JPH11352465A - Driving method for liquid crystal device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stably display a clear visible image on a liquid crystal display device utilizing chiral nematic liquid crystal by preventing the occurrence of display failure due to individual pixels which form a visible image displaying region. SOLUTION: A driving method for a liquid crystal device comprises a driving method of the device utilizing chiral nematic liquid crystal having two metastable states in which a reset voltage Vr is applied to generate Freedericksz transition and subsequently a selecting voltage Vs is applied to generate either of the two metastable states. When a reset period during which the reset voltage Vr is applied is expressed as Tr, a scanning period of a screen of the device as Tf, a relaxation period of liquid crystal, varying its state by applying a voltage, to vary from an initial state to a saturated state as Tk and a delay period as Td, these are specified to satisfy inequalities 800 μsec<=Tr<=Tf-(Tk+ Td).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for driving a liquid crystal device using a chiral nematic liquid crystal having two metastable states.

[0002]

2. Description of the Related Art TN (Twisted Nematic) liquid crystal, STN
A nematic liquid crystal known as a (Super Twisted Nematic) liquid crystal has a property that the alignment state of the liquid crystal immediately returns to the original state when the applied voltage is released. On the other hand, a chiral nematic liquid crystal having two metastable states is known to have a so-called self-holding property (or memory property) in which the state is maintained for a short time even when the applied voltage is released. Have been.

A TN liquid crystal or the like is a TFT (Thin Film Transistor).
or) driven by an active matrix method such as a liquid crystal display, whereas a chiral nematic liquid crystal with two meta-stable states has a self-holding property as described above, and realizes high-quality display by a simple matrix method. It is expected to be possible.

As a driving method for such a chiral nematic liquid crystal, Japanese Patent Publication No. 1-51818 discloses the following two methods. One is to obtain a 360 ° twist alignment by rapidly turning off a voltage applied to the liquid crystal of 60 Hz, 15 V (Peak to Peak) using a toggle switch. This is a driving method in which a voltage applied to the liquid crystal is slowly dropped using a variable voltage source for about one second to obtain a uniform orientation state of 0 °.

Another driving method is as follows. That is, first, after the low-frequency electric field is turned off, 1
When a high frequency of 500 KHz is immediately applied to the liquid crystal, 3
A 60 ° twist orientation is achieved. Then, when a high-frequency electric field of 1500 KHz is applied after a delay of about 1/4 second following the turn-off of the same low-frequency electric field, a uniform orientation state of 0 ° is obtained.

However, the above-mentioned conventional driving method is of a mere laboratory level and is not practical. In view of this, as a highly practical driving method, the present applicant has proposed the following driving method in Japanese Patent Application Laid-Open No. 7-175041. That is, in the driving method,
By applying a reset voltage of a predetermined magnitude to the chiral nematic liquid crystal for a reset period of a predetermined length, a Freedericksz transition is caused in the liquid crystal, and thereafter,
After a delay time of a predetermined length has elapsed, one of two kinds of selection voltages is selectively applied to the liquid crystal to cause one of two metastable states to occur in the liquid crystal.

According to this driving method, by providing a delay time of a predetermined length before applying the selection voltage, the selection period, that is, the writing period can be shortened, and a highly practical driving method can be realized.

[0008]

However, according to an experiment conducted by the present inventor, even if the conventional driving method disclosed in Japanese Patent Application Laid-Open No. 7-175041 is used, a visible image to be displayed may be occasionally displayed. Is blurred, or
It has been found that problems such as difficulty in seeing the displayed visible image due to low brightness occur.

The present inventor has conducted various experiments to solve such problems, and found that the problems depend on the quality of individual pixels forming the visible image display area of the liquid crystal device. Was found. It has been found that such a defective pixel largely depends on the length of the period for applying the reset voltage, that is, the reset period.

The present invention has been made on the basis of the findings, and an object of the present invention is to prevent the occurrence of display defects depending on individual pixels forming a visible image display area, thereby achieving chiral nematics. An object of the present invention is to enable a clear visible image to be stably displayed on a liquid crystal device using liquid crystal.

