Abstract:
This invention discloses an apparatus and method of determining the temperature of the core of an inductive coil sensor so that the effective inductance of the coil sensor can be temperature compensated to thereby provide an accurate measure of the level of fuel in a tank. The method comprises energizing the sensor with a prescribed voltage, de-energizing the sensor, measuring the resultant voltage across the sensor, and determining the core temperature from the measured resultant voltage across the sensor.

Description:
TECHNICAL FIELD 
     This invention relates to inductive coil sensors and more particularly to temperature compensation of such sensors. 
     BACKGROUND 
     Inductive coils have been used to determine liquid levels by measuring the change in the effective inductance of a coil sensor obtained as a magnetic core moves within a current carrying coil. The further the core is inserted into the coil, the greater the effective inductance and vice versa. By measuring this effective inductance, the relative position of the core inside of the coil can be determined. The core is connected by way of a lever arm to a floatation device residing in the liquid. As the level of the liquid increases, the core is inserted further into the coil thus, increasing the effective inductance of the coil and as the level decreases, the core is retracted from the coil thus decreasing the effective inductance. 
     The effective inductance of the coil is determined by the number of turns in the coil, the current carried by the coil, the geometry of the coil, and the position of the core within the coil. The effective inductance is also affected by the magnetic permeability of the core. The number of turns, the current, and the geometry of the coil are all fixed parameters in the design of the coil sensor. The position of the core depends upon the liquid level, however, the magnetic permeability of the core is temperature dependent. Thus, the effective inductance will vary not only with fuel level but also with the temperature of the core. The temperature-dependence of the core&#39;s magnetic permeability has more effect the farther the core is inserted into the coil. In order to make an accurate automotive fuel level sensor, the temperature of the core needs to be determined so that the effective inductance can be temperature compensated. 
     One method for determining the temperature of the core is to add a temperature-sensitive device in close proximity to the core. Such devices include, but are not limited to, thermistors, RTD&#39;s and thermocouples. However, adding such devices increases the cost and reduces the reliability of the inductive fuel level sensor. 
     SUMMARY OF THE INVENTION 
     This invention discloses a method of determining the temperature of the core of an inductive coil sensor so that the effective inductance of the coil sensor can be temperature compensated to thereby provide an accurate measure of the level of fuel in a tank. It is impractical to directly measure the temperature of the core of the coil sensor since the core moves. However, since the temperature in a fuel tank varies relatively slowly, it is possible to measure a nearby temperature and assign that temperature to the core. 
     The temperature of the core, T core , is determined by the temperature of the sensor coil, T coil , since the coil is in close proximity to the core. T coil  can be determined from the resistance of the coil, R coil . Thus a method of compensating for the temperature dependence of the core of a coil sensor is disclosed. The method comprises energizing the sensor with a prescribed voltage; after a prescribed time interval, measuring the resultant voltage across the sensor; and determining the core temperature from the measured resultant voltage across the sensor. 
     Once the temperature of the core is known, the effective inductance of the coil can be compensated for temperature changes. The inductive coil sensor is connected to a Fuel Control Unit. The Fuel Control Unit contains the electronics to measure the effective inductance of the coil and to read the coil resistance. The Fuel Control Unit uses the coil resistance to compensate the effective inductance of the coil and to provide an accurate measure of fuel level. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a generalized schematic diagram of an electromechanical system including an electric circuit having a coil sensor for determining the level of a liquid in a container; 
     FIG. 2 is a schematic diagram of an exemplary embodiment of the electric circuit of FIG. 1 including a model of a coil sensor for determining the temperature of a liquid in a container; 
     FIG. 3 is a graphical representation of experimental data depicting the effect of the temperature dependence of the magnetic permeability of the core of an inductive coil sensor on the temperature of the core, wherein a first graph shows the temperature dependence of the magnetic permeability of the core with the core substantially out of the coil, a second graph shows the temperature dependence of the magnetic permeability of the core with the core approximately half way within the coil and a third graph shows the temperature dependence of the magnetic permeability of the core with the core substantially fully within the coil; 
     FIG. 4 is a graphical representation depicting the relative timing of the square wave driving pulse voltage, V pulse , of FIG.  1  and the resultant voltage, V coil  across the coil sensor; 
     FIG. 5 is a graphical representation of the exponential decay of V coil  wherein the core of the coil sensor is not inserted into the coil; 
     FIG. 6 is a graphical representation of the exponential decay of V coil  wherein the core of the coil sensor is fully inserted into the coil; and 
     FIG. 7 is a schematic diagram of an exemplary embodiment of the electric circuit of FIG. 1 including a model of an inductive coil sensor for determining the level of a liquid in a container. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An inductive coil is constructed by winding a given number of turns of conductive wire onto a bobbin. Copper is used as the wire material due to its low cost and low electrical resistance. Although the resistance of the 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. 
