[0001] The invention relates to a method of enhancing the response of a liquid crystal display
at relatively low temperatures, and a control unit therefor. In particular, but not
exclusively, the invention relates to a method of enhancing the response of a liquid
crystal display of the type suitable for use in a vehicle to display a warning or
other status signal to the driver of the vehicle.
[0002] A liquid crystal display (LCD) typically includes two glass plates which are separated
by between 5 to 10 µm, and which define a cell which is filled with a liquid crystal
material. Two polarisers are arranged, one on each side of the cell, with their polarisation
axes oriented at 90 degrees to one another. The inner surfaces of the glass plates
may be coated with transparent electrodes, typically formed from indium tin oxide,
which define the characters, symbols or other patterns to be displayed. Between the
glass plates and the liquid crystal material, alignment or orientation layers of polymeric
material are treated to induce the adjacent liquid crystal molecules to maintain a
defined orientation angle. If the alignment or orientation layers are arranged at
90 degrees to one another, the molecules of the liquid crystal material introduced
between the plates will be caused to twist through a 90 degree helix and the polarised
light passing through the cell will be guided by the molecules through the 90 degree
helix so that the polarisation axis is rotated by 90 degrees.
[0003] By applying an electric field applied across the cell, the polarisation properties
of the cell can be varied as the liquid crystal molecules are caused to align with
the electric field and the 90 twist in the optic axis is distorted. By varying the
electric field applied across the cell, incident polarised light will therefore either
have its polarisation axis aligned with that of the exit polariser (an ON state),
in which case light is transmitted through the device, or will have its polarisation
axis orthogonal to the exit polariser (an OFF state) in which case light is not transmitted
through the device. In a multiplexed liquid crystal device, multiplex drive electrodes
are arranged in a matrix on the glass plates. Selective addressing of the electrode
matrix provides a means of varying the characters, graphics or others symbols to be
displayed, as molecules in only selected segments of the cell are caused to align
with the applied electric field.
[0004] As the viscosity of liquid crystal material increases at decreasing temperatures,
the switching response time of liquid crystal displays is reduced at low temperatures.
By way of example, when a nematic liquid crystal display is operated at room temperature,
the application of an electric field across the cell can cause twisting of the liquid
crystal molecules in a timescale of between 50 and 100 ms. However, when operated
at temperatures below -20° C, the response time can be of the order of minutes. At
even lower temperatures (around -40° C), it may not be possible to twist the liquid
crystal molecules at all such that the display is inactive.
[0005] A known technique for increasing the response time of a liquid crystal display is
to provide a heating element at the rear of the display which is activated when the
temperature of the liquid crystal material falls below a certain value. It has been
found that the use of a heating element on a liquid crystal display operating at a
temperature of -20° C gives a satisfactory response time for some applications. At
lower temperatures, around -40°C, it has been found that, even when a heater is provided,
the response time may nevertheless be as high as 70 to 80 seconds.
[0006] If the liquid crystal display is to be used in a vehicle to display information to
the driver about the status of the vehicle, it may be essential to display such information
as soon as possible after the driver enters the vehicle. For example, if the liquid
crystal display provides a warning signal to the driver in the event that the oil
level in the engine is too low, it is essential that the warning signal is displayed
before the engine is started, otherwise serious engine damage will occur. Although
the use of a heating element on the liquid crystal display can enable faster device
response times, for the purpose of providing certain warning signals to a driver,
this solution is unsatisfactory.
[0007] It is an object of the present invention to provide a method of controlling operation
of a liquid crystal display which overcomes the aforementioned problems.
[0008] According to the present invention, there is provided a method of enhancing the response
time of a liquid crystal display comprising a liquid crystal material, the method
comprising the steps of;
measuring the temperature of the liquid crystal material,
applying a drive voltage to the liquid crystal material at a selected frame refresh
frequency so as to display an image,
selecting the frame refresh frequency depending upon the measured temperature of the
liquid crystal material,
comparing the measured temperature with a predetermined temperature and, if the measured
temperature is less than the predetermined temperature,
determining a cold kick time period for which a negative bias voltage is applied across
the liquid crystal material, wherein the cold kick time period is dependent upon the
measured temperature, and
applying a negative bias voltage across the liquid crystal material for the duration
of the cold kick time period.
[0009] For the purpose of this specification, the "response time" of the liquid crystal
display shall be taken to have its usual meaning, i.e. the time taken, following application
of a drive voltage to the liquid crystal material, for the liquid crystal material
to switch state so as to display a demanded image.
[0010] Preferably, the step of applying the negative bias voltage comprises applying a maximum
negative bias voltage for the voltage source associated with the liquid crystal display.
