JPS61235897A - Driving of liquid crystal element - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a multiplex driving method for a liquid crystal device, particularly a liquid crystal display device using ferroelectric liquid crystal.

[Prior Art] and [Problems to be Solved by the Invention] Conventionally, as this type of display system, a twisted nematic display system using a nematic liquid crystal is known. This display method utilizes the dielectric anisotropy of nematic liquid crystal molecules, and displays by controlling the direction of liquid crystal molecules when an electric field is applied. In the case of this method, the polarity of the applied electric field does not matter, and it was possible to drive based on the effective value of the applied voltage. On the other hand, when using ferroelectric liquid crystals, which have been attracting attention in recent years, liquid crystal molecules respond due to the product of the spontaneous polarization in the phase that exhibits ferroelectricity (e.g., chiral smectic C phase) and the electric field, as shown in Equation 1 (2). In order to
The sign of the electric field is an important issue.

11-1-10000. ■ s-E However, τ: response speed, η: viscosity, PS: spontaneous polarization, E:
electric field.

In other words, the direction of the spontaneous polarization vector, that is, the direction of the director of liquid crystal molecules, is controlled by the positive or negative electric field.
When multiplex driving is performed, the average value of the voltage applied to the pixels according to write information does not become zero, but contains a large amount of DC component, causing a DC electric field to act on the liquid crystal molecules. Such effects cause electrolysis within the liquid crystal, lowering the phase transition point, and causing changes in physical constants such as fluctuations in spontaneous polarization, and are a cause of accelerated deterioration of the liquid crystal. Ta.

The present invention has been made to solve the problems in the prior art described above, and when driving a ferroelectric liquid crystal, it reduces the deterioration of the liquid crystal by reducing the DC component applied to the liquid crystal to zero on a time average basis. It is an object of the present invention to provide a method for driving a ferroelectric liquid crystal that prevents the above problems.

[Means for Solving the Problems] The present invention provides a first step of writing image information to pixels of a scanning line and a direct current component applied to the pixels in multiplex driving of a liquid crystal element using ferroelectric liquid crystal. exceeds a certain value, a second step is applied to apply a compensation pulse to cancel out this DC component, that is, a pulse voltage to make the average voltage applied to the liquid crystal zero. .

The ferroelectric liquid crystal used in the driving method of the present invention takes either a first optically stable state or a second optically stable state depending on the applied electric field, that is, it has a bistable state with respect to the electric field. A substance, in particular a liquid crystal having such properties, is used.

As the ferroelectric liquid crystal having bistability that can be used in the driving method of the present invention, a chiral smectic liquid crystal having ferroelectricity is most preferable, and chiral smectic C phase (StmC and H phase (SmH)) is most preferable. This ferroelectric liquid crystal is suitable for use as a liquid crystal.

"LE JOtlRN"("LE JOtlRN
ALDE PHYSIQUE LETTERS'')
11375, No. 3B (1,-1119), “Ferroelectric Liquid Crystals J (r
Ferroelectric Liquid Crls
taIsJ) ; “Applied physics Letters”
ers”) 1880, No. 38(11), “Submicro Second Bistiple Electro-Optic Switching in Liquid Crystals” (S
ubmicro 5econd Bistable
EIelectrooptic Switching in
Liquid Grystais J); "Solid State Physics", 1881, No. 141, "Liquid Crystal", etc., and the ferroelectric liquid crystal disclosed in these can be used in the present invention.

More specifically, as an example of the ferroelectric liquid crystal compound used in the method of the present invention, decyloxybenzylidene-P'-amino-2-methylbutylcinnamate (DOBAMBC
), hexyloxyben zylidene-P'-amino-2-chloropropylcinnamate (HOBACPC) and 4-o-(2-methyl)-butylresorsilitin-
Examples include 4'-octylaniline (MBRA 8).

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 Sac" phase or S+sH" phase. You can do it by doing.

