JPH0579968B2 - - Google Patents

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]

Conventionally, liquid crystal display elements are 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 that were put to practical use were, for example, “Applied Physics Letters.”
(“Applied Physics Letters”) 1971, 18(4)
M. Shatt (M.
Schadt) and W. Helfrich, “Voltage Dependant Optical Activity of a Twisted Nematic Liquid Crystal”
(“Voltage Dependent Optical Activity of a
It was a TN (Twisted Nematic) type liquid crystal shown in "Twisted Nematic 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 Clark and Lagerwall in Japanese Patent Application Laid-open No. 107216/1983 and US Patent No.
It has been proposed in the specification of No. 4367924, etc. As a bistable liquid crystal, a ferroelectric liquid crystal having a chiral smectic C phase (SmC ^* ) is generally used, and in these states, it changes to a first optically stable state in response to an applied electric field. It has the property of taking one of the second optically stable states and printing that state when no electric field is applied, that is, it has bistability, and also has quick response to changes in electric field, high speed, and memory. Widespread use is expected in the field of type display devices and the like.

The threshold characteristic of the ferroelectric liquid crystal element mentioned above is largely dependent on temperature, so when driving such an element by multiplexing, for example,
As disclosed in Publication No. 149899, the drive voltage at high temperatures is set to a lower voltage than the drive voltage at low temperatures, or the drive frequency at high temperatures is set to a higher frequency than the drive frequency at low temperatures (high frame frequency and ) is proposed.

However, the temperature compensation method in which the drive voltage is varied according to temperature changes requires a very large drive voltage on the low temperature side, which has the problem of increasing the cost of the drive circuit. Furthermore, in the temperature compensation method in which the drive frequency is varied in response to temperature changes, the frame frequency becomes smaller at low temperatures, which causes problems in that the writing speed becomes slow and flickering becomes noticeable.

[Summary of the invention]

An object of the present invention is to provide a liquid crystal device that solves the above-mentioned problems. In particular, an object of the present invention is to provide a liquid crystal device that enables temperature compensation over the entire operating temperature range without increasing the amount of variation in frame frequency or drive voltage.

That is, the present invention provides a liquid crystal element having a scanning electrode, a signal electrode, and a ferroelectric liquid crystal disposed between the scanning electrode and the signal electrode, means for applying a scanning selection signal and a scanning non-selection signal to the scanning electrode, A first voltage waveform having one and the other polarity and a first DC component on the one polarity side, with respect to the voltage of the scan non-selection signal, within the period of application of the scan selection signal to the signal electrode. selectively transmitting an information signal and a second information signal having a voltage waveform having an opposite phase with respect to the voltage waveform and a second DC component that is the same DC component as the first DC component; The present invention is characterized by a liquid crystal device having a means for applying a direct current component, and a means for varying the first and second direct current components in accordance with temperature changes.

[Detailed description of aspects of the invention]

The present invention will be explained below with reference to the drawings.

FIG. 10 is a schematic diagram showing a matrix electrode of a cell filled with ferroelectric liquid crystal.

The cell structure 10 shown in FIG. 10 is a cell in which a pair of substrates 1a and 1b made of glass plates are held at a predetermined distance by a spacer 4, and the periphery is bonded with an adhesive 6 to seal the pair of substrates. The structure has a plurality of transparent electrodes 2 on the substrate 1a.
A group of electrodes (for example, a group of electrodes for applying a scanning voltage in a matrix electrode structure) is formed in a strip pattern, and the transparent electrode 2a described above is formed on the substrate 1b.
An electrode group (for example, an information voltage application electrode group in a matrix electrode structure) is formed of a plurality of transparent electrodes 2b intersecting with each other. An inorganic insulating film of SiO ₂ and an organic alignment film of polyvinyl alcohol (PVA) are formed on the substrate provided with the transparent electrode.
Its surface has been subjected to rubbing treatment. The liquid crystal used was an ester-based mixed liquid crystal having the phase series shown below, and had a smectic phase.

Iso ←−−−− 80.5℃Ch ←−−−− 69.1℃SmA ←−−−− 54.4℃SmC ^* ←−−−− −21℃Crystal (CS1014) (Ch: cholesteric phase, SmA: smectic A phase, SmC ^* ; chiral smectic C phase,
Iso: isotropic phase, Crystal: crystal phase) FIG. 1 is a block diagram showing a liquid crystal device of the present invention, and FIGS. 2 and 3 are examples of driving wave types of the present invention.

