GB2423822A

GB2423822A - Capacitive proximity sensor with reduced sensitivity to water trickles

Info

Publication number: GB2423822A
Application number: GB0504467A
Authority: GB
Inventors: Anthony Moon; David Snell
Original assignee: AB Automotive Electronics Ltd
Current assignee: AB Automotive Electronics Ltd
Priority date: 2005-03-02
Filing date: 2005-03-02
Publication date: 2006-09-06
Also published as: KR20080012259A; JP2008532034A; WO2006092627A1; GB0504467D0; MX2007010790A; BRPI0607582A2; EP1856559A1; CN101167000A

Abstract

A capacitive proximity detecting method and a sensor comprising a sensor conductor with a large guard conductor behind it, in which the sensitivity to distant objects is enhanced by minimising the area of conductive material forming the sensor conductor particularly in vertical strips representing the path of water over the sensor. Sensitivity to trickles of water such as from rain that flow over the sensor is thereby reduced. The sensor is used in a vehicle bumper.

Description

Capacitwe Sensor and Method of Production The present invention relates to

a capacitive sensor in particular for attachment to a vehicle for sensing the proximity of the vehicle to other objects, for example to assist when manoeuvring such a vehicle.

In recent times, cars have been fitted with sensors, particularly on the rear of the vehicle, to warn the driver of objects. In this way when the vehicle is being reversed, a collision with unseen or obscured objects can be avoided whilst still being able to position the vehicle close to such objects, e.g. walls, bollards etc. In a common implementation, audible warnings are sounded as a series of "beeps" or tones as an obstruction comes into range. These tones get closer together as the obstruction is approached, culminating in a solid tone when the obstruction is close, for example 35cm away, signalling to the driver to stop. The range at which this stop tone is sounded is the stop distance.

Capacitive sensors have guard and sensor plates which are connected to a control unit.

In use, the control unit supplies A.C. signals to the sensor and guard plates. Objects in the vicinity of the vehicle present a capacitance to ground. This capacitance is formed by two capacitances in series, namely a capacitance between the sensor plate and the object in series with a capacitance between the object and ground. This latter capacitance is actually formed by the capacitance between the object and the surface on which the object and the vehicle are positioned in series, with the capacitance between that surface and the electrical ground of the vehicle. 1-lowever, this latter capacitance is very large compared to the other capacitances and so it can be considered as a direct connection between the surface and electrical ground in the vehicle sensor. The control unit measures this capacitance between the sensor plate and ground. The unit may be triggered automatically when reverse gear is engaged (for a rear mounted system), manually or otherwise.

A particularly useful sensor construction for mounting across the rear bumper of a vehicle is shown in Figure 1. On the front side of the sensor body 12, as illustrated, is a sensor means or plate 11 made of conductive material. The sensor plate 11 comprises a central portion with a uniform width and two end portions or lobes of larger width adjacent the ends. These lobes provide increased sensitivity at the edges of the vehicle, due to their increased surface area. This is because, were the sensor plate formed as a uniform strip, for objects positioned to one side of the sensor plate 11, its sensitivity may be reduced at the edges because the object can only couple capacitively with that side of the sensor, whereas an object directly in front of the sensor plate will couple capacitively with both sides of the plate 11. This feature is particularly important to ensure that the stop distance is approximately uniform across the width of the vehicle.

GB 2,348,505 describes providing enlarged lobes to increase the sensitivity in the corners of the vehicle. GB 2,376,075 describes a modified arrangement for a non- uniform sensor plate. 11'l this, the area of the sensor per unit length is varied to provide the same increased sensitivity in selected parts of the sensor whilst providing alternative designs of the sensor which is useful where the sensor is visible in use.

A first guard plate (not shown) is provided on the opposite side of the sensor body 12 from the sensor plate 11. This first guard plate acts as a shield to reduce the sensitivity of the device to anything behind the sensor plate. The first guard plate and the sensor plate are insulated from each other by the substrate 12.

A second guard plate 13, shown in the form of a uniform strip, is positioned on the same side as the sensor plate 11. This second guard plate is also made of conductive material and in this example is illustrated above the sensor plate. Alternatively, however, it may be positioned below the sensor plate 11 or above and below, depending on the geometry of the vehicle to which it is applied. This second guard plate is described in PCT Publication Number WO 02/084875.

The purpose of this second guard plate 13 is to protect the sensor plate 11 from interference, such as could result from water falling onto and rolling off a vehicle or vehicle bumper and past the sensor plate. The second guard plate 13 has a signal applied to it that corresponds, or substantially corresponds, to the signal applied to the sensor plate. In this regard the voltage applied to the second guard plate has the same phase and frequency as the sensor plate, and preferably greater amplitude, such as about 1.2 times that applied to the sensor plate.

Such a sensor 10 is typically provided inside the bumper of a vehicle, with the front side of the sensor with the sensor plate 11 and second guard plate 13 arranged outermost relative to the vehicle. Correspondingly the side of the sensor with the guard plate (not illustrated) is innermost, i.e. closest to the vehicle itself.

