Abstract:
This invention provides a method and apparatus for utilizing an inductive coil fluid level sensor to measure the temperature of the fuel, or fuel vapors, in a fuel tank depending upon the location of the sensor within the tank. The inductive coil sensor is connected to a Fuel Control Unit containing the sensor electronics to drive the inductive coil sensor and read the corresponding fuel or fuel vapor temperature.

Description:
TECHNICAL FIELD  
         [0001]    This disclosure relates to temperature sensors and more particularly to fuel, or fuel vapor, temperature sensing using an inductive fuel level sensor.  
         BACKGROUND  
         [0002]    Current automotive fuel or fuel vapor temperature sensing is performed with a thermistor positioned within a fuel tank. This requires an additional component (the thermistor) in the fuel system. It also requires two electrical connections, e.g., one for signal output and one for electrical ground.  
           [0003]    The ground connection can be shared. However, this still requires a minimum of one extra system electrical connection. The disadvantage to this approach is the cost of the thermistor and the extra electrical connections. Another concern is the ability of the thermistor to withstand being in contact with the fuels and fuel vapors. It is therefore advantageous to provide a fuel or fuel vapor temperature sensing apparatus and method that does not require either extra components nor extra electrical connections and that can provide long term reliability.  
         SUMMARY OF THE INVENTION  
         [0004]    This disclosure provides a method and apparatus for utilizing an inductive coil fluid level sensor to measure the temperature of the fuel, or fuel vapors, in a fuel tank depending upon the location of the sensor within the tank. The inductive coil sensor is connected to a Fuel Control Unit containing the sensor electronics to drive the inductive coil sensor and read the corresponding fuel or fuel vapor temperature.  
           [0005]    The method comprises charging the sensor to generate a voltage across the sensor, measuring the voltage across the sensor at the temperature of the sensor, measuring the voltage across the sensor at a reference temperature; and from the voltage measured across the sensor at the temperature of the sensor and the voltage measured across the sensor at the reference temperature, calculating the temperature of the sensor with respect to the reference temperature.  
           [0006]    The sensor comprises an inductive coil receptive of a magnetic core moveable within the coil, a device linked to the core and responsive to the level of the fluid in a container and a circuit charging the inductive coil generating thereby a voltage across the inductive coil indicative of the temperature of the fluid. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    [0007]FIG. 1 is a generalized schematic diagram of an electro-mechanical system having an electric circuit including an inductive coil sensor for determining the temperature of a fuel or fuel vapor in a container;  
         [0008]    [0008]FIG. 2 is a schematic diagram of a first embodiment of the inductive coil sensor of FIG. 1 immersed within the fuel;  
         [0009]    [0009]FIG. 3 is a schematic diagram of a second embodiment of the inductive coil sensor of FIG. 1 immersed within the fuel vapor;  
         [0010]    [0010]FIG. 4 is a schematic diagram of a first exemplary embodiment of the electric circuit of FIG. 1 including a model of an inductive coil sensor for determining the temperature of a fuel or fuel vapor in a container;  
         [0011]    [0011]FIG. 5 is a schematic diagram of a second exemplary embodiment of the electric circuit of FIG. 1 including a model of an inductive coil sensor for determining the temperature of a fuel or fuel vapor in a container;  
         [0012]    [0012]FIG. 6 is a schematic diagram of a third exemplary embodiment of the electric circuit of FIG. 1 including a model of an inductive coil sensor for determining the temperature of a fuel or fuel vapor in a container;  
         [0013]    [0013]FIG. 7 is a schematic diagram of an electric circuit, including a model of an inductive coil sensor, for determining the level of a fuel in a container;  
         [0014]    [0014]FIG. 8 is a graphical representation of the square wave driving pulse voltage, V pulse , of FIG. 1 and the resultant voltage, V coil , across the inductive coil sensor;  
         [0015]    [0015]FIG. 9 is a graphical representation of the exponential decay of V coil  wherein the core of the inductive coil sensor is not inserted into the coil; and  
         [0016]    [0016]FIG. 10 is a graphical representation of the exponential decay of V coil  wherein the core of the inductive coil sensor is fully inserted into the coil. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]    An inductive coil is constructed by winding a given number of turns of conductive wire onto a bobbin. Copper is typically used due to its low cost and low electrical resistance. Although the resistance of the inductive coil, R coil , is small, it is easily measurable. Copper has a very well defined change in resistance due to temperature. The temperature coefficient of resistance, α, for Copper as given by  The Engineers&#39; Manual  by Hudson is 0.00393 per degree C. at 20 degrees C. By analyzing the change in resistance in the copper coil, R coil , the temperature change of the coil, T coil , can be determined.  
         [0018]    Referring now to FIG. 1, a generalized schematic diagram is shown of an electro-mechanical system  100  having an electric circuit  100   a  including an inductive coil sensor  108  for determining the temperature of a fluid such as a fuel or fuel vapor in a container. The sensor  108  for measuring the temperature of the fluid  104 , comprises an inductive coil  108   b  receptive of a magnetic core  108   a  moveable within the coil  108   b.  A flotation device  106   a  is mechanically linked at  106  to the core  108   a  and responsive to the level of the fluid  104  in the container  102 , such as a fuel tank. A circuit  100   a  charges the inductive coil  108   b  generating thereby at  110   b  a voltage, V coil , across the inductive coil  108   b  indicative of the temperature of the fluid  104 .  
         [0019]    As the flotation device  106   a  rises and falls with the level of the fuel  104 , the core  108   a  falls and rises as the lever arm  106  pivots about point P. The movement of the core  108   a  within the coil  108   b  causes the effective inductance of the coil  108   b  to change in a measurable way. As seen in FIG. 1, the inductive coil sensor  108  may be located remote from the fuel tank  102  or as seen in FIG. 2 and  3 , may be located within the fuel tank  102 . To measure the temperature, T v , of the fuel vapor  104   a,  the inductive coil sensor  108  is located within the tank  102  above the fuel  104 . To measure the temperature, T f , of the fuel  104 , the inductive coil sensor  108  is located within the tank  102  immersed within the fuel  104 .  
         [0020]    In FIG. 1, an input terminus  110   a  of input resistor  110  is energized by a square wave signal, V pulse , having values of 0 volts and V cc  volts as seen for example at  202  in FIG. 8. Such a voltage input at  110   a  results in a corresponding coil voltage, V coil , at an output terminus  110   b  of the input resistor  110 . In FIG. 1, V coil  is amplified by an amplifier  130  which provides as output a signal, V out , which is filtered at  140 . The output of the filter is provided as input to an analog-to-digital converter (ADC)  146 .  
         [0021]    Referring to FIG. 4, a first exemplary embodiment of the circuit  100   a  of FIG. 1 is shown. In FIG. 4, V pulse  is provided by an oscillator  120  connected to the base of a pnp bipolar junction transistor  112  (Q 1 ) having a supply voltage, V cc , of 5 volts provided by a power source  118 . Q 1    112  is used to switch V cc  to the coil sensor through R in    110 . The coil sensor  108  of FIG. 1 can be modeled as a parallel RLC circuit  124 ,  126 ,  128 . In the circuit shown in FIG. 4, R in  is chosen to be much larger than R coil    128 . This allows the resistance of the coil, R coil , to be neglected in determining the effective inductance of the coil to determine fuel level. The value of V coil  is relatively low if R in  is much greater than R coil  as required to measure the effective inductance of the coil  108   a.    
         [0022]    A method of measuring R coil  is to measure the voltage, V coil , across the coil  108 . In order to measure V coil , the square wave  202  used to measure the effective inductance is halted temporarily at zero volts and transistor Q 1  in FIG. 4 would remain turned “on” (for about 100 msec) until the coil  108  is fully charged. Once the coil  108  is fully charged, the voltage across the coil is given by  
               V     c                 o                 i                 l       =         R     c                 o                 i                 l           R     c                 o                 i                 l       +     R     i                 n           ×       V     i                 n       .               (   1   )                               
 
