JP2512290B2 - Liquid crystal element driving method - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of driving a liquid crystal element , and more particularly to a method of driving a liquid crystal element 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 against 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 has not been clarified, and it has not been clarified what specific 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.

The object of the present invention is to eliminate the above-mentioned drawbacks, prevent the deterioration of the ferroelectric liquid crystal, and prevent the deterioration of the ferroelectric liquid crystal from the relationship between the applied voltage and the light transmission state of the ferroelectric liquid crystal found by the present inventors. Driving a liquid crystal element that can obtain a light transmission state at high speed
To provide a method .

[0011]

The feature of the present invention that achieves the above-mentioned object is that the threshold value of the light transmission characteristic of liquid crystal is
The first pulse voltage greater than the voltage and this pulse voltage
Immediately before the voltage is applied, this voltage pulse is equal to the peak value.
A pair of pulses consisting of a second pulse voltage with extremely different polarities.
By applying a scanning voltage to the first light transmitting state of the liquid crystal.
State is established and the polarity of this pair of pulse voltages is reversed.
Another pair of pulses consisting of third and fourth pulse voltages
By applying a voltage, the second light transmission state of the liquid crystal is changed.
I tried to maintain it .

[0012]

[Operation] A pair of pulse voltages having the same peak value but different polarities
One of the light transmission states is established and maintained by applying
Then, the other pair with the polarity of this pair of pulse voltages reversed
The other light transmission state by applying the pulse voltage of
Can be established and maintained. Furthermore, these paired pulse voltage
As a result, the direct current component can be reduced, and deterioration of the electrode or liquid crystal due to an electrochemical 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 000 Å Ln ₂ O ₃ , SnO ₂ , and a mixture thereof is provided, and the thickness is 100 to 10
If necessary, an alignment film 14 of organic resin, SIO _2, or the like, having a thickness of about 100 μm, is provided.
m), DOBAMBC10, which is a ferroelectric liquid crystal, is placed between 73 and 9
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. Further, the polarizing plates 131 and 132 are formed on the surfaces of the substrates 121 and 122 on which 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 spiral axis 2 when an electric field is applied from the front side of the paper surface to the direction penetrating the paper surface. 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 the case where an electric field of -E is applied, and at this time, the light (natural light) emitted from the front side of the paper surface 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 molecule 1, and passes through the 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 polarization plate 131, the component in the major axis direction of the ferroelectric liquid crystal molecule 1 is the refractive index n in the major axis direction, and the component in the minor axis direction is the refractive index in the minor axis direction. Liquid crystal layer 1 according to the rate n⊥
Since the light passes through 0, the light exiting the liquid crystal layer 10 becomes elliptical polarized light. Therefore, since it has a light component that passes through the lower polarizing plate 132, it looks bright on the display element.

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 to 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 the figure, it was found that the brightness B rapidly increased as the pulse voltage V _{P was} applied and the rise time t ₁ was short, but the recovery time t ₂ after the pulse voltage V _{P was} applied was long as shown. .

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 in order to eliminate display flicker 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 V _LC applied to the ferroelectric liquid crystal in a state where the incident light is transmitted, that is, when the display element displays a bright image, 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 V _LC and the brightness B in the case where the incident light is cut off, that is, when the display element displays a dark image. In FIG. 9, the peak value −V _P (5V to 20V) at time t ₀ ,
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. Thus, when two or more pulses are continuously applied within the predetermined period, the light transmission state is defined by the last pulse having a peak value higher than the threshold voltage Vc.

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.

Further, in this embodiment, immediately before the pulse voltage V _P that determines the light transmission state is applied, the pulse voltage −V having the same absolute value of the pulse width and the peak value and the reverse polarity is used.
_{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 concrete circuit for realizing the driving waveforms shown in FIGS. 9 and 10.

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 outputting the pulse voltage, and C is a signal that determines the phase of the output voltage E. By controlling C, the light transmission state can be determined.

[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 examples.

[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 make the DC component of the voltage applied to the ferroelectric liquid crystal zero. _P1 is made smaller than the threshold voltage V _{C and the} pulse width is widened accordingly. At this time, in order to make the direct current component zero as shown in Expression 1, the positive and negative pulse direct current components S ₁ and S ₂ 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 the ferroelectric liquid crystal is present. No deterioration occurs, 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, so that it is higher than that in the first embodiment. Then, the contrast ratio becomes large.

