EP0000594B1 - Electro-optical display device and method for operating same - Google Patents

Description

The present invention relates to an electro-optical display device for displaying symbols consisting of separately controllable display elements according to the preamble of claim 1 and to a method for controlling such a device.

From DE-A 2459058 a display device of the aforementioned type is known, in which partial electrodes are provided on the front panel, one of which, in the absence of a supply line, cannot be acted upon by electrical signals. The result of this is that no electrical field can form in the electro-optically activatable medium located between this partial electrode and the opposite rear electrodes and can influence the medium. Such display devices are designed for the principle of the twisted nematic liquid crystal and must therefore be provided with polarizers and surface treated. In such a display device, an electro-optically activatable medium is used, which absorbs light when it is in the field-free state and transmits light when signals are present at the front and rear electrodes, the differential voltage of which is greater than a transition voltage at which the medium is light lets through, bright symbols appear on the display surface on a dark background. From DE-OS 2450698 a display device is known in which each partial electrode surface of the front electrode is congruent with the surface of the display element. The liquid-crystalline medium located between the partial electrodes of the front and back electrodes of a symbol is optically activated by electrical activation of the corresponding electrodes and scatters the incident ambient light, so that a viewer can read the activated symbol. Such display devices are designed to take advantage of the dynamic scattering effect of liquid crystals and have a high energy consumption because they are driven with relatively high voltages. This display device is also not always to be used advantageously for electro-optical media, the activation of which is based on a different physical effect, since these devices also display bright symbols on a dark background, for example when using a cholesterol guest-host substance. This is not always advantageous, since the symbol shown is optimally readable if it is present like the letters of a newspaper as a dark marking against a light background. The invention seeks to remedy this. The invention, as characterized in the claims, achieves the object of providing a display device of the aforementioned type in which dark symbols are displayed on a light background by actuation with low voltages and without the use of polarizers, and in which the control is also carried out by a simple, low-energy method.

With the display device according to the invention, even with illumination with unpolarized light and with control with comparatively low voltages, extremely high-contrast representations of the displayed symbols on a light background can be achieved. It is particularly advantageous here that the device can be controlled using the same integrated circuits that are used in the very common rotary crystal display device.

In the drawing, exemplary embodiments of the subject matter of the invention are shown in simplified form below.

• Fig. 1 eine Aufsicht auf die Elektrodenmuster der Front- und der Rückelektrode einer Anzeigevorrichtung nach der Erfindung,
• Fig.2 eine Aufsicht auf die Anzeigeelemente der Anzeigevorrichtung nach Fig. 1,
• Fig.3 eine schematische Darstellung der Anzeigevorrichtung nach Fig. 1, wobei die Teilelektroden als zweizeilige Matrix geschaltet und drei Anzeigeelemente optisch angesteuert sind,
• Fig.4 eine schematische Darstellung der an die Elektroden der Anzeigevorrichtung nach Fig. 3 angelegten Ansteuersignale,
• Fig. 5 die funktionale Abhängigkeit der Helligkeit des Untergrundes der Anzeigevorrichtung nach Fig. 1 von der an die Elektroden angelegten wirksamen Spannungsdifferenz U [Volt],
• Fig. eine schematische Darstellung der Mittel zum Ansteuern einer bekannten Drehkristall-Anzeigevorrichtung,
• Fig. 7 eine schematische Darstellung der Mittel zum Ansteuern der Anzeigevorrichtung nach Fig.1,
• Fig. 8 ein Adressierschema zum Ansteuern der Anzeigevorrichtung nach Fig. 1,
• Fig. eine Aufsicht auf die Elektrodenmuster der Front- und Rückelektrode einer weiteren Anzeigevorrichtung nach der Erfindung, und
• Fig. 10 die zur Einstellung und Aufrechterhaltung der nematischen Phase eines cholesterinischen Flüssigkristalls in einer gemäs Fig. 8 angesteuerten Anzeigevorrichtung notwendige minimale Spannungsdifferenz in Abhängigkeit von der Frequenz des an die Teilelektrode i gelegten höherfrequenten Signals.

