Abstract:
A sensor device is described for detecting wetting, in particular rainfall and soiling, on a windshield that has at least one transmitter and at least one receiver for the light emitted by the transmitters where transmitters and receivers are arranged at the corner points of an imaginary triangle or parallelogram. As a result, the outside dimensions of the sensor are reduced, and the ratio of sensitive area to base area of the sensor is improved.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a sensor device for detecting wetting of a windshield. 
     BACKGROUND INFORMATION 
     German Patent No. 197 01 258 describes a sensor device that operates according to an optoelectronic principle for controlling wash/wipe systems for automotive windshields. The sensor device has multiple transmitters and at least one receiver for injecting and outputting via a coupling means a defined radiation into and out of the windshield whose wetting due to moisture or soiling is to be measured. The radiation is completely reflected at least once in the windshield on the (dry) surface of the windshield due to the injection angle and finally is output again to a receiver at a predetermined location. Total reflection is prevented by wetting of the surface of the windshield (air, water, ice, dirt, fog, etc.), resulting in wetting-dependent radiation losses due to output of a portion of the radiation from the original beam path, e.g., due to water droplets. 
     The diminished radiation detected by the receiver is outputted as a sensor signal to a signal processing arrangement, where it is analyzed with regard to controlling a wash or wipe system for an automotive windshield, for example. A controller controls the wiper motor in continuous or interval wiping mode as a function of the sensor signal analyzed. 
     According to German Patent No. 197 01 258, the transmitters are arranged concentrically around the receiver on the coupling means or concentrically in sections, the coupling means having a circular or toroidal design. Thus, a circular base area is spanned by transmitters, receivers and coupling means. For this reason, the sensor, i.e., the sensor casing, is in the form of a round cylinder. 
     The sensitive area of the sensor is defined by the sum of the measuring ranges of the windshield located approximately between a transmitter and the respective receiver, i.e., the one receiving the radiation. The measuring range is understood to be the range on the wettable side of the windshield within which the transmitter radiation is completely reflected in the absence of wetting, and therefore the transmitter radiation can be outputted more or less in the measuring range because of wetting of the windshield. 
     One disadvantage here is the circular base area defined by the transmitters, receivers and coupling means and the round cylindrical sensor casing, leading to a relatively great extent of the sensor and its contact area on the windshield and therefore making the sensor noticeable and causing interference for the driver of the vehicle when it is mounted within the wiping area on the windshield, as is customary today. 
     Another disadvantage here is the small proportion of sensitive area relative to the base area of the sensor, i.e., its contact area. This is due to the fact that with a predetermined number of transmitters, e.g., eight, the non-sensitive areas between the transmitters arranged on the outer perimeter of the base area are large, usually much larger than the sensitive areas. 
     To increase the size of the sensitive area, a great number of transmitters could be used. However, then the sensor would be more expensive without eliminating the disadvantages of the extent, i.e., contact area, of the sensor. 
     SUMMARY OF THE INVENTION 
     The sensor device according to the present invention has the advantage that the images of the transmitter and receiver as optical elements projected onto a plane parallel to the windshield form the corner points of a parallelogram or an isosceles triangle. The ratio of sensitive area to base area is increased by this arrangement of optical elements. 
     Due to the reduced base area, the outside dimensions of the sensor on the windshield and thus its contact area are likewise reduced. It is especially advantageous that due to the trapezoidal base area, rectangular outside dimensions of the sensor casing are selected, so that the base area can be arranged in the contact area with optimal utilization of the latter. In addition, a rectangular contact area, i.e., casing, is less expensive to manufacture. 
     The arrangement of the transmitters and receivers as a parallelogram is especially advantageous, such that two transmitters or two receivers are arranged at the opposite corner points. Thus four measuring zones, i.e., sensitive areas, of the sensor are implemented with only two sensors and two receivers by using transmitters whose transmitter light is bundled and deflected in two directions. The accuracy of the sensor in detecting wetting of the windshield is thereby greatly improved. This doubles the sensitive area in comparison with conventional transmitters emitting radiation to only one receiver. 
