EP1910812A1

EP1910812A1 - Capacitive rain sensor

Info

Publication number: EP1910812A1
Application number: EP06764186A
Authority: EP
Inventors: Yves GLAVERBEL - Centre R & D DELATTE; Thomas Schuler; Jürgen NIES; Heiko Hofmann
Original assignee: AGC Glass Europe SA; Valeo Schalter und Sensoren GmbH
Current assignee: AGC Glass Europe SA; Valeo Schalter und Sensoren GmbH
Priority date: 2005-07-19
Filing date: 2006-07-17
Publication date: 2008-04-16
Also published as: WO2007009973A1

Abstract

The invention relates to capacitive rain sensors. The inventive rain sensors are placed on a window panel, comprise electrodes and conductors, which supply the latter and which are made of a conductive material. Inside the sensors, the electrodes are provided with dimensions and are arranged relative to another so that they simultaneously have a number of levels of sensitivity. The inventive rain sensors are primarily used on motor vehicle windshields.

Description

Capacitive rain detector

The present invention relates to rain detectors, and in particular those used on motor vehicles.

The use of sensors to detect the presence of water on a glazing unit, for example to control an operation such as the starting of wipers for motor vehicles is usual. In this application the marketed sensors are of the type using the alteration of a light signal on the path of which are the drops of water to be detected. The sensor comprises a transmitter and a receiver of the light signal constituted for example by a reflected ray.

Detectors operating on these optical signals, when used in particular on automotive windows, have the disadvantage of leading to the presence on the glazing of non-transparent elements. Even miniaturized, the detector covers about ten square centimeters. To minimize annoyance on the windshields, the detector is usually hidden behind the interior rearview mirror. Even in this arrangement, its presence on the windshield remains unsightly, at least seen from the outside.

Another type of detector has been proposed previously, which implements a device in which the signal is generated by a variation of capacity. An electrode assembly is disposed on the glazing. The presence of water on the glazing, water which has a dielectric constant very different from that of air or glass significantly modifies the capacity of the electrode system. This variation constitutes the generated signal. A significant advantage of capacitive sensors lies in the possibility of forming the constituent electrodes of the sensor in a material essentially transparent to visible radiation. By substantially transparent is meant that the sensor leaves a visible transmission of at least 50%, and preferably at least 60%. This transmission in the most advantageous cases exceeds 70%. Of course, the higher the transmission, the more discreet the sensor appears in the observation of the glazing. The choice of materials of this type makes it possible to dispose the sensor on areas of the glazing in which aesthetically and functionally it is preferable not to have opaque elements. Not only the electrodes, but also the conductors feeding them, are advantageously transparent.

The capacitive detectors according to the invention are used in particular to control the starting of the wipers of the vehicles. A great variety of situations can condition this start-up. To speak only of the rain, this one can take various forms. An extreme case corresponds for example to a stormy shower which at first the drops are bulky but few. Another extreme case, unlike the first, is that of a drizzle formed by very small but relatively numerous drops. In both cases, however, the windshield wiper may be needed although the sensor is under very different conditions.

The sensitivity of the sensor requires that the electric field is sufficiently intense. To increase the field it is preferable to have the electrodes constituting the capacitive sensor at a short distance from each other. With an intense field, a small drop can be detected. The useful area between the electrodes on which small drops are detected may be relatively small. Because of the distribution of these drops, he always find a sufficient number on this useful surface. For larger drops but whose distribution is more random, conversely it is necessary to provide a useful area large enough to improve the probability of finding on this useful surface.

These two contrary requirements with regard to the sensitivity and the effective surface, lead the inventors to propose, contrary to the previous arrangements, electrodes which in their dimensions and their relative arrangement to each other simultaneously offer various levels of sensitivity and surface effective.

To obtain these different levels of sensitivity and effective surface, the inventors propose in particular to make sure that the sensor comprises several distances between the contiguous detection electrodes. Under these conditions, the electric field and the sensitivity are the strongest in the zone where the electrodes are the closest, and vice versa. The smallest and largest drops can thus be detected with the best probability, regardless of the large difference in their distribution and impact.

The sensor according to the invention may have two distinct distances corresponding to two detection levels. It is also possible to provide more than two levels with more than two distances. In the same way the distance between the detection electrodes may vary differently than in successive steps. A provision in this sense is constituted by electrodes whose distance from the edges of these which face each other varies continuously.