For example, if the display is used in a vehicle to display various engine and/or
vehicle operating parameters, the maximum negative bias voltage which can be derived
from the engine battery is, typically, around -13.5 V.
[0011] More preferably, the method includes the additional step of applying a heating effect
to the liquid crystal material if the temperature of the liquid crystal material falls
below the predetermined temperature.
[0012] Preferably the heater is activated in the event that the measured temperature of
the liquid crystal material falls below the predetermined temperature.
[0013] Typically, the predetermined temperature is between 0°C and-25°C and, preferably,
is substantially -20 °C.
[0014] Preferably, the liquid crystal material may be a twisted nematic (TN) or a super
twisted nematic (STN) material. The liquid crystal display may be of the type comprising
a double STN cell (DSTN) having an active cell to which drive signals are applied,
in use, to display a demanded image, and a passive cell which is provided for the
purpose of improving the contrast ratio of the display.
[0015] The method may comprise the step of determining the frame refresh frequency by reference
to a look up table or a data map comprising pre-calibrated frame refresh frequency
values as a function of the measured liquid crystal temperature.
[0016] Alternatively, the method may comprise the step of determining the frame refresh
frequency in real time.
[0017] The method enables the response time of the liquid crystal display to be increased
significantly at relatively low temperatures by applying a 'cold kick' to the liquid
crystal material. The 'cold kick' is achieved by applying a negative bias voltage
to the liquid crystal material for a period of time determined by the temperature
of the material and, in addition, by reducing the frame refresh frequency to a value
dependant upon the temperature of the liquid crystal material.
[0018] It will be appreciated that the method steps of the invention may be implemented
in software on a computer processor or control unit of the associated engine.
[0019] According to the present invention, there is further provided a control unit as defined
in claim 11 below.
[0020] Optional but preferred method steps as defined in the dependent claims below may
also be implemented in software on a computer processor or control unit of the associated
engine.
[0021] The invention provides a particular advantage when used to enhance the response time
of a liquid crystal display in a motor vehicle where the display is used to provide
warning or other status signals to the driver. It has been found that, even at temperatures
as low as -40°C, the response time of the display can be increased to around 10 to
12 seconds. Thus, if the liquid crystal display is used to display a warning signal
to indicate a low level of engine oil, the driver can be alerted in good time to prevent
the vehicle from being driven. The invention provides a significant advantage over
the known technique for enhancing the response time of a liquid crystal display through
use of a heater, for which the response time has been found to be between 70 and 80
seconds at - 40°C.
[0022] The invention may also be useful for the purpose of driving liquid crystal displays
of the type used in Global Positioning System (GPS) devices, and in particular on
hand-held GPS devices, for which enhanced display response times at low temperatures
may be particularly desirable.
[0023] The invention will now be described, by way of example only, with reference to the
accompanying drawings in which;
Figure 1 is a schematic side view of a double STN (DSTN) liquid crystal display,
Figure 2 is a perspective view of the double STN (DSTN) liquid crystal display shown
in Figure 1,
Figure 3 is a flow diagram to illustrate the method steps of a main background control
loop for displaying an image on the liquid crystal display in Figures 1 and 2,
Figure 4 is a flow diagram to illustrate a 'cold kick' sub-control loop in accordance
with an embodiment of the present invention,
Figure 5 is a timing sub-control loop to determine the length of time for which the
'cold kick' is applied,
Figure 6 is a flow diagram to illustrate a 'frame refresh adjust' sub-control loop
for selecting the frame refresh frequency depending on liquid crystal temperature,
and
Figure 7 is a graph to show frame refresh frequency data as a function of liquid crystal
temperature, for use in the sub-control loop illustrated in Figure 6.
[0024] Referring to Figures 1 and 2, there is shown a liquid crystal display device of the
type comprising a first, active cell 10 and a second, passive cell 12. The active
cell 10 includes front and rear glass plates (only the rear one 19 of which is shown
in Figure 2), between which a liquid crystal material is provided such as, for example,
a super twisted nematic material. The passive cell 12 is of similar construction and
the two cells 10, 12 are bonded together by means of a bonding layer 13 in a conventional
manner to form a double-cell arrangement, commonly referred to as a double super twisted
nematic (DSTN) device. A heating element 14 formed from a layer of indium tin oxide
(ITO) extends through the passive cell 12, in contact with the liquid crystal material.
Front and rear device polarisers 18, 20 are arranged on the front face of the active
cell 10 and the rear face of the passive cell 12 respectively and are oriented such
that their polarisation axes are orthogonal to one another. A temperature sensor (not
shown) is provided on the device to continuously monitor the temperature of the liquid
crystal material within the active cell 10.
