JPH04218023A - Ferroelectrric liquid crystal display device and driving method thereof - Google Patents

Abstract

PURPOSE:To provide the ferroelectric liquid crystal element which does not deteriorate a liquid crystal material and does not lose bistability and the driving method thereof. CONSTITUTION:The ferroelectric liquid crystal is so driven that the polarity of a reset voltage and the polarity of a gradation signal voltage invert at every specified period Tf and that the gradation signal voltage V1 after negative polarity resetting and the gradation signal voltage -V2 after positive polarity resetting corresponding thereto attain T(V1)+T(V2)=100 where the state inversion rate of the ferroelectric liquid crystal when the gradation signal voltage is V is designated as T(V)%. In addition, the electric element formed by using the ferroelectric liquid crystal is driven in the above-mentioned manner.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal element mounted on a display device or the like, and more particularly to a ferroelectric liquid crystal element and a method for driving the same.

0002

[Description of prior art] Ferroelectric liquid crystal (hereinafter referred to as FLC)
Electro-optical devices using 3D have been mainly applied to simple matrix display devices, taking advantage of their characteristics of rapid response to electric fields and bistability, but in recent years, they have been applied to active matrix display devices. Applications of this technology are also being considered. The characteristics of active matrix FLC elements are the first
Another advantage is that the scanning time (frame period) for one screen can be determined without depending on the response speed of the FLC. In a simple matrix FLC element, the liquid crystal must respond within the selection time of one scanning line, so the frame period cannot be lowered below (response speed of liquid crystal) x (number of scanning lines), and the number of scanning lines is There was a drawback that the frame period became longer as the number increased. On the other hand, active matrix FL
In the C element, it is only necessary to charge and discharge the pixels on the scanning line within one scanning line selection time, and after the selection period, the switching element of the pixel is turned off and the voltage applied to the liquid crystal is maintained. The response of the liquid crystal occurs within this holding time. Therefore, the frame period does not depend on the response speed of the liquid crystal, and even if the number of scanning lines increases, it is possible to operate at the 33 ms used in televisions.

A second feature of active matrix FLC elements is that they can easily display gradations. One of the gradation display methods for active matrix FLC elements is EP284134
The principle is that the pixel is reset to one stable state in advance, and then a charge amount Q is applied to the pixel electrode through the active element. The idea is to cause a switch to a stable state. When using this principle, if the area of the part where switching to the second stable state occurs is a, and the magnitude of the spontaneous polarization of the FLC is PS, then a charge of 2PS·a moves due to switching, and this is initially Switching to the second stable state continues until the applied charge Q is canceled out. Finally, the area of a=Q/2PS becomes the second stable state. By changing Q, control of a, that is, area gradation is realized.

FIG. 7 shows a sectional view of a specific example of a ferroelectric liquid crystal cell constituting such a ferroelectric liquid crystal element. This liquid crystal cell uses a TFT (thin film transistor).

In FIG. 7, a semiconductor film (amorphous silicon doped with hydrogen atoms, etc.) is formed on a substrate 70a made of glass or plastic with a gate electrode 71 and an insulating film (such as a silicon nitride film doped with hydrogen atoms) 72 interposed therebetween. 73 (film, etc.) and two terminals 74 and 75 in contact with this semiconductor film 73, and the terminal 7 of the TFT.
The pixel electrode (ITO: Indium Ti
n Oxide, etc.) 76a is formed.

Furthermore, an insulating layer (polyimide,
A light shielding film 78 made of polyamide, polyvinyl alcohol, polyparaxylylene, SiO2, or TiO2) 77a and aluminum, chromium, or the like is provided. A counter electrode (ITO:
Indium Tin Oxide) 76b and an insulating film 77b are formed.

A ferroelectric liquid crystal 80 is sandwiched between substrates 70a and 70b. Further, a sealing material 81 for sealing the ferroelectric liquid crystal 80 is provided around the substrates 70a and 70b.

Polarizers 82a and 82b in a crossed Nicol state are arranged on both sides of the liquid crystal element having such a cell structure.
Behind the polarizer 82b, a reflector plate (sheet or plate of diffused aluminum) 83 is provided so that the viewer A can see the display state of the liquid crystal element by the reflected light I1 from the incident light I0.

