GB2400666A

GB2400666A - Capacitive proximity sensor

Info

Publication number: GB2400666A
Application number: GB0307130A
Authority: GB
Inventors: David Snell; Anthony Moon; Ceri Clatworthy; Larry Jones
Original assignee: AB Automotive Electronics Ltd
Current assignee: AB Automotive Electronics Ltd
Priority date: 2003-03-27
Filing date: 2003-03-27
Publication date: 2004-10-20
Anticipated expiration: 2023-03-27
Also published as: GB0307130D0; GB2400666B

Abstract

A capacitive sensor for attachment to a vehicle for sensing proximity to objects comprises a substrate (21) with a sensor conductor (22) which is shorter than a guard conductor (23). The sensor is formed by folding the ends (26, 28) of the substrate over onto the sensor face such that the guard layer overlies the sensor conductor. Further, a sensor may be formed where conductive portions are adhered to the substrate on the sensor face (24) to overlay the sensor conductor which results in regions of reduced sensor sensitivity. A retention system is also disclosed for retaining a sensor against a vehicle body without the need for glues, this utilises a flexible retaining member curved to apply force to the sensor to retain it against a vehicle part.

Description

Capacitive Sensor and Method of Manufacture thereof The present invention

relates to the field of capacitive sensors in particular capacitive sensors for attachment to a vehicle for sensing the proximity of the vehicle to other objects when manoeuvring the vehicle. Further, the present invention relates to methods of manufacturing capacitive sensors, and methods for fitting such sensors to vehicles.

Capacitive sensors have been used in parts of cars for collision avoidance purposes, and in recent years a number of luxury cars have been fitted with sensors particularly on the rear of the vehicle to warn the driver of objects. in operation when the vehicle is being reversed, a collision with unseen or obscured objects, such as walls or bollards, can be avoided whilst still being able to position the vehicle close to such objects.

A capacitive sensor typically consists of two strips of metal, or other conductive material, insulated from each other and provided inside the bumper of a vehicle. The two strips of metal form a guard plate and the sensor plate. FIGURES 1A and 1B illustrate front and back views of an example of such a guard and sensor plate arrangement.

The plate which is outermost relative to the vehicle is called the sensor plate or sensor conductor and the plate which is innermost i.e. closest to the vehicle itself, is the called the guard plate or guard conductor. The guard plate acts as a shield to reduce the sensitivity of the device to anything behind the sensor plate. These plates are preferably elongate strips adapted for attachment to a vehicle bumper.

In use, the guard and sensor plates are connected to a control unit which supplies high frequency signals to the sensor and guard plates. Objects in the vicinity of the vehicle present a capacitance to ground. In fact this capacitance is formed by two capacitances in series, that is: the capacitance between the sensor plate and the object (or objects) being sensed; and the capacitance between the object and ground. The control unit monitors the capacitance between the sensor plate and ground. As the distance between the vehicle (and hence the sensor) and an object changes, the measured capacitance changes. The output from the control unit provides an indication of the distance between any objects in the vicinity of the vehicle and the sensor plate, and hence the rear of the vehicle. The control unit senses the change in capacitance and uses this to provide an indication ofthe distance to the object.

FIGURE IA illustrates the back of such a sensor and guard plate combination. The guard plate l l faces the vehicle so as to shield the sensor plate (not visible) to ensure it only detects changes in the outward direction (i.e. away from the vehicle). The guard plate 11 is formed on a plastic film 12, which is only visible in FIGURE 1A as a border around the guard plate. It is however to be appreciated that in some sensor arrangements, the guard plate wholly covers the plastic film surface so that the plastic film is not visible from the back view of the sensor and guard plate arrangement.

FIGURE 1 B illustrates the front of the sensor and guard plate combination. The sensor plate 13 is screen printed with conductive ink onto the sheet of plastic film 12. The sensor plate 13 is of irregular shape in that it has two lobes (14, 15) of increased width at either end of the sensor plate. This provides increased sensitivity at the edges of the vehicle. With a uniform strip the sensitivity would be typically reduced at the edges because obstructions can only couple capacitively with one side of the sensor, whereas an object directly opposite the middle of the sensor would couple capacitively with both sides of the sensor.

An optional feature is also shown in FIGURE 1B, being an extra conductive strip 16 which can be positioned above or below the sensor or both. This conductive surface carries an amplified guard signal and has the effect of making the guard appear bigger.

It is therefore effective in pulling the effect of drips of water running across the front of the sensor. This feature is described in greater detail in British patent application number 0129765 and will not be described in any further detail here.

To date to manufacture such a sensor and guard plate arrangement, the plates are created by screen-printing conductive ink on both sides of the sheet of plastic film 12.

The ink is protected by subsequently applying a dielectric coating. Screen-printing offers the advantage that the areas to be covered with the sensor and guard plates can be controlled accurately, and various shapes of sensor and guard plates can be accommodated. However, screen- printing does have the disadvantages of being expensive, especially if silver-based inks have to be used to make the electrical resistance of the conductive layers low enough.