FIG. 7 schematically depicts an example of a ferroelectric liquid crystal cell. 10 and 10' are Inz O3,5n02 or I
It is a substrate (glass plate) coated with a transparent electrode such as TO (Indium-Tin-Oxide), between which a 5T layer with a liquid crystal molecular layer 20 oriented perpendicular to the glass surface.
sC” phase liquid crystal is sealed. Shown with thick line!13
0 represents a liquid crystal molecule, and this liquid crystal molecule 30 is.

Dipole moment (P engineering) 4 in the direction perpendicular to the molecule
It has 0. When a voltage higher than a certain threshold is applied between the electrodes on the substrates 10 and 10', the helical structure of the liquid crystal molecules 30 is unraveled, and the liquid crystal molecules 30 are aligned so that all dipole moments (P) 40 are oriented in the direction of the electric field. You can change direction. The liquid crystal molecules 30 have an elongated shape and exhibit refractive index anisotropy in the long axis direction and the short axis direction. Therefore, for example, if polarizers are placed above and below the glass surface in a crossed nicol positional relationship, It is easily understood that this is a liquid crystal optical modulation element whose optical characteristics change depending on the polarity of applied voltage. Furthermore, when the thickness of the liquid crystal cell is made sufficiently thin (for example, IP), the helical structure of the liquid crystal molecules is removed even when no electric field is applied, as shown in Figure 7.
(non-helical structure), its dipole moment P or P' is either upward (40a) or downward (40b). As shown in FIG. 8, when an electric field E or E' with a different polarity above a certain threshold value is applied for a predetermined time to such a cell, the dipole moment will move upward 40a or The direction is changed from the downward direction 40b, and accordingly, the liquid crystal molecules are aligned in either the first alignment state 50 or the second alignment state 50'.

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. To explain the second point with reference to FIG. 8, for example, when an electric field E is applied, the liquid crystal molecules are aligned in a first alignment state 50, and this state remains stable even when the electric field is removed. When an electric field E' in the opposite direction is applied, the liquid crystal molecules are oriented to a second alignment state 50' and the orientation of the molecules is changed, but they remain in this state even after the electric field is turned off.

Further, as long as the applied electric field E 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 in general, the cell should be as thin as possible.
5IL to 20IL, especially 1IL to 5IL are suitable. A liquid crystal-electro-optical device having a matrix electrode structure using ferroelectric liquid crystals of this type has been proposed by Clark and Ragaval in US Pat. No. 4,387,924, for example.

[Function] By canceling out the DC component applied to the liquid crystal by applying a pulse voltage corresponding to the voltage value, the voltage applied to the liquid crystal within the -scanning period becomes zero on a time average.

[Example] A first example of the present invention will be described with reference to FIGS. 1 to 3. Figure 2 is a matrix circuit diagram of scanning electrodes and display electrodes.
FIG. 3 is a diagram showing a display example of pixels corresponding to each intersection point in FIG. 2. In addition, in Figure 2, the connection between the segment electrodes and the common electrode in the case of actual multiplex driving is redrawn to the electrode configuration shown in matrix form.
1-Ss indicate scanning signal lines, and 1 to I5 indicate display signal lines. In addition, the pixels shown in the shaded area in FIG. shall be taken as a thing.

The first timing chart shows the case where the matrix circuit shown in FIG. 2 writes the pixels shown in FIG. In FIG. 1, the 6th line indicates writing, < lus width, and when writing, it is assumed that white is written with an electric field in the child direction, and black is written with an electric field in one direction. Furthermore, the write pulse (pulse exceeding the threshold) has a pulse width of Δt and a peak value of ±3V.

Furthermore, let A represent the voltage waveform applied to pixel A in FIG.

