JP2894037B2 - Liquid crystal panel driving method and light valve - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for driving a liquid crystal panel and a light valve.

[0002]

2. Description of the Related Art A light valve using a liquid crystal panel has many features such as an easy large area, a thin shape and low power consumption. In particular, a light valve using a polymer-dispersed liquid crystal panel as a liquid crystal panel has been used in window glasses and the like, and has been receiving attention in recent years. The polymer dispersed liquid crystal panel is milky white when no voltage is applied, but becomes transparent when a voltage is applied. When this polymer-dispersed liquid crystal panel is used as a window glass, a fogged glass can be switched to a transparent glass by applying a voltage. Further, the polymer-dispersed liquid crystal panel has an advantage that, unlike a twisted nematic type panel, an alignment film and a polarizing plate are not required.

Hereinafter, a light valve using a liquid crystal panel will be briefly described. FIG. 6 is a schematic diagram of a light valve. The voltage generator 61, the switching unit 62, and the liquid crystal panel 63 are connected in series. The voltage generator 61 generates predetermined voltages of positive polarity and negative polarity based on the set voltage. The positive and negative voltages generated by the voltage generating unit 61 are alternately switched by the switching unit 62 to the liquid crystal panel 6.
3 is applied. The liquid crystal panel 63 functions as a light valve by changing the intensity of transmitted light according to the applied voltage.

Hereinafter, the polymer-dispersed liquid crystal will be briefly described. Polymer-dispersed liquid crystals are roughly classified into two types depending on the dispersion state of the liquid crystal and the polymer. One is a type in which a liquid crystal in the form of water droplets is dispersed in a polymer. The liquid crystal exists in a discontinuous state in the polymer. Hereinafter, such a liquid crystal is referred to as PDLC, and a liquid crystal panel using the liquid crystal is referred to as a PD light valve. The other type adopts a structure in which a polymer network is stretched around a liquid crystal layer. It looks just like a sponge with liquid crystal. Liquid crystals exist continuously without being in the form of water droplets. Hereinafter, such a liquid crystal is called a PNLC, and a liquid crystal panel using the liquid crystal is called a PN light valve. In order to use the above two types of liquid crystal panels as light valves, scattering and transmission of light are controlled.

[0005] PDLC utilizes the property that the refractive index differs in the direction in which the liquid crystal is oriented. In the state where no voltage is applied, the respective liquid crystal droplets are oriented in an irregular direction. In this state, a difference in refractive index occurs between the polymer and the liquid crystal, and the incident light is scattered. Here, when a voltage is applied, the alignment directions of the liquid crystals are aligned. If the refractive index when the liquid crystal is aligned in a certain direction is adjusted in advance to the refractive index of the polymer, incident light is transmitted without being scattered.

On the other hand, PNLC uses the irregularity of the alignment of liquid crystal molecules. In an irregular orientation state, that is, in a state where no voltage is applied, incident light is scattered.
On the other hand, when a voltage is applied to make the arrangement state regular, light is transmitted. The above description of the movement of the liquid crystal of PDLC and PNLC is based on a model-based concept. Although the present invention is not limited to one of the PD light valve and the PN light valve, the PD light valve will be described as an example for ease of explanation. PDLC and PN
LC is generically called polymer dispersed liquid crystal, and PD light valve and PN light valve are generically called polymer dispersed liquid crystal light valve. The liquid containing the liquid crystal to be injected into the polymer-dispersed liquid crystal light valve is generally called a liquid crystal solution, and the state in which the resin component in the liquid crystal solution is polymerized and cured is called a polymer.

The operation of the polymer dispersed liquid crystal (FIG. 2)
This will be briefly described using (a) and (b)). (FIG. 2 (a)
(B) is an explanatory view of the operation of the polymer dispersed liquid crystal panel. In FIGS. 2A and 2B, 31 is a glass substrate, 32 is a substrate electrode, 33 is a counter electrode, 34 is a water-drop liquid crystal, 35 is a polymer, and 36 is a counter substrate. A voltage generator 37 and a switching unit 38 are connected to the substrate electrode 32 and the counter electrode 33, and the voltage generator 37 is turned on and off by the switching unit 38 to form a pair with the substrate electrode 32.
A voltage is applied between the facing electrodes 33 to change the alignment direction of the liquid crystal on the substrate electrode to modulate light. As shown in FIG. 2A, when no voltage is applied, the liquid crystal droplets 34 are oriented in an irregular direction. In this state, a difference in refractive index occurs between the polymer 35 and the liquid crystal, and the incident light is scattered. Here, as shown in FIG. 2B, when a voltage is applied to the substrate electrode, the directions of the liquid crystals are aligned. If the bending index when the liquid crystal is oriented in a certain direction is previously matched with the refractive index of the polymer, the incident light is not scattered and the glass substrate 31 is not scattered.
It emits more.