Sensors of the configuration shown in Figure 1 are reasonably proficient in shielding the sensor plates from the effects of rain and the like. Other steps can be taken when processing the signal from the sensor plate to mitigate the effect of water. However, reducing the effect of rain is an important design criterion, and improving this feature will give better overall performance.

In designing a sensor with improved performance against near field interference, the following capacitance formula needs to be kept in mind: CE0ErA/L (1) where * C is the capacitance between two objects, such as a water droplet and the sensor plate * E0 is the permittivity of free space * Er is the dielectric constant for the material (Er = I for air and is in the range of 2 to 3.5 for most plastics) * A is the opposed conductive area of the objects (e.g. of the sensor plate and the water droplet) * L is the separation of the objects (e.g. distance between the water droplet and the sensor plate).

As a trickle of water runs across the sensor plate in the near field it couples capacitively to the sensor plate. This is not desirable, as the result of these trickles is sharp spikes in the detection signal, which detract from the true detection signal. So, to reduce the capacitance between the water droplet and the sensor plate, considering formula (1), it would be necessary to have a smaller sensor plate area.

However, for the effective detection of approaching objects in the far field, a large capacitance between the object and the sensor is desirable. To achieve this, based on the formula (1), a large sensor plate area would have to be used. This however adversely affects the near field interference, since increasing the plate size also increases the effect of water running over the surface of the bumper close to the sensor plate.

Formula (1) relates to an ideal parallel plate capacitor and is only an approximation.

The formula works well where the area A of the objects is much greater than the square of the separation between them (L2). In other words, when considering the interaction with objects that are close to the sensor (i. e. objects in the "close" or "near" field), formula (1) can be used to estimate the capacitance between them and the sensor.

Flowever, when considering the interaction with objects that are far away from the sensor (i.e. objects in the "distant" or "far" field), the above formula does not give an accurate value of the capacitance between them because A is much less than L2.

So, in summary, for effective detection in the far field, it is considered that a large sensor plate area is required. However, increasing the sensor plate area also increases the effect of water running over the sensor, which is not desirable. Conversely, for minimising the effect of water, the plate size and hence capacitance should be kept small. Hence the two considerations (i.e. good overall sensitivity and minimal spikes due to water trickles) are competing.

The overall sensitivity of the system can be adjusted by selection of resistor values in the amplification stages. However, in a practical application such as a parking aid, the far field range is limited by the sensitivity to uneven ground that cause the vehicle and therefore the sensor to "bounce", resulting in false triggering. In this regard, as the system is designed to detect changes in capacitance between sensor and ground, to have good sensitivity, a sensor needs to produce a large percentage change of capacitance as an obstruction is approached.

Overall, there is a need to provide a sensor with good sensitivity to far field objects within a certain range of a capacitive sensor, which would be objects that it is desirable to detect, such as objects close to the rear of a vehicle. This would be in conjunction with minimising the effect of near field interference, predominantly resulting from water passing over the sensor, the detection of which is undesirable. This is particularly applicable to parking aid sensors attached to the rear of a vehicle, with good sensitivity required to far field objects from about 50mm to 2 metres away, and reduced sensitivity to near field interference, occurring typically within 5mm of the sensor.

In one aspect, the present invention provides a capacitive sensor comprising: a substrate; and an elongate sensor plate formed of a pattern of conductive material on the surface of the substrate, the sensor plate having a central portion and two outer portions arranged at either side of the central portion along a longitudinal axis of the sensor plate, wherein the conductive material is arranged such that a plurality of imaginary strips of similar width, arranged side by side along the longitudinal axis of the sensor with each strip extending across the width of the substrate would each overlap a plurality of portions of the conductive material, and the total area of the conductive material which each of the strips overlaps is substantially constant, in the longitudinally central section of the sensor plate.

The strips may extend along the surface of the substrate in a first direction perpendicular to the longitudinal axis of the sensor plate.

The strips preferably relate to the expected path of water flowing over the sensor when it is in use on a vehicle bumper. Thus, the layout of the sensor plate may be arranged so that the distribution of conductive material is substantially constant in the strips even though the strips are not parallel. In other words, the direction which the strips take across the sensor can vary along the length of the sensor to take account of the different flow directions of water over the length a vehicle bumper. The arrangement of the sensor plate is such that it ensures the uniformity of material in each of these strips.

The outer portions can have different areas of conductive material in each strip compared to the central portion, to provide the effect of having lobes in the prior systems, such as to provide increased near and mid-field sensitivity around the corners of the car. The central portion is preferably longer than each of the outer portions in the longitudinal direction so that a uniform sensing region in the rear of a vehicle is maintained and also so that drips of water passing over the surface have a uniform effect. In this way, their effect can be more easily filtered out than if the size of the effect depended upon the position along the length of the sensor.

