Flowmeter for Use in a Flow Monitoring System

The present invention relates generally to fluid flow measurement and more particularly to an improved fluid flowmeter for generating highly accurate fluid flow and temperature data for use in a flow monitoring system.

Fluid flow monitoring systems are well known in the prior art. Typically, such systems include a fluid flowmeter device interposed in a path of the fluid whose flow rate is desired to be measured. The flowmeter normally includes a flow transducer which generates an electrical signal proportional to the sensed flow rate. Although such flowmeter devices have proven generally effective, it is often difficult to produce an electrical signal proportional to flow rate which is both thermally and electrically-stable. Instability in the electrical signal generated by the flow transducer creates measurement errors which decrease the accuracy of the device. To overcome this problem, complex and costly processing circuitry must often be used to compensate for electrical signal variations caused by thermal and other factors.

There is therefore a need for an improved fluid flowmeter for use in a flow monitoring system which overcomes these and other problems of prior art devices.

An improved flowmeter is described including a turbine assembly which cooperates with a high frequency oscillator to generate a highly accurate and stable electrical signal proportional to the measured flow rate of a fluid through the flowmeter. This electrical signal is filtered and then processed using a look-up table encoding technique to generate instantaneous estimates of the flow rate based on linearized values of flow rate generated during calibration of the device. The flow rate estimates are then averaged over a predetermined time period and corrected to a normalized temperature.

In the preferred embodiment, the flowmeter includes a housing having a longitudinal bore therein. The turbine assembly is supported for rotation in the longitudinal bore and includes one or more conductive blades. An inductor coil is supported adjacent the longitudinal bore, and an RF oscillator coupled to the inductor coil for generating a signal of predetermined RF frequency in the inductor coil to thereby induce a magnetic field in the bore surrounding the blades. When the blades are at rest, the signal in the inductor coil has a predetermined resonant frequency and amplitude. However, when the fluid is directed through the longitudinal bore, the blades of the turbine assembly are rotated to alter the resonant frequency and amplitude of the signal in the inductor coil. In particular, the resulting signal has a relatively low frequency amplitude-modulated signal component whose frequency is proportional to the speed of the blades and thus the flow rate of the fluid. The "sensed" signal is filtered to remove the high frequency RF signal component, and the resulting amplitude-modulated signal component is then shaped and amplified to produce a pulse signal having a plurality of pulses.

According to another feature of the invention, the pulse width of each pulse in the pulse signal is used as a pointer to one of a plurality of addressable storage locations in a look-up table. Each of the storage locations includes a predetermined flow rate value equal to a predetermined number of pulses per unit volume at a given flow rate of the fluid. These flow rate values are preset during calibration of the flowmeter. Specifically, a known volume of fluid is passed through the fluid system at a plurality of known flow rates. For each such flow rate, a pulse signal having a predetermined number of pulses per unit volume is generated in the manner previously described. These pulse values are then stored in the look-up table at predetermined addresses. The remainder of the values in the look-up table are generated by linearly-extrapolating between the predetermined flow rate values generated during the calibration process.

To measure an unknown flow rate, a digital processing circuit of the flowmeter receives a pulse signal representing the unknown flow rate and in response thereto determined the pulse width of each pulse therein. The digital processing circuit is responsive to each pulse width determination for selecting one of the plurality of addressable storage locations in the look-up table. The predetermined flow rate value is then retrieved from the selected addressable storage location to form an estimate of the flow rate for the given pulse used as the pointer to the look-up table. Successive flow rate estimates are then averaged over a predetermined time period to generate a flow rate signal. The flow rate signal is then corrected to a normalized temperature and output from the device.

In the preferred embodiment, the flowmeter is incorporated in an engine fuel monitoring system having a suitable monitor device. The monitor device includes a keyboard for entering control information, and a display for displaying the sensed flow rate as well as fuel consumption and temperature information. The fuel monitoring system also preferably includes an accumulator located between the flowmeter and the engine for dampening or removing pressure pulses in the fuel line. If the engine fuel system has both supply and return lines between a fuel tank of the system and the engine, a flowmeter of the type described above is used in each line.