[0008]

As described above, since the polymer dispersed liquid crystal panel does not use a polarizing plate, a light valve having high light use efficiency and high contrast can be obtained. But,
When the liquid crystal panel is used for a light valve, there are the following problems. It is the hysteresis property of polymer dispersed liquid crystal. An explanatory diagram of the hysteresis characteristic is shown in FIG.
As shown in FIG. 3, in the polymer-dispersed liquid crystal, the curve of the applied voltage (V) versus the transmittance (T) when the absolute value of the applied voltage is gradually increased, and the absolute value of the applied voltage is gradually decreased. It is not the same curve as when you let it. That is, it has a hysteresis characteristic. Thus, although the transmittance at the time of changing from the voltage V ₀ which is applied to the V ₁ was a T _2, V ₁ from the applied voltage V ₂
Transmittance when changing to become T _1. This phenomenon greatly hinders the gradation display of the light valve.

As described above, when the polymer-dispersed liquid crystal is used, the light utilization factor is increased and a light valve with high contrast can be obtained. However, desired good gradation display cannot be performed due to the hysteresis characteristic. For this reason, it has conventionally been difficult to construct a light valve that exhibits good gradation display using polymer dispersed liquid crystal.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method of driving a liquid crystal panel and a light valve which are free from the influence of the hysteresis characteristics of the polymer dispersed liquid crystal when the polymer dispersed liquid crystal is used for a light valve.

[0011]

According to the driving method of the liquid crystal panel of the present invention, in addition to the normal driving waveform to the liquid crystal between the substrate electrodes, an alternating signal not higher than the rising voltage of the liquid crystal is applied as a modulation voltage. . The frequency of the AC signal is lower than the response frequency of the liquid crystal.

A light valve according to the present invention has been invented for realizing the above-described method of driving a liquid crystal panel, and has a signal generator for generating a signal having a frequency lower than a response frequency of liquid crystal. The output from the generator is applied to a substrate electrode or a counter electrode of a liquid crystal panel as a modulation voltage. As a result, the hysteresis characteristics of the polymer-dispersed liquid crystal are removed to realize a good gradation display.

[0013]

The liquid crystal generally has a response frequency. The liquid crystal rotates in the direction of the molecular axis following the electric field change at a frequency lower than the response frequency. That is, it is possible to finely vibrate the liquid crystal by applying a modulated wave modulated by an AC voltage having a frequency equal to or lower than the response frequency to the liquid crystal. This vibration changes the refractive index of the liquid crystal and changes the panel transmittance.
Thus, by vibrating the liquid crystal molecules, the panel transmittance is averaged in the process of raising and lowering the voltage, and the above-described hysteresis can be reduced. Also, as the frequency of the AC voltage at this time is larger, the vibration of the liquid crystal molecules becomes more intense, so that the effect of reducing the hysteresis is greater. Further, if the amplitude of the AC voltage used for modulation is higher than the rising voltage of the liquid crystal, black display of the panel cannot be performed. Therefore, the amplitude needs to be lower than the rising voltage of the liquid crystal. Hereinafter, this will be described in detail with reference to FIG.

The polymer-dispersed liquid crystal is apparent from FIG.
It has a hysteresis characteristic. Voltage V₀From
V₁And the transmittance becomes T in FIG._TwoBecomes
Is the voltage V_TwoTo V₁When it falls, the point becomes d and the transmittance
Is T₁Becomes Also, the voltage V₀-V₁From a certain voltage to V₁
Is changed to another transmittance, and the voltage V _Two
-V₁From a certain voltage to V₁Different transmittance when changed to
Becomes From the above, the conventional liquid crystal panel driving method
The rising hysteresis curve and the rising hysteresis
Within the range of the hysteresis curve (hereinafter referred to as within the hysteresis)
It is not known which transmittance will be obtained in step (b).