When writing to the pixels shown in Figure 3 is performed by first writing the scanning signal line white and then writing black to the selected pixel on the display signal line (line clear line writing), the scanning signal is as shown in Figure 1. +3V1 and -2 as shown in
This becomes an asymmetrical signal of VI, and a DC component corresponding to a pulse having a pulse width Δt and a peak value Vl is applied to the pixel. Therefore, in order to compensate for the application of this DC component,
The compensation pulse applied to each scanning signal line is a pulse of one Vl indicated by 1 in the figure. The timing of applying this compensation pulse may be either after writing or before writing. FIG. 1 shows an example in which the compensation pulse is applied after writing one frame. On the other hand, if a similar operation is to be performed on the display signal line, the pulse shown in FIG. 1a may be applied to the display signal line.

FIG. 4 shows a second embodiment of the present invention. Generally, the larger the screen or the larger the number of pixels, the longer it takes to scan one frame. Therefore, in this embodiment, a compensation pulse is applied at the end of scanning. As in the previous embodiment, numeral 1 in the figure is a compensation pulse.

A third embodiment of the present invention is shown in FIGS. 5 and 6. In this embodiment, after scanning a plurality of lines, compensation pulses are applied to the display signal lines at once. This is particularly useful for waveforms that include a DC component.

In this case, it is difficult to cancel the DC component periodically, so if a certain amount (depending on the properties of the liquid crystal material) of the DC component is applied.

The method is to cancel that amount.

To explain with reference to Figure 1, 5l-S3 is a time chart of the scanning signal line, and 11 is a time chart of the display signal line. Assuming that the pixel written by the scanning signal line S1 and the display signal line 11 is A, when pixel A is written in to, 1. Up to this point, voltage imbalance occurs in pixel A for a period of 3Δt.

Using this point as a guide, the compensation pulse la is set on the display signal line I.
If input to f, the voltage imbalance will be resolved. Therefore, in this case the compensation pulse enters the display signal line.

Further, in this case, since the timing of compensation differs depending on the written content, it is necessary to consider a circuit configuration as shown in FIG. 6, for example, and determine the compensation interval based on the input content. In FIG. 6, 3 is an I/F, 4 is an image signal monitor, 5 is an S/R circuit, 6 is a latch circuit, 7 is a drive circuit, 8 is a scanning signal circuit, and 9 is a matrix panel.

Here, the input signal 2 from the outside is transferred from the I/F C interface 3 to the image signal monitor 4, and then transferred to the I/F C interface) 3.
/F Return to 3 and drive. In this case, the voltage imbalance can be eliminated by counting in the image signal monitor section and inserting a compensation pulse between the image signals when a predetermined image signal continues.

[Effects of the Invention] As explained above, in the present invention, when a liquid crystal element using ferroelectric liquid crystal is driven in a multiplex manner, the voltage imbalance is resolved by applying a compensation pulse, so that the voltage applied to the liquid crystal is The DC component becomes zero. As a result, deterioration and malfunction of the liquid crystal can be prevented, and display quality, reliability, and life of the display element can be improved.

[Brief explanation of drawings]

FIG. 1 is a timing chart diagram showing a first embodiment of the present invention, FIG. 2 is a matrix circuit diagram in each embodiment of the present invention, and FIG. 3 is a display example of pixels corresponding to each intersection in FIG. 2. 4 is a timing chart showing the second embodiment of the present invention, FIG. 5 is a timing chart showing the third embodiment of the present invention, and FIG. 6 is a timing chart showing the third embodiment of the present invention. The circuit configuration diagram, FIGS. 7 and 8 are perspective views schematically showing the ferroelectric liquid crystal used in the present invention. S1 to S5...Scanning signal line, I, -is...Display signal line, 1.1a...Compensation pulse.

Claims (3)

[Claims] (1) In multiplex driving of a liquid crystal device with a ferroelectric liquid crystal sandwiched between electrodes, the first step is to write image information to the pixels of the scanning line, and when the DC component applied to the pixels exceeds a certain value. , a second step of applying a compensation pulse for canceling the DC component to the pixel. (2) A method for driving a liquid crystal element according to claim 1, characterized in that a compensation pulse is applied after writing one scanning line. (3) A method for driving a liquid crystal element according to claim 1, characterized in that a compensation pulse is applied every frame writing.