     Referring now to FIG. 1, an electromechanical system is shown generally at  100 . The electromechanical system  100  comprises a circuit  100   a  for an inductive coil sensor  108 . The circuit  100   a  comprises a core  108   a  mechanically linked by way of a lever arm  106  to a floatation device  106   a  resting in a liquid  104  within a container  102 . The core  108   a  is moveable within an inductive coil  108   b . As the flotation device  106   a  rises and falls with the level of the liquid  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. 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.  4 . 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 . 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 . 
     Referring to FIG. 2, an exemplary embodiment of the circuit  100   a  of FIG. 1 is shown. In FIG. 2, 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 for about 100 msec through R in . The coil sensor  108  of FIG. 1 can be modeled as a parallel RLC circuit  124 ,  126 ,  128 . In the circuit shown in FIG. 2, R in  is chosen to be much larger than R coil . This allows the resistance of the coil to be neglected in determining the effective inductance of the coil to determine fuel level. 
     A method of measuring R coil  is to measure the voltage across the coil, V coil . In order to measure V coil , a square wave used to measure the effective inductance is halted temporarily and Q 1  in FIG. 2 would remain turned “on” until the coil is fully charged. To measure temperature, the square wave on V pulse  is stopped and V pulse  is set to 0 Volts to turn Q 1  on. Once the coil is fully charged, the voltage across the coil is given by                V   coil     =         R   coil         R   coil     +     R     i                 n           ×       V     i                 n       .               (   1   )                                
     as shown in FIG.  2 . If R in  and V cc  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 . Thus, V in ≈V cc , and                V   coil     ≈         R   coil         R   coil     +     R     i                 n           ×       V   cc     .               (1a)                                
     V cc  can vary somewhat with temperature but this can be neglected if the ADC is also powered by V cc . Therefore, the coil voltage can be approximated to vary in the same fashion as the temperature coefficient of resistance of copper (0.393% per degree C.). 
     FIG. 3 displays experimental data at  210 ,  212  and  214  that shows the effect of the temperature-dependence of the core&#39;s magnetic permeability with the distance that the core is within the sensor coil. It shows the effective inductance at the full position  210  varying by 4% from −40 degrees C. to 85 degrees C. 
     As seen in FIGS. 2 and 4, 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.  4 . 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 0 . It will be appreciated from FIGS. 5 and 6 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  204   b ,  204   c  will change. Thus, FIG. 5 is representative of the sensor  108  charging when the core  108   a  is substantially out of the coil  108   b  and FIG. 5B 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 0  (e.g., 100 msec) until V coil  settles to the substantially DC voltage level of V L  at  206   b  and  206   c . At such time, in the circuit model  108  of FIG. 2, 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,                  V   L          (     T   coil     )       =           R   coil          (     T   coil     )             R   coil          (     T   coil     )       +     R     i                 n           ×       V   cc     .               (   2   )                                
     In the circuit of FIG. 2, V L  is about 120 mV if R coil  is about 25 Ohms. If V L  has been measured at a reference temperature T 0 , then                  V   L          (     T   0     )       =           R   coil          (     T   0     )             R   coil          (     T   0     )       +     R     i                 n           ×       V   cc     .               (   3   )                                
     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)  
       
     
     where α is the temperature coefficient of resistance. Equations (2) and (3) can be substituted into equation (4) to give the difference between T coil  and T 0 :                  T   coil     -     T   0       =         1   α          [         (         V   L          (     T   coil     )           V   L          (     T   0     )         )          (         V   cc     -       V   L          (     T   0     )             V   cc     -       V   L          (     T   coil     )           )       -   1     ]       .             (   5   )                                
     It will be appreciated that in Eq. 5, V in  may be substituted for V cc . 
