Abstract:
The invention concerns a float arm assembly for use in a vehicle fuel tank and a method of operating the float arm assembly to determine a level of fuel in the tank. First and second resistor cards are electrically connected in series and each includes a pivoting float arm with a float attached to an end. The first and second resistor cards, with separate floats allow the fuel level to be more accurately determined, especially in fuel tanks with complex contours and indentations in the tank wall that create dead bands in a single float type of arrangement.

Description:
BACKGROUND OF INVENTION 
       [0001]    The present invention relates to a float arm assembly used for measuring the liquid level in a fuel tank, and in particular to a float arm assembly in a vehicle fuel tank. 
         [0002]    A typical float arm assembly used in a vehicle fuel tank includes a fixed support to which a ceramic resistor card is mounted. A float is pivotally mounted to the resistor card by a float arm. As the fuel level in the tank goes up and down, the buoyancy of the float causes it to move up and down with the level of fuel, thus changing the angle of the float arm relative to the resistor card. This change in angle causes a change in output from the resistor card, which is typically received and processed by a powertrain control module and displayed on the fuel gauge. 
         [0003]    In modern automotive vehicles, the desire to maximize the capacity of a fuel tank may require the fuel tank to have a greatly contoured shape. This may even include significant indentations in the fuel tank wall in order to avoid adjacent vehicle structure and components. The contoured shape—in combination with the indentations—may limit the amount of pivoting of the float and float arm that can occur without creating interference problems between the float arm assembly and the fuel tank wall. Consequently, the float and float arm may not be able to pivot all the way up to a full liquid level and/or all the way down to an empty liquid level. Such limitations in the travel of the float arm and float, then, create a dead band, which is the unreadable but useful fuel in the fuel tank. This dead band is undesirable because it limits the accuracy of the fuel gauge reading, which may reduce customer satisfaction with the vehicle and possibly even increase warranty costs. 
         [0004]    Attempts to minimize the dead band have included fuel tanks with two separate float arm assemblies, each mounted and operating separately. While this may reduce dead band concerns, it adds an additional sender opening for communicating the additional fuel level signal to the powertrain control module. This multiple float arm assembly, then, requires the powertrain control module to receive and process a second, separate fuel level signal, adds additional cost for fabricating and assembling an entire separate float arm assembly and cover for the second sender opening, and increases the potential for permeation out of the tank. 
       SUMMARY OF INVENTION 
       [0005]    An embodiment of the present invention contemplates a float arm assembly for a vehicle fuel tank. The float arm assembly comprises a first float support mountable in the fuel tank, a second float support mountable in the fuel tank, a first resistor card mounted to the first float support, a second resistor card mounted to the second float support, spaced from the first resistor card and electrically connected in series with the first resistor card, a first float arm pivotally connected to the first resistor card and extending therefrom, a second float arm pivotally connected to the second resistor card and extending therefrom, a first float fixed to the first float arm, and a second float fixed to the second float arm. 
         [0006]    An embodiment of the present invention contemplates a float arm assembly for a vehicle fuel tank comprising: at least one float support mountable in the fuel tank; a first resistor card mounted to the at least one float support; a second resistor card mounted to the at least one float support, spaced from the first resistor card and electrically connected in series with the first resistor card; a first float arm pivotally connected to the first resistor card and extending therefrom; a second float arm pivotally connected to the second resistor card and extending therefrom; a first float fixed to the first float arm; and a second float fixed to the second float arm. 
         [0007]    An embodiment of the present invention contemplates a method of detecting a level of a fuel in a vehicle fuel tank having a float arm assembly, the method comprising the steps of: connecting a first resistor card in electrical series connection with a second resistor card that is spaced from the first resistor card; allowing a first float, having a density less than the fuel, to pivot on a first float arm relative to the first resistor card due to the buoyancy of the first float in the fuel; allowing a second float, having a density less than the fuel, to pivot on a second float arm relative to the second resistor card due to the buoyancy of the second float in the fuel; and communicating a signal indicative of a position of the first float arm relative to the first resistor card and a position of the second float arm relative to the second resistor card to a powertrain control module. 
