JPS6283727A - Driving method 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 [Field of Industrial Application] The present invention relates to a method for driving a ferroelectric liquid crystal element, and more particularly, to a method for driving a ferroelectric liquid crystal element that enables gradation display as a display element.

[Summary of Disclosure] This specification and drawings describe a method for driving a ferroelectric liquid crystal, in which a voltage value is equal to or higher than a predetermined threshold value and a group of pulses containing different values is applied while being modulated.
The operating margin for gradation has been expanded to reproduce fine gradation display.

[Prior art] Examples of conventionally used liquid crystal elements include M,
Shut (M, 5chadt) and W.

Helfrich (W, He1frich) °°Applied Physics Letters (Applied Pb
ysicsLetters'') Volume 18, No. 4 (19
Voltage Dependen (February 15, 1971), pp. 127-12B, “Voltage Dependent Optical Activity of a Twisted Nematic Liquid Crystal”
Optical Activity of a Tw
isted Nematic LiquidCryst
↑N (twisted nema) shown in
tic) Those using liquid crystal are known. This TN-type liquid crystal has a limited number of pixels because it generates wolf talk when driving on the hour and minute sides using a matrix electrode structure with a high pixel density. Also, display elements are known in which a TPT switching element is connected to each pixel and each pixel is switched.
Because TPT has a complicated structure, it requires a large number of manufacturing steps and high manufacturing costs. Therefore, C1ark and Lagervall have developed a display element that utilizes the refractive index anisotropy of ferroelectric liquid crystal molecules to control transmitted light in combination with a polarizing element. ) and U.S. Patent No. 4.367.).
This is proposed in the specification of No. 924 and the like. This ferroelectric liquid crystal generally has a chiral smectic C phase (Safe or H phase) in a specific temperature range, and in this state, it responds to an applied electric field to a first optically stable state. It has the property of taking one of the first and second 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 has high speed and memory. It is expected that it will be widely used as a type display element.

In this way, it has been considered difficult to display gradations in display elements using ferroelectric liquid crystals that utilize the optical difference between two stable states, but ferroelectric liquid crystals sandwiched between a pair of electrodes On the other hand, a gradation display method has been proposed that utilizes the fact that the threshold value for "partial reversal", where transition between bistable states occurs partially, and the threshold value for "complete reversal", which occurs over the entire cell, are different.

FIG. 2 is a diagram illustrating such a gradation display method, and is a graph showing the optical response to the applied pulse voltage by changing the amount of transmitted light. In a surface-stable ferroelectric liquid crystal memory cell (5SFLC cell) as shown in the above-mentioned Japanese Patent Application Laid-open No. 107216/1987, which has been given bistability by reducing the distance between the substrates of the liquid crystal cell to about IJL11, 2 in a crossed nicol relationship before and after the cell
For example, two polarizing plates are arranged so that the molecular axis of the first stable state coincides with the polarization axis of one of the polarizing plates, and a voltage applied in a direction that moves the liquid crystal molecules to the second stable state is applied. When the change in the amount of transmitted light is measured, the result is as shown in FIG. 2. In the diagram, 0 and L are the corresponding levels of the amount of transmitted light throughout the cell in the first and second stable states, respectively. From this figure 2, it can be seen that the liquid crystal molecules change from the first stable state to the second stable state.
It can be seen that the voltage (partial reversal threshold value) Vthp at which a partial reversal begins to reach a stable state is different from the voltage Vthw at which the cell reversals over the entire cell (total reversal threshold).

That is, by applying a voltage intermediate between VLhp and VLhw, it is possible to obtain an amount of transmitted light intermediate between D and L, making it possible to display gradations. This difference in the amount of transmitted light at intermediate tones is due to the difference in the size of the domain that has been inverted to the second stable state, and each minute portion is binary in either the first or second stable state. Therefore, it is essentially different from the gradation expression method of nematic liquid crystal, which utilizes the fact that the direction of the molecular axis changes continuously depending on the magnitude of the applied voltage.

