WO1998053366A1

WO1998053366A1 - Method for ensuring the operational security of a liquid crystal display

Info

Publication number: WO1998053366A1
Application number: PCT/FR1998/001025
Authority: WO
Inventors: Gérard Voisin; Olivier Pernia
Original assignee: Sextant Avionique
Priority date: 1997-05-23
Filing date: 1998-05-20
Publication date: 1998-11-26
Also published as: FR2763734A1; FR2763734B1

Abstract

The invention concerns a method for ensuring the operational security of liquid crystal displays to prevent their breakdown from being unnoticed. The method consists in using the polarising layers with perpendicular polarising axes and in controlling the columns (203) of the pixel matrix ensuring the display such that in case one of the two control cards (204, 205) of said columns breaks down, this does not cause one of the colours to disappear but a coloured comb to be superposed on the image. The method further consists in commanding a polarisation reversal from one writing to the next as soon as possible in the chain generating the column control signals. The method enables to obtain, in case of breakdown, the display of parasitic light signals which attract the observer's attention.

Description

METHOD FOR SECURING A LIQUID CRYSTAL DISPLAY

The present invention relates to methods for securing liquid crystal displays. Securing means any system which makes it possible to clearly indicate to the observer who is looking at the display that it presents a fault, at least partial, and that the information or the absence of information, which appears on this display may be at least partly false. Such a method is particularly useful for displays intended to indicate in real time important or even vital information for the action to be taken, for example displays installed on aircraft.

The control and navigation instruments of an airplane were originally mechanical or electromechanical and it was common to include a device, called "flag", briefly presenting the appearance of a flag, which appeared in the field of vision to indicate the failure of the instrument.

Over time, all or part of these devices have been replaced by electronic display devices making it possible to display these indications, and many others in addition, in a form that is both much more flexible and much more meaningful.

These devices initially used cathode ray tubes powered by an image generator. The evolution of these devices has led to replacing cathode ray tubes with liquid crystal flat screens, generally backlit, which are at the same time more robust, lighter and less bulky, and which do not use high voltage.

In the case of a cathode ray tube, the securing of the tube itself is carried out by monitoring the currents and voltages of the different electrodes and of the scanning devices. This monitoring only involves around ten checkpoints, which is relatively easy to implement.

The liquid crystal screen, also called a panel or matrix, has about 1000 lines and 1000 columns defining elementary display points at their intersections and corresponding each with an input connection. The surveillance of all these entry points is not a priori conceivable. Even if it could be done, the means used would undoubtedly increase the risk of breakdown, so that the balance of the whole would surely be negative. To make it possible to secure such a device, the invention provides a method for securing a liquid crystal display, of the type comprising a layer of liquid crystal capable of being excited to rotate the plane of polarization of the light passing through it. , and two polarizing layers surrounding this layer of liquid crystal, mainly characterized in that the respective orientations of these three layers are chosen so that the assembly is transparent to light when the layer of liquid crystal is not excited.

According to another characteristic, the polarizing layers are oriented so that their axes of polarization are perpendicular. According to another characteristic, when the display further comprises means for exciting the liquid crystal layer which comprise a set of columns formed of elementary red, green blue dots grouped according to a set of pixels and two control members for these columns, the column controls are distributed between these two members so as to preserve, in the event of the breakdown of one of them, a degraded image comprising all the colors and on which a colored comb is superimposed.

According to another characteristic, when the pixels are organized in a square “QUAD” form comprising two elementary red and blue points along a diagonal and two green elementary points along the other diagonal, the control elements are made to control columns of rank 1 + 4N, and 2 + 4N and that the columns of rank 3 + 4N, and 4 + 4N are ordered to be ordered by the other control member. According to another characteristic, when the pixels are organized in an elongated shape comprising three successive red, green, blue elementary points corresponding to successively red, green, blue columns, all the red columns are controlled with one of the control members, green, blue row odd and with the other control member all the red, green, blue columns of even rank.

According to another characteristic, when the display of the columns is controlled by reversing the polarity of the control voltages between two successive writes, this reverse polarity is carried out before the input of the control members.