There is therefore a need to be able to create a sensor and guard plate arrangement at a lower cost.

There is also a need for a simplified process of creating a sensor and guard plate arrangement.

Further, where the sensor plate is an irregular shape, such as with the two lobes as shown in FIGURE I B. costly specialized equipment is required to create the irregularly shaped sensor plate as a unitary structure.

As it is desirable to minimise manufacturing costs as much as possible, there is therefore also a need for a cheaper and simplified process for creating a sensor plate and/or arranging it on a sensor substrate.

A further problem is the presence and movement of water in the detection area of the sensor. The presence of water such as that caused by steady rain tends to provide a fairly continuous effect on the sensor and can be compensated to a degree. However, where water collects and is then released suddenly, the sensor sees a sudden change that can lead to false triggering. For example, as rain falls onto the vehicle itself, the water runs down the back of the vehicle in drips that grow in size as they collect more water.

These drips may then roll down the metal bodywork or the car and across the outer skin of the bumper as a trickle or rivulet of water. As water is a conductor, the trickle behaves as a piece of wire.

As the trickle travels past the sensor, the sensor registers an increase in capacitance between the sensor plate and ground. This is due to capacitive coupling between the sensor and trickle of water in series with the capacitive coupling between the trickle and the car body. Where the trickle is very close to the sensor plate 1, the effect is quite significant even though the drip itself maybe quite small. This may have an effect on the sensor similar to a much larger or more conductive object that is much further away e.g. an iron bollard behind the vehicle. In addition, the sudden change of the measured capacitance as the drip travels rapidly from the top to the bottom of the bumper may also mislead the sensor into believing that there is an larger object close to or approaching the bumper and signal a false alarm as a result.

This problem was also addressed in British Patent Application Number 0129765, which discloses a guard plate extending over the entire interior surface of a vehicle bumper and the sensor between the guard plate and the bumper. The guard plate has a similar voltage to the sensor plate, so the effect of water as it couples to the sensor plate is reduced. However, while this arrangement does reasonably counteract the effects in most situations, there are still circumstances when it is not wholly effective. For instance, if a small pool of water has gathered somewhere high up on the vehicle and the vehicle motion causes the pool to be displaced, the water will run down the vehicle and over the bumper briefly forming a rivulet which then flows until the displaced water is exhausted. Such rivulets are particularly problematic since they amount to larger concentrations of water. Hence their effects can further exacerbate the size of a false measurement.

In British Patent Application Number 0129765, an alternative approach was disclosed whereby water was deflected from the tailgate so as to reduce the amount of water flowing over the bumper and therefore the sensor. This technique, however, can be difficult to implement on some vehicles, as drastic styling changes may be required.

There is therefore a need for an alternative approach for reducing the adverse effects of water in the detection area of the sensor.

Another problem that exists with capacitive sensors is that effectively and efficiently attaching them to vehicles. At present the sensors are attached to vehicle bumpers using glue. While this approach satisfactorily effects the connection, it is not ideal for recycling. Vehicle manufacturers and repairers like to recycle as much of a vehicle as possible, once it has been taken off the road. Where capacitive sensors are glued to the bumpers, recycling of the whole bumper is generally not appropriate. Also, it is not possible to replace the sensor alone without replacing the whole bumper.

There is therefore a need for an alternative approach for attaching capacitive sensors to vehicles.

According to a first aspect, the present invention provides a process of forming a sensor for use in a capacitive sensor system, comprising: attaching a strip of sensor conductor to a first surface of a length of substrate; attaching a guard conductor to a second surface of the length of substrate such that the guard conductor at least substantially covers the second surface; detaching a section of the substrate from the length of substrate to form the sensor, such that the sensor conductor extends from a first edge of the sensor to a second opposite edge; and decreasing the length of the sensor conductor by folding the first edge and the second edge of the sensor towards the first surface, so that sections of the guard conductor on the folded ends shield the underlying sensor conductor.

Further according to this first aspect of the invention, there is provided a process of forming a sensor for use in a capacitive sensor system, comprising: obtaining a substrate having a sensor conductor on a first surface of the substrate, the sensor conductor being elongate and extending from a first edge of the substrate to a second opposite edge and a guard conductor on a second surface of the substrate, the guard conductor at least substantially covering the second surface; and decreasing the length of the sensor conductor by folding the first edge and the second edge of the sensor towards the first surface, so that sections of the guard conductor on the folded ends shield the underlying sensor conductor.

Still further according to this first aspect of the invention, there is provided a sensor for use in a capacitive sensor system comprising: a substrate; a sensor conductor on a first surface of the substrate, the sensor conductor being elongate and extending from a first end to an opposite second end of the substrate; a guard conductor on a second opposite surface of the substrate, and with the first and second ends of the substrate folded onto the first surface such that sections of the guard conductor on the folded ends shield the underlying sensor conductor.

