JP2505761B2 - Driving method of optical modulator - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for driving an optical modulation element, and more particularly to a method for driving a ferroelectric liquid crystal element having at least two stable states.

[Description of Prior Art]

2. Description of the Related Art Conventionally, a liquid crystal display element in which a scanning electrode group and a signal electrode group are configured in a matrix, a liquid crystal compound is filled between the electrodes to form a large number of pixels, and an image or information is displayed is well known. . As a driving method of this display element, there is a time-division drive in which an address signal is sequentially and selectively applied to the scanning electrode group and a predetermined information signal is selectively applied to the signal electrode group in parallel in synchronization with the address signal. Has been adopted.

Most of these were put to practical use, for example, "Applied Physics Letters"("Appliedphysics").
Letters ") 1971, 18 (4), pages 127-128, co-authored by" M. Schadt "and" W. Helfrich "" Voltage Day Pendant Optical Activities Day " Of a twisted nematic liquid crystal ”(“ V ₀ ltage Dependent
Optical Activity of a Twisted Nematic Liquid Cryst
It was a TN (twisted nematic) type liquid crystal shown in al ").

In recent years, as an improved version of the conventional liquid crystal device, the use of a liquid crystal device having bistability has been disclosed by both Clark and Lagerwall in JP-A-56-10721.
No. 6, US Pat. No. 4,367,924 and the like. As the bistable liquid crystal, a ferroelectric liquid crystal having a chiral smectic C phase (SmC *) or an H phase (SmH *) is generally used. It has either the first optical stable state or the second optical stable state, and has the property of maintaining that state when an electric field is not applied, that is, stability, and has a quick response to changes in the electric field. Therefore, it is expected to be widely used in the field of high-speed and memory type display devices and the like.

[Problems to be solved by the invention]

However, a problem arises 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 the ferroelectric liquid crystal cell having bistability for a predetermined voltage application time is -Vt.
h ₁ and the threshold voltage for giving the second stable state is + Vt
If h ₂ is set, when the voltage is continuously applied for a long time even if these threshold voltages are not exceeded, the display state (for example, white state) in which the pixel is written is different from the display state (for example, black state). ) May be reversed. FIG. 1 shows the threshold characteristics of a bistable ferroelectric liquid crystal cell.

Figure 1 shows DOBAMBC (12 in the figure) as a ferroelectric liquid crystal.
And HOBACPC (11 in the figure) are used to plot the application time dependence of the threshold voltage (Vth) required for switching.

As is clear from FIG. 1, it is understood that the threshold value Vth has an application time dependency, and the shorter the application time, the steeper the gradient. From this, when the number of scanning lines is extremely large, and when applied to an element that is driven at high speed, for example, even if a bright state is switched at the time of scanning a certain pixel, an information signal of Vth or less is always obtained after the next scanning. It can be seen that there is a risk that the pixel is inverted to the dark state in the middle of the end of the scanning of one screen if the application is continued.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a novel method for driving a liquid crystal display device which solves the problems in the conventional liquid crystal display device or liquid crystal light shutter as described above.

Another object of the present invention is to provide a driving method of an optical modulator suitable for expressing gradation.

[Means for Solving Problems] and [Operation] The present invention relates to a method of driving an optical modulation element in which an optical modulation substance is arranged between a pair of substrates having a scanning electrode group and a signal electrode group. The groups are continuously selected by scanning, and a signal voltage corresponding to the information to be displayed is supplied to the signal electrode group during the continuous scanning selection period including the first to fourth periods to select the groups. After the pixel on the scanning electrode is once erased to a predetermined optical state, the step of determining the optical state of the pixel by either maintaining or changing the erased state is included. When the absolute value of the maximum pulse voltage is set to a predetermined value 2V ₀ , the absolute value of the signal voltage is selected from the range of 0 to 2V ₀ , and the erase voltage of the first polarity is applied to the pixel in the second period. And applying a write voltage of the second polarity during the third period. A first auxiliary voltage having a polarity different from the erase voltage is applied to the pixel during the first period, and a second auxiliary voltage having a polarity different from the write voltage during the fourth period is applied to the pixel. An auxiliary voltage is applied to the pixel to apply the first to fourth
This is a method of driving an optical modulation element in which a voltage waveform in which voltages of the same polarity are not continuous for two unit periods or more when each period of the period is set as a unit period is applied to each pixel. In the present invention, the voltage selection range according to the information to be displayed is set within a predetermined range based on the scanning selection signal, the function of the voltage pulse in the first to fourth periods is determined, and two unit periods (two phases) are set. The voltage of the same polarity is prevented from being continuously applied to the pixels over the above. In this way, so-called crosstalk in which the optical state of the pixel is inverted unintentionally can be prevented. Moreover, even when the scan electrodes are continuously selected (the scan electrodes are not provided with a pause period in which no scan is performed), continuous application of the same polarity voltage can be prevented, so that the frame frequency for rewriting one screen does not decrease. Also has features.