[0011]

(1) In order to achieve the above object, a method for driving a liquid crystal device according to the present invention comprises the steps of (a)
The liquid crystal molecules sealed between the substrates have a predetermined twist angle in an initial state, and as a relaxed state after applying a reset voltage that causes a Freedericks transition to the initial state,
A method for driving a liquid crystal device using a chiral nematic liquid crystal having two metastable states different from an initial state,
(B) applying a reset voltage for causing the Freedericksz transition to the liquid crystal during a reset period; and (c) providing a selection period after the reset period to generate one of the two metastable states. (D) providing a non-selection period after the selection period, and applying a non-selection voltage for maintaining the two metastable states to the liquid crystal during the non-selection period. (E) the reset period is Tr, the one-screen scanning period of the liquid crystal device is Tf, and the relaxation period, which is the period during which the liquid crystal that changes state by voltage application changes from the initial state to the saturated state, is defined as When Tk, 800 μsec ≦ Tr ≦ Tf−Tk.

Considering an ideal bistable chiral nematic liquid crystal, if a certain selection voltage is applied to the liquid crystal,
Liquid crystal molecules are always stably set in one of two metastable states, so that individual pixels are "bright"
Alternatively, either “dark” should be displayed. However, if the reset period Tr is too short, a clear black-and-white visible image with high contrast cannot be obtained even if a constant selection voltage is applied to the liquid crystal after the reset period Tr.

[0013] This phenomenon is that if the reset period Tr is too short, sufficient Freedericksz transition does not occur in the chiral nematic liquid crystal, so that the liquid crystal molecules cannot be set in an ideal homeotropic state. Even if a predetermined selection voltage is applied to the pixel, the selected state becomes unclear, and as a result, the individual pixels do not switch all at once, so that the dark area and the light area are included in each pixel. This is considered to be due to the fact that

On the other hand, if the reset period Tr is set to a sufficient length such that 800 μsec ≦ Tr as in the present invention, the switching characteristic is excellent in the visible image display region of the liquid crystal device and the contrast is high. A clear image can be obtained. It is considered that this is because by setting the reset period Tr to a sufficient length, sufficient Freedericksz transition can be generated in the chiral nematic liquid crystal.

By the way, a voltage as shown in FIG. 6A is applied to the liquid crystal in each pixel forming the visible image display area of the liquid crystal device. In this figure, Tr is a reset period, Tf is one screen scanning period of the liquid crystal device, and Ts
Indicates a selection period. Since a delay period Td may be provided between the reset period Tr and the selection period Ts, the delay period Td is shown in parentheses in the figure.

FIG. 6B shows how the luminance of the pixel changes when the scanning electrode is selected and the signal electrode is selected at the same time. In this graph, the time region indicated by the symbol A is a region where the liquid crystal molecules in the pixel are in a saturated state, and the luminance becomes the maximum value when the liquid crystal molecules are in this state. The period from the rising of the selection period Ts to the saturation of the liquid crystal state is the relaxation period Tk.

In the driving method of the present invention in which the reset period Tr is provided, as the reset period Tr is set longer, a sufficient Freedericks transition is obtained, and a stable switching characteristic between two metastable states is obtained. can get. However, if the reset period Tr becomes too long, the brightness of the image displayed in the visible image display area decreases, and the image quality deteriorates.

In this regard, in the present invention, the delay period T
Considering the case where d is not provided, it is regulated that the reset period Tr becomes longer than necessary, such as Tr ≦ Tf−Tk, so that the saturation region (that is, the effective lighting period) A is made sufficiently long. Therefore, it is possible to prevent the luminance of the displayed visible image from being lowered.

That is, according to the driving method of the liquid crystal device according to the present invention, in the liquid crystal device using the chiral nematic liquid crystal, the reset period Tr is regulated to an appropriate range, so that the liquid crystal can be switched between two metastable states. Switching operation can be stably maintained, and the display brightness can be maintained at a sufficient level.

(2) Regarding the driving method of the above configuration, a delay period Td is provided between the reset period Tr and the selection period Ts, and a delay voltage equal to or lower than the non-selection voltage is applied to the delay period Td. Can be applied to the liquid crystal. In that case, it is desirable to set the delay period Td as follows: 800 μsec ≦ Tr ≦ Tf− (Tk + Td).

If the delay period Td is provided between the reset period Tr and the selection period Ts as in this embodiment, the selection period, that is, the writing period can be shortened, and therefore the number of scanning electrodes in one screen scanning period can be reduced. It is possible to increase the resolution of the displayed image. In addition, according to the present embodiment, the switching operation of the liquid crystal can be stably maintained by regulating the reset period Tr to an appropriate range,
In addition, since the display brightness can be maintained at a sufficient level, an extremely clear display image can be obtained.

(3) Regarding the driving method of the liquid crystal device having the above configuration, it is desirable to set the reset period Tr so that 1 msec ≦ Tr ≦ 5 msec.