         [0008]    An advantage of an embodiment of the present invention is that the dead band is greatly reduced or even eliminated, even with tanks having complex contours and intrusive indentations. Thus, a more accurate fuel gauge reading over the entire fill level of the tank is provided. 
         [0009]    An advantage of an embodiment of the present invention is that only one fuel level signal needs to be communicated from the tank to the powertrain control module, even though two float arms and floats are employed. Moreover, with resistor cards connected in series, this communicated signal can be the same as with a tank having a single float arm and float. 
         [0010]    An additional advantage of an embodiment of the present invention is that the more accurate fuel level reading is obtained without the need to add an additional float arm assembly, fuel sender hole and cover to the fuel tank. Thus, increase potential for permeation is avoided, and additional parts and assembly time are minimized. 
     
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0011]      FIG. 1  is a somewhat schematic, partial section view of a portion of a fuel tank assembly, with floats shown in a full fuel tank position, in accordance with the present invention. 
           [0012]      FIG. 2  is a view similar to  FIG. 1 , but with the floats shown in an empty fuel tank position. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]      FIGS. 1-2  illustrate a fuel tank assembly, indicated generally at  20 , for a vehicle (not shown). The fuel tank assembly  20  includes a tank  22  (i.e., the tank wall and covers for openings in the tank walls), which has a complex shape needed to maximize the fuel capacity while fitting around/between various vehicle components. For example, the tank  22  includes a seat belt anchor indentation  24 , which is needed for the tank  22  to avoid interfering with a seat belt anchor (not shown). Also, for example, the tank  22  has a rear suspension indentation  26  that allows the tank  22  to avoid interfering with the vehicle rear suspension (not shown). The tank  22  has an overall shape with a shorter horizontally extending portion  28  and a taller vertically extending portion  30 . Again, the shape is dictated to a great extent by the need to fit around vehicle structure and components (not shown). The examples just discussed are some of many examples of overall shapes and indentations that may be needed in order to allow for maximum capacity of a fuel tank while avoiding vehicle structure and components. 
         [0014]    The fuel tank assembly  20  includes a float arm assembly  32  mounted therein, having a main support  34  that extends generally vertically and is connected to a cover  36 , which is mounted on and sealed to the top of the tank  22  (over a sender opening). The float arm assembly  32  also includes a first float support  38 , extending from the main support  34 , and a second float support  40 , which may extend from the main support  34  or the first float support  38 . While the first float support  38  and second float support  40  may be separate components, they also may be an integral part with two separate resistor cards mounted spaced apart if the shape of the tank  22  for the particular vehicle allows for such a configuration. Preferably, the first float support  38  and the second float support  40  are mounted on the same main support  34  so they move up and down by the same amounts as the floor of the tank  22  flexes due to pressure fluctuations in the fuel tank  22 . This mounting configuration, then, minimizes inaccuracies that can be introduced in fuel level readings. 
         [0015]    The first float support  38  includes a first ceramic resistor card  42  mounted to its end opposite the main support  34 . A first float arm  44  extends from and pivots relative to the first ceramic resistor card  42 . A first float  46  extends from a free end of the first float arm  44 . The second float support  40  includes a second ceramic resistor card  48  mounted to its end opposite the main support  34 . A second float arm  50  extends from and pivots relative to the second ceramic resistor card  48 . A second float  52  extends from a free end of the second float arm  50 . 
         [0016]    The float supports  38 ,  40  and resistor cards  42 ,  48  remain fixed in their positions in the tank  22  as the vehicle is operated. The floats  46 ,  52  have a density less than the fuel in the tank  22  and so their buoyancy will cause them to float up and down with the changing fuel level in the tank  22 . As the floats  46 ,  52  float up and down with the changing fuel level in the tank  22 , they will cause the float arms  44 ,  52  to pivot relative to their respective ceramic resistor cards  42 ,  48 . The pivoting causes the resistance output of the particular resistor card  42 ,  48  to vary, based on the angle of its float arm  44 ,  50 . 