To further enhance the sensitivity of a sensor whose electrodes have different spacings, it is also advantageous to ensure that the width of the electrodes forming the capacitor is not not uniform over the entire length of these electrodes. The widest corresponds to the largest capacity, and therefore also a relatively more intense field;

Preferably the width of the electrodes varies in the same direction, as the distance between these electrodes. In other words, the increase of the surface situated between the electrodes by increasing the distance of the electrodes, an increase which is accompanied by a less intense field, is partly at least compensated by an increase related to the capacitance of the electrodes. The combination of these two effects does not necessarily or even preferably lead to a field of uniform intensity. This is also not necessary since the distance of the edges of the electrodes is intended primarily to provide a useful surface area sufficient to first detect the large scattered drops, which are hypothetically more easily detected when they are present because of the substantial modification that they cause to the dielectric constant of the medium in which the measurement develops.

The variations indicated previously depend on the importance of those concerning the variations induced by the presence of the drops variable in dimension. It is endeavored to ensure that in all cases the generated capacitance variations remain of comparable order of magnitude so that the intensity-treated signal does not vary by more than a factor of 10, and preferably by a factor of not more than 5 in extreme conditions.

To obtain these conditions on the signal, the variation in the distance between the electrodes is advantageously at most of the order of 1 to 10 and preferably at most 1 to 5, and similarly the variation of the width of the electrodes is preferably more than 1 to 10, and preferably at most 1 to 5. The invention is described in detail hereinafter with reference to the appended figures in which:

FIG. 1 is a schematic perspective view of a windshield equipped with a capacitive sensor

FIG. 2 is a plan view of the design of the electrodes and the conductors which feed them, forming a capacitive sensor, in an arrangement of the electrodes not reproducing the characteristics of the invention;

- Figure 3 shows a schematic section of a glazing carrying a capacitive sensor of the type shown in Figure 2;

FIG. 4 shows a drawing of the electrodes of a capacitive sensor according to the invention, intended to better detect drops of variable number and size;

FIG. 5 represents another electrode design of a capacitive sensor similar to the preceding one, composed for a differential type measurement;

- Figure 6 is another embodiment of a sensor according to the invention, having a wide range of sensitivity;

FIG. 7 shows another drawing of the electrodes for a sensor offering a continuous range of sensitivity

- Figure 8 is similar to the previous for a three-electrode sensor.

For the sake of simplification in the rest of the description, the glazings presented are said to consist of glass sheets, which are also most often of the laminated type. The sensors according to the invention can also be used on glazings consisting of

organic "glasses", such as polycarbonates, well known and widely used to constitute glazing. Similarly, if most frequently the sensors in question are introduced into laminated assemblies comprising two rigid sheets assembled by means of polyvinyl butyral (PVB) type dividers or the like, it is possible to arrange the sensor on a glazing unit comprising only a rigid sheet. It is then in particular so-called "bilayer" glazing which comprise a glass sheet associated with a sheet of plastic material, in particular polyurethane, a material which simultaneously offers the plasticity ensuring the resistance against the eviction of the passengers in the event of an accident , and a sufficient surface quality to resist scratching. It can also be glazing comprising only a sheet of glass. In the latter case, however, it is preferable to ensure that the conductive elements of the sensor are electrically insulated and protected by an additional coating advantageously having the same properties of transparency.

Figure 1 shows a typical arrangement of a rain sensor on an automobile windshield (1). On the windshield (1) the rain sensor (4) is necessarily located in a zone (2, 3) swept by the windshield wipers. In the figure these areas are shown schematically by broken lines.

This arrangement is controlled by the fact that the sensor (4) is intended to trigger the movement of the wipers in the presence of water on the scanned areas. Outside of these areas, water may remain after the influx of rain into the swept areas has stopped. If the sensor were positioned outside the swept areas, the movement of the wipers could be unnecessarily maintained.

In optical detection systems which have non-transparent elements, the sensor (4) is preferably arranged at a point where it does not cause any inconvenience to the driver. If nevertheless it is still in the field of vision, preferably this location is already obscured by another functional element. Very usually the optical sensors are arranged behind the interior rearview mirror. In the case of capacitive sensors according to the invention, the fact that the electrodes are preferably very largely transparent to visible radiation, offers a greater latitude in the choice of this location, even if the surface occupied by the sensor is substantially larger. than that masked by traditional optical sensors.

Capacitive sensors work with a signal analysis and processing unit. Most usually the unit in question consists of a relatively small electronic circuit. It can even be reduced to a "chip" of a few square millimeters or less. This unit is usually non-transparent. For this reason it is advantageous to locate it outside the transparent part of the glazing. For the reasons indicated below, however, the analysis unit is as close as possible to the electrodes of the sensor. It is located for example behind the enamelled strips that are often placed at the edge of the glazing.

In the form shown in Figure 1, the sensor (4) is in the central high position, that is to say behind the rearview mirror. Given its predominantly transparent nature, another positioning is nevertheless possible.

