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Claims  |
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What is claimed is:
1. A liquid crystal apparatus, comprising:
a ferroelectric liquid crystal device having:
a group of scanning electrodes;
a group of signal electrodes disposed to intersect the scanning electrodes;
and
a ferroelectric liquid crystal having a first and a second threshold
voltage of one and another polarity, respectively, disposed between the
scanning electrodes and the signal electrodes so as to form a picture
element at each intersection; and
voltage signal application means for:
(a) applying to a selected scanning electrode a scanning selection signal
comprising;
a voltage of one polarity or another polarity with respect to the voltage
level of a non-selected scanning electrode; and
a same level voltage which is at the same voltage level as that of the
non-selected scanning electrode;
(b) applying to a selected electrode an information signal comprising:
a first voltage signal providing a voltage exceeding the first or second
threshold voltage in synchronism with said voltage of one polarity or
another polarity; and
an alternating voltage signal commencing with a voltage of a polarity
opposite to that of the first voltage signal with respect to the voltage
level of the non-selected scanning electrode in the application period of
said same level voltage; and
(c) applying to another signal electrode an information signal comprising:
a second voltage signal providing a voltage not exceeding the first or
second threshold voltage of the ferroelectric liquid crystal in
synchronism with said voltage of one polarity or the another polarity; and
an alternating voltage signal commencing with a voltage of a polarity
opposite to that of the second voltage signal with respect to the voltage
level of the non-selected scanning electrode in the application period of
said same level voltage.
2. The apparatus according to claim 1, wherein the average voltage of each
of the information signals is at the same level as the voltage level of
the non-selected scanning electrode throughout the application period of
the scanning selection signal.
3. The apparatus according to claim 1, wherein the voltage level of the
non-selected scanning electrode is zero.
4. The apparatus according to claim 1, wherein each of the first and second
voltage signals has a pulse duration T and each of the alternating voltage
signals comprises pulses having unit pulse duration T.sub.0 which is
shorter than T.
5. The apparatus according to claim 1, wherein said ferroelectric liquid
crystal is a chiral smectic liquid crystal.
6. The apparatus according to claim 5, wherein said chiral smectic liquid
crystal assumes a non-spiral structure.
7. The apparatus according to claim 5, wherein said chiral smectic liquid
crystal is in the C phase, the H phase, the I phase, the J phase, the K
phase, the G phase or the F phase.
8. The apparatus according to claim 1, wherein each of the alternating
voltage signals is applied after the first or second voltage signal and
another alternating voltage signal is applied before the first or second
voltage signal.
9. A liquid crystal apparatus, comprising:
a ferroelectric liquid crystal device comprising:
a group of scanning electrodes;
a group of signal electrodes disposed to intersect the scanning electrodes;
and
a ferroelectric liquid crystal having a first and a second threshold
voltage of one and another polarity, respectively, disposed between the
scanning electrodes and the signal electrodes so as to form a picture
element at each intersection; and
voltage signal application means for:
(a) applying to a selected scanning electrode a scanning selection signal
comprising:
a voltage of one polarity and a voltage of another polarity, respectively,
with respect to the voltage level of a non-selected scanning electrode;
and
a same level voltage at the same voltage level as that of the non-selected
scanning electrode;
(b) applying to a selected signal electrode an information signal
comprising:
a first voltage signal providing a voltage exceeding the first threshold
voltage in synchronism with said voltage of one polarity; and
an alternating voltage signal commencing with a voltage of a polarity
opposite to that of the first voltage signal with respect to the voltage
level of the non-selected scanning electrode in the application period of
said same level voltage; and
(c) applying to another signal electrode an information signal comprising:
a second voltage signal providing a voltage exceeding the second threshold
voltage of the ferroelectric liquid crystal in synchronism with said
voltage of another polarity; and
an alternating voltage signal commencing with a voltage of a polarity
opposite to that of the second voltage signal with respect to the voltage
level of the non-selected scanning electrode in the application period of
said same level voltage.
10. The apparatus according to claim 9, wherein the voltage of one polarity
and the voltage of the another polarity in the scanning selection signal
are consecutive in time.
11. The apparatus according to claim 9, wherein the voltage level of the
non-selected scanning electrode is zero.
