Title:
METHOD OF DRIVING LIQUID CELL DISPLAY PANEL
United States Patent 3863221


Abstract:
In case of addressing the predetermined liquid cells of a liquid cell display panel comprising a plurality of parallel X-axis electrodes, a plurality of parallel Y-axis electrodes disposed perpendicular to said X-axis electrodes, and liquid cells disposed at the points where said X-axis electrodes cross said Y-axis electrodes, a D.C. voltage whose polarity is inverted three times during one horizontal scanning period is sequentially and periodically applied to the X-axis electrodes while a D.C. voltage whose polarity is inverted three times only during the predetermined horizontal scanning periods is applied to the Y-axis electrodes, so that the light permeability of the addressed liquid cells may be changed.



Inventors:
KAJI TETSUNORI
Application Number:
05/345517
Publication Date:
01/28/1975
Filing Date:
03/28/1973
Assignee:
HITACHI, LTD.
Primary Class:
Other Classes:
345/209
International Classes:
G02F1/133; G09G3/36; (IPC1-7): G08B5/36
Field of Search:
307/149 350
View Patent Images:
US Patent References:
3746426LIQUID CRYSTAL CELL AND METHOD FOR ITS MANUFACTURE1973-07-17Masi
3740717LIQUID CRYSTAL DISPLAY1973-06-19Huener et al.
3701249SOLID STATE TIMEPIECE WITH LIQUID CRYSTAL DISPLAY1972-10-31Bergey
3645604LIQUID CRYSTAL DISPLAY1972-02-29Ngo



Primary Examiner:
Schaefer, Robert K.
Assistant Examiner:
Ginsburg M.
Attorney, Agent or Firm:
Craig & Antonelli
Claims:
1. A method of driving a liquid cell display panel having a plurality of X-axis electrodes, a plurality of Y-axis electrodes disposed perpendicular to the X-axis electrodes respectively, and liquid cells disposed respectively at the points where the X-axis electrodes cross the Y-axis electrodes, the method comprising producing a scanning signal voltage having a first voltage portion to be applied to the liquid cell when the liquid cell is addressed and a second voltage portion to be applied to the liquid cell when the liquid cell is not addressed, the polarities of the first and second voltage portions being opposite to each other with respect to a predetermined reference level and being inverted every predetermined number of time frames, producing a display signal voltage having a third voltage portion to be applied to the liquid cell when the liquid cell is addressed and a fourth voltage portion to be applied to the liquid cell when the liquid cell is not addressed, the polarities of the third and fourth voltage portions being opposite to each other with respect to the predetermined reference level and being inverted every predetermined number of time frames, and applying the scanning signal voltage to each X-axis electrodes sequentially and periodically while applying the display signal voltage of a predetermined display information

2. A method according to claim 1, including providing the first voltage portion with a value with respect to the predetermined reference level which is substantially three times as large as the value of the second

3. A method according to claim 1, wherein the first voltage portion has a value with respect to the predetermined reference level which is substantially three times as large as the value of the fourth voltage

4. A method according to claim 1, wherein the third voltage portion has a value with respect to the predetermined reference level which is substantially three times as large as the value of the fourth voltage

5. A method according to claim 1, wherein the first and second voltage portions and the third and fourth voltage portions are inverted each time

6. A system for driving a liquid cell display panel having a plurality of X-axis electrodes, a plurality of Y-axis electrodes disposed perpendicular to the X-axis electrodes, respectively, and liquid cells disposed respectively at the points where the X-axis electrodes cross the Y-axis electrodes, the system comprising means for producing a scanning signal voltage having a first voltage portion to be applied to the liquid cell when the liquid cell is addressed and a second voltage portion to be applied to the liquid cell when the liquid cell is not addressed, the polarities of the first and second voltage portions being opposite to each other with respect to a predetermined reference level and being inverted every predetermined number of time frames, means for producing a display signal voltage having a third voltage portion to be applied to the liquid cell when the liquid cell is addressed and a fourth voltage portion to be applied to the liquid cell when the liquid cell is not addressed, the polarities of the third and fourth voltage portions being opposite to each other with respect to the predetermined reference level and being inverted every predetermined number of time frames, and means for applying the scanning signal voltage to each X-axis electrodes sequentially and periodically while applying the display signal voltage of a predetermined display information to each Y-axis electrode for driving the liquid cell display panel.

