GB2331780A - Electrically heated window - Google Patents

Abstract

A window (20) includes an array (21) of fine, closely spaced wires (22) through which an electric current is passed to heat the window. Such windows may be used as vehicle windscreens, the heating dispersing ice or condensation from the window. The window of the invention includes a group of heating wires in which the wires extend along lines which deviate from the straight, so that some of the wires verge on a non-parallel edge (31,32) of the window at a point (37) part way along its length.

Description

2331780 ELECTRICALLY HEATED EMOW The present invention relates to an

electrically heated window, and more particularly but not exclusively to a laminated vehicle window comprising a number of plies2-the window including an array of fine, closely spaced wires carried by one of the plies and forming a heating element, the heating effect resulting from passage of electric current through the wires.

Such a window may be installed as the windscreen, rear window, or other window of a car or other vehicle, or as a window (especially the windscreen) of a commercial vehicle, locomotive or aircraft, or in a boat or ship. The electric heating is used to disperse condensation or ice on the surfaces of the window, and thereby maintain good visibility through the window.

At its simplest, a laminated window comprises an inner ply of interlayer material ("the interlayer") disposed between two outer plies of rigid transparent sheet glazing material, but more sophisticated laminated windows comprise greater numbers of plies of interlayer and glazing material, for example so as to confer greater resistance to impact damage upon the window, and reduce the risk of penetration of the window. The interlayer is normally a flexible plastics material, e.g. polyvinylbutyral, and the glazing material may be glass or a rigid plastics material. Laminated windows are also known comprising two or more plies, in which an exposed outer ply is a ply of flexible plastics material which may both increase penetration resistance and reduce the risk of laceration by fragments of glazing material during an impact. Such a plastics outer ply normally forms the inward-facing surface of the window, and is generally subjected to a surface treatment to increase its abrasion resistance.

A known window, which is suitable for use as a windscreen, is shown in Figure 1 Windscreens generally have the approximate shape of a trapezium, for they have two parallel edges which in practice are usually the two longer edges, and two non-parallel edges which in practice are the two shorter edges. The heating wires are laid along parallel straight lines, this being a limitation imposed by the wire-laying apparatus. Consequently, the area of the windscreen which is heated by the wires is generally rectangular, and so there is a generally triangular area adjacent each short edge of the windscreen which is left unheated. Any condensation or ice in these untreated areas will remain in place, impairing visibility through B916nGB.doc/nep 2 the windscreen. For the avoidance of doubt, in the present specification, "trapezium" is used to mean a quadrilateral with only one pair of sides parallel.

In the windscreens of modem vehicles, the unheated triangular areas are substantial, and may even include part of the area normally wiped by the windscreen wipers (the "wiped area"). The impairment of visibility resulting from the unheated areas is most uidesirable, and may have serious safety consequences. Furthermore, when ice remains present in the wiped area, damage to the wiper blades may occur as a result of scraping over the ice.

It has been proposed to heat substantially the whole area of a windscreen by arranging the heating wires to run from side to side, that is, when the window is in the shape of a trapezium, the wires run parallel to the two long, parallel, edges. However, because of the rake of the windscreen this arrangement of the wires may itself hnpair visibility, especially with the increasing angles of rake which are prevalent in modem vehicle designs (ie the installation angle of a windscreen is deviating flu-ther and further from the vertical). Furthermore, it is at the vehicle driver's eye level (in the so-called "primary vision are&) that maximum visibility is required, and because an arrangement heating the whole area of the windscreen includes heating the lower corner areas unnecessarily, a certain amount of electrical power is wasted.

