CAPACITIVE PROXIMITY SENSOR WITH CONNECTOR TONGUE

Field

The present disclosure relates to capacitive proximity sensors suitable for mounting to a body, for example to detect the presence of external objects.

Background

Capacitive proximity sensors have been used in various industrial applications for locating the presence of objects or materials. Various forms of capacitive proximity sensors are known and are suitable for use in different environments and applications including, for example, touch-operated systems, collision-prevention systems, occupancy-detection systems, and security/warning systems. In one field of application, capacitive proximity sensors have been fitted, for example, with the rear side and/or bumpers of cars. When the vehicle is reversed a warning signal is provided when the car approaches an object so that a collision can be safely avoided while still allowing the driver to conveniently position the car close to such object.

WO 01/08925 (AB Automotive Electronics Ltd.) describes a capacitive proximity sensor for a vehicle, which consists of two strips of metal, or other conductive material, insulated from each other and provided on the inside of the bumper of a vehicle. One strip, which faces outwardly from the vehicle, is referred to as the sensor plate and the other strip, which faces inwardly towards the vehicle is called the guard plate. Both plates are connected to a control unit. The control unit monitors the change that occurs in the capacitance between the sensor plate and (electrical) ground as the vehicle approaches an external object and provides an indication to the driver of the distance between the sensor plate (and, hence, the vehicle) and the object. Various geometries for the sensor plate are described, to increase the sensitivity of the proximity sensor at the corners of the vehicle.

GB- A-2 374422 (of the same Applicant) describes a modified form of such a capacitive proximity sensor, in which an extra conductive plate is provided to reduce the effect of rainwater on the sensitivity of the sensor. That extra conductive plate, which can be arranged above or below the sensor plate (with respect to ground level), is often referred to

as the superguard conductor. More generally, a superguard conductor can be used to address the problem of reducing the sensitivity of a capacitive proximity sensor to very close objects that the sensor is not required to detect.

GB- A-2 400 666 (also of the same Applicant) mentions the manufacture of a capacitive proximity sensor of the type described in WO 01/08925 by screen-printing the sensor and guard plates with conductive ink onto opposite sides of a plastic film substrate. GB- A-2 400 666 also describes that the sensor and guard plates may, as an alternative, be formed from aluminium foil that is laminated to the plastic film substrate.

The present disclosure is concerned with capacitive proximity sensors of the type comprising a dielectric substrate, for example a film, having a sensor conductor on one of its major surfaces and a guard conductor on at least one of its major surfaces to provide an electrical shield for the sensor conductor. The need to make electrical connections to the sensor and guard conductors can render the incorporation of such a sensor into, for example, the bumper of a car complicated and can adversely affect the reliability of the sensor during its lifetime.

In some embodiments, the present disclosure provides a capacitive proximity sensor to which electrical contact can be established comparatively easily and reliably, while complying with the requirements of mass production.

In some embodiments, the present disclosure provides a capacitive proximity sensor for mounting to a body for sensing external objects, the sensor comprising a dielectric film substrate having front and rear major surfaces which, in use of the sensor, face respectively outward from and towards the body; a sensor conductor on the front major surface, and a guard conductor on at least one of the major surfaces to provide an electrical shield for the sensor conductor; wherein the substrate comprises a tongue portion onto which the sensor and guard conductors extend to provide respective terminals by which external electrical connections can be made to the sensor.

Brief description of the drawings

Embodiments of the disclosure will be described below, by way of example only, with reference to the accompanying drawings, in which: Fig. 1 shows a diagrammatic plan view of a major surface of a capacitive proximity sensor in accordance with some embodiments of the present disclosure, from which a cover film has been omitted;

Fig. 2 shows an enlarged diagrammatic cross-section of the sensor of Fig. 1, taken on the line 2-2; Fig. 3 shows a similar view to Fig. 2 but includes the omitted cover film; ^•

Fig. 4 shows a plan view of part of a major surface of a modified capacitive proximity sensor, from which a cover film has been omitted;

Fig. 5 shows an enlarged diagrammatic cross-section of the sensor of Fig. 4, taken on the line 5-5; Fig. 6 shows a plan view of part of a major surface of another modified capacitive proximity sensor, from which a cover film has been omitted;

