JPS63306425A - Liquid crystal device - Google Patents

Abstract

PURPOSE:To obtain an excellent display which generates no panel crosstalk for a long period by superposing an alternating voltage which is applied to the intersection part of a scan electrode and a signal electrode upon a DC voltage and inverting the polarity of the DC voltage based upon a voltage applied to a scan unselected electrode at each specific period. CONSTITUTION:A liquid crystal element has ferroelectric liquid crystal between a group of scan electrodes 102 and a group of signal electrodes 103, and a voltage applying means applies the scan select signal to a scan electrode 102 and an information signal to a signal electrode 103 in synchronism with the scan select signal. This device has the voltage applying means superposes the alternating voltage applied to the intersection part of the scan electrode 102 and signal electrode 103 on the DC voltage and a means which inverts the polarity of the DC voltage based upon the voltage applied to scan unselected electrodes at each specific period. Consequently, no pulse crosstalk is caused, bistability is maintained for a long period, and driving characteristics are obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of the Invention] The present invention relates to liquid crystal devices such as display panels and shutter array printers using ferroelectric liquid crystals.

[Prior art]

2. Description of the Related Art Liquid crystal display elements have been well known in which a scanning electrode group and a signal electrode group are configured in a matrix, and a liquid crystal compound is filled between the electrodes to form a large number of pixels to display images or information. The driving method for this display element is time-division driving, in which an address signal is selectively and periodically applied to a group of scanning electrodes, and a predetermined information signal is selectively applied in parallel to a group of signal electrodes in synchronization with the address signal. It has been adopted.

Most of these were put to practical use, for example, in “Applied Physics Letters” (Applied Physics Letters).
ysics Letters”) 1971, 18 (
4) M, Shut (M, 5) published in issue 127-128
chadt and W, Helfrich.
) co-authored ``0 Voltage Dependent Optical Activity - of a Twisted Nematic Liquid Crystal'' (Voltage
Dependent Optical Activ
ity of aT, twisted Ne
TN (Twisted Nemati) shown in
c) type liquid crystal.

In recent years, the use of bistable liquid crystal elements as an improved version of conventional liquid crystal elements has been proposed by both C1ark and Lagerwall in Japanese Patent Application Laid-open No. 107216/1982 and U.S. Pat. No. 4,367,924.
It is proposed in the specification etc. As a bistable liquid crystal,
Generally, ferroelectric liquid crystals having a chiral smectic C phase (SmC") or H phase (SmH") are used, in which states a first optically stable state and a second optically stable state occur in response to an applied electric field. It has the property of being in one of the optically stable states and maintaining that state when no electric field is applied, that is, it has bistability, and also has a quick response to changes in the electric field, and is a high-speed and memory-type display. It is expected to be widely used in the field of equipment, etc.

Furthermore, many driving methods for such bistable ferroelectric liquid crystal elements have been proposed. For example, methods for driving ferroelectric liquid crystal elements are disclosed in US Pat. No. 4,548,476 (Kaneko) and US Pat. No. 4,655,561 (Kannabe et al.). Among these, a driving method has already been proposed in which a constant DC voltage is superimposed on a driving pulse in order to cancel out internal bias caused by molecular orientation in the liquid crystal layer.

In experiments conducted by the present inventors, it has been found that depending on the drive waveform, it is effective to provide a constant DC component to the drive pulse as described above. However, at least when the device is in operation, continuing to apply a DC component not only causes deterioration of the liquid crystal material, but also impairs bistability, making it difficult to control the driving conditions to obtain good images over a long period of time. It was extremely difficult.

[Summary of the invention]

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a liquid crystal device that eliminates the above-mentioned problems, particularly a ferroelectric liquid crystal device that has drive characteristics that maintain bistability over a long period of time.

In other words, the present invention includes a liquid crystal element in which a ferroelectric liquid crystal is arranged between a scanning electrode group and a signal electrode group, and a scanning selection signal is applied to the scanning electrode, and the signal electrode is synchronized with the scanning selection signal. In the liquid crystal device, the voltage applying means superimposes a DC voltage on the alternating voltage applied to the intersection of the scanning electrode and the signal electrode, and applies the voltage to the scanning non-selected electrode. The liquid crystal device is characterized by having means for inverting the polarity of the DC voltage based on the voltage at predetermined intervals.