Another arrangement of wires for an electrically heated windscreen is known from EP 506 521. This document is primarily concerned with forming the busbars from braid. These busbars extend part way along the nonparallel edges of the windscreen and Figure 1 of EP 506 521 not only shows heating wires connected to the part of the upper busbar which extends along the upper edge of the windscreen, but also shows heating wires connected to the parts of the upper busbar which extend part way along the non-parallel edges of the windscreen. This arrangement does have the effect of extending the area heated by the wires into the previously un-heated triangular areas of the windscreen, but it will be observed that the heating wires in this area are very short in comparison with the heating wires in the central area of the windscreen. The electrical resistance of these wires is therefore much lower, possibly 50% lower, than the resistance of the wires in the central area of the windscreen. Unless burdensome measures are taken to equalise the resistance of these wires, such as changing to a thinner gauge of wire, these wires will run very much hotter when a voltage is applied between the busbars. This necessarily results in an undesirable non- 3 uniform heating effect, and the wires may become so hot that they either bum out or cause damage to the interlayer.

There therefore remains a need for an electrical heating arrangement for a windscreen that provides good visibility for a vehicle driver, without excessive variation in heating effect, and without undue consumption of electric power. It has now been found possible to provide a windscreen satisfying these requirements by employing a novel arrangement of heating wires in which some of the heating wires are laid along lines which closely approach the nonparallel edges of the windscreen without being any shorter in length.

According to the present invention there is provided an electrically heated window having two generally parallel curved edges and two nonparallel edges, the window being laminated from at least two plies of glazing material and at least one ply of interlayer material extending between the plies of glazing material, the window including an array of fine, closely spaced electrical heating wires carried by one the plies, the wires extending in directions which are generally from one of the parallel edges to the other, characterised by a group of heating wires in which the wires extend along lines which deviate from straight, so that some of the wires verge on a non-parallel edge of the window at a point part way along its length. This group of wires will be referred to as the Meviating group".

In verging on an edge of the window, the wires closely approach the edge so as to lie immediately adjacent the edge. Where the window is provided with an opaque border, then it is sufficient for the wires to closely approach the edge of the transparent portion of the window.

This arrangement allows the full width of a windscreen to be heated in a region roughly corresponding to a drivees eye level, so that uninterrupted vision may be maintained at eye level from one side edge of a windscreen to the other. Furthermore, the arrangement provides a more uniform release of heat from the heating wires than the arrangement known from EP 506 521, and avoids the need to extend the busbars part way along the non-parallel edges of the windscreen. (It may, however, be desirable to extend the busbars along these sides of the windscreen in order to facilitate electrical connection.) Preferably the group of heating wires diverges away from the non-parallel edge of the window so as to leave a corner of the window unheated. The corner would be one through which good vision was not required, i.e. where the extra electric power needed to heat it could not be justified.

4 The deviating group of wires may extend along diverging lines or alternatively, the second group of wires may extend along curved lines. Either arrangement may be preferable according to the apparatus used to lay the wires, and to the aesthetic effect to be achieved in the windscreen.

In preferred embodiments of the invention, the ratio of the maximum heiang power density to the minimum heating power density measured in different locations within the heated area of the window may be less than 2.0. Preferably the ratio is less than 1.5, more preferably less than 1.2.

In a particularly preferred embodiment of the invention, the window may be a windscreen for a vehicle operated by a driver, wherein the deviating group of wires verges on a non-parallel edge of the windscreen at a point which approximately corresponds to the eye level of a driver of average height when the windscreen is installed in the vehicle.

lle invention will now be flu-ther described by way of the following specific embodiments, which are given by way of illustration and not of limitation, and with reference to the accompanying drawings in which:Fig. 1 is a general view of a known electrically heated window, viewed in a direction approximately normal to the surface of the window; Fig. 2 is a general view of a first embodiment of an electrically heated window according to the invention, viewed in a corresponding direction to Figure 1; Fig. 3 is a greatly enlarged cross-sectional view of a small marginal part of the window of Figure 2, taken on the line III-III of Figure 2; Fig. 4 is a general view of a second embodiment of an electrically heated window, again viewed in a corresponding direction to Figures 1 and 2; Fig. 5 is a perspective view of on apparatus for laying heating wires on interlayer material in the course of making a window according to the invention, including an endless support surface in the form of a cylindrical drum; Fig. 6 is a schematic diagram representing the cylindrical support surface of the drum of Figure 5 as a flat rectangle, so that the entire support surface may be seen at the same time.

