Abstract:
The invention relates to a sensor arrangement for determining a tank fill level of a liquid, especially a reducing agent for removing nitrogen from the exhaust gases of an internal combustion engine, which liquid is contained in a tank, at least two electrically conductive measuring electrodes being at least partially embedded in a carrier that is produced of an electrically non-conductive material. The sensor arrangement includes a temperature sensor, especially a resistor with negative temperature coefficient being used as a temperature sensor, which is integrated into the carrier.

Description:
CROSS-REFERNECE TO RELATED APPLICATION 
     This application is a 35 USC 371 application of PCT/EP2008/064827 filed on Oct. 31, 2008. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a sensor arrangement and to a method for producing a sensor arrangement. 
     2. Description of the Prior Art 
     In vehicles with an internal combustion engine, because of the increasingly stringent exhaust gas legislation expected in the next few years, the pollutant nitrogen oxide must among other things be reduced to an increased extent. A method that is used for this is the method of selective and catalytic reduction, in which the pollutant nitrogen oxide is reduced to nitrogen and water with the aid of a liquid reducing agent, such as an aqueous urea solution. In that method, the liquid reducing agent is stored in a tank and delivered from a tank to a metering point by means of a pump, via a line. A fill level sensor expediently indicates the fill level of the tank. 
     From German Patent Disclosure DE 101 62 269, a device for determining the fill level of a liquid is already known, which has measuring electrodes of various lengths injected into plastic. 
     From U.S. Pat. No. 6,063,350, it is also known to put a temperature sensor and a fill level sensor for a urea tank into contact via a common multiple-pin plug. 
     ADVANTAGES AND SUMMARY OF THE INVENTION 
     The sensor arrangement of the invention, and the method for producing such a sensor arrangement has the advantage over the prior art of furnishing compactly disposed means for sensing a plurality of measurement variables that are significant to the operation of a reducing agent tank in particular. 
     It is especially advantageous to dispose the temperature sensor in the interior of the support, in particular inside a tubular region inside the support, because as a result, a protected disposition of the temperature sensor is made possible in a support that is necessary anyway for the measuring electrodes that serve to measure the fill level. 
     It is especially advantageous in this respect moreover to provide a spray-coated covering region on the contact side of the measuring electrodes for the fill level and to join it integrally to material that locks the temperature sensor with good thermal conductivity inside the support. As a result, in a simple two-stage production method with a primary spray coating and after that a secondary spray coating, a compact multisensor arrangement that is resistant to the chemical effects of the reducing agent and moreover can react quickly to temperature fluctuations in the tank, is furnished. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiment of the invention are shown in the drawings and described in further detail in the ensuing description. In the drawings: 
         FIG. 1  shows a tank with a fill level sensor; 
         FIG. 2  shows a second fill level sensor; 
         FIG. 3  shows a third fill level sensor; 
         FIG. 4  is a fragmentary view of a sensor arrangement for determining a fill level; and 
         FIG. 5  is a schematic view of a sensor arrangement with an integrated temperature sensor. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows a tank  5 , with a fill level sensor  3  disposed inside the tank that extends, essentially parallel to the lateral tank wall  4 , from the bottom  9  of the tank to the highest liquid level  7  attainable in the tank of an aqueous urea solution (“Ad Blue”). On the top side of the tank, on which a closable opening  6  of the tank is disposed, the fill level sensor  3  is held in a vertical position by means of a covering region  18  joined to the top side of the tank. The fill level sensor  3  has a first measuring electrode  14 , embodied as a metal pin, and a second measuring electrode  16 , also embodied as a metal pin. Both measuring electrodes are embedded in a support  8 , which surrounds them completely with the exception of one measurement window each. The first measuring electrode  14  can enter into electrical contact with the liquid located in the tank via an upper measurement window  12 , which is formed by a suitable recess in the support  8 , if the level  7  of this liquid exceeds a certain maximum value. The second measuring electrode  16 , hereinafter also called the reference electrode, has a lower measurement window  10 , which is disposed near the bottom of the tank. On the side toward the tank top side  11 , the two measuring electrodes are joined by means of an electrical contact  20 , and at this point, the measuring electrodes protrude out of the support  8 , but, together with the part of the electrical contact  20  toward the measuring electrodes, they are surrounded by the covering region  18 . 
