JPS6051715B2 - liquid crystal display device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an image display method using a liquid crystal panel, and particularly to a method for adjusting brightness, contrast, etc.

Due to its electro-optical properties, liquid crystals are widely used as light-receiving display elements in numeric displays. Various characteristics are known as the electro-optical effect of liquid crystals. For example, there is a DSM effect that causes light scattering due to the fluctuation of domains of liquid crystal molecules, or a light polarization effect due to optical anisotropy of liquid crystal molecules. Various liquid crystal display systems have been devised by combining these characteristics with characteristics such as dielectric anisotropy of liquid crystal molecules. Here, FIG. 1 shows an example of display characteristics of a liquid crystal.

In FIG. 1, 1 is a characteristic curve of applied voltage and display contrast in a general TN liquid crystal. The horizontal axis is the voltage applied to the liquid crystal, and the vertical axis is the display contrast. When the applied voltage is zero, the contrast is 0%, and when the voltage exceeds a certain level, the contrast is 1.
It becomes 00%. In the figure, curves 1 and 2 indicate temperature in palmometers, and as the temperature decreases from state 1, the curve moves toward curve 2. Even in liquid crystal display systems other than TN, the correlation between temperature, voltage, and contrast is almost the same, so a description thereof will be omitted here. Furthermore, it is known that the characteristic curve changes depending on structural differences in the liquid crystal display panel, such as the thickness of the liquid crystal phase. Conventionally, the influence of temperature on the display effect of liquid crystals has been extremely large, and has been one of the reasons why liquid crystals are inferior to other display materials.

In other words, ordinary light-emitting display materials have a clear threshold voltage, and when the voltage is below the threshold voltage, the display does not light up, and when the voltage is above the threshold voltage, it lights up, making it possible to display two values. By controlling the amount of current, it is also possible to create gradations in brightness. On the other hand, for liquid crystals, as shown in Figure 1, in curve 1, the threshold voltage is 3, the voltage is 6 at 50% contrast, and 8 at 90% contrast, and as the temperature changes, the curve shifts to 2, and the threshold voltage is 5, 50. The % contrast voltage changes to 7, and the 90% contrast voltage changes to 9. In other words, the temperature dependence of the threshold voltage is high, and the voltage 13 at 10% contrast in curve 2 and the voltage 6 at 50% contrast in curve 1 have extremely close values.
If you display a liquid crystal with the characteristics shown in Figure 1, the display contrast for the same drive signal will change significantly even if the temperature conditions change.In order to keep the display contrast constant, the liquid crystal has as small a temperature characteristic as possible. Materials must be selected.

However, results have not been obtained that indicate that liquid crystals that are superior in temperature characteristics are also superior in other display performances. For this reason, the conventional methods have been to strictly limit and reduce the operating margin of the drive circuit, and to control the drive conditions using a temperature correction circuit that follows the characteristic curve of the liquid crystal. If the operating margin is small, it becomes necessary to set the constants of the grandchild circuit in detail, which causes complicated circuit design and assembly, and also limits the usage environment. Temperature correction is the most natural concept, and an example is shown in FIG. Reference numerals 17 and 18 indicate circuit power supplies; 14 is a constant voltage circuit for generating a reference voltage applied to the liquid crystal; and 15 is a resistor having temperature characteristics, which is a temperature sensor placed near the liquid crystal display element.

Reference numeral 16 denotes a liquid crystal drive circuit that applies the temperature-corrected output voltage of the constant voltage circuit 14 to the liquid crystal element in accordance with a display signal.

19 and 20 are signal terminals applied to the liquid crystal, and in the method in which the applied voltage is corrected by a temperature sensor, it is necessary to set initial values for the circuit conditions at a specific temperature, and it is also necessary to set initial values for the circuit conditions at a specific temperature. Obtaining the correction curve requires a complicated circuit, so a simple approximation must be used.