In Fig. 1, 11 is an FLC (ferroelectric liquid crystal) panel, 12 is a scanning side drive circuit, 13 is a signal side drive circuit, and 14 is a power supply controller, which has one polarity voltage V _{S1 and the other polarity voltage V S1} of the scan selection signal. Information signal voltages V _I and V ⁰ _I with controlled _S2 and DC (direct current voltage) components
(V _I + DC component) is output. 15 is a temperature sensor;
16 is a microprocessor unit.

In the device shown in FIG. 1, when applying a refresh drive in which the scan selection signal is scanned repeatedly, 1
For each frame or field, the microprocessor unit 1b adjusts the driving voltages (V _S1 , V _S2 , V _I and V ⁰ _I ) based on the temperature information from the temperature sensor 15.
The microprocessor unit 16 can select the image information and the frame frequency and send a command to the scanning side drive circuit 12 and the signal side drive circuit 13, and at the same time the microprocessor unit 16 can send the information signal V ⁰ output from the signal side drive circuit 13. The DC offset amount of _I can be controlled by temperature information from the temperature sensor 15.

FIG. 2 is an example of a drive waveform when an information signal with zero DC component is used, and FIG. 3A is an example of a drive waveform when an information signal on which a controlled DC component is superimposed is used.

In Figures 2 and 3A, S _S is the selected scanning waveform applied to the selected scanning line, S _N is the unselected scanning waveform, and I _S or I ⁰ _S is the selected data. The selection information waveform (black) applied to the line is set to I _N or
I ⁰ _N represents a non-selection information signal (white) applied to an unselected data line. Also, in the figure (I _S −
S _S ) or (I ⁰ _S −S _S ) and (I _N −S _S ) or (I ⁰ _N −S _S )
is the voltage waveform applied to the pixels on the selected scanning line, and the pixels to which the voltage (I _S −S _S ) is applied display black, and the voltage (I _N −S _S ) or (I ⁰ _N − The pixel to which S _S ) is applied assumes a white display state.

In the driving examples shown in FIGS. 2 and 3A, the minimum application time Δt of the unipolar voltage applied to the pixels on the selected scanning line corresponds to the time of the write phase t ₂ , and 1
Line clear t ₁ phase time is set to 2Δt. At this time, in the present invention, one line clear phase t ₁
Although it is possible to set the preferable time to 2Δt to 10Δt, it is particularly suitable to set the time of one line clear phase _t1 to 2Δt as shown in the figure. or,
In the driving examples shown in FIGS. 2 and 3A, the maximum amplitude V ¹ _R (=|−V _S |) of the voltage V _R applied to the pixel ( _IN − S _S ) at the 1-line clear phase t ₁ V ¹ _R < |Vsat|
Preferably, V ¹ _R ≦ | Vth between the minimum application time Δt and the inversion threshold Vth.
|, especially 1/3|Vsat|≦V ¹ _R ≦|Vth|. Furthermore, in the driving examples shown in Figures 2 and 3A, the maximum amplitude of voltage V ² _B |V _S2 +V ₁ | and the maximum amplitude of V _S1 are absolute values and saturation based on the minimum application time Δt. The maximum amplitude |V ₁ | of the voltage V ¹ _B is set to a value that does not exceed the inversion threshold Vth based on the minimum application time Δt in absolute value.

In the driving examples shown in FIGS. 2 and 3A, the scan selection signals applied to the selected scan line are V _S1 and -V _S2
AC voltage set to the voltage (positive and negative polarity are
(based on the potential of the unselected scan line), |
Although it is set to V _S1 |=3/2|−V _S2 |, in the present invention, it can be set to |V _S1 |≧|−V _S2 |.
Therefore, in the present invention, the voltage V _R applied to the pixel (I _N −S _S ) or (I ⁰ _N −S _S ) at the one line clear phase t ₁
The maximum amplitude of V ¹ _R is the voltage applied in the write phase t ₂
The maximum amplitude of V ¹ _B | V ₁ | is more than twice or more than three times,
The maximum amplitude V 2 _R of the voltage VR applied to the pixel (IS − S _S ) or (I 0 S −S S ) is preferably set to ₂ times or 3 times, and is applied to the pixel (I _S −S S ) or (I ⁰ _S −S _S ) at the 1 _line clear phase t ¹ . is the write phase
The amplitude can be set to be equal to or greater than the maximum amplitude |V _S2 +V ₂ | of the voltage V ² _B applied at t ₂ . Further, in the present invention, the maximum amplitude of the voltage V ² _B can be set to be twice or more, or three times or more, preferably twice or three times the maximum amplitude of the voltage V ¹ _B.