As the folded ends each include a portion of the guard conductor on their second side, by folding the ends, the portions of the guard conductor on the second side of the folded portion is brought to overlie the sensor conductor. By placing the guard conductor over the sensor conductor, the effect of the sensor conductor is cancelled out, so that the length of the sensor conductor is effectively reduced. It is preferable for the guard conductor to be longer than the sensor conductor, as this increases the sensitivity of the sensor conductor.

This first aspect of the invention provides an efficient technique for creating capacitive sensors that is suitable for a continuous manufacturing process.

According to a second aspect the present invention provides a process of forming a sensor for use in a capacitive sensor system, comprising: obtaining a portion of substrate having a sensor conductor on a first surface of the substrate; and attaching a conductive portion to the first surface of the substrate so that the conductive portion overlays a portion of the sensor conductor.

Preferably the conductive portion is attached to the sensor conductor using an adhesive.

The process of this second aspect may further comprise attaching a second conductive portion to the first surface of the substrate which overlays a portion of the sensor conductor and wherein the sensor conductor is elongate and the two conductive portions are positioned at or towards opposite ends of the elongate sensor conductor.

Further, according to this second aspect of the invention, there is provided a sensor for use in a capacitive sensor system comprising: a substrate; a sensor conductor on a first surface of the substrate; at least one conductive portion attached to the first surface of the substrate such that the at least one conductive portion overlays a portion of the sensor conductor.

In arriving at this second aspect of the invention, it has been realised that it is not always essential for the lobes to be unitary and in direct contact with the sensor conductor. It has been realised that it is possible to attach the lobes to the sensor conductor, such as by using glue, without adversely affecting the capacitive coupling between the lobes and the sensor conductor. The glue or other attachment means between the lobes and the sensor conductor will not adversely affect the capacitive coupling between the two so long as the adhesive separating the stick-on lobe and the sensor conductor is thin compared to the thickness of the substrate separating the sensor conductor and the guard conductor.

This aspect of the invention allows the sensor conductor of capacitive sensors to be created more simply and with greater flexibility in terms of the design of the sensor conductor.

The present invention also provides a process of forming a sensor for use in a capacitive sensor system, comprising: obtaining a portion of sensor substrate having a sensor conductor on a first surface of the substrate; and attaching a rear surface of an attachment portion to the first surface of the sensor substrate such that the attachment portion includes at least one of: a conductive lobe on the rear surface of the attachment portion such that the conductive lobe is configured to overlay a portion of the sensor conductor and electrically connect therewith; and a guard conductor on a front surface of the attachment portion configured to shield underlying portions of sensor conductor.

Preferably the attachment portion is attached to the substrate using an adhesive. It is also preferably for the guard conductor on the front surface of the attachment portion to be electrically connected to a second guard conductor on a second surface of the sensor substrate.

References in this specification to electrical connections are to be taken to include connection by capacitive coupling.

According to a third aspect the present invention provides a capacitive sensor for mounting to a body, the sensor comprising: a substrate; a sensor conductor on a first surface of the substrate, the first surface configured to face outward from the body; a first guard conductor overlying a portion of the sensor conductor such that the guard conductor shields the underlying region of the sensor conductor and defines a region of reduced sensor sensitivity.

Preferably the sensor further comprises a second guard conductor on the second surface of the substrate, which is electrically connected to the first guard conductor. This may be achieved using a rivet through the sensor substrate.

By utilising localised guard plates in areas where water interference is to be expected, the sensor conductor beneath the localised guard conductor is in effect buried and shielded from capacitive coupling. Therefore, in this region, few, if any, capacitive changes will be detected. Provided the localised guard plates are made as small as possible, this will have minimal effect on the performance of the sensor in detecting actual obstructions, while further reducing the incidence of adverse effects from water interference.

According to a fourth aspect the present invention provides a retention system for retaining a sensor to a vehicle, comprising: a vehicle part having first and second fastening portions; and a retaining member having a first and second end, such that in use, the sensor is positioned between the vehicle part and the retaining member, and secured with force applied by the retention member, which is affixed to the vehicle part by securing the first and second ends of the retaining member at the first and second fastening portions.

Preferably the vehicle part is curved and the retaining member has a corresponding curved profile in a static position sufficient to apply force to the sensor to retain it in place against the vehicle part.

Preferably to effect the curved profile, the retaining member has a plurality of elongate ribs on one surface to strengthen it and control the force applied. It is also preferable that the retaining member is flexible in order to assist in assembling and disassembling the sensor with the vehicle part.

The retention system may further comprise location retention means associated with the sensor, for retaining the sensor in position relative to the retaining member. Preferably the location retention means have ends which are secured through holes in the sensor and retention member.

This aspect of the invention provides a mechanism for attaching a sensor to a bumper whereby the sensor is readily attachable and detachable to the bumper without the need for invasive glues and the like which affect the ability to recycle the bumper as necessary.