Other features and advantages of the invention will appear clearly in the following description, presented by way of nonlimiting example with reference to the appended figures which represent: - Figure 1, a schematic exploded perspective view of the layers of a liquid crystal panel;

- Figure 2, a front view of such a panel provided with its control means;

- Figure 3, a partial sectional view of this panel and one of these control devices;

- Figure 4, a schematic view of the arrangement of elementary display points of such a panel in connection with their control means according to the prior art;

- Figure 5, the same view as Figure 4 but for a panel according to the invention; and

- Figure 6, the same view as Figure 5 but for a different arrangement of the basic display points.

The basic structure of a liquid crystal panel generally comprises, as shown in FIG. 1, a thin layer of liquid crystal 101 surrounded by two polarizing layers 102 and 103.

In the case, most used in instrumentation, of a backlit device, a light source 104 is located on one side of the assembly to allow the device to be viewed from the other side by transparency.

The liquid crystal layer 1 01 rotates the polarization of the light 90 ° in the normal state. It comprises means, not shown in the figure, which make it possible, by applying a voltage to this layer, to suppress this effect and so that the plane of polarization of the light entering through one face leaves the other face with a polarization identical to that at input for maximum voltage for which the panel is intended for. The arrow on the layer in the figure represents the polarization of the light leaving this layer.

The polarizing layers 1 02 and 1 03 on the other hand are of the conventional type and therefore only allow light having a polarization corresponding to the arrows shown in the figure, that is to say in the case of unpolarized light at the start 50% of the intensity of it.

While Figure 1 represents only one configuration, there is a choice in practice between two configurations, that of the figure where the polarizers are parallel, and another not shown where the polarizers are crossed.

In the configuration of FIG. 1, the light emitted by the lamp 104 is polarized in one direction by the layer 102. The layer 1 01 of liquid crystal not excited by an electric voltage rotates the plane of polarization of the light emerging from the layer 1 02 at 90 ° relative thereto. The light emerging from the layer 1 01 is then stopped by the layer 103 whose polarization direction is parallel to that of the layer 102.

When, on the other hand, the layer 101 of liquid crystal is excited by an electric field of adequate value, the latter no longer rotates the plane of polarization of the light coming from the layer 102 and transmits it as it is. This plane of polarization having not changed, the light having transmitted the layer 1 01 passes entirely through the layer 1 03 and can be perceived by the observer located on the other side of this layer relative to the lamp 104. This observer So observe a bright image. The content thereof varies according to the configuration of the electrodes applying the electric field on the layer 1 01 in an operational configuration of the liquid crystal panel.

Conversely, in a structure where the polarization planes of the layers 102 and 103 are crossed, the light polarized by the layer 102 sees its plane of polarization rotate when it passes through the layer 101 in the case where this is not excited . It can then pass through layer 1 03, the plane of polarization of which is crossed with that of layer 102. In this configuration with crossed polarizers, the image is then luminous when the liquid crystal layer is not excited. Conversely, it is black when the layer is excited.

These two basic configurations are the most common, but it is possible to find others depending on the liquid crystals used and the technology in which they are used.

From this basic configuration, where we can observe a uniformly black or luminous surface, we can obtain a device making it possible to display various pictograms, by depositing on the two faces of the layer 1 01 electrodes having the shape of these pictograms. These electrodes can also be optionally controlled selectively. The information thus displayable is relatively frozen.

To then obtain a device making it possible to display the information in a more versatile way, two sets of linear electrodes crossed between the two faces are used which define over the whole of the liquid crystal layer a tight network of elementary points than the you can address separately by selecting the different electrodes, which are called columns in one direction and rows in the other. The network of points can be very fine so you can display detailed images. Using a three-color filter, this image can be in color.

At this level, the sources of failures come essentially from the following causes:

- line or column break; - short circuit between adjacent rows or columns;

- short circuit of a line with a column; -aging of the cell.

To control such an LCD panel or matrix, use is generally made of an arrangement as shown in FIG. 2, in which electronic circuit boards receive multiplex signals containing the information to be displayed on the LCD matrix 201 and generate from these signals the actual control signals to be applied to the rows and columns.

According to a usual technique, a line card 202 receives the vertical and horizontal synchronization signals and generates the signals to be applied to the lines of the matrix 201, by means of line 203 “drivers” making it possible to obtain the necessary voltages and powers. There is usually only one line card that is attached to one side of the matrix, but in some layouts you can find two line cards, one on the right, the other on the left.

