US3891306A - Method for driving liquid cell display panel - Google Patents

Abstract

A method for driving a liquid cell in a scanning liquid cell display panel in which the liquid cell is interposed between two scanning electrode groups comprises the step of applying a DC voltage and an AC voltage having a frequency above the limiting frequency of the liquid crystal to said two scanning electrode groups during a period of semi-selection of the scanning while applying said DC voltage thereto during a period of full selection, an amplitude ratio of said AC to DC voltages during said period of semi-selection ranging from 1 to 2.

Description

United States Patent Mitomo et al.

METHOD FOR DRIVING LIQUID CELL DISPLAY PANEL Inventors: lsamu Mitomo. Hachioji; Tetsunori Kaji; Masakazu Fukushima, both of Kokbunji. all of Japan Assignee: Hitachi, Ltd., Japan Filed: July 20, 1973 Appl. No.: 381,003

Foreign Application Priority Data July 21. 1972 Japan 47-73119 US. Cl.. 350/160 LC; 340/324 M; 340/166 EL Int. Cl. G02f 1/28 Field of Search 350/160 LC; 340/324 M.

References Cited UNITED STATES PATENTS 4/1971 Nestor 350/160 LC h 1 t t t 1 1 June 24, 1975 Hucner ct al 350/161) LC X Torrcsi 350/161) LC X Prinmry E.t'uminerRonald L. Wibert Assistant Erumirwr-Paul K. Godwin Attorney, Agent, or Firm-Craig & Antonelli [57] ABSTRACT A method for driving a liquid cell in a scanning liquid cell display panel in which the liquid cell is interposed between two scanning electrode groups comprises the step of applying a DC voltage and an AC voltage hav ing a frequency above the limiting frequency of the liquid crystal to said two scanning electrode groups during a period of semi-selection of the scanning while applying said DC voltage thereto during a period of full selection, an amplitude ratio of said AC to DC voltages during said period of semi-selection ranging from 1 to 2.

6 Claims, 11 Drawing Figures PATENTEI] JUN 24 I975 SHEET FIG. I

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0 2O 4O 6O 80 I00 I20 I40 (V) DC. PULSE METHOD FOR DRIVING LIQUID CELL DISPLAY PANEL BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for driving a liquid cell display panel. and more particulary to a method for driving a liquid cell display panel. in which a dynamic scattering mode produced during a period of semi-selection (hereinafter referred to as a semiselection response) is controllable.

2. Description of the Prior Art In general. image or character display devices making use of a dynamic scattering effect of light in response to a voltage applied to a liquid cell of the nematic type require the function of effecting selective determination of display positions. that is. a scanning function. Upon application of a voltage to the liquid cell. there arises a big problem that an unpreferable dynamic scattering mode usually called a crosstalk takes place during a period of s-called semi-selection.

SUMMARY OF THE INVENTION An object of the present invention is to provide a driving method being capable of avoiding a semiselection response.

Another object of the present invention is to provide a driving method being capable of effecting display with good contrast.

In order to attain these objects. the present invention provides a driving method in which a high frequency voltage having a level necessary for cancelling the scattering mode is superimposed on a DC voltage applied during the period of the semi-selection. and during a period of non-selection the high frequency voltage is made not to be applied in order to prevent the falling time of the scattering mode from being reduced to an unpreferable extent.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic view showing a conventional liquid cell display panel.

FIG. 2 is a graph showing relative brightness ofa liquid cell upon application of a DC voltage.

FIGS. 3 and 4 are graphs showing conventional driving waveforms.

FIG. 5 is a graph showing relative brightness of a liquid cell upon application of a DC voltage with a high frequency voltage superimposed thereon.

FIGS. 6a and 6b are graphs illustrating an effect of the high frequency voltage upon the falling time of the dynamic scattering mode of a liquid cell.

FIGS. 7 to 9 are graphs showing embodiments of waveforms according to the present invention.

