JPS62118326A - Driving method for liquid crystal element - Google Patents

Abstract

PURPOSE:To perform temperature compensation and to obtain proper display and drive characteristics by imposing modulation on the voltage value and pulse width of inverted driving pulses corresponding to variation in external temperature. CONSTITUTION:A liquid crystal display panel 1 consists of signal lines and scanning lines in a matrix and is equipped with a signal line driving circuit 2 as a driver, its signal latch circuit 3, and a signal shift register on the signal line side and also with a scanning line driving circuit 5 and its scanning shift register 6 on the scanning-line side. The signal line driving circuit 2 and scanning line driving circuit 5 are supplied source electric power outputs (b) and (e) from a voltage supply circuit 7. A signal input (d) to the signal shift register 4 and a scanning input (f) to the scanning shift register 6 are shifted with a signal clock h1 and a scanning clock h2 generated by a clock circuit 8, but this clock circuit 8 inputs a basic clock (i) from an oscillation circuit 9. The voltage supply circuit 7 and oscillation circuit 9 are controlled on the basis of temperature information (j) from a temperature sensor 10.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to liquid crystal elements such as liquid crystal display elements and liquid crystal-optical shutter 7-rays, and more specifically, the present invention relates to liquid crystal elements such as liquid crystal display elements and liquid crystal-optical shutter 7-rays. and a method for driving a liquid crystal element with improved driving characteristics.

[Summary of the Disclosure] This specification and drawings describe how to change the voltage value and pulse width of a single inversion drive pulse depending on changes in external temperature in a method of driving a liquid crystal element used in a liquid crystal display element, a liquid crystal-optical shutter 7-ray, etc. The display and drive characteristics are improved by corresponding modulation.

[Prior Art] In recent years, the use of bistable liquid crystal elements has been developed by Clark (C1ark) as an improved version of the conventional TN type liquid crystal elements.
) and Lagerwal 1, published in JP-A-58-107218 and U.S. Pat. No. 43
This method has been proposed in the specification of No. 87!824, etc. Bistable liquid crystals are generally chiral smectic C phase (Sa
A ferroelectric liquid crystal having a C phase or an H phase (S phase) is used. This liquid crystal has a bistable state consisting of a first and a second optically stable state with respect to an electric field, and thus has a TN Unlike optical modulators used in type liquid crystals, for example, the liquid crystal is oriented in a first optically stable state for one electric field vector and in a second optically stable state for the other electric field vector. This type of liquid crystal has the property of responding to an applied electric field to very quickly take one of the above two stable states, and maintain that state when no electric field is applied. By utilizing such properties, considerable improvements can be obtained in many of the problems of the conventional TN type elements mentioned above.

[Problems to be solved by the invention] However, when this bistable liquid crystal is used in an optical modulation element, its driving characteristics vary greatly depending on the external temperature, causing crosstalk or a decrease in contrast. .

SUMMARY OF THE INVENTION An object of the present invention is to eliminate such drawbacks and provide a method for driving a liquid crystal element suitable for a display element or a liquid crystal light shutter, which can obtain driving characteristics appropriate for an external temperature.

Means for Solving the L Problem] The present invention provides a liquid crystal element in which a ferroelectric liquid crystal is sandwiched between two substrates on which transparent electrodes are formed, in which the ferroelectric liquid crystal is in a first stable state and a second stable state. A driving method for a liquid crystal element, characterized in that the voltage value and pulse width of an inverted driving pulse between these two stable states are modulated in response to changes in external temperature. be.

Particularly suitable liquid crystal materials for use in the present invention include:
It is a chiral smectic liquid crystal and has ferroelectricity. Specifically, the caitylsmectic C phase (
Liquid crystals having a Sac or H phase (SmH'') are preferred.

FIG. 5 schematically depicts an example of a cell for explaining the operation of a ferroelectric liquid crystal. 21 and.

21' is Inz03.5n02 or ITO (In
A substrate (glass plate) coated with a transparent electrode made of a thin film such as dium-Tin-Oxide, between which a liquid crystal molecular layer 22 is oriented perpendicular to the glass surface.
C" phase or S■H" phase liquid crystal is sealed. A thick line 23 represents a liquid crystal molecule, and this liquid crystal molecule 23 has a dipole moment (P
Engineering) 24. When a voltage higher than a certain threshold is applied between the electrodes on the substrates 21 and 21', the helical structure of the liquid crystal molecules 23 is unraveled, and the liquid crystal molecules 23 are oriented so that all the dipole moments (P↓) 24 are directed in the direction of the electric field. You can change direction. The liquid crystal molecules 23 have an elongated shape and exhibit refractive index anisotropy in the long axis direction and the short axis direction. Therefore, for example, if crossed Nicol polarizers are placed above and below the glass surface, the polarity of the applied voltage changes. It is easily understood that this results in a liquid crystal optical modulation element whose optical properties change.

