JP2601244B2 - Driving method of liquid crystal element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal display, and more particularly to a liquid crystal display using a ferroelectric liquid crystal.

[0002]

2. Description of the Related Art As ferroelectric liquid crystals, for example, liquid crystals exhibiting a chiral smectic C phase (Sm * C) and a chiral smectic H phase (Sm * H) as shown in Table 1 are known.

[0003]

[Table 1]

FIG. 1 shows the state of these strongly inductive liquid crystal molecules with respect to an applied electric field.

As shown in FIG. 1B, when the electric field E is not applied, the ferroelectric liquid crystal molecule 1 has an angle of θ (for example, 20 to 25 degrees in DOBAMBC) with respect to the axis 2. Helical orientation.

When an electric field E equal to or higher than the threshold electric field Ec is applied to the ferroelectric liquid crystal molecules 1 thus oriented,
As shown in (a), the ferroelectric liquid crystal molecules 1 are oriented on a plane perpendicular to the direction of the electric field E at an angle of θ with respect to the helical axis 2. When the polarity of the electric field E in FIG. 1A is reversed, as shown in FIG. 1C, the ferroelectric liquid crystal molecules 1 are aligned with the helical axis 2 on a plane perpendicular to the direction of the electric field E. It is oriented with an angle of θ.

This phenomenon is characterized by a very high speed, and it is known that if a sufficiently large electric field is applied, it responds to a voltage pulse having a pulse width on the order of μs.
It is expected to be applied to large displays, optical shutters, polarizers, etc. in which the number of pixels increases.

[0008]

However, conventionally,
The relationship between the applied voltage and the light transmission state was not clarified, and it was not clarified what voltage should be applied to drive the ferroelectric liquid crystal.

In order to maintain the brightness of the ferroelectric liquid crystal element, a DC voltage is applied. If the DC voltage is continuously applied, ions in the liquid crystal are attracted to the electrode, thereby shortening the life of the electrode or the liquid crystal itself. There was a problem.

[0010] An object of the present invention is to eliminate the above-mentioned disadvantages and to prevent the deterioration of the ferroelectric liquid crystal from occurring, based on the relationship between the applied voltage and the light transmission state of the ferroelectric liquid crystal which the present inventors have found. It is an object of the present invention to provide a method for driving a liquid crystal element capable of reliably obtaining a light transmitting state.

[0011]

A feature of the present invention that achieves the above object is that a plurality of pulse voltages whose peak value is larger than the threshold voltage of the light transmission characteristics of the liquid crystal and whose polarity alternates are different. Wherein a first light transmission state of the liquid crystal is established and maintained by applying a first pulse voltage train including a plurality of pulse voltages in which the polarity of the first pulse voltage is different from the polarity of the last pulse voltage; The second light transmission state of the liquid crystal is established and maintained by applying a second pulse voltage train composed of a plurality of pulse voltages obtained by inverting the polarities of the plurality of pulse voltages.

[0012]

[Function] Ferroelectric liquid crystal depends on the polarity of the applied voltage.
Is determined. Therefore, as in the present invention, the peak value
When multiple pulse voltage trains with the same
In this case, the light transmission state is maintained by the polarity of the last pulse voltage.
Be held. In addition, a plurality of pulse power
By applying a pressure train, it is confirmed that the light transmission state of the liquid crystal is established.
You can do it. Further, the plurality of pulse voltage trains
The polarity of the first pulse voltage is the same as the polarity of the last pulse voltage.
Are different, i.e., starting with a pulse voltage of one polarity.
Is terminated by a pulse voltage of the other polarity.
The average value of the applied voltage is zero and the electrochemical
Deterioration due to the reaction can be prevented.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is based on the following experimental facts that the present inventors have found experimentally.

As shown in FIG. 2, a pair of substrates 121 and 122 made of glass, plastic, etc.
A display electrode 11 made of Ln ₂ O ₃ , SnO ₂ , a mixture thereof, or the like having a thickness of 2,000 ° is provided.
If necessary, an alignment film 14 of organic resin, SIO _2, or the like, having a thickness of about 100 μm, is provided.
m) Between 73 and 9 DOBAMBC10, a ferroelectric liquid crystal,
Hold at 3 ° C. Incidentally, a sealing agent for enclosing DOBAMBC10 in 15 is used. At this time, the alignment film 14 is oriented so that the helical axis 2 of the ferroelectric liquid crystal molecules is substantially parallel to the substrates 121 and 122. Furthermore, polarizing plates 131 and 132 are provided on the surfaces of the substrates 121 and 122 where the display electrodes 11 are not provided.
Are adjacent.

