Abstract:
Methods and systems for detecting fraud caused by tampering with a fuel flow meter. In one embodiment, the method comprises providing a fuel flow meter for measuring the flow of liquid fuel. The flow meter has at least one shaft supporting a rotor. The method further comprises providing the flow meter with a rotary displacement sensor. Also, the method comprises measuring a first angular position of the shaft upon termination of a first fueling transaction and measuring a second angular position of the shaft upon initiation of a second fueling transaction. Finally, the method comprises comparing data indicative of the first and second shaft angular positions to determine whether fraud has occurred.

Description:
PRIORITY CLAIM 
       [0001]    This application claims the benefit of provisional application Ser. No. 61/421,011, filed Dec. 8, 2010, which is hereby relied upon and incorporated herein by reference for all purposes. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to fuel dispensers. More specifically, the invention relates to detection and prevention of fraud caused by tampering with a fuel flow meter associated with a fuel dispenser. 
       BACKGROUND OF THE INVENTION 
       [0003]    Fuel dispensers in retail service station environments include flow meters that measure the volumetric flow rate of fuel as it is dispensed. Such flow meters are typically required to comply with weights and measures regulatory requirements that mandate a high level of accuracy. This ensures that the customer is neither overcharged nor undercharged for the fuel purchase. Typically, either positive displacement meters or inferential meters have been used for this purpose. 
         [0004]    In modern service station fuel dispensers, a control system processes signals generated by a displacement sensor to monitor the amount of fuel delivered to a customer&#39;s vehicle. One displacement sensor for this purpose is referred to as a pulser. Pulsers are typically variable reluctance sensors operatively connected to the flow meter to measure rotation of a flow meter shaft. As fuel is dispensed, causing the shaft to rotate, the pulser generates a pulse train. Each pulse represents a known volume of fuel (e.g., 0.001 gallons) passing through the meter. 
         [0005]    However, other types of sensors have been used to sense flow rate of various fluids, including magnetic sensors and optical sensors. Magnetic sensors often comprise one or more magnets coupled to and rotating with a flow meter shaft. In some sensors, the magnet(s) may be disposed on a disc that attaches via a threaded aperture at an end of the flow meter shaft and is aligned with the shaft longitudinal axis. Magnetic sensors further include a flux detecting device, such as a Hall-effect sensor, to detect shaft rotation speed and direction. 
         [0006]    Optical sensors typically comprise a disc with a pattern of transparent and opaque segments which form a number of concentric tracks. The disc rotates through a read head, which may comprise a light source, a mask, and a photodetector. The read head photodetector outputs the light intensity reaching its surface as the disc rotates, thus providing a signal indicative of the motion of the disc. 
         [0007]    Attempts have been made to interfere with the displacement sensor on a fuel flow meter in order to modify the calculated volume of fuel dispensed. For example, a dishonest consumer may disconnect the sensor (or one of its components) from the fuel flow meter prior to a fueling transaction. Also, a dishonest consumer may disable either or both of the fuel dispenser or displacement sensor electronics and force fuel through the fuel flow meter. 
       SUMMARY OF THE INVENTION 
       [0008]    According to one aspect, the present invention provides a method for detecting fraud caused by tampering with a fuel flow meter. The method comprises the step of providing a fuel flow meter for measuring the flow of liquid fuel. The flow meter has at least one shaft supporting a rotor. The method further comprises providing the flow meter with a rotary displacement sensor. Also, the method comprises recording data indicative of a first angular position of the shaft upon termination of a first fueling transaction and recording data indicative of a second angular position of the shaft upon initiation of a second fueling transaction. Finally, the method comprises comparing data indicative of the first and second shaft angular positions to determine whether a difference exists. 
         [0009]    According to a further aspect, the present invention provides a fuel flow meter comprising a shaft supporting at least one rotor. The fuel flow meter also comprises a rotary displacement sensor comprising at least one sensing element, a processor, and memory. The displacement sensor is adapted to store first data indicative of an angular position of the shaft in the memory upon termination of a first fueling transaction. The displacement sensor is further adapted to generate second data indicative of an angular position of the shaft upon initiation of a second fueling transaction. Finally, the processor is adapted to compare the first and second data to determine whether a difference exists. 
         [0010]    In another aspect, the present invention provides a fuel dispenser comprising a control system having control system memory and internal fuel flow piping adapted for connection to a fuel flow path from a bulk storage tank (e.g., an underground storage tank) to a nozzle. The fuel dispenser further comprises a fuel flow meter having a shaft, wherein said fuel flow meter is located along the piping. Also, the fuel dispenser comprises a rotary displacement sensor coupled to the fuel flow meter and in communication with the control system, wherein the displacement sensor comprises displacement sensor memory. The displacement sensor is adapted to determine data indicative of the angular position of the shaft, and the data indicative of the angular position is stored in both the control system memory and the displacement sensor memory. 
