Abstract:
A sensor arrangement is suitable for detecting if a windscreen of a motor vehicle is fogging. The sensor arrangement includes a heat conducting layer disposed on a supporting surface, the layer exhibiting good heat conducting properties. A support plate is arranged on top of the heat conducting layer, including a recess and several electrical conductor tracks. A moisture-sensitive sensor element is arranged in the region of the recess of the support plate. The sensor element is electroconductively connected to the conductor tracks on the support plate. The temperature difference between the sensor element and the supporting plate may not exceed a given limit when measurement occurs.

Description:
FIELD OF THE INVENTION 
   The present invention is directed to a sensor system which is suited, in particular, for detecting the fogging or misting of the inside of a motor-vehicle windshield. 
   BACKGROUND INFORMATION 
   Numerous approaches are conventional for avoiding the fogging of windshields in automobiles. U.S. Pat. No. 4,408,660, for instance, describes regulating the motor-vehicle air-conditioning system on the basis of output signals from many different types of sensors, in order to prevent fogging. Among the sensors used or required is a humidity sensor, which measures the relative humidity or the dew point on the particular motor-vehicle windshield. However, the foregoing does not include any further references to the specific arrangement of the humidity sensor. Moreover, the control concept requires at least one further sensor for measuring the temperature, so that considerable outlay is entailed for sensor technology. 
   Another approach for dealing with the above problem provides for using an infrared sensor to determine the surface temperature of one region of the automobile windshield. With the aid of this sensor and other humidity sensors in the passenger compartment, the humidity and/or the temperature of the passenger compartment can be regulated via the motor-vehicle air-conditioning system, to reliably prevent the windshields from fogging up. The drawback of this variant is likewise the substantial outlay for sensor technology. 
   In summary, therefore, existing approaches typically require a multiplicity of sensors to reliably sense the fogging of the motor-vehicle windshield. 
   SUMMARY 
   It is an object of the present invention, therefore, to provide a sensor system that is simple to construct and is suited for reliably sensing the fogging of a supporting surface, in particular of a motor-vehicle windshield. 
   The proposed measures may ensure that, with the aid of a single sensor system according to the present invention, it is possible to reliably sense and, therefore, prevent the fogging of a supporting surface, for example the inside of a motor-vehicle windshield. For this purpose, the output signals from the sensor system according to the present invention are fed to a corresponding climate control, i.e., to a controlled air-conditioning system. The sensor system according to the present invention merely detects the relative humidity on the supporting surface. It may be ensured, in this connection, that the sensor element used precisely detects the actual relative humidity on the supporting surface, since, in the measuring operation, the sensor element has virtually the same temperature as the supporting surface. Various measures are applied to make certain that there is merely a slight temperature differential between the supporting surface and the sensor element. 
   Thus, the sensor system according to the present invention provides a reliable manner to overcome the fogging problem described above, while entailing minimal outlay for sensory technology. 
   Moreover, by eliminating additional sensor elements, there are altogether fewer system errors. This is because, ultimately, each individual sensor element contributes to the overall error in the context of detecting fogging. The result is greater system accuracy and reliability. 
   According to one example embodiment of the present invention, a sensor system for mounting on a supporting surface includes: a thermally conductive layer having a high thermal conductivity arranged on the supporting surface; a support plate on the thermally conductive layer including at least one recess, a plurality of electrical conductor tracks and at least one bore adjacent to the recess, the bore including a thermally conductive coating; a humidity-sensitive sensor element positioned in a vicinity of the recess of the support plate and electroconductively connected to the conductor tracks on the support plate, the sensor element positioned on a side of the support plate above the recess and oriented averted from the supporting surface; and a fastening arrangement configured to fasten the support plate on the supporting surface; wherein the sensor system is configured so that a predefined temperature difference is not exceeded between the sensor element and the supporting surface during a measuring operation. 
   According to another example embodiment of the present invention, a sensor system for mounting on a supporting surface includes: a thermally conductive layer having excellent thermal conductivity, placed on the supporting surface; a support plate on the thermally conductive layer, which includes at least one recess, as well as a plurality of electrical conductor tracks; a humidity-sensitive sensor element, which is positioned in the vicinity of the recess of the support plate and which is electroconductively connected to the conductor tracks on the support plate; it being ensured that a predefined temperature difference is not exceeded between the sensor element and the supporting surface during the measuring operation; as well as fastening means for securing the support plate and the sensor element to the supporting surface. 
