JPH0943576A - Ferroelectric liquid crystal element driving method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To attain a large screen by shortening one frame scanning time and increasing scanning lines without lowering a frame frequency in a ferroelectric liquid crystal element. SOLUTION: In pixels A-C positioned at intersections of a certain information signal line and nth to (n+2)th scanning lines, scanning periods of 1H(A), 1H(B) are made to be scanning periods of pixels A, B and a period of IH(C) equivalent to consecutive scanning periods is made to be the scanning period of the pixel C and then scanning of three lines is performed in the scanning time of two lines by impressing scanning signals having waveforms different from each other on nth. (n+1)th and (n+2)th scanning lines and impressing a synthesized information signal obtd. by synthesizing information signals respectively corresponding to the lines on the information signal line.

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 display device for displaying characters and images, particularly a matrix display device for displaying by two stable states shown by a ferroelectric liquid crystal.

[0002]

2. Description of the Related Art Conventionally, a matrix electrode for forming pixels on a matrix is provided on the inner surface of a liquid crystal cell, an electrode group on one substrate is a scanning electrode group, and an electrode group on the other substrate is an information electrode group. As a known liquid crystal element, a liquid crystal compound is filled between these electrodes to display image information.
Among them, a ferroelectric liquid crystal element, which has a characteristic that liquid crystal itself has spontaneous polarization and responds rapidly to an electric field and can stably realize two liquid crystal molecule alignment states, has been actively researched and developed since the 1980s. For example, Japanese Patent Laid-Open No. 56-1
It is proposed in Japanese Patent Publication No. 07216.

Various driving methods of ferroelectric liquid crystal elements proposed so far have been such that writing is performed for each scanning signal line. A typical driving method is shown in FIG. In FIG. 13, (A) shows the scanning signal waveform when selected, and (B) shows the scanning signal waveform when not selected.
(C) is an information signal waveform at the time of white writing, and (D) is an information signal waveform at the time of black writing. This information signal waveform is erased by black writing by the V ₁ pulse of (A) for each line by the line sequential writing method, and the V ₃ pulse of (C) or the V ₄ pulse of (D) is selected. ,
Whether to write in white is determined by the voltage difference from the V ₂ pulse in (A). Setting _{voltage, V 3 = -V 4 = V} 5, V 2
= 2 × V ₄ , V ₁ = V ₂ , and the setting of the pulse width is V ₁ : V ₂ :
V ₅ : V ₄ (C): V ₃ (D): V ₃ (C): V ₄ (D) =
It is 5: 2: 1: 1: 1: 2: 2. Concrete voltage value,
The setting of the pulse width depends on the gap between the electrodes of the cell, the temperature, and the liquid crystal material.

[0004]

Since the ferroelectric liquid crystal device described above has a memory property, the black or white display is sufficiently retained even if the frame frequency becomes low and the time until the next writing becomes long. To be done. Therefore, it is possible to increase the screen size by increasing the number of scanning signal lines, but the deterioration of the image quality due to the decrease of the frame frequency cannot be avoided.

The object of the present invention is to realize high-speed driving utilizing the high-speed response of the ferroelectric liquid crystal even in an element having a large number of scanning signal lines, and to reduce the image quality due to the decrease in frame frequency. It is to plan the screen.

[0006]

According to a method of driving a ferroelectric liquid crystal element of the present invention, scanning signals having different selection period widths and different waveforms are simultaneously applied to a plurality of scanning signal lines and synchronized with the scanning signals. Then, a combined information signal obtained by combining a plurality of different information signals corresponding to each scanning signal is applied to the information signal line.

That is, according to the driving method of the present invention, (1) scan signals are simultaneously applied to a plurality of scan signal lines, and (2) simultaneously applied scan signals have different selection period widths and waveforms.
(3) By writing a composite information signal obtained by combining the information signals corresponding to the plurality of scanning signals to the information signal line in synchronization with the application of the scanning signal, the pixels of the plurality of lines are simultaneously written. The frame scanning time can be shortened.

The driving method of the present invention uses the conventional TN device and S
This is completely different from the driving method of the TN element, and the difference will be described below.

