Water drop sensor

Abstract: A water drop sensor (20) comprises a pair of electrodes (21,22) disposed in association with a portion (13) of a surface of a vehicle window (10) within a wipe pattern and having edges (25) defining an extended insulating gap between the electrodes (21,22). The electrodes (21,22) form a gap (25) which varies in width over the range of diameters of drops to be sensed. The sensor shows more sensitivity to larger drops than to smaller drops as compared with a similar sensor having a constant gap width. The variation in gap width may be continuous or stepwise over the total sensing range.

Description: This invention relates to a water drop sensor for a motor vehicle window, and particularly to such a sensor having electrically conducting electrodes separated by a gap which may be spanned by a water drop. The prior art shows many examples of such sensors, which may be resistive, in which case the electrodes are placed on the outer surface of the window to be contacted directly by water drops, or capacitative, in which case the electrodes are separated from the water drops by a dielectric coating or a layer of glass so that the capacitance between the electrodes is changed by the water drops. An example of a construction incorporating a resistive sensor is shown in German Patent Specification DE-A-2 255264 whilst one incorporating a capacitive sensor is shown in United States Of America Patent Specification Number 4827198. In either case, however, an electrical parameter (resistance, capacitance) is varied by water drops on the window which overlie both electrodes. If such a sensor is placed within the wiping area of a vehicle window wiper system, it may be used for automatic control of wiper operation. Sensors of this type as shown in the prior art have been shown to operate successfully at a basic level, in that they are able to distinguish between the presence or absence of moisture on the windshield and are able to activate and deactivate wiper operation or vary the wipe cycle rate or wiper speed in some manner dependent on the amount of moisture on the windshield glass in the vicinity of the sensor. The prior art shows such sensors with conducting electrodes comprising, among other materials, transparent tin oxide affixed to the windshield glass within the wiper pattern so as to be cleared of moisture with each wipe cycle but generally located in a portion of the wipe pattern out of the main sight lines so as to minimize the visibility thereof to the vehicle occupants. However, such prior art sensors, although operable on a basic level, have not been sufficiently sophisticated to provide optimal wiper control in a commercial product. Such prior art sensors provide a gap between the electrodes of constant width. When used for wipe cycle or speed control, such sensors exhibit decreasing sensitivity as water drop size increases, so that a larger area of windshield must be covered with precipitation to produce a given probability of sensing the precipitation. This appears to be due at least partly to the fact that the surface area of a generally circular water drop on a window increases as the square of the diameter, so that a large diameter drop is more noticeable to a vehicle occupant than a smaller diameter drop, whereas the ability to bridge the insulating gap increases only directly with the diameter. This decrease in sensitivity with increasing drop size, for a prior art sensor with constant gap width, is illustrated in Figure 3, in which curves 30-32 show the percent glass coverage required to detect a predetermined number (1, 3, 8, respectively) of water drops as a function of drop diameter on the windshield. Curves 30-32 slope upwardly in this graph with increasing drop size, which shows that a greater portion of the window must be covered to detect the same number of drops. Thus, this slope of curves 30-32 means that the sensitivity of the sensor decreases with increasing drop diameter. However, one or more drops may fall on the window in the driver's line of sight before a detectable drop falls on the sensor; and each such drop affects an area of the window proportional to its area on the window. German Patent Specification Number DE-A-2 255264 discloses a vehicle mounted rain sensor that has a pair of electrodes separated by a high resistance. The electrodes are separated by a gap that varies along its length but the spacing is such that several drops of rain are required to bridge the electrodes over a substantial portion of their length and therefore does not provide a sensitive response to large and small drops within the range of drops sizes experienced by a vehicle. Thus, many vehicle operators would prefer a wiper control in which the sensitivity increases with increasing drop size, or at least in which the decrease