Abstract:
A fuel dispenser comprises a fuel nozzle configured to be connected to a vehicle fuel system. Fuel piping configured to transfer fuel from at least one fuel storage tank associated with the fuel dispenser through the fuel nozzle into the vehicle fuel system is also provided. A flow control valve and a flow measurement device are located along the fuel piping, the flow measurement device having a housing defining a flow path therethrough. The flow measurement device includes a first exciter for producing a first wave in fuel moving along the flow path. A second exciter produces a second wave in the fuel which passes through the first wave, wherein the second wave has a higher frequency than the first wave. At least one sensor is spaced apart from the first exciter and the second exciter, the at least one sensor being configured to detect at least one measurable characteristic of the second wave from which flow rate can be derived.

Description:
PRIORITY CLAIM 
       [0001]    This application is based upon and claims the benefit of U.S. provisional application Ser. No. 62/304,662, filed Mar. 7, 2016, incorporated fully herein by reference for all purposes. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to fuel dispensers. More specifically, the invention provides a fuel dispenser including a device and method for determining the flow rate of a flowing fluid by exciting the fluid in an oscillatory motion by acoustic means. 
       BACKGROUND OF THE INVENTION 
       [0003]    Fuel pumps and fuel dispensers are known in the art. A fuel pump includes a pump located within its housing for extracting fuel from a fuel source, as well as meters for measuring fuel flow and switches and valves for controlling fuel flow. A fuel dispenser, in contrast, is connected to a source of fuel which contains its own pump, typically an underground storage tank (UST) with a submersible turbine pump (STP). Thus, a fuel dispenser does not typically require that a pump be housed in the unit itself. Instead, the dispenser housing contains the appropriate meters, switches and valves for controlling fuel flow supplied to it under pressure. As used herein, the term “fuel dispenser” shall include both fuel pumps and fuel dispensers, unless the context clearly indicates otherwise. 
         [0004]    Fuel dispensers are designed in a variety of different configurations. A common type of fuel dispenser, often called a “lane-oriented” dispenser, has one or more fuel dispensing nozzles on each side of the unit. A lane-oriented multiproduct fuel dispenser typically has two or more fuel dispensing nozzles on each side of the unit. Each of the nozzles on each side of the unit is typically used to dispense a particular grade (e.g., octane level) of fuel. Alternatively, a single nozzle may be provided for dispensing multiple grades of fuel depending on the customer&#39;s selection. Each side of the unit generally includes a display for displaying the amount and cost of the fuel dispensed, and can also include credit or debit card verification and cash acceptance mechanisms. 
         [0005]    A variety of different meters have been used in prior art fuel dispensers. Typically, either positive displacement meters or inferential meters have been used for this purpose. For a variety of reasons, fuel volume or flow rate measurement technologies are typically limited in their measurement accuracies across a finite range of flow rates. Additionally, measurement technologies may be limited in their maximum flow rates at the desired, restricted-to and/or otherwise realistic operating pressures by internal restrictions or fluidic impedances including but not limited to bore, port or other orifice size. Moreover, these measurement technologies require periodic recalibration and/or special filters. 
         [0006]    Flow meters utilizing the Coriolis Effect to measure the mass flow rate of a fluid are also known. Generally, in such Coriolis meters an electromechanical actuator forces one or more fluid-filled flow conduits to vibrate in a prescribed oscillatory bending-mode of vibration. When the process-fluid is flowing, the combination of fluid motion and conduit vibration causes inertial forces which deflect the conduits away from their normal paths of vibration proportionally related to mass flow rate. Motion of the conduit is measured at specific locations along its length and this information is used to determine mass flow. Detailed information on the structure and operation of traditional Coriolis flow meters is disclosed in U.S. Pat. Nos. 7,287,438 and 7,472,606, both of which are incorporated herein by reference in their entireties for all purposes. 
