Secure display device with twisted nematic liquid crystal matrix
The field of the invention is that of cockpit displays in which the
display devices comprise a liquid crystal matrix, also called "LCD", standing
for "Liquid Crystal Display". These display screens are nowadays fitted in all
types of fixed or revolving wing aircraft in both civilian and military domains.
5 An LCD screen operates on the principle of matrix addressing. As
can be seen in Figure 1, the LCD screen is made up of coloured individual
pixels organized in a matrix. Six pixels P are represented in Figure 1 with
different shades of grey representing the colours of the pixels. Each pixel P is
made up of a thin thickness of liquid crystal contained between two j
10 electrodes, a first so-called control electrode and a second, back electrode B, !
called "backplane", common to all the matrix and symbolized by ellipses in Figure 1, The potential difference or ddp between the electrodes is controlled by a so-called "TFT" (thin film transistor) transistor. This potential difference acts on the orientation of the liquid crystal molecules and therefore on their
15 light transmission. Each transistor TFT is located at the intersection of a row
L and of a control column C. The gate G of each transistor TFT is linked to a
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particular row and the drain D to a particular column. I
A video image generator drives, through electronic circuits called
drivers, the transistors TFT located at the intersection of the different rows
20 and of the different columns according to the image to be displayed. The
drivers are organized as "row drivers" and as "column drivers". ,
The column drivers receive the video information to be displayed j
which is validated by the row drivers. The column drivers convert the digital j
information from the video generator into analogue voltages proportional to
25 the voltage references called "GMA", standing for GamMA voltage. These voltages are applied to the drains D of the transistors TFT.
The row drivers apply to the gates G of the TFTs of one and the same row a line switch-on voltage "VG_on" in order to apply to the liquid crystal the analogue voltages supplied by the column drivers, the other rows
30 being kept switched off by the row switch-off voltage "VG_off". Once the row is written, the column drivers present the new analogue voltages and the row drivers switch on the next row, and so on for all the rows of the matrix screen.

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The LCD screens used are generally based on the so-called "twist nematic" (TN) technology because it offers the advantage of offering more optical transmission than the other technologies.
Figures 2a and 2b represent, as a function of the time t, the 5 voltages applied in volts in two configurations of use. In the first, so-called "BLACK LCD", configuration, the liquid crystal is opaque; in the second, so-called "WHITE LCD", it is transparent. The "backplane" voltage represented by a dotted line in Figures 2a and 2b is continuous and set at around 6 volts. In both cases, in order not to create a marking, the liquid crystal receives an
10 alternately positive then negative DDP corresponding to the "positive phases"
and "negative phases" of Figures 2a and 2b. j
In the first configuration represented in Figure 2a, the liquid crystal I
is opaque if the DDP is greater than 4 volts. The column voltages then vary
between 0 and 12 volts, that is to say by ± 6 volts around the 6 volt |
15 backplane voltage.
In the second configuration, represented in Figure 2b, the liquid crystal is transparent if the DDP is virtually zero or less than 1 volt. The column voltages then vary between 5 and 7 volts, that is to say ± 1 volt around the 6 volt backplane voltage.
20 The drawback of this technology is that it naturally presents a
white screen that is said to be "normally white" when the latter is defective or j
uncontrolled. This white screen with strong brightness compared to a traditional display with dark background of the "PFD" (primary flight display) can be a nuisance to the pilot.
25 Figure 3 represents a set of two dual displays denoted LCD1 and
LCD2. The expression "dual display" should be understood to mean a display comprising a single LCD matrix arranged in such a way as to display two totally independent or segregated images. More specifically, a dual display comprises two independent power supplies, two independent light boxes, two
30 independent graphic generations and two independent sets of row and column drivers. Thus, a first failure of any kind does not result in the total loss of the display device. In Figure 3, the left hand display LCD1 is perfectly functional and presents two displays denoted D1G and Dip. The left hand part D2G of the right hand display LCD2 has failed and displays a white
35 screen.. Only the right hand part D2D is operational.