Fig. 7 shows an enlarged diagrammatic cross-section of the sensor of Fig. 6, taken on the line 7-7;

Figs. 8 and 9 illustrate alternative constructions of part of the sensor shown in Figs. 4 and 5;

Fig. 10 shows a diagrammatic plan view of a major surface of another capacitive proximity sensor from which a cover film has been omitted;

Fig. 11 shows an enlarged diagrammatic cross-section of the sensor of Fig. 10, taken on the line 11-11; Fig. 12 shows an enlarged diagrammatic cross-section of the sensor of Fig. 10, taken on the line 12-12, when the sensor is connected to an electronic control unit; and

Fig. 13 shows a diagrammatic cross-section of an alternative form of tongue portion for a sensor.

Detailed description

The term "film substrate" as used herein refers to an article having an extension in two directions which exceeds the extension in a third direction, which is essentially normal to said two directions, by a factor of at least 5 and more preferably by at least 10. More generally, the term "film" is used herein to refer to a flexible sheet-like material, and includes not only films but also sheetings, foils, strips, laminates, ribbons and the like.

The term "dielectric" as used herein refers to materials having a specific bulk resistivity as measured according to ASTM D 257 of at least 1 x 10 ¹² Ohm •centimeter (ωcm) and more preferably of at least 1 x 10 ¹³ ωcm. The term "electrically-conductive" as used herein refers to materials having a surface resistivity as measured according to ASTM B 193-01 of less than 1 Ohm per square centimetre (ω/cm ² ).

The capacitive proximity sensor 1 of Figs. 1 to 3 comprises a dielectric film substrate layer 2, the peripheral shape of the main part 4 of which is determined by the intended location of the sensor as described further below. In Fig. 1 , the main part 4 of the substrate layer 2 is shown diagrammatically as being generally rectangular in shape, and has an elongated tongue portion 5 extending from one (4a) of its longer sides. The function of the tongue portion 5 will be described below.

The major surface of the main part 4 of the substrate layer 2 shown in Fig. 1 carries a sensor conductor 6 and a superguard conductor 7 that are spaced apart on the surface of the substrate layer, and electrically-isolated from one another by the intervening substrate material. The superguard conductor 7 comprises a flat, electrically-conductive strip extending essentially along the length of the main part 4 of the substrate layer 2. The sensor conductor 6 exhibits a more complicated design and comprises four, optionally- flattened, conductive strips 6a extending parallel to one another essentially along the length of the main part 4 of the substrate layer 2 and, adjacent both ends of the strips 6a, three additional parallel (but shorter), optionally-flattened, electrically-conductive strips 6b forming lobe type regions. The strips 6a, 6b of the sensor conductor are connected together in both lobe regions by electrically-conductive strips 6c extending at an angle across the

whole array of strips 6a, 6b.

The opposite major surface of the main part 4 of the substrate layer 2, not visible in Fig. 1, carries a guard conductor 8 in the form of an electrically-conductive layer that preferably covers an area of the substrate corresponding in size at least to that occupied, on the other side, by the sensor conductor 6. In the sensor illustrated in Figs. 1 to 3, the guard conductor 8 essentially folly covers the surface of the main part 4 of the substrate layer 2 to which it is attached. The guard conductor 8 is electrically-isolated from the sensor and superguard conductors 6, 7 by the intervening dielectric substrate layer 2.