For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following Description taken in conjunction with the accompanying Drawings in which:

FIG. 1 is a schematic diagram of a flow monitoring system according to the invention for use in a conventional closed loop engine fuel system having supply and return lines between an engine and a fuel tank thereof;

FIG. 2 is an exploded perspective view of an improved fluid flowmeter of the present invention;

FIG. 3 is a simplified block diagram of an electronic control circuit supported in the housing of the flowmeter of FIG. 2; and

FIGS. 4A-4B are detailed schematic diagrams of the analog transducer circuit and the digital processing circuit, respectively, of the electronic control circuit of FIG. 3.

With references to the FIGURES wherein like reference characters designate like or similar parts throughout the several views, FIG. 1 is a schematic diagram of a flow monitoring system according to the invention for use in a conventional diesel engine fuel system 10. The flow monitoring system of the invention is not limited for use in diesel or other automobile fuel systems, and the reference to such a fuel system is merely exemplary. Fuel system 10 includes a conventional fuel tank 12 which normally delivers fuel via a supply line 14 to a fuel transfer pump 16. The output of the fuel transfer pump 16 is filtered by a fuel filter 18 of the fuel system and then delivered to a fuel injection pump 20 at the engine 22. In the diesel fuel system, unburned fuel is normally returned to the fuel tank via return line 24 to form a closed loop system.

According to the invention, a flow monitoring system is incorporated into the fuel system 10 to measure and display fuel flow rate information, as well as total fuel consumption and temperature information. To this end, the flow monitoring system includes a flowmeter 26 preferably located in the supply line 14 between the fuel tank 12 and the fuel transfer pump 16. Likewise, the flow monitoring system includes a flowmeter 28 located in the return line 24. Each of the flowmeters 26 and 28 includes an associated sight glass 30 connected to the output thereof for checking for the presence of air in the fuel path, and an accumulator 32 for dampening any pressure pulses which may be present in the fuel supply and return lines. As seen in FIG. 1, each accumulator 32 is connected at the end of the respective flowmeter closest to the fuel injection pump 20. Each flowmeter 26 and 28 also preferably includes a flow straightener 33 connected to the input thereof.

The flow monitoring system includes a flow monitor device 34 connected to the flowmeters 26 and 28. The flow monitor device 34 includes a suitable keyboard 36 (or other control switches) for entering information to the fuel monitoring system, and a suitable liquid crystal display (LCD) 38, or the like, for displaying monitored information. Of course, the two flowmeter system of FIG. 1 is not required when the engine or other fluid delivery system does not include a fuel return line. In such case, only the single flowmeter 26 is incorporated in the supply line 14.

Referring now to FIG. 2, an exploded perspective view is shown of one of the flowmeters of the flow monitoring system of FIG. 1. As will be described in more detail below, the flowmeter of the present invention advantageously includes all of the supporting electronic control circuitry in the device itself. In particular, each flowmeter includes a housing 40 having first and second end portions 42 and 44, first and second side portions 46 and and 48, and top and bottom portions 50 and 52. The top portion 50 of the housing 40 supports a removable cover 54 for enabling access to the interior of the housing 40 as will be described. Removable cover 54 supports an electrical connector 55 for connecting electrical signals to and from the device.

The housing 40 includes a longitudinal bore 56 formed therein between first and second female threaded connectors, one of which is shown by reference numeral 60. The longitudinal bore 56 is designed to be interposed in the fuel line for receiving the fluid flow between the first and second end portions 42 and 44 of the housing 40. The longitudinal bore 56 supports a turbine assembly 60 comprising a first straightener member 62, a second straightener member 64 and a blade member 66 located between the first and second straightener members. The first and second straightener members 62 and 64 support a spindle 68 on which the blade member 66 rotates as the fluid whose flow rate is to be measured is directed through the longitudinal bore 56. As will be described in more detail below, the blade member 66 includes one or more arcuate-shaped conductive blades 70 made of ferromagnetic or like material. The speed at which the blades of the turbine assembly 60 rotate on the spindle 68 is proportional to the rate of fluid flow through the longitudinal bore 56.