〔Example〕

FIG. 1 is a configuration diagram showing an example of a display device of the present invention.

101 is a display panel, which includes scanning electrodes 102 and signal electrodes 10.
3 and a ferroelectric liquid crystal filled between them, and at the intersection of a matrix consisting of a scanning electrode 102 and a signal electrode 103, the ferroelectric liquid crystal is aligned by an electric field caused by a voltage applied to the electrode. is controlled.

104 is a signal electrode drive circuit, a video data shift register 1041 that stores serial video data from the information signal line 106, and a line memory 104 that stores parallel video data from the video data shift register 1041.
2. According to the video data stored in the line memory 1042, a signal electrode driver 1043 applies a voltage to the signal electrode 103, and a DC offset voltage VDC to be superimposed on the AC voltage having voltages v4 and -v4 applied to the signal electrode 103. It has a VOC polarity switching circuit 1044 that switches the polarity according to a signal from the switching control line 108.

Further, this AC voltage is supplied from the information side power supply 1045.

105 is a scan electrode drive circuit, which connects the scan address data line 1
a decoder 1051 for receiving a signal from 07 and instructing one scanning electrode among all scanning electrodes;
A scan electrode driver 1052 receives a signal from 51 and applies a voltage to the scan electrode 102, and further voltages V, -V2 and 0 are applied to the scan electrode 102 from the scan side power supply 1.
Output from 053.

A CPU 109 receives clock pulses from an oscillator 110 and controls the image memory 111 and the transfer of signals to the information signal line 106, the scanning address data line 107, and the switching control line 108.

According to experiments conducted by the present inventors, so-called "panel crosstalk" is likely to occur, especially when a driving method with a short selection time for one scanning line is used, and as a means to reduce this crosstalk, certain Adding a DC component to the AC drive pulse was effective.

Hereinafter, using the drive waveform shown in Fig. 2 as an example, the above-mentioned "
We will discuss in more detail the effects of "panel crosstalk" and superimposing DC components.

Sl+S2+Ss+... shown in FIG. 2 represent the voltage application state on the time axis applied to the first scanning line, second scanning line, third scanning line,..., respectively, and II and I2 are Each represents the state of voltage application on the time axis applied to signal lines 1 and I2. At this time, signal line 1
Line 1 contains an information signal for day-to-day, and signal line 2 contains a black-to-black information signal. During the erase step, a voltage of 211°2 with a pulse width Δt is applied to the scanning line.
12.213. ... is applied uniformly, and at this time, a voltage of 221°222 with a pulse width Δt is uniformly applied to the signal line, so that at each intersection there is a voltage exceeding the threshold voltage of one of the ferroelectric liquid crystals. Voltage (1) 8 is uniformly applied, and the entire screen is erased to white (or black). During the subsequent write step, voltages 231, 232, 233, . ... is applied, and a white (or black) signal (C)V is applied to the signal line in synchronization with this scanning selection signal. −■
■. AC voltage) and black (or white) signal (ΦV0-θ■.

AC voltage) is selectively applied, and at the intersection where the black signal is applied, a voltage vw exceeding the other threshold voltage of the ferroelectric liquid crystal is applied, producing a black (or white) display, and a white signal is applied. At the applied intersection, a voltage (based on the pulse width Δt) that does not exceed the threshold voltage of the ferroelectric liquid crystal VH is applied, and the white (or black) display state during the erasing step is maintained as it is.

At this time, if the pulse width of the minimum unit pulse among the pulses constituting the drive pulse (scanning selection signal and information signal) is Δt, in this example, the selection time for one scanning line is 2Δt, excluding the erasing process. be.