Figure 1 shows a known electrically heated window 1 suitable for use as the windscreen of a vehicle. It comprises an area 2 heated by an array of fine, closely spaced wires 3. The wires are arranged in two heating elements 4 and 5 extending between respective busbars 6, by means of which electric current is supplied to the two elements independently.

It will be appreciated that as the heating wires are positioned very close to one another in heated windows of the type described in this specification, it is not possible to represent in the drawings all the wires that are actually present in such windows. Consequently it should be understood that only a proportion of the heating wires are shown in the drawings of this specification, and they are shown further apart than is actually the case. Furthermore, to avoid excessive repetition for the draughtsman, where the area covered by wires is relatively large, no wires have been drawn in over part of the area. Instead, dot-dashed lines have been used to indicate the extent of the area covered by wires.

The window 1 is generally in the shape of a trapezium, with generally parallel edges 8 and 9, and non-parallel edges 10 and 11. All these edges may be curved to some extent, indeed parallel edges 8 & 9 are normally curved.

As mentioned above, the result of providing a heated area 2 comprising wires extending along parallel straight lines is that the heated area is rectangular, and so two generally triangular areas 7 remain unheated. Unfortunately it is not in practice feasible to simply continue the straight, parallel wires across areas 7, because the resistance, and hence the length, of each wire 3 should not be varied excessively if satisfactory performance (i.e. moderately uniform heating) is to be obtained.

References in this specification to the form of a line along which a heating wire extends (e.g. as "straight") are references to the form of the line when the piece of interlayer material is placed on a flat surface. When a piece of interlayer material comprising wires extending along straight lines is assembled into a curved window, and is placed between curved plies of glazing material, the lines obviously adopt the curvature of the glazing material, and only appear absolutely straight at a particular point in the window when viewed in a direction normal to the surface of the window at that point. As will be described below, the wires themselves may have fine undulations imposed on their overall form, and so it is more accurate to describe the form of a line along which a wire extends as e.g., straight or curved, than to describe the wire itself as straight or curved.

Figure 2 shows an electrically heated window according to a first embodiment of the invention. This window 20 is suitable for use as a heated vehicle windscreen, and has long edges 29 and 30, and short edges 31 and 32, the long edges being substantially parallel 6 so that the window has the general shape of a trapezium, allowing for the curvature of the edges. When the window is installed in the usual orientation for a windscreen, long edge 29 forms the top edge, long edge 30 forms the bottom edge, and the non-parallel short edges 31,32 form the sides of the windscreen. The window is heated by a heating means comprising an array 21 of fine, closely spaced wires 22, some of which are arranged to verge on the non-parallel edges 31, 32 of the window at points part way along their length.

Figure 3 is a cross-section of the window 20 taken along the line III-III of Figure 2. The window comprises two plies of sheet glazing material which may be curved, the ply forming the external surface 44 of the window 20 being designated the outer ply 40, and the ply forming the inner surface 47 of the window (i.e. the surface of the window which, after glazing, faces towards the inside of the object, e.g. a vehicle, to be glazed by the window) being designated the inner ply 41. While plies 40,41 may be composed of any rigid transparent sheet glazing material (e. g. various plastics), a preferred material is glass. The plies have further surfaces 45,46 which are bonded together to form a laminate by a ply of interlayer material 42, which is transparent in the finished product, extending between the plies of glazing material. The interlayer is generally composed of a plastics material with suitable physical and chemical properties to bond the plies of glazing material together, and confer on the product the requisite performance for its application, e.g. in terms of safety, optical performance etc. A suitable interlayer material is polyvinylbutyral ("pvb"), but other interlayer materials may be used. As mentioned above, other constructions of laminated windows exist which involve more or fewer plies.