The capacitive sensor components may be formed directly on a sheet of glass or be placed on a support associated with the (or) glass sheet (s). Various materials can enter into the constitution of these sensors. The electrodes in particular are advantageously constituted by thin conductive layers.

The electrodes are formed for example of conductive films of the type constituting the infrared reflective coatings used to minimize the "greenhouse effect" in vehicles. These are films generally comprising a metal layer, in particular of silver, which layer is included in dielectric layers which have the role of both protecting the layer metal, and to prevent the reflection of wavelength rays from the visible range. When the glazing comprises an infrared reflective layer, it is advantageous to form the electrodes in this layer by removing the conductive layer according to the chosen drawing. This removal is carried out for example by means of a laser. The conductors are constituted in the same way directly in the conductive layer.

Other infrared reflecting films are made from conductive oxides, for example based on ITO ("indium tin oxide"). If their constitution and their deposit different from the metallic films, the formation of the electrodes and conductors in these layers is analogous. For example, by removal according to the chosen drawing.

The formation of the design of the sensor can also be done by masking the appropriate areas during the deposition of the layer.

Other techniques still make it possible to produce the electrodes and the conductors. This is in particular the application of a conductive composition by screen printing, stencil, inkjet printing or any other similar technique, a pattern reproducing the corresponding elements.

A capacitive sensor of the prior art is shown diagrammatically in FIG. 2. This sensor comprises electrodes (5, 6) connected to the signal processing unit by conductors (7, 8). In the traditional form presented the electrodes are arranged at a distance from each other, this distance being uniform over the entire length of the electrodes facing each other.

The principle of operation of a capacitive detector is illustrated in FIG. 3. The glazing in this example presented in partial section is composed of two glass sheets (8, 9) joined by a polyvinyl butyral (PVB) interlayer (10). . A drop (g) which is deposited on the glazing in the area corresponding to the gap between the electrodes (5, 6) as shown, is in the electric field generated by the electrodes, and modifies it because of the differences between the dielectric constants of water and air. The modification of the capacitance of the electrodes, measured mostly indirectly, serves as a measure. This depends on the size of the drops. For a uniform field, like that which develops in the configuration of Figure 2, the signal is all the more intense that the drop is larger. The drop (I) generates a stronger signal than the drop (II).

It is possible to some extent to choose the characteristics and sensor and associated electronics to obtain the best possible sensitivity to a given signal. But the choice of implementation characteristics of the capacitive sensor does not optimize the signal for all conditions that may arise with respect to the volume and number of drops. In such a provision, the choice is necessarily based on a "typical" average situation. In other words, the sensor thus formed does not respond in the most appropriate manner to the various situations encountered. The arrangements according to the invention illustrated in the following figures provide a way to widen the areas in which the drops are better detected.

Figure 4 is an exemplary embodiment for this improved detection.

The capacitive sensor is constituted as before by two electrodes (11, 12). The zone between the electrodes has two distinct parts (Z ₁ z ₂ ) corresponding to two distances (d _1, d ₂ ) between the edges of the electrodes facing each other.

The arrangement shown in Figure 4 introduces two levels of detection. In the first, corresponding to the distance d _1; the electrodes are relatively close to each other and the field is relatively intense. Droplets of small size (II) can be detected by this intense field. Although the area corresponding to this part is relatively small, the probability of finding small drops in this area is sufficient. In the portion in which the electrodes are at a distance d ₂ from each other, the field is less intense but sufficient to detect large drops (I). The increase in the area between the electrodes also appreciably increases the probability of receiving one of these hypothesized fewer drops. Of course, a large drop located in the distance zone U ₁ is not excluded. It simply leads to a more intense signal.

The increase in the sensitive area of the sensor of FIG. 4 could be obtained in a mode corresponding, for example, to two electrodes distant from d _x by lengthening the electrodes (11) and (12). But this way of proceeding would give only one level of sensitivity, and secondly lead to an elongation of the electrodes that would harm the desired compactness. The model shown in Figure 4 has two distances.

It is possible to multiply the levels in the same way to have multiple sensitivities. Nevertheless, in practice, the conditions encountered can be analyzed with a small number of sensitivity levels. A test device makes it possible to check the operation described above. In this device a conductive metal ball attached to the end of a non-conductive support simulates a drop of water. This element is approached to the glazing sample carrying the sensor. The device is powered at a frequency of 30 kHz. The measurement is conducted differentially. The relative variation is determined for a set of positions of the field modifying element vis-à-vis the electrodes. The mapping of the sensitivity of the device for different types of simulation of the drops corresponding to alterations of the fields of variable intensity is thus drawn up. The advantage of having several sensitivities is well verified by this means.