Depends on the liquid crystal material. The correction circuits for each type are different and must be designed separately. Regarding correction for display characteristics other than temperature characteristics, each correction method must be considered separately. Variations between panels must be adjusted individually manually. Conventionally, there have been problems as described above. An object of the present invention is to solve these conventional problems, to suppress variations in display characteristics caused by temperature effects of liquid crystals and other differences in conditions, and to always obtain liquid crystal image display with the best image quality.

The present invention will be explained below with reference to the drawings.

FIG. 3 is an example of the shape of the vicinity of the panel when implementing the present invention. In Figure 3, 21 is a glass plate sandwiching the liquid crystal, 2
2 is a liquid crystal, and 23 and 24 are transparent electrodes formed on the glass surface in contact with the liquid crystal.
In the case of a TN type liquid crystal display, etc., into which light enters, as shown in FIG. Here, in the conventional liquid crystal panel, a light detection element is newly provided as shown at 26 on the lower side of the panel. There are various types of detection elements, such as photodetection and amplification elements such as phototransistors, or photovoltaic materials such as CdS, but there are differences depending on how they are formed near the panel and the display method of the liquid crystal. Therefore, the detection element is not limited here. In FIG. 3, the detector 26 detects the amount of light passing through the liquid crystal panel. 2
Since the liquid crystal controls the amount of light transmission (or reflection) in accordance with the signal applied to the liquid crystal in front of the detector 6, the amount of light reaching the detector 26 varies in accordance with the liquid crystal display signal.

The position of the detector 26 does not necessarily have to be as shown in FIG. For example, it may be attached to the glass plate 21, or it may be placed inside the glass plate 21. or,
Of course, it is also possible to install it in the opposite direction on the side where the light 25 enters and detect the amount of light reflected from the liquid crystal panel. In short, any position may be used as long as the degree of change in the liquid crystal display effect is detected according to the applied signal. FIG. 4 is an example of a control system diagram when implementing the present invention.

Reference numeral 33 designates a display signal that should originally be displayed by the liquid crystal, 31 designates an image pattern generation circuit, and 32 designates a kind of mixing circuit that appropriately mixes both signals 31 and 33 and supplies the mixture to the liquid crystal drive circuit 27.

28 is a liquid crystal display, 29 is a liquid crystal operation level detector including the detector 26, and 30 is a liquid crystal detected by 29 while a signal corresponding to the image pattern generated by 31 is applied to the liquid crystal. It is detected whether or not the operation level detection signal is at the optimum position, and a feedback signal corresponding to the drive signal is applied to the liquid crystal drive circuit 27.

By forming a closed loop in the feedback circuit, the liquid crystal drive state is always at the best operating point, regardless of differences in temperature or other environments, variations in panels, or even differences in liquid crystal materials. Can be set to An example of the control method will be explained with reference to FIG.