It shows:

• 1 is a plan view of the electrode patterns of the front and rear electrodes of a display device according to the invention,
• 2 shows a plan view of the display elements of the display device according to FIG. 1,
• 3 shows a schematic illustration of the display device according to FIG. 1, the partial electrodes being switched as a two-line matrix and three display elements being optically controlled,
• 4 shows a schematic illustration of the control signals applied to the electrodes of the display device according to FIG. 3,
• 5 shows the functional dependence of the brightness of the background of the display device according to FIG. 1 on the effective voltage difference U [volt] applied to the electrodes,
• 1 shows a schematic representation of the means for driving a known rotary crystal display device,
• 7 shows a schematic representation of the means for controlling the display device according to FIG. 1,
• 8 is an addressing scheme for driving the display device of FIG. 1,
• Fig. Is a plan view of the electrode patterns of the front and rear electrodes of a further display device according to the invention, and
• 10 shows the minimum voltage difference necessary for setting and maintaining the nematic phase of a cholesteric liquid crystal in a display device controlled according to FIG. 8 as a function of the frequency of the higher-frequency signal applied to the partial electrode i.

1 shows on the left side the top view of the electrode pattern of the front electrode applied on a transparent front plate and on the right side the electrode pattern of the rear electrode applied on a back plate of a possible embodiment of the display device according to the invention. With a, b, ..., g, h are the partial electrodes of the front and i, j the partial electrodes of the rear plate. Leads extend from the electrodes to the connection lugs a ", b", ..., i ", j" located on the edge of the front panel. The lines between the electrodes denote recesses A, A ', B, B', through which the partial electrodes covering the front and rear plates almost completely are electrically insulated from one another. The thick black lines A 'and B _' indicate the delimitation of partial electrodes caused by the recesses in the area of a display element belonging to a symbol to be displayed.

2 shows the display elements a ', b', ... g 'of this display device that become visible through activation. The area of each of these display elements is completely covered by a corresponding partial electrode of the front panel on the one hand and completely by the area of the counter electrode j of the rear panel on the other. The limitation of each display element is given by the overlap area of the corresponding partial electrode of the front panel and the partial electrode j of the rear panel. In addition to the sub-electrodes a, ..., g 'required to display the display elements a,

..., g, the front plate has a further partial electrode h, the surface of which does not coincide with any of the display elements. In addition to the connecting lugs of the partial electrodes located on the front plate, the connecting lugs i ", j" of the two partial electrodes i, j of the rear plate are attached to the front plate. This front plate can, for example, be plugged into a standardized connection piece, so that each electrode of the display device can be easily controlled. The contact from the leads of the electrodes i and j of the rear plate to the connection lugs i "and j" can be carried out in a simple manner, since the leads of the electrodes i and j are also guided to the edge of the backplate and are directly opposite the associated connection lugs. The electrode h of the front plate completely covers the supply line of the partial electrode j of the rear plate and the electrode i of the rear plate completely covers the supply lines of the partial electrodes a, b, ..., g of the front plate, whereby the part of the partial electrode that is to be understood as the supply line does not coincide with the area of the display elements. This prevents the feed lines from becoming visible when the device is in operation.

A simplified representation of the display device according to FIG. 1, the partial electrodes being connected as a two-part matrix, is shown in FIG. 3. The two partial electrodes i and j of the rear plate are represented by two horizontal lines and the partial electrodes a, b ..., h of the front plate by eight vertical lines. The intersections of these lines correspond to those areas of the display device where the corresponding front and rear electrodes overlap and a local electric field can be applied.

If, for example, the number “7” is to be displayed, this means that, according to FIGS. 1 and 2, the display elements a ', b' and c 'are activated in such a way that they remain dark, ie that the liquid crystal layer in question is field-free, while the remaining liquid crystal layer forming the background is in a sufficiently strong electric field so that a light background is present.

This can e.g. by controlling the partial electrodes a, b, ..., j with the signals indicated in FIG. 4.

