Abstract:
In a liquid sensing system, an RF signal is applied to a series-resonant circuit. The coil of the resonant circuit is placed proximate to a fuel tank, causing electromagnetic radiation to propagate into the fuel space. The fuel acts as an electrical load to the resonant circuit in a manner proportionate to the volume of fuel in the tank and/or to variations in electrical properties of the fuel itself. The loading effect of the fuel can change the resonant frequency and/or the Q of the resonant circuit. The loading effect of the fuel is determined by monitoring a change in one or more electrical parameters associated with the excited resonant circuit, such as a voltage across the resistor in the resonant circuit. Changes in this voltage are analyzed by a controller, the result of which is used to output a value indicative of level and/or composition of the fuel.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application is a Non-Provisional Utility application which claims benefit of U.S. Patent Application Ser. No. 60/679,562 filed May 10, 2005, entitled “SYSTEM AND METHOD OF FUEL LEVEL SENSING USING EMF SENSING” which is hereby incorporated by reference. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     REFERENCE TO SEQUENCE LISTING OR COMPUTER PROGRAM LISTING APPENDIX 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     The present invention relates to systems and methods for sensing levels and electrical properties of liquids stored in fuel tanks and other containers. More particularly, this invention pertains to sensing liquid levels and properties by propagating electromagnetic waves into a liquid container. 
     Motor vehicle operators rely on fuel gauges to provide accurate information on the amount of fuel remaining in the fuel tank. The most common method of measuring the amount of fuel remaining in a motor vehicle fuel tank is to place a mechanical float and lever inside the tank. When the fuel level changes in the tank, the float causes the lever to pivot. When the lever pivots in response to changing fuel levels, an electrical signal is proportionately generated and/or varied. This variation in electrical signal is transmitted to a fuel gauge or vehicle data bus external to the tank. Such electromechanical fuel measurement systems are not particularly accurate and, of course, require installation of a mechanism inside the tank. Repair, replacement, or adjustment of an internal fuel level measurement mechanism is problematic. 
     Engine control systems in many motor vehicles, and particularly in flexible fuel vehicles, also have a need to know the type and/or composition of fuel that is inside the fuel tank. Conventional fuel composition sensors are complex, expensive, and are not capable of also measuring fuel levels. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides reliable, inexpensive, and accurate systems and methods for measuring liquid levels and properties in a tank using a mechanism that can be installed external or internal to the tank. 
     In one embodiment of the liquid level sensing systems and methods of the present invention, a substantially sinusoidal radio frequency (RF) signal of constant frequency is generated and coupled to a series-resonant Inductance, Capacitance, Resistance (LCR) circuit. The coil (inductor) of the resonant circuit is placed in close proximity to, or inside, a plastic fuel tank causing electromagnetic radiation to propagate into the fuel space. Consequently, the liquid fuel inside the tank acts as an electrical load to the series resonant circuit in a manner proportionate to the volume of fuel remaining in the tank. The loading effect of the fuel can cause a shift in the resonant frequency of the circuit and/or a change in the inductance Q of the resonant circuit. The loading effect of the fuel is determined by monitoring a change in one or more electrical parameters associated with the excited resonant circuit. For example, the voltage across the resistor in the series resonant circuit can be monitored. Changes in this voltage are detected and analyzed by a system controller, the result of which is used to output a signal indicative of fuel level. This output can be in the form of a digital or analog electrical signal. 
     In one embodiment of the invention, the resistive component of the series-resonant LCR circuit is provided by the internal resistance of the inductor rather than by a discrete resistor. In this embodiment, the measurement of changes in voltage in the resonant circuit may be taken across the inductor or a portion thereof. 
     Depending on the position and orientation of the system coil, and/or the use of ground planes and other RF directional devices, the measured electrical parameter can represent the volume of liquid in the entire container or the volume of liquid in only a portion of the container. 
     The system and method can sense and measure liquid levels in other containers including oil tanks and water tanks and is not limited to the examples used in this description. The system can be used in a wide variety of scientific, consumer, industrial, and medical environments. 