In FIG. 5, 45 is a display image signal output from the liquid crystal drive circuit 27. t represents the time axis. Of these, a portion 44 is an image pattern signal, and portions 34 and 35 corresponding to the signal generated by the block 31 correspond to an image display signal input from the terminal 33. Therefore, 45 is a signal obtained by time division multiplexing the image signals inputted from 31 and 33 by the mixing circuit 32. Curve 1 in Figure 5 is
Equivalent to curve 1 in FIG. 1 explained earlier, of the liquid crystal display element,
It shows the correlation between applied voltage and display contrast. (2) is a voltage applied to the liquid crystal, and C is an axis indicating contrast. From FIG. 5, the display contrast of the liquid crystal when an image signal of 45 is applied to the liquid crystal can be easily read. That is,
The contrast corresponding to an arbitrary point on the curve 45 can be determined by drawing a straight line with the same potential as the arbitrary point perpendicular to the voltage axis ■.
As a result, it is sufficient to find a point that intersects curve 1 and draw a perpendicular line from the intersection to contrast axis C. Suppose here and now,
Assume that an image signal of 45 is applied to the liquid crystal. here 38
, 36, and 37 correspond to the lowest contrast image, intermediate contrast image, and maximum contrast image, respectively, which should be obtained by the liquid crystal display under driving conditions. (Even if contrast is expressed as brightness, the lowest contrast state is dark, the highest contrast state is bright, and the intermediate contrast is a state exactly between dark and bright, the display characteristics are equivalent.) 38 , 36, and 37 are found on the C-axis by the above method, and are 3', 36'', and 37''.
The expected contrast of the liquid crystal here is 38 to 3.
The contrast that varies approximately linearly between 7 and corresponds to 36 is desired to coincide with the exact center of contrast values in the operating range (ie, between 38 and 37). However,
3 The contrast at each point of ``, 36'', 37'' is
This indicates an unsatisfactory relationship with the above expectations. FIG. 5 shows an example in which the amount of change in applied voltage and the amount of change in contrast satisfy a substantially linear relationship in the operating voltage range with respect to curve 1, and the operating range is set as wide as possible. The operating voltages are 38-1, 36-1, 37, respectively.
-1, the corresponding contrast value is 38-1
", 36-", 37-" have the same points. In the initial state, the signal shown in FIG. The operating potential expected from this state (for example, 38-1, 36-1
, 37-1) will be described.

If the operating potential can be brought close to the expected value by the following method, even if the set value of the liquid crystal application signal in the initial state deviates from the ideal value as the operating potential of the liquid crystal,
By automatically correcting the potential level, it becomes possible to obtain a proper display. Therefore, the circuits that were conventionally provided for setting and correcting the operating potential for the characteristics, variations, and temperature characteristics of liquid crystal materials or display cells are no longer necessary, and a single automatic adjustment means can always be used to adjust the operating potential. It becomes possible to obtain an optimal or nearly optimal display. FIG. 6 shows an example of a circuit that automatically obtains the intermediate point 36-1.
In FIG. 6, 39, 40, 41 are differential amplifier circuits,
At least 39 and 40 have mutually equal gains. 42
is an operational amplifier for obtaining an intermediate potential 36-1, and the output 43 finally settles at 36-1.

Input terminals 39 and 40 form a clamp circuit, respectively, and are connected to detector output signals corresponding to the maximum voltage level, intermediate voltage level, and minimum voltage level of the signal applied to the liquid crystal via switches 46, 47, and 48. be combined.

Here, one method of combination will be explained. The switch 48 is closed at Tl, t5 in the timing pulse waveform shown in FIG.

The switch 47 is closed at T2, tl, and t6. T3, t7
to close the switch 46. At times other than those listed above, each switch should be left open. As a result, while the image pattern for correcting the driving conditions is being applied to the liquid crystal, that is, within the time period shown in FIG.
Each input terminal of the detector is coupled to a contrast detection circuit (FIG. 4, 29) at each timing to sample the detector output signal. The potential level held in the capacitance of each terminal corresponds to the contrast of the liquid crystal display, and is 37'' and 36', respectively.
, 3', the potential is proportional to each contrast. below,
The potential of each capacitor is substituted with 37'', 36'', 3'' and similar symbols.

Differential amplifiers 39 and 40 output signals with amplitudes proportional to the difference between 37' and 36'' and the difference between 36'' and 38''. 41 outputs a signal proportional to the difference between the output signals of 39 and 40. do.

From FIG. 5, the outputs of 39 and 40 are zero or positive, and
It cannot be a negative value. Since the output of 41 corresponds to the difference between the outputs of 39 and 40, it can take values from negative to positive.

When the output of 41 is positive, i.e. In this case, the intermediate potential 36 is too low.

At this time, output 43 increases and therefore 36 becomes high. Conversely, when the output of 41 is negative, that is, the intermediate potential 36 is too high.