Now, as shown in the example (It 52) in FIG. 2, pay attention to the second scanning line. Depending on the image information, the pixels on this scanning line may have a low voltage when half selected, but the erase pulse A pulse with a long time width (in this case, pulse width 3Δt) appears in the opposite direction to 8. In this way, when half-selected, n of Δt
nΔ
This will be called t-crosstalk. Of course, due to the switching threshold characteristics of the ferroelectric liquid crystal, which is determined by the pulse width and pulse peak value, the drive pulse parameters (frequency, peak value) are set so that switching occurs with the write pulse Vw and inversion switching does not occur with nΔt crosstalk. must have been done. In other words, the write pulse v
There must be a driving condition, that is, a driving margin, in which switching is caused by w and not caused by nΔt crosstalk. However, in large-area ferroelectric liquid crystal cells, it is necessary to make the cell thickness or liquid crystal molecule orientation uniform throughout the cell. Difficult to control. As a result, it is currently difficult to make the above-mentioned drive margin the same over the entire cell. Such intra-cell variation in drive margin is caused by the short selection period of one scanning line and the drive method which has a narrow operating margin to begin with, as described above.
Image distortion becomes noticeable. In this way, the drive waveform n
Due to the non-uniformity of the liquid crystal cell, Δt crosstalk cannot be controlled to prevent inversion switching as a whole cell.
The drive margin may not be secured in some areas, resulting in image disturbances (for example, pixels displaying information different from the information, or pixels displaying an intermediate color due to a polarization domain occurring within one pixel). In a broad sense, this is called "panel crosstalk."

Next, a constant DC component is applied to the e side of the drive waveform shown in FIG. The liquid crystal cell itself has been subjected to symmetrical alignment treatment for both the upper and lower substrates, and is bistable at least in its initial state.By superimposing a DC component on the pixel to which an AC drive pulse is applied, the pulse crosstalk described above can be avoided. This reduces the amount of noise considerably, allowing a good image to be displayed across the entire screen. Although the detailed cause of the effect due to such a DC component is unknown, it is believed that the nΔt crosstalk generated by the above-mentioned drive waveform is reduced, and as a result, the drive margin is expanded and pulse crosstalk is reduced.

However, experiments conducted by the present inventors have revealed that it is extremely difficult to maintain the effect of the DC component described above over a long period of time, and that pulse crosstalk occurs again over time. The details of the reason why the effect of the DC component decreases over time are unknown, but because an alignment film or an insulating film is placed inside the cell, the DC component applied to the liquid crystal layer disappears over time. However, in any case, it is not preferable to continue applying a DC component of the same polarity.

However, according to experiments conducted by the present inventors, the above-mentioned situation can be solved by changing the polarity of the DC component at a predetermined period, for example, at a frame period (
or field period) or one scan selection period, it was considerably eliminated.

FIG. 3 shows an example of driving used in the present invention.

V shown in FIG. 3(A) is a scanning selection signal, and ■.

is a scanning non-selection signal, Iw is an information non-selection signal and corresponds to a "white" signal, and In is an information selection signal and corresponds to a "black" signal. The information signal has a voltage peak value of 1±Vs l and 1±V
4 Alternating voltage with 1, roaring, peak value 1±Vs
Between I and 1±va l, 1±V31<l±va
It is set to I. FIG. 3(B) shows the signal line I,
An example is shown in which a white-white-one-white signal is applied and a black-black-one-black signal is applied to the signal line I2. In the driving example shown in FIG. 3, the polarity of the DC component applied to the pixel is reversed every frame period, and a good display can be obtained for a long time despite the presence of 3Δt crosstalk.

The amount of DC offset voltage VDC used in the present invention is ±0.5% to ±10% with respect to the maximum voltage amplitude applied to the pixel.
0%, preferably about ±1.0% to ±5.0%.

The driving example shown in FIG. 4 is a modification of the driving example shown in FIG.
An example is shown in which the polarity of the scan selection signal at the same phase is inverted in synchronization with the inversion of the polarity of the DC offset voltage vDc every frame period. At this time, the polarity of the voltage applied to the scanning electrode and the signal electrode during the erasing step is also reversed every frame period.

Using the driving example shown in FIG. 4, Δt=40μsec,
l±Vt1=l±V, 1=18 volts.

When driven under the conditions of 1±V, l-8 volts, and 1±V4 l=9 volts, good display without pulse crosstalk was obtained for a long time.

5 and 6 show another driving example of the present invention. According to the driving examples shown in FIGS. 5 and 6, the polarity of the DC offset voltage VDC is inverted every frame period and every scan selection period.