12. The apparatus according to claim 9, wherein each of the first and
second voltage signals has a pulse duration T and each of the alternating
voltage signals comprises pulses having a unit pulse duration T.sub.0
which is shorter than T.
13. The apparatus according to claim 9, wherein said ferroelectric liquid
crystal is a chiral smectic liquid crystal.
14. The apparatus according to claim 13, wherein said chiral smectic liquid
crystal assumes a non-spiral structure.
15. The apparatus according to claim 13, wherein said chiral smectic liquid
crystal is in the C phase, the H phase, the I phase, the J phase, the K
phase, the G phase, or the F phase.
16. The apparatus according to claim 9, wherein each of the alternating
voltage signals is applied after the first or second voltage signal and
another alternating voltage signal is applied before the first or second
voltage signal.
17. The apparatus according to claim 9, wherein each of the information
signals integrally assumes a voltage of the same level as the voltage
level of the non-selected scanning electrode throughout the application
period of the scanning selection signal.
18. A liquid crystal apparatus, comprising:
a ferroelectric liquid crystal device comprising:
a group of scanning electrodes;
a group of signal electrodes disposed to intersect the scanning electrodes;
and
a ferroelectric liquid crystal having a first and a second threshold
voltage of one and another polarity, respectively, disposed between the
scanning electrodes and the signal electrodes so as to form a picture
element at each intersection; and
voltage signal application means for:
(a) applying to a selected scanning electrode a scanning selection signal
comprising:
a voltage of one polarity and a voltage of another polarity with respect to
the voltage level of a non-selected scanning electrode; and
a same level voltage which is at the same voltage level as that of the
non-selected scanning electrode;
(b) applying to all or a prescribed number of the signal electrodes a first
voltage signal providing a voltage exceeding the first threshold voltage
of the ferroelectric liquid crystal in synchronism with said voltage of
one polarity; and
(c) applying to a selected signal electrode an information signal
comprising:
a second voltage signal providing a voltage not exceeding the second
threshold voltage of the ferroelectric liquid crystal in synchronism with
said voltage of the another polarity; and
an alternating voltage signal commencing with a voltage of a polarity
opposite to that of the second voltage signal with respect to the voltage
level of the non-selected scanning electrode in the application period of
said same level voltage.
19. The apparatus according to claim 18, wherein the voltage of one
polarity and the voltage of the another polarity in the scanning selection
signal are consecutive in time.
20. The apparatus according to claim 18, wherein the voltage level of the
non-selected scanning electrode is zero.
21. The apparatus according to claim 18, wherein each of the first and
second voltage signals has a pulse duration T and the alternating voltage
signal comprises pulses having a unit pulse duration T.sub.0 which is
shorter than T.
22. The apparatus according to claim 18, wherein said ferroelectric liquid
crystal is a chiral smectic liquid crystal.
23. The apparatus according to claim 22, wherein said chiral smectic liquid
crystal assumes a non-spiral structure.
24. The apparatus according to claim 22, wherein said chiral smectic liquid
crystal is in the C phase, the H phase, the I phase, the J phase, the K
phase, the G phase or the F phase.
25. The apparatus according to claim 18, wherein the alternating voltage
signal is applied after the first or second voltage signal and another
alternating voltage signal is applied before the first or second voltage
signal.
26. The apparatus according to claim 18, wherein said voltage of one
polarity is applied to all or a prescribed number of the scanning
electrodes simultaneously.
27. The apparatus according to claim 18, wherein the average voltage of the
information signal is at the same level as the voltage level of the
non-selected scanning electrode throughout the application period of the
scanning selection signal.
28. A driving method for a liquid crystal device of the type comprising a
matrix electrode structure having a first group of stripe electrodes and a
second group of stripe electrodes disposed opposite to and intersecting
the first group of stripe electrodes, and a ferroelectric liquid crystal
displaying a first state and a second state and disposed between the first
and second groups of stripe electrodes so as to form a picture element at
each intersection of the stripe electrodes, said driving method comprising
the steps of:
applying a first voltage signal to a plurality of said picture elements for
orienting the ferroelectric liquid crystal in the first state in a first
phase for a duration .DELTA.T, and applying a second voltage signal to
said plurality of picture elements for orienting the ferroelectric liquid
crystal in the second state in a second phase for a duration .DELTA.T,
whereby writing is effected in the first and second phases; and
applying to the remaining picture elements an alternating voltage signal
such that the maximum duration during which any voltage of one polarity of
the alternating voltage is applied to the remaining picture elements is
3.DELTA.T.