Still referring to Figure 3, a substantially opaque band 43 (known in the vehicle glazing industry as an obscuration band) is disposed around the periphery of one face of one of the plies, preferably the inner face 45 of the outer ply 40. The obscuration band 43 may be in the form of a printed coating composed of a ceramic (ffit-based) ink. The obscuration band serves to hide the receiving flange of the vehicle bodywork when the windscreen is glazed in position; and when the window is adhesively glazed, the band not only conceals but also protects the adhesive and/or sealant from light, especially its ultra-violet component. In this specification, references to a transparent portion of the window are references to the portion not obscured by the obscuration band. In Figure 3, an imaginary line ZZ is shown dividing the opaque portion 48 from the transparent portion 49.

7 Preferably the obscuration band 43 is disposed on inner face 45 of the outer ply 40 because in this position, the band is additionally able to conceal from external view components, e.g. busbars, situated on a peripheral surface of the ply of interlayer 42. For this reason, the obscuration band has been omitted from Figure 2 in order to reveal the busbars. If the obscuration band is disposed on a different surface, for example surface 47, iTis possible to apply an organic primer to the inner face 45 instead, so that components situated on a peripheral surface of the ply of interlayer are still concealed.

Referring to Figure 2 again, the array 21 of wires 22 is preferably disposed on the inner face of the ply of interlayer material 42, although it is entirely possible to carry out the invention with the wires disposed on the outer face of the ply of interlayer material, or in the middle of the ply, especially if it is a composite ply. Indeed, the invention also includes windows in which the wires are disposed on a face of one of the other plies, providing they are suitably protected. The array may comprise two or more heating elements 23,24, each element having its own current supply. This arrangement is preferable for large windows since such windows, when supplied with power from a standard nominal 12 volt vehicle supply, draw a substantial current. Splitting the array into independently supplied elements reduces the load on the various electrical connection means which supply current to each element; clearly the number of elements may be varied according to the size of the windscreen.

In the window of Figure 2, the elements 23 and 24 extend between electrical connection means in the form of respective busbars. Accordingly there are two busbars 25,26 which extend along the top edge 29 of the windscreen, and two busbars 27,28 which extend along the bottom edge 30. The busbars are made from electrically conductive strip, normally metal strip, for example, copper strips 3-6 mm wide, preferably 6 mm wide and about 0.04-0.08 mm thick. The strips are preferably tinned with a surface layer of tin, or tin lead alloy to protect the copper from oxidation.

In this windscreen, the top busbars 25,26 extend around the top corners, and down the sides 31,32 of the windscreen. The busbars 25,26,27,28 may be connected to flying leads 33 by soldering, possibly via a small connector tag of known design for neatness (not shown), and the leads have terminations 34 appropriate for connection to a voltage supply. Alternatively, conventional spade connectors may be soldered to the ends of the busbars, and 8 leads attached to the spades. These leads, tags, terminations and connectors also constitute electrical connection means.

The wires 22 are preferably attached to the strip forming the busbars 25, 26,27,28 by use of a flu-ther piece of busbar strip (not shown) to form a sandwich with the wires between the two strips, a face of at least one of the busbar strips being provided with a surfacilayer of low melting point solder such that the solder melts on autoclaving to provide good electrical contact between the busbar strip and the wires. Such a technique is known from EP 385 791.

The wires 22 are preferably composed of tungsten, and are preferably 1030 gm thick for a nominal operating voltage of 12 volts; typically, the array 21 of wires for a windscreen may include between 300 and 900 wires according to the size of the windscreen and the wire spacing used. The wires may be considered to comprise two groups 35 and 36; in a first, or straight, group 35, the wires are arranged along lines which extend directly from edge 29 to edge 30, e.g. along parallel straight lines. In the second or deviating group(s) 36, the lines along which the wires extend deviate from a direct route from edge 29 to edge 30 so as to verge on one or both non-parallel edges 31, 32 of the windscreen at a point 37 partway along their length.