These aspects of the present invention also provide a vehicle bumper to which a sensor as described is attached. Further, these aspects of the present invention provide a vehicle having a sensor as described attached thereto.

These aspects of the present invention will now be described with reference to the accompanying drawings in which: FIGURE IA illustrates the back view of a known sensor and guard plate arrangement for a parking aid sensor; FIGURE 1B illustrates the front view of the sensor and guard plate arrangement of FIGURE IA; FIGURE 2A illustrates a continuous substrate utilised in a technique according to a first aspect of the present invention in creating a sensor and guard plate arrangement; FIGURE 2B illustrates a cross-section along XY of the substrate in FIGURE 2A; FIGURE 3 illustrates an exploded view of a sensor with a sensor conductor having separately attachable lobes according to a second aspect of the present invention; FIGURE 4A illustrates a plan view of a sensor according to the second aspect of the present invention; FIGURE 4B illustrates a cross-section along XY of the substrate in FIGURE 4A; FIGURE 5 illustrates a sensor arrangement according to a third aspect of the present invention as positioned on a vehicle; FIGURE 6 illustrates a section through a sensor arrangement according to the third aspect of the present invention; FIGURE 7 illustrates an exploded view of a sensor arrangement, which combines the second and third aspects of the invention; FIGURE 8A illustrates a top view of an attachable portion, according to an embodiment of the invention, for use in the sensor of FIGURE 7; FIGURE 8B illustrates a bottom view of the attachable portion shown in FIGURE 8A; FIGURE 9A illustrates a plan view of a capacitive sensor that may be used in a capacitive sensor retention system according to a fourth aspect of the present invention; FIGURE 9B illustrates a side view of the capacitive sensor of FIGURE 9A; FIGURE 9C illustrates cross-sections of three different capacitive sensors that may be used in a capacitive sensor retention system according to the fourth aspect of the present invention, such that each capacitive sensor has different sensor conductor configurations; FIGURE 10 illustrates a plan view of a capacitive sensor secured to a vehicle bumper in accordance with the fourth aspect of the present invention; FIGURE 11 illustrates a cross- section of the capacitive sensor and bumper combination of FIGURE 10.

FIGURES 12A and 12B illustrate a method of attaching a first retention device to a vehicle bumper in order to secure the capacitive sensor in place, in accordance with the fourth aspect of the present invention; FIGURES 1 3A and 1 3B illustrate a method of attaching a second retention device to a vehicle bumper in order to secure the capacitive sensor in place in accordance with the fourth aspect of the present invention.

With reference to FIGURE 2A a sheet of substrate 21 is illustrated, which is preferably a plastic film such as polyester. The substrate may be any width. In FIGURE 2A, the substrate width is illustrated as approximately 1 00mm. On the front surface of the substrate a thin strip of conductive material 22 has been affixed, such as by lamination.

In the illustrated embodiment the width of the strip is approximately 20mm wide. Any conductive material may be utilised for the conductive strip 22 including conductive inks, although preferably a relatively cheap material is used, such as aluminium foil.

This strip 22 forms at least part of the sensor conductor.

A wider conductive strip 23 is affixed, such as by lamination, on the back surface of the substrate 21 and preferably covers the whole width of the substrate, or substantially the whole width. This strip 23 is visible in the cross-sectional illustration in FIGURE 2B and is also preferably a relatively cheap material such as aluminium foil.

The substrate and conductor combination can be manufactured in a continuous process by laminating the conductive strips to the substrate, such as by gluing, and forming a roll. Sensor and guard plate combinations can then be formed by cutting lengths from the roll as required.

The sensor and guard plate combination that is formed by cutting lengths from the roll have the sensor with the same length as the guard conductor. That is, each of the conductors will extend the edges of the length of substrate. In FIGURE 2A, the sensor conductor 22 is shown extending to the edge 26. On the opposing side, the guard conductor 23 will similarly extend to edge 26, although this is not visible in FIGURE 2A.

It is, however, preferable for the guard conductor to be longer than the sensor conductor, as this increases the sensitivity of the sensor conductor. This is because the guard plate 23 acts as a shield to reduce the sensitivity of the sensor device to anything behind the sensor plate 22. It is desirable for the sensor to operate by detecting objects that are generally outward of the vehicle, and to make the sensor effectively blind on the vehicle side of the sensor plate. This however will not be achieved at ends 26 and 27 where the strips of sensor and guard conductors 22, 23 each abut the end edges 26, 27 of the substrate. Therefore at these points the guard plate will not wholly shield the sensor plate.

To address this problem, the present invention provides an approach whereby the ends 26, 27 of the sensor device are folded, so that the folded ends abut the front surface of the substrate and cover a portion of the sensor conductor 22. The technique is illustrated for one end in FIGURE 2A, where end 27 has been folded over so as to abut the front surface of the substrate and partially overlie the sensor conductor 22. This has the effect of shortening the length of the sensor conductor 22 and creating a boundary between the new end of the sensor conductor at point 28 and the new end of the substrate at point 29.