     Preferably, the system includes auto-calibration hardware and software that enables the system to automatically determine an optimum system operating frequency. In one embodiment of the system, the optimum system operating frequency is selected to be a frequency above or below the resonant frequency of the series LCR circuit. The choice of this operating frequency over the resonant frequency allows for larger changes in voltage drop relative to changes in liquid volume. Preferably, the system is tuned to operate at a frequency between a lower and upper value. 
     In one embodiment, auto-compensation is provided so that the measured electrical parameter provides an accurate indication of the liquid level in the tank, independent of variations in operating conditions, such as ambient temperature. In another embodiment, the system can measure—and be calibrated for—variations in the electrical properties of the liquid itself. 
     The system can include a physical or wireless data interface to facilitate external transmission of the compensated measurement from the system to a fuel gauge or to a central controller in the vehicle. In some embodiments, the system may transmit raw data to a receiver connected to a central controller, with compensation of the raw data being performed in the central controller. The data can be transmitted periodically, in response to a change, by request from the central controller, or by request from an external device such as a diagnostic device. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a perspective view of a motor vehicle with fuel system components shown in phantom. 
         FIG. 2  is block diagram of one embodiment of the liquid sensing system of the present invention. 
         FIG. 3  is an electrical schematic drawing of one embodiment of the liquid sensing system of the present invention. 
         FIG. 4  is an electrical schematic drawing of a second embodiment of the liquid sensing system of the present invention. 
         FIG. 5  is an enlarged side view of an antenna coil as used in the present invention further showing its position in relation to a ground plane element. 
         FIG. 6  is a top view of a vehicle fuel tank in which fuel levels are measured using the system of the present invention. 
         FIG. 7  is a plan view of one embodiment of a printed circuit board (system board) to which the electronic components of the system are mounted and interconnected. 
         FIG. 8(   a ) is a side view schematically illustrating the physical relationship between a liquid tank, externally positioned antenna coil, and ground plane as used in one illustrated embodiment the present invention. 
         FIG. 8(   b ) is a side view schematically illustrating the physical relationship between a liquid tank, internally positioned antenna coil, and ground plane as used in one illustrated embodiment of the present invention. 
         FIGS. 9(   a ) and  9 ( c ) together show the mounting of the system board in a first position with respect to the fuel tank of  FIG. 6 . 
         FIGS. 9(   b ) and  9 ( c ) together show the mounting of the system board in a second position with respect to the fuel tank of  FIG. 6 . 
         FIG. 10(   a ) is a perspective view of another embodiment of the system board of the present invention. 
         FIG. 10(   b ) is a perspective view of the system board of  FIG. 10(   a ) mounted to a fuel tank. 
         FIG. 11  is graphical representation of the frequency response of the series resonant output circuit of the system after initial frequency calibration. 
         FIG. 12(   a ) is graphical plot showing the effects of temperature on system fuel level readings before temperature compensation. 
         FIG. 12(   b ) is graphical plot showing a linearization of the effects of temperature on system fuel level readings. 
         FIG. 13  is graphical plot showing the effects of temperature on system fuel level readings, with and without temperature compensation, using a linearized temperature compensation algorithm in accordance with  FIG. 12(   b ). 
         FIG. 14(   a ) is a graphical plot showing the effects of temperature on system fuel level readings during a driving test from a full tank to ¼ tank of fuel, with and without temperature compensation, using a linearized temperature compensation algorithm in accordance with  FIG. 12(   b ). 
         FIG. 14(   b ) is a graphical plot showing the effects of temperature on system fuel level readings during a driving test from a full tank to ⅜ tank using a linearized temperature compensation algorithm in accordance with  FIG. 12(   b ). 
         FIG. 14(   c ) is a graphical plot showing the effects of temperature on system fuel level readings during a driving test from a ⅜ full tank to an empty tank using a linearized temperature compensation algorithm in accordance with  FIG. 12(   b ). 
         FIG. 15  is a graphical plot of system response as ambient temperature is varying through the entire operating temperature range. 
         FIG. 16  is a flow chart illustrating the steps associated with the temperature compensation algorithm used in one embodiment of the system. 