In the TFT shown in FIG. 7, the names ``source electrode'' and ``drain electrode'' corresponding to the terminals 74 and 75 refer to the case where the direction in which current flows is limited to one direction. FETs can generally function similarly if the source and drain are interchanged.

0010

[Problems to be Solved by the Invention] According to experiments conducted by the present inventors, the above-mentioned area gradation method using charge modulation has 1
There are one drawback. This is because the transition from the first stable state to the second stable state does not proceed until the charge is completely canceled as described above, and stops midway. This situation is shown in FIG. Figure 8 shows the voltage between pixel electrodes (Figure 8(a)) and the transmitted light intensity (Figure 8(b)) when resetting and gradation display are repeated at a 33ms cycle, similar to those used in ordinary televisions. ) is plotted over time. The voltage exhibits a rapid attenuation immediately after the active element (eg, TFT) is turned off, but the attenuation eventually becomes extremely gradual. Similarly, the transmitted light intensity changes rapidly immediately after the active element is turned off, but the change eventually becomes slow. In other words, it can be seen that although an electric field exists between the electrodes, the reversal between states progresses extremely slowly or has stopped.

Due to this phenomenon, a residual DC electric field continues to be applied to the liquid crystal, eventually causing deterioration of the liquid crystal material. Alternatively, in a liquid crystal element having an insulating layer between an electrode and a liquid crystal as shown in FIG. 7, impurity ions in the liquid crystal adhere to the insulating layer interface due to the DC electric field, and
Since it generates an electric field in the opposite direction to the C electric field, it acts in a direction that impairs the bistability of the FLC.

SUMMARY OF THE INVENTION An object of the present invention is to provide a ferroelectric liquid crystal element in which the liquid crystal material of the ferroelectric liquid crystal element does not deteriorate or lose its bistability, and a method for driving the same.

[0013]

[Means for Solving the Problems] In order to achieve the above object, in one aspect of the present invention, a switching element is provided for each pixel electrode arranged in a matrix, and a ferroelectric liquid crystal is sandwiched between the switching element and the opposing electrode. In a ferroelectric liquid crystal element, there is a reset voltage between both electrodes that resets the entire pixel to a first stable state of ferroelectric liquid crystal, and a reset voltage that partially resets the pixel to a second stable state with a polarity opposite to the reset voltage. Voltage application means that alternately switches and applies a grayscale signal voltage that causes a transition to;
Means for inverting the reset voltage at regular intervals and means for inverting the gray scale signal voltage at regular intervals are provided, and the state reversal rate of the ferroelectric liquid crystal when the gray scale signal voltage is V is expressed as T( V)%, the gray scale signal voltage V1 after negative polarity reset and the corresponding gray scale signal voltage -V2 after positive polarity reset satisfy the relationship T(V1)+T(V2)=100. There is.

In another aspect of the present invention, in a method for driving an electro-optical element using an FLC, the polarity of the reset voltage and the polarity of the grayscale signal voltage are inverted at regular intervals, and the grayscale signal voltage is Assuming that the state reversal rate of the ferroelectric liquid crystal at V is T(V)%, the grayscale signal voltage V1 after negative polarity reset and the corresponding grayscale signal voltage -V2 after positive polarity reset are T(V1 )+T(V2)=100.

[0015]

According to the present invention, since the polarity of the reset voltage and the polarity of the grayscale signal voltage are reversed at regular intervals, it is possible to prevent a situation in which a DC electric field is constantly applied to the liquid crystal.

Therefore, deterioration of liquid crystal materials and FLC
can prevent a decrease in bistability. [Example]
The ferroelectric liquid crystal used in the method for driving a ferroelectric liquid crystal element of the present invention takes either a first optically stable state or a second optically stable state depending on the applied electric field. A substance having a bistable state, in particular a liquid crystal having such properties, is used.

As the ferroelectric liquid crystal having bistability that can be used in the driving method of the present invention, chiral smectic liquid crystal having ferroelectricity is most preferable, and chiral smectic C phase (SmC*), H phase ( SmH
*), I phase (SmI*), J phase (SmJ*), K phase (S
mK*), G-phase (SmG*) or F-phase (SmF*) liquid crystals are suitable. Regarding this ferroelectric liquid crystal, “L
E JOURNAL DEPHYSIQUE L
ETTERS” 36 (L-69) 1975, “Ferr
oelectric Liquid Crysta
ls";"Applied Physics Le
tters” 36 (11) 1980 “Submic
roSecond Bistable Elect
rooptic Switching in L
iquid Crystals”; “Solid State Physics” 16
(141) 1981 "Liquid Crystal", etc., and the ferroelectric liquid crystal disclosed therein can be used in the present invention.