Thereafter, the electrode j is operated with a pulsed DC voltage signal Ug of the amplitude 2V and in a 50% cycle, while a continuous DC voltage signal U _{9i of} the potential V is applied to the second partial electrode i of the backplate. The front plate-side electrodes a, b, c of the display elements a ', b', c 'are operated with the same pulsed DC voltage signal U _S1 , which is applied to the rear plate-side electrode j. All other electrodes on the front plate side d, e, f, g and h are operated with a pulsed direct current signal Us which is 180 ° out of phase with the signal at the electrodes a, b, c and j and has the same amplitude as this. There is therefore a vanishing potential difference at the display elements a ', b' and c ', while at the optically not selected elements d', e ', f' and g 'and the overlap area between the electrodes h and j there is a potential difference of 2V and in all other areas there is a potential difference of V. No area of the display area receives a constant DC voltage, so that electrochemical decomposition of the liquid crystal is largely avoided.

The voltage level V is selected so that it lies above a critical voltage U _e at which the cholesteric phase of the liquid crystal completely changes into the nematic phase. This voltage is hereinafter referred to as the “transition voltage”. No further reorientation of the liquid crystal can take place above the transition voltage and the feature characterizing the electro-optical property reaches a saturation value.

+ 94% einer nematischen Flüssigkristallmischung auf Biphenylbasis, welche in einer 8 µ dicken Anzeigezelle untergebracht ist, die Abhängigkeit der Helligkeit H des von der Zellrückwand reflektierten Umgebungslichtes in Abhängigkeit von der an die Zelle gelegten Spannung U [Volt] aufgetragen. Wegen des steilen Anstiegs und des raschen Erreichens eines Sättigungswertes ist bei dieser Substanz das optische Ansprechvermögen einer oberhalb der Übergangsspannung U_c liegenden Spannung V ebenso gross wie das einer Spannung 2V. Daher weisen alle hellen, als Untergrund wirkenden Bereiche der Anzeigevorrichtung, also gemäss Fig. 3 alle Anzeigeelemente, an denen die Differenzspannung V oder 2V anliegt, das gleiche optische Erscheinungsbild auf. Von grosser Bedeutung ist hierbei natürlich der Energieverbrauch, da fast die gesamte Fläche der Anzeigezelle unter Spannung steht. Es ist daher ratsam, die Anzeige mit einer möglichst geringen Frequenz, etwa 2 Hz, zu betreiben, um die kapazitiven Verluste des Zellenstromes zu minimalisieren.5 shows the following composition for a substance showing the guest-host effect:

+ 94% of a nematic liquid crystal mixture based on biphenyl, which is accommodated in an 8 μ thick display cell, shows the dependence of the brightness H of the ambient light reflected from the cell rear wall as a function of the voltage U [volt] applied to the cell. Because of the steep rise and the rapid reaching of a saturation level, is at of this substance, the optical response of a voltage V above the transition voltage U _c is as great as that of a voltage 2V. Therefore, all bright areas of the display device acting as a background, that is, according to FIG. 3, all display elements to which the differential voltage V or 2V is present, have the same visual appearance. Of course, the energy consumption is of great importance, since almost the entire area of the display cell is under voltage. It is therefore advisable to operate the display with the lowest possible frequency, around 2 Hz, in order to minimize the capacitive losses of the cell current.

The method for driving the display device described above has the advantage that the same integrated circuits that are usually used when driving nematic rotary cells can be used. The only additional elements are a simple logic inverter and a voltage divider.

6 shows the means for driving a known nematic rotary crystal display. In this case, a DC voltage U is applied to an integrated circuit IC, from which 50% pulsed signals U _a , ..., Ug reach the front electrodes a, ..., g and a signal U _B p to the rear electrode BP of a liquid crystal cell D. . FIG. 7 shows the means for controlling the display device according to the invention according to FIG. 1. These means include the same integrated circuit IC, but this time the DC voltage 2U is present. However, part of the signal U _SP is now phase-shifted to the front electrode h and another part in an inverter I (for example one of the type RGA CD 4041 A) by 180 ° and then applied to the back electrode j. The other back electrode i is supplied with a continuous DC voltage V, which can be generated, for example, in a simple voltage divider R ', R _" . A further method for controlling the display device according to the invention is specified below. A simplified addressing scheme of this method is shown in FIG All signals are unipolar pulse trains with a 50% pulse-pause ratio and an amplitude U. The display signals a ', b' and c 'have the same signals on both the front and rear electrodes, so that The display elements d ', e', f 'and g' are given phase-shifted pulses on the front and on the back electrode, so that the potential difference is a bipolar square wave voltage of the amplitude U and is without a DC component. As long as U is greater than the transition voltage U _c , these indicators are present elements in the nematic state, absorbing little light.