Description:
BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of driving a liquid cell display panel comprising a plurality of lateral electrodes parallel to one another (hereafter referred to as X-axis electrodes), a plurality of longitudinal electrodes parallel to one another (hereafter referred to as Y-axis electrodes) and perpendicular to the lateral electrodes, and liquid cells disposed at the points where said X-axis electrodes cross said Y-axis electrodes.

2. Description of the Prior Art

In order to display a certain information on a conventional liquid cell display panel having such a structure as mentioned above, a D.C. voltage is applied to the X- and Y-axis electrodes. Accordingly, the liquid cells are placed in a D.C. field during addressing so that the useful life of the cells is shortened due to the electrolytic effect caused in the cells by the D.C. field. This is one of inevitable drawbacks of the conventional method of driving a liquid cell display panel.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a method of driving a liquid cell display panel, wherein an A.C. electric field is applied for addressing to the liquid cells so as to prolong the useful life of the cells.

According to the present method of driving a liquid cell display panel, the direction (hereafter referred to as "polarity") of the electric field applied to the liquid cells located at the cross points of the X- and Y-axis electrodes is inverted at a predetermined frequency so that the electrolytic effect may be prevented from taking place in the liquid cells.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a general view of the structure of a conventional liquid cell display panel.

FIG. 2 shows the waveforms of the voltages which are useful for explaining a first embodiment of the present invention.

FIG. 3 is a block diagram of a circuit used in the method according to the present invention.

FIG. 4 is a circuit of a single unit of the X-axis electrode drive circuit constituting a part of the circuit shown in FIG. 3.

FIG. 5 shows the waveforms of the voltages useful for explaining the operation of the circuit shown in FIG. 4.

FIG. 6 shows the waveforms of the voltages applied to the X- and Y-axis electrodes employed in a second embodiment of the present invention.

FIG. 7 shows the waveforms of the voltages useful for explaining the operation of the previous embodiments.

FIG. 8 shows the waveforms of the voltages applied to the X- and Y-axis electrodes employed in a third embodiment of the present invention.

FIG. 9 is a circuit of a single unit of the X-axis electrode drive circuit used in the third embodiment.

FIG. 10 shows the waveforms of the voltages useful for explaining the operation of the circuit shown in FIG. 9.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown in FIG. 1, the structure of a general liquid cell display panel mainly comprises X-axis electrodes (X1, X2, X3, ....., Xm), Y-axis electrodes (Y1, Y2, Y3, ....., Yn) disposed in perpendicular relation to the X-axis electrodes, and liquid cells (crystals) a11, a12, a13, ....., amn disposed at points where the X- and Y-axis electrodes cross each other.

The conventional method of driving such a liquid cell display panel as having the above described structure, has resorted to the application of a D.C. electric field intense enough to change the optical property or more particularly light permeability of a desired liquid cell. For example, when it is desired to address the liquid cell a23 located at the cross point of the X2 electrode belonging to the X-axis electrodes and the Y3 electrode belonging to the Y-axis electrodes, the X2 electrode and the Y3 electrode are simultaneously energized.

The inevitable drawback of the above described method of driving the liquid cell is the problem of electrolytic effect taking place in the addressed liquid cell during the energization thereof. Namely, since the voltage applied between any pair of electrodes to address a desired liquid cell is of direct current, the liquid cell is electrolyzed so that the useful life of the cell is shortened.

FIG. 2 shows the voltage waveforms to be applied to the X-axis electrodes and the Y-axis electrodes, useful for explaining the operation of the first embodiment of the present invention (in this figure, only an electrode Y1 of the Y-axis electrodes is shown).

In FIG. 2, the horizontal scanning period is designated by an interval 1H and the voltage waveform applied during 1H is a rectangular wave signal whose polarity is inverted three times with respect to the ground level or a certain predetermined D.C. level during the horizontal scanning period 1H. In this case, the rectangular wave signals as described above are sequentially applied to the X-axis electrodes X1 - Xm while the similar voltage is applied to the Y-axis electrode only during the predetermined horizontal scanning periods.