     To measure T coil , the oscillator  120  is stopped periodically (e.g., once every second) in the low state. Approximately 100 msec are allowed to pass whereupon V coil →V L , from which is found V L (T coil ). T 0 , V L (T 0 ), V cc  and α are known and T coil  can be determined from Eq. 5. Due to the proximity of the core  108   a  and the coil  108   b  to one another within the coil sensor  108 , T core =T coil . 
     Once the temperature of the coil is known, then the temperature compensated effective inductance of the sensor at T 0 , V sensor (T 0 ), can be calculated from the uncompensated effective inductance of the sensor at T coil , V sensor (T coil ), according to:                  V   sensor          (     T   0     )       =             V   sensor          (     T   coil     )       -     (       V   empty     +       β   empty          (       T   coil     -     T   0       )         )           (       V   full     +       β   full          (       T   coil     -     T   0       )         )     -     (       V   empty     +       β   empty          (       T   coil     -     T   0       )         )         ×     V   cc               (   6   )                                
     where β empty  is the temperature dependence of V empty  and β full  is the temperature dependence of V full . If V empty  is measured with the core outside of the coil, then V empty  will not be affected by temperature and then β empty =0. If we let β full =β, then Eq. 6 simplifies to:                  V   sensor          (     T   0     )       =             V   sensor          (     T   coil     )       -     V   empty           V   full     -     V   empty     +     β        (       T   coil     -     T   0       )           ×     V   cc               (6a)                                
     where V empty  is V sensor (T 0 ) at the empty position and at temperature T 0 , V full  is V sensor (T 0 ) when the core is substantially fully inserted into the coil at temperature T 0 , and β is the temperature dependence of the core&#39;s magnetic permeability. V sensor  is the output voltage, V op , of the integrating Opamp  130  of FIG.  7 . It will be appreciated that in Eqs. 6 and 6a, V in  may be substituted for V cc . 
     It will be appreciated that since R in &gt;&gt;R coil , Eqs. 2 and 3 can be simplified:                  V   L          (     T   coil     )       ≈           R   coil          (     T   coil     )         R     i                 n         ×     V   cc                   and             (2a)                   V   L          (     T   0     )       ≈           R   coil          (     T   0     )         R     i                 n         ×     V   cc               (3a)                                
     This leads to a simplification of Eq. 5; namely:                  T   coil     -     T   0       ≈       1   α          [           V   L          (     T   coil     )           V   L          (     T   0     )         -   1     ]               (5a)                                
     Equation 5a is a useful approximation for implementation on a microprocessor. 
     In FIG. 2, 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 4 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. The microcontroller ADC  146  then uses a look up table to adjust the fuel level as measured in above procedure. 
     In FIG. 7, in an exemplary embodiment of the circuit  100   a , diode D 1 , connected between nodes  110   b  and  110   c , causes the circuit  100  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 aforesaid 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 =V sensor  that corresponds to fuel level, which is read by a microcontroller (not shown). Resistor  156  is used to to set the offset voltage to the integrator integrator  134 . Capacitor  152  is connected to resistor  156  and to electrical ground. The positive terminal of the integrator  134  is connected to electrical ground at  154 . 
     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.