On the other hand, two column cards 204 and 205 are generally used which are fixed at the top and at the bottom of the matrix. These column cards receive image data information from fast parallel digital buses. The information arriving on these buses is decoded to separate the information corresponding to each column. This individual information is then converted to analog in order to obtain power signals applied by the column drivers 206 and 207 to the various columns of the matrix 201.

Generally the digital / analog conversion is done at the level of the column drivers, which thus combine the functions of analog conversion and power amplification to have the levels necessary for the attack of the columns.

Given the density of the connections to be connected to the panel, a system is used to make these connections, as shown in FIG. 3, in which the electronic chips 208 forming the drivers are transferred to a flexible support on which the tracks are engraved input and output of these chips. This arrangement corresponds to the technology known as "TAB". Under these conditions, the TAB is fixed on the printed circuit board 204 and input connections are joined by solderings 209 to the tracks of the printed circuit. The outlet connections 21 2, for their part, form a comb whose teeth are very tight and having a pitch corresponding to that of the rows or columns on the panel 201. The connection of these teeth to the rows or to the columns is carried out by bonding 10 using an adhesive which ensures both the electrical contacts between the TAB and the panel and the mechanical fixing of the TAB to the panel. This adhesive, of known type, makes it possible to have a low resistivity in the direction of the thickness of the adhesive film and high in the direction of the plane of this film, so as to obtain the desired connections without having short circuits between they. For reasons of space, it is generally necessary to fold the TAB so that the cards 204 are perpendicular to the plane of the panel 201, so as not to excessively increase the inactive surface of the display device finally obtained relative to the active surface presented by the liquid crystal panel. To allow this folding without breaking the connections forming the outlet comb connected to the rows or columns, the plastic support of the TAB is eliminated according to grooves, here 2 in number, extending at the fold parallel to the axis folding. This elimination, which is done chemically, thus exposes the connections 21 2 which can therefore bend without breaking. The plastic support, which remains at the ends of the windows thus released, allows the assembly to be held during bending to avoid making these connections bear tensile forces or lateral displacements, which limits the stresses to those resulting from the kinks and prevents stray contact.

Under these conditions, the sources of breakdowns at this level come essentially from the breaks in the connections in the tracks, and to a lesser extent the breakdowns of the integrated circuits and the breaks of the inputs arriving on these integrated circuits.

All the failures thus identified cause on the display a certain number of disorders, the inventors of which have found that they all correspond to a clarification of the zones (rows, columns, crossings) of the panel affected by the failures in the case where we use the configuration with crossed polarizers.

In order for this failure to surely attract the attention of the observer of the display device, the invention proposes using a configuration in which the absence of excitation of the rows or columns results in the whitening of the regions concerned. by the breakdown, which makes it possible to obtain a positive manifestation corresponding to this breakdown. This configuration is here that with crossed polarizers.

Furthermore, in civil aviation applications for which the invention is more particularly intended, the production volumes do not allow the use, in particular for drivers, of specific integrated circuits. We are therefore led to use circuits commercially integrated, in particular those intended for displays intended for PC type computers. These integrated circuits include at input 3 BUS in parallel, which we will call BUS 1, BUS 2 and, BUS 3 for the high card and BUS 4, BUS 5 and, BUS 6 for the low card. The role of the column drivers is to demultiplex the information circulating on these BUSes to apply them to the columns.

In a configuration, represented more particularly in FIG. 4, the elementary points of the panel 201, distributed according to the three colors red (R), green (G), and blue (B) are grouped in structures called "QUAD" corresponding to squares 21 3 comprising on one diagonal a red point and a blue point, and on the other diagonal two green points. There is thus a succession of red / green and green / blue columns.

The connection mode usually used to control these columns from the cards 204 and 205, consists, as in FIG. 4, of controlling the successive columns alternately with the high card and with the low card. Thus all the odd columns will be controlled by the high card from BUS 1, 2 and 3. All the even columns will be controlled by the low card from BUS 4,5, and 6.

We see that in this configuration all the red dots are controlled by the top card and all the blue dots by the bottom card. Green dots are controlled for half by the top card and for the other half by the bottom card. So a total failure on one of the two cards causes the disappearance of the red image or the blue image. This situation is particularly troublesome in the case of a display display, where pure colors are often used, unlike imaging displays, where the colors are generally colors obtained from red / green / blue mixtures. For a display display there is therefore a risk of losing all of the information corresponding to a color, which is particularly troublesome for red, which is generally assigned to information of vital importance.

