Liquid crystal cells – elements and systems – Particular structure – Detector of liquid crystal temperature
Reexamination Certificate
2000-05-02
2001-11-13
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Detector of liquid crystal temperature
C349S033000, C349S074000, C349S147000
Reexamination Certificate
active
06317178
ABSTRACT:
FIELD AND BACKGROUND OF THE INVENTION
The invention relates to a display device, in particular for a vehicle, with a viewing screen having a liquid crystal cell, the liquid crystal cell having a front cell wall and a rear cell wall and a liquid crystal substance arranged in the cell space between these cell walls, and with a heating device by means of which the display device can be heated.
In liquid crystal cells, which are operated at least temporarily at low ambient temperatures, as is the case, for example, in displays in vehicles, the problem arises that at low temperatures the switching times of the liquid crystal cells become undesirably long, with the result that displays fitted therewith require a substantial inertia. It is known for the purpose of remedying this defect to provide a heating wire for a light box serving to illuminate a liquid crystal cell in a liquid crystal display. The liquid crystal cell is heated by means of the heating wire and held at a temperature level which renders it possible despite low ambient temperatures for the liquid crystal display to be operated with the required short switching times. Because of the relatively large spacing between the heating wire and liquid crystal cell, it is necessary for the heating wire to be supplied with a large amount of energy and to be strongly heated in order to be able to produce the required heating of the liquid crystal cell. Moreover, the heating wire undesirably heats the entire display unit because of its arrangement in the light box and of its necessarily high level of thermal radiation.
SUMMARY OF THE INVENTION
It is the object of the invention to configure a display device with a liquid crystal cell such that the display unit can be operated reliably and with only short switching times even in the case of low ambient temperatures.
This object is achieved according to the invention by virtue of the fact that the heating device is arranged in the cell space of the liquid crystal cell directly or indirectly on the front cell wall and/or the rear cell wall.
This design produces a direct and very quick heating of the liquid crystal substance without giving rise to undesired energy losses when relaying heat. The heat output generated by the heating device can therefore be particularly low. Undesired heating up of parts of the display device which are not to be heated is excluded. Moreover, the heating device can be fabricated simply along with the production of the liquid crystal cell without the need for additional assembly measures.
The heating-up and switching times of the display device are particularly short because of the direct assignment of the heating device to the liquid crystal substance and of the absence of a thermally insulating layer. Consequently, the display device according to the invention is also particularly suitable as a tachometer display in a motor vehicle. In a tachometer, short response times and an immediate ability to operate are particularly important, since the display of the traveling speed is of great significance for traffic safety.
The display unit is particularly simple and cost-effective in design when the liquid crystal cell is preferably a TN cell or an STN cell.
The production costs of the display unit can advantageously be lowered when the heating device has an electrically conducting layer. When current is applied, this layer can then function as a resistance heater. It is conceivable to arrange the electrically conducting layer immediately adjacent to the liquid crystal substance. However, it is particularly advantageous for the simple connection of the liquid crystals with as little outlay as possible in energy when the electrically conducting layer is arranged between a front electrode and/or a rear electrode of the liquid crystal cell and the front cell wall and/or rear cell wall.
The production of the liquid crystal cell is simplified still further when, in accordance with another advantageous development of the invention, the electrically conducting layer is arranged in a plane with a front electrode and/or a rear electrode of the liquid crystal cell. It is possible in this way for the electrically conducting layer to be applied in one operation with an electrode of the liquid crystal cell.
It is conceivable to configure the electrically conducting layer with a large area; however, the heating effect is particularly good when the electrically conducting layer extends in a meandering fashion. It is also conceivable to arrange the electrically conducting layer only in the regions in which switchable display elements are present. In this way, the use of electric energy required for heating can be further reduced owing to the avoidance of heating regions of the liquid crystal cell which are not switched and therefore also are not subject to any negative influence from low ambient temperatures.
Particularly in the case of point matrix displays having a liquid crystal cell, it is customary for the liquid crystal cell to have a front electrode and a rear electrode with in each case a set of mutually parallel electrode strips, the electrode strips of the front electrode and the rear electrode being arranged crosswise relative to one another. In order to achieve a good heating effect which covers an area in the case of such liquid crystal cells, it is particularly advantageous when the electrode, in the plane of which the electrically conducting layer is arranged, has the shape of a strip, and when elongated segments of the electrically conducting layer are arranged in interspaces of the strips.
In order to achieve a connection of the elongated segments of the electrically conducting layer which does not disturb the connection between the strip-shaped electrodes, plated-through holes connecting the plane of the front electrode and the plane of the rear electrode preferably electrically interconnect the elongated segments of the electrically conducting layer by means of connecting segments.
The electrodes of liquid crystal cells usually consist of indium tin oxide. The production of a display device according to the invention is therefore substantially simplified when the electrically conducting layer has indium tin oxide, with the result that there is no need for any additional material to be handled and processed in producing the liquid crystal cell.
In accordance with another advantageous development of the invention, the functional reliability of the display device is enhanced by virtue of the fact that the electrically conducting layer is covered with an electric insulating layer. The insulating layer advantageously has glass. This can, for example, be applied as glass powder and sintered on or sealed in.
In order to avoid damage to the liquid crystals of the liquid crystal substance, in accordance with an advantageous development of the invention, an AC voltage can be applied to the electrically conducting layer.
It is particularly advantageous, for transillumination of the viewing screen in particular, that the electrically conducting layer be optically transparent. In this way, light emitted by a light source serving the purpose of illumination is used in very large measure without there being appreciable light losses or nonuniform illumination of the viewing screen, as would be the case with the use of, for example, heating wires, which lead to the formation of shadows.
When, in accordance with an advantageous development of the invention, the front cell wall and/or the rear cell wall is a glass plate, it is rendered possible both for the viewing screen to be transilluminated (when both cell walls are glass plates) and for the liquid crystal cell to be produced simply and cost-effectively. Liquid crystal viewing screens can also be illuminated by ambient light and/or floodlighting falling on to it from the front side. This requires a reflector, which is arranged in the rear region or behind the liquid crystal cell, to deflect the incident light and render the driven regions of the liquid crystal cell visible to a viewer. It is therefore particularly advant
Brandt Peter
Tippl Dietmar
Chowdhury Tarifur R.
Farber Martin A.
Mannesmann VDO AG
Sikes William L.
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