Abstract:
The invention relates to a measurement device ( 12 ) for determining a fluid fill level ( 14 ) in a fuel tank ( 10 ) for a vehicle. The position of a float ( 18 ) in relation to a signal generating unit ( 22 ) which can be attached on the fuel tank ( 10 ) can hereby be converted into a signal that is correlated with the fill level ( 14 ) of the undeformed fuel tank ( 10 ). A correction device ( 48, 54, 56 ) is designed for measuring a deformation of the fuel tank ( 10 ) and is used for correcting the signal. The invention also relates to a method for determining a fluid fill level ( 14 ) in a fuel tank ( 10 ) for a vehicle.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application claims the priority of German Patent Application, Serial No. 10 2010 045 212.2, filed Sep. 13, 2010, pursuant to 35 U.S.C. 119(a)-(d), the content of which is incorporated herein by reference in its entirety as if fully set forth herein. 
       BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to a measurement device for determining a fluid fill level in a fuel tank for a vehicle. The measurement device includes a float and a signal generating unit which can be fastened on the fuel tank. A position of the float relative to the signal generating unit can be converted to a signal which correlates with the fill level of the undeformed fuel tank. The invention also relates to a method for determining a fluid fill level in a fuel tank. 
         [0003]    The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention. 
         [0004]    Conventional measurement devices can measure and compensate for tilting of the fuel tank when driving, for example, downhill. However, particularly unfavorable relative positions of the float in relation to the signal generating unit due to the deformation of the fuel tank or due to tilting of components of the measurement device may cause incorrect fill level indications in the conventional measurement devices. 
         [0005]    In addition, even with measurement devices where the float is not coupled with the signal generating unit attached on the fuel tank, deformations of the fuel tank can occur which then cause an incorrect display of the fluid fill level in the fuel tank. 
         [0006]    It would therefore be desirable and advantageous to obviate prior art shortcomings and to provide an improved a measurement device and a corresponding method which improve the determination of the fluid fill level in the fuel tank. 
       SUMMARY OF THE INVENTION 
       [0007]    According to one aspect of the present invention, a measurement device for determining a fluid fill level in a fuel tank of a vehicle includes a float, a signal generating unit fastened on the fuel tank, wherein a position of the float a relative to the signal generating unit is converted to a signal which correlates with the fluid fill level of an undeformed fuel tank, and a correction device configured to determine a deformation of the fuel tank, wherein the signal is corrected with the correction device to account for the deformation of the fuel tank. 
         [0008]    The invention is based on the observation that the signal generated by the signal generation unit, which correlates with the fill level of the undeformed fuel tank, may cause an indication of a fill level which is significantly different from the actual fill level, due to a deformation of the fuel tank. This is particularly the case when the fuel tank is constructed as a pressurized tank, i.e., a closed tank, which does not communicate with the environment via an always open line (which is typically provided with an active charcoal filter). 
         [0009]    However, even when a fuel tank is connected with the environment by way of such line provided with the active charcoal filter, a slight vacuum or overpressure with respect to ambient pressure may be generated inside the fuel tank due to temperature changes. 
         [0010]    The deformation of the fuel tank occurring in conjunction with the vacuum or the overpressure is of particular importance in a closed fuel tank, where the measurement of the deformation of the fuel tank and the correction of the signal with the correction device can be particularly advantageously employed. 
         [0011]    The measurement device according to the invention enables an improved determination of the fluid fill level in the fuel tank, because the deformation of the fuel tank is taken into account, for example when using signal generating units where the float is mechanically coupled with the signal generating unit. The measurement device, however, may also be used with systems where the float is cooperating with a signal generating unit, which is stationarily held on the fuel tank, without being in direct contact. In particular, such signal generating unit may be a measuring board having a Reed switch or an electrode measuring changes in the capacity due to the movement of the float. 
         [0012]    Complex measures for a deformation-compensating support of components of the signal generating unit, as commonly used in the art, can be obviated by correlating the signal of the signal generating unit. 
         [0013]    Correcting the signal based on the measured deformation of the fuel tank safely prevents indication of an incorrect fluid level. A vehicle having a fuel tank disposed in the vehicle can advantageously be prevented from becoming stranded because the fuel actually present lasts only for a shorter route than is apparent from the fill level indication. 
         [0014]    According to an advantageous feature of the present invention, the correction device may be designed for measuring an extent of the deformation or a quantity determining the extent of the deformation. This allows a particularly accurate compensation of a misrepresentation of the signal generated by the signal generating unit due to the deformation of the fuel tank. 