         [0011]    According to a further aspect, the present invention provides a fuel flow meter comprising a shaft supporting at least one rotor and a rotary displacement sensor comprising a processor and memory. The displacement sensor comprises an optical encoder adapted to output an expected number of position signals per revolution of the shaft and one or more reference signals per revolution of the shaft. Also, the displacement sensor is adapted to store data indicative of the position signals and the one or more reference signals in memory. Finally, the displacement sensor is adapted to compare a first number of position signals received after receiving a reference signal before termination of a first fueling transaction and a second number of position signals received after initiation of a second fueling transaction to the expected number of position signals to determine whether fraud has occurred. 
         [0012]    In accordance with another aspect, the present invention provides a method for detecting fraud caused by tampering with a fuel flow meter. The method for detecting fraud comprises providing a fuel flow meter for measuring the flow of liquid fuel, the flow meter having a housing and at least one shaft supporting a rotor. Also, the method comprises providing the flow meter with a rotary displacement sensor having a housing and a shaft. Further, the method comprises providing a first coupling between the flow meter shaft and the rotary displacement sensor shaft. Notably, the first coupling is operative to cause one of the rotary displacement sensor shaft and the flow meter shaft to rotate relative to the other of the rotary displacement sensor shaft and the flow meter shaft upon removal of the rotary displacement sensor from the flow meter housing. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    A full and enabling disclosure of the present invention, including the best mode thereof directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended drawings, in which: 
           [0014]      FIG. 1  is perspective view of an exemplary fuel dispenser according to one embodiment of the present invention. 
           [0015]      FIG. 2  is a schematic diagram of internal fuel flow components of the fuel dispenser of  FIG. 1  according to one embodiment of the present invention. 
           [0016]      FIG. 3  is a schematic diagram illustrating the relationship between the control system, flow meter, and displacement sensor according to one embodiment of the present invention. 
           [0017]      FIG. 4  is a flow chart outlining the operation of the components of  FIG. 3  according to one embodiment of the present invention. 
           [0018]      FIG. 5  is a schematic cross-sectional view of a positive displacement flow meter according to one embodiment of the present invention. 
           [0019]      FIG. 6  is a flow chart outlining the operation of a fuel dispenser flow meter having an optical displacement sensor according to one embodiment of the present invention. 
           [0020]      FIG. 7A  is a partial cross-sectional view illustrating a coupling between a flow meter and a displacement sensor and their respective shafts according to one embodiment of the present invention. 
           [0021]      FIG. 7B  is a partial top view of the flow meter of  FIG. 7A . 
           [0022]      FIG. 7C  is a bottom view of the displacement sensor of  FIG. 7A . 
           [0023]      FIG. 7D  is a top view of the flow meter shaft of  FIG. 7A . 
           [0024]      FIG. 7E  is a bottom view of the encoder shaft of  FIG. 7A . 
       
    
    
       [0025]    Repeat use of reference characters in the present specification and drawings is intended to represent same or analogous features or elements of the invention. 
       DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0026]    Reference will now be made in detail to presently preferred embodiments of the invention, one or more examples of which are illustrated in the accompanying drawings. Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that modifications and variations can be made in the present invention without departing from the scope or spirit thereof. For instance, features illustrated or described as part of one embodiment may be used on another embodiment to yield a still further embodiment. Thus, it is intended that the present invention covers such modifications and variations as come within the scope of the appended claims and their equivalents. 
         [0027]    Embodiments of the present invention relate to detection and prevention of fraud caused by tampering with a fuel flow meter associated with a fuel dispenser. Generally, the fuel flow meter comprises a rotary displacement sensor capable of determining the absolute shaft angle of the flow meter. Data indicative of the shaft angle may be stored in one or more memory devices, for example at the end of each fueling transaction. Then, at the beginning of a new transaction or when power is applied to the dispenser, for example, an algorithm may be run to ascertain the current shaft angle. The algorithm may then compare the detected shaft angle to the shaft angle previously stored in memory. As discussed below, if fraud has occurred during the time between when data indicative of the shaft angle is stored in memory and when the algorithm is run, it is likely that the current shaft angle will differ from the previously stored shaft angle. In this case, appropriate action may be taken to alert the operator that fraud has occurred, such as generating an alarm or disabling the fuel dispenser. 
         [0028]    It is contemplated that the present invention may be used with many types of rotary displacement sensors. Thus, as used below, the term “displacement sensor” comprises any device which converts shaft angular position to an analog or digital signal that can be detected and further processed. The term includes, but is not limited to, any type of noncontact rotary position sensor or encoder. In preferred embodiments, the rotary displacement sensor is an absolute sensor. Further, as described in more detail below, the present invention may be used with both positive displacement and inferential fuel flow meters. 
         [0029]    Referring now to  FIG. 1 , a perspective view of an exemplary fuel dispenser  10  is provided according to one embodiment of the present invention. For example, fuel dispenser  10  may be the ENCORE® fuel dispenser sold by Gilbarco Inc. of Greensboro, N.C., U.S.A. Those of skill in the art will appreciate, however, that the present invention may be used with flow meters in any fuel dispenser. 