As a driving method of a liquid crystal element, there is a voltage averaging method used for the above-mentioned TN element and STN element (“Basics of application of liquid crystal display”: Corona Company / Katsumi Yoshino,
Masanori Ozaki / 117-130). In this method, the electric field strength applied to the liquid crystal molecules is controlled by changing the effective values of the selected point, the semi-selected point, and the non-selected point, and the alignment direction of the liquid crystal in the electric field is controlled. The response to the electric field in the TN element or the STN element uses the "cumulative response effect" that the liquid crystal does not respond to one selection pulse but gradually responds by applying several selection pulses. are doing. Such a writing method is possible because the driving principle causes a change in the orientation of the liquid crystal molecules so as to minimize the permittivity of the liquid crystal molecules in the electric field. Since it has (dielectric anisotropy), orientation change will occur. The driving torque of the liquid crystal is proportional to the square of the electric field and does not depend on the polarity of the electric field. That is, such a state change of the liquid crystal is determined by the effective value of the applied voltage.

On the other hand, in the ferroelectric liquid crystal element, there is no change in the effective value during selection, half selection and non-selection, which is an essential difference from the TN element and STN element.

Regarding this point, JP-A-5-10064
The same applies to the "active addressing driving method" in the STN element disclosed in Japanese Patent Laid-Open No. This "active addressing driving method" is one of the measures for avoiding the phenomenon of the contrast reduction of the STN element.

With the speeding up of liquid crystal materials for STN elements (development of low-viscosity materials), a response has been obtained for each (effective value) of applied selection pulses of liquid crystal. That is, when the response speed of the liquid crystal is slow, the orientation change of the liquid crystal molecules occurs over several frames according to the effective value of the applied voltage. However, when the response time becomes fast for about one frame time, the liquid crystal molecules receive the applied pulse. Each time it responds (changes orientation). Such a phenomenon is generally called a "frame response phenomenon". When this happens, the difference in transmittance between on and off becomes smaller,
The contrast is reduced.

When the "active addressing driving method" is used to improve this, by associating several scanning signals with information signals, the same effect as high frequency can be obtained and the "frame response phenomenon" can be suppressed. It is possible to prevent deterioration of contrast. Therefore, the scanning time is not shortened by the "active addressing driving method", which is different from the driving method of the present invention described below.

The response of the TN element or STN element to the electric field is 1 even if the liquid crystal material of the STN element is accelerated.
The above "cumulative response effect" is used instead of making the liquid crystal respond to the selection pulse once. On the other hand, the driving principle of the ferroelectric liquid crystal element is that writing is performed with one selection pulse, and the effective voltage value of the subsequent non-selection signal portion is not related to the response of the ferroelectric liquid crystal element.

It is considered that the response form of the ferroelectric liquid crystal to the electric field is the action of the first-order term of the electric field and the spontaneous polarization, and there is essentially no "cumulative response effect". However, considering the movement of the liquid crystal molecules having a bistable state, the liquid crystal molecules in one stable state start to move by receiving the electric field in the inversion direction, but before moving to the other stable state (that is, in one direction). Even if it receives an electric field in the direction of returning to the stable state of
When the degree is small, it can be reversed to the other stable state by applying the electric field in the reverse direction again.

With respect to the application of such a minute electric field, it is possible to maintain the relationship that it is inverted if the pulse area is constant. Specifically, when reversing after applying the erase pulse, it is ideal to apply only the voltage polarity in the opposite direction (the voltage value itself may vary), but when the voltage in the opposite direction is applied, Must not exceed the area of the selection pulse immediately after the application of the erase pulse. Further, such a condition is not necessary when the inversion is not performed after the erase pulse is applied.

At this time, when the other stable state is reached by the electric field in the opposite direction, the above relationship cannot be maintained and an error occurs, but the above relationship can be obtained by correcting with the information signal or the scanning signal. Can be kept. In order to maintain such a relationship that the pulse area is inverted if the pulse area is constant, the waveform of the information signal voltage applied to the ferroelectric liquid crystal at the time of line-sequential driving for line erasing is a complete AC and has a DC component. Since it is an indispensable condition that there is no selection, there is no change in the effective value during selection, half selection, and non-selection.

[0018]

The driving method of the present invention will be described with reference to FIG.

In the figure, (a) to (c) are n to n +, respectively.
The scanning signals applied to the first scanning signal lines A to C are shown. As shown in FIG. 1, scanning signal line A and scanning signal line C
And are being scanned at the same time. The scanning time for one line at this time is 1H (A), and the scanning time for the next one line is 1H.
Assuming that (B), the scanning line selected in 1H (B) is only the scanning signal line B and the other scanning selection signal lines are not selected, but the scanning signal line is selected in the period of 1H (A), 1H (B). The information signal (d) synchronized with C will be completed.

This is schematically shown in FIG. 1 (e). This figure shows the information content represented by the information signal, 1 is the information writing period to the pixel of the scanning signal line A, 2 is the pixel of the scanning signal line B, and 3 is the scanning signal line C.
Represents the period for writing information to the pixel. The information writing period 3 overlaps the periods 1 and 2. That is, (d)
Is a combined information signal of 1 and 3 in the period of 1H (A), and similarly is a combined information signal of 2 and 3 in the period of 1H (B). In the present invention, the scanning signal line C
By changing the waveform (c) of the scanning signal applied to (a) or (b), it is possible to separate and take out the respective information items 1 to 3 from the combined information signal (d). Therefore, during the period in which the image signal is displayed in the pixel located at the intersection of the information signal line to which the composite information signal (d) is applied and the scanning signal lines A and B, the intersection of the information signal line and the scanning signal line C is displayed. Since the image information is displayed on the pixel, the writing speed is 1.5 times.

Further, by setting information in 7 periods by overlapping 1 to 3 and 4 to 6 of the same combination, the number of scanning signal lines that can be written in the same period is increased to write one frame. The time can be further shortened. In this case, since 3, 6, and 7 are originally selected for scanning in the 4-line scanning selection period of 1, 2, 4, and 5, the writing speed is 1.75 times the original speed.

[0022]

【Example】

[Example 1] Physical property data of the ferroelectric liquid crystal used in this example are shown below.

[0023]

[Table 1]

This liquid crystal has a melting point of 3.0 ° C. and a phase transition temperature of 4 from the smectic C phase to the smectic A phase.
1.1 ° C., the phase transition temperature from the smectic A phase to the isotropic phase is 77 ° C.

The liquid crystal cell had a cell thickness of 2.0 μm and was orientated on the matrix electrode substrate. The alignment film was rubbed with a nylon flocking cloth (having 2-3 mm of hairs) using LP-64 (manufactured by Toray Industries, Inc.) on one side, and the alignment film on the opposite side was "ODS-". E "
(Manufactured by Chisso Corporation) was used.

To form a film of LP-64, a 1.0 wt% solution was applied using a spinner at 2000 rpm for 20 seconds to adjust the film thickness to 10 nm. The ODS-E film is formed by using a 0.5 wt% solution in a spinner at 2000 rpm.
For 20 seconds. When measuring the surface tension at the contact angle,
The surface tension after rubbing treatment of LP-64 is about 48 dyn.
The surface tension of ODS-E is about 35 dyne / c at e / cm.
m. The rubbing treatment was performed only on the LP-64 side and not on the ODS-E side.

FIG. 12 is a block diagram of the display device constructed in this embodiment. In the figure, 107 is a graphic controller, and the data transmitted from this is the drive control circuit 1.
The signal is input to the scanning signal control circuit 104 and the information signal control circuit 106 through 05 and converted into address data and display data, respectively. At this time, a plurality of address data are set in the control circuit, and display data are also set in consideration of writing to a plurality of lines (in this embodiment, eight ways). The scanning signal applying circuit 102 and the information signal applying circuit 103 generate voltage signals according to the address data and the display data, and apply the voltage signals to the scanning signal electrodes and the information signal electrodes of the display unit 101, respectively.

In this embodiment, 1 to 3 shown in FIG.
Is an example of driving in which 1 frame scanning time is shortened by 1.5 times. FIG. 2 shows the information signal waveform of this embodiment. FIG. 2 shows the writing contents (white or black) of the scanning signal lines C in the horizontal direction and the writing contents (white or black) of the scanning signal lines A and B in the vertical direction.
The case classification of all the display information of the pixels at the points where an arbitrary information signal line intersects with the scanning signal lines A, B and C is represented by eight ways in this figure.

In FIG. 2, the top of each frame is the information signal waveform of the pixels (pixels A and B) of the scanning signal lines A and B, and the second is the information signal of the pixels of the scanning signal line C (pixel C). The third waveform is the sum of these waveforms, which is the composite information signal waveform applied to the information signal line. The ability to synthesize such information is particularly effective in the case where the threshold value of the polarization inversion accompanied by the memory property of the ferroelectric liquid crystal is determined by the applied pulse area. In the liquid crystal element of the present embodiment, the product of the applied pulse width and the applied voltage is approximately 1.05 or less, which is close to 1, and the relationship between the applied voltage and the applied pulse width is nearly inversely proportional, and the threshold value for polarization inversion is applied. This is the case when it is determined by the pulse area. However, even if the product of the applied pulse width and the applied voltage is about 1.05 or more, the composite information signal can be configured by applying an appropriate correction coefficient during the calculation.

Since the pixels A to C are scanned in the time sequence shown in FIG. 1, the pixels A and B are written continuously in time, and the pixel C is written over both times. Become. The voltage value in FIG. 2 was set as follows.

V ₁ = -6 [V] V ₂ = 6 [V] V ₃ = 3 [V] V ₄ = -3 [V] V ₅ = -3 [V] V ₆ = 9 [V] V _{7 = -9 [V] V 8 =} 3 [V]

The scanning signals used in this embodiment are shown in FIGS.
The waveform shown in (c) was used. The voltage applied to each pixel is the scan signal minus the information signal. Each voltage value of the scanning signal was set as follows.

Vs ₁ = 16.94 [V] Vs ₂ = 0.67 [V] Vs ₃ = -16.94 [V] Vs ₄ = -6 [V] Vs ₅ = -12 [V]

FIG. 3 shows the range of usable pulse widths when a liquid crystal element at 30 ° C. is driven under such voltage conditions. In the figure, the shaded strips are black writing, and the white strips are white writing. The write margin (width of the usable pulse width range in this case) during matrix driving varies depending on the post-write content. However, when the post-write content is disadvantageous to the pixel, it is regarded as a bad condition margin. Expressed and investigated the range of usable pulse widths for this.

FIG. 3A shows the pulse width of A and B when white is written in the pixel C and white or black is written in the pixels A and B.
(2) is the pulse width of A and B when black is written in the pixel C and white or black is written in the pixels A and B, and (3) is the pixel A,
A pulse width of C when white is written in B and white or black is written in the pixel C, and (4) shows a pulse width of C when black is written in the pixels A and B and white or black is written in the pixel C. .
(1) and (2) are bad condition margins of the pixels A and B,
(3) and (4) are bad condition margins of the pixel C. In any case, it can be seen that the pixels A to C can be written simultaneously.

From FIG. 3, the pulse width range in which the pixels A and B and the pixel C can be written simultaneously is 15.4 to (1).
21.0 μs, (2) 11.0 to 18.2 μs,
In (3), 14.6 to 17.2 μs, and in (4), 13.
6 to 17.0 μs, and their common pulse width range is 15.4 to 17.0 μs.

FIGS. 4 to 7 show white writing to the pixels A and B in FIG. 3 (1), similarly, white writing in (2), black writing in (2), and (1). The waveform of the voltage applied to the pixel A or B (upper row) and the optical response waveform (lower row) in the case of black writing are shown. Similarly, in FIGS. 8 to 11, white writing to the pixel C in FIG.
White writing in (4), black writing in (3), (4)
The waveform of the voltage applied to the pixel C (upper row) and the optical response waveform (lower row) in the case of black writing in FIG. As is apparent from FIGS. 4 to 11, it is possible to write in both white and black regardless of the writing contents of the pixels that are simultaneously written.

[Embodiment 2] The same liquid crystal cell as in Embodiment 1 was used, and the voltage value of the scanning signal was set as follows. The voltage value of the information signal is the same as that in the first embodiment.