         [0007]    Coriolis flow meters utilize the acceleration effects that govern a mass moving relative to a noninertial, or rotating, frame of reference. For example, consider a fluid particle moving with a velocity v in a fluid stream in a conduit where the conduit is oscillated about an axis perpendicular its centerline with an angular velocity Ω. To an observer in the noninertial frame of reference, the particle appears to accelerate. The particle&#39;s acceleration, a p , is given by 2·Ω×v. Thus, the apparent force, F c , exerted on the particle is: 
         [0000]        F   c   =m   p   ·a   p =2· m   p   Ω×v  
 
         [0000]    where m p  is the particle&#39;s mass and x is the vector cross product operator. The Coriolis force acts in a direction perpendicular to both the particle&#39;s linear and angular velocities. Because the force is proportional to the particle&#39;s mass and velocity, measurement of the force&#39;s effect on the conduit allows the mass flow rate of the fluid to be determined. 
         [0008]    Current Coriolis mass flow technology has several desirable characteristics over positive displacement and inferential flow meters. For instance, Coriolis meters are highly accurate, they are not subject to wear or meter drift because they lack internal moving parts, they can measure flow in forward and backward directions, and they measure fluid mass directly. Some implementations also measure fluid density directly. 
         [0009]    However, although Coriolis meters have been widely used in some industries, they have not been widely adopted in the fuel dispensing industry because of several drawbacks. For example, current implementations are expensive and complex. Limits of current Coriolis meters may also constrain the diameter size, thickness, and/or overall geometry of a flow conduit. 
         [0010]    Therefore, room remains in the flow measurement art for a flow meter that utilizes the Coriolis Effect to achieve a highly accurate measurement of mass flow while overcoming the above difficulties. In particular, a flow meter that does not require excitation of the flow conduit could have a less complex geometry, fewer moving parts, and be more readily implemented in a flow to be measured. 
       SUMMARY OF THE INVENTION 
       [0011]    According to one aspect, the present invention provides a fuel dispenser comprising a fuel nozzle configured to be connected to a vehicle fuel system. Fuel piping configured to transfer fuel from at least one fuel storage tank associated with the fuel dispenser through the fuel nozzle into the vehicle fuel system is also provided. A flow control valve and a flow measurement device are located along the fuel piping, the flow measurement device having a housing defining a flow path therethrough. The flow measurement device includes a first exciter for producing a first wave in fuel moving along the flow path. A second exciter produces a second wave in the fuel which passes through the first wave, wherein the second wave has a higher frequency than the first wave. At least one sensor is spaced apart from the first exciter and the second exciter, the at least one sensor being configured to detect at least one measurable characteristic of the second wave from which flow rate can be derived. 
         [0012]    Another aspect of the invention provides a flow measurement device for determining the mass flow rate of a fluid flowing in a conduit. The flow measurement device comprises a first exciter for producing a first wave in the fluid and a second exciter for producing a second wave in the fluid passing through the first wave. The second wave has a higher frequency than the first wave. The first wave causes a change in at least one measurable characteristic of the second wave. The flow measurement device also comprises a sensor array downstream of the first and second exciters configured to detect the at least one measurable characteristic of the second wave. The flow measurement device further comprises a processor in electronic communication with the sensor array to receive signals representative of the at least one measurable characteristic of the second wave, and the processor is configured to calculate the mass flow rate of the fluid. 
         [0013]    Thus, embodiments of the present invention provide a novel flow measurement system using Coriolis accelerations caused by the acoustic displacement of the fluid to be measured. As explained below, in contrast to traditional Coriolis flow meters, preferably few or no moving parts are required to generate Coriolis forces acting on the fluid. It will be appreciated that this may increase the reliability of the measurement system. Additionally, many embodiments of the present invention may comprise a much smaller form factor than currently-available Coriolis meters. Further, microelectromechanical systems (MEMS) may be used to implement at least some of the components of the present invention, which may reduce the cost of the measurement system. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    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: 
           [0015]      FIG. 1  illustrates a perspective view of an exemplary fuel dispenser in accordance with an embodiment of the present invention. 
           [0016]      FIG. 2  illustrates a diagrammatic representation of internal components of the fuel dispenser of  FIG. 1  according to an embodiment of the present invention. 
           [0017]      FIG. 3A  is a schematic representation of a particle of fluid flowing along a streamline in a fluid conduit. 
           [0018]      FIG. 3B  is a schematic representation of the fluid particle of  FIG. 3A  in the presence of an acoustic wave, wherein the wave oscillates in a direction orthogonal to the flow direction. 
           [0019]      FIG. 4  is a schematic perspective view of a measuring cell coupled to a fluid conduit according to an embodiment of the present invention. 
           [0020]      FIG. 5  is a system level schematic diagram of a flow meter according to one embodiment of the present invention. 
           [0021]      FIG. 6  is a top, schematic cross sectional view of the flow meter of  FIG. 5 . 
           [0022]      FIG. 7  is a side, schematic cross sectional view of an alternative embodiment of a flow meter comprising two measuring cells. 
       
    
    
       [0023]    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 
       [0024]    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. 
         [0025]      FIG. 1  is a perspective view of an exemplary fuel dispenser  10  according to an embodiment of the present invention. Fuel dispenser  10  includes a housing  12  with a flexible fuel hose  14  extending therefrom. Fuel hose  14  terminates in a fuel nozzle  16  adapted to be inserted into a fill neck of a vehicle&#39;s fuel tank. Fuel nozzle  16  includes a manually-operated 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 fuel hose  14  and fuel nozzle  16  to a vehicle&#39;s fuel system, e.g. fuel tank. 
         [0026]    Fuel dispenser  10  has a customer interface  18 . Customer interface  18  may include an information display  20  relating to an ongoing fueling transaction that includes the amount of fuel dispensed and the price of the dispensed fuel. Further, customer interface  18  may include a display  22  that provides instructions to the customer regarding the fueling transaction. Display  22  may also provide advertising, merchandising, and multimedia presentations to a customer, and may allow the customer to purchase goods and services other than fuel at the dispenser. 
         [0027]      FIG. 2  is a schematic illustration of internal fuel flow components of fuel dispenser  10  according to an embodiment of the present invention. In general, fuel of a particular grade or type 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, hereby incorporated by reference in its entirety for all purposes. More specifically, 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 self-contained, meaning fuel is drawn to the fuel dispenser  10  by a pump unit positioned within housing  12 . 
         [0028]    Main fuel piping  24  passes into housing  12  through a 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. Pat. No. 8,291,928, 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 . 
         [0029]    Fuel from the shear valve  26  flows toward a flow control valve  30  positioned upstream of a flow meter  32 . Alternatively, flow control valve  30  may be positioned downstream of the flow meter  32 . In one embodiment, flow control valve  30  may be a proportional solenoid controlled valve. 
         [0030]    Flow control valve  30  is under control of a control system  34 . In this manner, control system  34  can control the opening and closing of flow control valve  30  to either allow fuel to flow or not flow through meter  32  and on to the hose  14  and nozzle  16 . Control system  34  may comprise any suitable electronics with associated memory and software programs running thereon whether referred to as a processor, microprocessor, controller, microcontroller, or the like. In a preferred embodiment, control system  34  may be comparable to the microprocessor-based control systems used in CRIND (card reader in dispenser) type units sold by Gilbarco Inc. Control system  34  typically controls other aspects of fuel dispenser  10 , such as other valves, displays, and the like. For example, control system  34  typically instructs flow control valve  30  to open when a fueling transaction is authorized. In addition, control system  34  may be in electronic communication with a point-of sale system (site controller) located at the fueling site. The site controller communicates with control system  34  to control authorization of fueling transactions and other conventional activities. 
         [0031]    A vapor barrier  36  delimits hydraulics compartment  38  of fuel dispenser  10 , and control system  34  is located in electronics compartment  40  above vapor barrier  36 . Fluid handling components, such as flow meter  32 , are located in hydraulics compartment  38 . Meter  32  typically comprises electronics  42  that communicates information representative of the flow rate or volume to control system  34 . In this manner, control system  34  can update the total gallons (or liters) dispensed and the price of the fuel dispensed on information display  20 . 
         [0032]    As fuel leaves flow meter  32  it enters a flow switch  44 , which preferably comprises a one-way check valve that prevents rearward flow through fuel dispenser  10 . Flow switch  44  provides a flow switch communication signal to control system  34  when fuel is flowing through flow meter  32 . The flow switch communication signal indicates to control system  34  that fuel is actually flowing in the fuel delivery path. Fuel from flow switch  44  exits through internal fuel piping  46  to fuel hose  14  and nozzle  16  for delivery to the customer&#39;s vehicle. 
         [0033]    In an example embodiment, a breakaway device  48  may connect the internal piping  46  to the hose  14 . The breakaway device  48  may be configured to detach from the dispenser  10  and/or internal piping  46  in response to a force applied to the breakaway device  48  exceeding a predetermined threshold, for example 100 lbs or more. An example of a suitable breakaway device  48  is disclosed in U.S. Pat. No. 7,252,112, incorporated by reference herein in its entirety for all purposes. 
         [0034]    A blend manifold may also be provided downstream of flow switch  44 . The blend manifold receives fuels of varying octane levels from the various USTs and ensures that fuel of the octane level selected by the customer is delivered. In addition, fuel dispenser  10  may comprise a vapor recovery system to recover fuel vapors through nozzle  16  and hose  14  to return to the UST. An example of a vapor recovery assist equipped fuel dispenser is disclosed in U.S. Pat. No. 5,040,577, incorporated by reference herein in its entirety for all purposes. 
         [0035]    According to an embodiment of the present invention, flow meter  32  is configured to measure fluid flow characteristics based on noninertial frame of reference acceleration effects on an excited fluid stream flowing in a conduit. Thus, unlike traditional Coriolis flow meters, which rely on physical displacement of a fluid conduit, the present invention utilizes excitation of the fluid itself to determine mass flow. 
         [0036]    To create a moving frame of reference and induce inertial forces in a notionally uniform steady, fully-developed, laminar flow, embodiments of the present invention may symmetrically and rapidly excite a “column” of the flowing fluid in a measurement cell coupled to the fluid flow conduit. The general principle of operation is described below in reference to  FIGS. 3-4 . 
         [0037]      FIG. 3A  illustrates a particle of fluid  110  flowing along a streamline in a fluid conduit. The motion of particle  110  is best visualized in reference to a set of axes labeled X, Y, and Z. The X axis is parallel to the centerline of the fluid conduit. In the absence of oscillation or excitation, fluid particle  110  flows with velocity vector v in a streamline along the X axis. At this point, there is a substantially uniform pressure distribution in the fluid flow that may be sensed by an array of sensors described below. 
         [0038]    In some embodiments of the present invention, an exciter will generate an acoustic wave to oscillate a column of the flowing fluid and thus create a moving frame of reference. Preferably, the acoustic wave is planar, but those of skill in the art will appreciate that this is not required in all embodiments. In this regard,  FIG. 3B  illustrates the particle  110  in the presence of a planar acoustic wave  112  that oscillates in a direction orthogonal to the flow direction. This causes particle  110  to have an angular velocity about the Z axis, which is represented by the angular velocity vector Ω. 
         [0039]    More specifically,  FIG. 4  provides a schematic representation of measuring cell  114  coupled to a fluid conduit  116  according to an embodiment of the present invention. Here, a first exciter  118  generates planar acoustic wave  112  at a specific frequency along the Y axis, which is orthogonal to the flow direction. Those of skill in the art are aware that acoustic waves in fluids are longitudinal, meaning the deflected particles move in the direction of the acoustic wave propagation. Passage of the acoustic wave affects the pressure, density, and particle velocity of the fluid. Thus, after the exciter  118  generates the oscillating wave, wave  112  displaces particle  110  radially along the Y axis. Thus, particle  110  rotates about the Z axis as noted above and has angular velocity vector Ω. 
         [0040]    The radial movement alters the flow&#39;s velocity profile as a function of length of the measuring cell. Further, the movement causes a deviation from the initial, uniform pressure distribution in the measuring cell. Therefore, a rotating frame of reference is created that allows observation of inertial forces acting on particle  110 . In this example, the Coriolis force F c  acting on particle  110  will act along the Y axis, perpendicular to both the fluid velocity vector v and the angular velocity vector of oscillation Ω. It can be seen that when the Coriolis forces acting on each deflected particle in a fluid flow are summed, they will create a gradient of measurable pressures acting on the conduit wall. 