A shorter sensor conductor 22 results, because when the fold is effected, the guard conductor on the rear of the folded portion is brought over to the front of sensor device.

This portion of the guard conductor then overlies a part of the sensor conductor and cancels the effect of the sensor conductor beneath it. In other words, the guard conductor that overlies the sensor conductor has the effect of masking the sensor conductor beneath it. This approach increases the sensitivity of the sensor in a simple manner, as the sensor conductor on the front surface of the device has a shorter length than the guard conductor 23 on the rear surface, which still extends to the edges of the device.

In an alternative embodiment of this first aspect of the invention, it would also be possible to affix a separate guard conductor over the top of the sensor conductor at the edges 26, 27. This would have the same effect as the folding embodiment just described, but be less preferable, as the technique would be more complex to achieve, as electrical connections between the guard conductor 23 and the separately affixed guard conductors would need to be effected.

A second aspect of the present invention is illustrated in FIGURE 3, which is directed towards forming a sensor conductor 22 of an irregular shape, such as with two lobes towards the ends of the sensor. In arriving at this aspect of the invention, it has been realised that it is not always essential for the lobes to be in direct contact with the main body of the sensor conductor. It has been realised that it is possible to attach additional portions, such as the lobes 31, 32, to the sensor conductor body 22, without adversely affecting the capacitive coupling between the lobes and the sensor conductor. It has been found that an adhesive layer, such as glue, separating the lobes 31, 32 from direct contact with the sensor conductor body 22 will not adversely affect the capacitive coupling between the two so long as the adhesive separating the stick-on lobe 31, 32 and the sensor conductor 22 is thin compared to the thickness of the substrate 21 separating the sensor conductor and the guard conductor.

For example, for a substrate of 250 microns thickness, it has been found that the capacitive coupling will not be adversely affected if the adhesive layer between the lobe and the conductor body is approximately 50 microns. As an approximate maximum bound, the adhesive layer could be of the order of 100 microns thick before adverse effects materialize.

Therefore, according to the second aspect of the present invention, a capacitive sensor can be manufactured by attaching separate lobes (31, 32) onto a sensor conductor 22 on a substrate 21, as shown in FIGURE 3.

In FIGURE 4A a sensor, with the separate lobes 31, 32 attached, is illustrated. The lobes are small conductive sheets that are secured on the top of the sensor conductor 22 with an adhesive, so that there is a small gap between the conductive lobe and the sensor conductor.

FIGURE 4B shows a cross section of FIGURE 4A along the line XY, which is not to scale. In FIGURE 4B the substrate 21 of the sensor is shown with the guard conductor 23 attached to its rear side and the sensor conductor 22 attached to the front side of the substrate 21. The front side of the substrate is intended to face away from the vehicle.

The lobe 31 is made up of two components, a lobe conductor layer 42 associated with a lobe substrate layer 43. The lobe 42, 43 is attached to the sensor conductor 22 using an adhesive 41, with the lobe conductor layer 42 abutting the adhesive 41. Typically the lobe conductor 42 is aluminium foil.

A protective coating 44 covers the lobe 42, 43. Similarly a protective coating 45 covers the guard conductor 23 on the rear side of the sensor. It is to be appreciated that the use of protective coatings 44 and 45 are preferred features.

In FIGURES 4A and 4B, the lobe is shown so that it overlays the whole width of the sensor conductor 22. This however is not essential to the invention, and it is adequate for the lobe to only partially overlay the sensor conductor 22. It is also to be appreciated that the structure of the lobe is not critical to the invention, so other arrangements andshapes of conductive materials are possible. Further, it is not essential that the sensor be manufactured with two lobes, each positioned towards opposite ends of the elongate sensor, as illustrated in FIGURE 3. This is a sensor shape that has been found to provide improved sensitivity on most generic vehicles. It is however possible to tailor the shape of the sensor conductor to the shape of a particular vehicle, which may require one or more lobes to be attached to different regions of the sensor conductor.

A third aspect of the present invention is illustrated in FIGURE 5. The rear of a vehicle is shown, with an attached sensor 50. The sensor 50 is illustrated in a ghosted outline since it is located behind the bumper or trim 51 of the vehicle. The ghosted outline of the sensor 50 comprises the substrate 21 and the sensor conductor 22 with two lobes, one lobe at each end of the sensor conductor 22. In this aspect of the invention, localised guard plates 52, 53 cover particular regions of the sensor conductor 22. For the illustrated vehicle, a typical hatchback car, the guard plates 52, 53 have been positioned between the tailgate and the lamp cluster, in the region where most trickles of water are expected to flow past the sensor conductor 22, as shown by arrow 54.