As so far described, the main part 4 of the substrate layer 2, with the sensor conductor 6, the guard conductor 8 and the superguard conductor 7, can be attached to any suitable surface, for example the inside of a bumper of a vehicle, to function as a capacitive proximity sensor. To that end, in the case of a vehicle bumper, the substrate layer is positioned with the major surface of Fig. 1 (i.e. the surface carrying the sensor and superguard conductors 6, 7) directed outwardly from the vehicle and the other major surface (i.e. the surface carrying the guard conductor 8) directed inwardly towards the vehicle. The conductors 6. 1, 8 are connected to an electronic control unit that can monitor the change that occurs in the capacitance between the sensor conductor 6 and (electrical) ground as the vehicle approaches an external object, and thereby provide an indication to the driver of the distance between the sensor conductor (and, hence, the vehicle) and the object. During the monitoring process, the guard conductor 8 acts as a shield to reduce the sensitivity of the sensor conductor 6 to anything behind it in the direction of the body of the vehicle, while an electrical signal is applied to the superguard conductor 7 to make the guard conductor 8 appear even bigger and so minimize the effect, on the signal from the sensor conductor 6, of water drops running over the bumper in rainy weather conditions. Further information on the operation of a capacitive proximity sensor of that type can be obtained from, for example, WO 01/08925, GB-A-2 374422, and GB-A-2 400 666 mentioned above. The measurement and processing of signals from a capacitive proximity sensor are described, for example, in WO 02/19,524 of the same Applicant.

The tongue portion 5 of the substrate layer 2 is provided to facilitate the connection of the sensor, guard and superguard conductors 6, 7, 8 to the control unit that monitors the capacitance between the sensor conductor 6 and ground when the proximity sensor 1 is in use.

As shown in Fig. 1, the tongue portion 5 extends generally parallel to the side 4a of the main part 4 of the substrate layer 2, and is joined to the side 4a towards one end of the latter. A conductive strip 9 extends from the adjacent end of the sensor conductor 6 onto the tongue portion 5 and then along the length of the latter to a terminal 10 at the free end of the tongue portion. To ensure an effective electrical connection between the conductive extension strip 9 and the sensor conductor 6, the extension strip is preferably connected to more than one of the conductive strips 6a, 6b but that it not essential. Similarly, a conductive strip 11 extends from the adjacent end of the superguard conductor 7 onto the tongue portion 5 and then along the length of the latter, parallel to the conductive strip 9, to a terminal 12 at the free end of the tongue portion. On the opposite surface of the substrate layer 2 (not visible in Fig. 1), the guard conductor 8 also extends onto the tongue portion 5 and along the length of the latter, preferably covering an area that corresponds in size at least to that occupied, on the other side, by the conductive strip 9. In the sensor illustrated in Figs. 1 to 3, the extension 8 ¹ of the guard conductor 8 onto the tongue portion 5 essentially fully covers the respective surface of the latter.

An electrical shield is provided for the conductive extension strip 9 of the sensor conductor 6 by laminating a strip 13 of conductive material over the extension strip, with an intervening strip 14 of dielectric material. The electrical shield could extend over the extension strip 1 1 of the superguard conductor also, but that is not essential. The entire sensor 1, including the tongue portion 5, is then encased in a protective cover film 15 (shown only in Fig. 3). It will be understood that the apparent gaps between the upper cover film 15 and the remainder of the sensor .in Fig.l are a result of the exaggerated dimensions of the underlying conductive strips 6a, 6b etc. and would not exist in practice.

The elongate tongue portion 5, when constructed as described above, effectively has the

form of a flat screened cable extending from the sensor 1 , and can be used to connect the sensor to an electronic control unit at a remote location. In automotive applications, when the sensor is positioned on the bumper of a vehicle, that remote location may be within the vehicle itself. The tongue portion 5 thus eliminates the need to provide a separate, comparatively expensive, coaxial cable in order to connect the sensor to the control unit.

Moreover, because the tongue portion 5 is an integral part of the sensor 1 , there are no electrical connection points at the sensor: when the sensor is located on a vehicle bumper, this eliminates the risk of the sensor system being damaged through exposure of such a connection point to the weather or to objects thrown up from the road.

The capacitive sensor 1 can be easily attached, for example by an adhesive, to a suitable supporting surface and is thus easy to install in a location such as the interior of the bumper of a vehicle. The installation is further assisted by the flexibility of the substrate 2 on which the sensor is formed, which facilitates its attachment to curved as well as flat surfaces. It will be understood that the rectangular shape of the main portion 4 of the sensor 1 is an example only, and that a sensor of the type described with reference to Figs. 1 to 3 can be cut to any suitable shape, for example by die-cutting, punching, or laser cutting, and can also be processed to have a contoured shape corresponding to shape of the surface on which it is intended to be mounted. It is also possible to incorporate features such as cuts and darts in the substrate 2 so that the sensor can be attached to a three- dimensionally curved surface, such as the inner surface of a vehicle bumper, without forming undesirable creases.