Namely, according to FIGS. 1 and 2, a predetermined electric field is applied during the first 1H period to the liquid cell a11 located at the cross point of the electrodes X1 and Y1 ; during the second 1H period the electrode X2 is energized while the electrode Y1 is not energized; and during the third 1H period the electrodes X3 and Y1 are energized so that an electric field is applied to the liquid cell a31 located at the cross point of the electrodes X3 and Y1.

As described above, according to the present invention, the polarity of the electric field applied to the liquid cells for addressing thereof is inverted (2l-1) times (l = 2.0 in the previous case) in order to prevent the electrolytic effect caused in the liquid cells during addressing. Here, l can assume any real value greater than unity.

The greater is the value for l, the more effective is the action of eliminating electrolytic effect in the liquid cells. If, however, l is too great, i.e., the frequency of polarity change in the rectangular wave signal is so high, the liquid cells cannot follow the input signal, that is, the change in the optical property of each cell cannot take place properly.

Of course, it is possible to make each liquid cell perfectly responsive to the signal having a large value of l by expanding the period of 1H. However, too long a period for 1H causes the flicker in display. Expecially for a panel having a large area to display much informations a single frame needs a rather long time, which also causes flicker.

The range of allowable values for l obtained through experiments is such that

1 ≤ l ≤ 10.

This range is justly applicable to general liquid cells represented by the compounds such as p-methoxybenzylidene-p-n-butylaniline.

Moreover, it will be more preferable if l is an integer satisfying the above inequality. For if l assumes an integral value, then the opposite polarities with respective to the ground level or the predetermined D.C. level occur the same times. This can perfectly avoid the electrolytic effect in the liquid cells. Also, even when l is of a large value with a fractional part, i.e., l = 9.5, the effect of suppressing the electrolysis in the liquid cells can be expected. Namely, for l = 9.5, the frequency of occurence of the positive polarity (the higher level of the rectangular wave signal is hereafter referred to as such for convenience sake) differs by unity from that of the negative polarity (the lower level of the rectangular wave signal is hereafter referred to as such for convenience sake). However, the same effect as in case where l is an integer can be obtained by adding every 1H period, every frame or every field a signal which compensates for the difference to the positive or negative polarity.

FIG. 3 is a block diagram of a drive means for a liquid cell display panel prepared to realize the driving method according to the present invention. In FIG. 3, a keyboard 1 delivers a coded character signal Sb and a coded display position signal Sp. A scanning signal generator 6 normally delivers a scanning position signal Ss and a coincidence circuit 5 delivers a pulse to be applied to a gate circuit 2 when the display position signal Sp and the scanning position signal Ss are identical with each other.

The gate 2 serves not only to apply the output of a refresh memory 3 to the same memory 3 when there is no pulse input from the coincidence circuit 5 so that the character signal previously applied may be repeatedly sent to a character generator 4, but also to apply the renewed character signal Sb from the keyboard 1 to the refresh memory 3 when there is a pulse received from the coincidence circuit 5.

A scanning circuit 7 supplies scanning pulses for the character generator 4 and a gate and one-line memory 9 in response to the output of the signal generator 6. The character generator 4 supplies for the gate and one-line memory 9 outputs reprentative of actual characters in response to the coded character signal from the refresh memory 3 and the scanning signal from the scanning circuit 7. Namely, the input to the character generator 4 is a coded signal comprising 6 bits or 8 bits, which is then converted through the character generator 4 into a signal corresponding to a specific character.

The gate and one-line memory 9 holds during the 1H period or a period nearly equal to 1H a signal which corresponds to a portion of a character per one line in response to the outputs from the scanning circuit 7 and the character generator 4. The outputs of the memory 9 and the gating signal generator 8 are applied to Y-axis electrode drive circuit 10, which produces a signal to be applied to the Y-axis electrodes Y1 - Yn of a liquid cell display panel 13.

An X-axis electrode scanning circuit 11 is actuated by the output from the scanning signal generator 6 and the outputs of the circuit 11 and the gating signal generator 8 are applied to an X-axis electrode drive circuit 12, which produces a signal to be applied to the X-axis electrodes X1 - Xm of the liquid cell display panel 13.

FIG. 4 is a circuit of one of like components constituting the X-axis electrode drive circuit 12. The Y-axis electrode drive circuit 10 can also be constituted of similar components.