         [0017]    The second float support  40  extends outward and upward farther from the main support  34  than the first float support  38 . The length and angle of extension of the second float support  40  locates the second ceramic resistor card  48  such that it will allow the second float  52  to freely pivot from at or near the highest level the fuel reaches for a full tank (indicated by dashed line  60  in  FIG. 1 ) to a level where the tank  22  is only partially filled with fuel. In the particular example of this embodiment, the second float  52  is allowed to pivot down to a level that is below that of the seat belt anchor indentation  24 . Consequently, neither the seat belt anchor indentation  24  nor the rear suspension indentation  26  restrict or inhibit the desired range of motion for the second float  52 . 
         [0018]    The length and angle of extension of the first float support  38  locates the first ceramic resistor  42  such that it will allow the first float  46  to freely pivot from near the seat belt anchor indentation  24  to a level where the tank  22  is almost completely empty of fuel (indicated by dashed line  62  in  FIG. 2 ). Neither the seat belt anchor indentation  24  nor the rear suspension indentation  26  restrict or inhibit the desired range of motion for the first float  46 . 
         [0019]    Ideally, the overlap between the lowest level to which the second float  52  extends and the highest level to which the first float  46  extends is zero. Thus, as the fuel tank  22  empties, the second float  52  would stop as the first float  46  begins to drop. But due to manufacturing tolerances, there may be a small designed-in overlap zone, which is preferably minimized based on manufacturing capability for the particular fuel tank assembly  20 . 
         [0020]    The first and second ceramic resistor cards  42 ,  48  are preferably wired in series (indicated schematically by dashed line  56 ), and in communication with a powertrain control module  54 . Thus, the powertrain control module  54  can receive a single signal from this fuel tank assembly  20  relating to the level of fuel in the tank  22 . Also, preferably (but not required), the two resistor cards  42 ,  48  have a total resistance equal to the resistance of a tank having a single float. In this way, the powertrain control module  54  can be used for a one float or a two float configuration. With the resistor cards  42 ,  48  wired in series, the overlap zone will provide a slight non-linearity in the signal from the resistor cards  42 ,  48  when the fuel level is in the overlap zone. But with the approximate amount of overlap known, the non-linearity of the signal in this zone is accounted for in the calibration of the powertrain control module  54 . 
         [0021]      FIG. 1  illustrates the positions of the floats  46 ,  52  when the level of fuel  60  in the fuel tank  22  is at its highest. Both floats  46 ,  52  are at their highest levels, so, with the resistor cards  42 ,  48  connected in series, the total resistance is the sum of that from the two cards  42 ,  48 —this resistance indicating a full tank  22 .  FIG. 2  illustrates the positions of the floats  46 ,  52  when the level of fuel  62  in the fuel tank  22  is at its lowest. Both floats  46 ,  52  are at their lowest levels. Again, the total resistance is the sum of that from the two cards  42 ,  48 —this resistance indicating an empty tank. Between full and partially empty, the second float  52  will drop with the fuel level, while the first float  46  remains stationary, so the change in resistance of the second resistor card  48  will indicate a drop in fuel level. 
         [0022]    Since the second float  52  extends essentially up to the full level  60  and the first float  46  extends essentially down to the empty level  62 , the amount of fuel in the tank  22  over the full range can be accurately determined. This is true even though the contours and indentations in the particular tank would otherwise prevent an accurate measurement from a float arm assembly with only a single float. Moreover, two separate float arm assemblies, with the extra hardware and multiple signals sent to the powertrain control module, are not required to obtain an accurate fuel level reading. 
         [0023]    While only two resistor cards connected in series (with respective float arms and floats) are shown in this embodiment, the float arm assembly can have three or more cards connected in series (and corresponding float arms and floats) if needed to obtain accurate fuel level readings for particularly complicated shapes of fuel tanks. Also, for vehicles having saddle tanks, each of the saddle tanks can have multiple resistor cards if so desired, with the resistor cards in each respective tank connected in series. 
         [0024]    While certain embodiments of the present invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention as defined by the following claims.