FIGS. 15 and 16 are drive waveforms showing a third embodiment of the present invention. FIG. 15 shows a case where a bright display is made and FIG. 16 shows a case where a dark display is made.

Also in FIGS. 15 and 16, the time integration 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 integration (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 show drive waveforms according to the fourth embodiment of the present invention. When bright display is shown in FIG. FIG. 18 shows a case where dark display is performed.

Also in FIGS. 17 and 18, the time integration 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 integration (S ₂ + S _{3) of} the second voltage signal.
As for the relationship with + S ₄ + S ₅ + S ₆ ), the polarities are opposite to each other and the absolute values are equal, as shown in Formula 3.

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 the 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 Formula 1, the polarities are opposite to each other and the absolute values are equal.

Also in this embodiment, the same effect as that of the above-mentioned embodiment can be obtained, and the period t _D in which the pulse voltage for determining the light transmission state is applied is the pulse for making the DC component zero. Since the period t _C during which the voltage is applied is sufficiently long, the contrast ratio becomes large. 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 spiral axis 2 of the ferroelectric liquid crystal molecule, in which case bright display and dark display are reversed in the first to fifth embodiments.

Further, in the first to fifth embodiments,
Immediately before and immediately after the pulse voltage that determines the light transmission state of the liquid crystal element, a voltage signal that makes the direct current component zero is applied, but the invention is not limited to this, and the pulse voltage that determines the light transmission state is applied. As long as it is inside, it is always good.

In the embodiment of the present invention, the static drive has been described as an example. However, the present invention can be applied to dynamic drive such as line sequential scanning and dot sequential scanning.
Furthermore, the present invention is applicable not only to DOBAMBC but also to other ferroelectric liquid crystals shown in Table 1, for example.

[0053]

As described above, according to the present invention, it is possible to obtain a liquid crystal display device which can prevent the deterioration of the ferroelectric liquid crystal and can obtain a desired light transmission state at high speed.

[Brief description of 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.

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

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;

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

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 drive waveforms in the 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.

Front page continuation (72) Inventor Fumio Nakano 4026 Kuji Town, Hitachi City, Ibaraki Prefecture Hitachi Research Laboratory (56) References JP-A-49-92948 (JP, A) JP-A-56-107216 (JP) , A)

Claims (6)

(57) [Claims] 1. A method of forcing between a pair of substrates having electrodes on opposite surfaces.
In a method of driving a liquid crystal element sandwiching an electronic liquid crystal, the first liquid crystal display device having a first voltage larger than a threshold voltage of the light transmission characteristic of the liquid crystal is used.
The pulse voltage and the voltage applied immediately before this pulse voltage
The second pulse whose peak value is the same as that of the voltage pulse of
By applying a pair of pulse voltages consisting of
The first and second light transmitting states of the liquid crystal are established and maintained, and the polarities of the pair of pulse voltages are inverted.
In applying a pair of pulse voltage consisting of pulse voltage
To establish and maintain the second light transmission state of the liquid crystal.
Characteristic liquid crystal element driving method. 2. The first and third parts according to claim 1, wherein
The loose voltage determines the first and second light transmission states of the liquid crystal.
Therefore, the second and fourth pulse voltages are applied to the liquid crystal.
Liquid crystal element characterized by reducing the DC component of the applied voltage
How to drive the child. 3. The first light transmitting device according to claim 1 or 2.
The overstate is a state in which the incident light on the liquid crystal is transmitted,
In the second light transmission state, the light incident on the liquid crystal is blocked.
A method for driving a liquid crystal element, characterized in that the liquid crystal element is in a ready state. 4. The first light transmitting device according to claim 1 or 2.
The over state is a state in which the incident light on the liquid crystal is blocked,
In the second light transmission state, the incident light on the liquid crystal is transmitted.
A method for driving a liquid crystal element, characterized in that the liquid crystal element is in a ready state. 5. The liquid according to claim 1, 2, 3 or 4.
The crystal elements are driven by static drive.
The driving method of the liquid crystal element. 6. The liquid according to claim 1, 2, 3 or 4.
The crystal elements are driven by dynamic drive.
The driving method of the liquid crystal element.