EP0142385A1 - Method and device for displaying symbols on a liquid-crystal matrix display - Google Patents

Abstract

The device according to the invention comprises an automaton controlled by a clock (H) which cyclically sends to the control circuit of a liquid crystal matrix (8), the bits representative of the elementary images contained in a random access memory (26) with a capacity greater than k (n + m) bits, k being the multiplexing coefficient and n and m being respectively the number of rows and columns of the liquid crystal matrix (8). The memory (26) is a random access memory refreshed by a central computer (27) during the bit transmission interval.

The invention applies in particular to the production of viewfinders or collimators mounted on board a vehicle.

Description

The present invention relates to a method and a device for displaying symbols using a liquid crystal matrix.

It applies in particular, but not exclusively to the production of viewfinders or collimators mounted on vehicles, for example helicopters in which it is necessary to superimpose, in the field of vision of an operator, for example of the pilot, visible symbols, to the external environment, for example to the external landscape.

In general, it is known that there are already many display devices or viewfinders of this kind using, for the generation of symbols, electromechanical systems or cathode ray tube systems.

In electromechanical systems, the symbols are produced by means of independent optical masks, either mobile or fixed. Usually, the movements of these symbols are obtained using mobile equipment possibly equipped with mirrors. However, the number of symbols generated by these systems is limited due to the complexity and the number of mechanical assemblies it makes intervene. In addition, these systems are relatively bulky and suffer from a certain fragility. In cathode ray tube systems, the screen of the cathode ray tube on which the symbols are generated, is arranged in the focal plane object of an optical system used for example to project these symbols on a semi-reflecting mirror. This solution has the advantage of visualizing very complex symbologies with high definition. On the other hand, it proves to be expensive and bulky, which considerably limits its field of application. In an attempt to avoid these drawbacks, we have therefore sought to produce display devices using, in optical systems similar to those used for cathode ray tubes, liquid crystal matrices, much cheaper and less bulky than cathode ray tubes.

On this subject, it is recalled that a liquid crystal matrix consists of two transparent, parallel glass plates, the intermediate volume of which contains a liquid crystal. Each of these glass plates supports a network of addressable electrodes forming the rows and columns of the matrix, which are connected to one or more electronic control circuits.

• Au repos, le cristal liquide est orienté de telle sorte que les molécules tournent de 90° d'un bord à l'autre de la matrice.

In addition, two light polarizers with parallel axes are arranged on either side of the glass plates. The effect used, well known as "TWISTED NEMA-TIC", is as follows:

• At rest, the liquid crystal is oriented so that the molecules rotate 90 ° from one edge to the other of the matrix.

Thus, a light beam applied perpendicular to the matrix thus produced will first of all undergo rectilinear polarization through the first polarizer. The polarized light will then undergo a rotation of 90 ° as it passes through the slice of liquid screen. The polarized light beam will then reach the second polarizer oriented perpendicular to its axis of polarization. We therefore obtain at the second polarizer a extinction of the light beam. This extinction is maintained as long as the electric field between the electrodes constituting the rows and the columns of the matrix is less than a predetermined electric field, generated by a threshold voltage Vth.

By applying a sufficiently large electrical voltage Von between a row electrode and a column electrode, the molecules will align, at the intersection of the row and the column, according to the electric field produced, so that in this area, the light will not be deflected and will appear oriented parallel to the axis of the second polarizer. This will therefore allow an elementary light point or pixel to pass (activation of a pixel).

The pixel deactivation or extinction is then obtained when the voltage Voff at the pixel level is less than a voltage Vth characteristic of the liquid crystal used.

At present, the only known way of generating an image on a liquid crystal matrix without memory effect, consists in carrying out an activation line by line, according to which one selects a line at a time and one activates all the electrodes columns.

Thus, for a matrix of n rows and m columns, the multiplexing rate is n.

• . la fréquence de rafraîchissement de l'image, qui est liée au temps de réponse du cristal, et
• . le contraste qui diminue lorsque le taux de multiplexage croît.

However, it turns out that in the liquid crystal matrices currently used, this multiplexing rate is limited by:

• . the refresh rate of the image, which is linked to the response time of the crystal, and
• . the contrast which decreases when the multiplexing rate increases.

Or, il s'avère que, dans la pratique, la tension Von se trouve limitée par la tension d'alimentation des "drivers". 2'est la raison pour laquelle, à l'heure actuelle, le taux 3e multiplexage est limité de sorte que le nombre de lignes est lui-même limité.It has been established that for this activation method, the maximum contrast is obtained by the formula (PLESKO formula)

However, it turns out that, in practice, the Von tension is limited by the supply voltage of the "drivers". This is the reason why, at present, the 3rd multiplexing rate is limited so that the number of lines is itself limited.