         [0030]    Fuel dispenser  10  includes a housing  12  with at least one flexible fuel hose  14  extending therefrom. Fuel hose  14  terminates in a manually-operated nozzle  16  adapted to be inserted into a fill neck of a vehicle&#39;s fuel tank. Nozzle  16  includes a fuel valve. Various fuel handling components, such as valves and meters, are also located inside of housing  12 . These fuel handling components allow fuel to be received from underground piping and delivered through hose  14  and nozzle  16  to a vehicle&#39;s tank, as is well understood. 
         [0031]    The fuel dispenser  10  has a customer interface  18 . Customer interface  18  may include an information display  20  that shows the amount of fuel dispensed and the price of the dispensed fuel. Further, customer interface  18  may include a media display  22  to provide advertising, merchandising, and multimedia presentations to a customer in addition to basic transaction functions. The graphical user interface provided by the dispenser allows customers to purchase goods and services other than fuel at the dispenser. The dispenser also preferably includes a payment card reader to allow the customer to pay for the fuel at the dispenser. 
         [0032]      FIG. 2  is a schematic illustration of exemplary internal fuel flow components of fuel dispenser  10 . In general, fuel may travel from an underground storage tank (UST) via main fuel piping  24 , which may be a double-walled pipe having secondary containment as is well known, to fuel dispenser  10  and nozzle  16  for delivery. An exemplary underground fuel delivery system is illustrated in U.S. Pat. No. 6,435,204 to White et al., hereby incorporated by reference in its entirety for all purposes. In many cases, a submersible turbine pump (STP) associated with the UST is used to pump fuel to the fuel dispenser  10 . However, some fuel dispensers may be equipped with a pump and motor within housing  12  to draw fuel from the UST to the fuel dispenser  10 . 
         [0033]    Main fuel piping  24  may pass into housing  12  first through shear valve  26 . As is well known, shear valve  26  is designed to close the fuel flow path in the event of an impact to fuel dispenser  10 . U.S. Patent App. Pub. No. 2006/0260680 to Reid et al., now U.S. Pat. No. 7,946,309, hereby incorporated by reference in its entirety for all purposes, discloses an exemplary secondarily-contained shear valve adapted for use in service station environments. Shear valve  26  contains an internal fuel flow path to carry fuel from main fuel piping  24  to internal fuel piping  28 , which may also be double-walled. 
         [0034]    After fuel exits the outlet of the shear valve  26  and enters into the internal fuel piping  28 , it may encounter a flow control valve  30  positioned upstream of a flow meter  32 . In some fuel dispensers, the valve  30  may be positioned downstream of the flow meter  32 . The valve  30  may preferably be a proportional solenoid controlled valve, such as described in U.S. Pat. No. 5,954,080 to Leatherman, hereby incorporated by reference in its entirety for all purposes. 
         [0035]    Flow control valve  30  is under control of a control system  34  via a flow control valve signal line  36 . Control system  34  may be a suitable microprocessor, microcontroller, or other electronics with associated memory and software programs running thereon. In this manner, the control system  34  can control the opening and closing of the flow control valve  30  to either allow fuel to flow or not flow through meter  32  and on to hose  14  and nozzle  16 . 
         [0036]    Flow control valve  30  is located below a vapor barrier  38  delimiting a hydraulics compartment  40  of the fuel dispenser  10 . The control system  34  is typically located in an electronics compartment  42  of fuel dispenser  10  above vapor barrier  38 . In this embodiment, after fuel exits flow control valve  30 , it flows through meter  32 , which measures the volume and/or flow rate of the fuel. 
         [0037]    Flow meter  32  may preferably be a positive displacement or inferential flow meter having one or more rotors which rotate on one or more shafts. Examples of positive displacement flow meter technology which may be modified for use with the present invention are provided in U.S. Pat. Nos. 6,250,151 to Tingleff et al., 6,397,686 to Taivalkoski et al., and 5,447,062 to Köpl et al., each of which is hereby incorporated by reference in its entirety for all purposes. Likewise, examples of inferential flow meter technology with may be modified for use with the present invention are provided in U.S. Pat. Nos. 7,111,520 to Payne et al. and 5,689,071 to Ruffner et al. and U.S. Patent App. Pub. No. 2010/0122990 to Carapelli. 
         [0038]    Meter  32  comprises a rotary displacement sensor  44  that generates a signal indicative of the volumetric flow rate of fuel and periodically transmits the signal to control system  34  via a signal line  46 . In this manner, the control system  34  can update the total gallons dispensed and the price of the fuel dispensed on information display  20  via a communications line  47 . 
         [0039]    As fuel leaves flow meter  32  it enters a flow switch  48 . Flow switch  48 , which preferably includes a one-way check valve that prevents rearward flow through fuel dispenser  10 , provides a flow switch communication signal to control system  34  via the flow switch signal line  50 . The flow switch communication signal indicates to control system  34  that fuel is actually flowing in the fuel delivery path and that subsequent signals from flow meter  32  are due to actual fuel flow. 