When the transmittance is T ₂ , the voltage applied to the liquid crystal layer is reduced by a voltage equal to or lower than the voltage V ₀ (hereinafter referred to as a rising voltage of the liquid crystal). That is, V ₁ −V _x (where V
_x <V ₀ ) is applied. Then, it moves to the point b in the hysteresis. Next, a voltage of V ₁ + V _x is applied to the liquid crystal layer. This time, it moves to the point c in the hysteresis. Thereafter, if the voltages V ₁ -V _x and V ₁ + V _x are applied alternately, the circuit reciprocates between the points c and b in the hysteresis. Therefore, the transmittance of the apparently becomes T _x. The transmittance T _x is the on curve substantially middle position of the hysteresis curve of the rising and the hysteresis curve and the edge of rise. That is, by applying a predetermined AC signal to the liquid crystal layer, the hysteresis becomes apparently very small. The period of the AC signal needs to be determined in consideration of the responsiveness of the liquid crystal, but it is preferable that the period be as short as possible within a range that the liquid crystal can follow.

[0016]

The liquid crystal material of the liquid crystal panel used in the light valve of the present invention is preferably a nematic liquid crystal, a smectic liquid crystal, or a cholesteric liquid crystal, and is a mixture containing one or more liquid crystal compounds and substances other than the liquid crystal compounds. It may be. Note that among the liquid crystal materials described above, a cyan biphenyl-based nematic liquid crystal is most preferable. As the resin material, a transparent polymer is preferable, and any of a thermoplastic resin, a thermosetting resin, and a photocurable resin may be used. However, an ultraviolet-curing type is used in view of easiness of a manufacturing process and separation from a liquid crystal phase. It is preferred to use the resin of As a specific example, an ultraviolet curable acrylic resin is exemplified, and a resin containing an acrylic monomer or acrylic oligomer which is polymerized and cured by irradiation with ultraviolet light is particularly preferable. When these are irradiated with ultraviolet rays, only the resin undergoes a polymerization reaction to become a polymer, and only the liquid crystal undergoes phase separation. At this time, when the amount of liquid crystal is small compared to the resin component, independent droplet-like liquid crystal is formed. On the other hand, when the amount of liquid crystal is large, the resin matrix has a particulate form in the liquid crystal material. ,
Alternatively, the liquid crystal exists in a network state and is formed so that the liquid crystal forms a continuous layer. At this time, unless the particle diameter of the water-droplet liquid crystal or the pore diameter of the polymer network is uniform to some extent and the size is in the range of 0.5 μm to several μm, the scattering performance of incident light is poor and the contrast is not improved. Preferably, the particle size of the droplet-shaped liquid crystal or the pore size of the polymer network is in the range of 0.8 μm to 2.0 μm. For this reason, the material must be a material that can be cured in a short time, such as an ultraviolet curable resin. The orientation ratio between the liquid crystal material and the resin material is 9: 1 to 1: 9, and particularly, 2:
A range of 1-1: 2 is preferred.

FIG. 4 is an explanatory diagram for explaining a method of driving a liquid crystal panel according to the present invention. (FIG. 4)
Reference numeral 43 denotes a glass substrate.
A substrate electrode 44 made of TO is formed. 41 is a pair
This is a counter electrode substrate, on one surface of which a counter electrode 42 is formed. A water-drop liquid crystal 49 and a polymer 50 are sandwiched between the counter electrode 42 and the substrate electrode 44. Substrate electrode 4
4. The counter electrode 42 is an electrode common to the entire surface of each substrate. A terminal for applying a voltage to the counter electrode is referred to as a counter electrode terminal 45. A terminal for applying a voltage to the substrate electrode 44 is called a substrate electrode terminal 46.

FIG. 5 is a waveform diagram of the applied signal voltage. A method of applying an AC signal to the substrate electrode terminal to reduce the hysteresis will be described with reference to FIG. The reference voltage A in FIG. 5 corresponds to the voltage waveform applied to the counter electrode in FIG. A rectangular signal 51 (hereinafter referred to as a modulation voltage) is output based on the reference voltage A, and the signal 51 is applied to the substrate electrode terminal 46. The cycle of the modulation voltage is set experimentally within a range in which the liquid crystal can substantially follow in consideration of the response time of the liquid crystal. As an example, a multiple of 60 Hz, such as 60 Hz, 120 Hz, and 180 Hz, is applicable. The voltage amplitude is set to be equal to or lower than the rising voltage of the liquid crystal. Normally, the rising voltage of the liquid crystal is 10 V or less, though it depends on the panel thickness. The voltage of the reference voltage A is applied to the counter electrode terminal 45.