   Further advantages of the sensor system according to the present invention and details pertaining thereto are derived from the following description of exemplary embodiments, on the basis of the figures. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of a motor-vehicle windshield, including a first example embodiment of the sensor system according to the present invention. 
       FIG. 2   a  is a part-sectional view of the sensor system illustrated in  FIG. 1 . 
       FIG. 2   b  is a part-sectional view of the sensor system illustrated in  FIG. 1 . 
       FIG. 3  is a part-sectional view of a second example embodiment of the sensor system according to the present invention. 
   

   DETAILED DESCRIPTION 
     FIG. 1  illustrates a part of the inside of a motor vehicle windshield  1 , which, in this exemplary embodiment, is used as a supporting surface and in which fogging is to be prevented. Sensor system  10  is mounted on the side of motor-vehicle windshield  1  facing the passenger compartment.  FIG. 1  also illustrates a first example embodiment of such a sensor system. The output signals from sensor system  10  are fed via a connecting cable  20  to a controlled motor-vehicle air-conditioning system. On the basis of the delivered humidity-dependent sensor signals, the air-conditioning system prevents the inside of motor-vehicle windshield  1  from fogging and, thus, a potential sight obscuration. For this reason, it is possible to vary the heating temperature and/or the ventilation flow rate in a defined manner, via the motor-vehicle air-conditioning system. 
   Using sensor system  10  according to the present invention, it is merely the relative air humidity that is determined on that side of motor-vehicle windshield  1  on which sensor system  10  is mounted. As soon as a predefined limiting value for the relative air humidity RH, e.g., RH=95%, is exceeded, as previously indicated, appropriate counter-measures are taken in terms of control engineering, via the motor-vehicle air-conditioning system. In comparison to conventional fog-prevention approaches which always focused on determining the dew point on the motor-vehicle windshield, sensor system  10  according to the present invention provides a comprehensive approach based on simple sensory technology. In particular, the approach according to the present invention manages without additional sensors, such as temperature sensors, etc. 
   In  FIG. 1 , of sensor system  10  according to the present invention a support plate  11  is illustrated on which is mounted a humidity-sensitive sensor element  12  over a recess  15 , as well as further electronic components  13   a – 13   e . Support plate  11  is configured as a conventional circuit plate of FR4 material and does not contain any electrical conductor tracks for contacting the components mounted thereon. The thickness of support plate  11  may be within the range of 1–2 mm. Via connecting cable  20 , sensor system  10  is connected to the downstream signal-processing unit, for example to a corresponding climate control of the motor-vehicle air-conditioning system. In this context, connecting cable  20  may be detachably connected to support plate  11 , for example using an appropriate connector. 
   In this example embodiment, sensor element  12  is configured as a conventional capacitive thin-film humidity sensor, which has a dielectric, e.g., a suitable polymer material, between two electrodes, which changes capacitance as a function of humidity. The electrodes and dielectric are usually placed on a suitable carrier substrate, such as glass. Accordingly, on the output side, sensor element  12  supplies electric signals in a generally conventional manner. They are a measure of the relative humidity in the particular ambient environment. 
   As illustrated in  FIG. 1 , sensor element  12  is positioned in the vicinity of a rectangular recess  15  of support plate  11 . Via recess  15  in support plate  11 , a certain air circulation is ensured in the area of the sensor. The longitudinal axis of rectangular recess  15  is aligned perpendicularly to the longitudinal axis of sensor element  12 . In this example embodiment, sensor element  12  is positioned on the side of support plate  11  which is oriented so as to face away from windshield  1 . For this reason, sensor element  12 , as well as the other components  13   a – 13   e  on support plate  11 , are configured as SMD components. Thus, on that side in the area of bearing surface on support plate  11 , sensor element  12  has electrically conductive contacts, which face support plate  11 . In this manner, a simple, efficient fabrication of sensor system  10  may be ensured, e.g. since support plate  11  is then able to be populated in automated fashion. 
   Also illustrated in  FIG. 1  is that sensor element  12  has a rectangular form and, at least in one dimension, is greater in length than recess  15 , so that sensor element  12  rests in two contact areas on support plate  11 . 
   The additionally provided components  13   a – 13   e  on support plate  11  are used for processing the output signals from sensor element  12 . A further signal processing of this kind may be implemented, for instance, in the form of a signal amplification; in addition, the conversion into a simply transmittable and further processable signal may be provided. Further components may be used for protecting sensor system  10  from electrical overvoltages and disturbing electromagnetic influences, etc. The various electronic components  13   a – 13   e  are provided on support plate  11  with a protective coating, which protects them from mechanical influences, as well as from humidity and pollution, etc. As an example, a protective lacquer manufactured by the firm Grace, marketed under the type designation Eccocoat, is suited for this purpose. 