[0025] The DSTN device shown in Figures 1 and 2 is of the type suitable for use in a vehicle
to display information to the driver regarding the status of the vehicle and/or the
engine. Typically, the display may be used to provide a warning signal if one of the
doors is open, if the boot is not closed or if the driver seat belt is not fastened.
Additionally, the display may be used to provide an indication of engine speed, engine
temperature, fuel economy or a warning signal that engine oil level is low, or may
be used to display Global Positioning System (GPS) information.
[0026] An appropriate matrix of electrodes (not shown) is provided on the active cell 10,
to which appropriate drive voltage signals are applied, in use, from an associated
voltage source (not shown) by means of first and second driver chips 22, 24 mounted
on the glass plate 19. Typically, the driver chips 22, 24 are off-the-shelf chips
such as, for example, SED157As. Cell 12 remains passive and is provided to improve
the contrast of the display.
[0027] An Electronic Interface Unit of the display is operated under the control of an Electronic
Control Unit. Appropriate drive voltage signals to the active cell electrode matrix
are applied in response to "Build Display Request" signals provided by the Electronic
control unit, the Build Display Request signals being input to the driver chips 22,
24 through respective connector wires 26, 27. When used in the aforementioned automotive
application, the display will typically be a high information density display, for
example 386 x 64 pixels, and may be driven to display the demanded image using a multiplex
addressing technique in a manner which would be familiar to a person skilled in the
art.
[0028] In use, current is supplied to the heating element 14 of the DSTN device through
an H-bridge circuit (not shown) by means of a first connection 16 (as shown in Figure
2) and a second connection through the second connector 26. The use of an H-bridge
circuit ensures minimal DC offset is applied to the heating element 14, which may
otherwise degrade the performance of the liquid crystal device. The heating element
14 is switched on and off by applying a ramped PWM drive voltage through the H-bridge
circuit. Ramping of the PWM drive voltage to the heating element 14 is advantageous
in that large currents are not rapidly drawn from the voltage source (for example,
the engine battery) which may otherwise effect the DSTN drive voltages derived from
the same voltage source. Typically, the power "ON" ramp for the heating element 14
from zero to maximum current takes 1.6 seconds and the power "OFF" ramp takes 3.2
seconds.
[0029] Figures 3 to 6 show the control method steps for activating the liquid crystal device.
As shown in Figure 3, at the onset of the main program control loop a check is made
as to whether a Build Display Request command signal has been generated by the Electronic
Control Unit. If there is no Build Display Request the main control loop exits and
is re-entered again after a predetermined period (typically 160 ms). If a Build Display
Request has been generated by the Electronic Control Unit, the appropriate display
data are input to a buffer (step 32) for the driver chips 22, 24. The data is input
to the driver chips 20, 24 at step 34 and the demanded drive voltage signals are applied
to the electrode matrix of the active cell 10 to display the demanded image. The image
is refreshed a number of times each second (referred to as the 'frame refresh frequency')
in a conventional manner.
[0030] At step 36, a 'cold kick' sub-control loop 36 is implemented for each frame refresh.
Figure 4 shows the method steps of the cold kick sub-control loop 36. For each Build
Display Request, the temperature of the liquid crystal material is compared with a
predetermined temperature, for example -20°C (step 38). If the measured temperature
is greater than -20°C, the cold kick sub-control loop 36 exits and the main program
control loop is restarted.
[0031] If the temperature of the liquid crystal material is less than -20°C, two steps are
carried out to enhance the response of the liquid crystal display. Generally, the
two steps may be referred to as (i) a 'cold kick bias voltage step' in which a relatively
large magnitude negative bias voltage is applied to the active cell 10 for a period
of time, T
coldkick, where T
coldkick depends upon the measured temperature and (ii) a 'frame refresh adjust step' in which
the frame refresh frequency is adjusted in accordance with the measured liquid crystal
temperature. In a practical embodiment, it is envisaged that the ITO heating element
and frame refresh frequency adjustments will be employed until a temperature of +7C
is reached.
[0032] In order to implement steps (i) and (ii), a check is made (step 40) as to whether
a cold kick timer is already running, such as would be the case if the measured temperature
of the liquid crystal material was less than -20°C for the preceding Build Display
Request. If the cold kick timer is not running, the cold kick time period, T
coldkick , is determined at step 42.