     It is also advantageous that the distances between the transmitters and receivers and thus the sides of the parallelogram are the same length. This converts the parallelogram to a rhombus. In particular, the distances are defined by the choice of only total reflection of the transmitter radiation in the windshield before the radiation is detected by a receiver. Consequently, the distances are minimal and thus the sides of the base area are also minimal. 
     It is likewise advantageous that the angles between the sides of the base area can be varied, and thus the base area can be adapted to the outside dimensions of the sensor and vice versa. For example, if only a narrow rain sensor can be used because of the automobile manufacturer&#39;s requirements or the specifics of a given vehicle, smaller acute angles can be selected accordingly without any great effort until the extent of the base area can be optimally integrated into the outside dimensions of the sensor casing. 
     Consequently, it is especially advantageous for the transmitters and receivers to be arranged within the contact area of the sensor device, with the base area being optimally integrated into the contact area while at the same time the ratio of the sensitive area to the base area is increased. This yields an inexpensive sensor with small outside dimensions. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a first embodiment of a sensor device according to the present invention. 
     FIG. 2 shows a through section of a sensor device according to the present invention. 
     FIG. 3 a  shows a first implementation of a second embodiment of a sensor device according to the present invention. 
     FIG. 3 b  shows a second implementation of a second embodiment of a sensor device according to the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIGS. 1 and 2 show a rectangular contact area  10  with width b and length  1  of a sensor device having a sensor casing  24  on a windshield  22 . Its contact area  10  is defined by contact of the sensor with windshield  22 , for example. In general, the outside dimensions of contact area  10  correspond to the outside dimensions of the sensor device. However, contact area  10 , for example, is also a projection of the outside dimensions of sensor casing  24  with a front view of windshield  22 . Therefore, as an alternative, sensor casing  24  may bulge over the contacted area of windshield  22 , accommodating optical elements of the sensor within it. 
     The sensor device is mounted, for example, in the wiping area of a windshield wiper on the inside of an automotive windshield  22  or integrated into the base of a vehicle rearview mirror. Not shown here is the mounting of sensor casing  24  or a fiber optic body on windshield  22 . 
     The fiber optic body or casing  24  is preferably mounted on windshield  22  by gluing, the former having the function of injecting light  26  emitted by a transmitter  12  into windshield  22  and outputting light  26  directed in windshield  22  by partial or total reflection to a receiver  14  at another predetermined location. This takes place at lenses  28 , refractive surfaces or mirrors integrally molded on the fiber optic body to bundle, deflect or divert beams  26  in the desired direction. 
     Above contact area  10 , i.e., the fiber optic body, two light emitting transmitters  12  and two light detecting receivers  14  are mounted on an essentially conventional mounting device within sensor casing  24 . A circuit board  30  carrying the electronic components or a sensor control is often available as a mounting device for attachment of transmitters  12  and receivers  14 . Transmitters  12  are preferably light emitting diodes (LEDs), receivers  14  are light receiving diodes (LRDs), with transmitter radiation  26  preferably being in the infrared (IR) range or in the visible (VIS) range. 
     Transmitters  12  and receivers  14  span a base area  16  (indicated with dotted lines) corresponding according to the present invention to a parallelogram or a double triangle having sides a′ and a″. Preferably two sides a′, a″ of base area  16  are of equal or approximately equal length because of the tolerances in the dimensions and in assembly of transmitters  12  and receivers  14 . 
     Distances a′ and a″ between transmitters  12  and receivers  14 , i.e., the sides of base area  16 , are defined, inter alia, by the wavelength of the emitted radiation of transmitter  12 , the thickness of windshield  22  and of the fiber optic body, the refractive index of windshield  22  as well as the angle of incidence and the point of incidence of radiation  26  into windshield  22  so that radiation  26  injected into windshield  22  is completely reflected only once at the surface of windshield  22 , preferably on the outside of the automotive windshield, and then guided out of windshield  22  to receiver  14 . 