The straight group of wires 35 may be absent, or may comprise only 1 or 2 wires in the centre of the windscreen (as indicated in Figure 6), or, as illustrated in Figure 2, it may comprise the majority of the wires. The latter arrangement is preferable because less time is required to lay a wire along a straight line than along a deviating line, so if the heating means is designed to have as many wires as possible in the straight group 35, manufacturing time is reduced. Furthermore, the heating effect is highly uniform over the straight group of wires.

In Figure 2 the wires of the deviating groups 36 extend for part of their length along lines which diverge in a direction from one long edge (top edge 29) to the other, generally parallel, long edge (bottom edge 30). The wires may be said to fan out, and as a result the wires adjacent the short edges 31 and 32 are substantially parallel to those respective edges for part of their length. Clearly, this arrangement of wires may also be described as converging in a direction from the bottom edge 30 to the top edge 29.

It is known to those skilled in the art, that when a bright light from a point source, such as one of the headlights of an oncoming vehicle, shines through a windscreen having an array of fine, closely spaced straight wires, undesirable secondary optical effects may occur (believed to be due to reflections from the wires) which disrupt the vehicle driver's vision 9 through the windscreen. As is common practice in the art, the wires 22 of window 20 normally have undulations to alleviate these effects, e.g. they are crimped in a sinusoidal pattern. Other patterns may be used, such as a zig-zag, or a helical spiral, or indeed the undulations may be random in nature.

Figure 4 shows a second embodiment of electrically heated window according to the invention, in which the wires of the deviating groups 36 extend along curved lines, whereas the wires of the straight group 35 extend along substantially parallel straight lines as before. The curved lines may have circular or non-circular curvature. This embodiment is also particularly suitable for use as a windscreen of a vehicle. Many aspects of this window 50 are the same as, or equivalent to, the corresponding aspects of the window 20 described in connection with the first embodiment (Figures 2 and 3) and so these aspects will not be described flurther here. Such aspects include the composition of the window, i.e. from plies of glazing material and interlayer material; use and details of any obscuration band; and the materials used for the wires, busbars and their connection.

In Figure 4 the array 51 is also divided into two heating elements, 53 and 54. Since in this embodiment the wires of the deviating group(s) 36 extend along curved lines, the '#verging points" 37 along the nonparallel edges 31, 32 of the windscreen should be regarded as the points at which the spacing of the wires from the edge of the transparent portions of the windscreen begins to increase; they may also be the points at which the spacing is at a minimum, or the spacing may be substantially constant up to point 37. In the latter case, the wire might be arranged along a line that was partly straight and partly curved, or it might be a curved line of variable radius.

In both the embodiments described above, it is important to ensure that the particular arrangement of wires selected for a window provides at least a moderately uniform heating effect (as measured, for example, in terTns of heating power density) over the heated area. It is preferred that the ratio of the maximum heating power density to the minimum heating power density measured in different locations on the window should be less than 2.0, more preferably less than 1.5, and most preferably less than 1.2. Assuming the supply voltage is fixed, heating power density depends on the resistance of the wires (which itself depends on their thickness and length), the spacing of the wires, and is also affected by the degree to which the wires are crimped. As can be seen, the present invention allows variations in the length of the wires to be minimised.

For satisfactory performance, the maximum spacing of the majority of wires in the transparent portion of a windscreen should not exceed 10 mm, preferably it is less than 5 mm, and it may be desirable to keep the spacing to 3 mm or less, especially in critical areas such as the primary vision area. Adjacent wires should not touch each other, so the minimum spacing is governed by the accuracy with which wires can be positioned during Gaydown, and the degree of crimp in use.