[0033] The cold kick time period, T
coldkick is determined by multiplying the absolute value of the measured liquid crystal temperature
by a predetermined factor, F. It has been found that, for a cold kick timer sub-control
loop having a repeat time of 80 milliseconds, the appropriate predetermined factor,
F, is [5 x 80] milliseconds. Thus, for example, if the measured temperature of the
liquid crystal material, T
LC, is -30°C, the cold kick time period is set to 12 seconds. Conveniently, the step
42 of determining the duration for which the cold kick timer is running is carried
out by reference to a look-up table or a data map having pre-stored calibrated data
relating the cold kick time period to the measured temperature, T
LC, of the liquid crystal material. Alternatively, for each cold kick sub-control loop,
the cold kick time period may be calculated in real time using the aforementioned
relationship.
[0034] The cold kick bias voltage step (ii) is applied at step 44 by applying negative bias
voltage across the liquid crystal material of the active cell 10 for the duration
of the cold kick time period. The negative bias voltage is preferably the maximum
negative voltage capable of being supplied by the associated voltage source which,
for a normal car battery, is typically -13.5 V, but may be as low as -20 V. At step
46, a kick timer running flag is set 'TRUE' to indicate that the cold kick sub-control
loop 36 is operational. It is this flag which is used at step 40 to determine whether
the cold kick timer is running, as mentioned previously.
[0035] Figure 5 shows the control loop for the cold kick time period count down. A check
is made at step 45 whether the cold kick timer is running and, if so (i.e. if the
cold kick sub-control loop flag is set 'TRUE'), the timer is decremented (step 47)
until the cold kick time period has expired. When the cold kick time period expires,
at step 48, the cold kick timer is reset to zero (step 50) and the maximum negative
bias voltage applied to the liquid crystal device is switched off allowing the main
bias voltage control loop to resume normal control and apply the normal bias voltage
for the current measured operating conditions.
[0036] As can be seen in Figure 4, if, at step 40, it is found that the cold kick timer
is already running when a new Build Display Request is received and the cold kick
sub-control loop receives an input from the Electronic Interface Unit that the temperature
of the liquid crystal material is still less than -20°C, the cold kick timer is reset
at step 54 and the bias voltage applied to the device is switched off (step 56). A
time delay of one second is introduced (step 58) before the next cold kick time period
is determined (step 42) for the subsequent Build Display Request.
[0037] The frequency with which the display is refreshed is also altered in accordance with
the measured temperature (the frame refresh adjust step) by means of a frame refresh
frequency control loop, as shown in Figure 6, which is run in parallel with the main
control loop. For each 160 millisecond period, a new frame refresh frequency is selected
depending upon the current measured temperature of the liquid crystal material. If
the frame refresh frequency is not equal to the value for the proceeding 160 millisecond
time period, a new frame refresh frequency is generated.
[0038] Figure 7 is a graph to show the relationship between optimum frame refresh frequency
and temperature for the DSTN device in Figures 1 and 2. It can be seen that the optimum
frame refresh frequency increases for increasing liquid crystal temperatures and,
at relatively low temperatures (typically below - 10°C), the optimum frame refresh
frequency decreases. As the LCD becomes warmer, a higher frame refresh frequency is
required to operate the display, otherwise the display will flicker. The frame refresh
frequency at normal temperatures, is carefully selected, so as not to cause beat frequencies
with the LED backlighting and street lighting. The cold kick frame refresh sub-control
loop illustrated in Figure 6 is conveniently implemented in software by reference
to a look up table or data map storing pre-calibrated values of frame refresh frequency
as a function of liquid crystal temperature. The use of data maps and look up tables
in control software is commonly used and would be familiar to a person skilled in
the art of Electronic Control System design.
[0039] It has been found that the application of a relatively large magnitude negative bias
voltage to the liquid crystal display device, in combination with an adjustment to
the frame refresh frequency depending upon measured temperature, has the effect of
enhancing the response time of the display when a Build Display Request is generated.
The response time is enhanced further if current is supplied to the heating element
14 (as shown in Figures 1 and 2) when the temperature of the liquid crystal material
falls below a predetermined amount. If the steps of applying a heating effect to the
liquid crystal material, of reducing the frame refresh frequency and of applying a
maximum negative bias voltage across the device are applied in combination, it has
been found that the response time of the device may be as low as 12 seconds for a
device operating at -40°C. Using conventional addressing techniques, even with the
use of a heating element 14, the response time of a liquid crystal display at a temperature
of -40°C is usually at least 70 seconds. For applications in which it is important
to display an image or character on the liquid crystal display as quickly as possible,
the method of the present invention provides a significant advantage over conventional
LC control techniques.