Consequently, these liquid crystal matrices do not make it possible to obtain images having a sufficient definition for many applications, in particular for the production of viewfinders of the type of those previously mentioned.

The invention therefore aims to eliminate this drawback by reducing the multiplexing rate while, however, using a liquid crystal matrix having a high number of lines making it possible to obtain images having good definition.

In order to achieve this result, the method according to the invention consists in decomposing the image into a determined number of elementary images each involving all the rows and all the columns of the matrix, each of these images being composed of a set of light points whose simultaneous activations are compatible, and successively activate these elementary images during the image refresh period.

According to this process, it is demonstrated, by calculation, that the contrast is maximum for voltages Von and Voff such that:

This ratio is therefore independent of the multiplexing rate, and consequently of the number of rows n and columns m of the liquid crystal matrix.

It should be noted that there remains however a limitation due to the rate of multiplexing. However, in this case, this limitation is introduced only by the complexity of the image to be displayed and is independent of the number of rows and columns of the liquid crystal matrix.

• Pendant la visualisation de l'image élémentaire d'indice i,

According to another characteristic of the invention, the generation of the elementary images is carried out in accordance with the following sequence:

• While viewing the elementary image of index i,

a symbol generator sends the (n + m) bits corresponding to the following elementary image (i + 1) in a memory associated with the control circuits (drivers) of the liquid crystal cell. A transfer signal then causes these bits to be transferred to a voltage generation system Von, Voff.

Preferably, the matrix control circuits are mounted and adjusted so that the voltages Von and Voff applied to the electrodes constituting the rows and columns of the matrix are in proportion

, k étant le taux de multiplexage.Furthermore, the complete image being refreshed at a frequency F, preferably greater than the flicker frequency, each of the elementary images will be displayed during a period of

, k being the multiplexing rate.

The invention also relates to a device for implementing the method defined above, this device comprising at least one automaton controlled by a clock which cyclically sends to the control circuit of the liquid crystal matrix the bits representative of the elementary images contained in a random access memory with a capacity greater than k (n + m) bits, k being the multiplexing coefficient and n and m being respectively the number of rows and columns of the liquid crystal matrix, this random access memory being refreshed by a central computer during the bit transmission intervals.

• La figure 1 est une représentation schématique d'un système de visée équipant un aérodyne tel qu'un hél-i-coptère
• La figure 2 est une vue éclatée permettant d'illustrer la structure d'une matrice à cristaux liquides ;
• La figure 3 est un schéma bloc indiquant l'architecture du générateur de symboles couplé à la matrice à cristaux liquides ;
• La figure 4 représente schématiquement une matrice à cristaux liquides sur laquelle se trouve générée une image complète ;
• Les figures 5 à 13 illustrent un mode de décomposition en images élémentaires de l'image complète représentée sur la figure 4.

Embodiments of the invention will be described below, by way of non-limiting examples, with reference to the accompanying drawings in which

• Figure 1 is a schematic representation of a sighting system equipping an aerodyne such as a helicopter
• Figure 2 is an exploded view to illustrate the structure of a liquid crystal matrix;
• Figure 3 is a block diagram showing the architecture of the symbol generator coupled to the liquid crystal matrix;
• FIG. 4 schematically represents a liquid crystal matrix on which a complete image is generated;
• FIGS. 5 to 13 illustrate a mode of decomposition into elementary images of the complete image represented in FIG. 4.

In the example shown in FIG. 1, the aerodyne has only been shown the canopy 1 through which the pilot can see the external environment. For simplicity, only the pilot's field of vision 2 has been indicated from his eye 3.

In this field of vision 2 is arranged an optical mixer 4 constituted for example by a partially reflecting mirror forming part of a display device 5 such as a viewfinder and / or even a head-up flight director. This display device 5 consists of a focused and suitably cooled light source 6 which illuminates via a filter 7, for example an infrared filter, and / or a monochromatic filter, a liquid crystal matrix 8 placed in the object focal plane of an optical system successively comprising a first lens 9, a mirror 10 at 45 ° and a second lens 11 focused at right angles to the first. The optical system makes it possible to project on the partially reflecting mirror 4 the image of the liquid crystal matrix 8 which is superimposed in the field of vision 2 of the pilot, on his vision of the external environment.

The liquid crystal matrix 8 is also connected, via a digital link 121, to an onboard computer of the aerodyne 12, which is located connected to various peripheral interfaces specific to the aerodyne (13).

As previously mentioned, the liquid crystal matrix 8 consists of two parallel transparent glass plates 14,15 between which is disposed a layer of liquid crystal 16 (fig. 2).