As the liquid crystal having bistability that can be used in the present invention, chiral smectic liquid crystal having ferroelectricity is preferable, and chiral smectic C phase (SmC"
) or H-phase (Sm)f”) liquid crystals are suitable.For this ferroelectric liquid crystal,
Physique Letter 1' (La Journal d
aPhysic 1etter'') Volume 36 (L-69
), 1975's "Ferroelectric Liquid
Crystals J (rFerr.

electric Liquid Crystal
“S”); “Applied Physics Letters (“A
pplied Physics Letters'')3
a@ (tt issue), 1980 submicron second bistiple electro-optic switching in liquid crystal J (rsubm
icr.

5econd B15table Electro
optic Switching 1nLiqui
d Crystals'): "Solid State Physics ts (14
1) 1981 r liquid crystal, and the ferroelectric liquid crystal disclosed in these documents can be used in the present invention.

More specifically, an example of the ferroelectric liquid crystal compound used in the present invention is decyloxybenzylidene-P'-amino-2-methylbutylcinnamate (DOBAMBC).
, hexyloxybenzylidene-P'-amino-2-chlorobrovir cinnamate (HOBACPC) and 4-o-(2-methyl)-butylresollylidene-4'
-octylaniline (MBRA8) and the like.

When constructing an element using these materials, the element is supported by a copper block with a heater embedded, etc., as necessary, in order to maintain the temperature state such that the liquid crystal compound becomes the SmC@ phase or the SmH'' phase. be able to.

Furthermore, in the present invention, in addition to the above-mentioned SmC'' and SmH'', chiral smectic F phase, ■ phase, and J phase are used.

It is also possible to use a ferroelectric liquid crystal that appears in a G phase.

FIG. 7 schematically depicts an example of a ferroelectric liquid crystal cell. 71a and 71b, In2O,.

SnO, ITO (indium tin oxide)
It is a substrate (glass plate) coated with transparent electrodes such as
In between, a liquid crystal of SmC" phase, in which a liquid crystal molecule layer 72 is oriented perpendicular to the glass surface, is sealed. A thick line 73 represents a liquid crystal molecule, and this liquid crystal molecule 73
has a dipole moment (P) 74 in the direction perpendicular to its molecule. When a voltage equal to or higher than a certain threshold is applied between the electrodes on the substrates 71a and 71b, the helical structure of the liquid crystal molecules 73 is unraveled, and the alignment direction of the liquid crystal molecules 73 is changed so that all dipole moments (P) 74 are oriented in the direction of the electric field. can be changed. The liquid crystal molecules 73 have an elongated shape and exhibit refractive index anisotropy in the long axis direction and short axis direction,
Therefore, it is easy to understand that, for example, if polarizers are placed above and below a glass surface in a cross-niffle positional relationship with each other, a liquid crystal optical modulation element whose optical characteristics change depending on the polarity of applied voltage can be obtained. Furthermore, when the thickness of the liquid crystal cell is made sufficiently thin (for example, 1μ), the helical structure of the liquid crystal molecules is unraveled even when no electric field is applied, as shown in Figure 8, and its dipole moment Pa or pb is Upward (84a
) or downward (84b). When such a cell is subjected to an electric field Ea or Eb with a different polarity above a certain threshold value for a predetermined time as shown in FIG. 8, the dipole moment will be directed upward 84a or downward 84b with respect to the electric field vector of the electric field Ea or Eb. The liquid crystal molecules change direction, and accordingly the liquid crystal molecules are either in the first stable state 83a or in the second stable state 83a.
is oriented in one of the stable states 83b.

There are two advantages to using such a ferroelectric liquid crystal as an optical modulation element. Firstly, the response speed is extremely fast, and secondly, the alignment of liquid crystal molecules has a bistable state. The second point will be explained with reference to FIG. 8, for example. When the electric field Ea is applied, the liquid crystal molecules are aligned in a first stable state 83a, and this state remains stable even when the electric field is turned off.

Moreover, when an electric field Eb in the opposite direction is applied, the liquid crystal molecules are oriented to the second stable state 83b and the orientation of the molecules is changed.
It remains in this state even if the electric field is turned off. Further, as long as the applied electric field Ea does not exceed a certain threshold value, each orientation state is maintained. In order to effectively realize such fast response speed and bistability, it is preferable for the cell to be as thin as possible, and generally the thickness is 0.5μ to 2μ.
0μ, especially 1μ to 5μ is passing through.