   To further clarify the first exemplary embodiment of the sensor system according to the present invention, reference is made to  FIGS. 2   a  and  2   b , each illustrating a part-sectional view of sensor system  10  illustrated in  FIG. 1  in the vicinity of recess  15  of support plate  11 . Illustrated here, in turn, is motor-vehicle windshield  1  having sensor system  10  positioned thereon. 
   As illustrated in  FIG. 2   a , support plate  11  is secured in this example embodiment via a thermally conductive layer  14  to motor-vehicle windshield  1 , i.e., to the supporting surface. In the illustrated exemplary embodiment, thermally conductive layer  14  has various functions. Of primary significance may be thermal conduction of this layer  14 . Furthermore, in this example embodiment, thermally conductive layer  14  also acts as a fastening arrangement for support plate  11  mounted thereon and for sensor element  12 . 
   In another example embodiment, the thermally conductive layer or adhesive layer is formed as a doubly-sided cementing adhesive film, as is available from 3M under the type designations 9882, 9885 or 9890 as an example. An important consideration in selecting a suitable adhesive layer in this context may be that it may have the most efficient possible thermal conduction properties. The customary thickness of the adhesive layer varies, depending on the type used, for instance between 0.05 mm and 0.25 mm. 
   Positioned above recess  15  is humidity-sensitive sensor element  12 . Suitable sensor elements  12  are marketed by the Applicant hereof under the type-designations HC102 or HC103. In  FIG. 2   a , on the bottom side of sensor element  12 , reference numerals  12   a ,  12   b  schematically denote the electrically conductive contacts, which are used to make electrical contact with and connect to the conductor tracks in support plate  11 . Contacts  12   a ,  12   b  are situated in sensor element  12  in the contact area, where the element  12  rests on support plate  11  outside of recess  15 . 
   Sensor element  12  used, which in this exemplary embodiment is configured as an SMD component, has a humidity-sensitive surface, which, in the same manner as contacts  12   a ,  12   b , is oriented toward support plate  11 . Accordingly, in this exemplary embodiment of a humidity-sensitive sensor element  12 , contacts  12   a ,  12   b  are positioned on the same side. 
   Illustrated in  FIG. 2   a  in support plate  11  are vertical bores  16   a ,  16   b  which are placed so as to be directly adjacent to recess  15 . Via these bores  16   a ,  16   b , an efficient thermal conduction may be ensured between the supporting surface, i.e., windshield  1  and sensor element  12 . In the plan view of a subsection of the sensor system illustrated in  FIG. 2   b , it is illustrated that, in this exemplary embodiment, a multiplicity of such bores  16   a – 16   h  are positioned adjacently to recess  15 . 
   The diameters of bores  16   a – 16   h  may be selected to be identical to the diameters of the plated-through holes or bores otherwise provided on support plate  11 , i.e., diameters between 0.3 mm and 0.8 mm are selected. As illustrated in  FIG. 2   b , a plurality of separate bores  16   a – 16   h  having smaller diameters, instead of fewer bores having larger diameters in support plate  11  may be provided. 
   In addition, in this exemplary embodiment, the various bores  16   a – 16   h  have a coating which may have good heat conduction properties, for example a copper coating, in the bore region, to ensure an excellent thermal conduction between supporting surface  1  and sensor element  12  via bores  16   a – 16   h . The thermally conductive coating in the bore region may be formed to be as thick as possible, for instance thicker than 30 μm. An additional tinning of a copper coating of this kind further improves the thermally conductive properties. 
   Thermal conduction may be improved when such thermally conductive coatings of bores  16   a ,  16   b  are thermally conductively interconnected on that side of support plate  11 , which is oriented in the direction of sensor element  12 . For this, a corresponding coating  17  of support plate  17  in a surface area around sensor element  12  and its terminal contacts  12   a ,  12   b  may be provided, as illustrated in  FIG. 2   b . In this manner, a thermally conductive connection is established between the various bores  16   a – 16   h.    
   Bores  16   a – 16   h  in support plate  11 , as well as the use of thermally conductive layer  14  having excellent heat-conduction properties, may ensure in this exemplary embodiment that sensor element  12  and supporting surface  1 , i.e., the inside of the windshield in measuring operation, have a substantially identical temperature. In addition, in this exemplary embodiment of the sensor system according to the present invention, the configuration of sensor element  12  as an SMD component also contributes to the efficient thermal coupling, since this may ensure an intimate thermal contact to the layers placed before sensor element  12 . 