The partial electrode i of the rear plate, which covers the leads of the front electrodes, is operated at a higher frequency, preferably a harmonic to the frequency of the signals applied to the electrodes of the front plate. The potential difference between the overlapping areas of the front electrodes and the rear electrode i then has the shape shown in FIG. 8. With a 50% pulse-pause ratio and a pulse height U of the higher-frequency signal, the polarity of these voltage differences changes with the period of the low-frequency signals. As long as U is sufficiently high above the transition voltage and the pulse frequency is sufficiently high, the liquid crystal layer delimited by the area of the back electrode i remains in the nematic and little light-absorbing liquid crystal state, regardless of the phases of the signals on the front electrodes.

The front electrode h is operated like the partial electrodes a to g. Therefore, the liquid crystal layer of the entire background is in the nematic state, while only the display elements a _' , b', c 'are in the light-absorbing cholesterol state.

Since the higher-frequency signal is tapped, for example, in clocks from the frequency chain of the means used to control conventional rotary crystal displays, a particular advantage of this method is that no two voltage sources, the levels of which have to be carefully coordinated with one another to avoid harmful DC components, are now necessary. In addition, this method can also be used for liquid crystals with transition voltages greater than 7.5 V in commercially available integrated circuits operated at 15 V voltage.

FIG. 10 shows the minimum voltage difference U necessary for setting and maintaining the nematic phase of a cholesteric liquid crystal in a display device (controlled as described above) as a function of the frequency f of the higher-frequency signal applied to the partial electrode i. Here, a liquid crystal with a transition voltage of 7.5 [V] is provided. Below 20 [Hz] the flickering of the display cannot be eliminated even when the voltage is raised. A suitable, almost flicker-free display results at U = 10 [V] and f = 30 [Hz].

Since the energy consumed is greatest at the locations of the display where the high-frequency voltages are present, because of the capacitive losses, it is advisable to reduce the areas of these locations. This can be done, for example, by changing the electrodes according to FIG. 9. In this arrangement, the leads of the electrodes a, ..., g of the front panel and corresponding to the electrode i of the rear panel are kept as small as possible. It is also possible to lead the leads of the electrodes a, ..., g to opposite edges of the front panel. Through this and the aforementioned measure, the energy loss of the display devices according to the invention can be increased by a factor of approximately gate 2 can be reduced.

The display device according to the invention is not only limited to cells with liquid-crystalline guest-host substances, it can also be used, for example, for the use of electro-phoretic suspensions as an electro-optically activatable medium, and above all because such substances cover the entire display area an electrical potential must be present in order to maintain or set a fully defined optical state. It is particularly advantageous here that, for example, the 7-segment display of a number can be switched without the feed lines to these segments becoming visible.

Label list

Claims (11)

1. An electro-optical display device for representing symbols consisting of separately controllable display elements (a', b', ..., g'), which is provided with a layer, arranged between a transparent front electrode and a rear electrode, of a medium which can be electro-optically activated, the front electrode being attached to a transparent front plate and the rear electrode being attached to a rear plate and both electrodes being provided with part-electrodes (a, b, ..., i, j) which are electrically insulated from one another by means of recesses and which cover the plate surfaces in the region of the medium which can be electro-optically activated, and of which part-electrodes the part-electrodes (a, b, ..., g, j) contained in the display elements are provided with feed lines, and electric control signals being applicable to the feed lines via a drive circuit, characterised in that each of the remaining part-electrodes (h, i) is also provided with a feed line, that an electro-optical medium is provided which absorbs light when the control signals (Ug., Ug, U_Si, U_Sll) are applied if the potential difference between the front electrode and the rear electrode is less than a medium-related transition voltage (U_c), and transmits light when this potential difference is greater than the said transition voltage (U_C), and that, between the part-electrodes (a, b, c) of the front electrode and the part-electrode (j) of the rear electrode of the display elements (a', b', c_') of the symbols to be represented, a potential difference exists during the operation of the display device, this potential difference being smaller than the transition voltage (U_c), whereas between the remaining part-electrodes (d, e, f, g, h) of the front electrode and the part-electrodes (i, j) of the rear electrode, during the operation of the display device, a potential difference exists which is at least equal to the transition voltage (U_c).