         [0040]    After the fuel leaves flow switch  48 , it exits through internal fuel piping  28  to be delivered through fuel hose  14  and nozzle  16  for delivery to the customer&#39;s vehicle. 
         [0041]    As noted above, embodiments of the present invention advantageously provide a fuel flow meter with a rotary displacement sensor capable of determining the absolute angle of the flow meter shaft. Thus, the rotary displacement sensor may preferably be an absolute, as opposed to an incremental, sensor. 
         [0042]    Incremental displacement sensors indicate the amount of change between a previous position of a shaft and the present position of the shaft. If a power loss or other disturbance, such as an error in signal transmission, causes information regarding the present position to be lost, an incremental sensor must be reset to place the sensor in a reference position. In contrast, absolute displacement sensors are capable of measuring the shaft&#39;s position relative to a predetermined point, rather than from a previous position. After a power loss when power is restored, an absolute sensor indicates the current sensor position without the need to be moved to a reference position. 
         [0043]    Those of skill in the art are able to identify suitable rotary displacement sensor technologies. As an example, the following companies offer rotary displacement sensor technology: Eltomatic A/S of Denmark and Metrom, LLC of Lake Zurich, Ill. In a preferred embodiment, the rotary displacement sensor may be a magnetic displacement sensor. Commercially available magnetic displacement sensor technologies that may be suitable for use in embodiments of the present invention include magnetoresistive, hall effect, inductive, and magnetic encoders. However, non-magnetic displacement sensors, such as optical or mechanical encoders, may also be used. 
         [0044]    Magnetic displacement sensors may typically comprise one or more permanent magnets coupled to a rotating shaft to apply a variable magnetic field over a sensing element and obtain a response indicating angular position. In some cases, the magnet(s) may be disposed on a disc coupled to the shaft and centered on the shaft&#39;s longitudinal axis. (See, e.g., U.S. Pat. No. 7,546,778 to Amante et al., hereby incorporated by reference in its entirety for all purposes.) Other displacement sensors may comprise one or more sensing elements positioned over a magnetic rotor having at least one region of discontinuity defined in its outer circumferential surface such that the rotor generates a characteristic magnetic flux. (See, e.g., U.S. Pat. No. 6,397,686 to Taivalkoski et al.) 
         [0045]    In many cases, the sensing element may be packaged as an integrated circuit. Further, the sensing element may provide a variety of outputs indicative of angular position, such as a multi-bit digital word for each distinct shaft angle, analog sine and cosine voltage outputs, or a change in electrical resistance. The absolute displacement sensor preferably has a high measurement resolution. 
         [0046]    In this regard,  FIG. 3  is a schematic diagram illustrating the relationship between control system  34 , fuel flow meter  32 , and displacement sensor  44  according to one embodiment of the present invention. For example, control system  34 , which preferably comprises memory  52 , may typically control various aspects of fuel dispenser  10 , such as valves, displays, and the like as is well understood. Control system  34  may be communicably coupled via signal line  46  to displacement sensor  44 , which may be operatively connected to flow meter  32 . Thereby, control system  34  may communicate with displacement sensor  44  to obtain data regarding operation of flow meter  32 , described in more detail below. In some preferred embodiments, communications between control system  34  and displacement sensor  44  are encrypted using suitable encryption algorithms known to those of skill in the art. 
         [0047]    Additionally, in a further aspect, a communication link  54  may provide communication between control system  34  and a site controller or the like. In some embodiments, the site controller functions may preferably be provided by the PASSPORT® point-of-sale system manufactured by Gilbarco Inc. Communication link  54  may be any suitable link for providing communication between control system  34  and the site controller, such as two wire, RS 422, Ethernet, wireless, etc. if needed or desired. By way of communication link  54 , control system  34  may communicate any of the data communicated thereto on to the site controller, which may use any of this information for reporting or decision purposes. For example, the site controller may communicate with a remote location for credit/debit card authorization or it may communicate information to a remote location for logging, tracking, or problem identification. 
         [0048]    Displacement sensor  44 , which may preferably comprise a magnetic rotary encoder adapted to determine absolute shaft position as described above, comprises sensor electronics  56 . Sensor electronics  56 , which may be formed as one or more programmable logic devices or application-specific integrated circuits (ASICs), preferably comprise memory  58  in electronic communication with a processor  60 . Processor  60 , which may be a microcontroller, microprocessor, or the like, is adapted to communicate with control system  34  via signal line  46 . Thus, for example, processor  60  may read from memory  52  and control system  34  may read from memory  58 . It should be understood that processor  60  may preferably include an operating program permanently stored in a read-only memory (ROM), and may also store information temporarily in a random access memory (RAM) on an as-needed basis. Processor  60  may typically employ a variety of conventional items, such as counters, registers, flags, and indexes as necessary or desired. 
         [0049]    Further, in some embodiments the sensor electronics may comprise signal processing circuitry. As shown, sensor electronics  56  comprise signal processing circuitry  62 . In embodiments where the sensing element outputs analog signals, the signal processing circuitry may comprise an analog to digital converter and/or an interpolator to increase measurement resolution. In addition, other signal processing operations may be performed, such as calculation of flow direction, flow rate from shaft rotation, or correction of measurement error at high or low flow rates. One skilled in the art will appreciate that signal processing circuitry  62  may be incorporated into processor  60 . 
