Apparatus and method for temperature compensation of liquid

Apparatus and method for temperature compensation of liquid
US005088806A
United States Patent {191
[11]
McCartney et a1.
[451
Attorney, Agent, or Firm-Dale E. Jepsen; D. J.
Lenkszus; A. Medved
[57]
John A. Rupp, Glendale, both of
Ariz.
the liquid crystal medium as a function of applied elec
tric ?eld varies with temperature. In order to ensure
that the temperature of the liquid crystal medium can
provide a practical response time, a heating element,
along with a sensor element and a thermostat, estab
lishes the temperature of the liquid crystal display in an
appropriate range. The output signal of the sensor ele
359/86
ment is digitized and is used as a ?rst portion of a mem
Field of Search .......... .. 350/331 T, 350 S, 331 R,
ory unit address. A second portion of the memory unit
350/332; 340/793, 784
address is provided by digitized command signals either
References Cited
U.S. PATENT DOCUMENTS
4/ 1985
1/1987
4,890,097 12/1989
4,923,285
4,952,032
ABSTRACT
In a liquid crystal display, the optical transmission of
[73] Assignee: Honeywell, Inc., Minneapolis, Minn.
[21] Appl. No.: 465,796
Jan. 16, 1990
[22] Filed:
[51] Int. Cl.5 .............................................. .. G02F 1/13
[52] US. Cl. .................................... .. 359/84; 340/784;
4,509,043
4,634,225
Feb. 18, 1992
Assistant Examiner-Ron Trice
[75] Inventors: Richard I. McCartney, Scottsdale;
[56]
5,088,806
Primary Examiner-John Zazworsky
[54] APPARATUS AND METHOD FOR
TEMPERATURE COMPENSATION OF
LIQUID CRYSTAL MATRIX DISPLAYS
[53]
Patent Number:
Date of Patent:
from an operator or from the processing apparatus con
trolling the image of liquid crystal display. The logic
signals stored at the location determined by the ?rst and
second address portions identify an electric ?eld (ap
Mossaides ......................... .. 340/721
Haim et a1. ........ ..
350/331 T
Yamashita et a1. ..
plied voltage) that provides compensation for tempera
..... .. 340/719
ture and compensation for non-linear characteristics of
5/1990 Ogino et a1. ....... ..
350/331 T
8/1990 Inoue et a1. .................... .. 350/350 5
the liquid crystal medium. The logic signals from the
memory unit are converted into a voltage to be applied
FOREIGN PATENT DOCUMENTS
to the liquid crystal display.
59-224827 12/1984
Japan ............................. .. 350/331 T
63-184487
Japan .
7/1988
17 Claims, 3 Drawing Sheets
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US. Patent
Feb. 18, 1992
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US. Patent
Feb. 18, 1992
Sheet 2 of 3
5,088,806
ILLUMINATION SIDE
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PRIOR ART
GRAYSCALE CHARACTERISTICS
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US. Patent
Feb. 18, 1992
Sheet 3 of 3
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APPARATUS AND METHOD FOR
predetermined relative orientation applied thereto and
enclose the liquid crystal material 40. The substrate 32
TEMPERATURE COMPENSATION OF LIQUID
CRYSTAL MATRIX DISPLAYS
ering the color ?lters 33 is a transparent common con
has red, green, and blue ?lters 33 coupled thereto. Cov
ductor 34. Associated with each color ?lter region 33 is
an electrode 30. The voltage applied between electrode
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to display technol
ogy and, more particularly, to liquid crystal displays.
The invention addresses the problem of the temperature
variation of the liquid crystal display characteristics
30 and the common electrode 34 determines the trans
mission of light through the liquid crystal in the region
of the associated ?lter 33. Therefore, the intensity of the
0 light transmitted through each color ?lter 33 can be
controlled and the cumulative effect can be used to
when the environmental temperature can not be opti
mized.‘
provide a color image.
The alignment of the liquid crystal molecules, in
2. Description of the Related Art
The transmission of light through a flat panel liquid
addition to the dependence on the applied electric ?eld,
is a function of the temperature, the temperature in turn
affecting such properties as the elastic constants which
in turn affect the electric ?eld required for the align
crystal display is determined by the alignment of the
liquid crystal molecules. The alignment of the liquid
crystal molecules is, in turn, determined by the electric
ment of the liquid crystal molecules. Referring to FIG.