         FIG. 17  is a flow chart illustrating the steps associated with auto-calibration of the RF signal generator at system initialization. 
         FIG. 18  is a block diagram of a test set-up used to determine the effects on system fuel level output caused by use of different fuel types in the vehicle fuel tank. 
         FIG. 19  is a graphical plot showing the effects on system fuel level output caused by use of different fuel types in the vehicle fuel tank. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A block diagram of the liquid measurement system  10  of this invention is shown in  FIG. 2 . A controller  30 , which can be a microcontroller, an application specific integrated circuit (ASIC), or another logical device, includes an RF generator  35 , an analog-to-digital converter (ADC)  40  and a pulse width modulator (PWM) or digital-to-analog converter (DAC)  45 . The controller  30  further includes embedded firmware/software functional to control the RF generator  35 , to receive and process data from ADC  40 , and to cause PWM/DAC  45  to transmit data external to the system  10 . The firmware/software in controller  30  also includes modules that implement the auto-calibration and compensation algorithms as described below. 
     Still looking at  FIG. 2 , the output of RF generator  35  is transmitted to antenna driver  55  which can include an RF amplifier and/or matching circuitry to effectively couple the RF signal to a series-resonant circuit that includes a resonant capacitor  60 , resonant inductor or coil  65 , and resistor  70 . One terminal of resistor  70  is electrically connected to system ground. The other terminal of resistor  70  is electrically connected to an analog input on ADC  40 . Thus, in accordance with one aspect of the invention, changes in voltage across resistor  70  are converted to digital signals by ADC  40  so that such digital signals can be further processed by controller  30 . Thus, the controller  30  converts the signal representing a change in the electrical parameter of the resonant circuit into a liquid level signal that is provided to the vehicle fuel gauge directly or through a vehicle data bus. 
     In the embodiment shown in  FIG. 2 , the series resonant circuit can be characterized as an antenna circuit in which resonant inductor  65  functions as a radiating component that directs RF energy into the fuel tank  15 . In other embodiments, a separate radiating component (not shown) may be coupled to the resonant circuit. Also, in the embodiment of  FIG. 2 , the resistive component R of the series-resonant LCR circuit is illustrated as a discrete resistor  70 . However, the resistive component R can also be provided as an internal resistance of the resonant inductor or coil  65  rather than as a separate discrete component. In such an embodiment, changes in voltage are measured across the resonant inductor  65  or a portion thereof. 
     Although the scope of the present invention is not limited to any particular circuit topology,  FIG. 3  is a schematic diagram of one embodiment of the system  10  shown in block diagram form in  FIG. 2 .  FIG. 4  is a schematic diagram of another embodiment of the system, as used in testing described below. 
     Preferably, the electronic components of the system  10  are mounted to a system board, such as a unitary printed circuit board (PCB)  85 , as shown in  FIG. 7 , with the coil  65  having a geometry, orientation, and position on the PCB  85  to provide optimum RF energy coupling external to the PCB  85 . The printed circuit board  85  can be rigid or flexible, with or without an adhesive back.  FIG. 6  shows one embodiment of a plastic vehicle fuel tank  15  which is conventionally mounted in the rear of a vehicle  20 , as shown in  FIG. 1 . The fuel tank  15  is fluidly coupled to the vehicle engine and emission systems via one or more fluid lines  25 . 
     As best seen in  FIGS. 9(   a )-( c ), the PCB  85  is attached to an external wall of fuel tank  15  in a position that will provide sensing of changes in the desired liquid level, either in the entire tank  15  or in only a portion of the tank  15 . 
     In some embodiments of the system  10 , a ground plane structure  75  is positioned proximate to and behind the coil  65  so as to focus and direct RF energy from the coil  65  into the tank  15 , as shown in  FIGS. 5 and 8 . In the embodiment of  FIG. 8(   a ), the ground plane  75  is spaced approximately 2 mm from the coil  65 , although other embodiments may use different spacings, as shown in  FIG. 5 , for example. Where a heat shield  14  is used with the fuel tank  15  (as shown on  FIG. 6) , the heat shield can optionally be used as the ground plane structure  75  as a further cost saving measure. 
     The coil  65  can be attached or incorporated into strap (not shown) that secures the fuel tank  15  to the vehicle. This would avoid the expense of modifying a conventional fuel tank to accept a direct-mounted coil  65  or PCB  85 . In addition, if the tank mounting strap is grounded to the vehicle, the strap itself can function as ground plane structure  75 , further reducing cost. 