Minimising or at least reducing the amount of conductive material that runs in the same direction as the direction in which near field objects tend to move (e.g. vertically), such as by having lines of conductive material with a small vertical height, serves to minimise the effect of water (which runs typically vertically). The capacitance between trickles of water and the sensor plate can be calculated by the parallel plate capacitance formula (1). Therefore, as the trickles generally travel vertically due to gravity, by reducing the amount of conductive material a trickle passes over, the smaller the value of A in equation (1). This means the coupling capacitance is between the sensor plate and the trickle is reduced and the effect of the trickle on the sensor should be reduced.

Reducing the effective area of the sensor means may cause some reduction in the far field sensitivity. However, this reduction is disproportionately smaller than the effect on the near field sensitivity and so can be compensated for. Therefore, this aspect of the invention allows the near field sensitivity to be minimised without unduly

compromising the far field range.

In the far field, the outer perimeter defined by the conductive material is more important than the actual area of the conductive material contained within the perimeter in determining the capacitance and hence sensitivity. Therefore, as long as the area enclosed by the outer perimeter defined by the portions of conductive material forming the sensor plate, encompasses an appropriately large area, the proportion of area of the area within which is conductive material has little effect on the sensitivity of the sensor.

Preferably, the conductive material within the perimeter forming the sensor means is distributed in a regular pattern across the length of the region. Further, by "appropriately large area" of the sensor means, typically the same perimeter area as for existing sensors is envisaged, albeit with reduced usage of conductive material therein.

It is to be appreciated that, where used, the terms "horizontal" and "vertical" are intended to be relate to an elongate sensor placed on a bumper on the rear of a vehicle, so that the horizontal direction corresponds to the direction along the length of the vehicle's bumper and vertical is the gravitational direction, i.e. perpendicular to the ground.

The near field interference is typically due to trickles or drips of water that run over or form on the sensor or an insulating surface behind which the sensor is mounted. The water may be from precipitation, condensation, melt water etc. due to the weather or local conditions.

According to another aspect the present invention there is provided a method of detecting the presence of objects in proximity to a sensor in the presence of rain, comprising providing a capacitive sensor such as that described above.

A further aspect the present invention provides for the use of a capacitive sensor in a vehicle bumper for mitigating the sensitivity to water flowing over the bumper, the bumper comprising an outer skin with the capacitive sensor provided on the inner surface of the skin, the capacitive sensor comprising: a substrate; and an elongate sensor plate formed of a pattern of conductive material on the surface of the substrate, the sensor plate being arranged such that imaginary strips extending along the surface of the substrate in a first direction perpendicular to a longitudinal axis of the sensor plate each contain a plurality of portions of the conductive material separated one or more regions free of conductive material.

A further aspect of the present invention provides a capacitive sensor comprising: a substrate; and an elongate sensor plate formed of a pattern of conductive material on the surface of the substrate, the sensor plate including a plurality of spaced apart parallel strips arranged along a longitudinal axis of the sensor.

A yet further aspect of the present invention provides a capacitive sensor comprising: a substrate; and an elongate sensor plate formed of a pattern of conductive material on the surface of the substrate, the sensor plate including a strip of conductive material provided around the outer perimeter of an elongate area of substrate extending along a longitudinal axis of the sensor.

A still further aspect of the present invention provides a capacitive sensor comprising: a substrate; and an elongate sensor plate formed of a pattern of conductive material on the surface of the substrate, the sensor plate including strips shaped as interconnected V-shapes arranged along a longitudinal axis of the sensor.

A specific embodiment of the present invention will now be described with reference to the accompanying drawings in which: Figure 1 illustrates a prior art sensor and guard plate arrangement; Figure 2 illustrates the capacitances associated with a capacitive parking aid sensor mounted behind a rear bumper of a vehicle.

Figure 3 illustrates a cross section of a rear bumper of a vehicle with a capacitive sensor and the capacitances associated with a trickle of water rolling down the bumper.

Figure 4 shows the typical output from any of the sensors illustrated in Figures 1, 6, 8 or 9 when a vehicle is reversed along a road slowly for 160 see, and then towards a large obstruction such as another vehicle. As the obstruction is approached, the voltage output from the sensor rises. Before the obstruction is reached, there are small fluctuations in sensor output due to undulations in the road.

Figure 5 illustrates an output sensor response when water is dripped on to its bumper for

the prior art sensor of Figure 1;

Figure 6 illustrates a sensor arrangement according to a first embodiment of the present invention; Figure 7 illustrates an output sensor response for the sensor arrangement of Figure 6, according to the first embodiment of the invention; Figure 8 illustrates a sensor arrangement according to an alternative embodiment of the invention; Figure 9 illustrates a sensor arrangement according to a further alternative embodiment of the invention; Figure 10 illustrates a cross-section of the sensor of Figure 9 about the line AB; Figure 11 illustrates the coupling that occurs between a trickle of water and a sensor plate of the sensor of Figure 9; Figure 12 is a graph comparing the area of the sensor plates for the sensors of Figures 1 ("solid"), 6 ("5-bar") and 9 ("wire") and charts their relative sensitivity at far field distances; and Figure 13 is a graph comparing the area of the sensor plates for the sensors of Figures 1, 6 and 9 and charts their relative sensitivity to rain.