The window of the invention may be provided with a window wiper, and normally such a wiper remains stationary on a particular portion of the window when not in use. The wiper is said to "parC on the window, and the particular portion of the window on which it parks is referred to as a "Nviper parking area". Preferably the array of wires is arranged to heat all of the wiper parking area, so that the wiper may be released when frozen to the window in frosty weather, or indeed prevented from freezing to the window. Generally, this is achieved by arranging the deviating group(s) of wires in the array so that the heated area of the window is sufficiently wide to cover the entire length of the parking area.

Figure 5 shows an apparatus 90 for laying heating wires along straight or diverging lines in various configurations so that, in the finished window, the array of wires verges on the edges of the transparent portion of a window, as described above. The apparatus comprises a base 105, a wiring head 95 and an endless support surface 91 for the piece or pieces of interlayer material 92 ("the interlayer). The support surface 91 is provided by the curved surface of a cylindrical drum 93, which is rotatable in the direction of arrow Y about an axis of rotation represented by dashed line X-X. The drum 93 is driven by belt 106 from a motor (not shown) within the base 105. Support surface 91 may be perforated and the internal air pressure of the drum 93 may be reduced to retain interlayer in contact with it. The interior of the drum may for instance be connected to an external suction means.

The preferred way of providing relative movement between the support surface 91 and the wiring head 95 is to mount the latter for sliding movement along one or more elongate members extending in a direction parallel to the axis X-X. As shown in Figure 5, the elongate member is in the form of two rails 94 which extend parallel to the axis X-X, and are displaced to one side of drum 93. Other arrangements for providing translational movement of the wiring head are possible.

The wiring head 95 comprises devices for the supply of wire and for laying it on the interlayer 92. Wire is supplied from a spool of wire 99, which may additionally be provided 11 with means for unwinding the wire, via wire guide means such as pulleys or eyes 100, and set in contact with the interlayer by means of a put-down roller 96. The spool and wire guide means are arranged to impart a slight tension to the wire, which assists in keeping the wire correctly threaded and under control.

Preferably, the wiring head also comprises means for imparting undulations to the wire for use when wiring interlayer for a windscreen, e.g. meshed pinions or bevel gears 101,102 through which the wire is fed so as to crimp it. The wire guide, crimp gears and put-down roller are all mounted on an arm 107, the crimp gears being mounted on the arm via an auxiliary fi-ame 108, and the put-down roller via a pivotable link 109. Different crimp gears with different sizes of teeth may be used to obtain different initial levels of crimp. Furthermore, as the wire is also preferably maintained under slight tension as it passes from the wiring head onto the drum, some of the crimp may be pulled out of the wire. By varying the tension, the degree of crimp may thereby be varied on the run.

A preferred way of ensuring that the wire adheres to the interlayer after being set in contact with it is to heat the wire, since interlayer becomes tacky when hot, so hot wire tends to adhere to the interlayer. A preferred method of heating the wire is to pass an electric current through it, so preferably the wiring head includes means for applying a voltage across a length of wire close to where it is set in contact with the interlayer. A convenient way of achieving this is to apply a voltage between the put-down roller 96 and the crimp gears 10 1, 102, by means of electrical leads 103,104 attached to a voltage source (not shown).

The put-down roller 96 is rotatable about an axis 110 passing through its centre, so that it can roll over the interlayer as it presses the hot wireinto the surface of the interlayer. In order to accommodate changes in the direction of the lines along which wire is laid, the putdown roller and part of the arm 107 can also be swivelled about a second axis substantially at right angles to the axis of rotation of the put-down roller. Such a second axis is shown by line V-V in Figure 5, and arrow W shows the direction of swivelling. The axis V-V about which the roller, link and arm may be swivelled is preferably oriented substantially at right angles to a tangent to the support surface 91 taken at the point at which the put-down roller 96 makes contact with it (or with the interlayer). As an alternative to swivelling the putdown roller and part or all of the wiring head 95, just the put-down roller 96 and its mounting link 109 may be swivelled by themselves. In this case, the swivel axis is translated towards the put-down roller so as to pass through the point of contact between the put-down roller and 12 the interlayer on the support surface, and a second wire guide means (not illustrated) may be provided adjacent the put-down roller to assist in keeping the wire in place on the put-down roller when the latter swivels.