Vs ₁ = 16.94 [V] Vs ₂ = 6.67 [V] Vs ₃ = -16.94 [V] Vs ₄ = -8 [V] Vs ₅ = -10 [V]

The drivable pulse width range in this embodiment is such that when the pixel C is white-written, the pixels A and B are 15.4-.
White and black writing is possible within the range of 21.0 μs, and 11.0 to 18.2 μs when the pixel C is black writing,
When the pixels A and B are white, the pixel C is 17.0 to 19.0 μs
It is possible to write in black and white in the range of 1 to 22.0 μs when the pixels A and B are black. That is, the overall margin is 17.0 to 18.2 μs, and the pixels A to C can be simultaneously written even with the set voltage of this embodiment.

[0041]

As described above, according to the present invention,
Since the one-frame scanning period is shortened, it is possible to increase the number of scanning signal lines and increase the screen size without lowering the frame frequency and lowering the image quality.

[Brief description of drawings]

FIG. 1 is a diagram illustrating the operation of the present invention.

FIG. 2 is a diagram showing an information signal waveform according to the first embodiment of the present invention.

FIG. 3 is a diagram showing a drive margin according to the first embodiment of the present invention.

FIG. 4 is a diagram showing applied voltage waveforms and optical response waveforms to pixels A and B when white is written in pixel C and white is written in pixels A and B in the first embodiment of the present invention.

FIG. 5 is a diagram showing applied voltage waveforms and optical response waveforms to pixels A and B when black is written in pixel C and white is written in pixels A and B in the first embodiment of the present invention.

FIG. 6 is a diagram showing a voltage waveform applied to pixels A and B and an optical response waveform when black is written in pixel C and black is written in pixels A and B in the first embodiment of the present invention.

FIG. 7 is a diagram showing applied voltage waveforms and optical response waveforms to pixels A and B when white is written in pixel C and black is written in pixels A and B in the first embodiment of the present invention.

FIG. 8 is a diagram showing a voltage waveform applied to a pixel C and an optical response waveform when white is written in the pixels A and B and white is written in the pixel C in the first embodiment of the present invention.

FIG. 9 is a diagram showing an applied voltage waveform and an optical response waveform to the pixel C when black is written in the pixels A and B and white is written in the pixel C in the first embodiment of the present invention.

FIG. 10 is a diagram showing a voltage waveform applied to the pixel C and an optical response waveform when white is written in the pixels A and B and black is written in the pixel C in the first embodiment of the present invention.

FIG. 11 is a diagram showing a voltage waveform applied to the pixel C and an optical response waveform when black is written in the pixels A and B and black is written in the pixel C in the first embodiment of the present invention.

FIG. 12 is a block diagram of a display device configured in an embodiment of the present invention.

FIG. 13 is a diagram showing a drive voltage waveform of a conventional ferroelectric liquid crystal element.

[Explanation of symbols]

 101 display unit 102 scanning signal application circuit 103 information signal application circuit 104 scanning signal control circuit 105 drive control circuit 106 information signal control circuit 107 graphic controller 108 temperature detection element 109 temperature detection circuit

 ─────────────────────────────────────────────────── ─── Continued Front Page (72) Inventor Tomoko Maruyama 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (72) Hiroyuki Kitayama 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Incorporated (72) Inventor Hirokatsu Miyata 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc.

Claims (2)

[Claims] 1. A method of driving a liquid crystal element, wherein a ferroelectric liquid crystal is sandwiched between a pair of matrix electrode substrates, wherein scanning signals having different selection period widths and waveforms are simultaneously applied to a plurality of scanning signal lines. Then, in synchronization with the scanning signal, a combined information signal obtained by combining a plurality of different information signals corresponding to each scanning signal is applied to the information signal line, and a method of driving a ferroelectric liquid crystal element. . 2. The nth and n + 1th scanning signal lines are
By successively applying scanning signals of the same scanning period and the same shape to perform selective scanning, at the same time, scanning signals of different waveforms are applied to the (n + 2) th scanning signal line in a scanning period twice as long as the above scanning period, In synchronization with the second scanning period, the information signal to be written to the pixel of the n-th scanning signal line and the information signal to be written to the pixel of the (n + 1) -th scanning signal line that follows the information signal are written to the pixel of the (n + 2) -th scanning signal. The method of driving a ferroelectric liquid crystal element according to claim 1, wherein a combined information signal obtained by combining the signals is applied to the information signal line.