JPH0651280A - Driving method for ferroelectric liquid crystal element - Google Patents

Abstract

PURPOSE:To accelerate the writing speed by specifying the voltage pulse applied to a scanning electrode and a signal electrode. CONSTITUTION:The pre-selection scanning voltage +Vr3 (or -Vr3) with two pulse widths 2t is applied immediately before the selective scanning voltage +Vr1, -Vr1 (or -Vr1, +Vr1) of a scanning electrode is applied, + or -Vr2 is applied as the nonselective scanning voltage of the scanning electrode, + or -Vc1 with the pulse width (t) is applied in the opposite polarity to the selective scanning voltage of the scanning electrode, + or -Vc2 is applied as the nonselective signal voltage of the signal electrode, and the nonselective scanning voltage of the scanning electrode is applied at an intermediate value between the selective signal voltage and the nonselective signal voltage of the signal electrode. The nonselective scanning voltage Vr2 satisfies the condition expressed by the inequality I, it satisfies the conditions expressed by inequalities II, III if the polarities of voltage pulses contained in the selective signal voltage and nonselective signal voltage of the signal electrode are the same, where Vth is the threshold value voltage at which the optical state is inverted in the time width (t) of the voltage pulse, and it satisfies the conditions expressed by inequalities IV, V if the polarities are opposite.

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 ferroelectric liquid crystal element, and more particularly to a time division driving method for a ferroelectric liquid crystal element.

[0002]

2. Description of the Related Art In 1980, NAClark, ST Lagerwal
l allows the so-called SSFLCD (Surface Stabilized Ferroel
Since the ectric Liquid Crystal Display) was proposed, the ferroelectric liquid crystal element has been attracting attention as a next-generation liquid crystal element because it has characteristics such as high-speed response and bistable memory characteristics which are not present in the nematic liquid crystal element. Since then, it has been found that the ferroelectric liquid crystal device can be driven in a time-division manner, and thus applied research as a display device has been actively conducted.

Ferroelectric liquid crystals correspond to chiral smectic liquid crystals and have chirality along the cone, but in a thin cell, the chirality disappears.
It comes in two stable states. FIG. 1 shows the structure of a ferroelectric liquid crystal cell (hereinafter referred to as a liquid crystal cell). Reference numeral 1 is a substrate on which an alignment film 3 is formed, and a ferroelectric liquid crystal 2, for example, S
It holds mC ^* liquid crystal. When the alignment films on the upper and lower substrates are processed so as to have uniaxial alignment, the uniaxial alignment directions of both substrates are aligned with the normal direction of the smectic liquid crystal layer, and the liquid crystal molecules are almost parallel to the substrate. Orient it. When viewed from above, two bistable states, that is, a first stable state 5 tilted by + θ and a second stable state 6 tilted by −θ with respect to the normal direction 4 of the layer, are generated, and each spontaneous polarization 7 The orientation is upward and downward.

Since the liquid crystal molecules move so that the directions of the spontaneous polarization are aligned with the direction of the applied electric field, one of the bistable states is selected by the positive and negative DC pulses. The two polarizing plates 8 optically identify the two stable states. For example, if the first stable state is a light blocking state (hereinafter referred to as black), it is converted into a second stable state, that is, a light transmitting state (hereinafter referred to as white).

FIG. 2 shows a matrix electrode for applying a drive voltage to the liquid crystal layer. Reference numeral 9 is a scanning electrode (hereinafter referred to as segment), and 10 is a signal electrode (hereinafter referred to as common). FIG. 3 shows a drive waveform applied to one matrix picture element (hereinafter referred to as a dot) in the line sequential drive using the AC bias averaging method. During the selection period in the first frame, positive and negative (common 10 reference) pulses P ₁ and P ₂ having a peak value equal to or higher than the threshold voltage in this pulse width are continuously applied. The positive pulse P ₁ causes the liquid crystal molecules to be aligned in the second stable state, and the subsequent negative pulse P ₂ causes switching to be aligned in the first stable state.