At this time, as shown in FIG. 3, the polarization axis direction 31 of the polarizing plate 131 and the polarization axis direction 32 of the polarizing plate 132 are made substantially orthogonal, and the polarization axis direction of one of the polarization axes is changed to the ferroelectric liquid crystal. The orientation direction of the ferroelectric liquid crystal molecules 1 when an electric field of 10 or more threshold electric field | Ec | In FIG. 3, the polarization axis direction 31 of the polarizing plate 131 is made to coincide with the direction of the helical axis 2 when an electric field is applied from the near side of the paper to the direction penetrating the paper. Hereinafter, the electric field in this direction is represented by -E with a negative sign, and the liquid crystal element having the structure shown in FIG. 2 will be described as an example. However, the present invention is not limited to this. For example, in FIG.
A reflecting plate is arranged adjacent to the substrate 122 instead of the polarizing plate 132,
The present invention can be applied to a case where a ferroelectric liquid crystal 10 mixed with a dichroic dye is used. In this case, the angle θ of the ferroelectric liquid crystal molecules with respect to the helical axis 2 is optimally 45 degrees.

FIG. 3A shows a case where an electric field of -E is applied. At this time, light (natural light) radiated from the near side of the drawing is polarized in the polarization axis direction 31 by the upper polarizing plate 131, and is intense. It becomes linearly polarized light having a vibration component only in the major axis direction of the dielectric liquid crystal molecules 1 and passes through the ferroelectric liquid crystal layer 10 as linearly polarized light according to the refractive index n in the major axis direction.

Thereafter, the light enters the lower polarizing plate 132,
Since the polarization axis direction 32 of the polarizing plate 132 is perpendicular to the polarization axis direction 31 of the polarizing plate 131, light is blocked and the display element looks dark.

FIG. 3B shows a case where + E is applied. At this time, the major axis of the ferroelectric liquid crystal molecule 1 is set to be the same as the polarization axes 31 and 32 of the upper and lower polarizers 131 and 132. They are not in the same direction. In this case, of the light linearly polarized by the upper polarizing plate 131, the component in the major axis direction of the ferroelectric liquid crystal molecule 1 is a refractive index n in the major axis direction, and the component in the minor axis direction is refraction in the minor axis direction. Since the light passes through the ferroelectric liquid crystal layer 10 according to the rate n⊥, the light exiting the ferroelectric liquid crystal layer 10 becomes elliptically polarized light. Accordingly, the display element has a light component that passes through the lower polarizing plate 132, so that the display element looks bright.

In this way, switching between light and dark can be performed by applying + E and -E, and can function as a display element, an optical shutter, and a polarizing element. When no electric field is applied, the brightness is almost intermediate between the two. This phenomenon will be referred to herein as the electro-optic effect of the ferroelectric liquid crystal.

As a result of closely examining the electro-optic effect,
It has become clear that it has characteristics as shown in FIG.

That is, when the applied voltage V _LC applied to the ferroelectric liquid crystal is increased from zero, the brightness B increases, and when the applied voltage V _LC exceeds the threshold voltage + V _C , the brightness B becomes a constant value.
Increasing the applied voltage V _LC in the negative direction in the same manner, the brightness B is reduced, saturation exceeds the threshold voltage -V _C.

Next, in order to investigate the correspondence with the pulse voltage V _P , a positive voltage pulse V _P having a peak value larger than the threshold voltage V _C as shown in FIG. 5 was applied to the ferroelectric liquid crystal. as shown in FIG rapidly increased brightness B accordance pulse voltage V _P applied, although the rise time t ₁ is short, recovery time t ₂ after the pulse voltage V _P applied was found to be long as illustrated .