?eld to which the molecules are subjected. Referring to
4, a plot of transmission of light versus voltage (i.e.,
FIG. 1A, FIG. 1B, and FIG. 1C, a pictorial representa 20 electric ?eld) for a liquid crystal enclosed between
tion of the effect of an electric ?eld on liquid crystal
generally parallel plates at several temperatures is illus
molecules of a twisted nematic liquid crystal material in
trated.
a typical display environment is illustrated. Substrates
The temperature dependence of the transmission of
29 and 32 enclose the liquid crystal material. Polarizing
elements 31, having a predetermined relative orienta 25 light through the liquid crystal flat panel cell is particu
larly critical in the region cf the on-set of transmission
_ tion, are coupled to substrates 29 and 32. Also deposited
of light. In this region, relatively modest changes in
on the substrates 29 and 32 are (transparent) electrodes
temperature can result in a relatively large change in the
transmission of light. However, it will be clear from
a voltage, that subjects the liquid crystal molecules to 30 review of FIG. 4 that the voltage for a selected trans
mission of light in one temperature range will provide a
an electric ?eld with a magnitude less than a threshold
different transmission of light at a different temperature.
value, is applied to electrodes 30 and 34, then the mole
While the effect of the change in transmission can be
cules of the liquid crystal material are not affected and
30 and 34 respectively. The liquid crystal molecules 40
are enclosed between the substrates. In FIG. 1A, when
light is not transmitted through the cell, i.e., through
polarizing elements 31. In FIG. 1B, a voltage is applied
negligible in an on-off type of liquid crystal display
35 when the operation is con?ned to the two extreme (and
to electrodes 30 and 34 of suf?cient strength to cause
complete element (of the relative orientation) of sub
stantially all of the liquid crystal molecules 40. With this
complete alignment (or saturation) of the molecules 40,
light is transmitted through polarizing elements 31. In
40
FIG. 1C, the voltage applied between electrodes 30 and
?at) portions of the transmission versus voltage charac
teristics of the liquid crystal material, in a display in
which a grayscale is employed, the change in transmis
sion can increase the difficulty of understanding the
material presented by the display. In the case of color
displays, the loss of accurate color tracking can com
34 provides for partial alignment of the liquid crystal
promise the bene?ts of providing a display with color.
molecules 40 and a portion of the light applied to the
cell is transmitted through polarizing elements 31.
A further problem can be identi?ed with reference to
FIG. 4. The grayscale characteristics of the liquid crys
In the prior art, a (?at panel) liquid crystal display can 45 tal display are not linear except for a limited region in
have a plurality of electrically separated display re
the mid-transmission range. The operator will typically
gions, each display region also being known as a display
wish to provide an input signal from settings for a
cell or, when the regions designate a small portion of
switch or similar apparatus which provide a linear scale.
the display, each display region is known as a pixel.
The conversion from a linear input signal to a signal
Referring to FIG. 2, the apparatus for activation of the 50
providing a linear transmission of the liquid crystal
areas of a flat panel display is shown. The electrodes of
display is typically referred to as the gamma correction
each display region form a capacitor 5. A thin ?lm
to the input signal.
transistor 20, acting as a switching element in response
A need has therefore been felt for apparatus and
to control and voltage signals applied thereto, controls
method
for providing temperature compensation for
the charge that is stored on the plates of the capacitor
the optical transmission characteristics of a liquid crys
and, therefore, controls the voltage and resulting elec
tal display. In addition, a need has been felt for appara
tric ?eld imposed on the liquid crystal material of the
tus and method in which the gamma correction can be
cell. When the gate terminal of transistor 20 is enabled
by means of a control voltage applied to the row n
_ combined with the temperature correction apparatus.
conductor, the capacitor 5 is charged to the voltage 60
applied to column m conductor. In this manner, a con
trollable charge is applied to each individual capacitor
of the display, and hence the transmission through each
individual capacitor of the display can be individually
controlled.
Referring next to FIG. 3, the technique for providing
a liquid crystal display for color images is shown. The
substrates 29 and 32 have polarizing elements 31 with a
FEATURES OF THE INVENTION
It is an object of the present invention to provide an
improved display apparatus.
It is a feature of the present invention to provide an
65
improved flat panel liquid crystal display.
It is another feature of the present invention to pro
vide temperature compensation for the activation of a
liquid crystal display.
-
3
5,088,806
It is a still further feature of the present invention to
4
to analog converter unit 56. The output signals from the
digital to analog converter 56 is applied to driver unit
57. The output signals from the driver unit 57 are ap
provide both temperature compensation and correction
for the non-linear optical transmission versus voltage
characteristic for a liquid crystal display.
plied to the appropriate portion of the liquid crystal
display unit 50 by addressing apparatus normally in
cluded in the liquid crystal display.