   It may, therefore, be important for sensor system  10  according to the present invention to have a best possible thermal contact between the side of windshield  1  on which fogging is to be prevented, and sensor element  12 . It may be provided at the least, however, that a specific—e.g., as low as possible—temperature difference between supporting surface  1  and sensor element  12  not be exceeded during measuring operation. In the present example, a maximum temperature difference of about 0.5° C. results between the side of windshield  1  facing sensor element  12 , and sensor element  12 . Such a temperature gradient is still considered as not being critical to the measurement. 
   A further measure for optimizing the heat-conduction properties in this area of sensor system  10 , in addition to the measures described above in connection with the borehole coating, may be, for instance, providing an additional, efficiently thermally conductive surface coating on that side of support plate facing thermally conductive layer  14 , and at least in the bore region. 
   A second exemplary embodiment of the sensor system according to the present invention is described with reference to  FIG. 3 , which illustrates a part-sectional view of the sensor system. 
   Mounted on the inside of motor-vehicle windshield  100 , i.e., on the corresponding supporting surface where fogging is to be prevented, is sensor system  110  according to the present invention. As in the first exemplary embodiment, this includes a thermally conductive layer  114 , formed as an adhesive layer, and, placed over it, a support plate  111  having a recess  115 . Humidity-sensitive sensor element  112  is placed on thermally conductive layer  114  in the vicinity of recess  115 . Also schematically illustrated in  FIG. 3  is an electronic component  113   a , which is used for further signal processing and is mounted on that side of support plate  111  which is oriented so as to face away from the motor vehicle windshield  100 . 
   With respect to suitable materials for the adhesive layer and for the support plate, reference is made at this point merely to the preceding example embodiment. 
   As a variation of the preceding example embodiment, sensor element  112  is positioned or placed in sensor system  110  according to the present invention. Thus, sensor element  112  in the area of recess  115  is directly positioned on thermally conductive layer  114  formed as an adhesive layer, the humidity-sensitive surface of sensor element  112  being oriented so as to face away from the motor-vehicle windshield. An SMD thin-layer humidity sensor is directly positioned, for example, on a doubly-sided cementing adhesive film, the humidity-sensitive surface of sensor element  112  being oriented toward support plate  111 . 
   Sensor element  112  is electrically contacted from that side of sensor element  112  which is oriented so as to face away from motor-vehicle windshield or supporting surface  100 . To this end, contacts  112   a ,  112   b , via which the connection to the electrical conductor tracks in support plate  111  is made, are positioned on this side of sensor element  112 . Accordingly, in this exemplary embodiment of sensor system  110  according to the present invention, the same humidity-sensitive SMD sensor element may be used as in the preceding example embodiment. The orientation of the humidity-sensitive surface or of contacts  112   a ,  112   b  is now selected to face away from the windshield. 
   Alternatively to the illustrated exemplary embodiment, it is also fundamentally possible to use bonding wires for electrical contacting, via which the contacts of the sensor element are electrically connected to the conductor tracks in the support plate. 
   The example embodiment of the sensor system according to the present invention illustrated in  FIG. 3  allows the sensor element  112  to be placed even closer to that side of motor-vehicle windshield or supporting surface  100  where a potential fogging is to be detected. It may thus be ensured to an even greater degree that a smallest possible temperature difference results between sensor element  112  and supporting surface  100 , i.e., that they exhibit nearly the same temperature as they do during measuring operation. 
   A further exemplary embodiment of the present invention may be provided to replace the thermally conductive layers formed as an adhesive layer, of the foregoing example embodiments, with a layer which merely has excellent thermal conductivity, but which is configured to be non-adhesive. Suited for this are, for example, ceramic-filled polymer films distributed under the designation Keratherm by the firm Kerafol, Keramische Folien GmbH. In such a case, appropriate mechanical fasteners may be used to secure the sensor system to the windshield or the supporting surface. For this, it may be provided, for example, to use the base of a rearview mirror as a fastening arrangement. Alternatively, a gap at the top windshield edge between the vehicle cover and the windshield may be used as a mechanical fastening arrangement, etc. 
   The present invention is, therefore, not limited to the described exemplary embodiments. Rather, within the scope of the considerations according to the present invention, there are a number of other variants of exemplary embodiments.