29. The driving method according to claim 28, wherein said first and second
phases are consecutive in time.
30. The driving method according to claim 29, wherein the average potential
of the alternating voltage signal throughout the application period of the
scanning selection signal is substantially equal to a reference potential,
wherein said reference potential is zero.
31. The driving method according to claim 28, wherein said ferroelectric
liquid crystal is a chiral smectic liquid crystal.
32. The driving method according to claim 31, wherein said chiral smectic
liquid crystal assumes a non-spiral structure.
33. The driving method according to claim 31, wherein said chiral smectic
liquid crystal is in the C phase, the H phase, the I phase, the J phase,
the K phase, the G phase or the F phase.
34. The driving method according to claim 28, further comprising the step
of:
applying to a selected first stripe electrode a scanning selection signal
comprising a voltage of one polarity and a voltage of another polarity,
respectively, with respect to the voltage level of a non-selected first
strips electrode, and in synchronism with the scanning selection signal,
and applying to a signal electrode an information signal which integrally
assumes the same voltage level as the voltage level of the non-selected
first stripe electrode throughout the application period of the scanning
selection signal.
35. The driving method according to claim 34, wherein the voltage level of
said non-selected first stripe electrode is zero.
36. A driving method for a liquid crystal device of the type comprising a
matrix electrode structure having a plurality of first stripe electrodes
and a plurality of second stripe electrodes disposed opposite to and
intersecting said first stripe electrodes, and a ferroelectric liquid
crystal displaying a first state and a second state and disposed between
the first and second stripe electrodes so as to form a picture element at
each intersection of the stripe electrodes; said driving method comprising
the steps of:
in a first phase, applying a voltage signal for orienting the ferroelectric
liquid crystal in the first stage simultaneously to the intersections of
all or a prescribed part of the first strips electrodes and all or a
prescribed part of the second stripe electrodes;
in a second phase,
applying to a selected first stripe electrode a scanning selection signal
comprising:
a voltage with a duration .DELTA.T of one or another polarity with respect
to the voltage level of the non-selected first stripe electrode; and
a same level voltage which is at the same voltage level as that of the
non-selected first stripe electrode; and
applying an information signal comprising an alternating voltage in
synchronism with the scanning selection signal; and
applying to the intersections of the second stripe electrodes and a
non-selected first stripe electrode an alternating voltage signal such
that the maximum duration during which any voltage of one polarity of the
alternating voltage is applied to said intersections is 3 .DELTA.T.
37. The driving method according to claim 36, wherein said ferroelectric
liquid crystal is a chiral smectic liquid crystal.
38. The driving method according to claim 37, wherein said chiral smectic
liquid crystal assumes a non-spiral structure.
39. The driving method according to claim 37, wherein said chiral smectic
liquid crystal is in the C phase, the H phase, the I phase, the G phase or
the F phase.
40. The driving method according to claim 36, wherein the voltage level of
said non-selected first stripe electrode is zero.
41. The driving method according to claim 36, wherein the average voltage
of the information signal is at the same voltage level as the voltage
level of the non-selected first stripe electrode throughout the
application period of the scanning selection signal.
42. The driving method according to claim 41, wherein the voltage level of
said non-selected first stripe electrode is zero. |
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Claims |
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Description |
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FIELD OF THE INVENTION AND RELATED ART
The present invention relates to a driving method for a liquid crystal
device such as a liquid crystal display device and a liquid crystal
optical shutter array, and more particularly, to a driving method for a
liquid crystal device having improved display and driving characteristics
through improved initial orientation of liquid crystal molecules.
As a conventional liquid crystal device, there has been known, for example,
one using TN (twisted nematic) type liquid crystals, as shown in
"Voltage-Dependent Optical Activity of a Twisted Nematic Liquid Crystal"
by M. Schadt and W. Helfrich, "Applied Physics Letters" vol. 18, No. 4
(Feb. 15, 1971), pp. 127-128. This TN-type liquid crystal device has the
disadvantage that a crosstalk phenomenon occurs when a device having a
matrix electrode structure arranged to provide a high picture element
density is driven in a time division manner, so that the number of picture
elements is restricted.