FIG. 10 is a graph showing relative brightness ofa liquid cell upon application of a DC pulse voltage.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1, there is schematically shown a liquid cell display panel ofthe nematic type adapted for use in the present invention, which is a well-known display panel. In FIG. 1 a group of longitudinal lines (X,, X X,,,) arranged in a lateral direction represents a series of horizontal scanning electrodes while a group of lateral lines (Y Y Y,,) represents a series of vertical scanning electrodes, both series of electrodes facing each other through liquid cells. With such an arrangement. the liquid cell located at an intersection P of the electrodes X and X effects a dynamic scattering mode upon the application of a predetermined voltage across the electrodes X and Y In FIG. 2 there is shown a graph illustrating the dynamic scattering mode in terms of the voltage applied to the liquid cell in the above-mentioned liquid cell display panel. In FIG. 2 an abscissa represents the voltage applied to one electrode and measured relative to the other electrode serving as a reference while an ordinate represents relative scattering brightness B. Curves u and b show characteristics upon the application of the voltage having positive and negative polarities. respectively. and marks V and V,,. show "a threshold voltage", that is. a voltage at which the dynamic scattering mode of the liquid cell is initiated.

Next. a general driving method and a semi-selection mode will be described in connection with FIG. 3. In FIG. 3 V shows a driving signal waveform applied to the electrode X with a voltage V applied thereto during the period of selection of I to I and V shows a driving signal waveform applied to the electrode Y with a voltage applied thereto during the periods of selection of r. to t and 1;, to 1 The distribution of the selection periods in FIG. 3 is arranged as a matter of conve nience for the purpose of simplification of the drawings and the description. This applies to the following similar description and drawings. In FIG. 3 there is shown a voltage (V,V between the desired electrodes X and Y at the intersection thereof to which the voltage 2V is applied during the period of selection of t. to that is. during a period of selection to both the electrodes X and Y (hereinafter referred to as a period of full selection); the voltage V is applied during the periods of selection of 1 to t and 1;, to t that is. during the period of a so-called semi-selection in which one electrode is in the period of selection and the other is in the period of no selection; and the voltage zero is applied during the period of selection 1 to I that is. during the period of no selection relative to both the electrodes (hereinafter referred to as a period of non-selection). In this case, it is of importance that the dynamic scattering mode of the liquid cell is adapted to be effected only during a period when the period of selection to both the electrodes coincides. that is. during the period of full selection, and hence no dynamic scattering mode thereofis preferably produced during the periods of semi-selection and non-selection.

The reason is that the generation of the dynamic scattering mode during the period of semi-selection or nonselection causes a ratio of the contrast of the dynamic scattering mode during the period of full selection to that during the above-mentioned periods to be degraded. thus necessarily resulting in the indistinct display of the images or characters to be desired.

Accordingly, in order to prevent the dynamic scattering mode during the period of semi-selection, it is necessary to restrict the voltage applied during the same period below the "threshold voltage" V shown in FIG. 2. The restriction of the applied voltage below V,,,. however. causes the voltage applied during the period of full selection to be below 2V because the voltage applied during the period of full selection is twice as great as that during the period of semi-selection as shown in the waveform V of FIG. 3. Assuming that the threshold voltage is about 8 volts, that level of the voltage becomes about In volts with reference to FIG. 2. This shows that the scattering brightness is restricted down below a half maximum at this level of the voltage with the result of no display of the satisfactory images or characters due to the reduced brightness.

As an improvement of such a driving system there has been proposed a conventional driving system represented by the waveform of the voltage in FIG. 4.

The driving system of FIG. 4. has the same general driving method and the same waveform of the voltage applied between both the electrodes as that of FIG. 3 with the characteristic exception that a DC bias near to the threshold voltage as shown in FIG. 2 is previously applied between both the electrodes X and Y.

In this embodiment. the DC bias is applied to the electrode X with the voltage V,,, applied during the period of no selection and with the voltage V applied during the period of selection while the voltage zero is applied to the electrode Y during the period of no selection with the voltage 2V,,, applied during the period of selection.

It will be appreciated from FIG. 4 that the voltage V between both the electrodes is 3V,,, during the period of full selection and V,,, and V during the periods of semi-selection and non-selection, respectively. and therefore the voltage applied during the period of full selection is increased at maximum three times as great as the threshold voltage. Accordingly. it will be calculated from FIG. 2 that the scattering brightness during the period of full selection reaches about 60 percent of its maximum. In any case, in the actual sanning system there is a requirement that the period of full selection should be as short as possible; however, it is unpreferable to restrict the driving voltage because the scattering brightness relative to the level of the applied voltage is always reduced to the level lower than that shown in FIG. 2 due to a relatively slow responce relative to the pulse voltage applied to the liquid cell sometimes without any production of the dynamic scattering mode even upon the application of the voltage as high as several tens volts. At the present time, the improve ment of the response by the use of a high voltage drive is effective to the above-mentioned problem, for the time being, other than the improvement of the liquid cell materials.