Suitable materials for the substrate layer 2 of the sensor 1 include, for example, polymeric films and layers, paper films and layers, layers of non-wovens, laminates (such as, for example, polyacrylate foams laminated on both sides with polyolefin films, and papers laminated or jig- welded with polyethylene terephthalate) and combinations thereof. Useful polymeric films and layers include, for example, polyolefin polymers, monoaxially oriented polypropylene (MOPP), biaxially oriented polypropylene (BOPP), simultaneously biaxially oriented polypropylene (SBOPP), polyethylene, copolymers of polypropylene and polyethylene, polyvinylchloride, copolymers having a predominant olefin monomer which

may be optionally chlorinated or fluorinated, polyester polymers, polycarbonate polymers, polymethacrylate polymers, cellulose acetate, polyester (e. g. biaxially oriented polyethylene terephthalate), vinyl acetates, and combinations thereof. Useful substrate materials may be subjected to an appropriate surface modification technique including, for example, plasma discharge techniques including corona discharge treatment and flame treatment, mechanical roughening and chemical primers.

The conductive strips 6a, 6b, 6c of the sensor conductor 6, and the extension 9 of the sensor conductor on the tongue portion 5, may be formed from any suitable electrically- conductive material, for example copper, and may be applied to the substrate 2 by an adhesive. As an alternative, the sensor conductor 6 may be formed by vapour deposition of a suitable metal onto the substrate 2 or by printing/die coating electrically-conductive ink onto the substrate, or it may be formed from a foil that is bonded to the substrate. As yet a further alternative, the sensor conductor 6 may be formed by removing zones of material from an electrically-conductive layer on the substrate 2, as described in our European patent application No. 06001155.8 of 19 January 2006. The sensor conductor 6 may assume a variety of shapes, although a discontinuous arrangement of conductive areas, such as the arrangement of conductive strips described above, exhibits an especially advantageous sensitivity and may be preferred.

The thickness of the sensor conductor 6 (i.e. its height above the substrate 2) may vary widely depending on the method by which it is manufactured. A sensor conductor comprising flattened metal strips will have a thickness of typically between 20 and 200 micrometers (μm) and, in some embodiments, between 25 and 100 μm. A sensor conductor obtained by vacuum metal vapour deposition may be as thin as 200 — 800

Angstroms (A) and, in some embodiments, 300 — 500 A. When using an aluminum foil for the sensor conductor, it may have a thickness of from 1 — 100 μm, in some embodiments, 2 — 50 μm and, in some embodiments, 3 — 30 μm.

The superguard conductor 7 and its extension 11 may be formed from any suitable electrically-conductive material in any of the ways described above for the sensor

conductor 6 and its extension 9, and will have a similar resulting thickness. The superguard conductor 7 is not an essential component of the sensor 1 but, if present, may assume a variety of shapes and, in automotive applications, may be arranged (relative to the road level) above or below the sensor conductor 6.

The guard conductor 8 and its extension 8' on the tongue portion 5 may be formed from any suitable electrically-conductive material, for example aluminium. They may be formed, for example, by adhesively-bonding a metal foil to the substrate 2 or by applying a metallic layer directly to the substrate, for example by vacuum metal vapour depositioa

The thickness of the guard conductor 8 and its extension 8' may vary widely depending on the method by which they are formed on the substrate 2. A metallic layer obtained by vacuum vapour deposition may be as thin as 200 — 800 A and, in some embodiments, 300 — 500 A. A metal foil, on the other hand, may have a thickness of from 1 — 100 μm, in some embodiments, 2 — 50 μm and, in some embodiments, 3 — 30 μm.

The electrical shield 13 for the conductive extension strip 9 on the tongue portion 5 may be formed from any suitable electrically-conductive material. It may, for example, comprise a metallic foil that is adhered to the tongue portion by an adhesive. That adhesive may provide the dielectric material 14, or an extra layer of dielectric material may be provided.