〔Example〕

The optical modulation substance used in the driving method of the present invention has at least two stable states, and particularly takes one of the first optical stable state and the second optical 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.

As a 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 them, a chiral smectic C phase (SmC *) or an H phase (SmC *). *) Liquid crystal is suitable. About this ferroelectric liquid crystal, "Le Journal dephysiove lett"
er ") 36 (L-69), 1975" Ferroelectric Liquid Crystals "(" Ferroelectric liquid
Crystals ");" Applied Physics Letters "36 (11) 1980
Year of "Submicron Second By Stable Electro-Optical Switching In-Liquid
Crystal ”(“ Submicro Second Bistable Electroop
tic Switching in Liquid Crystals) ”;“ Solid physics ”1
6 (141) 1981 “Liquid crystal” and the like, and the ferroelectric liquid crystal disclosed therein can be used in the present invention.

More specifically, examples of the ferroelectric liquid crystal compound used in the method of the present invention include desiloxybenzylidene-P'-
Amino-2-methylbutyl cinnamate (DOBAMBC),
Hexyloxybenzylidene-P'-amino-2-chloropropyl cinnamate (HOBACPC) and 4-o-
(2-methyl) -butyl resorcylidene-4'-octylaniline (MBRA8) and the like can be mentioned.

When using these materials to form an element, the liquid crystal compound is kept in a temperature state where it becomes the SmC * phase or SmH * phase. Can be supported.

Further, in the present invention, in addition to SmC * and SmH * described above, it is also possible to use a ferroelectric liquid crystal which appears in a chiral smectic F phase, I phase, J phase, G phase or K phase.

FIG. 2 is a schematic drawing of an example of a ferroelectric liquid crystal cell. 21a and 21b are substrates (glass plates) coated with transparent electrodes such as In ₂ O ₃ , SnO ₂ and ITO (Indium-Thein-Oxide), between which the liquid crystal molecular layer 22 is perpendicular to the glass surface. The liquid crystal of SmC * phase that is oriented like this is enclosed. A thick line 23 represents a liquid crystal molecule,
The liquid crystal molecule 23 has a dipole moment (P⊥) 14 in a direction orthogonal to the molecule. When a voltage above a certain threshold is applied between the electrodes on the substrates 21a and 21b, the liquid crystal molecules 23
The orientation of the liquid crystal molecules 23 can be changed so that the helical structure is unwound and all the dipole moments (P⊥) 24 are oriented in the electric field direction. The liquid crystal molecules 23 have an elongated shape, and exhibit refractive index anisotropy in the major axis direction and the minor axis direction thereof. Therefore, for example, if polarizers arranged in a crossed Nicols position above and below a glass surface are placed. It is easily understood that the liquid crystal optical modulation element has optical characteristics that change depending on the polarity of voltage application. Further, when the thickness of the liquid crystal cell is made sufficiently thin (for example, 1 μ), the helical structure of the liquid crystal molecules is unwound and the dipole moment Pa or Pb thereof is released even when no electric field is applied as shown in FIG. Takes either the upward (34a) or downward (34b) state. When an electric field Ea or Eb having different polarities equal to or greater than a certain threshold is applied to such a cell for a predetermined time as shown in FIG. 3, the bipolar moment is upwardly directed to the electric field vector of the electric field Ea or Eb,
The direction of the liquid crystal molecule is changed to the downward direction 34b and the liquid crystal molecule is changed to the first direction.
The stable state 33a or the second stable state 33b.

There are two advantages of using such a ferroelectric liquid crystal as the optical modulation element. First, the response speed is extremely fast, and second, the orientation of the liquid crystal molecules has a bistable state. The second point will be explained with reference to FIG. 2, for example.
When the electric field Ea is applied, the liquid crystal molecules are aligned in the first stable state 33a, but this state is stable even when the electric field is cut off. or,
When the electric field Eb in the opposite direction is applied, the liquid crystal molecules are oriented in the second stable state 33b to change the orientation of the molecules, but they remain in this state even when the electric field is turned off. Electric field
As long as Ea does not exceed a certain threshold, each alignment state is also maintained. With such a fast response speed,
In order to effectively realize the bistability, it is preferable that the cell is as thin as possible, and generally 0.5 μ to 20 μ, particularly 1 μ to 5 μ is suitable.