Figure 3 shows the DC component in the information signals I _S and I _N of Figure 2.
V _DC (DC component based on the voltage of the scan non-selection signal)
V _DC ) represents information I ⁰ _S and I ⁰ _N. The information signals I ⁰ _S and I ⁰ _N shown in FIG. 3A have asymmetrical alternating waveforms to which V _DC is applied, and are different from the voltage polarity of the scan selection signal at the 1-line clear phase t ₁ . ± the voltage that produces a DC component of the same polarity V _DC
It has V ⁰ _I. This voltage ±V ⁰ _I is set to a value smaller than the threshold voltage of the ferroelectric liquid crystal determined by the write phase period t ₂ . Further, the polarity of the DC component V _DC described above is not limited to the polarity described above, and may be the opposite polarity depending on the drive waveform.

FIG. 3B also shows another embodiment of the present invention. In this specific example, the drive pixel on the scanning line has 1
A DC component V _DC of voltage polarity during line clear phase t ₁ is applied.

FIG _{. 4A} shows the _drive waveform shown in FIG.
7.5V, S _N = 0V, Δt = 28μsec) is applied to the ferroelectric liquid crystal element shown in FIG. voltage, T; transmittance). Figure 4 shows white writing by an applied voltage of one line clear phase t ₁ in a pixel (I _S - S _S ) and black writing by an applied voltage of writing phase t ₂ in the same pixel (I _S - S _S ). Transmittance (in arbitrary units) is plotted. The measurement temperature at this time was 27°C.

According to FIG. 4A, the above-mentioned white write operation is possible within the voltage range of ±30V applied to the pixel, but the black write operation may not be possible within the above-mentioned applied voltage range. I understand.

On the other hand, FIG. 4B shows the electro-optical characteristics when the driving waveform shown in FIG. 2 is used under the same conditions except that the measurement temperature was 37° C. According to FIG. 4B, it can be seen that on the high temperature side, when the voltage applied to the pixel is within the voltage range of ±30V, a white write operation and a black write operation can be performed.

On the other hand, Fig. 5A shows that under the condition of a measurement temperature of 27°C,
The drive waveform shown in FIG. 3A mentioned above (however, V _S1 =
15.0V, −V _S2 = −15V, −V ⁰ _I = −6.5V, V ⁰ _I = 8.5V,
It shows the electro-optical characteristics when using S _N = 0 V and Δt = 28 μsec). Figure 5A is Figure 4A
According to this electro-optical characteristic, by superimposing a −DC component V _DC (+1.0V) and creating an asymmetrical alternating information signal, At the measurement temperature of 27° C., that is, on the low temperature side, even if the voltage applied to the pixel is within the voltage range of ±30 V, the white write operation and the black write operation can be performed.

Further, FIG. 5B shows the electro-optical characteristics when the driving waveform shown in FIG. 3 is used under completely the same conditions except that the measurement temperature is 37° C. Fifth
Figure B shows an electro-optical characteristic that is different from the electro-optical characteristic shown in Figure 4B.According to this electro-optical characteristic, when the aforementioned asymmetrical alternating information signal is used, drive on the high temperature side is shown. It can be seen that the voltage margin decreases.

Therefore, in a preferred embodiment of the present invention, the DC offset amount of the DC component V _{DC is set small on the low temperature side within the operating temperature range of the ferroelectric liquid crystal panel, and the DC offset amount of the DC component V DC is} set small on the high temperature side. It is appropriate to set it large. Also, the DC component V _DC
As a method for varying the amount of DC offset, for example, a method of changing the amount of DC offset in stages according to a rise (fall) in temperature can be used.