In FIG. 4, reference character A1 designates an input terminal to receive the output of the gating signal generator 8, B1 an input terminal to receive the output of the X-axis electrode scanning circuit 11, C1 a terminal connected with the X-axis electrode of the liquid cell display panel 13, 41 an AND circuit, 42 a NOT circuit, Tr1 and Tr2 transistors, R1 and R2 resistors, and D1 a terminal to which the positive polarity of a power source is coupled.

FIG. 5 shows the waveforms of signals A1 ' and B1 ' applied to the input terminals A1 and B1 in the circuit shown in FIG. 4 and the waveform of the signal C1 ' (in case where l = 1) delivered at the terminal C1 in the same circuit.

When the signal A1 ' from the gating signal generator 8 and the signal B1 ' from the X-axis electrode scanning circuit 11 are both applied to the AND gate 41, the transistor Tr1 is actuated in response to the output of the AND gate 41 corresponding to the signal A1 ' only during the period when the signal B1 ' is being applied to the AND gate 41. The signal B1 ' has its polarity inverted through the NOT circuit 42 and the polarity-inverted signal is applied to the transistor Tr2 to actuate the same. Therefore, such a signal C1 ' as shown in FIG. 5 appears at the terminal C1, the signal C1 ' being due to the resultant effect of both the voltage drop of the collector current for the transistor Tr1 across the resistor R1 and the voltage drop of the collector current for the transistor Tr2 across the resistors R1 and R2. It should be noted here that the present invention is by no means limited to the use of the circuit as shown in FIG. 4 but that any other circuit that can produce such a signal as the signal C1 ' shown in FIG. 5 can be utilized in the embodiment of the present invention. It is also a matter of course that the level of the D.C. component in the signal C1 ' shown in FIG. 5 may be arbitrarily chosen depending upon the circuit configuration employed.

FIG. 6 shows the waveforms of the voltages to be applied to the X- and Y-axis electrodes (in this figure, only the electrode Y1 of the Y-axis electrodes is shown), useful for the explanation of the second embodiment of the present invention. As seen from FIG. 6, this combination of the waveforms is similar to that shown in FIG. 2. Namely, the X-axis electrodes X1 - Xm are sequentially energized and the Y-axis electrode Y1 is energized during the first 1H and third 1H periods, as shown in FIG. 6, so that the liquid cells a11 and a31 are addressed. As shown in FIG. 6, according to the second embodiment of the present invention, the polarity of the electric field applied to each liquid cell is inverted every frame or every M frames (in this case the polarity inversion occurs every frame). According to this method, the polarity is inverted after a rather long period corresponding to a frame or a plurality of frames since the proper optical reaction cannot take place in the liquid cells for an A.C. field having too high a frequency, as described with the first embodiment. It is assumed in this embodiment that the polarity of the field is inverted every M frames. Then, the effect of suppressing electrolysis in the liquid cell decreases for too large values of M. The experiments has showed that the preferable range of M is given by the inequality

1 ≤ M ≤ 10.

It should here be noted that the circuit configurations as shown in FIGS. 3 and 4 can be used to realize the second embodiment of the present invention. For example, if a signal A1 " and a signal B1 "as shown in FIG. 7 are applied respectively to the terminals A1 and B1 of the circuit shown in FIG. 4, then a signal C1 " can be delivered at the terminal C1. Further, it should be noted that the polarity of the electric field may be inverted every N field instead of every M frames. Here, too, the allowable range of N can be given by the experiments and it follows that

3 ≤ N ≤ 10.

As described above, in the first embodiment in which the polarity inversion occurs every 1H period, the electrolytic effect can be prevented from taking place in the liquid cells since the polarity inversion is frequent for a short time, but the display on the liquid cell panel cannot be free from flicker since a single 1H period and therefore a period of a frame is appreciably long. On the other hand, in the second embodiment, the flicker can be suppressed since the 1H period can be shortened.

FIG. 8 shows the waveforms of the voltages to be applied to the X- and Y-axis electrodes (in this figure, only the electrode Y1 of the Y-axis electrodes is shown), useful for describing the third embodiment of the invention. The combination of the waveforms corresponds to the case where the liquid cells a11 and a31 as in the display panel shown in FIG. 1 are addressed.