Then, this state is maintained by the application of the AC pulse during non-selection. This is because the peak value of the AC pulse is below the threshold value. Thus, in the first frame, the first steady state black is written and maintained, followed by the second frame,
Also, the display state is switched. However, in this figure, P ₄ and P ₅ of the second frame are equal to or less than the threshold value, that is, the drive waveform in the state where the non-selected common signal is input, so that white is not written, but white is written. Black is maintained.

By the way, in such a general waveform, there is a problem that the contrast is lowered because the liquid crystal molecules are vibrated by the AC bias portion. Therefore, there is a method of increasing the contrast by using a liquid crystal having a negative dielectric anisotropy and increasing the value of θ shown in FIG. 1, that is, an apparent cone angle, by the dielectric interaction between the electric field and the liquid crystal. . This is an AC stabilization method that utilizes the AC stabilization effect. As a driving method for utilizing such an AC stabilization method, it is a general method to superimpose a high frequency voltage on the driving waveform as shown in FIG.

[0008]

FIG. 4 is a drive waveform diagram in which a high frequency voltage is superimposed on FIG. However, in order to superimpose such a high frequency, the drive circuit must be complicated. In addition, in the drive waveform as shown in FIG. 4, when writing an image, it is necessary to write white and then black, or to write black and then white, or to write one image. It requires a frame and takes a long time to write. There has been a demand for a drive waveform in which white and black can be simultaneously written in one frame and which also has an AC stabilization effect.

[0009]

SUMMARY OF THE INVENTION The present invention aims to solve the above-mentioned problems of the prior art, and the selective scanning voltages + Vr1, -Vr1 (or -Vr1, + Vr) of the scanning electrodes.
Immediately before the pulse of 1) is applied, + V of 2t for 2 pulses
r3 (or −Vr3) is applied as the preselection scanning voltage,
± Vr2 is applied as the non-selective scan voltage of the scan electrodes,
± Vc1 of pulse width t as the selection signal voltage of the signal electrode
Is applied with the opposite polarity to the selection scanning voltage of the scanning electrode, ± Vc2 is applied as the non-selection signal voltage of the signal electrode, and the non-selection scanning voltage of the scanning electrode is between the selection signal voltage of the signal electrode and the non-selection signal voltage. The value of is to be driven.

FIG. 5 shows the pre-selection scan voltage, the selection scan voltage, and the non-selection scan voltage applied to one scan electrode. The previous selection scan voltage is applied immediately before the selection scan voltage is applied, and has the same polarity as the first pulse of the selection scan voltage. FIG. 6 shows the voltage waveforms applied to the scan electrodes and the signal electrodes and the voltage waveforms applied to the respective intersections, that is, the actual dots. The voltage waveform applied to the dots shows the value of (scanning voltage-signal voltage). The non-selected scan electrode ± Vr2 of the scan electrode is set to an intermediate value between the select signal voltage ± Vc1 of the signal electrode and the non-select signal voltage ± Vc2.
In FIG. 6, since the selection scanning voltage and the non-selection scanning voltage of the signal electrode have opposite polarities, the relationship of Vr2 = (Vc1-Vc2) / 2 is established. Therefore, a voltage of ± (Vc1-Vr2) is applied to the non-selected scan lines, and a voltage of the opposite polarity is applied to the picture element depending on whether the selection signal voltage is applied to the signal electrode or the non-selection signal voltage is applied. Is applied.

[0011]

According to the present invention, by the above means, the white and black states are simultaneously written by one writing, the writing speed of one screen is improved, and the voltage applied to the liquid crystal is made alternating. Can be prevented from being deteriorated, and further, an AC stabilizer effect can be provided.

[0012]

The present invention will be described in detail below with reference to the drawings. FIG. 7 shows an example of waveforms when selection signals and non-selection signals are applied from the signal electrodes to the pre-selection scan voltage and selection scan voltage portions of the scan electrodes. (A) is a case where a selection signal is applied to both the pre-selection scanning voltage section and the selection scanning voltage section of the scanning electrode from the signal electrode, and two pulses of the pre-selection scanning voltage section and one of the selection scanning voltage section After one stable state (for example, black) of the bistable state is selected for the voltage application for three pulses of the eye pulse, white is written by the second pulse of the selected scan voltage of the scan electrode. That is,
The state of white is retained after the black pulse is written by the upward pulse and the white pulse is written by the downward pulse of (a).