         [0050]    The flow meter shaft angle is preferably stored in memory at least at the start and end of each transaction to enable a determination of whether the flow meter shaft angle has changed since it was last stored, which may indicate that fraud has occurred. However, the shaft angle may be recorded at any time, including when power is applied to the fuel dispenser after an outage and/or throughout each transaction. 
         [0051]    For example, the shaft angle may be stored in the control system memory or the displacement sensor memory. In a preferred embodiment, however, the shaft angle may be stored in both memories (at least at both the start and end of each transaction). Those of skill in the art will appreciate that this may provide an additional safeguard against fraud, in that even where a dishonest consumer modifies the memory of one of the control system and displacement sensor, the correct shaft angle stored at the end of a previous transaction will still be stored in the other&#39;s memory. Thus, the fraud may still be detected as described below. 
         [0052]    Memory  52  and memory  58  are preferably nonvolatile so that the data is preserved during electrical power loss. Desirable nonvolatile memory types include electronically programmable read only memory (EPROM), electronically erasable programmable read only memory (EEPROM), ferro-electric nonvolatile memory devices, flash memory devices, and other suitable types of alterable nonvolatile memory. The practice of the present invention contemplates using any suitable memory device as necessary or desired. 
         [0053]    As noted above, fraud may occur when a perpetrator disconnects a displacement sensor from the fuel flow meter prior to a fueling transaction or disables either or both of the fuel dispenser or displacement sensor electronics and forces fuel through the fuel flow meter. Thus, embodiments of the present invention preferably provide a checking algorithm to determine whether the flow meter, displacement sensor, or controller has been tampered with and fraud has occurred. As described below, either or both of the dispenser control system and the fuel meter displacement sensor may perform the checking algorithm. 
         [0054]    Generally, according to one embodiment, the algorithm may output whether a shaft angle detected prior to fuel dispensing, Θ S , is equal to a shaft angle stored in the control system or displacement sensor memory, Θ E , at the end of a previous transaction. If not, the control system or displacement sensor may take appropriate action to prevent fraud, such as generating an alarm or the like, disabling the fuel dispenser, and/or notifying appropriate authorities via the communication link. 
         [0055]    In this regard,  FIG. 4  is a flow chart outlining the operation of flow meter  32  according to one embodiment of the present invention. The process starts (step  100 ) and control system  34  receives a signal indicative of the start of a fueling transaction (step  102 ). For example, control system  34  may receive a signal that a customer has lifted a nozzle pedestal to the “on” position. Those of skill in the art will appreciate, however, that other signals may be used to indicate the start of fueling. 
         [0056]    Next, control system  34  may instruct displacement sensor  44  to determine the current shaft angle Θ S  (step  104 ). Data indicative of angle Θ S  may preferably then be stored in both control system memory  52  and displacement sensor memory  58 , although in some embodiments the data may only be stored in either memory  52  or memory  58  (step  106 ). 
         [0057]    Then, shaft angle Θ S  is compared to shaft angle Θ E  (step  108 ), which will have previously been stored in memory as described below. Regardless of where the shaft angles Θ S  and Θ E  are stored, either control system  34  or displacement sensor  44  may perform this comparison. Those of skill in the art will appreciate that because of the displacement sensor&#39;s high measurement resolution, it is exceedingly unlikely that a perpetrator will be able to reset the flow meter shaft and/or magnetic element to a position close enough to Θ E  to escape detection. Thus, if the values are not equal, either control system  34  or displacement sensor  44  may take appropriate action to report or prevent fraud (step  110 ). Preferably, however, both control system  34  and displacement sensor  44  may compare the values of Θ S  and Θ E  in their respective memories  52 ,  56 . Thereby, either control system  34  or displacement sensor  44  may take appropriate action to report or prevent fraud if the value of Θ E  differs from the value of Θ S . 
         [0058]    If angles Θ S  and Θ E  are equal, the fueling process begins and displacement sensor  44  may measure the position of a shaft of flow meter  32  (step  112 ). When control system  34  receives a signal indicative of the end of the transaction (step  114 ), it may instruct displacement sensor  44  to determine the current shaft angle, Θ E  (step  116 ). In some embodiments, the displacement sensor may reset this position as the reference point (or “zero position”) relative to which it measures angular position each transaction, although this is not required. Finally, shaft angle Θ E  may be stored in memory (step  118 ). As noted above, it is preferred that Θ E  be stored in both control system  34  memory  52  and displacement sensor  44  memory  58 . The process then ends (step  120 ). 
         [0059]    In a further embodiment, the above-described process may also be performed when power is applied to a fuel dispenser after an outage. This may be the case, for example, when a perpetrator disconnects the power from a dispenser and forces fuel through the fuel flow meter. 