In addition, although the voltage value is changed for a constant pulse width in the above example, when the pulse width is changed while keeping the voltage value constant, the pulse width partial inversion agony value TLhp and total inversion threshold By applying a pulse voltage having a pulse width intermediate between "rthw" and "rthw", it is possible to display halftones.

[Problems to be Solved by the Invention] However, as can be seen from FIG. 2, a characteristic of ferroelectric liquid crystals is that the difference between v thp and VLhw or the difference between Tthp and Tth+i is very small.
In other words, because the operating margin for displaying halftones is narrow, it has been difficult to display multivalued halftones with good reproducibility just by modulating the voltage value or pulse width.

The present invention has been made in view of these problems, and an object of the present invention is to provide a method for driving a ferroelectric liquid crystal element that displays multi-level halftones with good reproducibility.

Means for Solving the L Problem] The present invention provides a method for driving a liquid crystal element having a cell structure in which a ferroelectric liquid crystal having a bistable state is sealed between a pair of parallel substrates on which electrodes are formed. A ferroelectric liquid crystal characterized in that a pulse group consisting of a plurality of types of pulse voltages having voltage values, pulse widths, and pulse numbers that allow liquid crystal molecules in any of the states to coexist within the liquid crystal is applied between electrodes. In the device driving method, the pulse voltage is determined by the applied voltage value and voltage application time required for the entire liquid crystal sandwiched between a pair of electrodes to transition between the bistable states.
m or more, and at least two of the voltage value, pulse width, and number of pulses of the pulse group are set such that liquid crystal molecules in one of the bistable states coexist in the liquid crystal at a desired ratio. This is a driving method that simultaneously changes the values of seeds.

[Function] It has already been mentioned that a partial reversal phenomenon occurs when a voltage below the threshold is applied, but even when the voltage is above the threshold,
The inventors have discovered that a similar phenomenon occurs by applying a pulse group consisting of a plurality of pulses.

That is, by applying a pulse group consisting of multiple types of pulses with voltage values that are all higher than the reversal threshold, a portion that maintains the first stable state and a portion that has reversed to the second stable state appear, and these By controlling the ratio to a desired ratio, in one pixel of the liquid crystal sandwiched between a pair of electrodes,
An intermediate amount of transmitted light can be obtained, making gradation display possible.

[Example] Embodiments of the present invention will be described in detail below with reference to the drawings.

Figure 1 is a graph showing the change in the amount of transmitted light with respect to the number of applied pulses.
5lOV ) or more, and the white circles show the cases where all pulses are less than the inversion threshold (l taslOV ).

FIGS. 3 to 5 are micrographs showing the switching states of the liquid crystal shown in FIGS. 1 and 2. First, these switchings will be explained. As the ferroelectric liquid crystal material, C3S3 as shown in the following formula is used.

H3 H3 Here, the substrates on both sides in which the liquid crystal is sealed are glass plates on which ITO is vapor-deposited, and after coating a polymer film such as polyimide on the ITO, a rubbing treatment is performed.

In each figure from FIG. 3 to FIG. 5, the cell thickness is maintained at approximately 1 m by forming polyimide spacers 1 in stripes on the polymer film.

FIG. 3 shows a state in which the polarizing axes of a pair of polarizing plates arranged in a crossed nicol relationship on both sides of a liquid crystal cell are aligned with the molecular axis direction of the liquid crystal in the first stable state. This shows the case where the "dark" state is displayed.

In addition, Fig. 4 shows a state in which the liquid crystal molecules are transferred from the first stable state to the second stable state by applying a pulse voltage higher than the predetermined total inversion dark value, and the molecular axis direction of the liquid crystal is polarized. This represents a case where a "bright" state is displayed due to birefringence of incident light due to deviation from the polarization axis direction of the plate. Note that the total reversal threshold here was approximately 12V for 1ms.

