Ferroelectric liquid crystal apparatus having temperature compensation control circuit

A ferroelectric liquid crystal device is formed by disposing a ferroelectric chiral smectic liquid crystal between a pair of substrates respectively having thereon one and the other of a group of scanning electrodes and a group of data electrodes disposed so as to form an electrode matrix in combination. A liquid crystal apparatus is constituted so as to detect a temperature of the liquid crystal device and insert a pause period corresponding to the detected temperature in a drive waveform for driving the device.

FIELD OF THE INVENTION AND RELATED ART 
The present invention relates to a liquid crystal apparatus such as a 
display apparatus using a chiral smectic liquid crystal which shows 
ferroelectricity. 
Display apparatus using a ferroelectric chiral smectic liquid crystal have 
been known as disclosed in, e.g., U.S. Pat. Nos. 4,639,089, 4,681,404, 
4,682,858, 4,712,873, 4,712,874, 4,712,875, 4,712,877, 4,714,323, 
4,718,276, 4,738,515, 4,740,060, 4,765,720, 4,778,259, 4,796,979, 
4,796,980, 4,859,036, 4,932,757, 4,932,758, 5,000,545 and 5,007,716. 
Such a display apparatus includes a liquid crystal device comprising a cell 
structure formed by disposing a pair of glass plates each provided with 
transparent electrodes and an aligning treatment on their inner sides 
opposite to each other with a cell gap on the order of 1 to 3 .mu.m and a 
ferroelectric chiral smectic liquid crystal (hereinafter sometimes 
abbreviated as "FLC") filling the cell gap. 
Among such liquid crystal devices, a device containing FLC molecules in an 
alignment state providing a chevron structure as shown in FIG. 1 has been 
known to provide an excellent bright state and thus a sufficiently large 
contrast when combined with crossed nicol polarizers. More specifically, 
FIG. 1 is a sectional view showing an alignment state of FLC 13 disposed 
between substrates 11 and 12. The FLC 13 forms a plurality of layers 14 
each comprising plural liquid crystal molecules 15. The layers 14 are 
aligned substantially in a direction and each layer 15 is bent between the 
substrates. The long axis of each liquid crystal molecule 15 may 
preferably be inclined to form a pretilt angle .alpha. of at least 5 
degrees with respect to the substrates 11 and 12. The above-mentioned 
alignment state may preferably be formed by providing unidirectional 
alignment axes 16 and 17, which are parallel and in the same direction, to 
the substrates 11 and 12, e.g., by rubbing. 
FIG. 2 (including FIGS. 2A-2C) is a plan view of a device in which FLC 13 
assumes a chevron structure as described with reference to FIG. 1. The 
device in FIG. 2 is constituted by fixing the substrates 11 and 12 having 
unidirectional rubbing axes 16 and 17, respectively, to each other by 
means of a sealant 21 to leave a space which is filled with FLC 13. In the 
device, the substrate 11 is provided with a first group of plural stripe 
electrodes for voltage application (not shown), and the substrate 12 is 
provided with a second group of plural stripe electrodes (not shown) 
intersecting the first group of stripe electrodes, thus forming an 
electrode matrix. The normal 22 with a vector n.sub.s to the layers 14 of 
FLC 13 (more exactly the projection of the normal 22 onto the substrates) 
is substantially parallel to the rubbing directions 16 and 17 as shown in 
FIG. 2B. The liquid crystal molecules 15 in the device shown in FIG. 2 
(FIGS. 2B and 2C) are uniformly oriented leftwards at a tilt angle 
+.theta. with their spontaneous polarization directing from the front face 
to the back face of the drawing. 
According to our experiments, when the FLC in this state was supplied with 
a voltage (e.g., an AC voltage of .+-.8 volts and 10 Hz) applied between 
the opposite electrodes, a phenomenon was observed that the liquid crystal 
molecules 15 started to flow rightwards to result in regions 31 with less 
or lacking liquid crystal molecules 15 on the left side and a region 32 
with more liquid crystal molecules 15, when the voltage application was 
continued for a long period (e.g., 20-50 hours), as shown in FIG. 3 where 
P denotes the optical axis of a polarizer and A denotes the optical axis 
of an analyzer arranged in cross nicols. As a result, an interference 
color was observed over the extension of the device to impair the display 
quality. 
In case where the liquid crystal molecules 15 in FIG. 2B were uniformly 
oriented rightwards at a tilt angle -.theta. with their spontaneous 
polarization directing from the back face to the front face of the 
drawing, the liquid crystal molecules 15 were found to move leftwards in 
contrast to the above. 