6A to 6C are other preferred examples of drive waveforms used in the present invention. In FIG. 6, voltage V _c is a voltage for clearing all or a predetermined number of pixels at once prior to writing, and is applied to, for example, the scanning electrodes all at once. S _S is a scan selection signal having an alternating voltage of 2V ₀ and -2V ₀ , and S _N is a scan non-selection signal set to a reference voltage of 0. _IS is an information signal for inverting a cleared pixel, or I _N is an information signal for retaining a cleared pixel, and these information signals are combined with scan selection signals that are sequentially applied to the scan electrodes. The signals are selectively applied to the signal electrodes in synchronization. In the figure, S ⁰ _S , I ⁰ _S and I ⁰ _N are signals obtained by adding a DC component V _DC to the above-mentioned S _S , I _S and I _N , respectively, and form an asymmetrical alternating voltage. This DC component V _DC can be a DC component −V _DC with the opposite polarity to the voltage V _C (3V ₀ ), and this DC component −V _DC can also be superimposed on the information signal voltage or the scan selection signal voltage. It is possible. At this time, in the present invention, the DC component V _DC can be varied between 0 and a predetermined offset amount within the operating temperature range of the ferroelectric liquid crystal panel. Further, the polarity of the above-mentioned DC component -V _DC is not limited to the above-mentioned polarity, and may be the opposite polarity.

Also, the amount of offset of the DC component V _DC varies depending on the liquid crystal cell and drive waveform, but it is ±0.01V to ±2.0V,
Preferably, a range of ±0.05V to ±1.0V is suitable.

Note that polarity in this specification represents positive polarity and negative polarity with reference to the voltage of the scan non-selection signal.

In a preferred embodiment of the present invention, there is a step in which each scanning line is sequentially written using the driving waveform described above (the period of this step is one frame or one field).
A still image or a moving image can be displayed by repeating cyclically and sequentially.

FIG. 7 is a characteristic diagram showing the voltage application time dependence of the saturation threshold Vsat and the inversion threshold Vth. In FIG. 7, 71 indicates a characteristic curve of the inversion threshold Vth, and 72 indicates a characteristic curve of the saturation threshold Vsat.

In addition, the "inversion threshold Vth" described in this specification means that when a voltage that causes a pixel under one optical state to change to the other optical state is applied, the optical rate (transmittance or light shielding rate) of the pixel is equal to the applied voltage. This is the voltage when it starts to change rapidly in response to the rise in voltage, and the voltage in Figure 8 is
It is represented by Vth. Also, “saturation threshold Vsat”
is the voltage at which the change in optical index according to the increase in applied voltage is saturated, and is the voltage in Figure 8.
Represented by Vsat. FIG. 9 schematically shows the alignment state of ferroelectric liquid crystal within a pixel in response to an increase in applied voltage. Voltage b in Figure 8, c in Figure 9
corresponds to the voltage c in FIG. 8, d in FIG. 9 corresponds to the voltage d in FIG. 8, and e in FIG. 9 corresponds to the saturation threshold voltage Vsat in FIG. 8, respectively. According to Figures 9a-e,
It has been revealed that the area of the black domain 91 partially formed in the white domain 92 increases as the applied voltage increases.

As the liquid crystal having bistability that can be used in the present invention, chiral smectic liquid crystal having ferroelectricity is most preferable, and among these, liquid crystal in chiral smectic phase (SmC ^* ) or H phase (SmH ^* ) is preferable. Are suitable. Regarding this ferroelectric liquid crystal,
“Le Univers de Physique Letters”
(“Le Journal de Physic letters”) Volume 36 (L-
69), 1975's "Ferroelectric Liquid Crystal"
Crystals”; “Applied Physics Letters”, Volume 36 (11
``Submicron Second Bistable Electro-Optical Switching in Liquid Crystal'' (``Submicro Issue''), 1980
Second Bistable Electrooptic Switching in
``Liquid Crystal''; ``Solid State Physics 16 (141) 1981 ``Liquid Crystal'', etc., and the ferroelectric liquid crystal disclosed therein can be used in the present invention.

More specifically, examples of ferroelectric liquid crystal compounds used in the method of the present invention include decyloxybenzylidene-P'-amino-2-methylbutylcinnamate (DOBAMBC), hexyloxybenzylidene-P'-amino- 2-chloropropyl cinnamate (HOBACPC) and 4-o-(2-methyl)
-butylresolcylidene-4'-octylaniline (MBRA8) and the like.

When constructing an element using these materials, the element must be 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 SmC ^* phase or SmH ^* phase. I can do it.