     The fill level sensor  3  makes discontinuous fill level measurement possible. If both measuring electrodes are in electrical contact with the liquid located in the tank, then, because both measurement windows are plunging at least partway into the liquid, when electrical voltage is applied to the electrical contact  20  via the liquid and the measuring electrodes, an electric current can flow. A flowing electric current is the signal that the tank is full. If the liquid is located between the two measurement windows  12  and  10  of the two metal pins of equal length injected into the plastic housing, then the current circuit is interrupted; that is, no current flows, which means that the tank is not full. The same is true if the liquid level drops below the measurement window  10 . The covering region  18 , which protects the electrical contact from the effects of liquid and keeps the fill level sensor in its vertical position, is embodied here as secondary spray coating of plastic on the support  8  that is embodied as a primary spray coating; this ensures safe operation of the fill level sensor even when chemically aggressive materials, such as an aqueous urea solution, are employed. 
       FIG. 2  shows a variant  22  of a fill level sensor for radial mounting in the tank. Here, a support element  28  is integrated in liquid-tight fashion with a vertical tank wall  4 . The support element keeps two measuring pins parallel to one another in a horizontal position. The first measuring pin  24  is disposed above the second measuring pin  26 . An electrical contact  20  is extended out of the support element  28  for contacting the fill level sensor outside the tank. 
     Similarly to the disposition of  FIG. 1 , a current circuit between the two measuring pins  24  and  26  closes as soon as both measuring pins, because of a sufficiently high fill level in the tank, come into contact with the liquid located in the tank. 
       FIG. 3  shows a fill level sensor or sensor arrangement  30  for determining a fill level of Ad Blue in a reducing agent tank. It has a first measuring electrode  34  and a parallel, longer second measuring electrode  36 , which are embodied as measuring pins, with the longer electrode acting as a reference electrode. The two measuring electrodes are retained by a support  32 , which is produced by partial spray-coating of the electrodes with plastic. On the side hereinafter called the locking side  17 , both measuring electrodes have a locking means, embodied as a circumferential groove  38 , which is surrounded entirely by the (primary) plastic spray-coated region or support  32 . In a similar way to the sensor of  FIG. 1 , the sensor arrangement  30  is arranged for being built into a tank or part of a tank in its interior, vertically parallel to a lateral tank wall. The locking side  17  of the measuring electrodes is then located near the top side  11  of the tank ( FIG. 1 ). The circumferential grooves  38  are located at approximately the same level, while the two measuring electrodes protrude to different depths into the tank; the longer, second measuring electrode  36  here serves as a reference electrode. The shorter measuring electrode  34  is held only on its half oriented toward the locking side  17  by the support  32 , while the half facing away from the locking side  17  protrudes freely downward so that it can be wetted by liquid in this region over its entire circumference, given a suitable liquid level in the tank. The reference electrode  36  is not only spray-coated with plastic in the vicinity of the locking means  38 , but in addition is surrounded halfway in a middle region  40  by the primary plastic spray-coated region or support  32 . On the side  19  of the reference electrode  36 , facing away from the locking side  17 , the support  32  has a retention means  42  for partial radial fixation of the reference electrode. This retention means is joined integrally to the support  32  and surrounds the reference electrode completely along its circumference in a manner similar to a shaft bearing. On the far side of the locking side  17 , of the middle region  40 , and of the retention means  42 , the end of the reference electrode protrudes freely downward, similarly to the half of the first measuring electrode that faces away from the locking side. The region  43  of the support  32  forming the retention means  42  can be produced in one operation in the context of a suitably performed spray-coating of the measuring electrodes with plastic. The support  32  furthermore, as a component joined integrally to it, has a closure edge  46 , onto which a secondary spray coating of plastic is applied as the covering region  48 . This covering region  48 , in a manner similar to that in the sensor arrangement of  FIG. 1 , serves the purpose of liquid-tight covering of the electrical contacts, not identified by reference numeral in  FIG. 3 , of the measuring electrodes on the top side of a tank. On the underside  47  of the sensor arrangement, which is disposed in the vicinity of the bottom of a tank, a temperature sensor  44  is selectively located, for measuring the temperature of the liquid located in the tank. 