Further, a type of display device is known, in which each picture element
is provided with a switching element comprising a thin film transistor
connected thereto so that the picture elements are switched respectively.
This type of device, however, requires an extremely complicated step for
forming thin film transistors on a base plate, moreover, involves it is
difficult to produce a large area of display device.
In order to solve these problems, a ferroelectric liquid crystal device,
utilizing a ferroelectric liquid crystal placed under a bistability
condition, has been developed by Clark et al. in, e.g., U.S. Pat. No.
4,367,924.
This ferroelectric liquid crystal device exhibit a memory effect, as
explained hereinafter, but also has undesirable effects. More
specifically, when a device is constructed to have a matrix electrode
structure comprising scanning lines and data lines is driven in a time
division manner, a picture element which has been written in one signal
state by applying thereto a writing voltage above a threshold value of one
polarity, can reverse the its signal state (e.g., from the written "white"
state to an opposite "black" state) when continually subjected to a
voltage of reverse polarity for a long period of, e.g., 5 times or more,
as long as the writing voltage pulse duration, even when the voltage of
reverse polarity is below a threshold voltage. This reversal phenomenon
has been discovered by our experiments.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide a time
division driving method for a ferroelectric liquid crystal device having a
matrix electrode structure comprising scanning lines and data lines.
Another object of the present invention is to provide a driving method for
a ferroelectric liquid crystal device for preventing the occurrence of the
above mentioned reversal phenomenon.
According to the present invention, there is provided a driving method for
a liquid crystal device of the type comprising a matrix electrode
structure having scanning lines and data lines, and a ferroelectric liquid
crystal, the driving method comprising: in a first time period, applying a
scanning selection signal to a scanning line, and applying an information
signal to a data line in synchronism with the scanning selection signal,
and in a second time period, applying an alternating auxiliary signal to
the data line.
These and other objects, features and advantages of the present invention
will become more apparent upon a consideration of the following
description of the preferred embodiments of the present invention taken in
conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1 and 2 are schematic perspective views for explaining operating
principles of a ferroelectric liquid crystal device to be used in the
present invention;
FIG. 3 is a plan view schematically illustrating a matrix electrode
arrangement use in the present invention;
FIGS. 4, 5 and 6 respectively illustrated time-serial waveforms of signals
applied to scanning and data lines and voltages applied to picture
elements used in the driving method according to the present invention;
FIG. 7(a)-(e), FIG. 8(a)-(e) and FIG. 9(a)-(e) respectively show signals
and voltages applied in other embodiments of the driving method according
to the present invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Ferroelectric liquid crystals which can be suitably used in the present
invention are chiral smectic liquid crystals, particularly those showing
chiral smectic C phase (SmC*), H phase (SmH*), I phase (SmI*), J phase
(SmJ*), K phase (SmK*), G phase (SmG*) or F phase (SmF*) .
Ferroelectric liquid crystals are described in detail in, e.g., "LE JOURNAL
DE PHYSIQUE LETTERS" 36 (L-69) 1975, "Ferroelectric Liquid Crystals":
"Applied Physics Letters" 36 (11) 1980 "Submicrosecond Bistable
Electro-optic Switching in Liquid Crystals"; "Kotai Butsuri (Solid State
Physics)" 16 (141) 1981 "Liquid Crystals", etc. In the present invention,
ferroelectric liquid crystals disclosed in these publication may be used.
Specific examples of ferroelectric liquid crystals to be used in the
present invention include decyloxybenzylidene-p'-amino-2-methylbutyl
cinnamate (DOBAMBC), hexyloxybenzylidene-p'-amino-2-chloropropyl cinnamate
(HOBACPC), 4-o-(2-methyl)-butylresorcylidene-4'-octylaniline (MBRA 8) and
those disclosed in European published Patent Applications EP-A No. 110299
and EP-A No. 115693.