It was also found that the above phenomenon also depended upon a change in 
environmental temperature and particularly was promoted when the 
environmental temperature was elevated. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a liquid crystal apparatus 
having solved the above-mentioned problem. 
According to the present invention, there is provided a liquid crystal 
apparatus, comprising: 
(a) a liquid crystal panel comprising a plurality of scanning electrodes 
and a plurality of data electrodes intersecting the scanning electrodes so 
as to form an electrode matrix, and a chiral smectic liquid crystal 
disposed between the scanning electrodes and the data electrodes, 
(b) drive means for sequentially selecting a scanning electrode from the 
scanning electrodes by sequentially applying a scanning selection signal 
to the scanning electrodes, and applying voltage waveform signals to the 
data electrodes, each voltage waveform signal including a data signal, a 
voltage pulse of a polarity opposite to that of the data signal and a 
pulse of voltage zero, respectively with respect to the voltage level of a 
non-selected scanning line, in a scanning selection period for the 
selected scanning electrode, the data signal providing a voltage 
sufficient to orient the chiral smectic liquid crystal at an intersection 
of the selected scanning electrode and an associated data electrode to 
either one or another orientation state depending on the polarity of the 
voltage in combination with the scanning selection signal, 
(c) temperature detection means for detecting a temperature of the liquid 
crystal panel, and 
(d) control means for controlling the drive means so that the period of the 
pulse of voltage zero is increased corresponding to an increase in the 
detected temperature of the liquid crystal panel. 
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.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 4 is a block diagram of a liquid crystal display apparatus according 
to an embodiment of the present invention. Referring to FIG. 4, the 
display apparatus includes a liquid crystal display panel 401, a scanning 
signal applying circuit 402, a data signal applying circuit 403, a 
scanning signal control circuit 404, a drive control circuit 405, a data 
signal control circuit 406, a graphic controller 407, a temperature 
detection element 408, and a temperature data detection circuit 409. Data 
sent from the graphic controller 407 are sent via the drive control 
circuit 405 to the scanning signal control circuit 404 and the data signal 
control circuit 406 and converted into address data and display data, 
respectively. On the other hand, the temperature of the liquid crystal 
display panel is detected by the temperature detection element 408 and the 
temperature detection circuit 409 from which temperature data are supplied 
via the drive control circuit 405 to the scanning signal control circuit 
404. Based on the address data and display data, a scanning signal is 
generated by the scanning signal applying circuit 40 and applied to the 
scanning electrodes in the liquid crystal display panel 401. Further, data 
signals are generated by the data signal applying circuit 403 based on the 
display data and applied to the display electrodes in the liquid crystal 
display panel 401. 
FIG. 5 is an enlarged view of the liquid crystal display panel 401 and 
shows scanning electrodes S.sub.1 -S6 . . . S.sub.n and data electrodes 
I1-I6 . . . I.sub.n which are disposed to intersect each other to form an 
electrode matrix. FIG. 6 is a schematically enlarged view of a section 
including the scanning electrode S.sub.2 in FIG. 5. Referring to FIG. 6, 
the display panel includes oppositely disposed substrates (glass plates) 
601a and 601b having transparent electrodes 602a (constituting scanning 
electrodes) and 602b (constituting data electrodes), respectively, 
comprising, e.g., In.sub.2 O.sub.3 or ITO (indium tin oxide) on their 
opposite faces, which are further laminated with 200 to 1000 .ANG.-thick 
insulating films 603a and 603b (of SiO.sub.2, TiO.sub.2, Ta.sub.2 O.sub.5, 
etc.) and 50 to 1000 .ANG.-thick alignment control films 604a and 604b of, 
e.g., polyimide. The alignment control films 604a and 604b are rubbed in 
the directions denoted by arrows A and B, respectively, which are parallel 
and identical to each other. A ferroelectric smectic liquid crystal 605 is 
disposed between the substrates 601a and 601b which are spaced from each 
other with a spacing of, e.g., 0.1-3 .mu.m, which is sufficiently small to 
suppress the formation of a helical structure of the ferroelectric smectic 
liquid crystal 605 and develop a bistable alignment state of the 
ferroelectric smectic liquid crystal 605. The sufficiently small spacing 
is held by spacer beads 606 (of silica, alumina, etc.). 