The protective cover film 15 that encases the sensor 1 is a polymeric film that is applied to the sensor by, for example, an adhesive or heat-lamination. The dimensions of the film generally exceed those of the substrate 2 to provide a border that will form an edge seal around the sensor 1 to protect, in particular, the edges of the guard conductor 8 and its extension 8' against corrosion. The border may have a width of 1 — 50 mm, in some embodiments, 1 —40 mm and, in some embodiments, 2 — 20 mm.

Figs. 4 and 5 show an alternative layout for the extensions of the sensor and superguard connectors 6, 7 onto the tongue portion 5 of the substrate 2. In this case, the tongue portion 5 is shown extending in the opposite direction from the long edge 4a of the main portion 4

of the substrate 2 but it will be apparent from the following description that this is not essential. In this layout, the extension of the sensor conductor 6 onto the tongue portion 5 is by means of a conductive strip 20 that is a continuation of one of the strips 6c that connects together the various strips 6a, 6b of the sensor conductor. The strip 20 extends onto the tongue portion 5 through a break 21 in the superguard conductor 7, and then extends along the length of the tongue portion to a terminal 22. As a consequence of the break 21 in the superguard conductor 7, two conductive strips 23 (one on each side of the break 21) are required to extend from the superguard conductor 7 onto the tongue portion 5 and then along the length of the latter, parallel to the conductive strip 20, to respective terminals 24. On the opposite surface of the substrate layer 2 (not visible in Fig. 4), the guard conductor 8 has the same configuration as in Figs. 1 to 3, and extends similarly onto the tongue portion 5.

An electrical shield is provided for the extension strip 20 of the sensor conductor 6 by laminating a strip 24 of conductive material over the extension strip, with an intervening strip 25 of dielectric material. In this case, the conductive and dielectric strips 24, 25 also cover the extension strips 23 of the superguard conductor 7 but that is not essential. The entire sensor 1, including the tongue portion 5, is then encased in a protective cover film (not shown) as described above with reference to Fig. 3. It will be understood that the apparent gaps between the strips 24, 25 and the remainder of the tongue portion 5 in Fig. 5 are a result of the exaggerated dimensions of the underlying conductive strips 20, 23 and would not exist in practice.

The layout shown in Figs. 4 and 5 allows a greater degree of choice in the positioning of the tongue 5 on the sensor, and also enables a more compact arrangement to be obtained if desired.

Figs. 6 and 7 show another modification of the sensor shown in Figs. 1 and 2, in which the extension of the sensor conductor 6 onto the tongue portion 5 is by means of a conductive strip 30 that is again a continuation of one of the strips 6c that connects together the various strips 6a, 6b of the sensor conductor. In this case, however, to avoid the need for a

break in the superguard conductor 7 as in Fig. 4, the strip 6c is reoriented so that it extends at a different angle across the array of parallel conductive strips 6a, 6b, enabling the strip 30 to pass around one end of the sensor conductor 7. The remaining components of Figs. 6 and 7 are as described above with reference to Figs. 1 and 2, and carry the same reference numerals. Again, the electrical shield 13, 14 could be extended over the extension strip 11 of the superguard conductor, but that is not essential.

Alternative methods of forming the tongue portion 5 into the equivalent of a coaxial cable are illustrated in Figs. 8 and 9. These alternative methods eliminate the need to apply the separate shielding and dielectric layers 13,14 (Figs. 2 and 7) and 24,25 (Fig. 5). In both methods, the substrate layer 2 of the tongue portion 5, with the attached extension 8' of the guard conductor 8, are extended outwards on the side of the tongue portion remote from the main body 4 of the sensor. When the conductive strips (9,11 ; 20;23; or 30,1 1) are in position on the tongue portion, the extended portion of the substrate layer (with the attached extension 8' of the guard conductor) is folded around over the strips to enclose and shield them. Fig. 8 shows a single fold 35 being used, which has the effect of to bringing together the edges of the tongue portion 5, while Fig. 9 shows the tongue portion being wrapped-around further, entailing an additional fold 36. Figs. 8 and 9 show three conductive strips on the tongue portion 5, as in the sensor 4 of Figs. 4 and 5, but the arrangement is equally applicable to the sensors of Figs 1 to 3, and 6 and 7. Again, it will be understood that the apparent gaps between the folded parts of the tongue portion 5 in Figs. 8 and 9 are a result of the exaggerated dimensions of the conductive strips 20, 23 and would not exist in practice.