FIG. 1 is a view schematically illustrating an example of a liquid crystal
cell for the purpose of explaining the operation of a ferroelectric liquid
crystal. Reference numerals 11 and 11a denote base plates (glass plates)
coated with transparent electrodes comprising thin films of In.sub.2
O.sub.3, SnO.sub.2, ITO (Indium-Tin Oxide), etc. A liquid crystal having
SmC*- or SmH*-phase, in which liquid crystal layers 12 are oriented
vertically to the surfaces of base plates is hermetically disposed between
the base plates 11 and 11a. Full lines 13 denote liquid crystal molecules,
respectively. These liquid crystal molecules 13 have dipole moments
(P.sub..perp.) 14 perpendicular to the orientation of the molecules. When
a voltage higher than a certain threshold is applied between electrodes on
the base plates 11 and 11a, the helical structures of liquid crystal
molecules 13 are loosened. Thus, the orientation directions of liquid
crystal molecules 13 can be changed so that dipole moments (P.sub..perp.)
14 are all directed in the direction of the applied electric field. Liquid
crystal molecules 13 have elongated shapes, and show refractive index
anisotropy between the long and short axes. Accordingly, it is easily
understood that, for instance, when polarizers having a cross nicol
relationship to each other, (i.e., their polarizing axes are crossing or
perpendicular to each other) are arranged on the upper and lower sides of
the glass surfaces, a liquid crystal optical modulation device, having
optical characteristics which change, depending upon the polarity of an
applied voltage, can be realized. When the thickness of the liquid crystal
layer used in the liquid crystal cell is made sufficiently thin (e.g.,
about 1 .mu.), the helical structures of the liquid crystal molecules are
loosened even in the absence of an electric field as shown in FIG. 2.
Dipole moments P and Pa can change in either direction, i.e., in upper
(24) and lower (24a) directions, respectively. When electric fields E and
Ea having polarities different from each other and higher than a certain
threshold level are applied to the cell thus formed with voltage applying
means 11 and 11a, the dipole moments change in the upper (24) or lower
(24a) direction, depending upon the electric field vector of the electric
field E or Ea, respectively. In accordance with the changes, the liquid
crystal molecules are oriented to either of the first stable state 23 and
the second stable state 23a.
As previously mentioned, the application of such a ferroelectric liquid
crystal to an optical modulation device can provide two major advantages.
First, the response speed is quite fast. Second, the liquid crystal
molecules show bistability in regard to their orientation. The second
advantage will be further explained, e.g., with reference to FIG. 2. When
the electric field E is applied, the liquid crystal molecules are oriented
to the first stable state 23. This state is stably maintained even if the
applied electric field is removed. On the other hand, when the opposite
electric field Ea is applied, they are oriented to the second stable state
23a to change their direction. Likewise, the latter state is stably
maintained even if the applied electric field is removed. Further, as long
as the given electric field E or Ea is not above a certain threshold
level, they are maintained at their respective oriented states. For
effectively realizing such high response speed and bistability, it is
preferable that the thickness of the cell is as thin as possible and
generally 0.5 to 20 .mu., particularly 1 to 5 .mu.. A liquid crystal
electrooptical device having a matrix electrode structure in which the
ferroelectric liquid crystal of this kind is used is proposed, e.g., in
the specification of U.S. Pat. No. 4,367,924 of Clark and Ragerwall.
The operation of a ferroelectric liquid crystal device has been explained
above with reference to a somewhat idealistic mode. The microscopic
mechanism of switching due to an electric field applied to a ferroelectric
liquid crystal having bistability has not been fully clarified. Generally
speaking, however, the ferroelectric liquid crystal can retain its stable
state semi-permanently, if it has been switched or oriented to the stable
state by the application of a strong electric field for a predetermined
time and is left standing under absolutely no electric field. However,
when a an electric field of a reverse polarity is applied to the liquid
crystal for a long period of time, even if the electric field is
sufficiently weak (corresponding to a voltage below the threshold value in
the previous example) that the stable state of the liquid crystal is not
switched in a predetermined time for writing, the liquid crystal can
change its stable state to the other state, whereby correct display or
modulation of information cannot be accomplished. We have recognized that
the liability of such switching or reversal of oriented states under the
long term application of a weak electric field is affected by the material
and roughness of the base plate contacting the liquid crystal and the kind
of the liquid crystal, but have not clarified the effects quantitatively.
We have confirmed that a monoaxial treatment of the base plate such as
rubbing or oblique or tilt vapor deposition of SiO, etc., tends to
increase the liability of the above-mentioned reversal of oriented states.