Each of these glass plates supports a network of addressable electrodes forming the rows and columns of the matrix, which can be connected by means of flexible connectors, to an electronic control circuit.

The matrix further comprises two polarizers 17, 18 with parallel axes arranged on either side of the assembly formed by the two glass plates 14, 15 and of the liquid crystal layer 16, one of these polarizers playing the role of an analyzer.

The operating principle of this matrix which has been previously exposed, will therefore not be described again. More precisely, each of the row and column electrodes is connected to an amplifier (driver) (blocks 20) intended to supply either a Von voltage for the ignition of a pixel, or a Voff voltage for the extinction of the pixel, these amplifiers being preferably adjusted so that the Von / Voff ratio is equal to 3 (FIG. 3).

These amplifiers 20 are controlled from a buffer memory 21 (buffer), by means of the transfer control device 22 of the logic levels (bits) stored in the buffer memory 21 on the control electrodes of the amplifiers 20 (drivers).

In this device, the symbol generator consists of an automaton controlled by a clock H which attacks a sequencing counter 23 provided for cyclically sending the content of a random access memory RAM 26, of capacity greater than k (n + m) bits on the buffer memories 21 associated with the liquid crystal cell 8, (k being the multiplexing coefficient, n and m being respectively the number of rows and columns of the liquid crystal matrix) This RAM memory 26 is connected by its starters Ao ..... A15 to the BUS of the addresses of a computer 27 via a first selection circuit 28. Similarly, the inputs Ao ..... A15 of the RAM are connected to the output of the sequencer counter 23 via a second selection circuit 29.

The connection between the symbol generator and the central computer 27 also involves a command line 30 DIN (Data in) allowing the storage in the RAM 26 of the information coming from the central computer 27 (and its updating), a line 31 controlling the selection circuits 28, 29 so as to obtain addressing of the RAM 26 by the central computer 27 or by the sequencing counter 23 and a link 32 allowing the sequencing counter 23 to send a signal to the central computer 27 indicating the end of the display of an elementary image.

• . un circuit 33 reliant la sortie Dout (Data out) de la RAM 26 aux mémoires tampon 21 de ligne et de colonne des circuits de commande de la matrice à cristaux liquides 8 et ce, par l'intermédiaire d'un sélecteur ligne/colonne 34, et
• . un circuit 35 reliant le compteur séquenceur 23 aux dispositifs de transfert 22 mémoire tampon/amplificateurs 20 (drivers) du circuit de commande de la cellule à cristaux liquides 8, ce circuit permettant de transmettre un signal permettant ledit transfert.

Furthermore, the connection between the symbol generator and the control circuits of the liquid crystal matrix comprises, at least

• . a circuit 33 connecting the Dout output (Data out) of the RAM 26 to the line and column buffer memories 21 of the control circuits of the liquid crystal matrix 8 and this, by means of a row / column selector 34 , and
• . a circuit 35 connecting the sequencing counter 23 to the transfer devices 22 buffer memory / amplifiers 20 (drivers) of the control circuit of the liquid crystal cell 8, this circuit making it possible to transmit a signal allowing said transfer.

The memory 26 is refreshed by the central computer 27 (Address bus 36, DIN line 30) during the bit transmission intervals, thanks to the selection circuits 28 and 29.

• - les bits Ao à A3 correspondent à la sélection d'images élémentaires,
• - le bit A4 sert à la sélection ligne/colonne,
• - les bits A5 à A15 fournissent les numéros d'ordre des bits.

Furthermore, this RAM 26, organized by bits, is sequentially emptied by the sequencing counter 23 in the following manner, from the most significant bits:

• the bits Ao to A3 correspond to the selection of elementary images,
• - bit A4 is used for row / column selection,
• - bits A5 to A15 provide the sequence numbers of the bits.

FIG. 4 represents an example of image to be displayed on a liquid crystal matrix 8 comprising twenty rows and twenty columns. It should be noted that this limited number of rows and columns has been chosen solely for the clarity of the drawings. On this subject, it is recalled that the object of the invention, on the contrary, is the use of matrices having a much higher number of rows and columns, for example greater than 100 rows and greater than 100 columns so as to be able to obtain images presenting a relatively high definition.

The image generated on the liquid crystal matrix comprises six symbols 40, 41, 42, 43, - 44, 45 formed by an adequate distribution of lit pixels which, in this example, have a square shape.

The symbol 40 is obtained by means of eight pixels 46 to 53 dispersed around the circumference of a circle whose center is materialized by a pixel 54.

The symbol 41 of rectangular shape involves two rows of three pixels juxtaposed one on the other.

The symbol 43 has an L shape and comprises three pixels 56, 57, 58 juxtaposed on the same column and a pixel 60 on the same line as the pixel 58.