〔Effect of the invention〕

According to the present invention, when a ferroelectric liquid crystal element is driven by multiplexing, a good display without panel crosstalk can be obtained for a long time.

[Brief explanation of drawings]

FIG. 1 is a block diagram of a liquid crystal device of the present invention. Second
The figure is a waveform diagram showing a driving example other than the present invention. Figure 3 (
A) is a waveform diagram of a driving example used in the present invention, and FIG.
) is a waveform diagram on the time axis. Figures 4 and 5
6 and 6 are waveform diagrams showing another driving example used in the present invention. 7 and 8 are perspective views of the ferroelectric liquid crystal element used in the present invention. (H=t, Otsu3 ・=) CM=
/, z, y...Vs '-〇- IkoS shi'l
Vl octopus + 1l-I - ~ target c/) Li Fig. 6

Claims (16)

[Claims] (1) A liquid crystal element in which a ferroelectric liquid crystal is arranged between a scanning electrode group and a signal electrode group, a scanning selection signal is applied to the scanning electrode, and an information signal is applied to the signal electrode in synchronization with the scanning selection signal. In the liquid crystal device, the voltage applying means superimposes a DC voltage on the alternating voltage applied to the intersection of the scanning electrode and the signal electrode, and the voltage applied to the scanning non-selected electrode is used as a reference. A liquid crystal device having means for reversing the polarity of the DC voltage at predetermined intervals. (2) the information signal has one polarity voltage and the other polarity voltage based on the voltage applied to the scan non-selected electrode;
2. The liquid crystal device according to claim 1, wherein the voltage peak values of the one polarity voltage and the other polarity voltage are different from each other. (3) The liquid crystal device according to claim 1, wherein the polarity inversion period of the DC voltage is synchronized with a frame or field period. (4) The liquid crystal device according to claim 1, wherein the polarity inversion period of the DC voltage is synchronized with one scan selection period. (5) The liquid crystal device according to claim 1, wherein the scan selection signal has one polarity voltage or the other polarity voltage based on the voltage applied to the scan non-selection electrode. (6) The scan selection signal has one polarity voltage or the other polarity voltage based on the voltage applied to the scan non-selection voltage, and the voltage polarity in the same phase of the scan selection signal is reversed every predetermined period. A liquid crystal device according to claim 1. (7) The liquid crystal device according to claim 6, wherein the inversion cycle of the voltage polarity in the same phase of the scanning selection signal is synchronized with the frame or field cycle. (8) The liquid crystal device according to claim 6, wherein the inversion period of voltage polarity in the same phase of the scan selection signal is synchronized with one scan selection period. (9) The scan selection signal has one polarity voltage or the other polarity voltage based on the voltage applied to the scan non-selection electrode, and the voltage polarity in the same phase of the scan selection signal is changed every scan selection period. , and is inverted every one frame or one field cycle. (10) The liquid crystal device according to claim 1, wherein the scan selection signal has one polarity voltage and the other polarity voltage based on the voltage applied to the scan non-selection electrode. (11) The scan selection signal has one polarity voltage and the other polarity voltage based on the voltage applied to the scan non-selection voltage, and the voltage polarity in the same phase of the scan selection signal is reversed every predetermined period. A liquid crystal device according to claim 1. (12) The liquid crystal device according to claim 11, wherein the inversion cycle of the voltage polarity in the same phase of the scanning selection signal is synchronized with the frame or field cycle. (13) The liquid crystal device according to claim 11, wherein the inversion cycle of voltage polarity in the same phase of the scan selection signal is synchronized with one scan selection cycle. (14) The scan selection signal has one polarity voltage and the other polarity voltage based on the voltage applied to the scan non-selection electrode, and the voltage polarity in the same phase of the scan selection signal is 1.
2. The liquid crystal device according to claim 1, wherein the liquid crystal device is inverted every scanning selection period and every one frame or one field period. (15) The liquid crystal device according to claim 1, wherein the ferroelectric liquid crystal is a chiral smectic liquid crystal. (16) The liquid crystal device according to claim 15, wherein the thickness of the chiral smectic liquid crystal is set to be sufficiently thin to eliminate the helical structure inherent in the chiral smectic liquid crystal in the absence of an electric field. .