Prior to describing the present invention, two characteristics the liquid cell has will be described at first.

One of the characteristics is shown in FIG. 5 as a dynamic scattering characteristic of the liquid cell upon the application thereto of the DC voltage superimposed by the high frequency voltage. In FIG. 5 the abscissa indicates the high frequency voltage (the voltage being represented by a peak value) while the ordinate indicates the relative scattering brightness. Bent curves 0. d and e show the relative scattering brightness relative to the high frequency voltage superimposed on the respective different DC voltages with the DC voltage increased in the order of e, d and c. The same figure shows that the increase in the high frequency voltage relative to each DC voltage causes a reduction of the scattering brightness in more than a certain value thereof, eventually falling to zero at the voltages V V and V for each bent lines 0, d and e.

Experiments carried out by the inventors reveal that a ratio of the high frequency voltage to the DC voltage when the scattering brightness falls down to zero is about 2 in a normal state in the characteristic curves shonn in Il(i. 5 while the ratio decreases more in a scanning state depending on a time during the period of full selection. its cycle and the level of the applied voltage with a lower limit reduced to about I.

The other characteristic is shown as an influence of the high frequency voltage upon the falling time in the scattering mode as shown in FIG. 6a. in which a waveform g shows the waveform of the dynamic scattering mode produced by the application of the driving voltage (6()\' 20 milliseconds) having a waveform f with 50 percent attenuating time being l5 milliseconds. FIG. 6a further shows that the 50 percent attenuating time of the waveform i of the light scattering mode is shortened to 2 to 2.5 milliseconds when the liquid cell is drived by a waveform 11 further including a high frequency voltage (60V, 4KHz) superimposed on a por' tion ofthe waveformffollowing the period of selection.

On the other hand. a relation of a dynamic scattering amount (i.e.. scattering brightness) to the frequency of the high frequency voltage is as shown in FIG. 6b (in which the abscissa shows the frequency F and the ordinate shows the scattering brightness B) with a limiting frequency/b existing therein. That is the scattering disappears above the limiting frequency fiz The low frequency voltage (including the DC voltage) exists in the frequency region below fc while the high frequency voltage exists in the frequency region above fr'.

It will be. therefore. appreciated from the first characteristic (FIG. 5) that it is possible to prevent the scattering mode if the high frequency voltage at least greater than the voltage corresponding to V,. of FIG. 5 is superimposed on the DC voltage during the period of semi-selection. Further it will be understood from the second characteristic (FIG. 6) that the application of the high frequency voltage during the period of nonselection causes the falling time of the scattering mode to be reduced.

This means that the high frequency voltage is not preferably to be applied during the period of nonselection because it serves to cancel out a desirable memory effect.

In other words, the fundamental features of the present invention are, on the one hand, to superimpose on the applied DC voltage the high frequency voltage equal to or greater than the voltage V,. shown in FIG. 5 for cancelling the scattering mode during the period of semi-selection, and, on the other hand. to apply no high frequency voltage during the period of nonselection to prevent the falling time of the scattering mode from being shortened to an undesirable extent. It is to be noted that the DC voltage or low frequency voltage is primarily applied during the period of full selection as hithertofore to develop the scattering mode and that it is not hindered to superimpose the high frequency voltage having as low amplitude as the scattering mode is not prevented.

Next, the driving voltages applied during each period of selection will be described by way of embodiments.