[0023]

FIG. 3 shows an embodiment of a liquid crystal cell which can be driven by the driving method according to the present invention.
The liquid crystal device is manufactured by mounting auxiliary devices such as a backlight and a liquid crystal driving device to the liquid crystal cell 1. The liquid crystal cell 1 of the present embodiment has a TFT (Thin Film Tr).
This is a simple matrix type liquid crystal cell that does not use an active element such as an anistor or TFD (Thin Film Diode).

The liquid crystal cell 1 shown here has a pair of translucent substrates 5a and 5b and an ITO (Indium Tin Oxide) on them.
xide) and the like, and is configured to include the translucent electrodes 4a and 4b formed in a predetermined pattern and the alignment film 2 applied on those electrodes. The electrode 4a functions as a scanning electrode, and the electrode 4b functions as a signal electrode. The alignment film 2 is made of, for example, polyimide (for example, SP-74 manufactured by Toray Industries, Inc.).
0).

Rubbing treatment was performed on each alignment film 2 in directions different from each other by 180 °. A pair of translucent substrates 5
A spacer (not shown) was inserted between a and 5b to keep the distance between the two substrates constant. For example, the distance between the substrates, that is, the so-called cell gap was set to 2 μm.

Liquid crystal was injected into the cell gap of the liquid crystal cell 1. The liquid crystal used in the present embodiment is formed by adding an optical activator (for example, S-811 manufactured by E. Merck) to a nematic liquid crystal (for example, ZLI-3329 manufactured by E. Merck) to reduce the helical pitch of the liquid crystal. It is adjusted to 3 to 4 μm.
The pretilt angles θ1 and θ2 of the liquid crystal molecules 11 in the injected liquid crystal become several degrees, and the initial alignment becomes a 180 ° twist state. Next, a pair of polarizing plates 7a and 7b having different transmission polarization axes on the outer surfaces of the light-transmitting substrates 5a and 5b.
To form a liquid crystal cell 1. In addition,
Reference numeral 3 denotes an insulating layer, reference numeral 6 denotes a flattening layer, reference numeral 8 denotes a light shielding layer between pixels, and reference numeral 9 denotes a director vector of the liquid crystal molecules 11.

FIG. 4 shows an embodiment of a liquid crystal driving device for driving the liquid crystal cell 1. The liquid crystal drive device 10 shown here includes a scan drive circuit 13 connected to the scan electrodes (ie, row electrodes) 4a of the liquid crystal cell 1, a scan control circuit 15 for controlling the scan drive circuit 13, and a liquid crystal cell 1
, And a signal control circuit 16 for controlling the signal drive circuit 14.

The scanning drive circuit 13 and the signal drive circuit 14 are supplied with a predetermined applied voltage from a potential setting circuit 17. Further, a reference clock signal and a predetermined timing signal are supplied from the line sequential scanning circuit 18 to the scanning control circuit 15 and the signal control circuit 16. As shown in FIG. 4, if the illuminating device 19 is provided on one side of the liquid crystal cell 1, the liquid crystal molecules in the liquid crystal cell 1 are controlled by the liquid crystal driving device 10, and two metastable states are provided for each pixel. When the state changes between the two, the light that has exited the lighting device 19 and has reached the liquid crystal cell 1 passes through the liquid crystal cell 1 for each pixel or
Thus, a desired visible image is displayed on the opposite side of the lighting device 19.

Scan driving circuit 13 in liquid crystal driving device 10
Applies a drive signal having a waveform as shown in FIG. 1 or FIG. 2 to each scanning electrode 4a. These drive waveforms include 1
A reset period Tr, a delay period Td, a selection period Ts, and a non-selection period Tn are included in a frame period, that is, one screen scanning period Tf. The driving waveform of FIG.
This is a driving waveform in which the polarity of the voltage charged in the liquid crystal is inverted for each f and converted into an alternating current. Further, the drive waveform in FIG. 2 is a drive waveform in which the polarity of the voltage applied to the liquid crystal is inverted for each pulse having a pulse width of Ts / 2 to be AC.

In the reset period Tr, a reset voltage (ie, a reset pulse) Vr that is equal to or higher than a threshold value for causing Freedericksz transition in the nematic liquid crystal is applied.
In this embodiment, the reset voltage Vr has its peak value set to ± 30 V. The delay period Td is provided to delay the timing at which the selection voltage (that is, the selection pulse) Vs is applied to the liquid crystal during the selection period Ts after the reset voltage Vr is applied to the liquid crystal.