It has already been mentioned that a partial reversal phenomenon occurs when a voltage below the threshold is applied, but even when the voltage is above the threshold,
A similar phenomenon appears by opening a pulse group consisting of a plurality of pulses.

FIG. 5 shows the state of partial reversal caused by such an operation, where the "dark" part 2 is the part that did not invert and maintained the first stable state, and the "bright" part 3 is the part that maintains the first stable state. , which shows the inversion to the second stable state. Note that the voltage applied here is 1/2 duty, consisting of a 13V pulse.
This is a group of three pulses. That is, by applying a pulse group consisting of a plurality of types of pulses with voltage values that are all higher than the inversion threshold, a pixel in the liquid crystal sandwiched between a pair of electrodes is inverted between the points in FIG. 3 and FIG. 4. The amount of transmitted light can be obtained, and gradation display becomes possible.

In Fig. 1, the black circles indicate the change in the amount of transmitted light when the pulse group is composed of all pulses equal to or higher than the D transfer threshold. gradation display becomes possible.

Here, the amount of transmitted light on the vertical axis is normalized as 100%, which is the amount of transmitted light when the liquid crystal molecules are oriented in the second stable state throughout the liquid crystal cell with the liquid crystal cell and polarizing plate arranged as described above. It is.

On the other hand, even when a pulse group consisting of pulses that are all below the inversion threshold is applied, a similar change in the amount of transmitted light is obtained, as shown by the white circles in FIG.

However, it has been confirmed that when a pulse group consisting of pulses with a total inversion value of 121 or less is applied, it is difficult to obtain a total inversion state, that is, 100% transmitted light amount, unless the pulse width and voltage value of the pulses are optimized. .

In this embodiment, the pulse width and voltage value of the applied pulses are kept constant and the number of pulses is varied.

Considering FIG. 2, it goes without saying that display with even finer gradation is possible by changing the number of pulses and simultaneously modulating the pulse width and voltage value.

[Effects of the Invention] As explained above, according to the present invention, the operating margin of gradation can be improved by modulating the number of applied pulses or by simultaneously modulating the number of pulses, pulse width, and voltage value. It is possible to provide a driving method for a ferroelectric liquid crystal element that can widen the range of the ferroelectric liquid crystal element, display multi-level halftones, and display fine gradations with good reproducibility.

[Brief explanation of drawings]

Figure 1 is a graph showing the optical response to the number of applied pulses.
Figure 2 is a graph showing the optical response to applied voltage, Figure 3 is a graph showing the optical response to applied voltage.
5 through 5 are microscopic photographs showing the inverted state of the liquid crystal. 1... Spacer? ... "Dark" part, 3... "Bright" part.

Claims (1)

[Claims] 1) In a method for driving a liquid crystal element having a cell structure in which a ferroelectric liquid crystal having a bistable state is sealed between a pair of parallel substrates on which electrodes are formed, the device is in any of the bistable states. A method for driving a ferroelectric liquid crystal element, characterized in that a pulse group consisting of a plurality of types of pulse voltages having voltage values, pulse widths, and pulse numbers that allow liquid crystal molecules to coexist within the liquid crystal is applied between electrodes. 2) The pulse voltage is equal to or higher than a threshold value determined by the applied voltage value and voltage application time necessary for transitioning between the bistable states over the entire liquid crystal sandwiched between a pair of electrodes. A method for driving a ferroelectric liquid crystal element according to scope 1. 3) Adjust at least the voltage value, pulse width, and number of pulses of the pulse group so that the liquid crystal molecules in one of the bistable states coexist in the liquid crystal sandwiched between the pair of electrodes at a desired ratio. A method for driving a ferroelectric liquid crystal according to claim 1 or 2, characterized in that two types of values are varied simultaneously. 4) The method for driving a ferroelectric liquid crystal according to any one of claims 1 to 3, wherein the ferroelectric liquid crystal is a smectic liquid crystal having bistability.