To illustrate the theory behind the aspects of the present invention, Figure 2 illustrates the operative capacitances when a car reverses towards an obstruction 3 with a capacitive parking aid sensor I mounted behind its rear plastic bumper. A guard plate 2 is mounted between the sensor and the car body.

When there is no obstruction present, the capacitance between sensor and ground is composed of a number of capacitances in parallel, as follows: Ca is the capacitance between sensor and car body above the guard Cb is the capacitance between sensor and car body below the guard Cc is the capacitance between sensor and ground The car body is coupled capacitively to the ground (earth) by capacitance Cz, and the capacitance between the sensor and ground is in reality Cc in series with Cz. As Cz is quite large compared to Cc, it can generally be ignored. Thus, it is reasonable to consider the vehicle body and earth to be directly connected and references to ground herein relate to both vehicle body ground and earth ground.

There are also capacitances Cd (not shown) between the sensor and car body beyond the ends of the sensor and a small capacitance between the sensor and car body through the guard plate. This latter capacitance is due to the guard signal not perfectly following that of the sensor because of non-linearity in the guard amplifier.

The overall capacitance between sensor and car body is therefore the sum of all these capacitances in parallel: Co=Ca+Cb+Cc+Cd (2) All these capacitances depend on the area of the sensor plate. Therefore, if the area of the sensor plate is increased, all the capacitances will increase.

When the vehicle approaches the obstruction 3, it senses an additional capacitance, Cp in series with Cz. As explained above, we can generally ignore Cz. The output of the sensor changes approximately in proportion to the increase in capacitance: V=K*(Cp+Co)/Co (3) Where V is the voltage of the output (referred to as sensitivity) K is a constant of proportionality Cp is the additional capacitance introduced by approaching the obstruction Co is the background capacitance between sensor and ground as in (2).

The capacitance Cp also increases with area of the sensor. If the capacitance of the various elements (Ca, Cb, Cc, Cd, Cp) increased linearly with the area of the sensor

II

plate, then the change in voltage V would be independent of the actual area of the sensor plate.

This theory was tested in relation to Figure 12, where comparative far field sensitivity results of a number of different sensor plates having different areas of conductive material mounted on the same guard are illustrated. The test was done with three sensor layouts. The first layout was a single narrow strip or wire extending across width of the bumper. The second layout was a sensor similar to that shown in Figure 6 having 5 narrow strips above each other. The final sensor layout was similar to the prior art arrangements having a broad strip extending across the width of the bumper. This sensor had a similar outer perimeter to the second layout but with a much greater amount of conductive material within that area. Figure 12 shows that the change in output (sensitivity) does increase with area of conductive material, but not very much.

Next considering the near field effect, and referring to Figure 3, a trickle of water 4 is shown rolling down the bumper 5. Figure 3 is a section through the bumper 5, mounted on a metal car body 3, with sensor 1 and guard 2. The top portion of the trickle, above the guard, couples capacitively to the car body (Cx). There may also be a coupling to the car body below the guard if the trickle is long enough (not shown). This coupling about the guard is further described in PCT Publication Number WO 02/084875.

As the trickle passes over the sensor means 1, it couples capacitively to it, with capacitance Cy. Thus as the trickle passes over the sensor means, the capacitance to car body (ground) increases by Cx in series with Cy.

The trickles of water can be considered as short lengths of conductor that roll down over the bumper skin. Although pure water is not a good conductor, relative to the large impedances in a capacitance measuring circuit, the electrical resistance of the water in the trickles is comparatively low and so can be ignored.

If the sensor is mounted directly behind the bumper skin, the trickle passes very close to it. The surfaces of car bumpers are typically made of polypropylene, are about 3mm thick, and have a dielectric constant of approximately 3. Due to the closeness of the trickle to the sensor means, the capacitive coupling Cy between the trickle and the sensor means can be calculated using the parallel plate capacitance formula (1). The coupling Cy therefore depends on the opposed area between the sensor means and the length of conductor representing the trickle. In other words, the effective area is the area of overlap between the trickle and the conductive portions of the sensor plate.

Therefore, in order to minimise Cy, the contact area between the sensor and trickle needs to be minimised. As the trickle normally takes a vertical (or near vertical depending upon airflow) path, Cy can be minimised by arranging the sensor to have a small cross section of conductive material in a vertical strip across the surface of the bumper from top to bottom.

As described in detail below, the effect of varying the shape of the sensor plate on the detection of trickles of water is significant. Figure 13 shows this variation for the same three sensors used in the example shown in Figure 12.