The selection voltage Vs applied to the liquid crystal during the selection period Ts is based on a critical value that produces one of two metastable states of the nematic liquid crystal, for example, a twist alignment state of 360 ° and a uniform alignment state of 0 °. Is the voltage selected as

The peak value of the selection voltage Vs is 0 to 1.9 V
In this case, a twist orientation state of 360 ° is obtained. On the other hand, when a voltage of 2 V or more is applied to the liquid crystal as the selection voltage Vs, a uniform orientation state of 0 ° is obtained. Also,
In the non-selection period Tn, a non-selection voltage Vn having an absolute value smaller than the selection voltage Vs is applied, and the state of the liquid crystal selected in the selection period Ts is maintained.

As shown in FIGS. 1 and 2, the reset voltage Vr
When the selection voltage Vs is applied to the liquid crystal after a certain delay period Td is inserted after the application of, the liquid crystal can be switched between two metastable alignment states even when the length of the selection period Ts is set short. This means that the writing time per line in the matrix type display can be significantly reduced.

In this embodiment, the reset period Tr, the one-screen scanning period Tf of the liquid crystal device, the relaxation period Tk during which the liquid crystal whose state changes by applying a voltage changes from the initial state to the saturated state, and the delay period Regarding Td, the reset period Tr is set so that 800 μsec ≦ Tr ≦ Tf− (Tk + Td).

If the reset period Tr is set so as to satisfy 800 μsec ≦ Tr, the reset voltage Vr is applied to the liquid crystal for a sufficient length of time, so that a sufficient Freedericksz transition can be generated in the chiral nematic liquid crystal. Therefore, the entire liquid crystal molecules in each pixel can be made to be in a uniform homeotropic state. Therefore, when a selection voltage Vs is subsequently applied to the liquid crystal, all of the liquid crystal molecules are uniformly set to two states. The state changes simultaneously to one of the stable alignment states. As a result, the obtained visible image display is a clear display image in which light and dark are clearly separated.

On the other hand, when the reset period Tr is Tr ≦ Tf−
(Tk + Td), the reset period Tr
Can be prevented from becoming unnecessarily long, and therefore, the length of the saturation period of the liquid crystal molecules within one screen scanning period (ie,
It is possible to prevent the effective lighting time A (see FIG. 6) from becoming too short. As a result, the brightness when each pixel is controlled to be in a bright state can be maintained at a high level, so that a visible image displayed on the liquid crystal device can be displayed brightly.

FIG. 5 shows the relationship between the result of a dynamic simulation showing the behavior of the bistable liquid crystal used in this embodiment and the delay period Td and the selection period Ts. The horizontal axis represents time, and the vertical axis represents the tilt of the liquid crystal molecules at the center of the liquid crystal cell, that is, the tilt. In this graph, the start time is the time when the reset period Tr has expired.

According to this figure, after the liquid crystal molecules stand upright (ie, the homeotropic alignment state), they fall back slightly by the backflow phenomenon, and the voltage applied when returning again is reduced. Depending on the tilt is 0
° and those that move further 180 °. The former is a transition to a uniform orientation state of 0 °, and the latter is equivalent to a transition to a twist orientation state of 360 ° because a twist is added in addition to the change in tilt.

By the way, as can be seen from FIG.
Irrespective of the transition to the uniform orientation state, or the transition to the twist orientation state of 360 °, the reset pulse Vr
Immediately after the cutoff, the behavior is the same in that the same process called back flow of the liquid crystal is performed. That is, whether the alignment state of the liquid crystal becomes 0 ° or 360 ° depends on how to give a trigger after the backflow.

According to the driving method shown in FIGS. 1 and 2,
Delay period Td between reset period Tr and selection period Ts
Is inserted, and the time length of the delay period Td is adjusted, so that the trigger voltage after back-flow of the liquid crystal should be applied to the liquid crystal at the timing when a trigger should be applied regardless of the length of the selection period Ts. Vs can be applied. Therefore, even if the time length of the selection period Ts is greatly reduced, the alignment state of the liquid crystal can be switched.

In the driving waveforms shown in FIG. 1 and FIG. 2, the delay period Td is provided between the reset period Tr and the selection period Ts.
The present invention can also be applied to a driving method in which the selection period Ts is provided immediately after the reset period Tr without providing the delay period Td. In this case, the delay period Td
It is necessary to make a change to the set value of the reset period Tr for the absence of the data. Specifically, 800 μsec ≦ Tr ≦ Tf−Tk, where Tr: reset period, Tf: one screen scanning period, Tf:
k: Set as in the relaxation period.