in sensitivity with increasing drop size is reduced. The present invention seeks to provide an improved water drop sensor. According to the invention, there is provided a water drop sensor for sensing water drops of different diameters on a vehicle window, the sensor being adapted to provide a variable electrical signal related to the size of water drop sensed for use in adjusting the wiping rate of a vehicle window wiper system and comprising a pair of adjacent electrically conductive electrodes including adjacent edges forming one or more elongate gaps between the electrodes, characterised in that the one or more gaps have a varying width according to a calibrated range of drop diameters, wherein in use water drops overlying the gap or gaps cause the sensor to adopt an electrical characteristic dependent upon the position at which the water drops overlie the gap or gaps such that the electrical characteristic is a function of the diameter of said water drops within the calibrated range, which electrical characteristic is adapted to provide the varying electrical signal. This invention can provide a water drop sensor for a vehicle window having at least a reduction in the decrease in sensitivity, and preferably an increase in sensitivity with increasing drop size on the window. In a preferred embodiment, there is provided a water drop sensor comprising a pair of electrodes disposed in association with a portion of the outer surface of a vehicle window surface within the wipe pattern and having edges forming an extended insulating gap between the electrodes. The electrodes may provide a gap which varies in width over the diameters of drops to be sensed. The result is that the sensor can increase in sensitivity with drop diameter. The variation in gap width may be continuous or stepwise over the total range of drop diameters. The improved response is a characteristic of the sensor itself and can thus simplify the wiper control apparatus. An embodiment of the present invention is described below, by way of illustration only, with reference to the accompanying drawings, in which: Figure 1 shows a motor vehicle windshield with windshield wipers and an embodiment of water drop sensor; Figure 2 shows an enlarged view of the sensor of Figure 1; Figure 3 shows a graph of percent glass coverage against drop diameter for water drops on a windshield for a prior art resistive sensor with constant gap width; Figure 4 shows a graph of percent glass coverage against drop diameter for water drops on a windshield for the sensor of Figures 1 and 2; Figure 5 shows a second embodiment of water drop sensor; and Figure 6 shows a third embodiment of water drop sensor. Referring to Figure 1, a motor vehicle window or windshield 10 is provided with a pair of window or windshield wipers 11 and 12, which may be driven by an electric motor drive 13 including any known gear and/or linkage apparatus so as to sweep from an inner wipe position, as shown, through a wipe area 13, shown by a broken line 14, to an outer wipe position and back in repeated cycles. Wiper motor drive 13, which may include one or more wiper motors, may be controlled by an electronic controller 15 to provide intermittent operation with a variable delay between wipe cycles; and many such controllers are known. The cycle rate of the wipers is variable, either by variation of the speed of electric motor drive 13 for a variable wiper speed or by alternate activation and deactivation of electric motor drive 13 for intermittent operation. Variable cycle rate may further include fully automatic operation by turning the electric motor drive on when sufficient water is detected on windshield 10 and off when water is no longer detected. Windshield 10 is provided with a water drop sensor 20 indicated schematically in Figure 1. Sensor 20 is located within wipe area 13 and preferably near the top centre thereof, where it will not be in the driver's direct field of vision during driving and it may be partially hidden by the internal rear view mirror. Sensor 20 is shown enlarged in Figure 2 and comprises a pair of electrodes 21 and 22, each made of a coating of transparent, electrically conductive tin oxide on the outer surface of windshield 10. The edges of the electrodes may further provide a gap which comprises an arc similar to the arc of a point on the wiper blade moving across the windshield, so as to reduce streaking and/or damage across the gap. At least one embodiment of the sensor may be made by coating the entire windshield with electrically conductive material and removing material to form the gap, which significantly reduces cost. Of course, although the curved sensors shown are preferred, sensors with straight electrodes could also be used. Electrode 21 has a vertical lead portion 21a, a half length horizontal extending