Heated windscreen

Abstract

A heated laminated windscreen including two sheets of glass joined by an inserted sheet, and including a system of conductive layers covering most of a surface of one glass sheet of the windscreen. The system is electrically powered by busbars at a top and bottom of the windscreen, with windows which are devoid of layers arranged at the top in the middle of the windscreen. The windscreen further including at least one additional busbar arranged laterally on edges and at the top of the windscreen.

Claims

1. A heated laminated windshield formed of two glass sheets joined by an interlayer sheet, comprising: a conductive system of layers covering most of a surface of one glass sheet of the windshield, which system is electrically powered by busbars in a top portion and a bottom portion of the windshield, windows without layers being positioned in the top portion in a middle of the windshield; at least one additional busbar positioned laterally on an edge and at the top of the windshield, an additional power supply positioned in a form of a power supply busbar located at least beneath widest windows in an area masked by an enamelled coating, wherein the power supply busbar is substantially level with lower ends of the at least one additional busbar.

2. The windshield according to claim 1, comprising two symmetrical additional busbars.

3. The windshield according to claim 1, wherein the one or more busbars extend over edges in a direction of the bottom of the windshield, their lower end being, at most, level with a lower portion of a lowest window.

4. The windshield according to claim 1, wherein the power supply busbar is substantially at a same potential as the at least one additional busbar.

5. The windshield according to claim 1, wherein an end of the at least one additional busbar is, at a lowest point, at a boundary of a viewing area.

6. The windshield according to claim 1, wherein one of the busbars is positioned along an upper edge and is formed of plural separate elements.

7. The windshield according to claim 6, wherein the busbars are formed of metal strips.

8. The windshield according to claim 1, wherein the busbars are formed by conductive pastes printed by screenprinting.

9. A heated laminated windshield, comprising: two glass sheets; an interlayer sheet joining the two glass sheets; a conductive system of layers on a surface of one of the glass sheets of the windshield, which system is electrically powered by busbars in a top portion and a bottom portion of the windshield; windows without the conductive system of layers positioned in the top portion of the windshield; at least one additional busbar positioned laterally on an edge and at the top of the windshield; and a power supply busbar located in an area masked by an enamelled coating, wherein the power supply busbar is substantially level with lower ends of the at least one additional busbar.

10. The windshield according to claim 9, comprising two symmetrical additional busbars.

11. The windshield according to claim 9, wherein the one or more busbars extend over edges in a direction of the bottom of the windshield, and lower ends of the one or more busbars extending, at most, level with a lower portion of a lowest window.

12. The windshield according to claim 9, wherein the power supply busbar is substantially at a same potential as the at least one additional busbar.

13. The windshield according to claim 9, wherein an end of the at least one additional busbar is, at a lowest point, at a boundary of a viewing area.

14. The windshield according to claim 9, wherein one of the busbars is positioned along an upper edge and is foHned of plural separate elements.

15. The windshield according to claim 14, wherein the busbars are formed of metal strips.

16. The windshield according to claim 9, wherein the busbars are formed by conductive pastes printed by screenprinting.

Description

(1) The invention is described in detail by referring to the pages of drawings, in which:

(2) FIG. 1 schematically represents, as a front view, the structure of a conventional heated windshield;

(3) FIG. 2 is a representation analogous to FIG. 1 illustrating the prior arrangements that tend to make the heating of the windshield uniform;

(4) FIG. 3 represents a windshield comprising elements analogous to those from FIG. 2, but incorporating arrangements according to the invention;

(5) FIG. 4 illustrates the distribution of temperature increase over the windshield from FIG. 2 after a heating time;

(6) FIG. 5 is analogous to the previous figure for the windshield from FIG. 3;

(7) FIG. 6 is a graph representing the average temperature increase for the areas A and B;

(8) FIG. 7 is a graph analogous to the preceding graph for the temperature increase in the center of the glazing;

(9) FIG. 8 is a presentation of another embodiment of the invention analogous to FIG. 2 or 3;

(10) FIG. 9 represents, as in FIGS. 4 and 5, the distribution of temperatures for the embodiment from FIG. 8;

(11) FIG. 10 is a presentation of another embodiment of the invention;

(12) FIG. 11 represents the distribution of temperatures corresponding to the embodiment from FIG. 10.

