Window equipped with electrostatic protection circuit

A laminated window includes at least one rigid external substrate (5) and a sheet made of flexible plastic material (6) which is adjacent to the substrate (5). The window is equipped with an electrostatic protection circuit which is capable of dissipating the charges which have accumulated on the external sheet (7). This circuit includes conducting elements (2) which cross through at least the substrate and at least one part of the sheet made of plastic material (6). The conducting elements (2) are connected to a collector element (3) which is connected to the ground (4) and embedded in the sheet made of flexible plastic material (6).

SUMMARY OF THE INVENTION 
1. Field of the Invention 
This invention pertains to a laminated window equipped with an 
electrostatic protection circuit to dissipate electrostatic charges which 
are capable of accumulating on one side of the window, in particular a 
laminated window which may be used in the aviation industry and which is 
composed of a sheet of rigid plastic material such as methyl 
polymethacrylate (PMMA) and an external sheet. The invention also pertains 
to the method for fabricating such a window. 
2. Description of the Related Art 
Such windows, especially those used in the aviation industry, can be 
subjected in flight to an accumulation of electrostatic charges on the 
external sheet of the window. These charges can reach a potential which is 
capable of creating an electric discharge along the side of the window 
and/or a discharge within the body of the window, especially when the 
window is equipped with an electric heating circuit. This discharge can 
locally modify the properties of the window, that is, it can create holes 
and/or micro-carbonization, which deteriorates its optical properties. 
It is known from U.S. Pat. No. 4,078,107 that the external side of the 
window can be covered with a conducting coating in order to dissipate the 
electrostatic charges which are present on the external sheet. A surface 
discharge can thereby be avoided, but a discharge within the body of the 
window can be produced. In addition, this coating can be at least 
partially removed by, among other phenomena, abrasion caused by movement 
of the windshield wipers and by operations such as cleaning, polishing, 
etc., which reduces the efficiency of the coating. 
It is known from U.S. Pat. No. 4,590,535 that conducting elements which 
cross through the thickness of the external rigid sheet, are connected 
electrically to other conducting elements located on the inner side of the 
external sheet and are connected to a ground, can also be provided. 
Several independent circuits which are capable of draining off the 
electrostatic charges should be provided for reasons of safety. The time 
and cost of fabrication of such a window are increased by these measures. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a window equipped with 
an electrostatic protection circuit which prevents the formation of both 
surface discharges and discharges within the body of the window, this 
circuit not being impaired by any abrasion to which the window is 
subjected. 
It is a further object of the present invention to provide a window 
equipped with such a circuit, whose fabrication is relatively simple. 
It is yet a further object of the present invention to provide a window 
equipped with a single electrostatic protection circuit, without the 
latter impairing the reliability of the window. 
The above and other objects are achieved according to the present invention 
by a laminated window which includes at least one external rigid substrate 
and a sheet made of flexible plastic material, adjacent to the substrate, 
the latter including an external sheet which is capable of storing 
electrostatic charges, said window being equipped with an electrostatic 
protection circuit which is capable of dissipating the charges which 
accumulate on the external sheet of the substrate. This circuit includes 
conducting elements which pass through the substrate and pass through at 
least one part of the sheet made of flexible plastic material, these 
conducting elements being connected to a collector element which is in 
turn connected to the ground embedded in the plastic material. 
The fact that the collector element is embedded in a sheet of flexible 
plastic material and is not in contact with the external rigid substrate 
allows one to improve, according to the inventors, its mechanical and 
thermal resistance. The sheet of flexible plastic material absorbs, to a 
certain degree, the possible expansions and/or contractions which the 
external substrate might be subjected to, the latter being caused, in 
particular, by the great temperature differences to which a window in 
flight is subjected. According to one mode of implementation of the 
invention, the collector element is located in the sheet made of flexible 
plastic material in a plane different from that of the interface between 
the rigid external substrate, and said sheet made of flexible plastic 
material. 
By "external rigid substrate" is meant a monolithic substrate or laminated 
sheet of plastic material. By sheet of flexible plastic material we mean, 
according to the invention, one or several adjacent sheets, which are 
formed by one or several folds (or layers), of flexible polyurethane or, 
preferably, polyvinylbutyral. 
