is SmC ^* phase or SmH ^* phase liquid crystal. A thick line 13 represents a liquid crystal molecule, and this liquid crystal molecule 13 has a dipole moment 14 (P⊥) in a direction perpendicular to the molecule.
When a voltage higher than a certain threshold is applied between the electrodes on the substrates 11 and 11', the helical structure of the liquid crystal molecules 13 is unraveled, and the liquid crystal molecules 13 change their alignment direction so that all the dipole moments 14 are oriented in the direction of the electric field. Can be done. The liquid crystal molecules 13 have an elongated shape and exhibit refractive index anisotropy in the major and minor axis directions. Therefore, for example, if crossed Nicol polarizers are placed above and below the glass surface, voltage can be applied. It is easily understood that the liquid crystal modulation element has optical characteristics that change depending on the polarity. Furthermore, when the thickness of the liquid crystal cell is made sufficiently thin (for example, 1μ), the helical structure of the liquid crystal molecules unravels (non-helical structure) even when no electric field is applied, as shown in Figure 2, and its dipole The moment P or P' is either upward 24 or downward 24'. When such a cell is given an electric field E or E' with a different polarity above a certain threshold as shown in Fig. 2, the dipole moment will move upward 24 or E' corresponding to the electric field vector of the electric field E or E'. The direction is changed from downward 24', 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 bistability. For example, the second point
To explain with the drawing, when an 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. When an electric field E' in the opposite direction is applied, the liquid crystal molecules are oriented to a second stable state 23' and change their orientation, but they remain in this state even after the electric field is turned off. or,
As long as the applied electric field E does not exceed a certain threshold value, each orientation state is maintained. In order to effectively achieve such high response speed and bistability, it is preferable that the cell be as thin as possible, and generally 0.5μ to 20μ, particularly 0.5μ to 5μ is suitable. A liquid crystal electro-optical device having a matrix electrode structure using this type of ferroelectric liquid crystal has been proposed, for example, by Clark and Ragaval in US Pat.
This is proposed in Publication No. 4367924.

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

FIG. 3A is a schematic diagram of a cell 31 having a matrix pixel structure in which a bistable optical modulation material is sandwiched between scanning lines (scanning electrode group) and data lines (signal electrode group). 32 is a scanning electrode group, and 33 is a signal electrode group. Now, to simplify the explanation, an example will be shown in which a black and white binary signal is displayed. In FIG. 3A, the pixels indicated by diagonal lines correspond to "black" and the other pixels correspond to "white". To initially align the screen to "white" (this step is referred to as the erase step), the bistable optical modulating material may be aligned to a first stable state. For this purpose, a signal of a predetermined voltage pulse (for example, +2V ₀ , time width Δt) is applied to all the scanning electrode groups, and a predetermined pulse (for example,
A signal of voltage −V ₀ , Δt) may be applied. That is, in this erase step, the scanning electrode group is applied with an electric signal of opposite polarity to the scan selection signal during the write step, which will be described below, and the signal electrode group is applied with an electric signal of opposite polarity to the information selection signal (write signal) during the write step. The signals are each applied synchronously.

FIGS. 3B-a and 3B-b respectively show an electric signal given to a selected scan electrode (scan selection signal) and an electric signal given to other scan electrodes (unselected scan electrodes). B-c and B-
d is the electrical signal (information selection signal phase) given to each selected (black) signal electrode.
V ₀ ) applied at T ₁ represents the electrical signal applied to the unselected (white) signal electrode.
In each of FIGS. 3B-a to 3-d, the horizontal axis represents time and the vertical axis represents voltage. T ₁ and T ₂ represent the phase in which the information signal (and scanning signal) is applied and the phase in which the auxiliary signal is applied, respectively. In this example, T ₁ = T ₂ =
An example of Δt is shown.

The scanning electrode groups 32 are sequentially selected. Now, the threshold voltage at the application time Δt to give the first stable state (white) of the liquid crystal cell with bistability is −Vth ₂
Assuming that the threshold electrode at the application time Δt to give the second stable state (black) is Vth ₁ , the electrical signal given to the selected scanning electrode is as shown in Figure 3 B-a.
As shown in , the voltage is -2V ₀ at phase (time) T ₁ and 0 at phase (time) T ₂ .
Further, the other scanning electrodes are in a grounded state as shown in FIG. 3 B-b, and the electrical signal O is present.
On the other hand, the electric signal applied to the selected signal electrode is V ₀ at phase t ₁ as shown in FIG. 3B-c.
and the electrical signal applied to the unselected signal electrodes is -V ₀ at _{phase T 1 and +V 0 at phase} T ₂ as shown in FIG. 3B-d _. . In the above, the voltage value V ₀ is
It is set to a desired value that satisfies V ₀ <Vth ₁ <3V ₀ and −V ₀ >−Vth ₂ >−3V ₀ .