With reference to FIGURE 6 a cross section of the sensor in FIGURE 5 is shown in the region where it is covered by the guard sensor. The sensor substrate 21 has the guard conductor 23 on the rear surface and the sensor conductor 22 on the front surface, such that the front surface faces out wards from the vehicle. The guard conductor 23 has a protective coating 45 as does the sensor conductor, being protective coat 61. Atop the sensor conductor 22 and its protective coat 61 is an insulative cover or substrate 62.

This insulative cover preferably has a greater width than the sensor conductor so as to fully encompass it. Where an insulative cover is applied, the sensor protective coat is not necessarily required. It does, however, provide additional protection, such as against stones thrown up by a vehicle at high speed.

Atop the insulative cover 62 is an additional guard conductor 63. It is this guard conductor 63 which acts as a shield to prevent the sensor conductor 22 detecting changes in the region covered by the guard conductor 63.

The effectiveness of this aspect of the invention can be further improved by designing the vehicle shape to concentrate water now off the back of the vehicle in the particular regions where the guard conductors are located. This could also assist in minimising the size of the guard conductor 63.

In FIGURE 6, the top guard conductor 63 is electrically connected to the lower guard conductor 23. This connection may be effected by any suitable means, such as using a rivet 64. This enables the top guard conductor 63 to have the same voltage as the lower guard conductor 23 without requiring an additional voltage source. This connection between the two guard conductors is not essential, but it is preferred, as otherwise separate connections would be required, one to each guard. The guard conductors can be made from any conductive material, including conductive inks.

Protective coatings may be applied over the top guard 65 and over the top and bottom exposed surfaces of the rivet 66, 67.

A sensor embodiment is illustrated in FIGURE 7, which combines the principles of this third aspect of the present invention, with the second aspect of the invention, relating to attachable lobes. A substrate 21 is shown with a strip of conductive material 22 laminated thereon. Towards each end of the elongated substrate two attachment portions 71, 72 are positioned. In this embodiment, portion 72 is a mirror image construction of portion 71.

To illustrate the construction of the attachment portion 71, FIGURES 8A and 8B illustrate the top and bottom views respectively. Referring to the top view shown in FIGURE 8A, the attachment portion 71 has a substrate 81, which, for example, is a plastic film. A conductive guard layer 82 is placed onto the substrate such as by screen- printing. In this embodiment, the substrate 81 is rectangular and the conductive guard layer 82 is U-shaped, with the arms of the U extending along the two short sides of the rectangular substrate and the base of the U extending along one long side of the substrate 81. The guard conductor layer 82 may be any shape. The U-shape in the present embodiment has been devised for a particular vehicle and is intended to serve the combined purposes of shortening the sensor conductor 22 and also covering the sensor conductor 22 in a position where water is expected to concentrate. Alternatively, the guard conductor 82 may be a different shape to achieve these purposes or only be intended to one or other of these purposes.

With reference to FIGURE 7, as the placement of the attachment portions 71, 72 are indicated on the sensor conductor 22, so the functionality of the guard conductor. One of the short sides of the rectangular attachment portion 71 is placed along the short side of the sensor substrate 21. Along this short side of the attachment portion 71 is an arm 73 of the U-shaped guard conductor. This arm 73 covers an end of the sensor conductor 71, and so serves to shorten the effective length of the sensor conductor. The other arm 74 of the U-shaped guard is positioned on the attachment portion 71 so as to cover the sensor in a region where water is expected runs over the bumper.

FIGURE 8B illustrates the base of the attachment portion 71, where a conductive lobe 83 is illustrated. The lobe 83 is formed from a conductive sheet screen- printed onto the substrate 81. The positioning of the guard conductive layer 82 on the top surface of the attachable portion is illustrated in FIGURE 8B by a ghosted outline. Further protective coatings could be applied to protect the conductors. The attachment portions 71, 72 can be affixed to the sensor substrate using an adhesive, such as glue.

Referring again to FIGURE 7, when the attachment portions 71, 72 are positioned onto the sensor substrate 21, the lobes 31, 32 come into contact with the sensor conductor 22 since they are on the rear surface of the attachment portions. These conductive lobes 31, 32 improve the evenness of the sensor's sensitivity across the length of the sensor.

The guard conductors 82 are connected electrically to the guard conductor 23 (not shown) beneath the sensor substrate 21. This electrical connection may be achieved by any means, and is illustrated using rivets 75.

A fourth aspect of the present invention relates to retaining the sensor in close proximity to a vehicle bumper, and is illustrated in relation to FIGURES 9 to 13.

FIGURE 9A illustrates a top view of a retaining member according to this aspect of the present invention. The retaining member comprises a body 91 that is fabricated from a resilient material such as a polymer material. Since the retaining member is for affixing the sensor, it should be made from a non-conductive material.

To provide the retaining member with a curved profile, as illustrated in FIGURE 9B, which shows a top view of the retaining member of FIGURE 9A, a number of ribs 92 are provided along the length of the member, which keep the body 91 under compression. This curved profile assists in affixing a sensor to a curved vehicle bumper. The retaining member can be fitted directly behind a car bumper or can be fitted behind a trim strip that fits into the bumper.