A specific example of the ferroelectric liquid crystal compound used in the present invention is decyloxybenzylidene-p'-amino-2-methylbutylcinnamate (DOBAMBC).
, hexyloxybenzylidene-p'-amino-2-chloropropylcinnamate (HOBACPC), 4-o
-(2-methyl)butyl resol cylidene-4'-octylaniline (MBRA8) and the like.

When constructing a liquid crystal using these materials, the element may be heated with a heater as necessary in order to maintain the liquid crystal compound at a temperature such that it becomes a chiral smectic phase such as SmC phase C* or SmHC* phase. It can be supported by embedded copper blocks or the like.

FIG. 9 schematically depicts an example of a ferroelectric liquid crystal cell. 91a and 91b, In2O3, Sn
O2 or ITO (Indium Tin Ox)
A substrate coated with a transparent electrode made of a thin film such as
A liquid crystal of a chiral smectic phase such as SmC* or SmH* with a liquid crystal molecular layer 92 oriented perpendicular to the glass surface is sealed therebetween. A thick line 93 represents a liquid crystal molecule, and this liquid crystal molecule 93 has a dipole moment (P⊥) 94 in a direction perpendicular to the molecule. When a voltage higher than a certain threshold is applied between the electrodes on the substrates 91a and 91b, the liquid crystal molecules 9
The helical structure of 3 unravels and the dipole moment (P⊥)9
The alignment direction of the liquid crystal molecules 93 can be changed so that all of the liquid crystal molecules 93 are oriented in the direction of the electric field. The liquid crystal molecules 93 have an elongated shape and exhibit refractive index anisotropy in the long axis direction and short axis direction. Therefore, for example, if crossed Nicol polarizers are placed above and below the glass surface, the polarity of voltage application can be changed. It is easily understood that the liquid crystal optical modulation element has optical characteristics that change depending on the amount of the liquid crystal. Furthermore, when the thickness of the liquid crystal cell is made sufficiently thin (for example, 1 μm), the helical structure of the liquid crystal molecules unwinds and becomes a non-helical structure even when no electric field is applied, as shown in Figure 10, and its dipole moment Pa or P
b points upward (arrow 104a) or downward (arrow 104
Either state b) is taken. In such a cell, Figure 1
When an electric field Ea or Eb with a different polarity, which is equal to or higher than a certain threshold as shown in FIG. , and the liquid crystal molecules change accordingly to the first stable state 10
5a or the second stable state 105b.

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 second reason is that the orientation of liquid crystal molecules has bistability. Second
To further explain this point with reference to FIG. 10, for example, when the electric field Ea is applied, the liquid crystal molecules are aligned in a first stable state 105a, and this state remains stable even when the electric field is turned off. Furthermore, when an electric field Eb in the opposite direction is applied, the liquid crystal molecules are oriented to a second stable state 105b and change their orientation, but they remain in this state even after the electric field is turned off. Also,
As long as the applied electric field Ea or Eb 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 that the cell be as thin as possible, and generally 0.5 μm to 20 μm, particularly 1 μm to 5 μm is suitable. A liquid crystal-electro-optical device having a matrix electrode structure using this type of ferroelectric liquid crystal has been proposed by Clark and Ragabal, for example, in US Pat. No. 4,367,924.

According to the present invention, when the voltages applied between the drain electrode and the source electrode of an element having an FET (field effect transistor) structure such as a TFT constituting the active matrix are reversed, the respective drain and source electrodes are activated. This is based on the fact that it operates normally as an FET with the roles of the electrodes reversed. As the elements constituting the active matrix, any element having an FET structure, such as an amorphous silicon TFT or a polycrystalline silicon TFT, can be used. Further, switching elements other than FET structures, such as bipolar transistors that can operate with either positive or negative applied voltages, can be used as well. Furthermore, MIM (Metal-I
It is also possible to use a two-terminal switching element such as a diode or a diode.

[0024] In the N-type FET, VD is the drain voltage and VG is the drain voltage.
When is the gate voltage, VS is the source voltage, and VP is the threshold voltage between the gate and source, VD>VS and VG>VS+
It is in a conductive state when VP, and it is in a non-conductive state when VG<VS+VP.