FIG. 7B shows a case where the non-selection signal voltage is applied from the signal electrode to the pre-selection scanning voltage portion of the scan electrode and the selection signal voltage is applied from the signal electrode to the selection scanning voltage portion of the scanning electrode. It is a waveform. Also in this case, black is written by the voltage of 3 pulses of the first pulse of the previous selection scanning voltage section of the scanning electrode and the first pulse of the selection scanning voltage section, and then the second of the selection scanning voltage of the scanning electrode is written. The portion of the pulse of, is written in white by the pulse with the diagonal line downward. Then, the white state is retained.

If Vr1 + Vc1> Vth, black is always written by the upward pulse, and then white is written. Then, in order to maintain the written state in the non-selective scan voltage portion of the scan electrode,
It is sufficient if (Vc1-Vr2) <Vth. This is due to the non-selective scan voltage portion of the scan electrodes in FIGS.
A waveform with a pulse width of 2t is applied, which is Vc1- when the signal applied from the signal electrode switches from the selection signal to the non-selection signal or from the non-selection signal to the selection signal. This is because two Vr2 pulses will overlap.

In FIG. 7C, black is written by three pulses of two pulses of the pre-selection scanning voltage section of the scanning electrode and one pulse of the selection scanning voltage section. The conditions for this are: Vr3 + Vc1- (Vc1-Vr3) + Vr1-Vc2
> Vth That is, if 2Vr3 + Vr1-Vc2> Vth, black can be written. This also applies to the case of (d). Then, in both (c) and (d), since the non-selection signal is applied from the signal electrode to the selected scanning voltage portion of the scanning electrode, it is not written in white but held as black. 2 of the selective scanning voltage part of the scanning electrode, which is a pulse for writing white
The value of the second pulse (Vr1-Vc2) may be Vth or less.

That is, Vc1-Vc1 <Vth. As described above, driving is performed in which black is written by two pulses of the pre-selection scanning voltage section of the scan electrode and the first three pulses of the selection scanning voltage section, and white is written by the second pulse of the selection scanning voltage section. You can In the case of a ferroelectric liquid crystal device, such writing is possible because the transition between two stable states switches over a certain high energy state to the stable state of the other low energy state. This is because switching occurs if the product of the voltage applied and the application time exceeds a certain value.

Explaining with reference to FIG. 7D, in order to write in black by the two pulses of the pre-selection scanning voltage section of the scanning electrode and the first pulse of the selection scanning voltage section, in order to write in black, Assuming that the threshold voltage is Vth, (Vr3−Vc2) × t + (Vr3 + Vc2) × t +
The condition is (Vr1 + Vc2) × t> Vth × t, and 2Vr3 × t + Vr1 × t−Vc2 × t> Vth × t, that is, 2Vr3 + Vr1-Vc2> Vth, and the same expression as in FIG. Be derived.

The waveform examples of the present invention have been described above with reference to FIGS. 6 and 7. In this case, the selection signal voltage ± V of the signal electrode is used.
c1 and the non-selection signal voltage ± Vc2 have opposite polarities, and Vr1-Vc2 <Vth <Vr1 + Vc12 (Vc1-Vr2) <Vth <Vr1 + 2Vr3-V
If c2 is satisfied, one stable state of the bistable state is written at one end and then the other stable state is selectively written, that is, a drive for writing both white and black in one scan. Is possible.

Further, FIG. 8 shows a waveform example in the case where the selection signal voltage ± Vc1 of the signal electrode and the non-selection signal voltage ± Vc2 have the same polarity. In this case, white and black are obtained by one scanning. The writing condition is Vr1 + Vc2 <Vth <Vr1 + Vc1 2 (Vc1-Vr2) <Vth <Vr1 + 2Vr3 + V
It becomes c2.