This tendency is manifested at higher temperature, rather than lower
temperature.
In order to accomplish correct display or modulation of information, it is
advisable that electric field in one direction are prevented from being
applied to the liquid crystal for a long time.
Hereinbelow, a preferred embodiment of the driving method according to the
present invention will be explained with reference to the drawings.
FIG. 3 is a view schematically showing a liquid crystal device 31 having a
matrix electrode arrangement between which a ferroelectric liquid crystal
compound is interposed. Reference numerals 32 and 33 denote a group of
scanning lines composed of stripe electrodes and a group of data lines
composed of stripe electrodes, respectively.
FIG. 4 shows the waveforms of signals applied to scanning and data lines
and the voltages applied to the picture elements used in a preferred
embodiment according to the present invention.
In the embodiment shown in FIG. 4, a scanning selection signal of 2V.sub.0
in phase T.sub.1 and -2V.sub.0 in subsequent phase T.sub.2 is applied to a
scanning line S.sub.1 as shown at S.sub.1 in FIG. 4. In synchronism with
the scanning selection signal, an information signal of V.sub.0 (for
writing "white") or -V.sub.0 (for writing "black") is applied to data
lines (I.sub.1, I.sub.2, . . . ), whereby a voltage of 3V.sub.0 is applied
in phase T.sub.1 to a picture element (e.g., picture element B shown in
FIG. 3) to write "black" therein and a voltage -3V.sub.0 is applied in
phase T.sub.2 to another picture element (e.g., picture element A shown in
FIG. 3) to write "white" therein. Herein, the voltage value V.sub.0 is set
to satisfy the following relationships:
V.sub.0 <V.sub.th1 <3V.sub.0, and
-V.sub.0 >-V.sub.th2 >-3V.sub.0,
wherein V.sub.th1 is a threshold voltage for a first stable orientation
state, and V.sub.th2 is a threshold voltage for a second stable state.
Furthermore, voltage waveforms shown at D and C in FIG. 4 are applied to
picture elements D and C shown in FIG. 3, whereby these picture elements
are respectively written in "white" as shown in FIG. 3.
Then, in phases T.sub.3, T.sub.4, T.sub.5 and T.sub.6, an alternating
auxiliary signal is applied to data lines. Herein, the term "alternating
signal" means that the signal crosses a reference potential 10 volt or a
bias voltage level, if any) at least once. By application of the
alternating auxiliary signal, even if a signal for writing, e.g., "black"
is successively applied from a data line in phases T.sub.1 and T.sub.2 to
a picture element which has been written in "white", the period in which a
voltage signal of the same polarity as the signal for writing "black" is
restricted to 3T.sub.0 (T.sub.0 : unit pulse duration) at the maximum
because an auxiliary period comprising phases T.sub.3, T.sub.4, T.sub.5
and T.sub.6 for applying an alternating auxiliary signal is provided,
whereby a picture element in which a signal state has been written does
not reverse but retains the signal state for a period of substantially one
frame or one field. Herein, the total of phases T.sub.1, T.sub.2, T.sub.3,
T.sub.4, T.sub.5 and T.sub.6 corresponds to one horizontal scanning
period.
In a preferred embodiment according to the present invention, the average
potential of the combination of the alternating auxiliary signal and the
information signal applied to a data line may be a reference potential (a
bias voltage level when a bias voltage is applied or 0 volt when no bias
voltage is applied). The alternating auxiliary signal preferably comprises
a rectangular pulse, and the pulse duration T.sub.0 (T.sub.3, T.sub.4,
T.sub.5 or T.sub.6) is preferably equal to or shorter than the pulse
duration T (T.sub.1 or T.sub.2 =0.1 .mu.sec to 1 msec) of the information
signal, e.g., 0.1 to 1.0 times T.
FIG. 5 shows the waveforms of signals and voltages applied in another
preferred driving embodiment.
In the embodiment shown in FIG. 5, in phase T.sub.1, an electric signal
(i.e., a voltage) of 3V.sub.0 is applied to all the picture elements on a
scanning line S.sub.1 to be written, whereby the signal states written in
the preceding field or frame are erased into "white" states. Then, in the
subsequent phase T.sub.2, a scanning selection signal of -2V.sub.0 is
applied to the scanning line. In synchronism with the scanning selection
signal, an information selection signal of V.sub.0 for writing "black" or
an information non-selection signal of -V.sub.0 for holding the "white"
state is applied to data lines.