SUMMARY OF THE INVENTION
The aforementioned and other features are attained,
2. Operation of the Preferred Embodiment
according to the present invention, by providing a liq
uid crystal display with a heater element, a temperature
For many applications, such as in avionics, it is neces
sensing element, and a thermostat. These elements en
sure that the liquid crystal material has a suf?cient re
sary that the temperature of the liquid crystal material
of the display be high enough for sufficient display
sponse time for the display application. The output
signal of the temperature sensing element is digitized
response time. The heater element, the temperature
sensor element, and the thermostat of the present inven
tion ensure that the temperature of the liquid crystal is
in a range that provides an acceptable response time,
i.e., .when the display unit is operated over a wide tem~
perature range such as -l5' C. to 70' C. Once the
and used as a ?rst portion of a memory unit address. A
second portion of the memory unit address is deter
mined by a command signal either from an operator or
from image processing apparatus identifying a desired
transmission. The logic signals stored at the memory
general range of the temperature has been established,
then the signal from the temperature sensing element, in
unit address determined by the ?rst and second address
portions provide a digital signal representation of a
voltage which, when applied to the liquid crystal dis
play provides temperature compensation and compen
combination with the command signal, the command
sation for the nonlinear voltage excitation characteris
the display, is used to address a storage location in a
signal, identifying the required optical transmission for
tics of the liquid crystal display.
memory unit, the storage location containing a digital
These and other features of the invention will be 25 signal that identi?es the voltage needed, at the tempera
understood upon reading of the following description
ture determined by the temperature sensing element to
along with the drawings.
obtain the correct optical transmission. The logic signal
group is then applied to a digital to analog converter
BRIEF DESCRIPTION OF THE DRAWINGS
which provides the voltage to be applied to the display
region. As will be clear, the signal group stored in the
FIG. 1A is a pictorial representation of liquid crystal
molecules in the absence of an electric ?eld; FIG. 1B is
memory location can also compensate for the nonlinear
a pictorial representation of liquid crystal molecules in
ities of the voltage versus optical transmission, i.e., at
the measured temperature.
For low precision displays such as are used in certain
the presence of an electric ?eld with sufficient strength
to completely align the molecules; and FIG. 1C is a
pictorial representation of liquid crystal molecules in
the presence of an electric ?eld providing partial orien
tation of the liquid crystal molecules.
35 consumer products, the addition of a constant term to
the voltage applied to the display region may be satis
factory. For example, referring to FIG. 4, the shape of
FIG. 2 is a circuit diagram of the apparatus used in
the transmission versus voltage curves is generally the
same, but each curve appears to be off-set in voltage
from the other curves. To a ?rst approximation, this
charging the capacitor individual liquid crystal display
regions.
FIG. 3 is a cross sectional view of a liquid crystal
off-set in voltage can frequently be represented-by a
display capable of displaying a color image.
constant value. In the high precision displays, a more
FIG. 4 in plot of optical transmission as a function of
voltage (applied electric ?eld) for a liquid crystal dis
play for several temperatures.
FIG. 5 is a block diagram illustrating the apparatus
used in the temperature compensation of a liquid crystal
45
elaborate functionality for the stored logic signal groups
may be required.
For image displays wherein images are formed by a
multiplicity of pixel elements, the addressing circuits of
the liquid crystal display are synchronized with the
display according to the present invention.
temperature compensation circuits so that the voltage
applied to each pixel element is compensated for tem
perature.
The foregoing description is included to illustrate the
operation of the preferred embodiment and is not meant
DESCRIPTION OF THE PREFERRED
EMBODIMENT
1. Detailed Description of the Figures
Referring now to FIG. 5, a block diagram of the
present invention is shown. The liquid crystal display
to limit the scope of the invention. The scope of the
sensor 52 causes the thermostat 53 to maintain the
encompassed by the spirit and scope of the invention.
unit 50 has heater 51 and temperature sensor 52 in ther 55 invention is to be limited only by the following claims.
From the foregoing description, many variations will be
mal contact therewith. The thermal sensor 52 applies a
apparent to those skilled in the art that would yet be
signal to thermostat 53. The signal from temperature
heater 51 and, consequently, maintain the thermally
coupled liquid crystal display unit 50 at a preselected
What is claimed is:
temperature. An output signal from the temperature
at least one cell having a liquid crystal material en
sensor 52 is also applied to an analog to digital con
closed between two electrodes;
a temperature sensing element thermally coupled to
said cell for providing a temperature signal;
a heater unit thermally coupled to said cell;
a thermostat coupled to said temperature sensing
l. A compensated liquid crystal display comprising:
verter unit 54. The output signal from the analog to
digital converter unit 54 is applied to a portion of the
address terminals of P(rogrammable) R(ead) 0(nly)
M(emory) unit 55. A command word is also applied to
the address terminals of PROM unit 55. The output
signals from the PROM unit 55 are applied to a digital
65
element and to said heater unit, said thermostat
activating said heater unit in response to said tem
5
5,088,806
perature signal for providing a ?rst digital signal
group determined by said temperature signal;
signal group.