     To avoid malfunctions and to avoid corrosion in the interior of the sensors for the fill level and the temperature, the measuring elements must be suitably sealed off from the chemically aggressive Ad Blue. In this connection, a primary spray coating region or support  32  of the measuring electrodes, embodied as measuring pins, and an adjoining secondary spray coating of plastic at the covering region  48 , which forms the sensor head, are provided. This provision, that is, providing a primary and a secondary spray coating, is necessary since the inserted parts or measuring electrodes, or an electrical contact connected to them, cannot be injected in a free-floating fashion. Between the primary spray coating and the secondary spray coating, gaps can occur, which are reinforced to a greater extent upon a temperature change because of the differing coefficients of thermal expansion of the inserted parts of steel (the measuring electrodes) and the surrounding plastic. In continuous operation, gaps can also occur between the measuring pins and the spray-coated plastic region or support  32 . The measuring pins or measuring electrodes  34  and  36 , produced for instance from special steel, have a considerably lower coefficient of thermal expansion than the plastic support  32 . The construction principle of the sensor arrangement of  FIG. 3  is directed to this situation. The measuring electrodes are fixed axially firmly in the plastic only in the upper region, that is, on the locking side  17 , each by one circumferential groove  38 , while the measuring pins in the middle, half-spray-coated region  40 , or in the region of the retention means  42  or on the far side thereof, can move axially, especially since the pins on the face end, that is, on the side toward the underside  47  of the sensor arrangement, protrude out of the plastic. This construction ensures a different axial thermal expansion of the measuring pins relative to the plastic support  32 . 
     Alternatively, the sensor arrangement may also have more than two measuring electrodes, which are likewise disposed relative to one another but which have graduated lengths between the length of the shortest electrode  34  and the length of the longest electrode  36 , the latter used as the reference electrode. Thus on the basis of the electrical conductivity between the respective measuring pin and the reference electrode  36 , it is possible to conclude whether more than one fill level position in the tank has been reached or not. Alternatively, the temperature sensor  44  may also be omitted. In a further alternative embodiment, the reference electrode  36  and the electrodes that are embedded halfway in the support  32  in a middle region  40  can also be covered, on their side toward the tank interior, by a thin plastic film that leaves the expansion behavior of the measuring pins essentially unaffected. Instead of being embodied as a secondary spray coating, the covering region  48  can be embodied as a housing, which is mounted on the closure edge  46  of the support  32 . 
       FIG. 4  shows a fragmentary view  51  of a sensor arrangement for determining a fill level. What is shown is the region of the sensor head that does not plunge into the liquid and by way of which the measuring electrodes  34  and  36  are electrically contacted. Components that are the same or similar to those already described above are identified by the same reference numerals and will not be described in detail again. On the far side of the circumferential groove  38 , on the side facing away from the tank interior, the measuring electrodes protrude past the support  32 . There, they are each connected to a respective electrical contact  64 , which, relative to one another, merges with an electrically insulated manner with a common round cable  68 . This electric round cable  68  leads to an electric evaluation circuit or an electronic control unit, which may also be embodied as an engine control unit. On the upper end of the support, the measuring electrodes are sealed off by means of sealing elements  56  and  58 , respectively, embodied as O-rings. On the side toward the tank interior, the sealing elements are bounded by the support  32 , and on the side facing away from the tank interior, they are separated from the remaining structure of the sensor head by means of a separator plate  60  mounted on the support  32 . The separator plate  60  has ribs  62 , which engage the support  32  and serve the purpose of radial fixation of the separator plate during assembly. The measuring electrodes pass through the separator plate  60  so that they can be in contact, on the side facing away from the circumferential grooves  38 , with the electrical contact  64 . The separator plate  60  has a central hole  63 , through which an electric double line  72 , beginning at the electric round cable  68  and shown in simplified form, is extended in the direction of the tank bottom. This double line  72  serves the purpose of electrically contacting a temperature sensor, not identified by reference numeral. Here, the electric double line  72  is located in a tubular region  52 , which is filled with the same material  43  that comprises the covering region  48  embodied as a secondary spray coating. The covering region  48  and the tubular region  52  are integrally joined to one another and can be produced in a single suitably designed spray-coating operation. The same material or the same plastic can be used as that comprising the support  32 , or alternatively, a different material can be used that has a coefficient of thermal expansion similar to that of the material comprising the support  32 . The electrical contacts  64  can be made by soldering, welding, or pressing; the electric round cable  68  is fixed by means of a cable mount  66  that is secured peripherally to the separator plate  60 . 