Combining these near field interference considerations with those for far field object sensitivity, one embodiment of the invention provides a sensor means with a large lateral length and small vertical height. In other words, the above theory shows that since the capacitance between sensor and ground, Co, is related to the area of the sensor plate, its absolute value is not critical for far field detection. When considering far field objects, the sensitivity is more related to the total area contained within the outer perimeter of the sensor exposed to the obstruction, than its area.

Further, to minimise Cy, it is necessary to minimise the area of conductive material overlapping between sensor and trickle, and this can be achieved by arranging the sensor means with a small cross section in the direction rain interference would normally flow, typically being perpendicular to its length (i.e. vertically). Ideally the cross section of the sensor means is also small in directions oblique to the vertical, in order to take into account water that runs over the sensor in directions other than the vertical. This can occur when air or wind flow pressure is applied to rain running down a vehicle bumper and turbulent airflow behind a vehicle. The airflow would force the water off its natural vertical path (due to gravity) so that the resultant flow would be diagonal.

With reference to Figure 6, a capacitive sensor 20 according to a first embodiment of the invention is illustrated. The sensor 20 is formed on a substrate 21, which is preferably a plastic film. A first guard plate (not shown) is formed on the rear side of the sensor body. Alternatively, the first guard plate can be formed on the front side of the sensor body, to simplify manufacture, provided it is electrically separated from the sensor means 24.

The sensor means 24 in this embodiment is formed from a plurality of conductive strips.

In Figure 6, five conductive strips are shown. For ease of forming the sensor plate, the conductive strips are preferably formed in parallel rows, although this is not essential.

For example, there could be one or more rows and each row could be wavy, curved or zigzagged. It is also preferable that the conductive strips are as narrow as possible without compromising their mechanical strength or electrical resistance. The width therefore depends on the construction method chosen, but preferably they should be 1mm or less. Further, where more than one row is utilised, it is preferable that their separation is at least 5mm.

In the embodiment of Figure 6, the sensor plate also has lobes towards each end of the strip. In this embodiment, the lobes are formed by providing one or more additional strips of reduced length at each end of the sensor strip. In Figure 6, there is one such strip (26a, 26b) at each end of the sensor plate, each with a length corresponding to the length of the desired lobe. The lobe strips (26a, 26b) need not be positioned directly adjacent the ends of the sensor plate, for instance the lobe strips may be offset towards the centre. The lobes provide increased sensitivity in the corners of the vehicle to provide good sensitivity to objects which are not immediately behind the vehicle and to provide a substantially uniform sensitivity across the entire width of the vehicle. The principle of providing such lobes is well known in the art.

The rows of conductive strips, including the additional lobe strips, are electrically connected by a first conductive interconnect 25a formed at one end of the rows of strips and a second conductive interconnect 25b formed at the other end of the rows of conductive strips. Preferably, as shown in Figure 6, the conductive interconnects are positioned diagonally across the rows of conductive strips, extending from the corner end of the lobe strip to the upper conductive strip. The conductive interconnects are electrically connected to the upper conductive strip at a position which corresponds to the length of the lobe strip. By connecting the horizontal parallel lines with a diagonal strip, the area exposed to a trickle of water is minimised, wherever the trickle rolls. If the connection was vertical, a trickle to run down the surface of the bumper and couple strongly the connection giving a false reading. By angling the connection,any trickle will only pass by a small part of the connection. Furthermore, this extra material can be compensated for by thinning the parallel strips it connects to.

Other arrangements to connect the line together are possible. For example, they may be connected behind the sensor or even at a remote connection point.

In this embodiment, a second guard plate 22 is formed on the front surface of the substrate, where the front surface is the outermost relative to the vehicle on which the sensor 20 is positioned, as with the sensor in Figure 1. The second guard plate 22 can also have lobes (23a, 23b) towards the ends of the plate. These lobes on the guard plate provide increased guard protection against spikes on the output caused by drips. This helps to counter the increased area that the sensor lobes present to trickles of water. In this regard, as the diagonal line also contributes to the effective area of the conductive lobes, for trickles that roll across the lobes, rather than vertically down, the greater area of the lobes produces worse spikes. Making the second guard plate wider in the vicinity of the lobes can counteract this.

Overall, the aim of the design is to minimise the area and hence the capacitance between the plate and the trickle as it rolls down the front of the bumper. For instance, comparing this design to the prior art arrangement of Figure 1, a drip falling vertically over the central region of the sensor plate in Figure 6 would only couple with an area of the sensor corresponding to the width of the trickle multiplied by the width of each line in the sensor. There would be no coupling with areas in the spaces between the lines.

In the Figure 1 arrangement, however, there would be coupling across the whole width of the sensor plate, as there are no spaces. Thus the trickle would form a much greater capacitor with the sensor and thus have a much greater effect.

The conductive surface area and area within the outer bounds defined by the sensor plate 24 in this embodiment of the invention has been chosen in order to provide a desired range at which the sensor indicates that an object is close enough to require a stop.

As indicated above, the capacitance does increase with sensor plate area but the relationship between sensitivity and sensor area for more distant objects is non-linear.