For example, the present inventors have determined that a pulse voltage V _P (pulse width t ₀ ) having a peak value larger than a threshold voltage (5 to 10 V).
= 500μs) was applied to the ferroelectric liquid crystal, t ₁
= 120 μs and t ₂ = 8 ms.

[0024] As shown in respond 6 for negative pulse voltage -V _P, slow response at voltages removal than the response due to the pulse voltage application, recovery time is found longer.

When a pulse voltage train as shown in FIG. 7 is applied, a large difference in average brightness is caused by a positive pulse voltage train as shown in FIG. 7 and a negative pulse voltage train as shown in FIG. Is generated, and it is possible to set a binary light transmission state of light and dark.

In order to obtain a good display by such a method, the repetition period of the pulse voltage applied to the ferroelectric liquid crystal for eliminating flicker of the display must be at least 30 ms or less.

However, in such a driving method,
As long as the display unit does not equal the bright display time and the dark display time, a DC component exists in the voltage _VLC applied to the ferroelectric liquid crystal.

In an extreme example, a positive DC component is always applied to a segment that is always in a bright display state, and a negative DC component is always applied to a segment that is always in a dark display state.

It is well known that, when a DC component is applied during driving of a liquid crystal device, the deterioration of the device is accelerated due to an electrochemical reaction and the life is shortened. The method shown in FIGS. It has significant drawbacks in terms of:

[0030] FIGS. 9 and 10 is a driving waveform illustrating a first embodiment of the present invention, the pulse voltage V _P immediately before as shown in FIGS. 7 and 8, have opposite polarities, the same pulse width, the peak value applying a pulse voltage -V _P.

FIG. 9 shows the voltage _VLC applied to the ferroelectric liquid crystal when the incident light is transmitted, that is, when the display element performs bright display, and the light transmission state (brightness B) of the liquid crystal element shown in FIG. FIG. 10 is a diagram showing the relationship between the applied voltage _VLC and the brightness B when the incident light is blocked, that is, when the display element performs dark display. In FIG. 9, at time t ₀ , a peak value −V _P (5 V to 20 V),
When a negative pulse voltage of the pulse width T ₁ (500μs~100μs) is applied, but once darker, the peak value V _P at time t _0, a positive pulse voltage of the pulse width T ₁ is applied,
Suddenly bright, when the applied voltage at the time t ₂ is zero,
Brightness gradually decreases. By repeating this operation at a predetermined period T (1 ms to 30 ms) such that flicker does not occur, the average brightness can be sufficiently increased. When two or more pulses are successively applied within a predetermined period, the light transmission state is defined by the last pulse having a peak value larger than the threshold voltage V _C.

At this time, the pulse voltage V _P that determines the light transmission state is applied to the ferroelectric liquid crystal because a pulse voltage having the opposite polarity and the same absolute value is applied to the ferroelectric liquid crystal within a predetermined period T. The average value of the applied voltage is zero, and there is no DC component at all, so that the ferroelectric liquid crystal does not deteriorate due to the above-mentioned electrochemical reaction.

Furthermore, in this embodiment, immediately before applying a pulse voltage V _P which defines the light transmission state, the pulse width and the peak value of the absolute value of equal and opposite polarity pulse voltage -V
_{Since P} is applied, a state in which incident light is blocked can be obtained only by inverting the polarity of the pulse voltage as shown in FIG.

FIG. 11 shows an example of a specific circuit for realizing the driving waveforms as shown in FIGS.

In FIG. 11, 81 is an exclusive OR gate, 82 is an inverter, 83 and 84 are AND gates,
₁ , Q ₂ , Q ₃ , Q ₄ are switching transistors, R
1, R2 and R3 are resistors, A, B and C are input terminals, E is an output terminal, and LC is a liquid crystal element connected to the output terminal.

The timing of each signal in the circuit of FIG. 11 is as shown in Table 2, and the respective signal waveforms are shown in FIG.

A is a signal that determines the pulse width, B is a signal that determines the timing of generating the pulse voltage, C is a signal that determines the phase of the output voltage E, and the light transmission state can be determined by controlling C.