By "embedded" is meant that the collector element is completely embedded 
between two layers or two folds of the flexible plastic material or that 
the collector element is encrusted or partially encrusted in the side of 
the flexible plastic material layer opposite the substrate. 
In order not to harm the optical quality of the window the conducting 
elements are perpendicular to the plane of the window, the collector 
element being parallel to this plane. The collector element describes one 
or several lines connected to one another, parallel to the plane of the 
window. The spacing between the different conductor elements must be such 
that there is no danger of the occurrence of a short circuit. This 
distance is, for example, between 3 and 15 cm. 
Advantageously, the conductor elements consist only of a single wire or 
conductor filament. The filament can, possibly, form a loop near the side 
on which electrostatic charges might accumulate in order to strengthen, to 
some degree, this end which is subjected to external loads. 
According to a preferred variant of the invention the collector element and 
the conductor elements consist of the same wire or conductor filament, 
each conductor element forming, in addition, a loop. The fabrication of 
such a circuit is then particularly easy to implement. 
A single protection circuit according to the invention is generally 
sufficient to ensure the desired protection with complete safety. 
Nevertheless, the window can also be equipped with several electrostatic 
protection circuits for some applications. 
According to another feature of the invention, the window also includes a 
heating circuit in order to eliminate frost, snow and/or condensation 
which can form on the window. This circuit includes, in the traditional 
manner, heating elements located in a plane parallel to the window, such 
as thin electrical resistance wires or a conducting layer, connected 
electrically to at least one collecting strip connected to the ground. In 
order to improve efficiency of the heating circuit, the heating elements 
are, normally, located near the internal side of the substrate. 
Each conductor element of the electrostatic protection circuit in 
accordance with the invention crosses through the heating circuit. These 
two circuits, the electrostatic protection circuit and the heating 
circuit, are nevertheless independent. However, it is necessary to avoid 
creation of a short circuit which can be produced between these two 
relatively nearby circuits. According to a variant, each conductor element 
of the anti-static circuit in accordance with the invention will pass 
between two thin electrical resistance wires, the material constituting 
the flexible plastic material being sufficient to isolate these two 
circuits. According to another variant, the heating element is a 
conducting layer located in an intermediate plane between the external 
side of the window and the plane which contains the collector element. The 
layer can then form two heating circuits which are separated by an 
intermediate zone in which the conductor elements pass. The wire which 
forms the conductor can also be enclosed in an insulating material. 
According to a preferred variant, the collector element of the 
electrostatic protection circuit is connected electrically to the 
collecting strip of the heating circuit, which is connected to the ground 
located, for example, on the edge of the window. A single connection to 
the ground is therefore sufficient for these two circuits. The heating 
elements can advantageously be located in a plane different from the plane 
of the collecting strip of the heating circuit, these two planes being 
parallel to the plane of the window. Some thin electrical resistance wires 
are then advantageously turned by 180.degree. in the space which separates 
the edge of the window from the collecting strip in order to return into 
the plane of the collecting strip, in order to be connected there to the 
aforementioned collecting strip. This arrangement results in more 
flexibility of the heating network wires. 
The invention also pertains to a method of fabrication of a window 
according to the invention. According to this method the rigid external 
substrate is assembled with the sheet of flexible plastic material. The 
conductor elements are then introduced into passages which are previously 
provided in said assembly, the conductor element being embedded in the 
sheet of flexible plastic material before or after assembly, and it 
electrically connects each of the conductor elements and is connected to 
the ground. In a preferred manner, at least one of the sheets of flexible 
plastic material which form the window is provided with a heating circuit, 
the sheet being a sheet which is part of the make-up of the rigid external 
substrate, or the sheet of flexible plastic material is provided with the 
collector element. 
The window can also be fabricated according to another method without 
departing from the scope of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIGS. 1a-1d illustrate, by way of non-limiting example, several possible 
paths of the lines (e.g., electrically conductive wires) and elements of a 
circuit which is capable of eliminating electrostatic charges which have 
accumulated on an external sheet of window 1 according to the invention, 
generally a transparent sheet made of rigid plastic material such as 
acrylic, methyl polymethacrylate (PMMA), particularly polycarbonate, 
polyester or rigid polyurethane. The anti-static circuit according to the 
invention includes conductor elements 2 which are perpendicular to the 
plane of the window. One end of each of the conductor elements 2 is 
connected to a collector 3 whose end is connected to the ground 4. 