     In yet another embodiment of the system  10  as shown in  FIG. 8(   b ), the series-resonant LCR circuit, or at least the coil  65 , can be mounted inside the fuel tank  15 . If a ground plane  75  is used, it can also be positioned inside the tank  15 , between the coil  65  and the tank wall. 
     Another embodiment of the PCB  85  and fuel tank mounting is shown in  FIG. 10 . In this embodiment, the PCB  85  is attached to a mounting plate  17  which can also act as a heat sink. The mounting plate  17  is then attached a wall of the fuel tank  15 . The PCB  85  further includes a connector  16  that electrically connects the system  10  to the vehicle electrical system and to the vehicle data bus for purposes of transmitting a fuel level signal. The fuel level signal can be transmitted using a physical (hard-wired) connection or using a wireless connection. 
     When the PCB  85  and coil  65  are positioned proximate to or inside the tank  15  as shown, liquid in the tank  15  will electrically load the series-resonant circuit formed by capacitor  60 , coil  65 , and resistor  70 . Accordingly, when the controller  30  activates the RF generator  35 , the coil  65  is excited by a substantially sinusoidal RF signal at a constant frequency. The loading caused by fuel proximate the coil  65  will either reduce the Q of the resonant circuit and/or change its resonant frequency. In either case, the voltage measured across the resistor  70  (or across the internal resistance of the coil  65 ) will vary by an amount that is proportional to a change in fuel level, due to a corresponding change in impedance of the coil  65 . This change in voltage is converted to a digital signal in the ADC  40  and processed by the controller  30  so that a corresponding data output from the PWM/DAC  45  can be transmitted to a fuel gauge or vehicle central controller (not shown.) 
     Other embodiments of the system  10  can use a parallel resonant circuit with other means of measuring a parameter that represents a change in loading of the resonant circuit caused by changes in fuel level in the tank. 
     Preferably, the operating frequency of the RF generator  35  is adjusted so that it is slightly above the resonant frequency of the series-resonant LCR circuit. As shown on  FIG. 11 , the system operating frequency is selected to define a liquid level sensing window on a relatively steep portion of the frequency response curve, thereby providing maximum sensitivity to changes in liquid level. When the printed circuit board  85  and coil  65  are placed on a fuel tank  15 , the series resonant circuit will have a different resonant frequency that varies from tank to tank due to component tolerances, tolerance on tank dimensions, coil dimensions, track width, etc. In order to compensate for these variations, an auto-calibration method is preferably used. In one embodiment of such a method, the controller  30  includes a calibration module that finds the resonant frequency (fc) of the series-resonant circuit after first power up (or on request), then adjusts the operating frequency of the RF generator  35  to a frequency (f 1 ) so that the system is operating on the linear slope throughout the entire operating temperature range (−40 to +80 C). 
     Referring to the flow chart in  FIG. 17 , the controller  30  varies the frequency f of the RF generator  35  in steps from 0 to 255, where 0 corresponds to the lowest frequency (6.34 MHz) and 255 corresponds to the highest frequency (9.66 MHz). However, use of the system is not restricted to these frequencies. In one embodiment, the lowest frequency is 7.4 MHZ and the highest frequency is 8.3 MHz. For each frequency, the controller  30  samples the data from ADC  40  and reads V, the voltage across resistor  70 . The controller  30  varies the operating frequency f (sweeping the frequency from low to high or from high to low) in order to find the resonant frequency of the series-resonant circuit. The controller  30  then adjusts and fixes the frequency f 1  to a point in a substantially linear section of the frequency response curve. The variation from full to empty tank should be in the substantially linear zone (f 1  to f 2 ) as shown on  FIG. 11 . Once the nominal operating frequency of the system  10  is selected, it can remain fixed for as long as the system  10  remains in the vehicle. 
     Electronic components and systems in motor vehicles will preferably operate properly over a wide range of ambient temperatures. Changes in temperature can induce system output errors. This is shown with raw system data from the ADC  40  graphed in  FIG. 12(   a ) and linearized in  FIG. 12(   b ). In order to compensate for the effects of temperature, the system output is characterized over the entire range of operating temperatures (−40 to +80 C) at Empty and Full tank.  FIG. 15  shows system voltage output as a function of temperature through a full temperature range. As an approximation, the graph is divided into sections, where each section is linear following the equations below:
 