FIG. 3C shows a voltage waveform applied to each pixel when such an electric signal is applied.

In Figure 3C, a and b are on the selected scanning line, respectively, and "black" and "white" are to be displayed on the pixels, and c and d are on the unselected scanning line, respectively. This is the voltage waveform applied to the pixel.

On the scanning line to which the scan selection signal -2V ₀ is applied in the first phase T ₁ , the information selection signal +V ₀ is applied to the pixel that should display "black", and therefore the threshold voltage Vth A voltage of 3V ₀ greater than ₁ is applied to orient the bistable liquid crystal to a second optically stable state and the pixel will be written to "black" (write step). In addition, for pixels that are on the same scanning line and should be displayed as "white", the applied voltage in the first phase _T1 is a voltage that does not exceed the threshold voltage _Vth1 .
Since V ₀ , it remains in the first optically stable state, ie white.

On the other hand, on unselected scanning lines, the voltages applied to all pixels are ±V or O, neither of which exceeds the threshold voltage. Therefore, the liquid crystal molecules are
The orientation corresponding to the signal state when scanning is maintained without changing the orientation state. That is, in a state in which one of the electrodes is aligned to the optically stable state of "white", when the scanning electrode is selected, one line's worth of signals is written in the first phase _T1 , and one frame is completed. This means that the signal state can be maintained even after the

Now, FIG. 3A shows an example of an image formed in one screen by the erasing step and writing step in this manner. Next, FIG. 3D shows an example when this screen is partially rewritten. As shown in the figure, when rewriting the X-Y area consisting of the scanning line X and data line Y with new image information, the rewritten portion (X
The scanning lines S ₁ , S ₂ and _{S 3} corresponding to the -Y area) are connected to electrical signals (for example, 2V ₀ shown in FIG. 4 is applied at one time _or sequentially, and the information selection signal (for example, the _fourth An electrical signal with a polarity opposite to the polarity of I ₁ (V ₀ ) shown in the figure (for example, -V ₀ of I ₁ shown in FIG. 4)
By applying , it is possible to erase only a partial part of one screen, that is, the XY region (partial erasing step).

Next, the data lines of the area partially erased in this manner are supplied with an information selection signal (+V 0 ) and an information non-selection signal (-V ₀ ) according to the rewritten image information in the same manner as in the previous write step. ₀ ) in synchronization with the scan selection signal of -2V ₀ .
Writing in this area can be performed (partial write step).

On the other hand, in the area that is not rewritten, that is, the X'-Y' area consisting of the scanning line X' and the data line Y', this An electrical signal below the threshold voltage of the ferroelectric liquid crystal is applied to each pixel in the region.

Specifically, during the partial erase step, a signal during the erase step (for example, a 2V signal shown in FIG. ₀ ) with the same polarity as the electrical signal (for example, V ₀ of I ₃ shown in Figure 4)
Apply. Additionally, during the partial write step,
The data lines corresponding to the X'-Y' area are supplied with electricity of the same polarity in synchronization with the scan selection signal (for example, S ₁ , S ₂ , S ₃ -2V ₀ shown in FIG. 4) during the partial write step. A signal (for example, -V ₀ of I ₃ shown in FIG. 4) is applied. Further, the potential of the scanning line corresponding to this X'-Y' region is set to a reference potential (for example, 0 volts).

FIG. 4 shows the drive signals described above in chronological order. S ₁ to S ₅ are electric signals applied to the scanning electrodes, I ₁ and I ₃ are electric signals applied to the signal electrodes, and A, C and D are the pixels A, C and D shown in FIG. 3A and D, respectively. This is the voltage waveform applied to C and D.

Further, according to the present invention, a rewritten portion can be specified using a cursor.