For instance, for a thin strip sensor plate one metre long, and an object 20cm away, the ratio of the capacitances between the strip being 1mm wide and 2mm wide is approximately 1.5. Therefore, a doubling of strip size only results in a 1.5 times increase, where a doubling would have been expected for parallel plate capacitors, considering the standard capacitance formula (1). Similarly, a halving of the strip size only results in a decrease of only about a third, whereas a halving would have been expected from formula (1).

Hence, it follows that forming the sensor plate, for example, from five lines of 1mm thick with a 5 mm separation rather than just one solid 25mm thick (i.e. equivalent to the total thickness of the five separated thin lines), will result in only a slight reduction in the far field sensitivity. As can be seen from Figure 12, the sensor of Figure 6 has around a fifth of the area of the Figure 1 sensor, but around 90% of the far field sensitivity with around a third of the sensitivity to rain.

A further advantage of the form of conductive sensor shown in Figure 6 is that it reduces the capacitance between the sensor plate 24 and the rear guard plate (not shown). A reduced capacitance is desirable, as too much capacitance imposes a heavy load on the amplifier driving the guard plate. Consequently, it is possible to reduce the thickness of the substrate 20 without exceeding the maximum allowed capacitance between the sensor plate and the rear guard plate. As an illustration of this, the sensor of Figure 1 typically utilises a film substrate of 250 micron thickness, whereas the sensor of Figure 6 utilises a film 125 microns thick. A further advantage of the reduced substrate thickness is a saving in weight and cost.

Figure 8 illustrates an alternative form of sensor plate 50 according to a second embodiment of the invention. This sensor plate 50 has a substrate 52 and a first guard plate (not shown) on the reverse side. A second guard plate 53 is also positioned on the front surface of the sensor, above the sensor plate 51. In this embodiment of the invention, the sensor plate 5 1 is in the form of an outline. The outline is fully interconnected and has two lobes 54a and 54b formed towards the ends of the sensor plate.

In testing, it was found that this outline sensor had a similar sensitivity range to the prior art sensor of Figure 1 in the far field, and had enhanced water performance (i.e. reduced sensitivity to rain and the like) in the near field.

A still further alternative form of sensor is illustrated in Figure 9. Figure 10 is a cross section of the Figure 9 embodiment, about the line AB, so like reference numerals will be used in relation to both Figures. In this embodiment of the invention, a main guard plate 61 is formed on a substrate 62. The guard plate 61 is made of a conductive material, such as metal foil and the substrate is preferably a plastic film. A second guard plate 63, such as a strip of metal foil, is placed over the main guard plate 61, but electrically insulated therefrom via an insulating means 64, which is preferably another plastic film. The plastic film/second guard plate combination is attached to the surface of the main guard plate using an adhesive 65.

In this embodiment of the invention, the conductive material is laid down onto in the form of a series of vertical and interconnected V-shapes. The conductive material is copper wire with a diameter of 0.2mm, which may be at least partially flattened. The copper wire has an enamel coating 67. This coating serves to insulate the copper wire from the main guard plate 61 when it is laid thereon. The enamelled copper wire is affixed to the main guard plate using an adhesive 65. A protective film 68 is then placed over the top of the sensor and is preferably affixed by lamination.

As with the Figure 6 embodiment, this sensor plate requires less conductive material to form the sensor plate. The reduced use of conductive material to form the sensor means enables the thickness of the substrate 61 to be reduced, saving weight and cost. Further, the capacitance between the sensor means 66 and the main guard plate 61 are advantageously reduced in view of the reduced sensor means 66 area.

To illustrate the advantages that result from forming the sensor plates in accordance with the embodiments illustrated, some comparative examples will be provided.

Firstly, the sensors of Figures 1 and 6 were attached to a vehicle bumper and underwent a water performance test. In the test, the vehicle bumper was about 3mm thick, and was dripped with water from a titrator at a rate of O.03ml per second. In this way, natural rain was simulated, in that the water built up on the bumper periodically before a trickle rolled down over the bumper.

Figures 5 and 7 demonstrate the resulting sensor output for the sensors of Figure 1 and 6 respectively. Each trickle couples to the sensor because of its close proximity to it and causes a spike as it couples with the sensor. It is to be appreciated that the second guard conductor in each of the sensors tested caused many of the spikes to be negative.

By comparing Figures 5 and 7, it is apparent that the sensor according to Figure 6 produces spikes of reduced amplitude. It is considered that the water has less effect on the Figure 6 sensor, as compared with the prior art Figure 1 sensor, as formula (1) does apply to near field water interference. That is, because the water is so close to the sensor (i.e. 3mm away) the parallel plate capacitance formula holds in the near field, for the coupling between the water trickle and the sensor.

Referring to Figure 13, an alternative graph is illustrated, which compares the sensitivity of the Figure 1, 6 and 9 sensors in the near field to their cross-sectional area.

To obtain this data, water from a titrator was dripped on a car bumper with each of the sensors attached. Each trickle produced a spike in the sensor output of varying size, and the graph illustrates the average size of the drip spike recorded over a 4-minute period.