As seen in FIG. 8, in the third embodiment, the polarity of the electric field applied to each liquid cell for display is inverted every frame. It is also possible here to invert the polarity each time one field, a predetermined number of frames or fields or have been scanned. The notable feature of this embodiment is the application of a bias voltage to each liquid cell after the cease of display. Namely, as seen in FIG. 8, each display level h1 whose duration is 1H is followed by a bias level h2 having a polarity opposite to that of the display level in each frame. In this way, the difference in the intensity of the field applied to the addressed liquid cell from that of the field applied to the unaddressed one is large so that the contrast may be improved, that is, the contrast is improved by applying a bias voltage having the opposite polarity after the drive voltage has vanished.

In this embodiment, any liquid cell can be addressed by applying a voltage of +h1 to a selected X-axis electrode and a voltage of -h1 to a selected Y-axis electrode, these selected electrodes crossing each other at the cell to be addressed, while other liquid cells are kept unaddressed by applying a voltage of -h2 to corresponding X-axis electrodes and a voltage of +h2 to corresponding Y-axis electrodes. Thus, a voltage of 2h1 is applied to liquid cells to be addressed while a voltage of 2h2 or h1 - h2 is applied to liquid cells not to be addressed. In this case, the voltage h1 and h2 are such that h1 > h2, especially h2 = 1/3 h1 for the waveforms in FIG. 8.

By the way, when the waveforms as shown in FIG. 6 is used, a voltage of h1 is applied to both the X-axis electrodes and the Y-axis electrodes which cross at liquid cells to be addressed while the X-axis electrodes and the Y-axis electrodes which cross at liquid cells not to be addressed are all grounded. In such a case, a voltage of h1 is applied also to liquid cells not to be addressed which are located along those X-axis or Y-axis electrodes which lie on liquid cells to be addressed, so that the contrast sometimes tends to be degraded.

On the other hand, in the third embodiment, since there is a relation that h2 = 1/3 h1, a voltage of 2/3 h1 is applied to those liquid cells which are not to be addressed. Therefore, the contrast can be improved.

It is therefore concluded that if one employs the drive method described with FIG. 8 then the useful life of one's liquid cell display device can be prolonged and its contrast can be improved.

The last described drive method can be used to drive the liquid cell display panel 13 shown in FIG. 3. In such a case, the circuit of one of components of the X-axis electrode drive circuit 12 must be changed.

FIG. 9 shows such a modified circuit, in which reference character A2 indicates an input terminal to receive a signal from the gating signal generator 8 shown in FIG. 3, B2 an input terminal to receive a signal from the X-axis electrode scanning circuit 11, C2 an output terminal from which a signal is sent to the X-axis electrodes of the liquid cell panel 13, 91 an AND circuit, 92 an AND gate with two inhibit inputs, 93 an AND gate with an inhibit input, Tr1 - Tr3 transistors, R3 - R5 resistors and D2 a terminal to which the positive polarity of a power source is coupled. Here, the resistors R3 to R5 have their resistance values such that R5 >R3 >R4.

FIG. 10 shows the voltage waveforms of the signals A2 ', B2 ' and C2 ' (in case where l = 1) associated respectively with the input terminals A2 and B2 and the output terminal C2 of the circuit shown in FIG. 9. Namely, if the signals A2 ' and B2 ' are applied respectively to the terminals A2 and B2, the signal C2 ' can be obtained at the terminal C2.

Further, the value h2 of the voltage mentioned above is not limited to the relation that h2 = 1/3 h1, but asymmetric voltages may be applied to the X and Y-axis electrodes.

As has hitherto been described, according to the method of driving the liquid cell display panel, herein proposed by the inventors, the useful life of the driven display panel can be prolonged by alternately inverting the polarity of the electric field applied to each liquid cell. Moreover, in the third embodiment described above, the contrast can also be improved by applying a suitable voltage to the liquid cells not to be addressed. Furthermore, by using the drive methods described as the second and third embodiments of the present invention, a large-sized liquid cell display panel on which a great deal of information is displayed can be driven without flicker and any appreciable degradation in the liquid cells so that the useful life of the display panel can be prolonged. The present invention can be claimed to provide improved methods of driving liquid cell display panel.

This specification has referred mainly to the case where the liquid cells are actuated by a voltage source, but it should be noted that the present invention can be applied to the case where the liquid cells are energized by a current source if a part of the used circuits is somewhat modified.