         [0041]    Additionally, a second exciter  120  is provided to produce a second wave  122  at a higher frequency than the planar wave&#39;s frequency. The second wave may preferably be emitted at an angle θ to the flow and through the column of fluid oscillated by the first wave  112 . As is well known, any change in the path through which a wave propagates will result in a change in the wave&#39;s characteristics. Thus, the first wave&#39;s movement will alter measurable characteristics of the second wave  122  (e.g., frequency, phase, velocity, intensity, etc.). An array of sensors  124  is provided downstream of the first and second exciters to detect variables related to the mass flow rate of the fluid. The sensor array  124  may preferably be implemented using microelectromechanical systems (MEMS). For example, the sensor array  124  may comprise receivers to detect changes to the second wave as the first wave oscillates, such as changes in the second wave&#39;s frequency, velocity, and/or intensity. The sensor array  124  may also comprise high-accuracy pressure sensors to detect alterations in fluid pressure. 
         [0042]    Then, using the measured variables, a processor  126  may calculate the fluid&#39;s mass flow rate. As described in more detail below, the processor may be in electronic communication  128  with the first and second exciters  118 ,  120  and the sensor array  124 . The mass flow rate of a fluid can be written as: M=m p ·v/l, where M is the mass flow rate and l is the length of the measurement section. Rewriting the formula for the Coriolis force above yields: m p ·v=F c /(2·Ω). Therefore, by way of substitution, M=F c /(2·Ω·l). As an example, the sensors may detect a change in pressure gradient over the measurement area and determine F c . Plus, because the second wave is at a higher frequency than the first wave, the sensors may detect incremental changes in a fluid velocity vector field to determine Ω. Thus, using the above relationships the processor could calculate the mass flow rate M. 
         [0043]    The flow meter of the present invention can be adapted for many different flow measurement applications. For example, it is contemplated that the flow meter could be used in commercial and industrial fluid measurement systems where a high degree of accuracy is required. Although the flow meter may be used to measure many different fluids, the flow meter of the present invention may be particularly suited for measurement of mass flow of liquid fuel, such as but not limited to gasoline, diesel, or ethanol, in a retail fuel dispensing environment. It is contemplated that the present invention could be placed in various locations in a fuel dispensing environment and used with one or more fuel dispensers to facilitate the retail sale of fuel. Fuel dispensing equipment is shown and described in the following U.S. patents, each of which is incorporated herein by reference in its entirety for all purposes: U.S. Pat. Nos. 6,935,191; 6,253,779; and 5,630,528. Additionally, U.S. Pat. No. 8,342,199, entitled “Dispensing Equipment Utilizing Coriolis Flow Meters,” issued Jan. 1, 2013, contains information on fuel dispensing equipment utilizing traditional Coriolis flow meters and is incorporated by reference herein in its entirety for all purposes. 
         [0044]    Therefore, in an embodiment where the present invention is used in conjunction with a retail fuel dispensing environment, the processor  126  may also calculate the volumetric flow rate of the fluid and the total volume of fluid dispensed in a dispensing transaction. Alternatively, processor  126  may transmit the mass flow rate to a control system  130  (e.g., the control system in a fuel dispenser) for calculation of the volumetric flow rate and total volume dispensed. The density of the fluid may be calculated in a known fashion (e.g., a flow switch (see U.S. Pat. No. 6,935,191) that employs temperature probes in the fluid) so that the volumetric flow rate can be determined using the relationship: Q=M/ρ, where Q is the volumetric flow rate and ρ is the fluid density. The processor  126  or control system  130  then preferably displays the volume dispensed for a customer. 
         [0045]    An embodiment of the flow meter of the present invention will be described in more detail in reference to  FIGS. 5-6 . First,  FIG. 5  is a partial side view of a system level schematic diagram of a flow meter  150  according to one embodiment of the present invention. Flow meter  150 , generally indicated by a broken line, may comprise a measuring cell  152  and a measuring cell processor  154 . Flow meter  150  may be coupled in line with fluid conduit  156 , through which the fluid to be measured, indicated by arrows  158 , flows. 