(13) The representation of the windshield from FIG. 1 is limited to the elements necessary for the description of the invention.

(14) On the windshields, the heating layer is as described in numerous prior publications. It is a question of achieving the best results, namely the smallest possible sheet resistance of sets of metal layers protected by dielectric layers. The best performing systems comprise two, three or even four silver layers. Under the best conditions, the heating layers achieve sheet resistances of the order of 1 / or less. Despite these very small resistances, the actual dimensions of the windshields often exceeding one meter in height do not make it possible to obtain the power required in order to meet the needs of the manufacturers. This power is of the order of 400 W/m.sup.2, using the potentials available on private vehicles (12-14 V).

(15) FIG. 1 shows the conventional arrangement of the heating layer which extends over virtually the whole of the surface of the glazing. The boundary of the layer is marked by the line 1. Only the edges of the glazing are not in contact with the conductive layer in order to avoid possible impairments by contact with ambient moisture.

(16) The conductive layer is also interrupted at the location of various devices conventionally present. This is the case for example for that which is customarily denoted by the name telecommunication window 2, 3, of electronic toll collection type, or windows for driving assist cameras, especially night driving assist cameras, 4. The windows in question are made in order to let through waves, especially infrared waves, which do not cross, or are excessively attenuated by, the conductive layers incorporated in the heating system of layers.

(17) Other areas of the windshield may also be free of layers, such as the locations of rain sensors 5 when they also operate by infrared radiation. Generally, the heating systems of layers blocking a substantial portion of the infrared transmission, any instrument requiring the transmission in question, the area in which this instrument operates is free of the system of layers.

(18) In order to power the system of layers, the windshield comprises busbars 6, 7 formed of conductors that are weak enough to conserve as much as possible the power available for the elements directly of use in heating the windshield. These busbars are conventionally either metal strips or strips of conductive enamelled pastes.

(19) The main busbars are positioned on the top and bottom edges of the sheet 1. This arrangement is chosen so as to limit the distance that separates them in order to reduce the resistance between these busbars and increase the available power per unit of surface area for a limited available potential difference.

(20) The busbars 6 and 7 are connected by means of connectors (not shown) to the electric power supply.

(21) The busbar 7 in the bottom portion is often at a distance from the lower edge of the glazing in order to make, according to modes described previously, a particular heating area for the resting of the windshield wipers. These arrangements are not represented in the interests of clarity.

(22) The windshields ordinarily comprise enamelled portions intended to mask all of the busbars and the beads of adhesive for attaching the windshield to the body. The enamelled masking areas are positioned at position 2 on the outer sheet according to the conventional designation of the faces of the glass sheets of a laminated assembly.

(23) The enamelled area extends beyond the edges at the locations that receive the supports for the interior rearview mirror and other devices such as a camera, which supports are often adhesively bonded to the windshield.

(24) The presence of the various areas without a heating layer very significantly modifies the distribution of the current lines around these areas and in the continuation thereof in the direction of the busbar 7 located at the bottom of the windshield. Consequently, the heating cannot be uniformly provided in the vicinity of these areas. In order to to minimize this lack of uniformity, according to the arrangements described previously, an additional busbar 8 is positioned in the area masked by the enamel and beneath the main layer-free areas. The busbar 8 is electrically coupled to the busbar 6. This coupling may in addition adapt the potential of the busbar 8 so that it is more or less that which the layer would have at this level, in the absence of these windows. In other words, it is endeavored to re-establish the same potential over the entire width of the sheet for a certain uniformity of the current in the direction of the bottom of the windshield. In order to obtain this result, the prior art proposes to connect the busbars 6 and 8 via a conductor having substantially the same resistance as the layer between these same busbars.

(25) Represented schematically in FIG. 1 by dotted lines are the viewing areas that are distinguished following the regulations. These are respectively the area A, the one that is the most directly affected, for the vision of the driver. The area B is bigger than the preceding area, which it encompasses entirely. This area covers practically all the portions of the glazing that are not masked. The remainder of the surface corresponds to the area C.