The liquid crystal cell preferably used in the optical modulation element of the present invention can have a sufficiently thin thickness (for example, 10= or less).As the liquid crystal layer becomes thinner, as shown in FIG. Even when no electric field is applied, the helical structure of the liquid crystal molecules unwinds and becomes a non-helical structure, and the dipole moment P or P' is either upward (34) or downward (34'). When an electric field E or E' with a different polarity, which is equal to or higher than a certain threshold value, is applied to such a cell by the voltage applying means 31 and 31' as shown in FIG. The direction is changed to upward 34 or downward 34' in response to
Accordingly, the liquid crystal molecules are aligned in either the first stable state 33 or the second stable state 33'.

As mentioned earlier, there are two advantages to using such a ferroelectric liquid crystal as an optical modulation element.

The first is that the response speed is extremely fast, and the second is that the alignment of liquid crystal molecules has bistability. To further explain the second point, for example, with reference to FIG. 6, when an electric field E is applied, the liquid crystal molecules are oriented in a first stable state 33, and this state remains stable even when the electric field is turned off. Furthermore, when an opposite electric field E' is applied, the liquid crystal molecules enter a second stable state 33.
', and changes the orientation of the molecule, but it remains in this state even when the electric field is turned off. Also, the electric field E
The respective orientation states are maintained as long as the values do not exceed a certain threshold. In order to effectively realize such fast response speed and bistability, it is preferable that the cell be as thin as possible.

[Function] The driving characteristics of a liquid crystal element having bistability are characterized by pulse width and threshold voltage. FIG. 7 is a logarithmic curve diagram showing an example of the relationship between the pulse width ΔT (vertical axis) and the threshold voltage vth (horizontal axis) in a desired liquid crystal element at each temperature;
shows an example at 10°C, (b) at 20°C, and (C) at 30°C. From this vth-61 curve, the temperature dependence of the drive characteristics is clear, and the higher the temperature, the more it becomes possible to reverse the voltage at a lower voltage or in a shorter time. This is thought to be because the viscosity of the liquid crystal decreases as the temperature rises, and for example, the pulse width Δ
When T is set to 1 tasec, the value of threshold voltage vth is 14.2V (10°C).9. OV (20℃
), 5.8 V (30°C), which varies greatly. In the case of ΔT = 1 m5ec, the orientation of the liquid crystal is normally reversed within the range of Vth-Vth+3V at each temperature.
If a higher electric field is applied, a phenomenon of returning to the other stable state before inversion is observed, which causes a decrease in contrast.

Therefore, since the range of voltage values that can be reversed between the first stable state and the second stable state is limited depending on the temperature, temperature compensation is performed using a pulse voltage.

Furthermore, from the above vth-61 curve, it is possible to reduce the width of change in the voltage value by simultaneously modulating the voltage V and the pulse width ΔT with respect to the external temperature. -x' along a straight line (ΔT,
When performing modulation of V), temperature compensation can be performed by varying the voltage value V between 13 and 17 V and the pulse width ΔT between 0.26 and 0.5 SeC.

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

FIG. 1 is a block diagram showing an example of the configuration of a driving circuit for a liquid crystal element used in the driving method of the present invention, and (a) and (a) in FIG. 2 are diagrams of a liquid crystal cell used in the liquid crystal element. FIG. 2 is a plan view and a vertical cross-sectional view.

First, in FIG. 2, a striped electrode group 1 is placed on a pair of glass or plastic substrates 11a and llb.
2a and 12b are ITO (Indium-Tin-Ow
ide) to a thickness of 100 OA, and polyimide coatings 14a and 14b were formed thereon to a thickness of 100 OA. Furthermore, in order to maintain the thickness of the liquid crystal layer, a dot-shaped spacer group 13 made of bolyimide was interposed on the upper layer (not shown in FIG. 2(a)).With this spacer group 13, The liquid crystal layer 15 is kept constant over a wide range. After rubbing the two substrates 11a and llb, they were assembled into cells, and the chemical structure was as shown below.The phase transition of this liquid crystal was as follows.