         [0060]    In a further embodiment, the shaft angle Θ E  may be stored in both the control system and displacement sensor memories and these values may be compared for parity. Those of skill in the art will appreciate that this may provide additional fraud deterrence where a perpetrator attempts to alter one of the control system memory and displacement sensor memory (e.g., to hide the fact that fraud has occurred). Either or both of the control system and the displacement sensor may perform this comparison, preferably at all dispenser states (e.g., when power is applied and the start and end of each transaction, among others). Additionally, a site controller or the like in electronic communication with the fuel dispenser may perform this function. If the comparison does not yield equal shaft angles Θ E , appropriate action may be taken as described above. 
         [0061]      FIG. 5  is a schematic cross-sectional view of a positive displacement flow meter according to one embodiment of the present invention. In particular, flow meter  200  comprises a body  202  defining a longitudinal bore therethrough between an inlet  204  and an outlet  206 . As shown, flow meter  200  comprises a displacement sensor  208  positioned in a sensor housing  210 . Displacement sensor  208  is preferably analogous to displacement sensor  44 , described above, and thus displacement sensor  208  may comprise a magnetic encoder. Sensor housing  210  may be removably attached to an end of flow meter  200 . 
         [0062]    Flow meter  200  further comprises a rotor assembly  212  comprising a pair of screw spindles  214 ,  216 , shown in simplified form to facilitate illustration. Screw spindles  214 ,  216  may be elongate cylindrical rotors defining a helical blade, and as those of skill in the art will appreciate, spindles  214 ,  216  may mesh and rotate together. Spindles  214 ,  216  may rotate on shafts  218 ,  220 , respectively, which are mounted at each end on bearings  222 . 
         [0063]    Displacement sensor  208 , which may preferably be an absolute displacement sensor, comprises a magnetic element  224  which may be coupled to shaft  218  via an encoder shaft  226  (although in other embodiments element  224  may be coupled to shaft  220 ). For example, in one embodiment, encoder shaft  226  may be operatively connected to magnetic element  224  and threadably received in an axial bore  228  defined in an end of shaft  218 . Magnetic element  224 , which as shown comprises a disc having one or more magnets disposed thereon, is preferably adapted to apply a variable magnetic field to a sensing element  230  as shafts  218 ,  226  rotate. However, magnetic element  224  may take other forms, as those of skill in the art will appreciate. In some embodiments, for example, magnetic element  224  may simply be a magnet partially or completely received in a bore defined in an end of shaft  218 . Element  224  may also comprise more than one magnet in some embodiments. Sensing element  230 , which may comprise an integrated circuit, is preferably adapted to detect the variable magnetic field applied by element  224  and provide outputs indicative of the angular position of shaft  218 . 
         [0064]    Sensor housing  210  may comprise a radial measurement structure  232  defining an electronics compartment  234  therein. To maintain sensing element  230  isolated from flowing fuel, sensing element  230  may preferably be positioned in electronics compartment  234 . Thereby, sensing element  230  may be positioned proximate to and in axial alignment with element  224 . Electronics compartment  234  also houses other displacement sensor electronics  236  in electronic communication with sensing element  230 . Displacement sensor electronics  236  are preferably analogous to sensor electronics  56  described above, and thus sensor electronics  236  may comprise a processor, memory, and signal processing circuitry. As described above, the sensor electronics  236  may include one or more ASICs. 
         [0065]    In operation, fuel may flow from internal fuel dispenser piping into inlet  204 . As fuel flows through meter  200 , screw spindles  214 ,  216  rotate on their associated shafts  218 ,  220 . Magnetic element  224  rotates with shaft  218  to apply a varying magnetic field over sensing element  230 . Sensing element  230  detects changes in the magnetic field and produces signals indicative of the absolute angular position of shaft  218 . Sensor electronics  236  (or, in some embodiments, control system  34 ) may then process these signals to determine the volume of fuel flowing through meter  200 . Further, as described above, the absolute angle of shaft  218  may be stored in memory at various points during a fueling transaction. 
         [0066]    In a further embodiment, the displacement sensor may comprise an optical encoder. Incremental optical encoders typically output quadrature signals indicative of the motion of a flow meter shaft to a counter. Signal generation techniques include using geometric masking, Moiré fringing, or diffraction. However, incremental optical sensors are not suitable for storing absolute shaft angular position information. For example, digital output signals from these encoders consist of two square waves 90° out of phase and these signals have only four possible states. Analog signals from these encoders, which consist of sine and cosine signals output a number of times per revolution, are likewise insufficient. Further, problems in signal quality (e.g., quadrature separation and pulse “jitter”) can cause errors in encoding. 
         [0067]    Some incremental optical encoders may include a reference signal or “check pulse” in a fixed location in order to define a reference position. Because the number of signals per revolution is known, these reference signals have been provided to check for counting errors which can occur due to an error in signal transmission, for example. However, as noted above, if power loss occurs, position information is lost and the sensor must return to the reference position to reset its counter. 