(26) As indicated above, the objective of the invention is to favor a differentiated heating. The priority is to obtain the heating of the area A as quickly as possible.

(27) By way of example of implementation of the invention, a comparison is made between the operation of a windshield of conventional type represented in FIG. 2 and of a windshield according to the invention in FIG. 3.

(28) In FIG. 2, the potential of the various busbars has been indicated. The busbar 7 is grounded (0 V). The busbar 6 of the upper edge is at 14 V. The additional busbar 8 is at around 11 V.

(29) The same windshield is equipped in the manner represented in FIG. 3. In this figure, the upper busbar 6 is extended over the sides by two portions 9 and 10 which are at the same potential of 14 V. Likewise, the busbar 8 is at 14 V tending to reproduce, in a certain manner, an equipotential area level with the ends of 9 and 10 on the one hand and of the busbar 8 on the other hand.

(30) The two windshields are compared under their heating conditions.

(31) For the comparative test, the heating system of layers used is that described in Belgian patent application no. 2011/0218 filed on Apr. 12, 2011. It is an assembly comprising several thin layers of silver with dielectric layers that protect these metal layers. The resistance R/ of the layer is 0.786 /.

(32) The test windshield is composed of two glass sheets having a thickness of 2.1 mm for the outer glass and 1.6 mm for the inner glass, and of a PVB sheet having a thickness of 0.76 mm. The heating layer is at position 3 in the laminate.

(33) The temperatures are measured at the surface on the outside of the glazing. The initial temperature is 20 C. The external and internal convection provided by the air dissipates a power of 10 W/m.sup.2K.

(34) In the test, the result of which is represented in FIGS. 4 and 5, the temperature variation with respect to the initial temperature is measured over the entire surface of the glazing. This measurement is made after heating for 8 minutes. The temperatures are represented by the degree of gray. The temperature scale is attached to these figures.

(35) The test shows firstly that the area A is lighter for the glazing according to the invention. The temperature difference is around 5 C. higher for the glazing according to the invention. In the same way, generally a temperature increase is observed for the whole of areas A and B.

(36) FIGS. 6 and 7 establish the change in both cases in the temperature difference over time, on the one hand for the whole of areas A and B, and on the other hand at the center of the windshield. In all cases, the result according to the invention leads to a greater temperature increase.

(37) Additionally, the comparison of FIGS. 4 and 5 shows a lower heating in the case of the invention in the upper portion of the glazing, which is explained without difficulty due to the lesser role of the current lines in this portion, to the advantage of the lines coming from the side busbars 9 and 10.

(38) The structure according to the invention also introduces a modification regarding the location of the hot spots. Unsurprisingly, the end of the side busbars 9 and 10 is the site of hot spots that do not exist in the comparative example. The area located directly beneath the additional busbar 8 is also hotter, and the temperature increase continues beyond this area in order to also improve the temperature at the center of the glazing.

(39) Conversely, the entire portion comprising the windows is substantially colder than in the comparative example. Since the vision in this area is practically zero, a lower temperature has no effect on the expected behavior.

(40) The arrangement of FIG. 8 differs from that of FIG. 3. The side busbar elements 11 and 12 are not directly in the continuity of the busbar 6 but are connected by conductive wires to a power supply, the potential of which is no longer that of the busbar 6. In the height of the windshield, since the ends of the side elements are located lower than the busbar 8, a lower voltage of 10 V is applied. FIG. 9 shows, like for FIG. 5, that the arrangement proposed is substantially more effective than in the comparison mode from FIG. 4.

(41) In the windshield of FIG. 10, all the busbars are again at the same potential of 14 V. In this composition, the busbar bordering the upper edge of the windshield is divided into two portions 13, 14 which do not extend to the side edges. The elements located on the sides have their lower ends more or less level with the busbar 8.

(42) In FIG. 9, it is once again observed that the viewing areas A and even B are better heated than in the comparative FIG. 4. Conversely, the arrangement results in a more limited heating of the upper corners and of the area located above the busbar 8.