Various configurations of the ribs 92 are possible, and FIGURE 9C illustrates three side profiles of the retaining member each with different rib configurations. The first configuration has two ribs spaced near to the top and bottom edges of the retainer's body 91, the second configuration has three equi-spaced ribs and the third configuration has two ribs positioned so that they abut the top and bottom edges of the body.

The retaining member may be formed by an extrusion process, with a finishing process to form the ribs indicated at "A" in FIGURE 9B. During the extrusion process, a bow can be formed in the retaining member to achieve the inherent spring effect which serves to retain the sensor in place. Alternatively the retaining member could be formed by moulding.

FIGURE l O illustrates a sensor 95 affixed to a vehicle part 96, such as a vehicle bumper, according to this fourth aspect of the present invention. It is important that the vehicle part is non-conductive, so that it does not adversely affect the operation of the sensor. The vehicle part 96 is provided with fastening portions 93, 94. The retaining member 91 is used to hold the sensor 95 in place. This can be achieved by any means.

In FIGURE 10, it is achieved by each end of the retaining member 91 being inserted against or under the fastening portions 93, 94 to hold it in place. The retaining member 91 in turn holds the sensor 95 in place against the vehicle part 96.

FIGURE 11 illustrates a side view of the arrangement in FIGURE 10. The retaining member 91 is secured in place at each of its ends by fastening portions 93, 94. The retaining member 91 is bowed and therefore provides pressure to the sensor 95 to secure it against the vehicle part 96. The fastening portions 93, 94 are illustrated as rods which form niches with the vehicle part 96 into which the ends of the retaining member 91 are fitted. This is one possible form of the fastening portions, and others are possible, such as straps or tabs associated with vehicle part or recesses into the vehicle part itself.

FIGURES 12A and 12B illustrate an approach for installing the retaining member 91.

The sensor 95 is firstly positioned against the vehicle part 96, which in this illustration is a vehicle bumper. One end of the retaining member 91 is clipped under fastening portion 93 of the bumper, which is marked as "X" in FIGURE 12B. To insert the other end of the retaining member 91 under fastening portion 93, which is marked as "Z" in FIGURE 12B, the central area "Y" ofthe retaining member 91 is pulled away from the bumper 96. The central area "Y" of the retaining member 91 is then toggled overcentre to spring back to its resting curved position and thereby apply pressure against the sensor 95 to hold it in place.

FIGURES 13A and 13B illustrate a further feature ofthis fourth aspect ofthe invention, being location retention means 97. These location retention means are affixed to the sensor 95. In FIGURES 13A and B the location retention means 97 are T-shaped and extend through the substrate of the sensor, such that the top of the "T" shape is located on the side of the sensor abutting the vehicle part 96, and the body of the "T" extends through the sensor 95 towards the retention member 91. The retention member 91 has holes 98, 99 which are adapted for receiving the ends of the location retention means 97, thereby ensuring that the sensor 95 is correctly positioned relative to the retention member 91. The location retention members 97 may be of any shape or form for example, they may be of the form of knobs on the sensor associated with corresponding recesses in the retaining member 91.

The location retention members therefore enables the sensor to be attached to the vehicle bumper without the need for glues and other such materials which affect the ability to recycle the bumper once the vehicle has been taken offthe road. It also provides a simple approach for replacing and repairing the sensor, as required, as the retention member is removable and replaceable.

Alterations and additions are possible within the general inventive concepts. The embodiments of the invention are to be considered as illustrations of the inventions and not necessarily limiting on the general inventive concepts.

For example the shape of the sensor conductor described is to be taken as only an illustration of a preferred sensor conductor shape. Other shapes and configurations are possible. For example, to address adverse effects on sensor sensitivity caused by metallic objects attached to the bumper, such as number plates, the width of the sensor conductor can be made wider in the region of the metallic object. This can be achieved by attaching a lobe in the appropriate location.

The sensors of the present invention are primarily intended for mounting on the rear of a vehicle to assist a driver when reversing. However, the sensors are also suitable for front or even side mounting, e.g. for avoiding collisions with objects at low-level which are obscured from view below the bonnet. Additionally, when a vehicle is manoeuvring, either forwards or backwards, there is a danger that the side wings may strike an object if the vehicle is turning at the same time. *