In a P-type FET, when VD<VS, it becomes conductive when VG<VS+VP, and when VG>VS+
It becomes non-conductive at VP.

Whether the FET is P-type or N-type, which terminal of the FET acts as the drain and which acts as the source is determined by the direction of voltage application. That is, for N-type, the lower voltage serves as a source, and for P-type, higher voltage serves as a source.

In a ferroelectric liquid crystal element, which of the positive and negative voltages applied to the liquid crystal cell will be in the "bright" state and which will be in the "dark" state is determined by the arrangement above and below the cell. The polarization axes of a pair of polarizers in a crossed nicol state,
It can be set freely depending on the direction with respect to the long axis of the liquid crystal molecules.

Since the present invention performs display by controlling the electric field applied to the liquid crystal cell by controlling the voltage between the terminals of each element of the active matrix, the voltage level of each signal is determined according to the following embodiment. Without being bound,
This can be implemented if the potential difference between each signal is maintained relatively.

Embodiment 1 FIG. 1 shows the configuration of an FLC panel and a drive system for driving it according to an embodiment of the present invention. In the figure, 1 is an active matrix drive type FLC panel using TFT as an active element, 2 is an X driver consisting of a shift register and a hold circuit, etc., 3 is a Y driver consisting of a shift register and a latch, etc., 4 is a timing controller, 5 is a polarity inversion circuit for a video signal, 6 is a polarity inversion circuit for a reset signal, and 7 is a switching circuit between a video signal and a reset signal.

In this embodiment, the timing controller and Y driver generate the first gate pulse ■ and the second gate pulse ■ whose timing is slightly delayed (by Td) as shown in FIG. 2(b). is generated and applied to each gate line 9 in sequential horizontal cycles. If we focus on one line or pixel, the frame period up to the next gate pulse is Tf, and the pulses ■ and ■ in FIG. 2(b) correspond to this. And this gate pulse■
, ■, ■, ■, ..., the output to the input signal line 10 to the X driver is a negative reset voltage, a positive gray scale signal voltage, a positive reset voltage, and a negative gray scale, respectively. The operation timings of the polarity inversion circuits 5 and 6 and the switching circuit 7 are controlled so that the signal voltage becomes . . . (the same process is repeated thereafter). Therefore, the driving signal shown in FIG. 2(a) is applied to the pixel of interest through the TFT.
applied through. Furthermore, the TFT is in an off state when these signals are not applied, and the FLC described above
Due to the charge cancellation effect caused by the spontaneous polarization PS, the inter-electrode voltage waveform at the pixel of interest as a capacitive load becomes
c).

In FIG. 2(c), ■ corresponds to negative polarity reset, and at this time, all FLCs in the pixel return to the first stable state, and the entire black state as shown in FIG. 3(a) occurs during time Td. becomes. Thereafter, when a charge Q (=CV1, C: interelectrode capacitance of the pixel, V1: gradation signal voltage) is given to the pixel by applying a positive polarity gradation signal (■), a=Q/
The area (domain) corresponding to 2PS is reversed to white (
3(b)), at this time, the charge Q is canceled by the PS of the FLC, so that voltage attenuation 12 (FIG. 2(c)) occurs. This state continues for a period of time (Tf - Td) (however, Tf >>
Td) Display the gradation state. The reversal rate at this time is T(V1
) [%], then T(V1)=a/S (S: area of the entire pixel), which is approximately equal to the transmittance.

Next, the process moves to state (2). 3 corresponds to a positive polarity reset, and at this time, all FLCs in the pixel shift to the second stable state, becoming completely white as shown in FIG. 3(c). Then, by applying the negative polarity gradation signal (■), the charge Q (=
CV2) is applied to the pixel, and the area (domain) corresponding to a=Q/2PS is inverted to display black as shown in FIG. 3(d). 14 represents the voltage attenuation that occurs at this time. If the reversal rate at this time is a/S=T(V2), the transmittance is 100-T(V2). Therefore, the relationship of the transmittance to the signal voltage is complementary between the positive polarity gray scale signal and the negative polarity gray scale signal described above. Therefore, for example, the relationship between the positive polarity gradation signal V1 and the negative polarity gradation signal -V2 for producing a predetermined transmittance is T(V1)+T(V2)=100.
becomes. Then, the processes of (1), (2), (2) and (2) are repeated to perform display on the FLC panel. In particular, if the sum of the time Td required for resetting ■ and ■ and the gradation signal pulse application time is kept within the horizontal scanning time, the display states of ■ and ■ will be maintained for the frame period, and the pixels will be completely white or black due to reset. There is almost no effect of display.