A preferred specific example of the driving method of the present invention is shown by the following figures.

FIG. 4 is a schematic view of a typical ferroelectric liquid crystal cell 41 having a matrix pixel structure in which a bistable ferroelectric liquid crystal is sandwiched between a scan electrode group 42 and a signal electrode group 43. The present invention can be applied to multi-value or analog gradation display, but for simplification of description, a case of displaying three values of white, one intermediate level, and black will be described as an example.
In FIG. 4, the pixel A indicated by cross-hatching corresponds to “black”, the pixel B indicated by half-hatching corresponds to “gray” (intermediate level), and the other pixels C correspond to “white”. To do.

FIG. 5 shows an example of specific drive waveforms when image erasing and writing are sequentially performed line by line, and the image after writing corresponds to FIG.

FIG. 5 (a) shows each scan electrode S _S , S _NS and each signal electrode I.
_The voltage waveforms applied to _S , I _HS , and _{IN S} , and the voltage applied to the pixel liquid crystal sandwiched between the respective scan electrodes and signal electrodes are shown. Here, the horizontal axis represents time and the vertical axis represents voltage.

Here, S _S is a line in which image information is written, that is, a drive waveform applied to the selected scan electrode, and S _NS is a line in which image information is not written at that time, that is, a drive waveform applied to a non-selected scan electrode. Is. I _S is a driving waveform for writing “black” between the intersection with the selected line, I _HS is an intermediate level, and I _NS is a driving waveform for writing “white”. .

At this time, the electrodes respectively applied to the liquid crystals forming the pixels are I _S −S _S , I _HS −S _S , I _NS −S _S , I _S −S _NS , I _HS −
S _NS , I _NS −S _NS .

Here, the inversion threshold of the bistable ferroelectric liquid crystal used is Vth
, | ± 2V between the inversion threshold Vth and the drive voltage V _0
The drive voltage V ₀ is selected so that ₀ | <| Vth | <| ± 3V ₀ |. Here, the inversion threshold Vth may be slightly different between the sides due to the alignment process or the like applied to a normal liquid crystal cell, but in this case, the drive potentials on the side and the side are slightly different in each drive waveform. A correspondence such as correction is taken, and here, for convenience of explanation, | + Vth | (side inversion threshold) = | −Vth | (side inversion threshold) is set.

In the above case, when the voltage applied to each pixel is 2V ₀ or less in absolute value, liquid crystal molecules do not invert, and when it is 3V ₀ or more, inversion occurs,
As the absolute value becomes larger, the reversal becomes stronger.

 Here, each waveform will be described.

The scan selection signal applied to the selected scan electrode, S _S is 1
There are four phases within the line writing period, line erasing is performed in the second phase, and pixel writing is performed in accordance with the signal applied to the signal electrode in the third phase. 2V ₀ to 2 phase, the pulse waveform of 2V ₀ third phase is applied. Further, a potential of substantially zero (reference potential) is auxiliary applied to the first phase and the fourth phase.

On the other hand, the scanning non-selection signal SNS applied to the unselected scanning electrodes is fixed to the reference potential (here, OV).

Next, the potential waveform applied to the signal electrode is applied with the erase signal 2V ₀ in the second phase thereof substantially in synchronization with each phase of the scan selection signal, and in this phase, each signal electrode and the selected scan Since a voltage of 4V ₀ is applied between the electrode S _S and the inversion threshold −Vth of the liquid crystal is exceeded,
Invert all this lines to the erase side (white). next,
In the third phase, a voltage corresponding to each gray scale is applied to the signal electrodes that intersect the selected scan electrode S _S.
Here -2 V ₀ this time pixel as the potential to "black", -V ₀ as the potential to an intermediate level (gray), and zero as potential kept at the "white" (reference potential). In this way, in the third phase, the voltages applied to the pixels on this line are + 4V ₀ , + 3V ₀ , and + 2V ₀ , respectively, and "black" and intermediate level "white" are written to the pixels, respectively.

Next, the first phase and the fourth phase that are auxiliary applied will be described. First, as the voltage applied to the pixel on the scan electrode selected in the fourth phase, −2V _0, which is the same polarity as the voltage polarity in the erase phase and is equal to or less than the threshold value, is applied.

Further, as the first phase, a voltage corresponding to the write signal in the pixel of the second phase is applied, and this potential has the same polarity as the voltage applied to the pixel in the third phase (based on the scanning non-selection signal as a reference). ) Or the same voltage level as the scanning non-selection signal. At this time, in the present invention, in order to eliminate the flicker on the display screen, the absolute voltage value in each phase applied to the pixel on the selected scan electrode; the absolute voltage value in the first phase | V ₁ | Absolute voltage value in 2 phases | V ₂
|, The absolute voltage value | V ₃ | in the third phase and the absolute voltage value | V ₄ |; in the fourth phase are | V ₁ | + | V ₃ | = | V ₂ | + | V ₄
It is preferable to have a relationship of |.