Figs. 10 to 12 show another proximity sensor 40 comprising a sensor conductor 6, a superguard conductor 7 and a guard conductor 8 located on a substrate layer 2 in a similar configuration to that of the sensor 1 of Figs. 1 to 3. The substrate 2 of the sensor 40 likewise comprises a tongue portion to facilitate the connection of the sensor, guard and superguard conductors 6, 7, 8 to a control unit but, in this case, the tongue portion (indicated by the reference 41) is located within the main portion 4 of the substrate, in the area between the superguard conductor 7 and the sensor conductor 6. The tongue portion

41 is rectangular in shape and is formed by a cut 42, into the substrate and guard conductor layers 2, 8, that extends around two long sides and one short side of a rectangle, leaving the tongue portion attached at the other short side of the rectangle.

A conductive strip 43 (which is a continuation of one of the strips 6c that connects together the various strips 6a, 6b of the sensor conductor 6) extends onto the tongue portion 41 at the attached end of the latter and then along the length of the tongue portion to a terminal 44 adjacent its free end. Similarly, a conductive strip 45 extends from a convenient point on the superguard conductor 7 onto the tongue portion 41 and then along the length of the latter, parallel to the conductive strip 43, to a terminal 46.

The entire sensor 40 is encased in a protective film 15, as described above with reference to Fig. 3, except for the surface of the tongue portion 41. One way in which that can be achieved is as follows. Prior to the application of the protective film 15 and the formation of the cut 42 that defines the tongue portion 41 , a piece of siliconized paper or film in the intended size and shape of the tongue portion, is applied over the terminals 44, 46 in the intended location of the tongue portion. A similar piece of siliconized paper or film is applied to the guard conductor 8 in a similar position. The protective film 15 is then applied, and the cut 42 is made through the laminate, following which the pieces of siliconized paper (to which the protective film 15 does not adhere) can be removed, leaving the terminals 44, 46 exposed on one side of the tongue portion 41 and an area of the guard conductor 8 exposed on the other. If the cut 42 is made from the guard conductor side of the laminate, it does not need to extend through the protective film layer 15 on the other side of the laminate because the pieces of siliconized paper can be removed from both sides of the tongue portion 41 by lifting the latter up on the guard conductor side of the laminate.

It will be understood that the apparent gaps between the upper cover film 15 and the remainder of the sensor in Fig.l 1 are a result of the exaggerated dimensions of the underlying conductive strips 6a, 6b etc. and would not exist in practice.

In use of the sensor 40, the terminals 44, 46 on the tongue portion 41 are connected directly to an electronic control unit that is located in close proximity to the sensor. That is illustrated, by way of example, in Fig. 12 which shows the sensor 40 as it would be used on the bumper of a vehicle. The tongue portion 41 is lifted up on the guard conductor side of the sensor and the electronic control unit 50 is placed underneath it so that the terminals

44, 46 are connected into their respective circuits in the control unit. Conductive adhesive pads 55 (of which only one is shown in Fig. 12) can be used, if required, to make the electrical connection between the terminals 44, 46 and the control unit 50, and an external spring or pressure plate 51 can be positioned to act on the tongue portion 41 to maintain its position. Finally, an electrical connection is established between the exposed guard connector 8 on the tongue portion 41 and the control unit 50, for example by means of a conductive metal foil tab 56 extending between the two and secured in position by an electrically-conductive adhesive 57. A sealed enclosure, indicated by the dotted line 52 if Fig. 12, can be formed around the control unit 50 and tongue portion 41 of the sensor if required, and the sensor is then ready to be attached, at its opposite surface, to the inside of a vehicle bumper by a layer of adhesive 53. It will be understood that the comments made above with reference to Fig. 11 regarding the apparent gaps between the upper cover film 15 and the remainder of the sensor apply here as well.