     Before the secondary spray coating or the production of the covering region  48  is done, the aforementioned O-ring seals  56  and  58  are sealed off from the secondary spray coating by means of the separator plate or mounting plate  60  also already described. By being guided closely in the vicinity of the measuring pins and circumferentially, this mounting plate, caught in the support  32  via the ribs  62 , prevents liquid plastic from entering the O-ring space in the production of the covering region  48 . The fine ribs  62  reinforce an axial sealing off of the O-ring space. The cable mount  66  ensures that the cable is fixed in its position in the secondary spray coating. In the arrangement in  FIG. 4 , the closure edge  46  of the support  32  known from  FIG. 3  is formed by an encompassing rib on the primary spray coating, and this rib furthermore has a melting edge embodied as a hook  54 . The secondary spray coating flows from above over this rib or melting edge. As a result of the shrinkage of the secondary spray coating on cooling during the production process, the secondary spray coating is pressed tautly on the encompassing rib and will thus in terms of force relieve the melting edge, which during the injection operation, as a result of the melting, has created a partial sealing location: The finished primary spray coating is cold, while the secondary spray coating during the spray-coating is initially hot. On cooling down, the secondary spray coating shrinks, and particularly in the region between the hook  54  and the closure edge  46  of the support, it builds up pressure stresses on the primary spray coating. As a result, the melting edge that is responsible for the sealing is relieved in terms of force. The round cable  68 , which protrudes out of the secondary spray coating of the covering region  48 , is sealed off via a mechanical seal by means of a sealing element  69  of an elastomer material and a tension element  70  fitted over it. The sealing element  69 , for instance an O-ring, is a radial seal that is reinforced by an axial force component by means of the fitted-over tension element  70 . The tension element  70 , embodied for instance as a plastic sleeve, is joined to the secondary spray coating via a plastic weld. 
       FIG. 5  shows a partly schematic view of a fill level sensor with an integrated temperature sensor  74 , embodied as an NTC resistor. Details of how the measuring electrodes required for fill level measurement and the electrical contacting of these measuring electrodes are not shown in detail in  FIG. 5 ; it serves to explain the construction and integration of the temperature sensor with the sensor arrangement for determining a fill level. The electric double line  72  already schematically shown in  FIG. 4  leads to the underside  47  of the sensor arrangement, where it electrically contacts the NTC resistor, that is, a resistor with a negative temperature coefficient. The tubular region  52  extends here from the covering region  48  to the underside  47  of the sensor arrangement and is essentially completely filled with the material comprising the secondary spray coating. Toward the underside, the tubular region is sealed off from the tank liquid by means of a closure  76 , and the closure is joined to the support  32  via a circumferential welded region  78 . 
     The combination of a fill level sensor and a temperature sensor, that is, the disposition of both measuring elements in one housing with a common cable exit, makes a compact construction that is resistant to chemically aggressive materials possible. The temperature should expediently be measured in the vicinity of the bottom of the tank, that is, near or at the level of the lowest still-detectable liquid level. Accordingly, the temperature sensor  74  is disposed approximately at the level of the contact region of the longest measuring electrode, which makes an electrical contact of this measuring electrode with liquid in the tank possible. To that end, the primary spray coating region or support  32  of the fill level sensor contains at the center the tubular region  52  in which the NTC resistor can be placed. The separator plate  60  described above has a central hole  63 , through which the cable of the temperature sensor is extended into the covering region. It is expedient for the temperature sensor, as described, to be injected; it can thus detect the direct temperature in the vicinity of the bottom of the tank quickly and correctly; if it were disposed in an only air-filled tube inside the support  32 , on the one hand it would measure only the temperature in the air tube and on the other it would react only in very delayed fashion to temperature changes in the tank. Fast detection of the temperature in the tank, however, is desirable so that by suitable heating of the tank liquid, freezing can be prevented, or a heater can be switched off in good time to prevent premature aging of the liquid in the tank. Here, during the production process, the secondary spray coating passes through the opening of the mounting plate into the continuous tube  52  and thus surrounds the temperature sensor. Finally, this tube is closed by a welding operation with a capsule or closure  76 . It is thus ensure that the temperature sensor can operate, fully sealed off from the Ad Blue, in the lower region of the fill level sensor. 
     The foregoing relates to the preferred exemplary embodiments of the invention, it being understood that others variants and embodiments thereof are possible with the spirit and scope of the invention, the latter being defined by the appended claims.