The wiring head 95 also includes drive means 97, e.g. an electric motor, for propelling the wiring head back and forth along rails 94 in a reciprocating sliding manner, as indicated by arrow U. This reciprocating motion is in addition to a continuous background advancing motion indicated by arrow T; in other words, the drive means is capable of advancing the wiring head from one end of the drum to the other, as well as causing it to reciprocate during the course of such advancement. The wiring head may pause in its reciprocating movement during a change of direction, so that wire is laid along a straight line. The various motions of which the drive means 97 is capable are controlled by control means 98, which is preferably an NC (numerical control) control means, and which co-ordinates the reciprocating movement of the wiring head with the rotation of the drum. Further details of the known aspects of this apparatus are available from EP 443 691.

Although only one piece of interlayer 92 is visible in Figure 5, it is desirable to be able to wire two or more pieces at once.

Figure 6 shows, an arrangement of two pieces of interlayer 92 for making the window of Figure 2. In Figure 6, the endless support surface 91 of the drum 93 has been represented as a rectangle, as if the surface had been cut open and flattened out.

The longer edges 30 of the two parallel edges of each of the two interlayer pieces are placed adjacent each other, so that the two pieces are in an opposed relationship having substantial mirror symmetry about a line extending midway between the two longer edges. As seen in Figure 6, the lines along which the wires 22 are laid meet between the pieces of interlayer, making it possible for the wiring head 95 to traverse directly from one piece of interlayer onto the other without turning any sharp corners. The same arrangement may be used to make the window of Figure 4. Clearly, if the support surface is large enough, the number of pieces could be increased to four, or even more. Incidentally, Figure 6 shows a version of the first embodiment in which only one wire (the central wire) is laid along a straight line, and the remainder of the wires all deviate towards an adjacent non-parallel edge.

The manufacture of a heated window according to the invention, including the operation of the wiring apparatus 90, will now be described with particular emphasis on the novel aspects.

13 A piece of interlayer cut to the appropriate shape and size for the window to be manufactured is placed on a horizontal surface in a clean environment. Lengths of tinned copper strip are placed in position on the interlayer to serve as part of the busbars, and secured in place. The piece of interlayer is then transferred to the endless support surface 91 of the wiring apparatus 90, and secured in place by conventional means as knov.; from EP 443 69 1. A second piece of interlayer may similarly be placed on the drum as explained above.

During the laying down of wire, the drum 93 rotates and the wiring head 95 slowly advances along the rails 94, as Imown from EP 443 691. Additionally, reciprocation of the wiring head along the rails 94 is coordinated with the rotation of the drum. The head may perform an integral number of reciprocations for each revolution of the drum, so that after each complete revolution of the drum, the wiring head almost returns to its starting position, but not quite, the difference being equal to the spacing of successive turns of wire on the drum. When the drum holds two pieces of interlayer for a windscreen as shown in Fig 5, the movement of the wiring head is synchronised with the rotation of the drum, so that the wiring head performs one reciprocation for each revolution of the drum. The rotation of the drum and the movement of the wiring head may be arranged so that each wire is laid on the piece of interlayer along either a curved line, or a diverging straight line. Changes in the direction of translational movement of the wiring head occur as the put- down roller crosses an imaginary line joining points 37, this line being shown as a dashed line on Figure 6. As a result of the change in the direction of movement of the wiring head, the line along which a wire is being laid deviates from its previous direction. Such changes in direction also occur when the wiring head is between the pieces of interlayer, while crossing from one to the other.