         [0015]    The measurement device may include a control device for controlling a display, wherein at least one characteristic curve is stored in the control device, which provides at least one correction value which depends on the extent of the deformation of the fuel tank for correcting the signal of the signal generating unit. The characteristic curve can be used by the control device to correct the signal of the signal generating unit which does not indicate the actual fluid fill level due to the deformation of the fuel tank. When using this type of measurement device, no changes need to be made to the signal generating unit, so that an existing measurement device can be easily retrofitted. 
         [0016]    According to an advantageous feature of the present invention, the correction device may include at least one pressure sensor for measuring a vapor pressure in the fuel tank. The extent of the pressure-dependent deformation of the fuel tank can be deduced from the vapor pressure in the fuel tank. In particular, the pressure sensor may be employed with a fuel tank where a deviation from standard pressure causes a significant deformation of the fuel tank, for example with a fuel tank made of plastic. However, the pressure-dependent deformation of the fuel tank can also be deduced by measuring the vapor pressure in the fuel tank if the fuel tank made of a metal, in particular steel. The pressure-dependent deformation can be specifically determined for the type of the respective fuel tank, i.e., depending on the material, the nominal volume and the shape. 
         [0017]    According to an advantageous feature of the present invention, the at least one pressure sensor may be arranged in the fuel tank and coupled with the control device. The signal generated by the signal generating unit, which does not indicate the actual fluid fill level due to the deformation of the fuel tank and the signal provided by the pressure sensor are processed in the control device. In particular, if a pressure sensor is provided anyway to monitor the pressure in the fuel tank, the signal supplied by the pressure sensor can be easily used for correcting the signal provided by the signal generating unit. To this end, only the specific characteristic curve for the type of the fuel tank needs to be stored in the control device and the control device needs to be designed for processing both signals. 
         [0018]    Alternatively or in addition, the at least one pressure sensor may be integrated in the signal generating unit or in the float—or also in a coupling element which couples the signal generating unit with the float. A corrected signal can then be transmitted from the signal generating unit to the control unit via a signal path of the signal generating unit. In this case, a characteristic curve for correcting the signal need not be stored in the control device, because the signal corrected by the pressure sensor is transmitted directly to the control device. Such pressure sensor may include a membrane arranged, for example, on the float or on the signal generating unit, wherein the pressure-dependent excursion of the membrane supplies a measurement value which corrects the signal generated by the signal generating unit. The control device then does not need to evaluate two signals, and the supplied signal can instead be directly converted to control the display for indicating the fluid level. In this embodiment of the invention, no changes need to be made to a control device which is only designed for processing the signal of the signal generating unit to correctly control the display. 
         [0019]    According to an advantageous feature of the present invention, the correction device may include at least one temperature sensor for measuring a temperature of the fuel tank. In this way, material expansions or contractions due to a temperature can be taken into account. This is advantageous in particular if the fuel tank is made of a plastic material which has a greater thermal expansion coefficient than a steel tank. 
         [0020]    When using the temperature sensor in addition to a pressure sensor of the correction device, a characteristic curve stored in the control device may take into account both quantities, namely the pressure and temperature, when providing a correction value for correcting the signal of the signal generating unit. 
         [0021]    According to another advantageous feature of the present invention, the correction device may include at least one distance transducer for measuring a change in length. It is hereby unimportant if the deformation is caused by pressure and/or temperature. A characteristic curve stored in the control device can then also take into account the change in length when providing a correction value for correcting the signal of the signal generating unit. At least one distance transducer can be provided in addition to a pressure sensor and/or a temperature sensor of the correction device. 
         [0022]    According to yet another advantageous feature of the present invention, an actual position of the signal generating unit and the fuel tank which is different from a nominal position may be taken into account by coupling at least one distance transducer with, on one hand, the signal generating unit and, on the other hand, a wall of the fuel tank. It is hereby unimportant if the measured actual position depends on pressure stress and/or temperature stress applied to the fuel tank. A deformation or tilting of components of the measurement device caused by a mechanical load can also be measured and processed. 
         [0023]    According to still another advantageous feature of the present invention, the correction device may be designed for correcting the signal for a deformation of the fuel tank caused by an overpressure and/or a vacuum. Different deformations of the fuel tank caused by a vacuum and an overpressure can then be taken into account. The pressure in the fuel tank under vacuum can reach 100 mbar below ambient pressure, whereas the pressure in the fuel tank under overpressure can reach 300 mbar above ambient pressure. 
         [0024]    According to another aspect of the present invention, a method for determining a fluid fill level in a fuel tank of a vehicle includes the steps of supplying with a signal generating unit a signal indicating the fluid fill level, measuring a deformation of the fuel tank, and correcting the signal based on the measured deformation. 