In reality, the relationship between the gray scale signal voltage and the transmittance is not necessarily linear, but is nonlinear as shown in FIG. FIG. 4 is a plot of the area ratio of reversal to a white state when a voltage V (charge CV) is applied to a pixel in a black state. Since the white and black states are symmetrical, the area ratio when a pixel in a white state is reversed to black by applying a voltage of -V is an inversion of the curve in FIG. 4. In both cases, the reversal is not linearly proportional to the applied voltage. The reason why this happens is not necessarily clear, but it seems to be related to the fact that domain inversion hardly progresses in response to a weak electric field. However, even if the relationship between the applied voltage V and the inversion area ratio T (V) is not linear, if V1 and V2 are selected as shown in Figure 4, the relationship T (V1) + T (V2) = 100 can be established. be satisfied. In other words, if you want to display a 70% halftone level, for example, set the voltage for black reset and white writing to T1 in Figure 4.
= 70%, and set the white reset black writing voltage on the opposite side to voltage V2 (which gives T2 = 30%).
However, the polarity is set to negative). As described above, it is clear that the method of the present invention can be applied to any inversion characteristic T(V).

Furthermore, if this reset and the reversal period of the positive polarity and negative polarity of the gradation signal are set as the horizontal scanning period, and the reset voltages of adjacent scanning lines are driven so as to have opposite polarities, the entire white and Black display is averaged and flickers become less noticeable.

When such a driving method is adopted, the image shown in FIG. 2(c)
As can be seen from the figure, the DC electric field applied to the FLC layer is averaged without being biased in either the positive or negative direction, which prevents the aforementioned impurity ions from attaching and deteriorating the liquid crystal material, resulting in stable display over a long period of time. can be done.

In the above writing method, the width of each pulse in FIG. 2(a) is set to 5 μS, and the height of the reset pulses
v (volt), Ta in Fig. 2(b) is set to 200 μS, Tf is set to 33 mS, and the height of the gray scale signal pulse is selected as described above from the characteristic curve in Fig. 4. Halftone levels up to 100% could be displayed stably.

Embodiment 2 FIG. 5 shows another embodiment of the present invention, in which, unlike FIG. 1, a two-terminal switching element is used. Possible switching elements include MIM elements, diodes, and combinations of multiple elements.
In the following, explanation will be given taking MIM as an example. One terminal of the MIM is connected to the pixel electrode 51, and the other terminal is connected to the scanning signal line 59.
Information signal electrodes 58 are patterned in stripes on the opposite substrate side. M used in this example
The IM has a structure in which a thin film of tantalum pentoxide is sandwiched between two tantalum layers, and has a threshold value of about 1V. FIG. 6 shows the timing of the drive signal in this embodiment, in which (a) shows the voltage applied to the information signal electrode 58, and (b) shows the voltage applied to the scanning signal line 59.
(c) is the voltage waveform appearing at both ends of the pixel. At the time of reset (2), a negative voltage of -7V is applied to the scanning line 59 and 0V is applied to the information electrode. At the time of writing (2), a positive selection voltage +7v is applied to the scanning signal line, and a voltage from 0v to +7v is applied to the information electrode according to the gradation level. In the opposite periods ■ and ■, pulses of opposite polarity to ■ and ■ are applied, but the gradation signal level is
Not only are the polarities reversed, but also the amplitudes are generally different, and are selected so as to satisfy T(V1)+T(V2)=100%, as in the first embodiment.

Comparative Example 1 When driven by the one-polarity reset method shown in FIG.
The display disappeared after 2 or 3 seconds. The pulse width and frame period at this time were the same as in Example 1. The reason why the display disappeared is because ions moved within the liquid crystal due to the residual DC voltage, creating a built-in electric field on the opposite side of the writing electric field, reducing the effective writing electric field.

Comparative Example 2 The same drive waveform shown in FIG. 2 as in Example 1 was used, except that the positive side inversion rate T (V1) was 70% and the negative side inversion rate T (V2) was 2.
When I set the voltage for writing so that it was 5%,
Flicker was noticeable and display quality deteriorated. In addition, the positive side reversal rate T (V1) is 70%, and the negative side reversal rate T (V2) is 3
Even when set to 5%, flicker was visible.