In the drive waveform of the present invention, the non-selection scanning voltage ± Vr2 of the scanning electrode is set to an intermediate value between the selection signal voltage ± Vc1 of the signal electrode and the non-selection signal voltage ± Vc2.
An AC waveform of ± (Vc1-Vr2) is applied to the non-selection scanning unit. The pulse width may be 2t when the signal voltage applied from the signal electrode is switched between the selected signal voltage and the non-selected signal voltage in the case of t, and the voltage Vst at which the AC stabilization effect appears when the pulse width is 2t. Than (V
If c1−Vr2) is large, the AC cone stabilization effect widens the apparent cone angle between the two bistable states, and the contrast can be improved. Since the AC stabilization effect occurs when Δε of the liquid crystal is negative, the liquid crystal of Δε <θ must be used in order to use the AC stabilization effect.

As described above, in the application of the scanning signal to the scanning electrodes, the pre-selection scanning voltage is provided immediately before the selection scanning voltage to write in one of the bistable states, and then the other display state is selectively selected. Depending on the writing method, the selection scanning period includes the period of the pre-selection scanning voltage as conventionally proposed. That is, in the case of the scanning waveform as shown in FIG. 5, four pulses are set as one scanning period, but in the present invention, the scanning period for one line can be halved and the time for writing one screen can be halved. .

Further, the selection scanning voltage ± Vr2 of the scanning electrode is set to the selection signal voltage ± Vc1 of the signal electrode and the non-selection signal voltage ± V.
By setting the value between c2, the AC voltage having the value of Vc1-Vr2 is applied to the non-selective scanning voltage, and as described above, if 2 (Vc1-Vr2) <Vth, the written state is stable. Held in. Further, from the voltage Vst at which the Δst of the ferroelectric liquid crystal is negative and the AC stabilization effect occurs when the pulse width is 2t,
If the value of Vc1-Vr2 is large, the apparent cone angle is enlarged by the AC stabilization effect in the non-selective scanning portion, and high contrast is achieved. This drive waveform does not need to have a high-frequency AC waveform superimposed and applied, unlike the drive waveforms that use the AC stabilization effect up to now, and has the advantage that a simple drive circuit is sufficient.

6 and 8 show the driving method of the present invention in a general form, the non-selective scanning voltage of the scanning electrodes can be set to 0V as shown in FIG. Then, the selection signal voltage ± Vc1 of the signal electrode and the non-selection signal voltage ± Vc2 have the same value and opposite polarities. That is, Vc1 = Vc2. Therefore, the output value from the driver can be reduced and the driving circuit can be simple. Further, as shown in FIG. 10, it is possible to set the non-selection signal voltage of the signal electrode to 0V. In that case, Vc2 = 0V, so 2Vr2 = V
c. In this case as well, the output value from the driver can be reduced and the circuit can be simplified as in the above case.

Further, Vr1 = Vr3 may be set in order to reduce the output value of the driver. In FIG. 9, Vr1 = V
With r3, a simple waveform is obtained. By the way, in this driving method, a complete AC voltage is applied to the signal electrodes, but since the pre-selected scan voltage is a DC voltage in the waveform applied to the scan electrodes, a DC voltage component of this amount must be applied. I don't get it. In the case of a normal liquid crystal element, if a direct current voltage is continuously applied, the life is shortened, which is not preferable. Therefore, the scanning waveform applied to the scanning electrodes can be inverted every frame so that the DC component is not applied in two frames.

For example, if FIG. 10 has a specific waveform,
This waveform is set as an even frame, the polarity of the voltage applied to the scan electrode shown in FIG. 11 is inverted, the polarity of the voltage applied to the signal electrode is inverted, and the drive waveform in which selection and non-selection are switched is set as an odd frame. The DC component can be canceled by continuously applying the voltage.

If the drive waveform in FIG. 10 is a waveform in which black is first written and then white is selectively written, FIG. 11 shows a waveform in which white is once written and then black is selectively written. Therefore, for example, in order to continue writing black, a non-selection signal is continuously applied to the signal electrode in the even numbered frame, and in the odd numbered frame, white is written once and then black is written, that is, in FIG. The ± Vc1 indicated as the non-selection signal voltage will be continuously applied.