(Embodiment) The liquid crystal cell 1 shown in FIG.
Is applied to the scanning electrodes, the delay period Td, the selection period Ts, and the non-selection period Tn are held at a fixed length, and the reset period Tr is set to 500.
μsec, 750 μsec, 1.0 msec, 2.5 msec, 5.0
Visible images were displayed on the liquid crystal cell while being changed to seven types of msec, 7.5 msec, and 9.0 msec, and the states of the visible images corresponding to each voltage value were visually determined. As a result, the results shown in Table 1 were obtained.

[0043]

[Table 1]

In this table, the "switching state" indicates whether or not the whole of one pixel switches uniformly between the two metastable states, and "x" indicates that the switching state is non-uniform. In the above, “○” indicates that the switching state is uniform. The “display luminance” is the luminance when one pixel is in a bright state. “O” indicates that the pixel is sufficiently bright, and “x” indicates that the luminance is insufficient. Is shown.

As can be seen from the above experimental results, regarding the switching state, the reset period Tr is 1.0 msec.
A defect occurred when the time was shortened to a certain value between and 750 μsec. Further, regarding the display luminance, a defect occurred when the reset period Tr was increased to a certain value between 5.0 msec and 7.5 msec. From this, the reset period Tr for obtaining a clear and bright visible image on the display surface of the liquid crystal device is 800 μsec ≦ Tr ≦ Tf− (Tk + Td), where Tr: reset period, Tf: one screen scanning period, Tf
It is preferable that k: relaxation period, Td: delay period, and more preferably, 1 msec ≦ Tr ≦ 5 msec.

[0046]

According to the method of driving a liquid crystal device according to the present invention, in a liquid crystal device using a chiral nematic liquid crystal, the reset period Tr for causing Freedericksz transition in the liquid crystal is regulated to an appropriate range. Switching operation can be stably maintained, and the brightness of the displayed visible image can be maintained at a sufficient height. Therefore, a very clear and bright visible image can be displayed on the display surface of the liquid crystal device. .

[Brief description of the drawings]

FIG. 1 is a voltage drive waveform illustrating an embodiment of a method for driving a liquid crystal device according to the present invention.

FIG. 2 is a voltage driving waveform showing another embodiment of the driving method of the liquid crystal device according to the present invention.

FIG. 3 is a cross-sectional view of an example of a liquid crystal cell driven by a driving method of a liquid crystal device according to the present invention.

FIG. 4 is a block diagram showing one embodiment of a liquid crystal driving device for driving the liquid crystal cell of FIG.

FIG. 5 is a diagram showing a result of a dynamic simulation showing a behavior of a bistable liquid crystal used in the liquid crystal cell of FIG. 3;

6 is a diagram showing a relationship between a voltage driving waveform applied to the liquid crystal cell of FIG. 3 and a luminance of a visible image displayed on a display surface of the liquid crystal cell.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Liquid crystal cell 2 Alignment film 4a Scan electrode 4b Signal electrode 5a, 5b Translucent electrode 10 Liquid crystal drive device 11 Liquid crystal molecule 19 Illumination device Tf One screen scanning period Tr Reset period Td Delay period Ts Selection period Tn Non-selection period

Claims (3)

[Claims] 1. A liquid crystal molecule sealed between substrates has a predetermined twist angle in an initial state, and a relaxed state after application of a reset voltage for causing a Freedericks transition with respect to the initial state. Is a method of driving a liquid crystal device using a chiral nematic liquid crystal having two different metastable states, wherein a reset voltage for causing the Freedericksz transition is applied to the liquid crystal during a reset period, and selected after the reset period. A period is provided, and a selection voltage for generating one of the two metastable states is applied to the liquid crystal during the selection period. A non-selection period is provided after the selection period, and a non-selection period for maintaining the two metastable states is provided. In a method for driving a liquid crystal device, wherein a selection voltage is applied to the liquid crystal during the non-selection period, the reset period is Tr, the one-screen scanning period of the liquid crystal device is Tf, When a relaxation period, which is a period during which the liquid crystal that changes state by voltage application changes from the initial state to the saturation state, is Tk, 800 μsec ≦ Tr ≦ Tf−Tk is satisfied. Method. 2. The liquid crystal display according to claim 1, wherein a delay period is provided between the reset period and the selection period, and a delay voltage equal to or lower than the non-selection voltage is applied to the liquid crystal during the delay period. A driving method of a liquid crystal device, wherein when a delay period is Td, 800 μsec ≦ Tr ≦ Tf− (Tk + Td). 3. The method according to claim 1, wherein 1 msec.
≦ Tr ≦ 5 msec. A method for driving a liquid crystal device.