[0040]

Effects of the Invention As explained above, by reversing the polarity of the reset voltage and the polarity of the gray scale signal at regular intervals, deterioration of the liquid crystal material and FL caused by impurity ions can be prevented.
This makes it possible to prevent the bistability of C from decreasing.

Furthermore, by setting the period of these polarity inversions as a horizontal period and driving the reset voltages of adjacent scanning lines so that they have opposite polarities, flickers and the like can be prevented.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of an FLC panel and a drive system according to a first embodiment of the present invention.

FIG. 2 is a diagram of various signal waveforms in the driving method according to the first embodiment.

FIG. 3 is a display state diagram of a predetermined pixel (pixel of interest) in the first embodiment.

FIG. 4 is a diagram showing the relationship between tone signals and transmittance in a ferroelectric liquid crystal element.

FIG. 5 is a block diagram of an FLC panel and drive system according to a second embodiment of the present invention.

FIG. 6 is a diagram of various signal waveforms in a driving method according to a second embodiment.

FIG. 7 is a layer configuration diagram of an FLC element.

FIG. 8 is a characteristic diagram in an area gradation method using charge modulation.

FIG. 9 is a diagram schematically showing an example of a ferroelectric liquid crystal cell.

FIG. 10 is a diagram schematically showing an example of a ferroelectric liquid crystal cell in which ferroelectric liquid crystal molecules have a non-helical structure.

[Explanation of symbols]

1 FLC panel 2 )59 Scanning signal lines 70a, 70b Substrate 71 Gate electrode 72 Insulating film 73 Semiconductor film 74 Source (drain) terminal 75 Drain (source) terminal 77a Insulating layer 77b Insulating film 78 Light shielding film 80 Ferroelectric liquid crystal 81 Sealing material 82a, 82b Polarizer 83 Reflector 91a, 91b Substrate 92 Liquid crystal molecule layer

Claims (8)

[Claims] 1. A ferroelectric liquid crystal element in which a switching element is provided for each pixel electrode arranged in a matrix, and a ferroelectric liquid crystal is sandwiched between the counter electrode and the entire pixel between the two electrodes. a reset voltage for resetting the dielectric liquid crystal to a first stable state, and a polarity opposite to that of the reset voltage;
voltage applying means for alternately switching and applying a grayscale signal voltage that partially transitions the pixel to a second stable state; means for inverting the reset voltage at regular intervals; and means for inverting the reset voltage at regular intervals; means for inverting the signal voltage, and the state inversion rate of the ferroelectric liquid crystal when the gray scale signal voltage is V is T(
V)%, the grayscale signal voltage V1 after negative polarity reset
and the corresponding gradation signal voltage after positive polarity reset -V
A ferroelectric liquid crystal element characterized in that 2 and 2 satisfy the relationship T(V1)+T(V2)=100. 2. A ferroelectric liquid crystal element according to claim 1, wherein the period is a scanning period of one screen. 3. The ferroelectric liquid crystal device according to claim 1, wherein the reset voltages of adjacent scanning lines have opposite polarities. 4. A ferroelectric liquid crystal device according to claim 1, wherein the first stable state corresponds to a black state. 5. A driving method for driving a liquid crystal display element in which a switching element is provided for each pixel electrode arranged in a matrix, and a ferroelectric liquid crystal is sandwiched between a counter electrode and a pixel electrode. Applying a reset voltage that resets the entire ferroelectric liquid crystal to the first stable state, and partially transitioning the pixel to the second stable state by using a gray scale signal voltage of opposite polarity to the reset voltage. If the polarity of the reset voltage is reversed at regular intervals, and the state reversal rate of the ferroelectric liquid crystal when the gray scale signal voltage is V is T(V)%, then the gray scale after negative polarity reset is A driving method characterized in that a tone signal voltage V1 and a corresponding tone signal voltage -V2 after positive polarity reset satisfy the following relationship: T(V1)+T(V2)=100. 6. The driving method according to claim 5, wherein the period is a scanning period of one screen. 7. The driving method according to claim 5, wherein the reset voltages of adjacent scanning lines have opposite polarities. 8. The driving method according to claim 5, wherein the first stable state corresponds to a black state.