At this time, the voltage waveform applied to the liquid crystal panel is the sum of the reference voltage A and the modulation voltage, as shown by the drive waveform in FIG. As a result, the transmittance of the liquid crystal as shown in (FIG. 3) will be varied between bc As discussed in the action, the influence of the average transmittance T _x becomes hysteresis characteristics hardly. In other words, the modulation voltage reciprocates between the two transmittances T _i and T _j within the hysteresis. The visual transmittance T is (T _i + T _j ) / 2. If the signal frequency of the modulation voltage is too high, the liquid crystal will not follow. If the amplitude of the modulation voltage is too large, black display cannot be performed. Therefore, the frequency and amplitude of the modulation voltage should be set experimentally according to the response speed and the rising voltage of the liquid crystal.

Hereinafter, the light valve of the present invention will be described. FIG. 1 is a configuration diagram of the light valve of the present invention. The components other than the modulation circuit section 4 are the same as those of the conventional light valve, and therefore the description will be focused on the modulation circuit 4. The modulation circuit section 4 has a function of generating a modulation voltage and applying it to the substrate electrode terminal 46 or the counter electrode terminal 45. Further, the voltage generator has a function of applying the reference voltage A to the terminal to which the modulation voltage is not applied among the two terminals. The modulation circuit section has an amplifier inside, and can variably amplify the reference modulation voltage and apply it to the counter electrode terminal 45 or the substrate electrode terminal 46. The liquid crystal panel used for the light valve of this example had a panel thickness of 15 microns, a rising voltage of 5 V, and a voltage at which the transmittance became 100% was 15 V. As a reference voltage, a signal having a frequency of 60 Hz was continuously applied from 0 V to 30 V, and a modulation voltage having a frequency of 120 Hz and a voltage amplitude of 4 V was applied. As a result, the hysteresis characteristics were improved in a halftone region. At this time, the amplitude of the modulation voltage was adjusted within a range that did not affect the display while gradually increasing the amplitude of the modulation voltage while applying no reference voltage signal to the liquid crystal panel.

[0021]

[0022]

As described above, in the method of driving a light valve and a liquid crystal panel according to the present invention, a reference voltage is applied to one of the electrodes.
A modulation voltage is applied to the other electrode to perform gradation display. The polymer-dispersed liquid crystal has a problem that the voltage-transmittance characteristic curves at the time of rise and fall are not the same locus. Conventionally, the gradation display characteristics were inferior. However, the hysteresis characteristics are greatly reduced by the present invention, and The tone display characteristics can be improved.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a light valve of the present invention.

FIG. 2 is a sectional view of a polymer dispersed liquid crystal panel.

FIG. 3 is an explanatory diagram of a hysteresis characteristic.

FIG. 4 is an explanatory diagram of a method for driving a liquid crystal panel according to the present invention.

FIG. 5 is a waveform diagram of a voltage applied to a liquid crystal panel.

FIG. 6 is a configuration diagram of a conventional light valve.

[Explanation of symbols]

 4, 48 Modulation circuit part 34, 49 Water-droplet liquid crystal 35, 50 Polymer

──────────────────────────────────────────────────続 き Continuation of front page (72) Inventor Hideaki Mochizuki 1006 Kazuma Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. (56) References JP-A-61-245142 (JP, A) JP-A-57- 205785 (JP, A) (58) Field surveyed (Int. Cl. ⁶ , DB name) G02F 1/133 G02F 1/1333

Claims (4)

(57) [Claims] 1. A least how one of which drives the polymer and liquid <br/> crystal panel of the polymer dispersion type sandwiching the water droplet-shaped liquid crystal between a pair of transparent electrode substrates, the liquid crystal-panel
A first AC signal is applied to one of the electrodes, and the other electrode has a frequency higher than that of the first AC signal and the droplet-like liquid is applied to the other electrode.
A method for driving a liquid crystal panel, wherein a second AC signal having a frequency equal to or lower than a response frequency of a crystal is applied . The amplitude of 2. A second AC signal, the driving method of the liquid crystal panel according to claim 1, wherein a is less than or equal <br/> the rising voltage of the water droplet-like liquid. Wherein at least one of the liquid <br/> crystal panel of the high molecular dispersion type sandwiching the polymer and water droplet-shaped liquid crystal between a pair of transparent electrode substrates, the first alternating signal consisting of a positive polarity and negative polarity a voltage generating means for <br/> for generating a switching unit for applying switching the positive polarity and negative polarity of the voltage by the voltage generating means is generated at a predetermined period to one electrode of the <br/> liquid crystal panel,
Than the first AC signal to the other electrode of the liquid crystal panel
At a high frequency and below the response frequency of the droplet-like liquid crystal.
Light valve, characterized by comprising a modulation circuit for applying a second alternating signal. 4. The light valve according to claim 3 , wherein the amplitude of the second AC signal is equal to or lower than the rising voltage of the liquid crystal in the form of water droplets .