Further, as a method for achieving complete alternating current, FIG.
As shown in FIG. 2, in the application of the scan signal to the scan electrode, immediately before the application of the pre-selection scan voltage, Vr2 having the opposite polarity to the pre-selection scan voltage is applied and the pulse width is set to 2t (this voltage pulse is As shown in FIG. 12, hereinafter, referred to as AC scanning voltage), the DC component is canceled by the preselection scanning voltage and the AC scanning voltage, so that the scanning waveform applied to the scanning electrodes is completely AC-converted. Even during the period in which this AC scanning voltage is applied, the selection and non-selection signals are applied from the signal electrodes, but immediately after that, the pre-selection scanning voltage and the selection scanning voltage are written in, so the AC scanning voltage is applied. Even if the display state changes during the period, it does not affect the overall display state.

By adopting this method, as described with reference to FIGS. 10 and 11, complete alternating current can be achieved without reversing the polarities of the voltage waveforms applied to the scan electrodes in even and odd frames. FIG. 13 shows the voltage waveform (a) applied to the picture element when the drive waveform of the present invention is applied, and the transmitted light intensities (b) and (c) of the picture element at that time. As the driving waveform, an example in which an AC scanning voltage unit is provided as a scanning waveform to be applied to the scanning electrodes so as to be completely AC and Vr1 = Vr2 is shown. It is a waveform in which a selection signal is applied from the signal electrode to each of the alternating scan voltage section, the pre-select scan voltage section, and the select voltage section. The transmitted light intensity at that time is shown in (b) and (c), but (b) is for a general ferroelectric liquid crystal element having no AC stabilizer effect, and (c) is the AC stabilizer effect. Of a ferroelectric liquid crystal device having The display states before the selective scanning are both black and white, respectively.

In the waveform of FIG. 13, since the voltage of the same peak value is applied to the alternating voltage section, the pre-selection voltage section and the selection voltage section, all the display states are switched by the pulse of the peak value. Therefore, in the alternating voltage section, one end is written in black,
It is written in white by the first pulse of the previous selection voltage section and the selection voltage section, and then written again in black by the second pulse of the selection voltage section. Comparing (b) and (c), it can be seen that the display contrast is better in (c) where the AC stabilization effect due to the AC waveform of the non-selected voltage portion is apparently produced.

On a glass substrate having 200 scanning electrodes and a glass substrate having 200 signal electrodes, 1H,
1H, 7H-dodecafluoro-1-heptanol was spinner coated with an aligning agent having 1% of diisopropyl polyfumarate dissolved therein, heated and dried, and then rubbed in parallel so that the rubbing directions were the same. A cell was assembled so that the gap between the two substrates was 1.5 μm, and the following liquid crystal composition was injected and sealed.

[0031]

[Chemical 1]

Polarizing plates were placed in crossed Nicols on the assembled liquid crystal cell and connected to a module substrate, and the contrast was measured with an actual drive waveform. As the drive waveform, the waveform shown in FIG. 2 having values of Vr1 = Vr3 = 10V and Vr2 = 8V is applied to the scan electrodes, and Vc1 = 16V is applied to the signal electrodes.
The signal voltage waveform shown in FIG. 10 was applied.

The ON / OFF contrast of the display can be obtained from the pulse width of 33 μs to 50 μs, the contrast is better when the pulse width is small, and the pulse width is 40 μse.
At c, 16: 1 was obtained as the ON-OFF contrast of the picture element portion. This is the drive waveform of ± 8V and pulse width of 40μsec that is applied when not selected.
It shows that the stabilizer effect is generated.

When the pulse width was 40 μsec, one selective scanning period was 80 μsec, and a screen rewriting speed of 62.5 screen / sec was obtained with this 200 liquid crystal cells.

[0035]

By using the driving method of the present invention, black and white can be written by one scanning in a ferroelectric liquid crystal device having a matrix electrode structure, and the rewriting speed of one screen can be doubled. did it. Furthermore, the voltage applied to the picture element was completely AC, and the problem of element deterioration when a DC voltage was applied could be eliminated. Further, by using the ferroelectric liquid crystal with Δε <0, the AC stabilization effect can be provided and high contrast can be achieved.

[Brief description of drawings]

FIG. 1 is a perspective view of a ferroelectric liquid crystal.

FIG. 2 is an electrode layout diagram of a liquid crystal cell.

FIG. 3 is a conventional drive waveform diagram of a ferroelectric liquid crystal cell.

FIG. 4 is a conventional drive waveform diagram of a ferroelectric liquid crystal cell.

FIG. 5 is a scan voltage waveform diagram applied to scan electrodes.

FIG. 6 is a diagram showing a scanning voltage waveform, a signal voltage waveform, and a voltage waveform applied to a pixel.

FIG. 7 is a voltage waveform diagram applied to a pixel.

FIG. 8 is a diagram showing a scanning voltage waveform, a signal voltage waveform, and a voltage waveform applied to a pixel.

FIG. 9 is a diagram showing a scanning voltage waveform, a signal voltage waveform, and a voltage waveform applied to a pixel.

FIG. 10 is a diagram showing a scanning voltage waveform, a signal voltage waveform, and a voltage waveform applied to a pixel.

FIG. 11 is a diagram showing a scanning voltage waveform, a signal voltage waveform, and a voltage waveform applied to a pixel.

FIG. 12 is a scan voltage waveform diagram applied to scan electrodes.

FIG. 13 is an output showing the voltage waveform applied to a pixel and the transmitted light intensity when the waveform is applied.

[Explanation of symbols]

 1 Substrate 2 Deflection Plate 8 Polarizing Plate 9 Scanning Electrode 10 Signal Electrode

Claims (4)

[Claims] 1. A ferroelectric liquid crystal composition is sandwiched between a substrate having a plurality of scanning electrodes and a substrate having a plurality of signal electrodes,
In a method of driving a ferroelectric liquid crystal device, in which a pixel is formed at the intersection of the scan electrode and the signal electrode and a stable optical state is inverted, the pulse width t and the size are increased during a selection period of the scan electrode. Sa V
A selective scan voltage including two voltage pulses of r1 having different polarities is applied to the scan electrode, and a non-selective scan voltage that does not exceed the magnitude Vr2 is applied to the scan electrode during a non-selection period of the scan electrode, In the period 2t immediately before the selection of the scan electrode, the polarity is the same as that of the first voltage pulse of the selection scan voltage, the pre-selection scan voltage having the magnitude Vr3 is applied to the scan electrode, and the signal electrode is A selection signal voltage including a bipolar voltage pulse having a pulse width t and a magnitude Vc1 and having a polarity opposite to that of the selection scanning voltage and a non-selection signal voltage having a pulse width t and a magnitude Vc2 are applied. When writing the brightness information in the picture element, the non-selection scanning voltage Vr2 satisfies the following equation (1), and Vc1>Vr2> Vc2 (1) Threshold at which the optical state is inverted in the time width t of the voltage pulse. Value voltage is Vt When the polarity of the voltage pulse included in the selection signal voltage and the non-selection signal voltage of the signal electrode is the same, the following expressions (2) and (3) are satisfied, and Vr1 + Vc2 <Vth <Vr1 + Vc1 ( 2) 2 (Vc1-Vr2) <Vth <Vr1 + 2Vr3 + Vc2 (3) When the polarity of the voltage pulse included in the selection signal voltage and the non-selection signal voltage of the signal electrode is opposite, the following formula (4) And (5) are satisfied: Vr1-Vc2 <Vth <Vr1 + Vc1 (4) 2 (Vc1-Vr2) <Vth <Vr1 + 2Vr3-Vc2 (5) A method for driving a ferroelectric liquid crystal element. . 2. AC when the time width of the voltage pulse is 2t
The method for driving a ferroelectric liquid crystal device according to claim 1, wherein Vst <Vc1-Vr2 (6) that satisfies the following expression (6), where Vst is a voltage at which the stabilizing effect occurs. 3. The polarity of the scanning signal voltage applied to the scanning electrodes is reversed between the even frame and the odd frame, the polarities of the signal electrodes are also reversed in synchronization with it, and the selection-non-selection signals are reversed. The method for driving a ferroelectric liquid crystal element according to claim 1, wherein 4. A pulse of 2t having a polarity opposite to that of the pre-selection scanning voltage Vr3 corresponding to 2t is applied immediately before the pre-selection scanning voltage, where t is a time width of the voltage pulse of the rectangular wave AC to be applied. The method for driving a ferroelectric liquid crystal element according to claim 1, wherein