JPH01267619A - Method of driving liquid crystal display element - Google Patents

Abstract

PURPOSE:To display medium contrast by impressing an AC square wave voltage having the amplitude to exceed the threshold voltage necessary for transfer of a ferroelectric liquid crystal to one or the other of bistable states between electrode substrates to change the ratio between the time width on a positive side and the time width on a negative side. CONSTITUTION:The square wave voltage having the amplitude to exceed the threshold voltage Vth necessary for transfer from one to the other of the bistable states is impressed between the electrode substrates to change the ratio between the time width on the positive side and the time width on the negative side of the AC square wave voltage. The positive and negative pulse voltages having the absolute crest exceeding the threshold voltage Vth necessary for transfer from one to the other of the bistable states are alternately impressed between the substrates to change the ratio between the time width from the rise of the positive pulse of the pulse voltages till the fall of the next negative side pulse and the time width from the fall of the negative side pulse till the rise of the next positive side pulse. Switching is thereby executed at a high speed between the bright TH and dark TL2 states of the ferroelectric liquid crystal element and the medium contrast is displayed by the liquid crystal element from such time ratio.

Description

[Detailed Description of the Invention] [Objective of the Invention] (Industrial Application Field) The present invention relates to a novel method for driving a liquid crystal display element, and in particular to a method for driving a liquid crystal display element that has excellent high-speed response and is capable of displaying halftones. Regarding drive method.

(Prior Art) Liquid crystal display elements have been widely used in calculators, watches, etc. as the mainstay of small flat panel displays.

Recently, they have also come to be used in automobile displays, personal computer displays, and even moving picture displays such as small liquid crystal televisions.

By the way, twisted nematic liquid crystals have been commonly used in calculators, watches, etc., but display methods using this liquid crystal have problems such as insufficient response speed and crosstalk. Therefore, it has been difficult to apply liquid crystal display elements using twisted nematic liquid crystals to applications that require a fast response speed, such as large-screen displays for moving images.

For this reason, display systems using ferroelectric liquid crystals are attracting attention as an alternative to display systems using twisted nematic liquid crystals.

This ferroelectric liquid crystal has a liquid crystal form that takes a chiral smectic C phase, and this method was developed in 1975 with R, B,
Published by Mayer et al.

Display devices using ferroelectric liquid crystals have the advantage of faster response speed and memory effect compared to systems using twisted nematic liquid crystals.

Focusing on the characteristics of such ferroelectric liquid crystal elements, we have developed a dot matrix display type liquid crystal using ferroelectric liquid crystal.
There was a problem that only 0N-OFF type binary display was possible, and halftone display was not possible.

For this reason, various attempts have been made to display halftones using ferroelectric liquid crystal elements, but none of them have shown sufficient performance. For example, by creating fine irregularities on the cell surface, a distribution is created in the cell gap within the cell, thereby giving a distribution to the bistable transition threshold voltage Vth, and displaying an intermediate tone in the 0N-OFF binary transition. However, with this method, it is difficult to mechanically process the surface when creating the cell, and the unevenness of the inner surface of the cell appears as bright and dark, resulting in an uneven display surface when displaying halftones. It had a major drawback.

(Problems to be Solved by the Invention) The present invention has been made to solve these conventional problems, and provides a liquid crystal display element that enables halftone display using a ferroelectric liquid crystal that takes a chiral smectic C phase. The purpose is to provide a driving method for

[Structure of the Invention] (Means for Solving the Problems) An object of the present invention is to provide a method for driving a liquid crystal display element in which a gap between electrode substrates disposed opposite to each other is filled with ferroelectric liquid crystal. , ■ Apply an AC rectangular wave voltage with an amplitude exceeding the threshold voltage required to transition from one side of the bistable state to the other, and calculate the difference between the positive side time width and the negative side time width of this AC rectangular wave voltage. Change the ratio or ■ alternately apply positive and negative pulse voltages with absolute value peaks exceeding the threshold voltage necessary to transition from one side of the bistable state to the other, and This is achieved by changing the ratio of the time width from the rise of the negative pulse to the fall of the next negative pulse and the time width from the fall of the negative pulse to the rise of the next positive pulse.

In other words, the present invention aims to switch a ferroelectric liquid crystal element between two states of bright (high transmittance) and dark (low transmittance) at high speed, and display an intermediate tone on the liquid crystal display element depending on the time ratio. It is.

In the method (2), the vibration of brightness and darkness of the display has a high enough frequency that it does not change visually, and the time on the positive side or negative side is long enough to transition from the writing state to the decoloring state within one cycle. Width (τ-1μsec~lom
sec) is used.

In addition, in the method (■), the repetition period (T -10m5
ec ~LOOmsec) and a pulse width (τ-
A pulse voltage having a duration of 1 μsec to 10 m5 sec) is used.

Note that the positive and negative polarities of the square wave voltage or pulse voltage do not mean the brightness or darkness of the liquid crystal display element, and the brightness or darkness of the liquid crystal display element is determined by the relative angle formed within the plane of the liquid crystal display element and the polarizing plate. However, for convenience of explanation, the positive and negative sides of the rectangular wave voltage or pulse voltage will be explained below as corresponding to the brightness and darkness of the liquid crystal display element.

(Function) In the method (2) using an AC rectangular wave, halftones are displayed in the following manner.

In other words, in FIG. 1, the amplitude exceeds the threshold voltage vth necessary for transitioning from one side of the bistable state to the other, the frequency is high enough that the oscillation of brightness and darkness of the display does not change visually, and the period (T ) between the electrode substrates of the ferroelectric liquid crystal element. When applied to , the element becomes bright on the positive side and dark on the negative side.

Since this vibration of brightness and darkness occurs at a high frequency that does not change when visually observed, the degree of brightness and darkness of the display element is such that a halftone of TH/T is displayed with respect to a completely bright state.

This method is as follows: (a) The cycle (T) of the AC rectangular wave voltage is kept constant and the positive/negative duty ratio (R-TH/TL) is varied.

(b) The period (T), that is, the frequency, is changed while keeping the time width (T)l) on the positive side of the AC rectangular wave voltage constant.

This is realized by methods such as

Furthermore, in the method (3) of changing the pulse interval, halftones are displayed in the following manner.

That is, in FIG. 2, the repetition period (T) has an absolute value wave height exceeding the threshold voltage vth necessary for transitioning from one side of the bistable state to the other, and has a high repetition period (T) at which the oscillation of brightness and darkness of the display does not change visually. and a positive and negative pulse voltage having a time width (τ) on the positive or negative side sufficient to transition from the writing state to the decoloring state within one cycle is alternately applied between the electrode substrates of the ferroelectric liquid crystal element. to be applied.

In this way, the liquid crystal display element is in a bright state from the rise of the positive pulse of the pulse voltage to the fall of the next negative pulse (To), and from the fall of the negative pulse to the fall of the next positive pulse. Until the rise (TL), it is in a dark state.

Since this vibration of brightness and darkness occurs at a high frequency that does not change when visually observed, the degree of brightness and darkness of the display element is such that an intermediate tone of (TH/'T) is displayed with respect to a completely bright state. .

This method is as follows: (a) The repetition period (T) is kept constant, and the period (TH) from the rise of the positive pulse of the pulse voltage to the fall of the next negative pulse is varied.

(b) While keeping the repetition period (T) constant, the pulse width (τ) is varied.

(c) Repetition period of either positive or negative pulse (
T) is varied by an integer multiple of the other.

This is realized by a method etc.

In method (2), if the voltage value within one period is integrated to become zero, no DC voltage will be applied to the element in any display state, and therefore there will be no concern about DC voltage being applied to the liquid crystal display element. There are no problems such as electrolysis or seizure.

Note that each of the above methods can be applied not only to a complete memory type ferroelectric liquid crystal but also to a liquid crystal display element using an incomplete memory type ferroelectric liquid crystal.

(Example) Hereinafter, the present invention will be explained in detail by giving specific examples.

Example 1 A liquid crystal display element was constructed by attaching polarizing plates to the top and bottom of a ferroelectric liquid crystal cell having a threshold voltage vth of 3V.

When a DC voltage was applied to this cell, the voltage-transmittance characteristic exhibited hysteresis as shown in FIG. This liquid crystal display element has an amplitude of ±3.5V as shown in FIG.
A rectangular wave voltage with a pulse width of 100 μsec and a frequency of I K t (z) was applied. One period of this rectangular wave (7−
1m5ec), time TH showing +3.5V and -3
, 5 ■, that is, the duty ratio (TH/TL) X10Q (%) from 0 to 100.
It was changed continuously until. The transmittance of the device varied in the range of 5 to 95 to 6 as the duty ratio changed, as shown in FIG.

In addition, in the following cases, depending on the change in duty ratio, 5 to 9
Changes in the transmittance of the element were observed within a range of 5%.

(2) A rectangular wave voltage having an amplitude of ±3.5■ and a frequency of 1 kHz is applied to a ferroelectric liquid crystal cell with a threshold voltage vth of 3■.

■ ■ Ferroelectric liquid crystal cell with ±3.5■ amplitude, 0.1k
A square wave voltage with a frequency of Hz is applied.

■ ±4 for a ferroelectric liquid crystal cell with a threshold voltage vth of 4V
．． A square wave voltage with an amplitude of 5V and a frequency of 1kHz is applied.

(2) A rectangular wave voltage having an amplitude of ±3 V and a frequency of 1 kHz is applied to the ferroelectric liquid crystal cell (2).

Example 2 The threshold voltage required to invert the display state of the element is ±
As a pulse signal voltage, a pulse signal having a period of 10w5ecs, a wave height of 12v1, and a pulse width of 100μSee shown in FIG. 6 was applied to a 10V ferroelectric liquid crystal display element. here,
When the time TH between the rising and falling edges of the positive and negative pulses was changed without changing the pulse period, the transmittance of the cell changed as shown in FIG. 7, showing an intermediate tone.

In addition, for a ferroelectric liquid crystal display element whose threshold voltage is ±6V, a pulse signal voltage with a period of 5isec and a wave height of ±6V is applied.
Even when a signal of 6.5 V and a pulse width of 50 μm was applied, the transmittance of the device varied in the range of 5 to 95% in accordance with the change in the time TH between the rising and falling edges of the positive and negative pulses.

Embodiment 3 As shown in FIG. 8, a large number of gate lines 11 to 1° and signal lines 21 to 2Il are crossed to form a matrix, and a voltage controlled pulse stretcher (PS) is installed at the intersection of this matrix.
A pixel electrode 4 was connected through the substrate 3 to form an active matrix substrate 5 for a television. Next, this active matrix substrate 5 and a counter electrode substrate 8 having a counter electrode 6 and a color filter 7 are made to face each other with a small distance therebetween.
Both were fixed and the gap between them was filled with ferroelectric liquid crystal 9 to produce a liquid crystal display element for a color television.

In this electrode substrate, a gate pulse is supplied from each gate electrode to each gate line 1 at a predetermined frame period, the signal voltage of each signal line 2 is sampled, and a predetermined signal is applied to each voltage control pulse stretcher 3 at each intersection of the matrix. is converted into an AC square wave voltage. Note that the gate pulse has a repetition period (T - LOmsec - LOO
txsec).

In this embodiment, as shown in FIG. 10(a), when the signal voltage Vo is TH/T with respect to the reference voltage, as shown in FIG. 10(b), the necessary threshold voltage is v
A positive pulse with an absolute value wave height exceeding th and a pulse width (TH) proportional to TH/T, and the remaining pulse width (TL) obtained by subtracting the positive pulse from the time width of the gate pulse.
A negative side /l/ bus with 6 (TH=f (V
o) Vo).

As a result, the transmittance of the liquid crystal display element becomes as shown in FIG. 3(c), and since the light and dark vibrations are repeated at a frequency that does not change visually, an intermediate color proportional to TH/T is displayed.

Example 4 The voltage-controlled pulse stretcher 3 in Example 3 was replaced with a voltage-controlled pulse duty ratio adjustable oscillator (PDC).
A liquid crystal display element for a color television was constructed using the same active matrix substrate as in Example 3, except that the same active matrix substrate as in Example 3 was used, and halftones were displayed.

In this embodiment, as shown in FIG.
is converted into a pulse train with a duty ratio (R) proportional to the signal voltage (Vo) within the range of the gate pulse (
R-TH/TL VO).

This pulse train also has an absolute value wave height that exceeds the threshold voltage vth necessary for transitioning from one side of the bistable state to the other, and has a high repetition period (T) such that the oscillation of brightness and darkness of the display does not change visually. And the time width (τ) on the positive or negative side is sufficient to make a transition from the writing state to the decoloring state within one cycle.

As a result, the transmittance of the liquid crystal display element becomes as shown in the same figure (C), and since this vibration of brightness and darkness has a repeating cycle that does not change visually, an intermediate color proportional to the duty ratio (R) is displayed. Ru.

Example 5 A liquid crystal display element for a color television was constructed using the same electrode substrate as in Example 3, except that the voltage-controlled pulse stretcher 3 in Example 3 was replaced with a voltage-controlled oscillator (VCO), and halftones were displayed. did.

In this embodiment, as shown in FIG. 12, the signal voltage Vo
is converted into a pulse train with a frequency (F) proportional to the signal voltage VD within the range of the gate pulse (F-f (
VO) Vo).

This pulse train also has an absolute value wave height that exceeds the threshold voltage vth necessary for transitioning from one side of the bistable state to the other, and has a high repetition period (T) such that the oscillation of brightness and darkness of the display does not change visually. In addition, it has a time width (τ) on the positive side or negative side that is sufficient for transitioning from the writing state to the decoloring state within one cycle.

As a result, the transmittance of the liquid crystal display element becomes as shown in the same figure (c), and since this bright and dark oscillation has a repeating cycle that does not change visually, the transmittance becomes approximately proportional to the frequency (F). A neutral color is displayed.

[Effects of the Invention] As explained above, according to the present invention, the ferroelectric liquid crystal element is switched between the two bright (high transmittance) and dark (low transmittance) states at high speed, and the liquid crystal display is changed depending on the time ratio. Since the device displays halftones, it is possible to provide a liquid crystal display device with a wide viewing angle and quick response, which has great practical advantages.

[Brief explanation of the drawing]

1 and 2 are diagrams showing the principle of the present invention, FIG. 3 is a graph showing a transmittance-voltage hysteresis curve of a ferroelectric liquid crystal display element, and FIG. 4 is an AC rectangle used in the first embodiment. FIG. 5 is a graph showing the effect of the first embodiment. FIG. 6 is a diagram showing the pulse train used in the second embodiment. FIG. 7 is a graph showing the effect of the second embodiment. A graph showing the effect, FIG. 8 is a diagram showing the structure of the active matrix substrate used in the third example, FIG. 9 is a cross-sectional view of the color television liquid crystal display element used in the third example, and FIG. Figures 1 through 12 are graphs for explaining the effects of the third through fifth embodiments. 1......Gate line 2...Signal line 3...Voltage control pulse stretcher 4
...... Pixel electrode 5 ...... Electrode substrate 6 ...... Counter electrode

Claims (3)

[Claims] (1) In a method for driving a liquid crystal display element in which a gap between electrode substrates arranged oppositely is filled with ferroelectric liquid crystal, the ferroelectric liquid crystal is transitioned from one bistable state to the other between the electrode substrates. Displaying intermediate tones by applying an AC rectangular wave voltage with an amplitude exceeding the threshold voltage required for the AC rectangular wave voltage and changing the ratio of the positive side time width to the negative side time width of this AC rectangular wave voltage. A method for driving a liquid crystal display element characterized by: (2) In a method of driving a liquid crystal display element in which a gap between electrode substrates arranged oppositely is filled with ferroelectric liquid crystal, the ferroelectric liquid crystal is transitioned from one bistable state to the other between the electrode substrates. Positive and negative pulse voltages having an absolute value wave height exceeding the threshold voltage required for the pulse voltage are applied alternately, and the time width from the rise of the positive pulse of this pulse voltage to the fall of the next negative pulse and the negative pulse 1. A method for driving a liquid crystal display element, characterized in that halftones are displayed by changing the ratio of the time width from the falling edge of a pulse to the rising edge of the next positive pulse. (3) The method for driving a liquid crystal display element according to claim 1 or 2, wherein the liquid crystal display element is a liquid crystal display element for a television.