The collector can describe only a single line parallel to the plane of the 
window, as illustrated in FIG. 1a. It can also describe several lines 
parallel to the plane of the window, as illustrated in FIGS. 1b, 1c and 
1d, each of these lines being connected to the adjacent line by one of 
their ends, alternately, so as to form a sort of crenel, as shown in FIG. 
1b. Each of the lines can also be connected to the others by one of their 
ends, as shown in FIG. 1c, or to their two ends, as shown in FIG. 1d. 
Other paths can also be provided to the extent that collector 3 forms only 
one circuit connected to the ground 4, which electrically connects all 
conductor elements 2. In a preferred manner, the circuit describes a 
network with simple geometry so as to limit the number of lines parallel 
to the plane of the window, the presence of these lines in too great a 
number possibly decreasing the optical quality of the window. 
FIGS. 2a-2c present views, along section A--A, of embodiments of the window 
shown in FIG. 1a. These windows include a rigid external substrate 5 and a 
sheet made of flexible plastic material 6. The sheet made of a flexible 
plastic material, e.g., polyvinylbutyral or flexible polyurethane, 
normally improves the properties of the window, especially its shock 
absorption. A sheet 6 of flexible plastic material can also be provided as 
an external sheet, on the interior compartment side, of a cockpit for 
example, as shown in FIG. 2c. This sheet can impart desirable surface 
properties, such as resistance to scratching, and anti-brightness 
protection. This can be a sheet of polyurethane, for example. 
In these Figures, the conductor elements 2 pass through the substrate 5 and 
at least one part of the sheet of flexible plastic material 6. The rigid 
external substrate can include an external sheet 7, as shown in FIG. 2a or 
FIG. 2b. It can also include two rigid sheets 7 and 8 between which two 
sheets of flexible plastic material 22 and 23 are inserted. The window can 
include, in addition to substrate 5 and the sheet of plastic material 6, 
other elements such as a rigid sheet 24, as shown in FIGS. 2a and 2b. One 
can see in these figures that the conductor element 2 can pass through any 
number of rigid and flexible sheets, according to the anticipated use. The 
sheet of flexible plastic material 6 can be formed of two sheets 6a and 6b 
(or more), as shown in FIG. 2b. 
Passages 9 are provided in order to allow conductor elements 2 to pass 
through substrate 5 in at least one part of the sheet of flexible plastic 
material 6. These passages 9 are, for example, mechanically perforated 
holes. According to a preferred variant, the external sheet 7 is a drawn 
acrylic material which has the advantage of improved resistance to 
occurrence and propagation of cracks. This characteristic is particularly 
advantageous in the case when several passages 9 are perforated at regular 
intervals. The parts of the passages at the level of the sheet of flexible 
plastic material can be obstructed by the movement of the plastic material 
when the window is being assembled. 
The electrostatic protection circuit is such that the conductor elements 2 
electrically connect, when the circuit is placed in the window, external 
side 10 of the external sheet 7 to a collector 3 which is connected to the 
ground. The electrostatic protection circuit according to the invention 
thus allows the trajectory of the electrostatic charges present on side 10 
of sheet 7 to be controlled across the window to the ground, and this is 
accomplished upon first use of the window. Discharges due to a 
considerable potential difference between side 10 and the remaining part 
of the window can thus be avoided. 
The conductor element 2 can pass through any number of rigid and flexible 
sheets if one of its ends is embedded in a sheet of flexible plastic 
material, so that the collector is itself embedded in the same sheet of 
flexible plastic material. The flexibility of this sheet of plastic 
material will impart an improved mechanical resistance to the conductor 
elements 2 and, in particular, to the collector 3. 
The collector 3 is, in this case, shown near side 11 of a sheet of flexible 
plastic material which is the most distant from the side 12 of the 
external sheet 7. This preferred arrangement allows one to improve the 
flexibility of conductors 2 and of collector 3. In all cases, the 
collector can be approximately in the middle of the sheet of flexible 
plastic material or in proximity of side 12 of the external sheet, to the 
extent that the collector is embedded in the sheet of flexible plastic 
material and is not in contact with the rigid external substrate. 