         [0068]    Absolute optical encoders, on the other hand, may be suitable for determining and storing absolute shaft angle. Each track on the disc of an absolute optical sensor represents one bit of a binary number. As the disc rotates past a read head, a photodetector outputs a unique digital word for each shaft angle. Absolute optical encoders typically employ Gray code over direct binary coding because with Gray code, only one bit of data changes between representations of two consecutive positions. Those of skill in the art will appreciate, however, that to provide an absolute encoder with sufficient measurement resolution, the encoder should include a disc with a large number of tracks and sophisticated signal processing circuitry. For example, to provide an encoder with a resolution of 0.1° (i.e., 360°/2 12 ), twelve tracks are needed. Thus, while these encoders are within the scope of the present invention, a less expensive alternative may be preferable for some embodiments. 
         [0069]    Thus, according to one embodiment of the present invention, the displacement sensor preferably comprises an optical encoder having a shaft-mounted disc analogous to that of an incremental optical encoder. The encoder disc may be provided with two tracks for analog or digital quadrature output signals and a third track for a reference position output signal. The displacement sensor further comprises sensor electronics which may preferably be analogous to sensor electronics  56 . Thus, the sensor electronics may receive a predefined and known number of output signals per revolution of a flow meter shaft. Further, the sensor electronics are preferably adapted to record in nonvolatile memory at the end of a fueling transaction the number of quadrature signals received since the last reference position output signal. This number, N E , is indicative of absolute shaft angle. 
         [0070]    Depending on the needs of the operator and the memory available, recording of each output signal may occur for each revolution of the flow meter shaft, for the entire transaction, or for a plurality of transactions. Further, N E  may be recorded in either or both of the control system and displacement sensor memories. Then, if a dishonest customer attempts fraud by disconnecting power or disconnecting the displacement sensor from the flow meter prior to forcing fuel through the fuel flow meter, when a new valid transaction begins the control system and/or displacement sensor will know the angular position of the shaft at the end of the last valid transaction. 
         [0071]    In one embodiment, the displacement sensor may count the number, N S , of quadrature output signals received until the next reference position signal is received and transmit this information to the control system. If either or both of the control system and displacement sensor determine that, when subtracted from the expected number of signals per revolution, N S  does not yield a number within one of N E , appropriate action may be taken to prevent or report fraud. As noted above, this may include generating an alarm or the like, disabling the fuel dispenser, and/or notifying appropriate authorities via the communication link. Those of skill in the art will appreciate that this arrangement may be less expensive than traditional absolute optical encoders and thus may be suitable for deployment in a retail fueling environment. 
         [0072]      FIG. 6  is a flow chart outlining this process according to one embodiment of the present invention. This process may in many respects be similar to the process described with reference to  FIG. 4 . In particular, the process starts (step  300 ) and a fuel dispenser control system (preferably analogous to control system  34 ) receives a signal indicative of the start of a fueling transaction (step  302 ). Next, as fuel begins to flow through the flow meter, an optical displacement sensor as described above may count the number of quadrature output signals received until the next reference position signal is received (step  304 ). The result, N S , may be stored in either or both of the control system and displacement sensor memories, where the number of quadrature output signals received since the previous reference position output signal, N E , will have preferably been stored at the end of the previous transaction. 
         [0073]    Regardless of where N E  is stored, either or both of the control system and displacement sensor may then perform the following steps. N S  may be subtracted from the expected number of quadrature output signals per revolution of the flow meter shaft (step  306 ). Then, the result of this calculation may be compared to N E , which has been previously stored in memory (step  308 ). Specifically, in one embodiment, if the absolute value of the result minus N E  is not equal to one, fueling may be interrupted and appropriate action may be taken to report or prevent fraud (step  310 ). Those of skill in the art will appreciate that in other embodiments, the values of N S  and N E  may be determined differently, such that the result of the above calculation yields a different expected amount. For example, the outcome could be zero rather than one. 
         [0074]    However, in this embodiment, where the calculation yields an answer of one, fueling may not be interrupted and the transaction may continue (step  312 ). The displacement sensor preferably maintains the count of output signals in memory throughout the transaction. When the control system receives a signal indicative of the end of the transaction (step  314 ), it may instruct the displacement sensor to store a new value of N E  in memory, and this information may also preferably be stored in the control system memory (step  316 ). The process then ends (step  318 ). 
         [0075]    In a further embodiment, it may be desirable to force a nonnegligible rotation of an encoder shaft of a displacement sensor when a displacement sensor is removed from a flow meter. As explained below with reference to  FIGS. 7A-7E , this may aid in the detection of fraud because it decreases the likelihood that a perpetrator could replace the displacement sensor such that its encoder shaft is in the same angular position it was before removal. 
         [0076]    In this regard,  FIG. 7A  is a partial cross-sectional view illustrating a coupling between a flow meter  400  and a displacement sensor  402  and their respective shafts  404 ,  406  according to one embodiment of the present invention. Shafts  404 ,  406  may rotate in axial bores defined in respective housings  408 ,  410 . To facilitate illustration, flow meter  400  and displacement sensor  402  are decoupled in  FIG. 7A .  FIG. 7B  is a partial top view of flow meter  400  and  FIG. 7C  is a bottom view of displacement sensor  402 . Also,  FIG. 7D  is a top view of flow meter shaft  404  and  FIG. 7E  is a bottom view of encoder shaft  406 . 