Now, the mechanism of switching of a ferroelectric liquid crystal due to an electric field in a bistable state is not necessarily clear from a microscopic perspective, but generally, after switching to a predetermined stable state with a strong electric field for a predetermined time, the electric field disappears completely. If left unapplied, it is possible to maintain that state almost semi-permanently; ), if an electric field of opposite polarity is applied for a long time, the orientation state will be reversed again to the opposite stable state, and as a result, correct information display and modulation cannot be achieved. can occur. The inventors have discovered that the ease with which the orientation state transition reversal phenomenon (a type of crosstalk) occurs due to the long-term application of such a weak electric field is affected by the substrate surface material, roughness, liquid crystal material, etc. Although we have recognized this, we have not yet grasped it quantitatively. However, it has been confirmed that when a uniaxial substrate treatment for aligning liquid crystal molecules is performed, such as rubbing or oblique evaporation of SiO, etc., the tendency for the above-mentioned inversion phenomenon to occur tends to increase. especially,
It was also confirmed that this tendency appears more strongly at high temperatures than at low temperatures.

In any case, it is preferable to avoid applying an electric field in one direction for a long time in order to achieve correct information display or modulation.

Therefore, the second phase T ₂ in the driving method of the present invention
is a phase to prevent a weak electric field from being continuously applied in a certain direction, and as a preferred example, as shown in Fig. 3B-c and d, the electric field is applied to the signal electrode group at phase _T1. Information signal (c is black,
d corresponds to white) and a signal having a different polarity is applied at phase _T2 . For example, when trying to display the pattern shown in FIG. 3A, if a driving method without phase T ₂ is used, pixel A becomes black when scanning electrode S ₁ is scanned, but after S ₂ , The electrical signal applied to the signal electrode I ₁ is -V ₀ continuous,
Since that voltage is directly applied to the pixel A, there is a high possibility that the pixel A will eventually turn white.

In advance, the display is formed by an erasing step in which the entire screen becomes "white", and a writing step in which the corresponding pixels are written as "black" in accordance with the information in the first phase _T1 . In the example, the voltage for writing "black" in the first phase _T1 is _3V0 , and the application time is Δt. Also, the maximum voltage applied to each pixel except during scanning is |±V ₀ |,
The longest time this is applied continuously is the fourth
2△t at 40 points shown in the figure, and when the information signal continues from white → white → black, the second "white"
The most severe condition is when it corresponds to scanning, but even this is 4△t(41), the application time is short, no crosstalk occurs, and once the entire screen has been scanned, the display The displayed information is retained semi-permanently, so there is no need for a refresh process as in display elements using ordinary TN liquid crystals, which do not have bistability.

Now, the optimal time interval for the second phase _T2 depends on the magnitude of the voltage applied to the signal electrode in this phase, and the optimum time interval for the second phase _T2 depends on the magnitude of the voltage applied to the signal electrode. When applying a voltage with the opposite polarity to the voltage applied, it is generally preferable to shorten the time interval when the voltage is high, and to lengthen the time interval when the voltage is low; however, if the time interval is long, It takes a long time to scan the entire screen. Therefore, it is preferable to set T ₂ ≦T ₁ .

FIGS. 5 and 6 show time-series representations of other drive configurations based on other aspects of the invention. In these driving configurations, the threshold for orientational transition is |V ₀ | and 2|V ₀ | for pulse width Δt.
The value of V ₀ is set to be a value between .

In FIG. 5, the signal for erasing the image is +V ₀ to the scanning electrode group and -V ₀ to the information signal electrode group in the erasing step.
After applying the electrical signal, immediately
In the next writing step, a scanning signal of -V ₀ is applied sequentially from S ₁ , and in synchronization with this scanning signal, an information selection signal +V ₀ is applied to the signal electrode, and writing is performed in the state shown in FIG. 3A. .

Next, in the partial erasing step, the X-
An electric signal of -2V ₀ is applied to the pixel written in the previous step in the Y area and erased at 1 o'clock (at this time, as shown in the example of erasing at 1 o'clock in Fig. 5, V ₀
(The partial erasing step may be performed by sequentially applying electrical signals of 1 to 1 to the scanning lines as scanning selection signals). Next, by applying an electric signal corresponding to new image information to the X-Y area, it is possible to partially write the X-Y area into the state shown in FIG. 3D.