FIGS. 3a-3d presents several enlarged views of conductor elements 2. The 
conductor elements 2 consist preferably of a filament or wire. This 
filament can, possibly, form a loop 13 so that it will produce a back and 
forth movement in passage 9. This loop can, possibly, be coiled. It can 
extend beyond side 10 of sheet 1 forming a sort of packing, as shown in 
FIG. 3a, and its end can, possibly, be in contact with side 10. It can 
also be flush with the surface, or it can be located just below it, as 
shown in FIGS. 3b and 3c respectively. 
According to a variant which is particularly preferred, collector 3 and 
conductor elements 2 form only a single conductor wire, as shown in FIG. 
3d, which facilitates fabrication of the electrostatic protection circuit 
in accordance with the invention. 
The conductor elements and the collector are made of a material which can 
conduct electricity. It could be tungsten or a tungsten-based alloy. Their 
diameter is, for example, on the order of 0.015 mm. 
FIGS. 4 to 7 give preferred variants of the invention according to which 
the window includes a heating circuit and an electrostatic protection 
circuit. 
FIG. 4 shows a side view of the window of the invention. The electrostatic 
protection presented here occupies only one part of the window. The window 
includes a substrate 5 made of methyl polymethacrylate (PMMA) and a sheet 
of plastic material 6 which is formed, in this case, by several layers of 
PVB and a sheet 24 of PMMA. The conductor elements 2 are located in the 
dense part of the heating network which is normally in the central part of 
the window. Possible optical deteriorations are thereby limited. Note that 
the heating wires 14 and the collector 3 are located in two parallel 
planes, as can be seen better in FIGS. 6 and 8, so that they are not in 
contact with but separated by at least one part of the thickness from the 
sheet made of flexible plastic material, a dielectric 6, as shown in FIGS. 
7 and 8. 
The heating circuit shown here includes thin electrical resistance wires 14 
which are embedded, at least partially, in a sheet 6 made of flexible 
plastic material. Each of the ends of these wires is connected 
electrically to two connecting strips 15 and 16 of which one, for example 
the collecting strip 15, is connected to the ground, while the other, for 
example collecting strip 16, is connected to a common power supply. The 
thin electrical resistance wires 14 are, preferably, located near side 12 
of the flexible plastic material sheet, side 12 constituting, to a certain 
degree, the interface with the external sheet 7 which makes up the 
external substrate 5: the efficiency of the heating circuit is thereby 
improved. The two collecting strips are located on one side of a sheet 
made of flexible plastic material other than the side on which the heating 
wires are found. 
In these figures, collecting strips 15 and 16 are located on side 11 of 
sheet 6 made of flexible plastic material, while heating wires 14 are 
located on side of sheet 6 near side 12 of the external sheet 1. In order 
to connect the heating wires 14 to the collecting strip 15, the latter are 
turned by 180.degree. in the space located between the collecting strip 15 
and the edge of the window. This turning back is shown at 18. For reasons 
of convenience, the thin electrical resistance wires 14 are straight at 
19, while they are wavy in the central part of the window or, more 
precisely, in the main part of the space separating the two collecting 
strips 15 and 16 in order to improve their heating capability. 
FIG. 5 shows, in a more detailed way, the connection of the heating wires 
14 to the collecting strip 15. The heating wires 14 are, normally, 
inserted between two foils of 15a and 15b which form the collecting strip. 
The electrostatic protection circuit is, for its part, made of conductor 
elements 2, which are connected electrically to a collector 3 located on 
the same side of a sheet of flexible plastic material as the collecting 
strip 15 which is connected to the ground. The collector 3 is connected 
electrically to the collecting strip 15, by soldering for example, as 
shown in FIG. 5. Of course, according to the position of the collector in 
the sheet of flexible plastic material 6, it can be connected on side 20 
or 21 of the collecting strip 15. 
The configuration and the position of the two circuits, the heating circuit 
and the anti-static circuit, are such that the optical quality of the 
window is not altered and the two circuits are insulated electrically from 
one another in order to avoid any short circuit. Otherwise, the 
configuration and the position of the circuits take into consideration the 
constraints and loads which the window can be subjected to in flight. 
According to the invention, the two circuits are overlapped, as one can 
see in FIGS. 4, 7 and 8, the conductor elements being located between two 
heating wires. The distance between a heating wire 14 and a conductor 
element 2 is such that the electrical insulation is sufficient between 
these two materials and is greater than the diameter of passage 9. 