So as to restore a black screen, the failure of this type of screen
results either in the automatic switching off of the light box which lights the
matrix so as to restore a black screen, or the manual switching off of the J
equipment by the pilot. This switching off results in the complete loss of the |
5 equipment. This outcome is highly detrimental in the case of a dual display !
presenting two different and independent display types because it causes the loss of the second display, thus disrupting the screen reconfigurations
needed to satisfy the aircraft certification and safety constraints. j
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10 The device according to the invention does not present these !
drawbacks. It makes it possible to force a black display on an LCD screen of twisted nematic type even if the latter is no longer controlled after, for example, a failure of the row drivers or of the column drivers. In the case of use of a dual screen, the device according to the invention also makes it
15 possible to keep the second display operational without the pilot being hampered by the failed display that has become white. Thus, neither the system nor the pilot has to be concerned with the failure of the LCD. The device according to the invention therefore has a dual action: to not present an erroneous image and to avoid hampering the pilot.
20 More specifically, the subject of the invention is a display device
comprising an active liquid crystal matrix of twisted nematic type,
said matrix comprising a first set of pixels, a first set of rows and of j
columns and a first electronic control device;
each pixel being controlled by a transistor comprising a source, a
25 drain and a gate, the gate being linked to a..row, the drain to a column and
the source to the control electrode of the pixel; |
the voltage applied to each control row being either a switch-on voltage VG_on sufficient to switch on the transistors, or a switch-off voltage VG_off sufficient to switch off the transistors;
30 the GMA voltage applied to each column being dependent on the
predetermined optical transmission of the pixel;
the display device comprising first means for detecting the correct operation of the first electronic control device,
characterized in that, in case of detection of incorrect operation of
35 the first electronic control device, the display device comprises first cut-off

means arranged to force to zero the switch-off voltages VG_off of the rows and the GMA voltages of the columns, so that the transmission of all the pixels of the first set of pixels is minimum.
Advantageously, the device is dual, that is to say that the liquid
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5 crystal matrix comprises:
a second set of pixels, a second set of rows and of columns, a
second electronic control device, second detection means and second cut-off
means independent of the first set of pixels, of rows, of columns, of the first
electronic control device and of the first cut-off means.
10 Advantageously, the cut-off means are transistors arranged
between the power supply circuits of the rows and of the columns and the electrical ground, said transistors being controlled by the first or the second correct operation detection means.
Advantageously, the correct operation detection means comprise:
15 either means for detecting fixed images, an image corresponding
to all the GMA voltages applied to the different pixels during an individual display time;
or means for checking the current consumption of the rows and/or
of the control columns. j
20
The invention will be better understood and other advantages will become apparent on reading the following description given as a nonlimiting example and using the appended figures in which:
Figure 1, already discussed, represents an arrangement of pixels
25 of an active LCD matrix; .. _.._
Figures 2a and 2b, also discussed, represent the variation of ddp in volts applied to a pixel to make it either transparent or opaque;
Figure 3, previously discussed, represents a cockpit display system according to the prior art comprising two dual display devices, in 30 which one of the displays has failed;
Figure 4 represents an electrical schematic diagram of the row voltage cut-off means according to the invention;
Figure 5 represents an electrical schematic diagram of the column voltage cut-off means according to the invention;

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Figure 6 represents a cockpit display system according to the invention comprising two dual display devices in which one of the displays has failed.
5 Hereinafter, the same terms as previously have been adopted to
designate the main elements of a display device.
The display device according to the invention conventionally comprises an active liquid crystal matrix of twisted nematic type comprising:
- a first set of pixels, a first set of rows and of columns, a first
10 electronic control device and first means for monitoring or
detecting the correct operation of the first electronic control device;
- each pixel being controlled by a transistor comprising a source,
a drain and a gate, the gate being linked to a row, the drain to
15 a column and the source to the control electrode of the pixel;
- the voltage applied to each control row being either a switch-on
voltage VG_on sufficient to switch on the transistors, or a switch-off voltage VG_off sufficient to switch off the transistors;
- the GMA voltage applied to each column being dependent on
20 the predetermined optical transmission of the pixel.
The display device also comprises electronics for monitoring the correct operation of the screen.
The display device according to the invention also comprises first
cut-off means. The aim of these cut-off means is to equip the display device
25 so that it is possible to obtain a black screen regardless of the control of the
video generator and regardless of the state of the screen, whether functional
or failed.
To perform this function, two actions are needed. Firstly, all the
rows that are switched off must be switched on by forcing the switch-off
30 voltage VG_off to zero volt. Thus, whatever the state of the row drivers, all
the transistors TFT will leak and allow the analogue voltages presented by
the column drivers to pass.
A second action is needed on the column drivers in order to apply,
regardless of the video control, an analogue voltage that makes it possible to
35 display black. This is done by forcing all the GMAs voltages to zero volt.