(In SmC: chiral smectic C phase, SmA"
: smectic A phase, SmB" : smectic B phase, ch: cholesteric phase) The temperature of the liquid crystal layer of this liquid crystal element 7 is raised until it reaches the isotonic phase state, and at a cooling rate of 0.5 degrees Celsius, the Sac phase is formed. Then, by applying a rectangular pulse voltage to each electrode 12a and 12b shown in Fig. 2, the switching under crossed Nicols was observed, and its driving characteristics were determined. What was obtained from the investigation is the vth-ΔT curve diagram shown in FIG.

Figure 1 is a block diagram of a drive circuit that provides temperature compensation to this liquid crystal element. 1 in Figure 1 is a liquid crystal display panel, which is configured in a matrix of signal lines and scanning lines, and the signal line side is a driver. The signal line drive circuit 2 includes a signal line drive circuit 2, its signal latch circuit 3, and a signal shift register 4, and the scanning line side includes a scanning line drive circuit 5, which is a driver, and its scan shift register 6. and scanning line drive circuit 5
are supplied with power supplies S and e from a voltage supply circuit 7. The signal input d to the signal shift register 4 and the scanning input f to the scanning shift register 6 are shifted by the signal clock hl and scanning clock h2 generated from the clock circuit 8, which is generated by the oscillation circuit 9.
10 in the figure is a temperature sensor installed close to the liquid crystal display panel to detect the external temperature, and the voltage supply circuit 7 and the oscillation circuit 9 are Control is performed based on temperature information j from temperature sensor 0.

FIG. 3 is a waveform diagram showing an example of the drive waveform of the liquid crystal element, where is a scanning waveform, L is a signal waveform, and M is a voltage waveform applied to a pixel when these signals intersect. Here, if the maximum value of the voltage applied to the liquid crystal is V and the time is τ, then
V is expressed as the voltage difference between V' and V', that is, v=v'-v', and as shown in Figure 3, the relationship between L and M holds true.
V/ is the voltage of the power supply power e supplied from the voltage supply circuit 7 to the scanning line drive circuit 5, and V'' is the voltage of the power supply power e supplied to the signal line drive circuit 2, and both Both are controlled based on the temperature information j transmitted from the temperature sensor O after temperature-electrical conversion.
Figure (a) is a graph showing an example of the input/output characteristics of the voltage supply circuit 7, in which the horizontal axis shows the input as a value equivalent to the temperature T, and the vertical axis shows the output voltage. Note that in reality, it is not necessary to directly apply this input/output characteristic to the power supplies b and e; either b or e may be set to a constant value and the other may be set to a variable value, or both may be set to a variable value. Good too.

Similarly, FIG. 4(0) is a graph showing an example of the input/output characteristics of the oscillation circuit 9, where the horizontal axis shows the input as a value equivalent to the temperature T.
The vertical axis shows the pulse width τ. That is, the period of the oscillation circuit 9 is also controlled based on the temperature information j, and the drive characteristics of the liquid crystal display panel 1 can be temperature compensated from both the voltage and pulse width. Note that the drive waveform shown in FIG. 3 is only an example, and any drive method may be used as long as the voltage value and pulse width can be controlled.

[Effects of the Invention] As explained above, according to the present invention, temperature compensation can be performed by modulating the voltage value and pulse width of the inversion drive pulse in response to changes in external temperature, and proper display and It is possible to provide a method for driving a liquid crystal element that provides good driving characteristics and is suitable for use as a display element or a liquid crystal light shutter.

[Brief explanation of drawings]

Figure 1 is a drive circuit diagram of an embodiment of the present invention, Figure 2 (a)
and (a) are a plan view and a vertical cross-sectional view of one embodiment, FIG. 3 is a drive waveform diagram, FIG. 4 is a graph of input/output characteristics of voltage and pulse width, and FIGS. 5 and 6 are bistable liquid crystals. FIG. 7 is a graph of voltage/pulse width according to temperature. 1-...Liquid crystal display panel, 2...Signal line drive circuit. 5... Scanning line drive circuit, 7... Voltage supply circuit, 8.
... Clock circuit, 9... Oscillation circuit, 10... Temperature sensor.

Claims (1)

[Claims] 1) In a method for driving a liquid crystal element in which a ferroelectric liquid crystal is sandwiched between two substrates on which transparent electrodes are formed, the ferroelectric liquid crystal has first and second stable states. A method for driving a liquid crystal element, characterized in that the voltage value and pulse width of an inverted drive pulse between these two stable states are modulated in response to changes in external temperature. 2) The method for driving a liquid crystal element according to claim 1, wherein the ferroelectric liquid crystal is a liquid crystal having a smectic phase.