The detection level in the sensors according to the invention can still vary continuously. For this type the distance separating the edges of two neighboring electrodes are increasing continuously. For electrodes with straight edges, the defined surface is then wedge-shaped type. One embodiment is shown in FIG. 7. The sensor of FIG. 7 comprises two electrodes (13,

14) as before. These electrodes define between them a sector (15). The distance between the electrodes, in other words, the width of this sector, increases continuously from one end to the other of the electrodes. In the presented mode the distance from the narrowest to the widest varies in a ratio of the order of 1/5. The electric field and the sensitivity of the sensor are obviously smaller in the most distant parts and vice versa, to conveniently detect the drops of water regardless of their size.

In the mode shown in FIG. 5 the variation of detection sensitivity related to the distance between the electrodes is modulated by a simultaneous variation of the width of the electrodes themselves.

The ratios between the narrowest and widest electrode portions may vary depending on the importance of the desired effect. In the mode shown, this ratio is as before of the order of 1/5.

The configuration is still flexible in the ratio of the respective dimensions of the electrodes and the sectors (x / d) therebetween. In the form presented the dimensions are comparable. They are advantageously in a ratio of 1/2 to 2/1 to have a sensibility adjusted best with a sensor that remains as compact as possible.

As a function of the signal analysis mode, the capacitive sensors used may have more than two electrodes. In so-called differential treatments, two series of measurements are performed simultaneously between two sets of electrodes. In the absence of rain, two measurements show a specific ratio which is modified when drops occur between the sets of electrodes.

In differential systems one electrode can be common to both sets. FIG. 5 is an illustration of an embodiment of a three-electrode differential sensor (16, 17, 18) according to the invention, having a configuration similar to the sensor shown in FIG. 4. The electrodes (16) and (17) delimit as previously two zones Z ₁ and Z ₂ of different sensitivities. Between the electrodes (17) and (18) only one sensitivity is shown. A multilevel configuration can also be included between these electrodes. Since the determination is differential, it is less important to provide a "measure" than a variation in the level of the detected signals. In this case the important thing is to have on a set of electrodes at least the differentiated sensitivity levels according to the invention. Figure 6 shows another embodiment of a three-electrode sensor (19, 20, 21). In this system the central electrode (19) common to both sets, envelops the electrodes (20) and (21). The zones of sensitivity Z ₁ and z ₂ are arranged on both sides of the electrode (20). As for Figure 5 the electrode (21) has only one distance vis-à-vis the central electrode (19). As above a symmetrical arrangement is also possible.

The advantage of the configuration of Figure 6 is to promote the compactness of the detector. Otherwise the electrodes in parallel should have a double length, which could advance inappropriately in the viewing areas of the glazing. The compactness makes it possible to maintain the entire sensor in a part of this glazing possibly already obscured by the presence of objects such as the rearview mirror.

FIG. 8 shows an embodiment of the electrodes similar to that of FIG. 7 for the implementation of a differential detection. The embodiments presented obviously have no limiting character. These embodiments aiming to have several levels of sensitivity can correspond to extremely varied electrode designs without departing from the scope of the invention.

Claims

A capacitive rain sensor disposed on a window, comprising electrodes and conductors feeding them, formed of a conductive material, wherein the electrodes are configured in size and arrangement relative to each other, so as to present simultaneously several levels of sensitivity.

2. capacitive rain sensor according to claim 1 wherein the different levels of sensitivity correspond to different effective surfaces.

A capacitive rain sensor according to claim 1 or claim 2 wherein the distance (d) between the facing edges of two adjacent sensing electrodes is not uniform.

4. A capacitive rain sensor according to claim 3, comprising at least two distinct distances (d _1, d ₂ ) between the facing edges of two contiguous detection electrodes.

A capacitive rain sensor according to claim 3 or claim 4, wherein in addition to the distance (d) between the edges of the electrodes, the sensitivity is modulated by that of the extent (x) of the electrodes facing each other. .

6. Capacitive rain sensor according to one of claims 1 to 3 wherein the distance between the neighboring detection electrodes varies continuously.

7. Capacitive rain sensor according to one of claims 3 to 6 wherein the ratio of the extreme distances between the detection electrodes is at most 1/10 and preferably at most 1/5.

A capacitive rain sensor according to claim 6 or claim 6 wherein the ratio of the extreme (x) ranges of the detection electrodes, is at most 1 to 10 and preferably at most 1 to 5.

9. Capacitive rain sensor according to claim 8 wherein the trapezoidal shaped electrodes delimit between them also trapezoidal sectors.

10. A capacitive rain sensor according to claim 8 or claim 9 wherein the ratio, extended from the electrode to the distance between the electrodes, at the same level (x / d) is between 1/2 and 2/1.