The invention therefore proposes to distribute the commands in the manner presented in FIG. 5, where the high card controls the two first columns, the low card the two following columns, the high card the two columns after, and so on .... In other words, the high card commands the columns 1 + 4 n and 2 + 4n and the low card columns 3 + 4n and 4 + 4n. Under these conditions, in the presence of a total failure of one of the two cards, on the one hand the corresponding image does not disappear completely, since the symbols drawn on the display are always of a thickness greater than 1 pixel . On the other hand, and taking into account the configuration in crossed polarizers explained above, the observer notices the appearance of a colored vertical comb, which is superimposed on the degraded image, but nevertheless remaining, and indicates without any risk of confusion the appearance of a breakdown on one of the two cards.

This mode of column commands is not limited to matrices of the QUAD type, but it extends, using an adequate sequencing of the column commands, to other types of matrices where the elementary points are distributed in a different manner. Thus, in the case of a panel of the type known under the name of “STRIP”, where, as represented in FIG. 6, the pixel 223 is formed of three elementary points red, green, blue aligned, a distribution of the commands between the high card 224 and the low card 225 in which the high card will control the first three columns, the low card the following three, the high card the next three and so on ... high card all odd red, green, blue columns and with the low card all even red, green, blue columns.

The main thing is that in all configurations the loss of one of the two control boards for the columns does not lead to the complete disappearance of a color, but instead leads to the appearance of a colored vertical comb indicating the breakdown. Finally, it is known that a reverse polarity system is commonly used to control liquid crystal displays, intended to remedy the defects due to the presence of ions in the liquid crystal, in which the polarity of the command signals from one write to the next, of course reversing the value of the command applied as input to obtain the display of the same given. In the known art, this reversal takes place essentially at the level of the column drivers.

The invention proposes to effect this inversion at the level of image generation, more particularly at the level of the devices proceeding to smooth information. In this way, the black will be obtained by having all the wires of the BUSes at “0” during a writing, and all these same wires at “1” during the following writing, the corresponding information arriving by the input buses of the cards. high and low. Under these conditions a failure on these BUS will not allow this alternation of 0 and 1 on the links, and the background of the image will become bright instead of being black. This circumstance will therefore allow the observer to detect the existence of a breakdown.

This control mode obviously requires, to obtain the correct display, to switch the reference voltages between two successive writes. A breakdown in these generators will also result in the appearance of a bright background. The use of this means therefore makes it possible to secure the reference voltage generators at the same time.

Claims

1 - Method for securing a liquid crystal display, of the type comprising a layer of liquid crystal (101) capable of being excited to rotate the plane of polarization of the light passing through it, and two polarizing layers (1 03 , 102) surrounding this layer of liquid crystal, characterized in that the respective orientations of these three layers are chosen so that the assembly is transparent to light when the layer of liquid crystal is not excited.

2 - Method according to claim 1, characterized in that one directs the polarizing layers (1 03, 102) so that their axes of polarization are perpendicular.

3 - Method according to any one of claims 1 and 2, wherein the display further comprises means for exciting the liquid crystal layer which comprise a set of columns formed of elementary red points, blue green grouped according to a set of pixels (203) and two organs (204,205) for controlling these columns, characterized in that the commands for the columns are distributed between these two organs so as to preserve, in the event of one of them failing, a degraded image with all the colors and overlaid with a colored comb.

4 - Method according to claim 3, wherein the pixels (213) are organized in a square form "QUAD" comprising two elementary red and blue points along a diagonal and two green elementary points along the other diagonal, characterized in that l '' the columns of rank 1 + 4N and 2 + 4N are ordered by one of the control members (21 4) and that the columns of rank are ordered by the other control member (21 5) 3 + 4N, and 4 + 4N.

5 - Method according to claim 3, wherein the pixels (223) are organized in an elongated shape comprising three successive red, green, blue elementary points corresponding to columns successively red, green, blue, characterized in that one controls with one of the control members (224) all the red, green, blue columns of odd rank and with the other control member (225) all red, green, blue columns of even rank.

6 - Method according to any one of claims 3 to 5, in which the display of the columns is controlled by reversing the polarity of the control voltages between two successive writes, characterized in that I 'this polarity reversal is carried out before l entry of the control members (204,205).