The section in FIG. 8 illustrates the return of the heating wires 14. This 
characteristic allows one, moreover, to heat the marginal parts of the 
window located between the edge and the collecting strip 15. Otherwise it 
improves the flexibility of the heating wire, and therefore its thermal 
resistance, especially for temperature differences which can occur during 
flight. 
In FIGS. 4 to 8, the conductor elements cross through the substrate which 
includes the rigid external sheet 7, which here makes up the substrate, 
and at least one part of a sheet 6 made of flexible plastic material. The 
substrate can also include other rigid and/or flexible sheets, as, for 
example, shown in FIGS. 1 to 3, independently of the existence of the 
heating circuit. 
In order to fabricate the window described in FIGS. 4 to 8 one can first 
fabricate a sheet of flexible plastic material 6 which includes a heating 
network, the thin electrical resistance wires and the collecting strips 
being on two opposite sides of said sheet. This sheet is then, for 
example, heat sealed with the external rigid sheet 7. Passages 9 are then 
perforated, for example mechanically, in order to introduce the conductor 
elements 2. The latter are inserted and then connected, for example 
soldered, to a collector 3 which is previously deposited on sheet 6, or a 
conductor filament is inserted in each of the passages while connecting 
them in order to make up conductor elements 2 and collector 3. One of the 
ends of collector 3 is connected to the ground, by soldering for example. 
It is preferably connected to the collecting strip of the heating circuit 
connected to the ground. The assembly made of sheets provided with heating 
and anti-static circuits is then assembled, possibly with other sheets 
becoming part of the make-up of the window. 
EXAMPLE 
Three samples of windows were made and tested. These samples were made of 
two rigid sheets of methyl polymethacrylate (PMMA) between which two 
flexible sheets made of polyvinylbutyral (PVB) were inserted. A heating 
network was embedded in the depth of one of the two sheets made of PVB. 
The thicknesses of these sheets were 1.3 and 4 mm for rigid sheets and 
0.63 and 1.3 mm for the PVB sheets. 
Sample 1 did not have electrostatic protection. One can observe a 
perforation in the thickness of the window for a voltage of 160 kV, which 
corresponds to a current on the order of 300 mA/m.sup.2 or on the order of 
80 mA for the tested sample. This test illustrates the critical threshold, 
which is 80 mA, not to be exceeded in order to avoid any chance of 
discharge across the thickness of the window. 
Eight electrodes were placed at 3 cm from the samples. These electrodes 
simulated the production of electrostatic charges on the surface of the 
rigid sheet made of PMMA. 
Sample 2 included three conductor elements which were separated by 8 cm 
according to the configuration shown in FIGS. 2b and 3d. 
Sample 3 included six conductor elements which were separated by about 5 cm 
and are arranged according to the same configuration as Sample 2. 
The conductor elements were flush with the external surface of the rigid 
sheet made of PMMA for the two samples. 
The strength of the current in the conductor elements and the collector was 
then measured. 
The results were the following: 
______________________________________ 
Sample 2 Sample 3 
Applied voltage 
Measured current 
Measured current 
(kV) (mA) (mA) 
______________________________________ 
20 11 11 
24 23.5 32.5 
26 36 46 
30 62 110 
32 83.5 145 
36 130 
38 150 
______________________________________ 
Measurable currents were observed beginning with relatively weak voltages. 
The electrostatic charges were removed, to a certain degree, as they 
appeared. Note that the value of 80 mA was observed for voltages of 32 and 
29 kV, for Samples 2 and 3 respectively. 
The values which were obtained for the two types of protection appear to 
have relatively similar efficiency with regard to the break down voltage 
threshold (160 kV), the voltage values being 5 to 6 times less than the 
break down voltage. Otherwise, the applied voltage was increased for each 
sample in order to obtain a current on the order of 150 mA in order to 
test the resistance of the circuit for an abnormally elevated current. The 
two types of protection did not undergo any deterioration. 
This sample shows the effectiveness of the protection circuit which can be 
relatively simple (3 conductor elements) when the conductor elements are 
placed in the zone of the window which is subjected to these electrostatic 
charges. 
Obviously, numerous modifications and variations of the present invention 
are possible in light of the above teachings. It is therefore to be 
understood that the invention may be practiced otherwise than as 
specifically described herein.