At this time, the potential of the output 43 of 42 decreases, and therefore 36 becomes low. When the output of 41 is zero, that is, intermediate contrast 3
When 6'' is exactly centered between 37'' and 38'', 43 does not change.

At this time, the value of 36 is set to the optimum potential for 37 and 38. Next, a method for correcting the potentials of 38 and 37 will be explained in accordance with the above explanation. In the above description, the center value is adjusted assuming three values of the liquid crystal applied voltage: the maximum value, the minimum value, and the intermediate value, but sampling is also performed for these intermediate values.

According to the sampling at the three points described above, the potential can be set so that the contrast at the middle point is always at the center of the contrast at the other two points. If the gradient of the potential is limited in advance here, the point where the gradient of the potential set by the sampling breaks the limit can be determined as the minimum value or maximum value of the liquid crystal drive potential. Figure 7 shows the case of determining the minimum value. vinegar.

7, 1 is equal to 1 in FIG. 1 or 5, in particular:
The threshold voltage part is enlarged. When dividing the characteristic curve by uniform contrast difference ΔC based on an arbitrary standard, C
When the driving potentials corresponding to the contrasts of d, C2, C3, and C4 are obtained according to the above procedure, as shown in FIG.
■3, V4 is obtained. At this time, the interval between each potential is ΔVl
, ΔV2, Δ■3, ΔV4, ΔV5, there is almost no difference in Δ■ near the potential at which the center contrast is obtained, and Δ■ increases near the threshold voltage and saturation voltage. By setting the value of ΔVO in advance and setting the potential near the point where the above formula is not satisfied as the limit value of the performance range, the maximum operation can be achieved in the area where the correlation between the applied voltage and the contrast of the liquid crystal is almost linear. You can set the range.

In addition, as a method of comparing ΔV and ΔVO, if the signal Δ■ to be compared is supplied to the positive input signal of FIG. 6, 41, and ΔVO is constantly supplied to the other input terminal. good. In this state, by fixing the drive potential when the polarity of the output potential becomes negative for the first time, upper and lower operating potential limits can be obtained. In FIG. 6, the input terminals 39 and 40 are used as sampling and hold circuits, but if the response of the amplifier circuit system in FIG. There is no need to make the input circuit a sample and hold circuit.

In this case, one sample-and-hold circuit may be set for the output signal 43, and the sample-and-hold circuit may be operated with each timing signal shown in FIG. Furthermore,
In FIG. 5, the liquid crystal application signal 45 is obtained by temporally dividing and multiplexing the original image display signal and the image pattern for setting the operating voltage level, but it depends on the driving method of the liquid crystal display panel. Therefore, the configuration of the waveform can be different.

In the liquid crystal display panel shown in FIG. 3, if it is possible to constantly apply an image pattern for setting the operating voltage level to the electrode of the display picture element corresponding to the detector 26, that is, for detecting the level set value. If the display picture element corresponding to the detection element is dedicated to this, the control from FIG. 6 onward becomes extremely simple.

That is, a test image pattern signal necessary for level setting is always applied to a dedicated picture element, and the correspondence between the applied signal and display contrast is statically compared. Therefore, in this case, a timing signal or a data hold circuit is not necessarily required. a display pixel and a contrast detection circuit for the pixel;
and the potential level setting circuit are in a completely closed loop state, so that the operating potential can always be controlled to be maintained at an optimal value. FIG. 8 shows the correlation between applied voltage and display contrast when an alternating current voltage is applied to the liquid crystal.