         [0023]    If R in  and V in  do not vary with temperature, then R coil  would be the only temperature dependent variable. To accomplish this, R in  is chosen to be a discrete resistor with a low temperature coefficient as is common with carbon resistors. The voltage difference between V cc  and V in  is negligible for low currents flowing through Q 1 . V cc  can vary somewhat with temperature but this can be neglected if the analog-to-digital converter (ADC)  146  is also powered by V cc . Therefore, the coil voltage, V coil , can be approximated to vary in the same fashion as the temperature coefficient of resistance of copper (0.393% per degree C).  
         [0024]    As seen in FIGS. 1 and 8, V in  is alternately energized and de-energized at  110   a  by a square wave pulse, V pulse ,  202  having values of zero volts and V cc  volts. When V pulse  is positive (Q 1  off), V coil  grows exponentially as seen at  208  in FIG. 8. When V pulse  is zero (Q 1  on), the inductor  126  is charging and V coil  decays exponentially as seen at  204   a.  Depending upon the time constant, τ L , of the coil sensor  108 , as seen at  206   a,  V coil  will decay to a substantially constant value V L  after a prescribed time interval, t o . It will be appreciated from FIGS. 9 and 10 that as the core  108   a  moves into and out of the coil  108   b,  the time constant, τ L , of the coil sensor  108  changes and the rate of the exponential decay will change. Thus, FIG. 9 is representative of the sensor  108  charging when the core  108   a  is substantially out of the coil  108   b  and FIG. 10 is representative of the sensor  108  charging when the core  108   a  is more fully encompassed by the coil  108   b.  Q 1  is left turned on for a sufficiently long time interval, t 1 &gt;t o  (e.g., 100 msec) until V coil  settles to the substantially DC voltage level of V L . At such time, in the circuit model  108  of FIG. 4, inductor  126  acts as a short circuit and capacitor  124  acts an open circuit. Thus, at t 1  a voltage divider is created between V in  at  110   a,  V coil  at  110   b  and electrical ground at  148 . Thus, since V in  approximates V cc ,  
                 V   L          (     T     c                 o                 i                 l       )       =           R     c                 o                 i                 l            (     T     c                 o                 i                 l       )             R     c                 o                 i                 l            (     T     c                 o                 i                 l       )       +     R     i                 n           ×       V     c                 c       .               (   2   )                               
 