The Figure 1 sensor, with the solid sensor means, has the greatest average spike size, of the order of 609 units. The average cross sectional conductive area of this Figure 1 sensor is also the greatest, being of the order of 40000 mm2. It is to be appreciated in these graphs that 1024 units corresponds to 5 volts output from the A to D converter associated with the sensor output. Also, the average spike size was calculated using the area of a graph of the A to D output for the spike against time. The A to D converter was measuring the output every 2Oms.

The Figure 6 sensor, with the sensor means formed of five bars of conductive material has a substantially reduced average spike size, being of the order of 211. Similarly, the average cross-sectional conductive area of the Figure 6 sensor is also substantially reduced, being approximately 8850 mm2.

Finally, the Figure 9 sensor, with the sensor means formed in wire in a vertical V- shaped arrangement has the smallest average water spike size, at about 147. The average cross sectional conductive area of this sensor is also the smallest, being of the order of 1540 mm2.

Therefore, considering firstly the Figure 6 sensor, compared with the prior art sensor of Figure 1, the Figure 6 sensor is approximately 78% smaller in terms of its average cross-sectional conductive area, and this reduction corresponds with about a 65% improvement its performance due to near field water effects.

The Figure 9 sensor is approximately 96% smaller than the Figure 1 sensor, and this reduction corresponds to about a 76% performance improvement over near field interference. Therefore, the desired near field sensitivity is dependent on the area of conductive material in any vertical cross-sectional area of the sensor means.

Next, consider Figure 12, which graphically illustrates the far field performance of the Figure 1, 6 and 9 sensors and compares this against their average cross-sectional conductive area. This far field sensitivity was measured by bringing a 25mm diameter vertically grounded steel pole up from 3 metres towards the vehicle. As the pole approached the vehicle, there was a change in sensor output voltage. The sensitivity was measured as the distance of the pole from the vehicle when its movement produced a I Volt change in output of the sensor.

The Figure 1 sensor with the solid sensor means has the best far field sensitivity, with the distance being of the order of 310mm. The 5 bar sensor of Figure 6 had the next best sensitivity, being approximately 276mm. The wire sensor of Figure 9 had a sensitivity of about 262mm.

Therefore, the Figure 6 sensor has about an 11% reduction in far field sensitivity as compared to the prior art Figure 1 sensor. It is to be remembered though that the near

field improvement was of the order of 65%.

The Figure 9 sensor has about a 15% reduction in far field sensitivity, while having a

76% improvement in the near field.

Therefore it is apparent that the far field sensitivity reduction is not nearly as significant as the improvements over near field interference, such as would be produced by rain.

The far field sensitivity does increase with sensor area, but the change is not very great and is non-linear. For example, a 30-fold increase in the sensor area would increase the sensitivity of the sensor by only about 20%.

The marginal reduction in sensitivity of sensors of Figures 6 and 9 can be corrected by increasing the amplification in the electronics.

It is considered that the enhanced near field sensitivity, particularly in the Figure 9 embodiment, is due to minimising the area that couples with a droplet of water that vertically falls down the vehicle bumper. This is illustrated in Figure 11 where a trickle of water 81 passes across the surface of the vertical V-shaped sensor means 82 of the Figure 9 sensor. Due to the shape of the sensor means 82, the area 83 where the trickle would couple with the sensor is reduced, and accordingly the interference generated by this trickle, in the form of a voltage spike, is also reduced.

This Figure also shows that even if the water rivulet 83 fell other than vertically across the sensor means 82, the rivulet would still only couple with a small portion of the conductive sensor 82. The angle of the zigzags can be adjusted to compensate for expected rain angles. For example, if in a given application, significant off vertical rain flow is expected, the zigzag can be flattened to ensure that trickles always pass over the lines obliquely.

Therefore, overall, by selecting an area for the sensor means that is suitably sized to produce a desired far field sensitivity, or at least a sensitivity within a tolerance of that desired far field sensitivity and distributing conductive material about that area to form the sensor means, a desired near field sensitivity can also be obtained, based upon the distribution of the conductive material. For instance, where the crosssectional area of the conductive material is minimised, the near field sensitivity is also minimised, in that the sensor is least susceptible to near field interference effects. The embodiments of the present invention therefore present a balance between the far field performance of the

sensor and its near field performance.

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 vertical width of the sensor conductor can be made less in the region of the metallic object. Alternatively, the sensing plate may have a central portion of uniform width with two end portions having progressively increasing widths.

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.

The sensors described here can be used in applications where the outer cover of the sensor can get wet, and the sensor needs to be able to operate in such conditions.