In this driving mode wherein writing and erasure are effected line by line
for scanning lines to form an image, the above mentioned reversed
phenomenon also occurs. In this embodiment, phases T.sub.3, T.sub.4 and
T.sub.5 are provided for applying an alternating auxiliary signal, so that
the above mentioned reversal phenomenon can be prevented. This alternating
auxiliary signal may have an opposite polarity in phase T.sub.3, the same
polarity in phase T.sub.4 and an opposite polarity in phase T.sub.5 with
respect to an information signal applied to the data line in phase
T.sub.2. The average potential of the electric signals applied to a data
line throughout one field or frame period is a bias voltage or 0 volt as
in the driving example explained with reference to FIG. 4.
In this embodiment, as seen from FIG. 5, the maximum period in which one
polarity of voltages is continually applied to a picture element is
2T.sub.0 (T.sub.0 : unit pulse duration), whereby the above mentioned
reversal phenomenon does not occur at all. The total period of phases
T.sub.1, T.sub.2, T.sub.3, T.sub.4 and T.sub.5 corresponds to one
horizontal scanning period.
FIG. 6 shows waveforms of signals and voltages applied in still another
embodiment according to the present invention.
In the embodiment shown in FIG. 6, all or a part of the picture elements on
the whole picture written in the previous field or frame is erased
(written in "black") at the same time and then successively written (in
"white"). More specifically, in an erasure step C.sub.1, -2V.sub.0 is
applied to the scanning lines simultaneously while V.sub.0 is applied to
the data lines, whereby a voltage of -3V.sub.0 is applied to all the
picture elements to erase the whole picture into "black". In a subsequent
writing step C.sub.2, a scanning selection signal of 2V.sub.0 is applied
to the scanning lines line by line, and in synchronism with the scanning
selection signal, an information selection signal of -V.sub.0 for writing
"white" or an information non-selection signal of V.sub.0 for retaining
the "black" state is applied to data lines in phase T.sub.1.
In this embodiment, phases T.sub.2, T.sub.3 and T.sub.4 are provided for
applying an alternating auxiliary signal. The alternating auxiliary signal
is a signal having an opposite polarity in phase T.sub.2, the same
polarity in phase T.sub.3 and an opposite polarity in phase T.sub.4 with
respect to an information signal applied to the data line in phase
T.sub.1. By applying the alternating auxiliary signal to data lines in
phases T.sub.2, T.sub.3 and T.sub.4, the maximum period wherein one
polarity of voltage is applied to a picture element is restricted to
3T.sub.0 (T.sub.0 : unit pulse duration), so that the above mentioned
reversal phenomenon does not occur. The total period of phases T.sub.1,
T.sub.2, T.sub.3 and T.sub.4 corresponds to one horizontal scanning
period.
Waveforms indicated at D and C in FIG. 5 and at A and C in FIG. 6 are those
of voltage applied to picture elements D, C and A shown in FIG. 3, while
the displayed states do not accurately correspond respectively.
In the above embodiments, the pulse durations of each alternating auxiliary
signal applied in different phases may be the same or different from each
other, and the peak value or height of the pulse can be varied depending
on the pulse durations.
FIG. 7 shows a modification of the alternating auxiliary signal used in the
driving mode shown in FIG. 6, wherein the whole picture elements are
erased simultaneously and then written successively. FIG. 7(a) shows a
scanning selection signal of 2V.sub.0 applied to a scanning line S, while
FIG. 7(b) and 7(c) show an information non-selection signal NS at
I.sub.OFF and an information selection signal SS at I.sub.ON,
respectively, combined with alternating auxiliary signals AS. FIGS. 7(d)
and 7(e) show a voltage waveform S/I.sub.OFF applied to a picture element
to which the information non-selection signal is applied and a voltage
waveform S/I.sub.ON applied to a picture element to which the information
selection signal is applied, respectively, on a scanning line to which the
scanning selection signal is applied.