portion 21b, a vertical edge connecting portion 21c and a plurality of substantially parallel full length horizontal fingers 21d-21g. Electrode 22 similarly has a vertical lead portion 22a, a half length horizontal extending portion 22b, a vertical edge connecting portion 22c and a plurality of substantially parallel full length horizontal fingers 22d-22h. The fingers of electrodes 21 and 22 project between each other in an interdigital manner. For example, finger 22d of electrode 22 projects from vertical edge connecting portion 22c towards vertical edge connecting portion 21c between half length horizontal extending portion 22b and finger 21d of electrode 21 and extends between half length horizontal extending portion 21b and finger 21d. Finger 22e of electrode 22 projects similarly between fingers 21d and 21e of electrode 21; finger 22f extends between fingers 21e and 21f; and finger 22g extends between fingers 21f and 21g, the latter of which extends from vertical edge connecting portion 21c between fingers 22g and 22h. The result is that electrodes 21 and 22 form between them an extended gap 25 having a plurality of portions. For example, gap portion 25a is formed between half length horizontal extending portion 21b of electrode 21 and finger 22d of electrode 22; gap portions 25b and 25c are formed between the left and right sides, respectively, of fingers 22d and 21d; and gap portions 25d and 25e are formed between the left and right sides, respectively, of fingers 21d and 22e. In addition, gap portions 25f and 25g are formed between the left and right sides of fingers 22e and 21e; gap portions 25h and 25j are formed between the left and right sides, respectively, of fingers 21e and 22f; and gap portion 25k is formed between fingers 22f and 21f. Furthermore, gap portion 25m is formed between fingers 21f and 22g; gap portion 25n is formed between fingers 22g and 21g; and gap portion 25p is formed between fingers 21g and 22h. Finally, several short gap portions 25r, 25s, 25t and 25v are formed in the centre between finger pairs 22d/21d, 21d/22e, 22e/21e and 21e/22f, respectively. Although a gap 25x is formed between half length horizontal extending portion 22b and finger 22d, this portion does not contribute to the sensor since it is a gap between two portions of the same electrode 22. It is included only for appearance and convenience of manufacture and could be eliminated, if desired, by merging the portions of electrode 22 which form it. The half length horizontal extending portions 21b and 22b and the horizontally extending fingers 21d-g and 22d-h are all slightly curved to follow generally the arc of the wiper blade 11 which wipes across sensor 20. However, the curvature of the elements of electrode 21 is slightly greater than the curvature of the elements of electrode 22. The result is that each of gaps 25a-p has a width or distance between the electrodes forming it which varies continuously along the gap. The maximum gap width may be, for example, 3.7 mm; while the minimum gap width may be 1.1 mm. A larger sensor could provide a larger maximum gap width; but the minimum gap width is a result of practical limitations in the mass production techniques for applying tin oxide to windshield 10. As the state of the art in such processes advances, the minimum gap width of the continuously varying portion of the sensor may decrease. The reason for the varying gap width is to provide a sensor output with greater sensitivity to water drops on the window of larger sizes. A single large drop on the window has been found to be more annoying to many vehicle drivers attempting to look past it than many small drops. In a gap bridging resistive or capacitive sensor, the gap bridging effect of water drop size increases with the diameter of the drop on the window. However, the area of window affected by that water drop increases at a faster rate, closer to the square of the water drop diameter. A gap bridging sensor is generally provided with a constant gap width of the order of the diameter of the smallest water drops to which it is to respond. Assuming that a water drop has an equal chance of falling on any point on the sensor, the chance of it bridging the constant gap is proportional to the area formed by the drop centres of all water drop locations in which the water drop bridges the gap. For a constant gap width, such an area will be a rectangle which grows with drop diameter only in one dimension (across the gap). This produces a response as shown in the curves 30-32 of Figure 3, which shows a graph of percent glass coverage against drop diameter for water drops on a windshield with a resistive sensor having a constant gap width. The sensitivity of the sensor decreases as the required percent glass coverage increases. Thus, as mentioned above, for a given