The mechanism of switching by the electric field of the ferroelectric liquid crystal in the bistable state is not necessarily microscopically clear, but generally after switching to a predetermined stable state with a strong electric field for a predetermined time, no electric field is generated. If left unapplied, it is possible to maintain that state almost semi-permanently, but a weak electric field that does not switch for a predetermined period of time (in the example explained earlier, it corresponds to a voltage below Vth. ), When an electric field of opposite polarity is applied for a long time, the orientation state is inverted again to the opposite stable state, and as a result, correct information display and modulation cannot be achieved. Can occur. The inventors of the present invention have found that the ease of occurrence of the transition reversal phenomenon (a kind of crosstalk) of the alignment state due to the application of such a weak electric field for a long time is
We have recognized that it is affected by roughness and liquid crystal material, but we have not yet grasped it quantitatively. However, it was confirmed that uniaxial substrate treatment for aligning liquid crystal molecules such as rubbing or oblique vapor deposition of SiO 2 tends to increase the inversion phenomenon. In particular, it was confirmed that the tendency was stronger when the temperature was high than when the temperature was low.

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

In the present invention, the above-mentioned problems can be solved by preventing the voltages of the same polarity from being applied successively in two or more phases.

 FIG. 6 shows another embodiment.

FIG. 6 only changes the voltage value of the first phase applied to the selected scan electrode with respect to the drive waveform of FIG.
Therefore, the effect on the crosstalk generated in the pixels in which the scanning signals are not selected and the phases are continuous, or the effect on the stabilization of the gradation expression is exactly the same as in the example of FIG. The newly characterized point in FIG. 6 is that
Of the voltages applied between the selected scan electrode and signal electrode, the voltage applied as the line erase signal, that is, before the second phase, that is, the voltage applied in the first phase must be less than or equal to the threshold | Vth | The absolute value of the voltage is set. By doing so, for example, FIG. 5 (a)
In the case of S _S −I _HS and S _S −I _{NS in} the above example, that is, a write signal exceeding the threshold is added to the first phase before line erasure, so that the line is temporarily erased before the line erase. Flickering of pixels due to writing "black" or an intermediate level is prevented.

In the above description, a ternary image was described, but in FIGS. 5 and 6, the potential in the third phase of the drive waveform applied to the signal electrode is -2V _0.
From 0 to -2V ₀ by dividing the potential in the first phase into multi-values or by analog-selecting the value to give multi-gradation or analog gray-scale. Image is obtained.

FIG. 7 shows a matrix cell in which pixels are formed by the driving waveforms shown in FIGS.

Here, I _{1 to} I ₅ are signal electrodes formed of a transparent conductive film such as ITO, a portion connected to S _{0 to} S ₅ is a low resistance scanning electrode formed of Al, Au, or the like, or S ₀ The part sandwiched between ~ S ₅ is SnO _{2 and} other transparent high resistance films (10 ⁵ ~ 10 ⁸ Ω / □).

The drive waveform shown in FIG. 5 or 6 is applied to the scan electrode selected in the above configuration. Here, S ₀ was always set to zero (reference) potential. In this structure, a potential gradient of about 2V ₀ can be formed between the selected scan electrode and the non-selected scan electrode during pixel writing. That is,
For example, when a voltage of 2V ₀ is applied to S ₁ during writing, the potential becomes V ₀ at the midpoint between S ₀ and S ₂ .

On the other hand, when a predetermined signal voltage is applied to the signal electrode group, since the voltage applied to the liquid crystal is different at each position of the resistance film, “black” is written only in the portion where the voltage exceeds the threshold value. become. In FIG. 7, the portion surrounded by the alternate long and short dash line including each scanning electrode corresponds to one pixel.

This operation will be described in more detail using the line erase driving method. Scanning electrode S ₁ is selected, further if the signal voltage to each signal electrode is applied, the range of the range or "black" is erased in the line is written, the dashed line is approximately equidistant from each other than S ₁ A ₁ -B represented between _1. That is, if all of this range is once made “white” and then the signal voltage is “black”, almost all of this range around the scan electrode S ₁ becomes “black”, and if it is at an intermediate level. Part of it becomes "black", and when it is "white", it is kept as it is. Next, when the scan electrode S ₂ is selected, the range in which the entire surface is first set to “white” is between A _{2 and} B ₂ , and then “black” and the intermediate level “white” are determined within this range. . Therefore, by sequentially selecting the scan electrodes, the image shown in FIG. 7 is formed.

On the other hand, by appropriately selecting the maximum absolute value of the voltage applied to the pixel, the pixel can be divided with the centers between the scanning electrodes separated as shown in FIG. This means that when the absolute value of the potential gradient formed between the selected scan electrode and the non-selected scan electrode is 2V ₀ according to FIGS. 5 to 6, a value that is approximately | V ₀ | This is achieved by setting such a maximum voltage value. In other words, it is sufficient to perform gradation expression within a voltage that is about half the magnitude of the potential gradient, and therefore, for example, FIGS.
In the illustrated example, the maximum value may be set between | ± 3V ₀ | and | ± 4V ₀ |. In this case, there may be a difference between the voltage value for making all one pixel “black” and the voltage value for making “white”,
At this time, the voltage value can be corrected by appropriately changing it.

Further, at this time, the scanning can be performed without necessarily performing the scanning line sequentially, and it is possible to perform the driving, for example, every 1ine.

〔The invention's effect〕

According to the present invention described above, it is possible to stably perform gradation expression even in a matrix optical modulator having high density pixels.

[Brief description of drawings]

FIG. 1 is a characteristic diagram showing a change in threshold voltage of a ferroelectric liquid crystal with respect to a voltage application time. FIG. 2 and FIG.
FIG. 3 is a schematic perspective view of a ferroelectric liquid crystal element used in the present invention. FIG. 4 is a plan view of the matrix electrode structure used in the present invention. FIGS. 5 (a) and 6 (a) are drive voltage waveform diagrams, and FIGS. 5 (b) and 6 (b) are their time-series waveform diagrams. FIGS. 7 and 8 are plan views of the matrix electrode structure used in the present invention to express gradation.

Claims (7)

(57) [Claims] 1. A method of driving an optical modulation element in which an optical modulation substance is arranged between a pair of substrates having a scanning electrode group and a signal electrode group, wherein the scanning electrode groups are continuously and sequentially selected for scanning. By supplying a signal voltage corresponding to information to be displayed to the signal electrode group during the scan selection period including the first to fourth periods, the pixels on the selected scan electrode are once erased to a predetermined optical state. After that, the step of determining the optical state of the pixel by either maintaining or changing the erased state is performed. In the step, the absolute value of the maximum pulse voltage of the scan selection signal is set to a predetermined value 2.
When V ₀ is set, the absolute value of the signal voltage is selected from the range of 0 to 2V ₀ , and the erase voltage of the first polarity is applied to the pixel in the second period,
A write voltage having a second polarity is applied to the pixel in the third period, a first auxiliary voltage having a polarity different from the erase voltage is applied to the pixel in the first period, and the fourth voltage is applied to the pixel. When a second auxiliary voltage having a polarity different from that of the write voltage is applied to the pixel during the period of, and each period of the first to fourth periods is set as a unit period,
A method for driving an optical modulation element, wherein a voltage waveform in which voltages of the same polarity are not continuous for a unit period or longer is applied to each pixel. 2. The method for driving an optical modulation element according to claim 1, wherein the optical modulation substance is a chiral smectic liquid crystal. 3. The scanning selection signal comprises pulses of the same polarity and voltage in both the first and third periods, and pulses of the same voltage in the second and fourth periods having opposite polarities to the pulse. The signal voltage is a constant voltage pulse in the second and fourth periods, a voltage pulse changing in accordance with information to be displayed in the third period, and the first period is The method of driving an optical modulation element according to claim 1, wherein the pulse train is a pulse train composed of pulses of a voltage that changes according to the voltage of the third period. 4. The scan selection signal has a reference voltage in the first period, a pulse having the same polarity and the same voltage in the second and fourth periods, and a pulse having the opposite polarity to the pulse in the third period. A pulse train composed of voltage pulses, the signal voltage having a constant voltage pulse in the second and fourth periods, a pulse having a voltage changing in accordance with information to be displayed in the third period, the first pulse The method of driving an optical modulation element according to claim 1, wherein the period is a pulse train composed of pulses of a voltage that changes according to the voltage of the third period. 5. The information to be displayed is multi-valued information corresponding to gradation information, and the voltages in the first and third periods change according to the gradation information. A method for driving the optical modulator according to the item 1. 6. The method of driving an optical modulation element according to claim 1, wherein each pixel has a pixel structure in which a potential difference gradient is generated between the scanning electrode and the signal electrode in accordance with an applied voltage. 7. The method for driving an optical modulation element according to claim 1, wherein the optical modulation substance is a ferroelectric liquid crystal.