JPH0291614A - Driving method for liquid crystal element - Google Patents

Abstract

PURPOSE:To obtain a multiplex driving method to be able to improve the contrast and the transmittance of a liquid crystal element by minimizing the response of liquid crystal by a voltage pulse applied to liquid crystal in a non- selected period. CONSTITUTION:The liquid crystal element consists of a glass substrate 21, a scanning electrode 22, a signal electrode 23, an insulating layer 24, an oriented film 25, and a liquid crystal layer 26. A voltage pulse having a crest value V3 is the correction pulse to eliminate a DC component to '0' and is applied just before selection of the scanning electrode. Contrast ratio of 1:16 and transmittance of 30% are obtained by driving on this condition. Two polarizing plates are put one over the other so that their oscillation directions are parallel, and the transmittance is based on the quantity of light obtained at this time. By driving in a conventional driving method, contrast ratio of 1:8 and transmittance of 15% are obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application 1 The present invention relates to a method for driving a display body, a light valve, etc., and more particularly to a display using a bistable liquid crystal, particularly a ferroelectric liquid crystal. Concerning how the body is driven.

[Prior Art] As a conventional method for driving a ferroelectric liquid crystal, Seiko Electronics Industries has developed Japan Displa7 '86 PP-4.
Driving method reported in 1986, Driving method reported by Toshiba in SID″88 Digest, JP-A-61-180219
(Figure 4) and JP-A-Sho-+56o46 (Figure 5)
Many drive waveforms have been proposed, such as the drive waveforms described in .

For example, in the drive waveform (Fig. 4) presented in Japanese Patent Application Laid-Open No. 61-180219, the scanning type FfA (4 in Fig. 4)
01), a first voltage pulse (Vl in FIG. 4) whose absolute value is larger than the saturation value of the liquid crystal for aligning the bistable liquid crystal to the first stable state during the selection period tlO and t20; A second voltage pulse (V2 in Figure 4) with a smaller absolute value than the saturation value is applied with the opposite polarity to orient the liquid crystal to a second stable state, and the voltage is 0 volts during the non-selection period. On the other hand, a third voltage is applied to the signal electrode (402 in FIG. 4) which, when combined with the second voltage pulse, can increase the saturation value of the liquid crystal at the polarity on the second stable state side. pulse(
V3 in Figure 4), or a fourth voltage pulse with opposite polarity and equal DC component to the third voltage pulse (V4 in Figure 4) to lower the threshold value of the liquid crystal on the same polarity side. is applied within a period corresponding to the first voltage pulse, and has opposite polarity to the third and fourth voltage pulses, has an equal DC component, and when combined with the first voltage pulse, the first voltage pulse This is a driving method in which a fifth voltage pulse (V3 in FIG. 4) is applied that is equal to or higher than the saturation value of the liquid crystal on the voltage pulse polarity side. This driving method is configured such that when not selected, the average value of the DC component of the positive and negative voltage pulses is always zero below the threshold of the liquid crystal, regardless of the selected content and multiplicity of each pixel. In addition, this voltage pulse is not continuously applied in the same polarity direction for a period longer than twice the second voltage pulse width, and furthermore, the threshold value of the liquid crystal differs depending on the pulse width of the applied pulse. It is also characterized in that changes in the selection contents of the liquid crystal due to cumulative response effects are prevented to some extent.

In the driving method disclosed in Japanese Patent Application Laid-open No. 60-156046, a first voltage pulse is applied to a pixel on a scanning electrode to orient the liquid crystal in a first stable state in a first phase within a selection period (t10 in FIG. 5). and in a second phase, a second voltage pulse is applied to selected one of the pixels to orient the liquid crystal in a second stable state, selecting the pixel on the scanning electrode, and applying a second voltage pulse to a selected one of the pixels, selecting the pixel on the scanning electrode This driving method applies alternating voltage pulses of zero volts during non-selection periods. In this driving method, the voltage pulses applied during the non-selection period are 0 volts, so that the voltage pulses are continuously applied in the same polarity direction with a length that is twice or more than the pulse width of the first and second voltage pulses. The feature is that it is designed so that no voltage is applied.