In the waveform shown in FIG. 7(d), the phase periods may be set to satisfy
the relationship: .DELTA.T.sub.3 =.DELTA.T.sub.6 =.DELTA.T, .DELTA.T.sub.1
=.DELTA.T.sub.2 =.delta..sub.1, .DELTA.T.sub.4 =.DELTA.T.sub.5
=.delta..sub.2, .delta..sub.1 <.DELTA.T and .delta..sub.2 <.DELTA.T. In
this case, the maximum period in which an electric field in a reverse
direction is continually applied is either .DELTA.T+.delta..sub.2 or
.DELTA.T+.delta..sub.1 which is anyway shorter than 2.DELTA.T. In this
embodiment, as shown in FIGS. 7(b) and 7(c), the alternating auxiliary
signals applied before and after the information signals are reverse in
directions between those combined with the information non-selection
signal and those combined with the information selection signal. Moreover,
the portions of the alternating auxiliary signals immediately before and
after an information signal are mutually opposite in direction or polarity
with respect to the reference potential. Because of these features, a
period in which one polarity of voltage is continually applied to a
picture element does not exceed 3 .DELTA.T.
As shown in FIG. 7, a first alternating auxiliary signal and a second
alternating auxiliary signal are respectively applied before and after a
phase for applying an information signal, whereby the above mentioned
reversal phenomenon is effectively prevented.
FIGS. 8 and 9 respectively show a modification of a driving mode wherein
picture elements on one scanning line are written in "black (dark)" or
"white (bright)" simultaneously. More specifically, FIGS. 8 and 9
respectively show an embodiment wherein phases for applying a first
alternating auxiliary signal and a second alternating auxiliary signal are
added respectively before and after a phase for applying an information
signal.
In the embodiment shown in FIG. 8, a scanning selection signal of 2V.sub.0
in phase T.sub.3 and -2V.sub.0 in phase T.sub.4 (T.sub.3 =T.sub.4
=.DELTA.T) is applied to a scanning line. In synchronism with the scanning
selection signal, an information signal BS for writing "black" is applied
to a data line I.sub.(DARK) and an information signal WS for writing
"white" is applied to a data line I.sub.(BRIGHT). Further, phases for
applying a first auxiliary signal AS and a second auxiliary signal AS are
provided respectively before and after the phases for applying these
information signals, whereby a period in which a voltage in a reverse
direction is applied can be shortened to 2 .DELTA.T. In this instance, the
unit pulse duration of the alternating auxiliary signal is not necessarily
the same as that of the information signal. FIG. 8(d) shows a voltage
waveform S/I.sub.(DARK) applied to a picture element which is written in
"black" and FIG. 8(e) shows a voltage waveform S/I.sub.(BRIGHT) applied to
a picture element which is written in "white".
FIG. 9 shows a modification of the embodiment shown in FIG. 8. In this
embodiment, alternating auxiliary signals corresponding to but having
different waveforms from the alternating auxiliary signals used in the
embodiment of FIG. 8 are used. FIG. 9(a) shows a selection scanning
signal, FIG. 9(b) a combination of a signal BS for writing "black" with
auxiliary signals AS, FIG. 9(c) a combination of a signal WS for writing
"white" with auxiliary signals AS, FIG. 9(d) a voltage waveform applied to
a picture element for writing "black", and FIG. 9(e) a voltage waveform
applied to a picture element for writing "white".
Hereinbelow, the present invention will be explained with reference to a
specific example.
EXAMPLE 1
A pair of glass plates provided with patterned transparent electrodes of
ITO so as to form a matrix of 500.times.500 picture elements were
respectively coated with an about 300 .ANG.-thick polyimide film by spin
coating. These coated glass plates were respectively subjected to a
rubbing treatment with a suede-finished cotton cloth wrapped around a
roller and applied to each other with their rubbing directions in
alignment, whereby a cell was formed. A ferroelectric liquid crystal
DOBAMBC was injected into the cell and gradually cooled from its isotropic
phase to assume an SmC* phase in a monodomain state. While the cell was
kept at a temperature of 70.degree. C., an image was formed by a driving
mode as explained with reference to FIG. 4, whereby an excellent image was
formed with no irregularity in image caused by reversal phenomenon during
image formation.
The driving method according to the present invention can be widely
applicable to the fields of optical shutters such as liquid
crystal-optical shutters and display devices such as liquid crystal
television sets.
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