JP2934727B2 - Driving method of ferroelectric liquid crystal device - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to a method for driving a ferroelectric liquid crystal display device applied to a display, an optical shutter and the like.

2. Description of the Related Art Conventionally, as a driving method of a ferroelectric liquid crystal display device having a plurality of matrix electrodes, there are a two-field method, a four-pulse method, a plural simultaneous erasing method, and the like.

In the two-field method, as shown in FIG. 2A, the selection time of one scanning electrode is composed of four phases, one frame for one pixel display is divided into two,
Two phases are arranged in each of the first half field and the second half field, and black writing and white writing are performed in different fields.

Here, "the selection time is composed of four phases"
This means that there are four pulses within the selection time of one scan electrode in one frame.

Also in the four-pulse method, as shown in FIG.
The selection time of the scanning electrode is divided into four phases and the direction of spontaneous polarization of the ferroelectric liquid crystal molecules is inverted or held to display white and black.

In the multiple simultaneous erasing method, an electric field for simultaneously orienting the ferroelectric liquid crystal molecules on a plurality of arbitrary scanning electrodes in one direction during a selection time of one scanning electrode is applied, and then the scanning is performed. The selection time is divided into two phases for each electrode, and an electric field in a direction opposite to the previous direction is applied according to the control signal.

 FIG. 2 (c) shows an example of the driving waveform of the multiple simultaneous erasing method.

[Conventional Problems] In the above-described two-field method and four-pulse method, the selection time of one scanning electrode is divided into four phases, and the time during which the ferroelectric liquid crystal molecules are effectively moved is one of them. There are only two phases. For example, assuming a ferroelectric liquid crystal display device having 640 control electrodes and 400 scanning electrodes, if there are 30 screens per second generally called a video cycle,
The width T _{1 of the} effective pulse applied to the ferroelectric liquid crystal molecules on one scanning electrode is T ₁ = (30 × 400 × 4) ⁻¹ ≒ 21 [μsec]. Therefore, a fast response speed has to be required for the ferroelectric liquid crystal material. Alternatively, the magnitude of the applied electric field had to be increased.

In the multiple simultaneous erasing method, for example, assuming that eight lines are simultaneously erased, the effective pulse width T ₂ applied to the ferroelectric liquid crystal molecules is T ₂ = (30 × 400 × 2 × 9/8) ≒ 37 [μsec When driving by the multiple simultaneous erasing method,
A ferroelectric liquid crystal material having a response speed about 0.57 times slower than that of the two-field method or the four-pulse method can be used. However, since the driving waveform is complicated, there is a problem that peripheral driving circuits are complicated, and the number of steps for fabricating the circuit is increased.

[Structure of the Invention] In order to solve the above problems, the present invention provides a method in which a scanning signal applied to a scanning electrode for writing one line of a selected scanning electrode is composed of two phases, Is characterized in that the control signal applied to the signal is composed of two phases.

By using the present invention, the driving waveform can be simplified as compared with the related art, the cost can be reduced in terms of circuit fabrication, and furthermore, both the scanning signal and the control signal can be composed of two types of pulses at the time of writing. For example, a ferroelectric liquid crystal having a relatively slow response speed can be used as described below.

In general, a clear threshold voltage does not exist in a ferroelectric liquid crystal. Therefore, even when a very small electric field is applied for a long time, the direction of spontaneous polarization of the ferroelectric liquid crystal molecules changes. That is, in the time-division driving, only a direct current component of 0 [V] can be applied to ferroelectric liquid crystal molecules on average when not selected. Even if the average is 0 [V], an electric field for changing or maintaining the direction of spontaneous polarization of the ferroelectric liquid crystal molecules must not be applied within the time when one phase is applied.
Since the present invention describes time-division driving of a ferroelectric liquid crystal device, it is needless to say that these requirements are satisfied and matrix driving is possible.

When the above-described 640 × 400 dot ferroelectric liquid crystal display device displays 30 screens per second by using the present invention, the effective pulse width T ₃ applied to the ferroelectric liquid crystal molecules is T ₃ = ( 30 × 400 × 2) ^-1 ≒ 42 [μsec], which is twice as large as the 2-field method and 4-pulse method.
A ferroelectric liquid crystal material that is 1.1 times slower than the multiple simultaneous erasing method can be used. Further, the complexity of the driving circuit is almost the same as the two-field method and the four-pulse method.

Also, if the comparison is made using the same ferroelectric liquid crystal material, the driving voltage can be reduced to almost half,
The power consumption of the ferroelectric liquid crystal display device is calculated by the two-field method,
Compared to the pulse method, it became possible to reduce it to about 1/4.

In addition, it has been found that the response speed of the ferroelectric liquid crystal material increases as the ambient temperature increases. Therefore,
When comparison was made using the same ferroelectric liquid crystal material, display was possible to a lower temperature region by using the driving method of the present invention.

This will be briefly described with reference to Table 1. The data in Table 1 is for a ferroelectric liquid crystal having the structural formula shown below.

The upper part of Table 1 shows the temperature, and the lower part shows the response speed (unit: μsec).

In the case of using the conventional two-field method or the four-pulse method, in order to drive 30 screens per second in a 640 × 400 dot ferroelectric liquid crystal device, as described above, 42 pixels are required.
Since the ferroelectric liquid crystal must have a faster response speed than the μsec limit, as can be seen from Table 1,
It could only be performed in a temperature range of at least 35 ° C. Similarly, when the multiple simultaneous erasure method is used, about 20
Driving is possible in a temperature range of not less than ° C.

In addition, when the present invention is used, driving can be performed in a temperature range of about 15 ° C. or more, and it can be seen that the temperature range in which the ferroelectric liquid crystal device can display is greatly widened.

[Example] ITO (I) was formed on a soda glass substrate by a known sputtering method.
After forming a 2000-nm thin film of ndium tin oxide, a scan electrode side substrate having 400 scan electrodes and a control electrode side substrate having 640 control electrodes were manufactured by a known photolithography method. Thereafter, a polyamic acid is applied to the substrate having the scanning electrodes by using a known offset method,
Firing was performed at 280 ° C. for 2 hours to obtain a polyimide film having a thickness of 500 °. Then, after performing known rubbing using a cotton cloth, a 2.5 μm spherical insulator was used as a spacer and sprayed on the scan electrode side substrate. On the other hand, an epoxy-based adhesive was printed on the periphery of the control electrode side substrate, and the scan electrode side substrate and the control electrode side substrate were bonded. Thereafter, a ferroelectric liquid crystal material was injected between the substrates using a known vacuum injection method.

 Then, driving was performed by connecting the electrode and the drive circuit.

FIG. 1 shows a driving waveform used in this embodiment. Where is the waveform applied to the control electrode side,
The waveform applied to the scanning electrode side and the composite waveform (waveform applied to the liquid crystal) are shown.

FIG. 3 (a) shows an oscilloscope photograph showing a change in the electric field applied to the ferroelectric liquid crystal molecules of an arbitrary pixel of the ferroelectric liquid crystal device and a change in the amount of transmitted light when the present invention is used. FIG. 3 (b) shows the explanation. However, this figure shows the case where the number of scanning electrodes is 100, and the upper part of the photograph shows the optical response and the lower part shows the driving waveform.

FIGS. 4A to 4Q show examples of driving waveforms. (C), (d), (i), (m), and (q) are other examples of the drive waveform of the present invention, and the others are reference examples. However, for simplicity, the case of a 4 × 4 dot ferroelectric liquid crystal device is shown. In each of the figures (a) to (q),
Indicates a waveform applied to the control electrode side, a waveform applied to the scanning electrode side, and a composite waveform (a waveform applied to the liquid crystal).

[Effects] As described above, the use of the present invention makes it possible to simplify the driving circuit, and further, it is possible to use a ferroelectric liquid crystal material having a relatively slow response speed. Further, the temperature range in which the device can be driven has been widened.

Further, the scanning signal applied to the scanning electrode and the control signal applied to the control electrode both had two phases for one pixel and a signal of a different voltage at each phase, and were unselected and turned off. By setting the voltage of the pixel to 0 V instead of alternating current, power consumption can be reduced, minute fluctuations in the amount of transmitted light in the pixel that is turned off can be significantly reduced, and reduction in contrast can be minimized.

[Brief description of the drawings]

FIG. 1 shows a driving waveform of this embodiment. 11: ON when selected 12: OFF when selected 13: ON when not selected 14: OFF when not selected Fig. 2 shows conventional driving when driving a ferroelectric liquid crystal display device in a time division manner. The waveform is shown. (A) Two-field method (b) Four-pulse method (c) Multiple simultaneous erasing method FIG. 3 (a) shows a change in the light transmission amount of an arbitrary pixel when the driving method according to the present invention is used and the pixel 3 shows an oscilloscope photograph showing an oscilloscope waveform added to the oscilloscope. FIG. 3 (b) shows a description of the display of the ferroelectric liquid crystal device when the oscilloscope photograph of FIG. 3 (a) is taken. 31 White on a white background 32 Black on a white background 33 White on a black background 34 Black on a black background 35 White on a houndstooth lattice 36 Black on a houndstooth lattice Fig. 4 (a) to ( q) shows an example of the drive waveform.

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Claims (1)

(57) [Claims] 1. A method of driving a ferroelectric liquid crystal device in which a ferroelectric liquid crystal is interposed between a substrate having a scanning electrode and a substrate having a control electrode, wherein a scanning signal applied to the scanning electrode and the control signal The control signals applied to the electrodes each have two phases for one pixel and signals of different voltages at each phase, and the voltage of the scanning signal is selection or non-selection of the pixel, and the voltage of the control signal. Determines ON or OFF of the pixel, respectively, wherein the voltage of the scanning signal determines non-selection, and the voltage of the pixel when the voltage of the control signal determines OFF is 0V. A method for driving a dielectric liquid crystal device.