         [0046]    As described in more detail below, measuring cell processor  154  may be in electronic communication with the measuring cell  152  to evaluate various characteristics of the second wave and the flow described above and thereby calculate the fluid&#39;s mass flow rate. Measuring cell processor  154  is preferably a digital signal processor (DSP) or the like capable of accurately correlating many signals in real time. In this case, communication occurs via data line  160 , but those of skill in the art will appreciate that operative communication could occur via various wired and wireless methods, such as a local area network, wireless area network, or the like. Measuring cell processor  154  may preferably be located remote from measuring cell  152  in a suitable enclosure, such as a fuel dispenser electronics compartment. However, in other embodiments processor  154  may be directly coupled to the measuring cell  152  or in another suitable location. 
         [0047]    Additionally, in many embodiments measuring cell processor  154  is in electrical communication with a control system  162  via data line  124 . Control system  162 , preferably in the form of a microprocessor located in the upper portion of a fuel dispenser housing, controls the operation of various gasoline dispenser components, such as valves, pumps, display electronics, and payment and transactional electronics. For example, control system  162  may correspond to control system  34  described above. Control system  162  may also be in electrical communication with a site controller or a remote network. Measuring cell processor  154  transmits calculated mass or volumetric flow rate information to control system  162  to enable control system  162  to calculate the precise quantity of fuel dispensed and the monetary amount a customer owes. 
         [0048]      FIG. 6  provides a top cross sectional diagrammatic view of a flow meter  150  according to an embodiment of the present invention. Meter  150  has housing  164  having inlet  166 , outlet  168 , and axis A along its centerline. Fluid to be measured (illustrated by flow profile  170 ) flows through the meter  150  from inlet  166  to outlet  168  with streamlines preferably parallel to axis A. As noted above, the fluid  170  is preferably a substantially steady, fully developed, laminar flow to facilitate accurate measurement. Those of skill in the art are familiar with suitable materials for housing  164 . However, to facilitate mass flow rate measurement, in some embodiments housing  164  may be formed of a material having low roughness (such as glass or quartz). In the alternative, the interior of housing  164  may be lined with such a material. 
         [0049]    An exciter  172  is provided in meter housing  164  to generate an acoustic wave  174 , which may be planar as described above. In this embodiment, exciter  172  comprises two acoustic transducers  176 ,  178  provided opposite each other in meter housing  164  in ports  180 ,  182 , respectively. Acoustic transducers  176 ,  178  may preferably be directly coupled with the fluid flow to each propagate an acoustic wave in opposite directions orthogonal to axis A and the fluid flow. Preferably, the two waves are sinusoidal with the same amplitude, frequency, and wavelength such that they sum to result in wave  174 , which is a standing wave. Wave  174  induces a steady displacement of fluid particles per unit cycle time. However, in other embodiments exciter  172  could comprise one acoustic transducer to generate an acoustic wave that accelerates fluid molecules in generally reciprocating displacements in the direction of the acoustic wave&#39;s propagation. Acoustic wave  174  has a frequency f 1 , which may be between 1 kHz and 20 kHz. Wave  174  may induce a displacement ξ according to the relationship ξ=P/(2πf 1 Z) in the fluid molecules where P is the pressure of the wave  174  and Z is the fluid&#39;s acoustic impedance. 
         [0050]    Transducers  176 ,  178  are preferably magnetostriction drivers that may be made of nickel or aluminum. Alternatively, they could each comprise a piezoelectric disc or an acoustic vibratory node. Transducers  176 ,  178  comprise acoustic generators to provide the electrical signals that the transducers convert into acoustic waves. Measuring cell processor  154  may be in electronic communication with transducers  176 ,  178  to control their operation. Those of skill in the art will appreciate that in other embodiments transducers  176 ,  178  could comprise miniature, high-speed DC motors with eccentric rotors mounted on two shafts. In such a case the resulting wave would not be acoustic, but such motors have a good driving force and the principle of the invention would not change. 