A ferroelectric liquid crystal display panel of the above-described 
structure was subjected to continuous display of a display pattern 
including black display stripes 71 and white display stripes 73 for 
prescribed hours, after which the panel was subjected to measurement or 
observation of drive margin, cell thickness, color tone and occurrence of 
liquid crystal-void portions which are items most sensitively reflecting 
the occurrence of the liquid crystal molecular movement, whereby no change 
was observed in any of the above-mentioned items, thus showing good 
results. The cell thickness was measured at points 7201-7215. The set of 
driving waveform used was one as shown in FIG. 8 including waveforms 
(scanning selection signals) applied to scanning electrodes S.sub.1, 
S.sub.2, S.sub.3 . . . and data signal waveforms including no pause period 
(period of voltage zero) applied to data electrodes I1, I2, I3 . . . and 
having a voltage amplitude of 15 volts. The surface temperature of the 
liquid crystal panel at that time was 20.degree. C. 
Good results with no change in any of the above-mentioned items were 
observed when the panel was driven by using a set of driving waveforms 
shown in FIG. 9 including a pause period in an overall data signal applied 
to data electrodes within a period (scanning selection period) for a 
scanning line of 2.DELTA.t + the pause period under two panel surface 
temperature conditions of 20.degree. C. and 30.degree. C., respectively. 
Further, good results with no change in any of the above-mentioned items 
were observed when the panel was driven by using a set of driving 
waveforms shown in FIG. 10 including a pause period in an overall data 
signal applied to data electrodes within a scanning selection period for a 
scanning line of 2.DELTA.t + the pause period at three panel surface 
temperature conditions of 20.degree. C., 30.degree. C. and 40.degree. C., 
respectively. 
In contrast to the above, when the panel was driven continuously by using 
the set of driving waveforms shown in FIG. 8 at a panel surface 
temperature of 30.degree. C. and then subjected to similar measurement, 
whereby the change in color tone or the occurrence of liquid crystal void 
was not observed but the cell thickness was increased by 2-3% compared 
with the original value at points 7211, 7213 and 7215 (rightmost points) 
in black display stripes 71 and at points 7202 and 7204 (leftmost points) 
in white display stripes 73, thus failing to provide a good results. At 
these points, an increase in threshold value was observed corresponding to 
the increase in cell thickness, thus resulting in an adverse effect with 
respect to the drive margin. Further, when the panel was driven at 
40.degree. C. by using the driving waveform shown in FIG. 8, the cell 
thickness increase was raised to 6-8%, resulting in a corresponding 
increase in threshold value and a change in color tone. 
Further, when the panel was driven continuously at a panel surface 
temperature of 40.degree. C. by using the driving waveforms shown in FIG. 
9 and then subjected to similar measurement, the cell thickness was 
increased by 1-2% resulting in a corresponding increase in threshold 
value, but some improvement was attained than in the case of using the 
driving waveforms shown in FIG. 8. 
The above results are summarized in the following Table 1. 
TABLE 1 
______________________________________ 
Driving Panel surface temp. 
waveform 
20.degree. C. 
30.degree. C. 
40.degree. C. 
______________________________________ 
FIG. 8 Normal Cell thickness 
Cell thickness 
increased by increased by 
2-3%. 6-8%. 
Threshold value 
Threshold value 
increased increased. 
FIG. 9 Normal Normal Cell thickness 
increased by 
1-2%. 
Threshold value 
increased. 
FIG. 10 
Normal Normal Normal 
______________________________________ 
The pulse width .DELTA.t of the data signal, pause period and scanning 
selection period (period of overall data signal for a scanning line) used 
in the above-mentioned measurement under the temperature conditions of 
20.degree. C., 30.degree. C. and 40.degree. C. are summarized in the 
following Table 2. 
TABLE 2 
______________________________________ 
(Time in .mu.sec) 
20.degree. C. 
30.degree. C. 
40.degree. C. 
______________________________________ 
(1) FIG. 8 waveform 
.DELTA.t 125 100 75 
Pause period 0 0 0 
Scan selection 500 400 300 
period (4.DELTA.t) 
(2) FIG. 9 waveform 
.DELTA.t 125 100 75 
Pause period 125 175 225 
Scan selection 375 375 375 
period (2.DELTA.t + pause) 
(3) FIG. 10 waveform 
.DELTA.t 125 100 75 
Pause period 250 300 350 
Scan selection 500 500 500 
period (2.DELTA.t + pause) 
______________________________________ 
As described hereinabove, according to the present invention, there is 
provided a liquid crystal apparatus by which an optimum drive waveform is 
selected depending on a detected liquid crystal panel temperature so that 
the liquid crystal molecular movement is suppressed to a level practically 
free of problem.