In the figure, 36, 37, and 38 correspond to 36, 37, and 38 in FIG. The voltage and contrast in AC voltage driving of the liquid crystal are symmetrical with respect to zero voltage. Therefore, 38,
Display contrast 53 corresponding to applied voltages 36 and 37
, 54, 55 with opposite polarity voltages 50, 51, 52
are 38, 36, and 37, respectively, which are almost the same in absolute value.
As a method for setting the operating potential level during AC drive, basically any of the methods described above can be considered. One method is to find out the correlation with the display contrast across both poles of the applied voltage, and to
It is conceivable to set each of the 52 operating potential levels. In this case, it is expected that about twice as many steps as the control described in connection with FIG. 5 will be required. On the other hand, as another method, it is conceivable to set the AC drive potential level in the same steps as the potential level setting for drive using a unipolar voltage, which was explained in connection with FIG. That is,
8, 38, 36, and 37 are set according to the procedure described above.
50, 51, and 52 have the same absolute values as 38, 36, and 37 as described above, and only differ in polarity. 50 each for 37
, 51, 52 are obtained.

Alternatively, the polarity of the unipolar image signal obtained by setting 38, 36, and 37 may be converted by the polarity conversion amplifier. As described above, the present invention uses a first voltage, a second voltage, and an intermediate voltage having an intermediate level between the first voltage and the second voltage as pattern signals in time series to generate the liquid crystal. a pattern signal generating means for applying the pattern signal to the liquid crystal; when the pattern signal is applied to the liquid crystal, the amount of light transmitted through the liquid crystal corresponding to the first voltage, the second voltage, and the intermediate voltage is detected in time series, and a transmitted light amount signal is generated; a first comparing means for comparing a difference between a transmitted light amount signal corresponding to the first voltage and a transmitted light amount signal corresponding to the intermediate voltage; a transmitted light amount signal corresponding to the second voltage and the intermediate voltage; a second comparing means for comparing the difference in the amount of transmitted light corresponding to the voltage; a third comparing means for comparing the difference between the output signal of the first comparing means and the output signal of the second comparing means; and from the third comparing means. The fourth comparison means compares the output signal of the output signal with the reference voltage level and applies the difference resulting from the comparison as a correction signal to the drive circuit of the liquid crystal. Even if there is a variation in contrast, it is possible to always obtain a desired constant voltage-contrast through automatic adjustment without any manual correction. Brief explanation of the drawings Figure 1・
...Voltage and contrast correlation of liquid crystal display, Figure 2...
・An example of a conventional drive voltage temperature correction circuit, FIG. 3... A cross-sectional view of a panel according to the present invention, FIG. 4... A block diagram of a control system according to the present invention, FIG. 5... An example of a correlation diagram between a liquid crystal applied voltage and a display contrast, FIG. 6: a detection control circuit diagram, FIGS. 7 and 8: a voltage contrast correlation diagram related to FIGS. 5 and 6.

1... Voltage contrast curve, 15...
Temperature detector, 16...Liquid crystal drive circuit, 26...
...Display contrast detector, (light sensor), 28
... Display section, 29 ... Detector, 45.
...Liquid crystal applied signal, 39, 40, 41, 42...
...Differential amplifier.

Claims (1)

[Claims] 1. In a liquid crystal display device in which a liquid crystal is sandwiched between two electrodes, a first voltage, a second voltage, and an intermediate voltage having an intermediate level between the first voltage and the second voltage are each applied as a pattern signal in time series. A pattern signal generating means for applying to the liquid crystal, when the pattern signal is applied to the liquid crystal, time-series detection of the amount of light transmitted through the liquid crystal corresponding to the first voltage, second voltage, and intermediate voltage, and generating a transmitted light amount signal. a first comparing means for comparing a difference between a transmitted light amount signal corresponding to the first voltage and a transmitted light amount signal corresponding to the intermediate voltage;
a second comparing means for comparing the difference between the transmitted light amount signal corresponding to the voltage and the transmitted light amount corresponding to the intermediate voltage; and comparing the difference between the output signal of the first comparing means and the output signal of the second comparing means. It is characterized by comprising a third comparing means, and a fourth comparing means for comparing the output signal from the third comparing means with a reference voltage level and applying the difference resulting from the comparison to the driving circuit of the liquid crystal as a correction signal. LCD display device.