         [0051]    A second exciter  184  is also provided upstream of exciter  172  to generate a second wave  186 . As described above and in more detail below, second wave  186  is preferably used to observe changes in the flow&#39;s velocity vector field as a result of disturbance by the first wave  174 . Exciter  184  preferably comprises an ultrasonic transducer such that second wave  186  is an ultrasonic wave. In many embodiments, exciter  184  may preferably be similar to ultrasonic transducers found in known ultrasonic flow meters. Thus, wave  186  has a frequency f 2  which may be between 20 kHz and 0.5 MHz. The transducer of exciter  184  also is preferably a wetted transducer that is flush-mounted in meter housing  164  through a port  188 . However, those of skill in the art are aware that it may be desirable for exciter  184  to be nonwetted (e.g., to reduce turbulence or increase strength) and thus exciter  184  may be mounted in a recess in housing  164  or on the exterior of housing  164  as needed or desired. Also, wave  186  preferably travels at an angle θ to axis A along or parallel to axis B. Angle θ can be in the range of zero (0) to ninety (90) degrees, but θ may preferably be 45 degrees. 
         [0052]    In addition, a sensor array  190  comprising a plurality of sensors is provided downstream of exciter  172 . Sensor array  190  may be flush-mounted in meter housing  164  through a port  192 . Sensor array  190  preferably comprises at least two types of sensors. First, in a preferred embodiment sensor array  190  may comprise one or more high resolution ultrasonic sensors to detect characteristics of second wave  186 , such as frequency and intensity. The ultrasonic sensors may preferably be adapted to calculate the instantaneous velocity of the fluid at multiple points. Additionally, in preferred embodiments the ultrasonic sensors may be less than 3 mm in diameter, and may be implemented as MEMS. Those of skill in the art will be able to select suitable sensors that are commercially available. 
         [0053]    Second, sensor array  190  may preferably comprise high resolution pressure sensors to calculate the pressure created by the Coriolis force acting on the displaced fluid. Because they will be sensing highly dynamic pressures, the pressure sensors in sensor array  190  may preferably be piezoelectric. As with the ultrasonic sensors, the pressure sensors are commercially available and can be selected by those of skill in the art. Further, the pressure sensors are typically similar in size to the ultrasonic sensors and may also preferably be implemented as MEMS. 
         [0054]    Because second wave  186  may be at a much higher frequency than the first wave  174 , the ultrasonic sensors in sensor array  190  can take many measurements of flow velocity vectors at multiple positions as the first wave  174  oscillates through one cycle. Thus, the sensor array can determine the incremental change in the flow&#39;s velocity profile of the fluid induced by the first wave&#39;s oscillation. Additionally, the pressure sensors in the sensor array  190  may dynamically detect changes in pressure over their area caused by the oscillation of the fluid by the first wave. The sensor array  190  sends these signals to measuring cell processor  154 , which calculates the angular velocity Ω and the Coriolis force F c . Using those variables and the relationships described above, the measuring cell processor calculates the mass flow rate of the fluid. 
         [0055]    Although the flow meter of the present invention is highly accurate, for some applications it may be desirable to increase its accuracy. In this regard,  FIG. 7  shows an alternative embodiment of a flow meter  200  comprising two measuring cells  202 ,  204 . Here each measuring cell  202 ,  204  comprises an exciter  206 ,  208 , respectively. Exciters  206 ,  208  each comprise two acoustic transducers as described above to generate a plane acoustic wave. However, in this case exciter  208  generates an acoustic wave with a phase opposite to that generated by exciter  206 . Measuring cell processor  154  is in electronic communication with measuring cells  202 ,  204  and exciters  206 ,  208  to provide electronic control. 
         [0056]    Additionally, measuring cells  202 ,  204  each comprise a second exciter  212 ,  214 , respectively, to generate a second wave  216 ,  218  as described above. However, in this example second exciter  214  is provided downstream of first exciter  208  and generates the second wave  218  in a direction opposite second wave  216 . Those of skill in the art are aware that many ultrasonic flow meters have a first set of transducers to determine flow velocity and also have a second set of transducers that generate an ultrasonic wave in the opposite direction to improve accuracy. Thus, an embodiment having two measuring cells would utilize known techniques to improve the accuracy of flow velocity measurements. 
         [0057]    Each measuring cell  202 ,  204  comprises a sensor array  220 ,  222  as described above. The ultrasonic sensors in each sensor array collect velocity vector field data to determine angular velocity. Measuring cell processor  154  compares the pressure measurements by the pressure sensors of the sensor arrays to improve accuracy of the measurement of the Coriolis force. 
         [0058]    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.