Claims

CLAIMS: 1. A process of forming a sensor for use in a capacitive sensor

system, comprising: obtaining a portion of substrate having a sensor conductor on a first surface of the substrate; and attaching a conductive portion to the first surface of the substrate so that the conductive portion overlays a portion of the sensor conductor.
2. The process of claim I wherein the conductive portion is attached to the sensor conductor using an adhesive.
3. The process of claim 1 or 2 further comprising attaching a second conductive portion to the first surface of the substrate which overlays a portion of the sensor conductor and wherein the sensor conductor is elongate and the two conductive portions are positioned at or towards opposite ends of the elongate sensor conductor.
4. A sensor for use in a capacitive sensor system comprising: a substrate; I a sensor conductor on a first surface of the substrate; at least one conductive portion attached to the first surface of the substrate such that the at least one conductive portion overlays a portion of the sensor conductor.
5. The sensor of claim 4 further comprising an adhesive for attaching the at least one conductive portion to the sensor conductor.
6. A process of forming a sensor for use in a capacitive sensor system, comprising: obtaining a portion of sensor substrate having a sensor conductor on a first surface of the substrate; and attaching a rear surface of an attachment portion to the first surface of the sensor substrate such that the attachment portion includes at least one of: a conductive lobe on the rear surface of the attachment portion such that the conductive lobe is configured to overlay a portion of the sensor conductor and electrically connect therewith; and a guard conductor on a front surface of the attachment portion configured to shield underlying portions of sensor conductor.
7. The process of claim 6 further comprising: attaching the attachment portion to the substrate using an adhesive.
8. The process of claim 6 or 7 comprising: electrically connecting the guard conductor on the front surface of the attachment portion to a second guard conductor on a second surface of the sensor substrate.
9. A process of forming a sensor for use in a capacitive sensor system, comprising: attaching a strip of sensor conductor to a first surface of a length of substrate; attaching a guard conductor to a second surface of the length of substrate such that the guard conductor at least substantially covers the second surface; detaching a section of the substrate from the length of substrate to form the sensor, such that the sensor conductor extends from a first edge of the sensor to a second opposite edge; and decreasing the length of the sensor conductor by folding the first edge and the second edge of the sensor towards the first surface, so that sections of the guard conductor on the folded ends shield the underlying sensor conductor.
10. A process of forming a sensor for use in a capacitive sensor system, comprising: obtaining a substrate having a sensor conductor on a first surface of the substrate, the sensor conductor being elongate and extending from a first edge of the substrate to a second opposite edge and a guard conductor on a second surface of the substrate, the guard conductor at least substantially covering the second surface; and decreasing the length of the sensor conductor by folding the first edge and the second edge of the sensor towards the first surface, so that sections of the guard conductor on the folded ends shield the underlying sensor conductor.
A sensor for use in a capacitive sensor system comprising: a substrate; a sensor conductor on a first surface of the substrate, the sensor conductor being elongate and extending from a first end to an opposite second end of the substrate; a guard conductor on a second opposite surface of the substrate, and with the first and second ends of the substrate folded onto the first surface such that sections of the guard conductor on the folded ends shield the underlying sensor conductor.
l 2. A capacitive sensor for mounting to a body, the sensor comprising: a substrate; a sensor conductor on a first surface of the substrate, the first surface configured to face outward from the body; a first guard conductor overlying a portion of the sensor conductor such that the guard conductor shields the underlying region of the sensor conductor and defines a region of reduced sensor sensitivity.
13. The sensor of claim 12 further comprising a second guard conductor on the second surface of the substrate.
14. The sensor of claim 13 further comprising a connection means for electrically connecting the first guard conductor to the second guard conductor.
15. The sensor of claim 14 wherein the connection means is a rivet through the substrate to connect the first guard conductor and the second guard conductor.
1 6. The sensor of any one of claims 12 to 15 wherein the body is a vehicle bumper.
17. A retention system for retaining a sensor to a vehicle, comprising: a vehicle part having first and second fastening portions; a retaining member having a first and second end, such that in use, the sensor is positioned between the vehicle part and the retaining member, and secured with force applied by the retention member, which is affixed to the vehicle part by securing the first and second ends of the retaining member at the first and second fastening portions.
18. The retention system of claim 17 wherein the vehicle part is curved and the retaining member has a corresponding curved profile in a static position sufficient to apply force to the sensor to retain it in place against the vehicle part.
19. The retention system of claim 17 or 18 wherein the retaining member is curved and has a plurality of elongate ribs on one surface to retain the curved-shape when in a static position.
20. The retention system of any one of claims 17 to 19 wherein the retaining member is flexible in order to assist in assembling and disassembling the sensor with the vehicle part.
21. The retention system of any one of claims 17 to 20 wherein the fastening portions are shaped to latch the ends of the retaining member in position.
22. The retention system of any one of claims 17 to 21 further comprising location retention means associated with the sensor, for retaining the sensor in position relative to the retaining member.
23. The retention system of claim 22 wherein the location retention means have ends which are secured through holes in the sensor and retention member.
24. The retention system of any one of claims 17 to 23 wherein the retaining member comprises a non-conductive material.
25. A retention system substantially as herein described with reference to Figures 9 to 13.
26. A sensor substantially as herein described with reference to the accompanying drawings.
27. A process of forming a sensor for use in a capacitive sensor system substantially as herein described with reference to the accompanying drawings.