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Thus, the ddp seen by the liquid crystal is directly linked to the voltage
applied to the backplane.
These actions are controlled by the electronics monitoring the
correct operation of the screen. There are various technical principles that 5 allow for such monitoring. A first example that can be cited are fixed image
detection means, an image corresponding to all the GMA voltages applied to
the different pixels during an individual display time. The fixed images are
considered to be critical in avionics systems in as much as they can pass
unseen by the crew for a certain time.
10 A second example that can also be cited are means for checking
the electrical current consumption of the rows and/or of the control columns. The result of this monitoring will on the one hand control the
forcing to zero of the voltage VG__off and on the other hand collapse the
GMAs voltages to force the screen to black.
15 It is also possible to force the voltage VG_on to zero so as not to
see the row which is in the process of being written at the time of the failure. As a first example, Figure 4 represents the cut-off means to be
implemented to carry out the function for resetting to zero the voltages
VG_on and VG_off. These are generally obtained using a voltage "boost" 20 converter as can be seen in Figure 4. This "boost" converter simply has to be
blocked in order to force these two voltages to zero volt. This is done by
means of three transistors TFTc, the first arranged at the input of the
converter which isolates the converter from its power supply voltage VREF, the
other two on the two output channels VG_on and VG_off of the converter
;::.;. 25 which are connected to the ground, which three transistors are controlled by
the monitoring electronics EdS of the display device.
As a second example, Figure 5 represents the cut-off means to be
implemented to perform the function for resetting to zero the GMAs voltages.
These are produced by a resistor bridge R. The power supply voltage for this 30 bridge is cut off by means of two transistors TFTc which isolate the resistor
bridge from its power supply voltage VREF' and which connects the resistor
bridge R to the ground. As previously, these two transistors are controlled by
the monitoring electronics EdS of the display device.

These simple arrangements make it possible, by slightly modifying
the basic electronics of an LCD screen, to be able to force a "normally white"
TN LCD to display black and to do so regardless of the operational state of
said screen.
5 This device still offers the advantage of never presenting an
incorrect image to the pilot.
In the case of a single display, it makes it possible to minimize the visual nuisance to the pilot in as much as the display screen, even when failed, does not cause glare.
10 In the case of a dual display, the preceding cut-off means are
duplicated for each display area. The system is no longer required to cut off all the equipment when just one of the displays is effected, as can be seen in Figure 5 which uses the same notations as Figure 3. The still-functional half-screen D2D of the display LCD2 is maintained, thus making it possible to
15 retain the reconfiguration capabilities as demanded by the aeronautical safety requirements.
Thus, in the display system of Figure 3, the failure of the display of the half-screen D2Q resulted both in the appearance of the white screen and the loss of the so-called "PFD" information display, information that is
20 essential for piloting. In the case of the display system of Figure 6, the failure of the display of the half-screen D2G results in both the appearance of the much more ergonomic black screen and the reconfiguration of the so-called "PFD" information display on the still-functional part D2Q of the display LCD2.

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CLAIMS
1. Display device comprising an active liquid crystal matrix of twisted nematic type,
said matrix comprising a first set of pixels (P), a first set of rows (L)
and of columns (C) and a first electronic control device,
5 each pixel being controlled by a transistor (TFT) comprising a
source, a drain (D) and a gate (G), the gate being linked to a row, the drain to a column and the source to the control electrode of the pixel,
the voltage applied to each control row being either a switch-on voltage VG_on sufficient to switch on the transistors, or a switch-off voltage 10 VG_off sufficient to switch off the transistors,
the voltage applied (GMA) to each column being dependent on the predetermined optical transmission of the pixel,
the display device comprising first means (EdS) for detecting the
correct operation of the first electronic control device,
15 characterized in that, in case of detection of incorrect operation of
the first electronic control device, the display device comprises first cut-off
means (TFTc) arranged so as to force to zero the switch-off voltages VG_off
of the rows and the GMA voltages of the columns, so that the transmission of
all the pixels of the first set is minimum.
20 2. Display device according to Claim 1, characterized in that the
device is dual, that is to say that the liquid crystal matrix comprises:
a second set of pixels, a second set of rows and of columns, a second electronic control device, second detection means and second cut-off means independent of the first set of pixels, of rows, of columns, of the first 25 electronic control device and of the first cut-off means.
3. Display device according to one of Claims 1 or 2, characterized
in that the cut-off means are transistors (TFTC) arranged between the power
supply circuits of the rows and of the columns and the electrical ground, said
transistors being controlled by the first and/or the second correct operation
30 detection means.
4. Display device according to one of the preceding claims,
characterized in that the correct operation detection means comprise means

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for detecting fixed images, an image corresponding to all the voltages (GMA) applied to the different pixels during an individual display time,
5. Display device according to one of the preceding claims, characterized in that the correct operation detection means comprise means 5. for checking current consumption of the rows and/or control columns.