         [0077]    According to one embodiment, the coupling may prevent rotation of the displacement sensor housing relative to the flow meter housing when the displacement sensor is removed. For example, flow meter  400  preferably defines a socket  412  adapted to receive a protrusion  414  of displacement sensor  402  such that a bottom surface  416  of displacement sensor housing  410  rests flush against a top surface  418  of flow meter housing  408  when coupled. Socket  412  may preferably be non-round, defining a key  420  adapted to be received in a keyway  422  defined in protrusion  414 . Similarly, protrusion  414  may define a key  424  adapted to be received in a keyway  426  defined in socket  412 . One skilled in the art will recognize that other suitable methods for preventing relative rotation of the displacement sensor housing  410  and flow meter  400  during removal may be used and are within the scope of the present invention. 
         [0078]    At the same time, the coupling may force rotation of one of the encoder shaft and flow meter shaft relative to the other when the displacement sensor is removed. In one example, encoder shaft  406  may define a bore  428  at a proximal end  430  thereof. Bore  428  may preferably have a depth equal to the height of protrusion  414 . Also, at least at an end portion thereof equal to the depth of bore  428 , flow meter shaft  404  may define a slightly smaller diameter D 1  than outer diameter D 2  of encoder shaft  406  such that shaft  404  may be snugly received in bore  428  when displacement sensor  402  is coupled to flow meter  400 . 
         [0079]    Further, flow meter shaft  404  may include pins  432  diametrically opposed on its periphery. The diametric distance between the distal ends of pins  432  may be equal to diameter D 2 . In addition, encoder shaft  406  may define slots  434  extending in an upward helical fashion from bottom edge  436  and adapted to receive pins  432  when shafts  404 ,  406  are coupled together. In one embodiment, slot  434  may travel through 90° of rotation from the bottom edge  436  to its terminus  438 . Thus, when displacement sensor  402  is coupled to flow meter  400 , flow meter shaft  404  may be inserted into bore  428  of encoder shaft  406  in a twisting motion as pins  432  follow the rotation of slot  434 . 
         [0080]    A greater torque is preferably required to rotate flow meter shaft  404  than to rotate encoder shaft  406 . Thus, when displacement sensor  402  is removed from flow meter  400 , it will be appreciated that encoder shaft  406  may be forced to rotate in a clockwise direction while flow meter shaft  404 , flow meter  400 , and the housing  410  of displacement sensor  402  remain stationary. 
         [0081]    Because shaft  406  has changed position, it will be extremely difficult, if not impossible, for a dishonest customer to replace displacement sensor  402  with shaft  406  in the same position as prior to removal. It will also be appreciated that because of the coupling between socket  412  and protrusion  414 , shafts  404 ,  406  are inaccessible until displacement sensor  402  is decoupled from flow meter  400 . Thus, a dishonest customer cannot lock either shaft to prevent rotation during removal of displacement sensor  402 . 
         [0082]    Additionally, in a further embodiment, the optical encoder disc or magnetic element (depending on the type of displacement sensor used) may be coupled to encoder shaft  406  via a one-way clutch which transmits torque in only one direction of rotation. In other words, shaft  406  may transmit torque when rotated clockwise but may “freewheel” when rotated counterclockwise. Thereby, reassembly cannot reproduce the exact prior angular position of the optical encoder disc or magnetic element because it will not turn as displacement sensor  402  is replaced (although shaft  406  will turn). Those of skill in the art can select a suitable one-way clutch, such as a roller clutch or the like. 
         [0083]    In a further embodiment, the above-described physical couplings between flow meter  400 /displacement sensor  402  and flow meter shaft  404 /encoder shaft  406  may be reversed. In particular, when displacement sensor  402  is coupled to or decoupled from flow meter  400 , a physical screw or bayonet fitting may force rotation of displacement sensor  402  relative to flow meter  400 . At the same time, shafts  404 ,  406  may be coupled in a manner that prevents relative rotation during attachment and removal of displacement sensor  402 . For example, pins  432  and slots  434  may not be provided and shafts  404 ,  406  may be coupled using keys and keyways as described above. As a result, the relative position of the sensor element inside the housing  410  and the optical encoder disc (or magnetic element) will change. Those of skill in the art will appreciate that this embodiment may provide maintenance advantages in some applications. 
         [0084]    While one or more preferred embodiments of the invention have been described above, it should be understood that any and all equivalent realizations of the present invention are included within the scope and spirit thereof. The embodiments depicted are presented by way of example only and are not intended as limitations upon the present invention. Thus, it should be understood by those of ordinary skill in this art that the present invention is not limited to these embodiments since modifications can be made. Therefore, it is contemplated that any and all such embodiments are included in the present invention as may fall within the scope and spirit thereof.