         [0025]    In the circuit of FIG. 1, V L  is about 120 mV if R coil  is about 25 Ohms and R in  is 1000 Ohms. If V L  has been measured at a reference temperature T 0 , then  
                 V   L          (     T   0     )       =           R     c                 o                 i                 l            (     T   0     )             R     c                 o                 i                 l            (     T   0     )       +     R     i                 n           ×       V     c                 c       .               (   3   )                               
 
         [0026]    R coil  varies with temperature T coil  according to the equation:  
           R   coil ( T   coil )= R   coil ( T   0 )[1+α( T   coil   −T   0 )],   (4)  
         [0027]    where α is the temperature coefficient of resistance. Equations (2) and (3) can be substituted into Eq. (4) to give the difference between T coil  and T 0 :  
                 T     c                 o                 i                 l       -     T   0       =         1   α          [         (         V   L          (     T     c                 o                 i                 l       )           V   L          (     T   0     )         )          (         V     c                 c       -       V   L          (     T   0     )             V     c                 c       -       V   L          (     T     c                 o                 i                 l       )           )       -   1     ]       .             (   5   )                               
 
         [0028]    As best understood from Eq. 5, V in  may be used therein for V cc .  
         [0029]    Depending upon the location of the inductive coil sensor  108  within the tank  102  (FIGS. 2 and 3), due to the intimate contact between the fuel  104  or fuel vapor  104   a  and the coil  108   b,  the temperature of the coil is equal to the temperature of the fuel  104  or fuel vapor  104   a  respectively, i.e., T coil =T f  or T coil =T v .  
         [0030]    To read a low voltage accurately, a higher resolution ADC  146  is required. A method to reduce the accuracy requirements of the ADC  146  is to amplify the V coil  signal as shown at  130  in FIG. 5. In FIG. 5, in a second exemplary embodiment of the circuit  100   a,  the amplifier  130  of FIG. 1 comprises an operational amplifier  134  having resistors  132  and  138  and capacitor  136  in a negative feedback circuit. The operational amplifier  134  accepts as input thereto V coil , at a positive terminal, and provides as output V out . V out  is an amplified V L  (Gain=R 138 /R 132 =33.2, V out  is about four volts, given that R coil  is about 25 Ohms) which is filtered by an RC lowpass filter  142 ,  144  and provided as input to a microcontroller ADC  146  to determine coil temperature T coil .  
         [0031]    A second method to increase V coil  is to use a smaller R in , such as R in(temp) &lt;R in , as seen in FIG. 6. In FIG. 6, in a third exemplary embodiment of the circuit  100   a,  the square wave  202  used to drive Q 1  is halted temporarily while Q 2  is turned “on” until the coil  108  is fully charged. The voltage across the coil is then given by  
               V     c                 o                 i                 l       =         R     c                 o                 i                 l           R     c                 o                 i                 l       +     R     i                   n        (     t                 e                 m                 p     )               ×       V     i                   n        (     t                 e                 m                 p     )           .               (   6   )                               
 
         [0032]    Referring to FIG. 7, a schematic diagram of an electric circuit, including a model of an inductive coil sensor  108 , for determining the level of a fuel in a container, is shown generally at  100   b.  Diode D 1 , connected between nodes  110   b  and  110   c,  causes the circuit  100   a  to analyze the negative portion  208  of the V coil  waveform. The negative voltage  208  is used rather than the positive voltage  204 ,  206  because a wiring harness short to either electrical ground or battery voltage will produce a zero output at the Opamp  134 . Resistor  144  provides the discharge resistance with current flowing through the diode  140  and determines the time constant for exponential decay in combination with the inductive coil (L coil /R 144 ). Resistors  146 ,  132  and capacitor  148  filter the input signal V out , to the operational amplifier  134 . The Opamp  134  acts as an integrator to provide an analog voltage output, V op , that corresponds to fuel level, which is read by a microcontroller (not shown).  
         [0033]    While preferred embodiments have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustration only, and such illustrations and embodiments as have been disclosed herein are not to be construed as limiting the claims.