[0038] In the embodiment of FIGS. 9 and 10, the pulse voltage V _P which defines the light transmission state is a pulse voltage and the absolute value opposite polarities are equal and applied to the liquid crystal, the absolute value must always equal Absent. That is, as long as the DC component is reduced even a little, the effect of the present invention can be considerably obtained. Therefore, in the present invention, it is only necessary to apply a voltage of the opposite polarity. The same applies to the following embodiments.

[0039]

[Table 2]

FIGS. 13 and 14 show driving waveforms according to the second embodiment of the present invention. FIG. 13 shows a case where bright display is performed, and FIG. 14 shows a case where dark display is performed.

The difference from the first embodiment shown in FIGS. 9 and 10 is that the pulse height V of the reverse pulse voltage newly provided to reduce the DC component of the voltage applied to the ferroelectric liquid crystal to zero. _P1 is smaller than the threshold voltage V _C , and the pulse width is increased accordingly. At this time, in order to make the DC component zero as shown in Expression ₁ , the DC components S ₁ and S ₂ of the positive pulse and the negative pulse have opposite polarities and equal absolute values.

S ₁ = −S ₂ (Equation 1) Also in this embodiment, the average value of the voltage applied to the ferroelectric liquid crystal is zero, and there is no DC component, so that the ferroelectric liquid crystal is deteriorated. Does not occur, and a desired light transmission state can be obtained at high speed.

Further, in this embodiment, the peak value of the pulse voltage for making the DC component zero is smaller than the threshold voltage V _C of the ferroelectric liquid crystal, and therefore, compared to the first embodiment. , The contrast ratio increases.

FIGS. 15 and 16 show driving waveforms according to the third embodiment of the present invention. FIG. 15 shows a case where a bright display is performed, and FIG. 16 shows a case where a dark display is performed.

Also in FIGS. 15 and 16, the time integral S ₁ of the pulse voltage, which is the first voltage signal that determines the light transmission state of the liquid crystal element, and the time integral (S ₂ + S ₃ +) of the second voltage signal.
The relationship with S ₄ ) is that the polarities are opposite to each other and the absolute values are equal, as shown in Equation 2.

S ₁ = − (S ₂ + S ₃ + S ₄ ) (Equation 2) FIGS. 17 and 18 are driving waveforms showing the fourth embodiment of the present invention, and FIG. 17 shows a case where bright display is performed. FIG. 18 shows a case where dark display is performed.

Also in FIGS. 17 and 18, the time integral S ₁ of the pulse voltage, which is the first voltage signal that determines the light transmission state of the liquid crystal element, and the time integral (S ₂ + S _{3) of} the second voltage signal.
+ S ₄ + S ₅ + S ₆ ), as shown in Expression 3, the polarities are opposite to each other and the absolute values are equal.

S ₁ = − (S ₂ + S ₃ + S ₄ + S ₅ + S ₆ ) (Equation 3) FIGS. 19 and 20 are driving waveforms showing the fifth embodiment of the present invention. FIG. 19 shows a bright display. FIG. 20 shows a case where a dark display is made.

[0049] Also in FIGS. 19 and 20, the time integral S ₁ of the pulse voltage is a first voltage signal for determining the light transmission state of the liquid crystal element, the relationship between the time integral S ₂ of the second voltage signal, As shown in Equation 1, the polarities are opposite to each other, and the absolute values are equal.

In this embodiment, the same effect as in the above-described embodiment can be obtained. Further, the period t _D during which the pulse voltage for determining the light transmission state is applied is shorter than the pulse voltage for making the DC component zero. Since the applied period is sufficiently longer than t _C , the contrast ratio increases. The first to the present invention described above.
In the fifth embodiment, as shown in FIG.
1, the polarization axis direction 31 is made to coincide with the direction of the helical axis 2 of the ferroelectric liquid crystal molecules when the electric field -E is applied.
May be made to coincide with the direction of the helical axis 2 of the ferroelectric liquid crystal molecules when applied. In this case, in the first to fifth embodiments, bright display and dark display are reversed.

Further, in the first to fifth embodiments,
Immediately before and immediately after the application of the pulse voltage that determines the light transmission state of the liquid crystal element, a voltage signal that makes the DC component zero is applied. However, the invention is not limited to this. Any time within.

Although the embodiments of the present invention have been described by taking static driving as an example, the present invention can be applied to dynamic driving such as line-sequential scanning and point-sequential scanning.
Further, the present invention is not limited to DOBAMBC, but may be applied to other ferroelectric liquid crystals shown in Table 1, for example.

[0053]

As described above, according to the present invention, the deterioration of the ferroelectric liquid crystal is prevented and the desired light transmission state is ensured.
The liquid crystal display device can be obtained.

[Brief description of the drawings]

FIG. 1 is a diagram showing a state of a ferroelectric liquid crystal with respect to an applied electric field.

FIG. 2 is a cross-sectional view showing one embodiment of a liquid crystal element to which the present invention can be applied.

FIG. 3 is a diagram showing the relationship between the direction of the helical axis 2 of the ferroelectric liquid crystal molecules 1 in FIG. 2 and the polarization axis directions 31 and 32 of the polarizing plate.

FIG. 4 is a diagram showing an example of light transmission characteristics of a ferroelectric liquid crystal to which the present invention can be applied.

It illustrates the response of the light transmitting state to the pulse voltage V _P of the present invention; FIG.

It illustrates the response of the light transmitting state to the pulse voltage V _P of the present invention; FIG.

FIG. 7 is a diagram showing a response of a light transmission state to a pulse voltage train.

FIG. 8 is a diagram showing a response of a light transmission state to a pulse voltage train.

FIG. 9 is a diagram showing driving waveforms according to the first embodiment of the present invention.

FIG. 10 is a diagram showing driving waveforms in the first embodiment of the present invention.

FIG. 11 is a diagram showing an example of a specific circuit for realizing the driving waveforms shown in FIGS. 9 and 10;

FIG. 12 is a diagram showing timing of each signal of the circuit of FIG. 11;

FIG. 13 is a diagram showing driving waveforms in a second embodiment of the present invention.

FIG. 14 is a diagram showing driving waveforms in a second embodiment of the present invention.

FIG. 15 is a diagram showing driving waveforms according to a third embodiment of the present invention.

FIG. 16 is a diagram showing driving waveforms according to a third embodiment of the present invention.

FIG. 17 is a diagram showing driving waveforms in a fourth embodiment of the present invention.

FIG. 18 is a diagram showing driving waveforms in a fourth embodiment of the present invention.

FIG. 19 is a diagram showing driving waveforms according to a fifth embodiment of the present invention.

FIG. 20 is a diagram showing driving waveforms according to a fifth embodiment of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Ferroelectric liquid crystal molecule, 11 ... Display electrode, 121, 12
2 ... substrate.

Claims (6)

(57) [Claims] 1. A method for driving a liquid crystal element is sandwiched ferroelectric liquid crystal between a pair of substrates having electrodes on opposing surfaces comprising, consisting of four or more of the pulse voltage peak value equal polarity alternately changes A first pulse voltage train , wherein the peak value is larger than the threshold voltage of the light transmission characteristic of the liquid crystal and the polarity of the first pulse voltage is different from the polarity of the last pulse voltage; The first light transmission state of the liquid crystal is established and maintained by applying the second pulse voltage sequence having the polarity inverted from the first pulse voltage sequence, and the second light transmission state of the liquid crystal is applied by applying the second pulse voltage sequence. And a method for driving a liquid crystal element. 2. The liquid crystal device according to claim 1, wherein the first pulse voltage of the first and second pulse voltage trains is used to control the first liquid crystal.
And a second light transmission state, and a DC component of a voltage applied to the liquid crystal is reduced by first and second pulse voltage trains excluding the last pulse voltage, respectively. Method. 3. The liquid crystal display according to claim 1, wherein the first light transmitting state is a state in which incident light to the liquid crystal is transmitted.
The method of driving a liquid crystal element, wherein the second light transmission state is a state in which light incident on the liquid crystal is blocked. 4. The liquid crystal display according to claim 1, wherein the first light transmission state is a state in which light incident on the liquid crystal is blocked.
The method of driving a liquid crystal element, wherein the second light transmission state is a state in which light incident on the liquid crystal is transmitted. 5. A method according to claim 1, wherein said liquid crystal element is driven by static driving. 6. A method according to claim 1, wherein said liquid crystal element is driven by dynamic driving.