number of detected water drops, the sensitivity of the sensor decreases with increasing drop diameter. However, for a varying gap width, such an area will be a triangle which grows with drop diameter in both width (across the gap) and height (along the gap) simultaneously. Thus, the chance of detection of a water drop with a varying gap width increases with the area of the triangle, which is a faster rate with drop diameter than the chance of detection with a constant gap width. This can be seen in curves 35-37 of Figure 4, which are similar to those of Figure 3 except that the sensor is that of Figure 2, with a continuously varying gap width from 1 to 3.7 mm. The portion of each curve in this range of gap width shows precisely the opposite slope, which indicates sensitivity increasing with drop size in this range. If the range of gap width was extended beyond 3.7 mm, the range of this slope would be similarly extended. Even with this maximum gap width, above 3.7 mm the curves show a lower decrease in sensitivity than the similar portions of curves 30-32. Since practical manufacturing considerations limit the variation in sensor gap width for the tin oxide coating applied to windshield 10 to widths above 1 mm, gap portions 25r-v of much smaller width are formed after application of the tin oxide by coating removal through scoring or similar means to provide response to drops of smaller diameter. Gaps 25r-v are responsible for sensitivity to water drops of a diameter less than 1 mm, as shown in curves 35-37 of Figure 4. These gaps are most useful in detecting the small drops of fine mist. As such, they need not cover a large area of the windshield, since such fine mist tends to cover the entire windshield when it occurs and an occupant is generally tolerant of some degree of such fine mist before desiring windshield wiper operation. Clearly, however, advances in manufacturing techniques allowing smaller gaps within the coating process could render these short gap portions and the extra steps required for their formation unnecessary. In the case of a resistive sensor 20, a transparent insulating coating 40 may be applied over vertical lead portions 21a of electrode 21 and 22a of electrode 22 to prevent the continuous shorting of electrodes 21 and 22 outside wipe area 13, where wiper blades 11 and 12 do not clean away water drops during each wipe cycle. This coating 40 may be continued over the outer edge of half length horizontal extending portions 21b and 22b and vertical edge connecting portions 21c and 22c to provide additional abrasion protection for these connecting portions of the sensor. The exposed portions of lead portions 21a and 22a are, of course, separated sufficiently that they are unlikely to cause a water drop detection before some other part of the sensor. A capacitive sensor does not require additional insulation from the water but may include such a coating to reduce the contribution of the vertical lead members to sensor capacitance. Vertical lead portions 21a and 22a provide connection to electrical circuit apparatus for sensing the resistance or capacitance between electrodes 21 and 22 to detect the presence of water drops. Another embodiment of sensor is shown in Figure 5. Only a portion of this sensor 120 is shown, since the structure of the remainder will be clear from the embodiment of Figure 2 and the accompanying description. Sensor 120 comprises a pair of electrodes 121 and 122 similar in material, location, structure, shape and size to electrodes 21 and 22 of Figure 2 except that the horizontally extending portions and fingers forming gap 125 vary in a stepwise manner rather than in a continuous manner. It is only necessary for one surface of each portion of gap 125 to be stepped; the other surface may be straight and smooth. There may be, for example, five different gap widths distributed across the range of 1.1 mm to 3.7 mm. In any case, there is a sufficient number of step sizes, and the step sizes are sufficiently distributed in gap length across the total range, that gap 125 approximates the continuously varied width of gap 25 in the embodiment of Figure 2. The short gap portions 125r, 125s, 125t and 125v are identical with the similarly numbered short gap portions of Figure 2. Once again, gap portion 125x does not contribute to sensor operation and could be eliminated. Thus, the sensor of Figure 5 will provide substantially the same performance as that shown in curves 35-37 of Figure 4. Another embodiment of sensor is shown in Figure 6. As with sensor 120 of Figure 5, only a portion of this sensor 220 is shown in Figure 6. Sensor 220 comprises a pair of electrodes 221 and 222 similar in material, location, structure, shape and size to electrodes 21 and 22 of Figure 2 except that each of the horizontally extending portions or