In addition, in the present invention, in addition to the above-mentioned SmC ^* and SmH ^* ,
It is also possible to use a ferroelectric liquid crystal that exhibits a chiral smect F phase, I phase, J phase, G phase, or K phase.

FIG. 11 schematically depicts an example of a ferroelectric liquid crystal cell. 111a and 111b are In ₂
It is a substrate (glass plate) coated with transparent electrodes such as O ₃ , SnO ₂ and ITO (indium-tein-oxide), and between them, a liquid crystal molecular layer 112 is made of SmC ^* phase oriented perpendicular to the glass surface. Liquid crystal is enclosed. A thick line 113 represents a liquid crystal molecule, and this liquid crystal molecule 113 has a dipole moment (P⊥)1 in the direction perpendicular to the molecule.
It has 14. When a voltage equal to or higher than a certain threshold is applied between the electrodes on the substrates 111a and 111b, the helical structure of the liquid crystal molecules 113 is unraveled, and the orientation direction of the liquid crystal molecules 113 is changed so that all dipole moments (P⊥) 114 are oriented in the direction of the electric field. can be changed. The liquid crystal molecules 113 have an elongated shape and exhibit refractive index anisotropy in the major and minor axis directions. Therefore, for example, if polarizers are placed above and below the glass surface in a crossed nicol positional relationship, polarizers can be placed above and below the glass surface. For example, it is easily understood that the liquid crystal optical modulation element is a liquid crystal optical modulation element whose optical characteristics change depending on the polarity of applied voltage. Furthermore, if the thickness of the liquid crystal cell is made sufficiently thin (for example,
1μ), the helical structure of the liquid crystal molecules unravels even in the absence of an applied electric field, as shown in Figure 12, and the dipole moment Pa or Pb increases upward by 1μ.
24a or downward 124b. When an electric field Ea or Eb of different polarity above a certain threshold value is applied to such a cell for a predetermined period of time as shown in FIG. The liquid crystal molecules change direction, and accordingly the liquid crystal molecules are either in the first stable state 123a or in the second stable state 123a.
is oriented in one of the stable states 123b.

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. For example, change the second point to the first
To explain with reference to FIG. 2, when an electric field Ea is applied, the liquid crystal molecules are oriented in a first stable state 123a, and this state remains stable even when the electric field is turned off. or,
When an electric field Eb in the opposite direction is applied, the liquid crystal molecules are oriented to a second stable state 123b 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 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, generally from 0.5μ to 20μ, especially
1μ to 5μ is suitable.

〔Effect of the invention〕

When driving a ferroelectric liquid crystal element, it is possible to perform temperature compensation over the entire operating temperature range for driving the ferroelectric liquid crystal element without greatly varying the frame frequency and driving voltage, and at a low voltage. can.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing a liquid crystal device of the present invention. FIGS. 2 and 3 are waveform diagrams of the driving waveforms used in the present invention, and FIGS. 4 and 5 are characteristic diagrams showing electro-optical characteristics when the driving waveforms are used. FIG. 6 is a waveform diagram of another drive waveform used in the present invention. FIG. 7 is a characteristic diagram showing the dependence of applied voltage and application time on the inversion threshold and saturation threshold of a ferroelectric liquid crystal pixel. FIG. 8 is a characteristic diagram showing the amount of transmitted light characteristics when a voltage is applied to the pixel, and FIG. 9 is an explanatory diagram schematically showing the domain state at that time. FIG. 10 is a plan view of the ferroelectric liquid crystal element used in the present invention. Figures 11 and 12
The figure is a perspective view of a ferroelectric liquid crystal element used in the present invention.

Claims (1)

[Claims] 1. A liquid crystal element having a scanning electrode, a signal electrode, and a ferroelectric liquid crystal disposed between the scanning electrode and the signal electrode, means for applying a scanning selection signal and a scanning non-selection signal to the scanning electrode, and a means for applying a scanning selection signal and a scanning non-selection signal to the signal electrode, A first information signal having a voltage waveform having one and the other polarity and a first DC component on the one polarity side, with respect to the voltage of the scan non-selection signal, within the scan selection signal application period; means for selectively applying a second information signal having a voltage waveform having an opposite phase to the voltage waveform and a second DC component that is the same DC component as the first DC component, depending on the information; and a liquid crystal device comprising means for varying the first and second direct current components in accordance with temperature changes.