If t&lt;t1 : V=a 0 T+b 0
 
If t1&lt;t&lt;tref:  V=a 1 T+b 1
 
If tref&lt;t&lt;t2: V=cst
 
If t2&lt;t&lt;t3 : V=a 2 T+b 2
 
If t&gt;t3:  V=a 3 T+b 3
 
     One embodiment of the system  10 , as shown in  FIG. 4 , was installed in a test vehicle similar to that shown in  FIG. 1 . The system coil  65  was wound and configured as follows: 
     Number of turns=35 
     Physical Size=50×50 mm, Spiral 
     Track width=0.15 mm 
     Distance between tracks=0.4 mm (center to center) 
     The series-resonant circuit included the following component values: 
     L=68 uH 
     R=33 ohms 
     C=10 pF 
     The ground plane was configured to be 50×50 mm and positioned a few centimeters away from coil  65 , as shown in  FIG. 8 . The fuel tank  15  had the following dimensions: w=950 mm, l=670 mm, h=210 mm, with a fuel capacity of approximately 80 liters. Note that if the resistance in the series-resonant LCR circuit is provided by the internal resistance of the coil rather than by a discrete resistor, the actual resistance will likely be lower, on the order of 20 ohms or less. 
     In a first test, the effects of temperature on system output were measured as follows: The vehicle was driven for 10 minutes then stopped for 20 minutes in order to obtain a fuel level signal (at ADC  40 ) at different temperatures. This test was repeated at three different fuel levels (full, ¾ and empty). The test results are shown on the graph in  FIG. 12(   a ). As can be seen, the ADC  40  output vs. temperature varies according to a pattern (oscillating around a line) at all liquid levels. Therefore, to compensate for the temperature, as a first approximation, a simple linearization algorithm was implemented as shown in  FIG. 12(   b ). 
     After the temperature compensation algorithm was programmed into the system controller  30 , the vehicle was driven for 20 minutes starting from cold (outside temperature=−7 C) with an empty tank. The temperature, fuel level, and temperature compensated fuel level signals were recorded as displayed in the graph shown in  FIG. 13 . The compensated fuel level ADC  40  output remains stable around 65 counts (corresponding to an empty tank) when the temperature and the measured fuel level varies. Thus, the temperature compensation algorithm compensates for the changes due to temperature so that the fuel gauge always shows the actual empty level. 
     In a further test, the vehicle was driven for 230 miles starting with a full tank, stopping at regular intervals (approx. every 30 miles). The temperature, fuel level, and temperature compensated fuel level signals were recorded. The results are shown in  FIGS. 14(   a )-( c ). The compensated fuel level signal from ADC  40  reading varies linearly from 115 to 90 when the temperature and the measured fuel level vary up and down. This demonstrates that the temperature compensation module compensates for the changes due to temperature, so that the fuel gauge always shows the real level. 
     The system  10  of the present invention can also be used to detect variations in the electrical properties associated with different liquids placed into the tank  15 . For example, if diesel fuel is placed into a fuel tank of a vehicle that runs on gasoline (or vice-versa), this mistake can be detected upon activation of the system. Using a voltage measurement taken across a portion of the series-resonant LCR circuit, it is possible to determine the type or composition of liquid fuel in the tank due to the variation in the electrical properties of the liquid. Using the test set-up of  FIG. 18 ,  FIG. 19  shows system output profiles corresponding to the different fuel types described in the table below when placed in a fuel tank. 
     
       
         
               
               
               
             
           
               
                   
               
               
                 NO 
                 TITLE 
                 FUEL RECIPE 
               
               
                   
               
             
             
               
                 1 
                 MS-9368 
                 Unleaded gasoline 
               
               
                   
                 Reformulated 
               
               
                 2 
                 MS-9368 
                 Unleaded gasoline with 0.05% 
               
               
                   
                 Reformulated 
                 thiophene added 
               
               
                 3 
                 MS-9368 
                 Unleaded gasoline with 10% by volume, 
               
               
                   
                 Reformulated 
                 ethanol, with .5% aggressive water. 
               
               
                 4 
                 MS-9368 
                 Unleaded gasoline with 25% 
               
               
                   
                 Reformulated 
                 by volume, ethanol 
               
               
                 5 
                 Leaded 
                 Indolene 30 (3 g/gal lead) 
               
               
                   
                 gasoline 
               
               
                 6 
                 M25 
                 M25 − [75% MS8004] + [25% methanol 
               
               
                   
                   
                 solution consisting of: methanol (MS2585), 0.5% 
               
               
                   
                   
                 Aggressive water, and 0.028 ml/L Formic acid] 
               
               
                 7 
                 MS-9368 
                 Unleaded gasoline with 20 mm 
               
               
                   
                 Reformulated 
                 water added (0.5 gal in 10 gal tank) 
               
               
                   
               
             
          
         
       
     
     Accordingly, the system  10  of the present invention can be used as a fuel composition sensor, including detecting fuel type, prior to or in addition to measuring actual fuel level. In flexible fuel vehicles which can operate with different fuel compositions (for example, E85, E10, E20), the engine control systems are preferably informed electronically of the composition of the fuel in the tank so that the necessary engine control adjustments can be made. For example, by comparing actual system output with stored output profiles associated with electrical properties of certain fuel compositions, the system of this invention can provide that functionality (along with fuel level measurement) without the added cost of conventional fuel composition sensors, such as that described in U.S. Pat. No. 6,927,583, which is incorporated herein by reference. 
     The system output profiles determined as illustrated in the examples of  FIGS. 18 and 19  can also be used to compensate the liquid level reading according to the type of liquid in the container. 
     Preferably, the RF generator  35  will provide RF power levels within the constraints and requirements of the FCC/ETSI regulations as appropriate. 
     Thus, although there have been described particular embodiments of the present invention of a new and useful System and Method for Sensing the Level and Composition of Fuel in a Fuel Tank, it is not intended that such references be construed as limitations upon the scope of this invention except as set forth in the following claims.