[Problems to be Solved by the Invention] However, in the conventional driving method, as shown in FIGS. 4 and 5, depending on the selection content of each pixel, pixels may be continuously or A voltage is directly applied in the same polarity direction via zero volts. It is known that ferroelectric liquid crystals respond cumulatively, and the apparent response is approximately 2
When a voltage pulse of twice the length is applied, the operation margin becomes narrower depending on the pixel selection due to the effect of the pulse width dependence, and the optical characteristics and special contrast deteriorate.
Flickering is likely to occur. Furthermore, for some reason, the threshold characteristics change asymmetrically after being energized and after being left unused.

On the other hand, in order to prevent the above-mentioned reduction in contrast and flickering, a waveform in which, for example, a high frequency signal is superimposed during the non-selection period was also published by Akada et al. 9426. However, such a driving method has a problem in that the power consumption of the driving circuit increases when the peak value of the high-frequency signal voltage is high.

The present invention is intended to solve the above problems, and its purpose is to minimize the response of the liquid crystal due to the voltage pulse applied to the liquid crystal during the non-selection period.
The object of the present invention is to provide a multiplex driving method that can improve the contrast and transmittance of a liquid crystal element.

1 Means for Solving the Problems 1 In order to solve the above problems, the method for driving a liquid crystal element of the present invention provides a method for driving a liquid crystal element using a method for driving a ferroelectric device between substrates in which electrode surfaces of a substrate having a scanning electrode and a substrate having a signal electrode face each other. In a method for driving a liquid crystal element having a liquid crystal sandwiched between them, during the selection period, at least the alignment direction of the liquid crystal molecules is aligned in the first stable state.
A first period in which a first voltage pulse whose absolute value is greater than or equal to the saturation value of the element is applied, and then a polarity opposite to that of the first voltage pulse is applied to change the alignment direction of liquid crystal molecules from the first stable state to a second one. A second period of applying a second voltage pulse whose absolute value is below the threshold value or above the saturation value of the element, and the absolute value of the applied voltage pulse, in order to select whether or not to invert the arrangement to a stable state. A third period in which the value is zero is provided, and at any point within the third period, the scanning electrode is in a state where a resistor is interposed between it and the device that supplies the voltage pulse, that is, the first and The impedance is in a sufficiently high state compared to the second period, and on the other hand, during the non-selection period, the scanning electrode maintains the state given in the third period within the selection period for any period of time, and at least the next selection period is maintained. Before the period, the impedance is returned to the impedance in the first period of the selection period, and a voltage pulse whose absolute value is zero or less than the threshold value of the element is applied to the signal electrode, and the voltage pulse tends to be at least It is characterized in that the voltage pulses are not continuously applied in the same polarity direction with a length longer than the pulse width of the first and second voltage pulses.

[Operation] According to the above configuration of the present invention, the potential of the scanning electrode in the non-selection period is Vk, and the potential of the signal electrode is Vk (k=1 to N:
N is the number of signal electrodes, and ■ is ±V assuming that Ov is the average potential.
(Vs is the peak value of the signal electrode signal), Re is the resistance inserted after the selection period, and R2 is the internal resistance of the pixel, ignoring the transient state and approximating the steady state to the following equation. holds true.

FIG. 3 shows a conceptual diagram of the drive waveform generation circuit of the present invention. Seiko Epson (
+1SED1610F, signal electrode driver (308)
5E01600F was used.

The switch (310) is a non-selection potential changeover switch in the driver, and in reality, it is only necessary to connect the non-selection drive potentials V1 and 4 of the driver to the average potential of the scanning electrode signal via the resistor Re.

Hereinafter, the details of the present invention will be shown by examples.

The voltage applied to the pixel during the non-selection period is Vc-Vk.
) Since VC is within ±Vs (when Re is sufficiently large), the voltage applied to the pixel is within ±2Vs, and is almost Ov when displaying normal display contents. Furthermore, when considering a transient state in which a signal is applied to a pixel during a non-selection period, the liquid crystal becomes even more difficult to respond. Therefore, during the non-selection period, the pixels are in an extremely stable memory state, resulting in high contrast and high transmittance.

[Example] (Example 1) FIG. 1 shows a driving voltage waveform according to the present invention.

(101) is the scanning electrode waveform, (102) is the signal electrode waveform, (103) is the voltage waveform actually applied to the liquid crystal layer, and (104) is the optical response of the liquid crystal to the voltage waveform (103). FIG. 2 shows a schematic cross-sectional view of a liquid crystal element. 21 is a glass substrate, 22 is a scanning electrode, 23 is a signal electrode, 24 is an insulating layer, 25 is an alignment film, 26 is a liquid crystal layer, 27
is a spacer, and 28 is a polarizing plate. Also, the number of pixels is 4
00x600, and the pixel size is 0. 3xQ,
3i+m2.

A rubbing process was performed using CS-1015 manufactured by Chisso Corporation as a liquid crystal material and using polyimide as an alignment film.

Then, Rc=1, Ω, Pw=100 μsec,
VI=1.2.5v, V2=-17.5v, V3=5
．． Ov, V4=V6=2.5v, V5=V1=-2
, 5v so that the DC component becomes 0. The voltage pulse having a peak value of v3 is a correction pulse for reducing the DC component to 0, and is applied immediately before the scanning electrode is selected. When driven under these conditions, a contrast ratio of 1:16 and a transmittance of 30% were obtained. however,
The transmittance is based on the amount of light when two polarizing plates are stacked so that their vibration directions are parallel. As a comparative example, when driven according to a conventional driving method, the contrast ratio was 1:8 and the transmittance was 15%.

(Example 2) Fel 1x001 manufactured by Hoechst was used as a liquid crystal material, polyimide was used as an alignment film, and rubbing treatment was performed. According to the drive voltage waveform shown in Fig. 1, Rc=5
0Ω, VI=17. Ov, V2=-23, Ov
, V3=6. Ov, V4=V6=3. Ov
, V5=VT=-3, Ov, Pw=200 μsec, contrast ratio of 1:16 and 30%.
The transmittance was obtained. As a comparative example, when driven according to a conventional driving method, the contrast ratio was 1 near and the transmittance was 18%.

(Example 3) Using DOFOOO4 manufactured by RODIC as a liquid crystal material,
A rubbing treatment was performed using aminosilane as an alignment film. Further, a 15 Hz, ±30 V square wave was applied to the ferroelectric phase to perform energization. Then, according to the drive voltage waveform shown in Fig. 1, Rc=2MQ, V1=
9. OV, V2=-23, Ov, V3=1
4. Ov, V4=V6=3. Ov.

V5=V? = -3, Ov, and Pw = 200 μsec, a contrast ratio of 1:35 and a transmittance of 80% were obtained. As a comparative example, when driven according to a conventional driving method, the contrast ratio was 1:20 and the transmittance was 70%.

(Example 4) This example has the same configuration as (Example 3), but V3
= Ov. That is, the DC component is not Ov. When driven under the above conditions, a contrast ratio of 1:32 and a transmittance of 78% were obtained.

As a comparative example, when driven according to the conventional driving method,
The contrast ratio was 1:18 and the transmittance was 67%.

(Example 5) This example has the same configuration as (Example 3), but after producing the liquid crystal element, an accelerated test of three-month reliability was performed, and then driving was performed again. However, the bistability did not change at all compared to when it was created. Driving conditions (Example 3)
It is similar to As a result, a contrast ratio of 1:35 and a transmittance of 80% were obtained. However, when driven according to the conventional driving method, the contrast ratio was 1:1. The reason for this is thought to be as follows. Since the spontaneous polarization of liquid crystal molecules was held in one fixed direction for a long period of time, impurity ions moved due to the polarization field, causing asymmetry in the electro-optical characteristics (threshold characteristics) of the liquid crystal element. As a result, the signal voltage applied during non-selection causes the liquid crystal molecules to be attracted to one of the two stable orientation directions of the liquid crystal molecules, causing an apparent loss of bistability and a contrast ratio of 1=1. Oops. In contrast, according to the driving method of the present invention, the voltage actually applied to the liquid crystal layer when not selected is lower than the voltage applied from the outside, and its influence is relatively small, so that bistability is maintained. A sufficient contrast ratio can be obtained.

Although the embodiments have been described above, the present invention is effective not only for the above embodiments but also for other liquid crystal materials and alignment methods, and the external resistance value may be 50Ω or more, preferably It is 50 to 2 MΩ. Although the effects of the present invention can be obtained even if a resistor of 2 MΩ or more is used, it is not easy to manufacture.
0 to 2 MΩ is preferable. The present invention can be applied to displays, light valves, optical switches, spatial light modulators, etc.

[Effects of the Invention] As described above, according to the present invention, the impedance between the scanning electrode and the voltage pulse supplying device can be changed at any time within the third period within the selection period. The impedance during the selection period inner tube 1 is maintained at a sufficiently high level compared to the impedance of the selected period, and the impedance is maintained for an arbitrary period during the non-selection period, and at least before the next selection period, the impedance during the period of the selection period inner tube 1 is maintained. As a result, the voltage actually applied to the liquid crystal layer when it is not selected is lower than the voltage applied externally, and the decrease in contrast ratio and light transmittance due to that voltage is reduced, resulting in better optical performance than before. characteristics can be obtained. In addition, asymmetry occurs in the electro-optical characteristics due to some reason, and although it is bistable when no voltage is applied, when a signal voltage pulse is applied for multiplex drive, the bistability is lost in the conventional method, resulting in contrast. Since multiplex driving is possible even when the ratio is 1:1, it becomes less susceptible to variations in characteristics when manufacturing liquid crystal elements and changes in characteristics due to changes over time. Furthermore, even if the signal voltage is increased, the voltage actually applied to the liquid crystal layer does not become that high, so even if there is a distribution of liquid crystal layer thickness or temperature, it is possible to drive the entire surface uniformly, and the drive margin is reduced. It has the effect of being wider than before.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a driving voltage waveform according to the present invention,
FIG. 2 is a schematic cross-sectional view of the cell used in the example, FIG. 3 is a circuit diagram for generating the drive voltage waveform of the present invention, and FIGS. 4 and 5 are conventional drive voltage waveforms. FIG. Glass substrate scanning 1! Polar signal electrode insulating layer alignment layer 26 ・ ・ 27 ・ ・ 28 ・ ・ Liquid crystal layer spacer Polarizing plate Clock input part Drive voltage input part Data input part Clock input part Latch signal input part Drive voltage input part Running i-electrode driver Signal electrode driver resistance R0 Switch Signal Electrode Scanning Electrode Optical Response Above Applicant Seiko Epson Co., Ltd. Agent Patent Attorney Kizobe Tsuzuki Suzuki (and 1 other person) 101.401,501... Scanning electrode waveform 102.
402,502... Signal electrode waveform 103.403,
503... Composite waveform Figure 3 Section 3I/2 Figure 2 Figure 4

Claims (3)

[Claims] (1) In a method for driving a liquid crystal element in which a ferroelectric liquid crystal is sandwiched between substrates in which the electrode surfaces of a substrate having a scanning electrode and a substrate having a signal electrode face each other, at least liquid crystal molecules are In order to align the arrangement direction of to the first stable state,
A first period in which a first voltage pulse whose absolute value is greater than or equal to the saturation value of the element is applied, and then a polarity opposite to that of the first voltage pulse is applied to change the alignment direction of liquid crystal molecules from the first stable state to a second one. A second period of applying a second voltage pulse whose absolute value is below the threshold value or above the saturation value of the element, and the absolute value of the applied voltage pulse, in order to select whether or not to invert the arrangement to a stable state. A third period in which the value is zero is provided, and at any point within the third period, the scanning electrode is in a state where a resistor is interposed between it and the device that supplies the voltage pulse, that is, the first and The impedance is in a sufficiently high state compared to the second period, and on the other hand, during the non-selection period, the scanning electrode maintains the state given in the third period within the selection period for any period of time, and at least the next selection period is maintained. Before the period, the impedance is returned to the impedance in the first period of the selection period, and a voltage pulse whose absolute value is zero or less than the threshold of the element is applied to the signal electrode, and the voltage pulse is at least the same as that of the first period in the selection period. A method for driving a liquid crystal element, characterized in that voltage pulses are not continuously applied in the polarity direction with a length longer than the pulse width of the first and second voltage pulses. (2) In any period within the non-selection period, apply a voltage pulse whose absolute value is equal to the difference between the absolute values of the first voltage pulse and the second voltage pulse, and apply the DC component applied during the selection period to 2. The method of driving a liquid crystal element according to claim 1, wherein the voltage is applied with canceling polarities. (3) During the third period within the selection period and the non-selection period, the electrical resistance value of the resistor interposed between the scanning electrode and the voltage pulse supply device is 10% of the electrical resistance value of the liquid crystal element.
2. The method of driving a liquid crystal element according to claim 1, wherein the driving ratio is 1/5 or more.