The configuration illustrated in Fig. 12, in which the electronic control unit 50 for the sensor is actually located adjacent the sensor eliminates the need for a screened cable to carry electrical signals from the sensor to the control unit. In the case in which, in automotive applications, the sensor and electronic control unit are located on the bumper of a vehicle, this means that only conventional electrical wiring is required to convey the output of the control unit 50 to visual and/or audible warning devices inside the vehicle. If the sensors of Figs. 1, 4 and 6 are employed in a similar situation (i.e. one in which the electronic control unit is adjacent the sensor) the electrical shield 13, 24 on the tongue portion 5 can be omitted.

The sensors described above with reference to the drawings can be manufactured by a method as described in our European patent application No. 06001155.8 of 19 January

2006 entitled "Capacitive sensor film and method for manufacturing the same", in which the sensor conductor 6 arid the superguard conductor 7 (when present) are at least partly surrounded by a front conductor on the same major surface of the substrate layer 2, being electrically isolated against the front conductor by zones where the front conductor is removed (for example, by laser ablation). When that method is applied to sensors of the type described above, it will be appreciated that the front conductor may extend onto the tongue portion 5, 41 of the substrate layer 2, where it will at least partly surround the conductive extension strips 9,11; 20,23; 30,11 while being electrically-isolated therefrom by zones where the front conductor is removed. The front conductor may be electrically- connected to the guard conductor 8 on the other major surface of the substrate layer 2, and thereby contribute to the electrical shielding effect of the guard conductor. Alternatively, in the case of sensors of the type shown in Figs. 1 , 4 and 6, the guard conductor 8 on the main part 4 of the sensor can be omitted whereby the electrical shielding of the sensor conductor 6 is provided only by the front conductor (as described, for example, with reference to Fig. 7 of our International patent application claiming priority from the above-mentioned

European patent application No. 06001155.8 [attorney reference 61263]). In that case, if the front conductor extends onto the tongue portion 5 of the substrate layer, the tongue portion may have the configuration illustrated in the cross-sectional view of Fig. 13, in which the extension of the front conductor is indicated by the reference numeral 60. Fig. 13 illustrates a tongue portion 5 having two extension strips 9, 1.1 only (one (9) from the sensor conductor 6 and the other (11) from the superguard conductor), each of which is defined within, and electrically isolated from, the surrounding front conductor 60 by zones 61 where the front conductor has been removed e.g. by laser ablation. The tongue portion is completed by the provision of an electrical shield for the conductive strip 9, comprising a layer 13 of conductive material laminated over the strip with an intervening layer of dielectric material 14 (as in Fig. 2) and a corresponding layer 8" of conductive material on the opposite side of the substrate layer 2. The electrical shield layers 8", 13, 14 could extend over the second conductive strip 11 also, but that is not essential. The tongue portion 5 of Fig. 13 may be encased in a protective film (not shown) as described above.

Sensors of the type shown in Figs. 1 to 9 are also described and claimed in our co-pending International patent application of even date entitled "Proximity sensor with an edge connection, and method for manufacturing the same" (attorney docket No. 61219WO00X).

The capacitive proximity sensors described above with reference to the drawings can be easily installed and, because they are flexible, they can be applied to shaped substrates having, for example, curved surfaces. It is particularly advantageous that the capacitive sensors can be electrically contacted in an easy and reliable way. In view of these advantages the sensors are especially suited for use in the automotive industry.

It will be understood that the particular configurations shown in the drawings for the sensor and guard conductors and the optional superguard conductor are for the purposes of illustration only and are not an essential feature of the invention. The proximity sensors described herein with reference to the drawings are particularly appropriate for use on vehicle bumpers but the manner in which electrical connection is made from the sensor and guard conductors (and, when present, the superguard conductor) is applicable to capacitive proximity sensors intended for use in other applications and to capacitive proximity sensors with differently-configured conductors including, for example, those with a sensor conductor of serpentine or spiral form or with two interdigitated sensor conductors, or with a multiplicity of guard conductors.