Applications include sensors to detect if vehicle doors, bonnets and tailgates are liable to hit an obstruction as they are opened or closed. This is particularly important if they are driven by motors. Another set of applications includes the use of capacitive sensors to monitor an opening, which can be traversed by a sliding panel to close it. This includes systems designed to detect an obstruction such as a person's arm when closing a car window or sunroof A further application is a sensor system to detect an obstruction while closing the roof on a convertible car. All these systems may have to Further, it is to be appreciated that the techniques described relating to creating the sensor of Figure 9 can similarly be used to make the sensors of the other embodiments of the invention.

Capacitive sensors according to the embodiments of the invention may be mounted on any opening and closing part, so that the sensor is used to detect the presence of an object that could be struck by the opening or closing of the opening and closing part.

The opening and closing part may be a vehicle door, tailgate or bonnet. Alternatively, the opening and closing part may be a powered sliding window, powered sunroof or a powered opening and closing hood, such as on a convertible car.

Claims

CLAIMS: 1. A capacitive sensor comprising: a substrate; and an elongate

sensor plate formed of a pattern of conductive material on the surface of the substrate, the sensor plate having a central portion and two outer portions arranged at either side of the central portion along a longitudinal axis of the sensor plate, wherein the conductive material is arranged such that a plurality of imaginary strips of similar width, arranged side by side along the longitudinal axis of the sensor with each strip extending across the width of the substrate would each overlap a plurality of portions of the conductive material, and the total area of the conductive material which each of the strips overlaps is substantially constant, in the longitudinally central section of the sensor plate.
2. A capacitive sensor according to claim 1 wherein the strips extend along the surface of the substrate in a first direction perpendicular to the longitudinal axis of the sensor plate.
3. A capacitive sensor according to claim 1 wherein the strips extend along the surface of the substrate in a direction corresponding to the desired direction of water flow over the surface of the sensor, in use.
4. A capacitive sensor according to claim 1, 2 or 3 wherein the conductive material is formed as one or more lines of conductive material that extend along the longitudinal axis of the sensor plate.
5. A capacitive sensor according to claim 4 wherein the lines of conductive material are straight and parallel with each other.
6. A capacitive sensor according to claim 4 wherein the lines of conductive material are angled and/or curved along their length.
7. A capacitive sensor according to claim 6 wherein the lines of conductive material are shaped as interconnected V-shapes.
8. A capacitive sensor according to claim 7 wherein the point of each V is curved.
9. A capacitive sensor according to any one of the preceding claims wherein the conductive material is a formed from a wire attached to the substrate surface.
10. A capacitive sensor according to any one of the preceding claims wherein the conductive material is formed as one or more lines of conductive material having a width in the range 0.2mm to 2mm, and more preferably within the range 0.5mm to 2mm.
11. A capacitive sensor according to any one of the preceding claims wherein the central portion is longer than each of the outer portions in the longitudinal direction.
12. A vehicle bumper incorporating a capacitive sensor according to any one of claims I to Ii.
13. A proximity sensor configured to be mounted on a vehicle in order to detect the presence of an object proximate to the vehicle as the vehicle and the object approach each other, the proximity sensor comprising a capacitive sensor as claimed in any one of claims ito 11.
14. A method of detecting the presence of objects in proximity to a sensor in the presence of rain, comprising: providing a capacitive sensor having: a substrate; and an elongate sensor plate formed of a pattern of conductive material on the surface of the substrate, the sensor plate having a central portion and two outer portions arranged at either side of the central portion along a longitudinal axis of the sensor plate, wherein the conductive material is arranged such that imaginary strips extending along the surface of the substrate in a first direction perpendicular to the longitudinal axis of the sensor plate, each overlap a plurality of portions of the conductive material, and the total area of the conductive material which each of the strips overlaps is substantially constant, in the central portion of the sensor plate.
15. Use of a capacitive sensor in a vehicle bumper for mitigating the sensitivity to water flowing over the bumper, the bumper comprising an outer skin with the capacitive sensor provided on the inner surface of the skin, the capacitive sensor comprising: a substrate; and an elongate sensor plate formed of a pattern of conductive material on the surface of the substrate, the sensor plate being arranged such that imaginary strips extending along the surface of the substrate in a first direction perpendicular to a longitudinal axis of the sensor plate each overlap a plurality of portions of the conductive material separated by one or more regions free of conductive material.
16. A capacitive sensor comprising: a substrate; and an elongate sensor plate formed of a pattern of conductive material on the surface of the substrate, the sensor plate including a plurality of spaced apart parallel strips, each arranged along a longitudinal axis of the sensor.
17. A capacitive sensor comprising: a substrate; and an elongate sensor plate formed of a pattern of conductive material on the surface of the substrate, the sensor plate including a strip of conductive material provided around the outer perimeter of an elongate area of substrate extending along a longitudinal axis of the sensor.
18. A capacitive sensor comprising: a substrate; and an elongate sensor plate formed of a pattern of conductive material on the surface of the substrate, the sensor plate including strips shaped as interconnected Vshapes arranged along a longitudinal axis of the sensor.
19. A method substantially as herein described with reference to Figures 2 to 10.
20. An apparatus substantially as herein described with reference to Figures 2 to 10.