     With more than a desired total reflection on the wettable outside of windshield  22 , distances a′ and a″ of transmitters  12  and receivers  14 , respectively, would be selected to be larger accordingly. Furthermore, distances a′ and a″ from a transmitter  12  and receiver  14  that do not belong together, i.e., transmitter  12  does not emit radiation to this receiver  14 , may be selected as desired. 
     Sensitive areas  20  illustrated here correspond to the areas on the wettable side of windshield  22  where there is total reflection of radiation  26  when windshield  22  is not wetted. The extent of sensitive areas  20  will vary depending on the arrangement of transmitters  12  and receivers  14  relative to windshield  22 , the thickness of windshield  22  and the diameter of transmitter beam  26 . 
     According to FIG. 1, distance a″ for example corresponds to total reflection in windshield  22  of light  26  emitted by transmitter  12  to receiver  14 . Distance a′ and thus the distance between independent transmitters  12  and receivers  14  can be selected as desired. Therefore, the dimensions of the parallelogram and thus of base area  16  and of contact area  10  are determined essentially by the choice of distance a′ and angle α. 
     Angles α and β between two sides of the parallelogram can be selected freely, angle α here by definition referring to each smaller angle or acute angle of the parallelogram. Angle α is assigned an angle range from almost zero to ninety degrees. 
     Criteria for the selection of an angle include, for example, the outside dimensions of the sensor, in particular the ratio of length  1  to width b of its contact area  10 , as well as the dimensions of sensitive areas  20  between transmitters  12  and receivers  14 , so that there is no unwanted overlapping of sensitive areas  20  and thus a restriction of the sensitivity of the sensor. In addition, the beam diameter and the dimensions of sensors  12  and receivers  14  must also be taken into account. Furthermore, distance a″ between transmitter  12  and receiver  14  and thus the number of total reflections achieved in windshield  22  must also be considered. 
     For example, angle α will usually be selected to be small when contact area  10  of the sensor is long and narrow, i.e., rectangular, e.g., α=45°. On the other hand, angles α, β may form right angles, yielding a square or rectangular design of the sensor. An angle range of 65° to 70° is preferred. 
     In an improved version of this embodiment, four measuring zones are implemented with two transmitters  12  and two receivers  14  by bundling radiation  26  emitted by transmitters  12  in two directions using lenses  28  or the like, and deflecting it to two receivers  14 . Consequently, all distances a′, a″ are to be selected so as to yield a given number of total reflections in windshield  22 . Distances a′, a″ are preferably equal. Base area  16  is thus a rhombus, i.e., a double triangle, composed of two isosceles triangles. The four measuring zones thus double sensitive areas  20  of the sensor and therefore improve its sensitivity in detection of rainfall with the same design. 
     FIGS. 3 a  and  3   b  show two implementations of a second embodiment of the sensor device having a contact area  10  of width b and length  1 . A light emitting transmitter  12  and two light detecting receivers  14  are arranged above contact area  10 , i.e., the fiber optic body. According to the present invention, transmitter  12  and receivers  14  span an isosceles triangle having sides a′ and a″ indicated by a dashed line. Two sides a″ of base area  16  are preferably the same length or approximately the same length, and base side a′ can be selected as desired. 
     Distances a″ between transmitter  12  and two receivers  14  are defined, as already described regarding the first embodiment, so that radiation  26  injected into windshield  22  is reflected completely once or a plurality of times at the surface of windshield  22  and then is guided out of windshield  22  to both receivers  14 . 
     Merely for the sake of thoroughness, it should be pointed out that a different length of sides a″ requires a separate analysis by a signal processing arrangement of measuring zones  20  and the two sensor signals output by two receivers  14 . In particular, amplification of the signal which is usually necessary must be adjusted separately. 
     Therefore, the dimensions of the triangle and thus of contact area  10  are essentially determined by the choice of distance a′ and angle α. FIGS. 2 a  and  2   b  show different angles α and resulting different dimensions b, l of contact area  10  of the sensor device.