A first embodiment is shown in FIG. 7 wherein numeral I indicates the waveform V, of the driving signal applied to the electrode X (dotted lines hereinafter denoting the DC voltage) with the DC voltage of 1 volt superimposed by the high frequency voltage In of 2 volts during the period of selection of z, to 1 and with the voltage reduced to zero during the period of no selection of 1 to Numeral 2 indicates the waveform V of the driving signal applied to the electrode Y [dotted lines hereinafter denoting the DC voltage) with the DC voltage of 2 volts superimposed by the high frequency voltage of 2 volts during the periods of selection of t, to and I;; to l and with the voltage of zero applied during the periods of no selection of 1 to I and t to 1 In this case. it is assumed that the high frequency voltage lu has the same frequency. phase and voltage as the high fre quency voltage 2a. In this figure. numeral 3 shows the voltage waveform V, between the electrodes X and Y at the intersection thereof to which the waveforms I and 2 are applied. respectively. with the DC voltage of 3 volts applied during the period of full selection of r, to 1 with the DC voltage of 1 volt and the high frequency voltage 312 of 2 volts applied during the period of semi-selection of to 1;, and the DC voltage of 2 volts and the high frequency voltage 3c of 2 volts applied during the period of 1;, to r and with the voltage of zero volt applied during the period of non-selection. These values meet the above-mentioned fundamental conditions of the present invention.

The above-mentioned embodiment shows that a ratio of the high frequency voltage to DC voltage becomes 2 at maximum during the period of semi-selection when the DC voltage is applied to the electrode X while the ratio thereof becomes 1 at maximum during the period of semi-selection when the DC voltage is applied to the electrode Y.

A second embodiment will be shown in FIG. 8 in which numeral 4 shows the waveform V, of the driving signal applied to the electrode X with the DC voltage of I volt applied during the period of selection of r, to 1;, and with the high frequency voltage 4b of 2 volts applied during the period of no selection of to Numeral 5 indicates the waveform V, of the driving signal applied to the electrode Y with the DC voltage of 2 volts applied during the periods of selection of t, to 1 and I" to 1 and with the high frequency voltage 5b of 2 volts applied during the periods of no selection to I and I to 1 In this case. the high frequency voltage 4b is assumed to have the same frequency phase and voltage as the high frequency voltage 5h. Further. in FIG. 8 numeral 6 indicates the waveform V of the voltage between both the electrodes, which is the same as the waveform 3 shown in FIG. 7.

It will be appreciated that although the different waveforms of the driving voltage are applied to the electrodes X and Y in the first and second embodiments, the same scattering effect of the liquid cell is obtainablc.

A third embodiment shows a composite combination of the first typical embodiment with the second typical embodiment with the waveform of the driving signal shown in FIG. 9. In FIG. 9, numeral 7 indicates the waveform V, of the driving signal applied to the electrode X with the DC voltage of 4/3 volts and the high frequency voltage 7a of 2/3 volts applied during the period of selection oft to 1;, and with the high frequency voltage 711 of2 volts applied during the period of no selection of to 15. The high frequency voltage 70 is herein assumed to have the same frequency as the high frequency voltage 7h an 1 the phase opposite thereto.

Further. in the same figure numeral 8 shows the waveform V of the driving signal applied to the electrode Y with the DC voltage of 2 volts applied during the periods of selection of to 1 and 1;, to t and with the high frequency voltage 8!) of 2 volts applied during the periods of no selection of 1 to and I, to In this case, the high frequency voltage 7b is assumed to have the same frequency. phase and voltage as the high frequency voltage 8h.

Further. in FIG. 9 numeral 9 indicates the waveform V ofthe voltage between both the electrodes with the DC voltage of 3%; volts and the high frequency voltage 9a of 2/3 volts applied during the period of full selection. with the DC voltage of l /is volts and the high frequency voltage 9b of 2% volts applied during the period of semi-selection of to with the DC voltage of 2 volts and the high frequency voltage 9c of 2 volts applied during the period of selection of 1 to 1 and with the voltage of zero volt applied during the period of non-selection.

An advantage of the third embodiment is that the DC voltage applied during the period of full selection is increased by I/3 volts although the amplitude of the driving voltage ranges the same as that of the first and second embodiments. No problem arises with respect to the high frequency voltage (9a in FIG. 9) subjected to the simultaneous application because it is low frequency voltage of 2/3 volts.

The use of the driving system as mentioned above causes no dynamic scattering mode during the period of semi-selection. thus permitting an improvement of an optical contrast.