The amplitude of reciprocation is varied according to the position of the wiring head 95 along the rails 94. For a windscreen, the amplitude may be greatest when the wiring head is at either extreme of its travel along the rails, and smallest at the midpoint. In fact, if the windscreen is to have one or more wires laid along a straight line, such as along or parallel to its centre line (i.e. its axis of mirror symmetry), then during the laying of these wires the wiring head does not reciprocate at all; it merely advances so as to space the wires. Such wires are laid substantially parallel to the edges of the drum, allowing for the slight slant due to the wiring head advancing.

14 The wiring head does not reciprocate therefore during the laying-down of the parallel wires in the straight group 35. Reciprocation only occurs during the laying-down of the wires in the deviating groups 36, which are laid along curved or diverging lines. It is possible to run the wiring machine faster (i.e. a greater number of revolutions of the drum per minute) when the wiring head is not reciprocating, and so the Figures 2 and 4 versions ofithe embodiments have the advantage that they can be made faster than the Figure 6 version.

The speed at which the wiring head slides along the rails as it reciprocates during the wiring of a piece of interlayer varies according to the amplitude of reciprocation, for a given rate of revolution of the drum. When the amplitude is greater, the wiring head has to travel a greater distance along the rails per reciprocation, and so it slides at a correspondingly greater speed.

When wiring is complete, the pieces(s) of interlayer are removed from the drum and again placed flat on a horizontal surface. Further lengths of tinned copper strip are laid on top of the previously positioned strips where the wires cross them, and soldered in position so that the busbar is of two-layer or "sandwich" construction, as taught by EP 3 85 79 1, in the region where the wires make contact with the busbars.

The window is completed by performing steps which are known, and hence described only briefly. Further electrical connection means (e.g. flying leads, connectors etc.) are attached, and the wired piece of interlayer is placed between complementary curved plies of glazing material. Air is then removed from the assembly, and it is subjected to high temperature and pressure in an autoclave so that the interlayer material bonds the plies of glazing material together. If low melting point solder was preapplied to the lengths of busbar strip, then this fuses in the autoclave and ensures good electrical connection between the wires and the busbar.

Claims (1)

1. An electrically heated window having two generally parallel curved edges and two nonparallel edges, the window being laminated from at least two plies of glaziU material and at least one ply of interlayer material extending between the plies of gI g material, the window including an array of fine, closely spaced electrical heating wires carried by one of the plies, the wires extending in directions which are generally from one of the parallel edges to the other, characterised by a group of heating wires in which the wires extend along lines which deviate from straight, so that some of the wires verge on a non-parallel edge of the window at a point part way long its length.

2.

A window as claimed in claim 1, wherein the group of heating wires also diverges away from the non-parallel edge of the window so as to leave a comer of the window unheated.

A window as claimed in claim 1 or claim 2, wherein the group of wires extends along diverging lines.

A window as claimed in claim 3, wherein for part of their length the wires of said group extend along diverging lines, and for part of their length they extend along parallel lines.

A window as claimed in claim 1 or claim 2, wherein the wires of said group extend along curved lines.

A window as claimed in any preceding claim, wherein the ratio of the maximum heating power density to the minimum heating density measured in different locations on the window is less than 2.0.

A window as claimed in claim 6, wherein the ratio is less than 1.5.

16 8. A window as claimed in any preceding claim, wherein the spacing of adjacent wires is less than 1 Omm.

9. A window as claimed in claim 8, wherein the spacing is less than 5mm.

10. A window as claimed in any preceding claim, being a windscreen for a vehicle operated by a driver, wherein the group of wires verges on a nonparallel edge of the windscreen at a point which approximately corresponds to the average driver's eye level when the windscreen is installed in the vehicle.

11. A window as claimed in any preceding claim, provided with a window wiper which parks on a particular portion of the window when not in use, wherein the array of wires is arranged to heat all of the particular portion.

12. An electrically heated window substantially as hereinbefore described with reference to and as illustrated in Figures 2 and 3, or Figure 4, of the accompanying drawings.