The symbol 44 has a C shape comprising three pixels 61, 62, 63 juxtaposed on the same column and two pixels 64, 65 juxtaposed with pixels 61, 63 respectively on the same lines.

The symbol 45 has a D shape of the type described above with, in addition, pixels 66 to 70, a pixel 71 arranged on the same line as the pixel 68, but offset by one pixel.

The symbol 42 comprises a pattern 72 comprising ten pixels superimposed in the same column and a pattern 73 comprising six pixels superimposed in the next column.

As previously mentioned, the decomposition of this image into independent elementary images is carried out by taking takes into account the sets of pixels whose simultaneous activations are compatible.

• - une première image élémentaire dans laquelle les pixels 46, 50 54 du premier symbole 40 qui se trouvent disposés dans une même colonne se trouvent allumés (figure 5).
• - une deuxième image élémentaire dans laquelle les pixels 48 et 52 du symbole 40 qui se trouvent disposés sur une même ligne sont allumés (figure 6).
• - une troisième image élémentaire comprenant les pixels 47, 49, 51, 53 situés à l'intersection de deux lignes et de deux colonnes (figure 7).
• - une quatrième image élémentaire reprenant en bloc le symbole 41 (figure 8).
• - une cinquième et une sixième images élémentaires comprenant respectivement les motifs 72 et 73 du symbole 42 (figures 9 et 10).
• - une sixième, une septième et une huitième images élémentaires comprenant chacune les pixels [56, 61, 64, 66, 67] [57, 62, 63, 71] [58, 60, 63, 65, 69, 70] des symboles 43, 44, 45 (figures 11, 12 et 13).

Thus, the image shown in FIG. 4 can be broken down into nine elementary images, namely:

• - A first elementary image in which the pixels 46, 50 54 of the first symbol 40 which are arranged in the same column are lit (Figure 5).
• a second elementary image in which the pixels 48 and 52 of the symbol 40 which are arranged on the same line are lit (FIG. 6).
• a third elementary image comprising the pixels 47, 49, 51, 53 located at the intersection of two rows and two columns (FIG. 7).
• - A fourth elementary image reproducing in block the symbol 41 (FIG. 8).
• - A fifth and a sixth elementary images respectively comprising the patterns 72 and 73 of the symbol 42 (Figures 9 and 10).
• - a sixth, a seventh and an eighth elementary image each comprising the pixels [56, 61, 64, 66, 67] [57, 62, 63, 71] [58, 60, 63, 65, 69, 70] of the symbols 43, 44, 45 (Figures 11, 12 and 13).

It is clear that in the example described above, in the case of line-by-line activation, the multiplexing rate should be at least 15.

On the other hand, in the context of an activation according to the elementary images previously described, the multiplexing rate is reduced to 9.

Claims (7)

1. Method for displaying symbols constituting an image, using a liquid crystal matrix, method in which the image is periodically refreshed,
characterized in that it consists in decomposing the image into a determined number of elementary images each involving all the rows and all the columns of the matrix, each of these images being composed of a set of points whose simultaneous activations are compatible, and successively activate these elementary images during the image refresh period. 2. The method of claim 1 involving a symbol generator and a memory associated with the control circuits of a liquid crystal cell include an _- ing n rows and m columns,
characterized in that, during the display of the elementary image of index (i), the symbol generator sends the (n + m) bits corresponding to the following elementary image of index (i + 1) and
in that a transfer signal then causes the transfer of these bits to a system generating a Von or Voff voltage on the corresponding row and column electrodes of the liquid crystal matrix. 3. Method according to one of the preceding claims,
characterized in that the voltages Von and Voff applied to the electrodes constituting the rows and columns of the liquid crystal matrix are in a proportion

little different from 3. 4. Method according to one of the preceding claims,
characterized in that, the complete image being refreshed at a frequency F, each of the elementary images is viewed read for a period equal to

, k being the multiplexing rate. 5. Device for implementing the method according to one of the preceding claims,
characterized in that it comprises an automaton controlled by a clock (H) which cyclically sends to the control circuit of the liquid crystal matrix (8), the bits representative of the elementary images contained in a random access memory (26) of capacity greater than k (n + m) bits, k being the multiplexing coefficient and n and m being respectively the number of rows and columns of the liquid crystal matrix (8). 6. Device according to claim 5
characterized in that the random access memory is a random access memory refreshed by a central computer during the bit transmission intervals. 7. Device according to one of claims 5 and 6, characterized in that the matrix (8), illuminated by a light source (6) is arranged in the object image plane of an optical system designed to project the image produced by said matrix (8) on an optical mixer (4) intended to be placed in the field of vision (2) of an operator.

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