         [0025]    In this way, it can be ensured that the actually indicated fluid fill level is not falsified by a deformation of the fuel tank. The deformation of the fuel tank can be measured and corrected, in particular, by a correction device. 
         [0026]    The advantages and preferred embodiments described for the measurement device according to the invention apply likewise also to the method according to the invention for determining the fluid fill level. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0027]    Other features and advantages of the present invention will be more readily apparent upon reading the following description of currently preferred exemplified embodiments of the invention with reference to the accompanying drawing, in which: 
           [0028]      FIG. 1  shows, in a schematic diagram, an undeformed fuel tank for a vehicle, with a fill level sensor, wherein a float of the fill level sensor is coupled to a signal transducer via an arm; 
           [0029]      FIG. 2  shows, in a schematic diagram, a deformation of the fuel tank caused by a vacuum, wherein an angular position of the arm relative to the signal transducer is changed by the deformation; 
           [0030]      FIG. 3  shows, in a schematic diagram, a change of the angular position of the arm caused by a deformation due to an overpressure in the tank; 
           [0031]      FIG. 4  shows a change in the angular position of the arm, wherein the arm is deformed due to a change of the pressure in the fuel tank, causing the position of the signal transducer to deviate from its nominal position; and 
           [0032]      FIG. 5  shows a height-volume curve, which shows the dependence of the fill level indicated by the signal transducer from the fuel volume present in the fuel tank, wherein the actual curves are corrected to match a nominal curve. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0033]    Throughout all the figures, same or corresponding elements may generally be indicated by same reference numerals. These depicted embodiments are to be understood as illustrative of the invention and not as limiting in any way. It should also be understood that the figures are not necessarily to scale and that the embodiments are sometimes illustrated by graphic symbols, phantom lines, diagrammatic representations and fragmentary views. In certain instances, details which are not necessary for an understanding of the present invention or which render other details difficult to perceive may have been omitted. 
         [0034]    Turning now to the drawing, and in particular to  FIG. 1 , there is shown in a schematic diagram a fuel tank  10  and a measurement device  12  for determining a fill level  14  of the fuel  16  in the fuel tank  10 . The fuel tank  10  is constructed as a pressurized tank of a vehicle, i.e. a closed tank, which is not in communication with the environment of the fuel tank  10  via an always open vent line. In particular, such pressurized tank can be used in a hybrid vehicle. In a hybrid vehicle, gaseous fuel should be prevented from being released into the environment even if the internal combustion engine of the automobile is not operated for an extended period of time, i.e. when fuel vapors cannot be dissipated by combustion in the internal combustion engine. A pressurized tank can be particularly inexpensively produced from plastic. 
         [0035]    The measurement device  12  includes a float  18  floating on the fuel  16 , which is coupled to a signal transducer  22  via an arm  20 . The changing angular position of the arm  20  relative to the signal transducer  22 , which changes with the changing the fill level  14  in the fuel tank  10 , is converted into an electrical signal, for example a resistance value. The resistance value indicates for an undeformed fuel tank  10  how much fuel  16  is left in the fuel tank  10 . 
         [0036]    A corresponding characteristic curve for the undeformed fuel tank  10  is shown in  FIG. 5  as a nominal curve  24  in a coordinate system where the fill level is indicated on the ordinate  26  and the actual fill volume of the fuel  16  in the fuel tank  10  is indicated on the abscissa  28 . The fill level is used to display an (absolute or relative) display volume or a range on a tank display of the vehicle. 
         [0037]    The signal transducer  22  transmits hereby the electrical signal via a signal path  30  to a control device  32  which in turn controls the (unillustrated) tank display in a combined instrument of the automobile, as illustrated in  FIG. 1  by an arrow  34 . 
         [0038]    In the present example, the signal transducer  22  is arranged on a feed unit  36  which is supported on the bottom  38  of the fuel tank  10  by an (unillustrated) support under spring bias. A line  40  extends from the feed unit  36  via a tank flange  42  to outside the fuel tank  10  and from there to an (unillustrated) internal combustion engine of the vehicle. 
         [0039]      FIG. 2  shows a situation where the fuel tank  10  is under vacuum, causing deformation of the fuel tank  10 . The deformation of the walls  44  of the fuel tank  10  causes an angular position of the arm  20  in relation to the signal transducer  22  to be different from that in an undeformed fuel tank  10 . According to a first embodiment, when the fuel tank  10  is deformed by the vacuum, the signal transducer  22  transmits to the control device  32  a signal which corresponds to a greater fill height than in an undeformed fuel tank  10 . Such correlation of the displayed fill height and the actual fill volume is shown in  FIG. 5  by an actual curve  46 . 