[0040] It will be appreciated that the temperature values, bias voltage values and frame
refresh frequencies referred to previously are given by way of example only and will
vary depending on the particular configuration, material and performance characteristics
of the liquid crystal display device. For liquid crystal materials of different type,
for example ferroelectric materials as opposed to nematic materials, operating characteristics
will be different and the predetermined temperature at which the cold kick sub-control
loop is activated may be different to that described previously. Additionally, if
the display device is driven through a voltage source other than a normal car battery,
the maximum negative bias voltage which may be applied to the device may be significantly
greater than -13.5 V.
[0041] It will be appreciated that the liquid crystal display device to which the control
method of the present invention is applied may take the form of a single cell liquid
crystal display, and need not be a double cell arrangement such as that shown in Figures
1 and 2. In a single cell device, the heating element may conveniently be provided
on the rear glass plate of the liquid crystal cell. It will further be appreciated
that the method is applicable to a display of any size (for example 8 × 1, 64 × 1,
64 × 64, or 256 × 256 pixels), and including a simple ON/OFF display, and is compatible
with both multiplex drive or direct drive control methods.
[0042] It is envisaged that in embodiments of the invention, a control means may be included
for controlling the degree of heating supplied by the heating element. The control
means may control the heating such that the LCD temperature is raised to above +7
degrees C. Above 7 degrees C the LCD display will function normally. The ITO Heating
element control may include a control algoirthm which estimates the required amount
of I2Rt enery required for the heating cycle and limits the energy applied to the
ITO heater. The heater current and voltage may also be monitored to ensure that the
RMS current is maintained below 1.5 amps, even when the supply voltage changes.
1. A method of enhancing the response time of a liquid crystal display comprising a liquid
crystal material, the method comprising the steps of;
measuring the temperature of the liquid crystal material,
applying a drive voltage to the liquid crystal material at a selected frame refresh
frequency so as to display an image,
selecting the frame refresh frequency depending upon the measured temperature of the
liquid crystal material,
comparing the measured temperature with a predetermined temperature and, if the measured
temperature is less than the predetermined temperature, determining a cold kick time
period which is dependent upon the measured temperature and for which a negative bias
voltage is applied across the liquid crystal material, and
applying a negative bias voltage across the liquid crystal material for the duration
of the cold kick time period.
2. The method as claimed in Claim 1, wherein the negative bias voltage is derived from
a voltage source associated with the liquid crystal display, the method comprising
the step of applying a maximum negative bias voltage derivable from the voltage source
for the duration of the cold kick time period.
3. The method as claimed in Claim 2, comprising the step of applying a negative bias
voltage of around -13.5 V.
4. The method as claimed in any of Claims 1 to 3, comprising the further step of applying
a heating effect to the liquid crystal material if the temperature of the liquid crystal
material falls below the predetermined temperature.
5. The method as claimed in any of Claims 1 to 4, wherein the predetermined temperature
is between 0°C and-25°C.
6. The method as claimed in any of Claims 1 to 5, comprising the step of determining
the selected frame refresh frequency by reference to a look up table or data map comprising
pre-calibrated frame refresh frequency values as a function of the measured temperature.
7. The method as claimed in any of Claims 1 to 5, comprising the step of determining
the selected frame refresh frequency in real time.
8. The method as claimed in any of Claims 1 to 7, when applied to a liquid crystal display
comprising a super twisted nematic liquid crystal material.
9. The method as claimed in any of Claims 1 to 8, when applied to a liquid crystal display
of a Global Positioning System (GPS) device.
10. The method as claimed in any of Claims 1 to 8, when applied to a liquid crystal display
in a vehicle.
11. A control unit for enhancing the response time of a liquid crystal display having
a liquid crystal material, the device comprising:
means for measuring the temperature of the liquid crystal material;
means for applying a drive voltage to the liquid crystal material at a selected frame
refresh frequency so that the liquid crystal material can display an image;
means for selecting the frame refresh frequency depending upon the measured temperature
of the liquid crystal material;
means for comparing the measured temperature with a predetermined temperature and,
if the measured temperature is less than the predetermined temperature, for determining
a cold kick time period which is dependent upon the measured temperature and for which
a negative bias voltage is applied across the liquid crystal material; and
means for applying a negative bias voltage across the liquid crystal material for
the duration of the cold kick time period.
12. The control unit according to claim 1, comprising means for applying a heating effect
to the liquid crystal material if the temperature of the liquid crystal material falls
below the predetermined temperature.
13. The control unit according to claim 1, comprising means for determining the selected
frame refresh frequency by reference to a look up table or data map comprising pre-calibrated
frame refresh frequency values as a function of the measured temperature.