2. A display device according to Claim 1, characterised in that, as the medium which can be electro-optically activated, a liquid-crystal guest- host substance and particularly a cholesteric liquid crystal which is provided with at least one pleochroic dye, or an electro-phoretic suspension which is provided with an electrica Ily-charged pigment is provided.

3. A display device according to Claim 1 or 2, characterised in that in the front electrode every part-electrode (a, b, ..., f, g) except one (h) at least partly overlaps the display elements (a', b', ..., fl, g'), that this one part-electrode (h) of the front electrode overlaps the feed lines of the part-electrodes (i, j), arranged in the rear electrode, except for one (i), and that this one part-electrode (i) of the rear electrode overlaps the feed lines of every one of the part-electrodes (a, b, ..., f, g), arranged in the front electrode, which at least partly overlap the display elements, but does not overlap the feed line of the one part-electrode (h), which does not overlap any of the display elements, of the front electrode.

4. A display device according to one of Claims 1 to 3, characterised in that at one edge of the front plate, connection tabs (a", ..., j") are provided, to which the feed lines of all the part-electrodes (a, ..., j) are connected.

5. A display device according to one of Claims 1 to 4, characterised in that on the rear electrode a metallic-reflecting and electrically insulating coating is provided in the form of a layer.

6. A method for operating the display device according to Claim 3, characterised in that, during the whole operating time, unipolar rectangular signals (U_Sl, U_gl), having identical amplitudes, pulse frequencies, phases and pulse shapes, are applied to the part-electrodes (a, b, c) of the front electrode and the part-electrode (j) of the rear electrode, which overlap the display elements (a', b', c' ) of the symbols to be represented, that, to the front part-electrodes (d, e, f, g) of the display elements (d', e', f', g') contained in the background, and to the front electrode (h) which does not overlap any of the display elements (a', ..., g'), signals (U_sll) are applied which are phase-shifted by half a period with respect to the signals (U_s,, U_gl) which are applied to the part-electrodes (a, b, c, j) of the display elements (a', b', c^l) of the symbols to be represented, but that these signals have the same amplitude, pulse frequency and pulse shape as the latter signals, and that a signal (U_gi) having a higher pulse frequency than that of the other signals is applied to the part-electrode (i) which does not overlap any of the display elements, of the rear electrode.

7. A method according to Claim 6, characterised in that the signal (Ug.) with the higher frequency is a signal which is harmonically related to the signal (Us,) which is applied to the part-electrodes (a, b, c, j) of the display elements (a', b', c') of the symbols to be represented.

8. A method for operating the display device according to Claim 3, characterised in that, during the whole operating time, unipolar rectangular signals (Us), having identical amplitudes, pulse frequencies, phases and pulse shapes, are applied to the part-electrodes (a, b, c) of the front electrode and the part-electrode (j) of the rear electrode, which overlap the display elements (a', b', c') of the symbols to be represented, the amplitudes (2V) of which signals are at least twice the transition voltage (U_c), that, to the remaining front part-electrodes (d, e, f, g, h), signals (U_sll) are applied which are phase-shifted by half a period with respect to these signals (Us,) but have the same amplitude, pulse frequency and pulse shape, and that a direct-voltage signal the potential (V) of which has half the amplitude of the aforementioned signals, is applied to the part-electrode (i), which does not overlap any of the display elements, of the rear electrode.

9. A method according to one of Claims 6 to 8, characterised in that the pulse frequency of the signals (Us) applied to the display elements (a,, b', c' ) of the symbols to be represented is at most 10 [Hz].

10. A method according to Claim 7, characterised in that the pulse frequency of the signal (U_gl) having the higher frequency is between 20 and 50 [Hz].

11. A method according to Claim 10, characterised in that, with a medium which can be electro-optically activated and which has a transition voltage (U_c) of approximately 7.5 V, the amplitude of the signal (Ug.) having the higher frequency is about 10 [V] and the pulse frequency of the signal is about 30 [Hz].

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