The examples shown in FIGS. 5 and 6 are cases in which there is no auxiliary signal, and the example in FIG. 7 is a case in which there is an auxiliary signal. The voltage value of each drive pulse is indicated in the figure, but in the example of FIG. 7, the erase signals applied to the scanning electrode and the signal electrode during the erase step have polarities different from those during information writing. It is inverted and the absolute value is smaller (2/3Vo),
Moreover, it is an electrical signal with a large pulse width (2Δt). Such an erasing method is possible if the threshold voltage of the liquid crystal has pulse width dependence and the threshold value Vth ² △ ^t for the width 2 △t satisfies Vth ² △ ^t ≦4/3V ₀ . be.

Also, in the partial erase step, an electric signal of -4/3V ₀ is applied to the pixels in the X-Y area to partially erase them, and in the next partial write step, a new image is written in the X-Y area. .

In addition to DOBAMBC used in Example 1, examples of ferroelectric liquid crystal compounds include hexyloxybenzylidene-P'-amino-2-chloropropyl cinnamate (HOBACPC), 4-0-(2-methyl) -Butyl-resolcylidene-4'-octylaniline (MBRA8) and the like can be used. In addition, as a ferroelectric liquid crystal, the above-mentioned SmC ^*
In addition to SmH ^* , chiral smectic phase I,
F phase, G phase, J phase and K phase can be used.

When constructing an element using these materials, the element may be supported by a copper block with a heater embedded in it, if necessary, in order to maintain the temperature at which the liquid crystal compound enters the chiral smectic phase. can.

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

Hereinafter, specific examples of the present invention will be shown.

Example 1 A transparent conductive film (ITO) was applied to one of a pair of glass plates patterned to form a 500 x 500 matrix by spin coating.
A 300 Å polyimide film was formed. The substrate was subjected to a rubbing treatment using a roller whose surface was wrapped with a terrene cloth, and then bonded to the other substrate not coated with the polyimide film to form a cell. The cell spacing at this time is approximately 1.6μ. In this cell,
Desyloxybenzylidene-P′-, a ferroelectric liquid crystal
By injecting amino-2-methylbutyl cinnamate (DOBAMBC) and slowly cooling it from the heated molten state, a uniform monodomain state in the SmC state was obtained. The cell temperature was controlled at 70°C, and based on the driving method shown in Figures 3 and 4,
Set V ₀ = 10V, T ₁ = T ₂ = △t = 80μsec,
When line sequential scanning was performed, an extremely good image was formed, and when a part of this screen was then rewritten as a new image using the method shown in Figure 4, a good image with the rewritten screen was obtained. Ta.

[Brief explanation of the drawing]

1 and 2 are perspective views of a liquid crystal element used in the driving method of the present invention. FIGS. 3A and 3D are plan views of electrode structures used in the driving method of the present invention. FIGS. 3B a to 3D are explanatory diagrams showing waveforms of electrical signals applied to the electrodes. Figure 3
Ca-d are explanatory diagrams showing voltage waveforms applied to pixels. Figures 4, 5, 6 and 7
The figure is an explanatory diagram showing a voltage waveform when voltage is applied in time series.

Claims (1)

[Claims] 1. In a liquid crystal device in which a pixel is formed by arranging a ferroelectric liquid crystal at the intersection of a scanning electrode group and a signal electrode group, a pixel formed by applying a voltage exceeding the threshold voltage of one of the ferroelectric liquid crystals A pixel in a 1-orientation state,
A liquid crystal device that forms an image area by pixels in a second orientation state formed by applying a voltage exceeding the other threshold of the ferroelectric liquid crystal, wherein an area to be rewritten is specified in the image area. Then, in the rewriting target area, in the first step, a voltage exceeding the threshold voltage of one of the ferroelectric liquid crystals is applied to the intersection of the scanning electrode group and the signal electrode group, and in the second step, scanning selection is applied to the scanning electrodes. an information selection signal for applying a signal and, in synchronization with the scanning selection signal, applying a voltage to the selected signal electrode that exceeds the other threshold voltage of the ferroelectric liquid crystal by combination with the scanning selection signal; the auxiliary signal is applied to the pixel on the scanning non-selected electrode,
Before the application time of the voltage of the same polarity applied to the pixel reaches the application time that reverses the stable state of the ferroelectric liquid crystal selected at the time of scan selection, by combining with the voltage applied to the scan non-selection electrode, A liquid crystal device characterized in that a voltage of opposite polarity to the voltage of the same polarity is applied.