That is. in FIG. I0 there is shown a relation of the applied voltage to the relative brightness for each duty ratio of one-tenth to one-fiftieth when the DC pulse voltage is applied to the liquid cell. Accordingly, with the conventional driving waveform as shown, for example, by V, in FIG. 4 a maximum driving voltage is 3 X 8 (volts) 24 (volts) when the threshold voltage is set to be 8 volts. but the maximum driving voltage experimentally reaches 36 volts in a scanning state (in the duty ratio of one-tenth) because the threshold voltage increases more than 8 volts. As a result, the relative brightness becomes 0.36 from the characteristic curve with the contrast where 0.06 represents background brightness.

In contrast thereto, according to the present invention in which no dynamic scattering mode occurs during the period of semi-selection. the level of the driving voltage is experimentally as high as I00 volts taking into account the break-down voltage or life span of the liquid cell. so that the relative brightness is 1.5 with the contrast As a result, the present invention permits the contrast to be improved about four times as much as that of the conventional one.

In the description above mentioned, a ratio of the high frequency voltage to the DC voltage applied during the period of semi-selection has been selected to be I and 2. but suitably ranges from 1 to 2 depending on the characteristics of FIG. 5. Further. it will be appreciated that the driving waveform is not limited to the above embodiments but only required to be of alternating waveform.

We claim:

I. A method for driving a liquid cell display panel including a plurality of first electrodes arranged in parallel to each other, a plurality of second electrodes re spectively intersecting with each of said first electrodes and arranged in parallel to each other, and liquid cells of nematic type disposed at the respective intersections of said first electrodes with said second electrodes, said method comprising the step of applying a voltage to a predetermined liquid cell of said liquid cells through said first and second electrodes to drive said predetermined liquid cell, wherein said voltage includes a DC voltage and a high frequency voltage superimposed thereon during periods of two types of semi-selection of said liquid cell and said voltage includes at least a DC voltage during a period of full selection thereof, an amplitude ratio of said high frequency voltage to DC voltage during the period of at least one of said two types of semi-selection being in the range of at least 1 and not more than 2.

2. A driving method as set forth in claim 1, wherein a low frequency voltage is substituted for said DC volt age.

3. A driving method as set forth in claim 1 including applying a DC voltage having a high frequency voltage superimposed thereon to a first electrode or a second electrode associated with a respective liquid cell during the period of semi-selection of said liquid cell.

4. A driving method as set forth in claim 1, wherein the DC voltage and the high frequency voltage are applied during the entire period of semi-selection so as to provide said amplitude ratio.

5. A driving method as set forth in claim 1, wherein the amplitude ratio of the high frequency voltage to the DC voltage is 2.

6. A method as set forth in claim 1, wherein the amplitude ratio of high frequency voltage to DC voltage is in the range of more than 1 and not more than 2.

Claims (6)

1. A method for driving a liquid cell display panel including a plurality of first electrodes arranged in parallel to each other, a plurality of second electrodes respectively intersecting with each of said first electrodes and arranged in parallel to each other, and liquid cells of nematic type disposed at the respective intersections of said first electrodes with said second electrodes, said method comprising the step of applying a voltage to a predetermined liquid cell of said liquid cells through said first and second electrodes to drive said predetermined liquid cell, wherein said voltage includes a DC voltage and a high frequency voltage superimposed thereon during periods of two types of semi-selection of said liquid cell and said voltage includes at least a DC voltage during a period of full selection thereof, an amplitude ratio of said high frequency voltage to DC voltage during the period of at least one of said two types of semi-selection being in the range of at least 1 and not more than 2.

2. A driving method as set forth in claim 1, wherein a low frequency voltage is substituted for said DC voltage.

3. A driving method as set forth in claim 1 including applying a DC voltage having a high frequency voltage superimposed thereon to a first electrode or a second electrode associated with a respective liquid cell during the period of semi-selection of said liquid cell.

4. A driving method as set forth in claim 1, wherein the DC voltage and the high frequency voltage are applied during the entire period of semi-selection so as to provide said amplitude ratio.

5. A driving method as set forth in claim 1, wherein the amplitude ratio of the high frequency voltage to the DC voltage is 2.

6. A method as set forth in claim 1, wherein the amplitude ratio of high frequency voltage to DC voltage is in the range of more than 1 and not more than 2.

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