fingers has smooth edges to form a constant gap width. However, the widths of the gap portions increase from one to the next proceeding downwardly from the top of the sensor. For example, gap portion 225a defined between horizontally extending portion 221b and finger 222d of constant width (such as 1.1 mm), is the narrowest. The next lower gap portions 225b and 225c, may have a slightly greater constant width (such as 1.4 mm); and the increase in gap width may continue with lower gap portions to the largest gap, at the bottom of the sensor and therefore not shown, having a width near 3.7 mm. Of course, alternative arrangements are possible. For example, each of the half length gap portions such as 225a-225j may be a different gap width to increase the number of different widths. In addition, the order of size variation with gap portion may be different. There is a sufficient number of gap portions of different width, and they are sufficiently distributed in gap length, that gap 225 approximates the continuously varied width of gap 25 in the embodiment of Figure 2. The short gap portions 225r, 225s, 225t and 225v are substantially identical with the similarly numbered short gap portions of Figure 2. Thus, the sensor of Figure 6 will provide substantially the same performance as that shown in curves 35-37 of Figure 4. It will be apparent to those skilled in the art that the stepwise and continuous gap variation techniques illustrated in the embodiments of Figures 2, 5 and 6 may be combined in a single sensor to produce a great number of equivalent variations. Some gap portions may be continuously varied, some may be stepwise varied, some may be of constant width and some may combine the techniques in a single gap portion, as long as the gap widths vary across the range of drop diameters to be measured and there is a sufficient number of different gap widths to approximate a substantially continuous variation.

Claims: A water drop sensor for sensing water drops of different diameters on a vehicle window, the sensor (20) being adapted to provide a variable electrical signal related to the size of water drop sensed for use in adjusting the wiping rate of a vehicle window wiper system and comprising a pair of adjacent electrically conductive electrodes (21,22) including adjacent edges forming one or more elongate gaps (25,125,225) between the electrodes, characterised in that the one or more gaps have a varying width according to a calibrated range of drop diameters, wherein in use water drops overlying the gap or gaps cause the sensor to adopt an electrical characteristic dependent upon the position at which the water drops overlie the gap or gaps such that the electrical characteristic is a function of the diameter of said water drops within the calibrated range, which electrical characteristic is adapted to provide the varying electrical signal. A water drop sensor according to claim 1, wherein the sensitivity of the sensor is adapted to increase with water drop size in the sensing range. A water drop sensor according to claim 1 or 2, wherein the or at least one gap (25,125,225) varies substantially continuously in width between the electrodes. A water drop sensor according to claim 1, 2 or 3, wherein the or at least one gap (25,125,225) varies in a stepwise manner in width between the electrodes. A water drop sensor according to any one of claims 1 to 4, wherein the electrodes are resistive and are adapted to be disposed on an outer surface of a vehicle window for direct contact by water drops, wherein in use a water drop overlying the gaps or a gap alters the resistance of the sensor. A water drop sensor according to any one of claims 1 to 4, wherein the electrodes are capacitative and are adapted in use to be separated from water drops on a vehicle window by a dielectric, wherein in use a water drop overlying the gaps or a gap alters the capacitance of the sensor. A water drop sensor according to any preceding claim, wherein the gap or gaps are substantially of the same shape of a wiping arc of a wiper blade of a vehicle window wiper system. A water drop sensor according to any preceding claim, wherein each electrode comprises a connecting lead portion and a plurality of fingers (21, 22;121,122;221,222) extending therefrom, the fingers of the two electrodes being interdigitated, a gap being provided between each pair of fingers. A water drop sensor according to claim 8, wherein the fingers of the electrodes form arcs having differing radii of curvature. A water drop sensor according to claim 8 or 9, wherein the gaps between one pair of fingers and another have a different gap width.

Inventors: BUSCHUR JEFFREY JAMES, BUSCHUR, JEFFREY JAMES
Applicants: GEN MOTORS CORP, GENERAL MOTORS CORPORATION
Application year: 1992
Main classification: B60S1
IPC codes: B60S1