         [0040]    In the present example, a pressure sensor  48  measures the vapor pressure in the fuel tank  10  and transmits the vapor pressure to the control device  32 . The control device  32  processes the signal supplied by the signal transducer  22  and the signal supplied by the pressure sensor  48 , wherein a characteristic curve is stored in the control device  32  which provides correction values that depend on the extend of the deformation of the fuel tank  10 .  FIG. 5  shows individual exemplary correction values in form of arrows  50 . Values on the actual curve  46  are corrected with the corresponding correction values, so that the course of the actual curve  46  approaches the nominal curve  24 . 
         [0041]    In the deformation of the fuel tank (see  FIG. 2 ) caused by the vacuum, the signal generated by the signal transducer  22  may indicate a lower fill level than the actual fill level, depending on the shape of the fuel tank and the mounting location of the signal transducer  22 . The correction values according to the arrows  50  also enable the actual curve  52  to approach the nominal curve  24 . 
         [0042]      FIG. 3  indicates a situation where deformation of the fuel tank  10  is caused by overpressure in the fuel tank  10 . The is again causes in angular position of the arm  20  in relation to the signal transducer  22  which is converted by the signal transducer into a lower fill level than the actual fill level in the fuel tank  10 . The actual curve  52  shown in  FIG. 5  is an example for such correlation between the indicated fill level and the associated fill volume. 
         [0043]    The correction value supplied by the pressure sensor  48  also ensures for a deformation due to overpressure that the control device  32  directly controls the tank display. With the signals supplied from the pressure sensor  48  to the control device  32 , the values on the actual curve  52  can be corrected by the corresponding correction values based on a corresponding characteristic curve stored in the control device  32  and matched to the nominal curve  24 . 
         [0044]      FIG. 4  lastly illustrates a situation where a pressure-induced deformation of the fuel tank  10  causes the feed unit  36  and thus also the signal transducer  22  attached to the feed unit  36  to tilt relative to the arm  20 . The pressure sensor  48  then also enables a correction of the signal supplied from the signal transducer  22  to the control device and hence a correct indication of the fill level. 
         [0045]    In an alternate embodiment, a pressure sensor may be arranged on the float  18  or on the signal transducer  22 . Advantageously, such pressure sensor is then coupled via the signal path  30  with the control device  32 . The electrical signal supplied by the signal transducer  22  to the control device  32  can then already be corrected before the signal reaches the control device  32 . Both the pressure sensor  48  arranged in the fuel tank and the pressure sensor coupled with the control device  32  via the signal path  30  may be provided. 
         [0046]    Preferably, a temperature sensor  54  is also provided which is also coupled with the control device  32 . Correction values are then determined from the pressure signal transmitted from the pressure sensor  48  and from the temperature signal determined by the temperature sensor  54 , which can be used to correct the respective actual curve  46 ,  52 . In alternative embodiments, the temperature sensor  54  may be arranged in the float  18  or on the signal transducer  22  or on the arm  20  to transmit a corrected signal to the control device  32  via the signal path  30 . 
         [0047]    Alternatively or in addition, distance transducers  56  may be provided which measure a change in length of the walls  44  of the fuel tank  10  and transmit corresponding measurement values to the control device  32 . The control device  32  then converts these measurement values into corresponding correction values, whereby the values of the actual curves  46 ,  52  can be matched to the nominal curve  24 . Corresponding characteristic curves, which allow correction of the actual curves  46 ,  52  according to the arrows  50 , supply deformation-dependent correction values determined for the respective type of the fuel tank  10 . The characteristic curves are stored in the control device  32 . 
         [0048]    According to another embodiment, when the tank display is controlled by the control device  32 , a tilt of the feed unit  36  (see  FIG. 4 ) can be taken into account by coupling the distance transducers  56 , on one hand, with the wall  44  of the fuel tank and, on the other hand, with the signal transducer  22  or the feed unit  36 , even if the tilt is not caused by a pressure or temperature variation in the fuel tank  10 . 
         [0049]    While the invention has been illustrated and described in connection with currently preferred embodiments shown and described in detail, it is not intended to be limited to the details shown since various modifications and structural changes may be made without departing in any way from the spirit and scope of the present invention. The embodiments were chosen and described in order to explain the principles of the invention and practical application to thereby enable a person skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. 
         [0050]    What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims and includes equivalents of the elements recited therein: