JP2615690B2 - Driving method of optical modulation element - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for driving an optical modulation element which is effective for a display device having a memory property such as a ferroelectric liquid crystal panel.

2. Description of the Related Art In recent years, in the field of information devices centered on computers and the field of video devices centered on televisions, video tape recorders, and the like, demand for large-screen, thin display devices has been increasing. As this type of display device, a liquid crystal display device having the characteristic of low power consumption is widely used.

Since the ferroelectric liquid crystal panel itself has a memory property, the display quality does not deteriorate even when time-division driving is performed.

Hereinafter, an example of a method of driving a ferroelectric liquid crystal panel as an example of a method of driving a conventional optical modulation element will be described with reference to the drawings.

As a matrix driving method for a ferroelectric liquid crystal panel, there is a driving method in which a conventional voltage averaging method is slightly modified. FIGS. 5 and 6 show an example of the driving voltage waveform when the pattern shown in the configuration diagram of the matrix panel in FIG. 4 is displayed. In FIG. 4, 11, 12, 2
Numerals 1 and 42 denote off pixels and numerals 22, 31, 32 and 41 denote on pixels, respectively. FIG. 5 (a) is a voltage waveform diagram applied to the off pixel 21, and FIG. 21 is a schematic diagram of the amount of transmitted light. FIG. 6 (a) is an ON pixel shown in FIG.
FIG. 6B is a schematic diagram showing the amount of transmitted light of the ON pixel 22. The drive waveform is a waveform based on the 1/4 bias voltage averaging method, and the polarity of the pulse and the on-voltage and the off-voltage are inverted within one scanning period (1H). When an absolute value voltage higher than the threshold voltage Vt is applied, the optical state is inverted, but when an absolute value voltage smaller than the threshold voltage Vt is applied, the optical state does not change. Therefore, the off state can be determined by the first two pulses of the scanning period, and the on state can be determined by the second two pulses. For example, Harada, Taguchi, Iwasa,
Kai: S.I.D '85 Digest, 1985,
135 pages (T.HARADA, M.TAGUCHI, K.IWASA, M.KAI: SID '85 Di
gest (1985) p.131].

Problems to be Solved by the Invention However, in the conventional method, the on-voltage and the off-voltage are inverted within one scanning period, so that the driving circuit becomes complicated and the minimum time of one pulse is restricted. Since four pulses are required during the scanning period, there is a problem that the display capacity cannot be increased when the frame frequency is fixed.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems and to provide a method for driving an optical modulation element which enables large-capacity display.

Means for Solving the Problems In order to solve the above problems, a method for driving an optical modulation element according to the present invention selects at least two scan electrodes at the same time and divides a scan electrode selection period into at least two sections. At least one of the scanning electrodes simultaneously selected in each of the sections is selected so as to be different from each other, and each of the sections is divided into two phases, and the phases of the two phases are mutually different in each of the sections. A voltage having a different polarity, the second and third phases in the selection period bringing the optical modulator into the first or second stable state, and the fourth phase into the optical modulator. A first voltage for inverting the stable state of the optical modulator or a second voltage for maintaining the stable state of the optical modulation material is applied to the scan electrode even when the stable state of each pixel is not changed. Electric The polarity of the voltage is inverted for each frame cycle, and the signal voltage applied to the signal electrode is relatively changed for each frame cycle so that the stable state of each pixel is the same as the state of the previous frame within the selection period, The voltage waveform applied to each pixel is made different for each frame period.

Further, a ferroelectric liquid crystal having a chiral smectic phase as an optical modulation material is used.

Function In the present invention, since at least two scanning electrodes are selected at the same time, the effective one scanning period can be reduced to half of the conventional one, and the display capacity is compared with the conventional one when the frame frequency is fixed. Even if the stable state of each pixel is not changed, the polarity of the applied voltage of the same pixel in the same phase is inverted for each pixel even if the stable state of each pixel is not changed. A stable state is refreshed by applying a voltage, and the electrical imbalance of the optical modulator can be reduced.

Further, by using a ferroelectric liquid crystal having a chiral smectic phase as the optical modulation substance, an optical modulation element having bistability can be easily produced.

Embodiment Hereinafter, a method for driving an optical modulation element according to an embodiment of the present invention will be described with reference to the drawings.

FIG. 1 is a voltage waveform diagram of a quarter bias applied to off pixels and on pixels of a ferroelectric liquid crystal panel according to one embodiment of the present invention. In FIG. 1, voltage waveform diagrams (a), (b), (c), (d), (e), (f),
(G) and (h) are the pixels 11, 1 shown in FIG.
2, 21, 22, 31, 32, 41, and 42 show the applied voltage waveform diagrams,
s ₁ , Ts ₂ , Ts ₃ , and Ts ₄ indicate selection periods of the scan electrodes 10, 20, 30, and 40, respectively. The ferroelectric liquid crystal has a characteristic that a bistable state is determined by the magnitude of the time integral of the applied voltage. Now, let the minimum pulse width be τ in FIG. 1 (therefore, one selection period is 4τ).
When the absolute value of the time integral of the applied voltage is smaller than 4 Vn τ, the element is in the reset state when the voltage is 4 Vn τ or less, and is in the set state when the voltage is 4 Vn τ or more.
In Figure 7 a waveform voltages applied off pixels 21 shown in FIG. (C), the time integral value of the pulse 50 in the selection period Ts ₂ 6
Since it is Vn τ, it becomes an effective set pulse, and the ferroelectric liquid crystal is set. Since the time integration value of the subsequent pulse 51 is 4Vn τ, it becomes an effective reset pulse, the ferroelectric liquid crystal is inverted to the reset state, and the absolute value of the time integration of the pulse of the applied voltage during the non-selection period is 2Vn τ at the maximum. For this reason, the ferroelectric liquid crystal keeps the previous reset state, and as a result, the pixel 21 is turned off. In addition, the applied voltage of the ON pixel 22 shown in FIG.
_Since the time integral of the pulse 52 in ₂ is 4Vn τ, it is an effective reset pulse, the ferroelectric liquid crystal is in a reset state, and the time integral of the subsequent pulse 53 is 3Vn τ, so it is an invalid reset pulse, The liquid crystal maintains the set state. Since the non-selection period is the same as that of the off-pixel 21, the pixel 21 is turned on after all. Other pixels in FIG.
11, 12, 31, 32, 41, and 42 are likewise shown in FIGS.
(B), (e), (f), (g), and (h) are states determined by the hatched pulses.

(A), (b), (c), and (d) of FIG. 2 are waveform diagrams of applied voltages applied to the scanning electrodes 10, 20, 30, and 40 of FIG. 4 to obtain the voltage waveform of FIG. From (a) and (b) in FIG.
4 shows a waveform diagram of an applied voltage applied to the signal electrodes 1 and 2 in FIG. As shown in FIG. 2 and FIG.
By this selection, the effective scanning period is reduced to half, and the display capacity can be doubled.

Further, in FIG. 1, T _v denotes the period or frame period to scan the entire scan electrodes, a stable state of each pixel even when held at a state of FIG. 4, the scanning electrode selection period T in _{s1, T s2, T s3,} T s4, and the polarity in the period T _v of the scanning electrode waveform and the signal electrode waveform shown in FIG. 3 the second frame shown in Figure 2 is inverted, the The voltage waveform applied to each pixel in FIG. 1 is different from that of the first frame.

However, although the voltage waveform applied to each pixel is different, the voltage that determines the stable state of the pixel is determined by the time integral value of the voltage in the hatched portion of the waveform in FIG. It is in the same stable state as one frame.

That is, by inverting the polarity of the scanning voltage waveform shown in FIG. 2 for each frame period and relatively changing the voltage waveform applied to the signal electrode side to a stable state to be set for each pixel, Each pixel can be brought into a predetermined stable state.

In particular, when each pixel is kept in the same stable state without changing the stable state, when the optical modulation material is ferroelectric liquid crystal, if the assembling uniformity of the liquid crystal display panel is poor, the held stable state may be reduced. For example, there was a problem that a phenomenon such as burn-in occurred, and even if the display was rewritten, the previous state remained as an afterimage.However, by adopting a configuration in which the voltage always changes as in the present embodiment, it is always stable. Can be displayed.

Further, as a material of the ferroelectric liquid crystal, a substance exhibiting a chiral smectic C phase [for example, + DOBAMBC (+ P-decyloxybenzylidene-P'-acino-2-methylbutylcinnamate)] has a fast response to an electric field and a liquid crystal panel. In many cases, the orientation with respect to the substrate at the time of forming the liquid crystal panel is easy to obtain and exhibits good bistability. Therefore, by forming a liquid crystal panel using this kind of substance, a large-capacity liquid crystal display device with good display uniformity is obtained. Can be realized.

In this embodiment, four pulses are applied within one selection period. However, four or more pulses may be applied. Also,
Although two scanning electrodes are selected at the same time, three or more scanning electrodes may be selected at the same time, and the scanning electrodes selected at the same time are not limited to adjacent scanning electrodes. Although the example of the applied voltage waveform of each pixel is shown as a 1/4 bias, the waveform is not limited to 1/4 but may be a waveform suitable for the characteristics of an optical modulation element such as a ferroelectric liquid crystal panel. I just need.

As described above, the present invention selects two or more scan electrodes at the same time, divides one selection period into at least four phases, and determines a bistable state by two phases and the final phase. As can be understood from FIGS. 1, 2 and 3, the number of pulses in 1H can be set to two, so that a double display capacity can be obtained when the frame frequency is fixed. Even when the applied voltage waveform of each pixel does not change in the stable state of each pixel, the polarity is inverted every frame period, so even if there is an electrical imbalance in the optical modulation element, the stability is not impaired. When used in a display device that can be driven, it is possible to obtain an effect that a burn-in phenomenon that easily occurs when the display is not changed can be largely suppressed.

Furthermore, by using a ferroelectric liquid crystal having a chiral smectic phase as the optical modulation material, a large-capacity display device can be easily realized.

[Brief description of the drawings]

FIGS. 1 (a), (b), (c) and (h) are waveform diagrams of applied voltages to off pixels according to an embodiment of the present invention, and FIGS. 1 (d), (e), (f) and (h). g) is an applied voltage waveform diagram of the ON pixel, FIGS. 2 (a), (b), (c) and (d) are scan electrode applied voltage waveform diagrams in one embodiment of the present invention, and FIG. 3 (a). , (B) is a waveform diagram of a voltage applied to a signal electrode in one embodiment of the present invention, FIG. 4 is a diagram showing a configuration and a display example of a matrix panel for explaining the embodiment of FIG. 1 of the present invention,
FIGS. 5 (a) and 5 (b) are waveform diagrams of applied voltage to an off pixel and a schematic diagram of a change in transmitted light amount of the pixel in the conventional driving method, and FIGS. 6 (a) and (b) are conventional driving methods. 3A and 3B are a schematic diagram of a voltage waveform applied to an ON pixel and a change in the amount of transmitted light of the pixel. 1,2 ... Signal electrode, 10,20,30,40 ... Scan electrode, 11,12,2
1,22,31,32,41,42 ... display pixels, 50,52 ... valid set pulse, 51 ... valid reset pulse, 53 ... invalid reset pulse.

 ──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Naohide Wakita 1006 Kazuma Kadoma, Kadoma, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. (56) References JP-A-63-138316 (JP, A) JP-A-64- 88523 (JP, A)

Claims (3)

(57) [Claims] An optical modulation element having an optical modulation substance having bistability with respect to an electric field disposed between a scanning electrode group and a signal electrode group, wherein at least two of the scanning electrode groups are provided. The scanning electrodes are simultaneously selected, the selection period of the scanning electrodes is divided into at least two sections, and at least one of the scanning electrodes simultaneously selected in each section is selected so as to be different from each other, and each of the sections is divided into two sections. And the phases of the two phases are voltages having polarities different from each other in the respective sections, and the second and third phases in the selection period cause the optical modulation material to be divided into two phases. The first or second stable state is set, and at the fourth phase, a first voltage for inverting the stable state of the optical modulator or a second voltage for maintaining the stable state of the optical modulator is applied. So that
Even when the stable state of each pixel is not changed, the polarity of the voltage applied to the scanning electrodes is inverted every frame period,
The signal voltage applied to the signal electrode is relatively changed in each frame period so that the stable state of each pixel becomes the same as the state of the previous frame during the selection period, and the voltage waveform applied to each pixel is changed in the frame period. A method for driving an optical modulation element, wherein the driving method is different for each. 2. The method according to claim 1, wherein the bistable optical modulation material is a ferroelectric liquid crystal. 3. The method according to claim 2, wherein the ferroelectric liquid crystal is a liquid crystal having a chiral smectic phase.