9. The method of claim 8 wherein said converting a
conversion means responsive to a combination of said
third digital signal group includes the step applying said
?rst digital signal group and a second digital signal
group for providing a third digital signal group,
said second digital signal group determined by a
desired optical transmission through said cell, said
third digital signal group determining a desired
voltage for application to said electrodes, said de
sired voltage resulting in said desired transmission;
and
a digital to analog converter coupled to said elec
trodes for converting said third signal group to said
third digital signal group to a digital to analog converter
to provide said actual voltage.
10. A temperature compensated liquid crystal display
comprising:
15
20
conversion means includes a memory unit, said combi
nation of said ?rst signal group and said second signal
voltage means for providing said activation voltage
to said cell, said voltage means including;
temperature means thermally coupled to said cell
for providing a ?rst logic signal group identify
ing a temperature of said cell, and
conversion means responsive to a combination of -
group providing an address in said memory unit
wherein said third signal group is stored.
3. The liquid crystal display of claim 2 wherein said 25
memory unit is a PROM unit.
4. The liquid crystal display of claim 2 wherein said
third signal group includes compensation for a tempera
ture dependence of said liquid crystal cell.
5. The liquid crystal display of claim 2 wherein said
at least one cell containing a liquid crystal material,
an optical transmission of said cell determined by
an activation voltage, wherein a desired optical
transmission of said cell is identi?ed by a second
logic signal group; and
desired voltage, said desired voltage compensating
for properties of said liquid crystal material.
2. The liquid crystal display of claim 1 wherein said
6
8. The method of claim 7 wherein said determining
step includes the step of addressing a memory unit loca
tion with said combined ?rst and second digital signal
group, said memory unit location storing said third
perature signal when a temperature of said cell is
below a predetermined range;
an analog to digital converter responsive to said tem
30
said ?rst logic signal group and to said second
logic signal group for providing a desired activa
tion voltage, said desired activation voltage pro
viding said desired optical transmission by said
cell when applied to said cell;
a heater element thermally coupled to said cell; and
a thermostat coupled to said temperature means and
to said heater element, said temperature means and
said temperature causing said heater element to
third signal group includes compensation for nonlinear
maintain said cell within a predetermined tempera
ity of an optical transmission of said liquid crystal cell as
ture range.
a function of voltage applied to said electrodes.
11. The liquid crystal display of claim 10 wherein said
6. A method of providing a compensated voltage to
activate a liquid crystal display cell, said method com 35 conversion means has a memory unit, said combination
of said ?rst and said second logic signal groups are
prising the steps of:
applied to address terminals of said memory unit.
identifying a desired optical transmission of said liq
12. The liquid crystal display of claim 11 wherein said
uid crystal cell by a ?rst digital signal group;
conversion
means includes a digital to analog converter
' identifying a temperature of said liquid crystal cell by
for converting a third logic signal group stored in said
second digital signal group;
memory means at a location determined by said combi
combining said ?rst and said second digital signal
nation of said ?rst and said second logic signal group to
groups to provide a combined ?rst and second
said
desired activation voltage.
digital signal group;
'
13. The liquid crystal display of claim 12 wherein said
third logic signal group is selected to provide a desired
activation voltage compensating an optical transmission
identi?ed by said ?rst logic signal group for a tempera
determining a third digital signal group from said
combined ?rst and second digital signal group, said
third digital signal group identifying an acutal volt
age to activate said liquid crystal cell, said actual
voltage resulting in said desired optical transmis
ture dependence of said cell for a temperature of said
cell identi?ed by said second logic signal group.
14. The liquid crystal display of claim 13 wherein said
third logic signal group is also selected to provide com
pensation for the nonlinear characteristics of an optical
sion of said liquid crystal cell when applied thereto;
converting said third signal group to said actual volt
.
age;
applying said actual voltage to said liquid crystal cell;
transmission of said'cell as a function of said activation
and
maintaining said liquid crystal cell within a predeter
voltage.
55
mined temperature range.
7. The method of claim 6 wherein said identifying a
15. The liquid crystal display of claim 11 wherein said
memory unit is a PROM unit.
16. The liquid crystal display of claim 14 further
temperature step includes the steps of:
measuring a temperature of said liquid crystal cell by
means of a temperature sensing unit, said tempera
ture sensing unit providing an analog signal related
to said liquid crystal cell temperature; and
converting said analog signal to said second digital
comprising a transistor coupled to said cell and to said
voltage means, said transistor controlling when said
desired activation voltage is applied to said cell.
17. The liquid crystal display of claim 12 further
comprising a plurality of cells, wherein said cells are
arranged in a matrix con?guration.
signal.
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65
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