The housing 40 includes a first recess 72 in an upper portion thereof for supporting a plurality of circuit boards 73. The circuit boards 73 support an electronic control circuit, to be described in more detail below, for controlling the operation of the flowmeter. The bottom surface 74 of recess 72 includes a recess 76 also having a bottom surface 77. Bottom surface 77 of recess 76 includes a first recess 78 for supporting a temperature sensor 80 therein, and a second recess 82 for supporting a coil assembly 84 therein. Coil assembly 84 comprises an inductor coil 86 supported on a suitable bobbin 88 within a coil form 89. The recess 76 provides easy access to the temperature sensor 80 and the coil assembly 84 for removal and replacement thereof if necessary. As seen in FIG. 2, the inductor coil 86 of the coil assembly 84 is supported adjacent to the longitudinal bore 56 for the purpose of inducing a magnetic field therein. As described above, when a fluid is directed through the longitudinal bore 56 between the end portions 42 and 44 of the flowmeter, the blades 70 of the blade member 66 of the turbine assembly 60 are rotated at a speed proportional to the flow rate of the fluid through the flowmeter. Rotation of the conductive blades 70 through the magnetic field generated by the inductor coil 86 causes both frequency and amplitude deviations in the electrical signal flowing in the inductor coil 86 as will be described below. These frequency and amplitude fluctuations are sensed to determine the flow rate.

Referring now to FIG. 3, a simplified block diagram is shown of an electronic control circuit 90 for use in measuring the flow rate of the fluid in the flowmeter of FIG. 2. As described above, all of the necessary components of the electronic control circuit 90 are supported on circuit boards 73 supported in the recess 72 of the housing 40. The electronic control circuit 90 includes two basic functional sections, an analog transducer circuit 92 and a digital processing circuit 94. Analog transducer circuit 92 includes an RF oscillator 96 connected to inductor coil 86, an RF filter 98, a reference signal generator 100, a comparator 102 and a temperature sensing circuit 104. Digital processing circuit 94 includes a programmable microprocessor 106, associated memory devices 108 for storing suitable control programs and data for use by the microprocessor 106, a control logic circuit 110 for controlling the microprocessor 106, and an input/output circuit 112 for interfacing the electronic control circuit 90 to the flow monitor device 34 of FIG. 1. The memory devices 108 include a read only memory 108a (ROM) for storing control programs, a programmable read only memory (PROM) 108b for storing a look-up table to be described, and a random access memory (RAM) 108c for storing data being processed by the microprocessor 106. Though not shown in detail in FIG. 3, the electronic control circuit 90 also includes a suitable voltage regulator device. External power to the electronic control circuit 90 is provided via the electrical connector 55 from the flow monitor device 34 of FIG. 1 or some other external power supply, e.g. an automobile battery.

Referring now simultanously to FIGS. 2 and 3, the operation of the electronic control circuit 90 will be described. In particular, the RF oscillator 96 is connected to the inductor coil 86 of the coil assembly 84 for generating a signal of predetermined RF frequency in the inductor coil 86. The oscillations in the inductor coil 86 induce a magnetic field in the longitudinal bore 56 supported adjacent thereto. The oscillations in the inductor coil 86 are at a predetermined amplitude and resonant frequency when the conductive blades 70 are at rest, i.e., when no fluid is flowing through the flowmeter. However, when a fluid is directed through the longitudinal bore 56, the blade member 66 is rotated, thereby causing the conductive blades 70 66 to rotate through the magnetic field induced in the longitudinal bore 56 by the inductor coil 86. Movement of the conductive blades 70 through the induced magnetic field changes the resonant frequency of the RF oscillator 96 and generates a low frequency amplitude modulation component on the resulting "sensed" RF signal flowing through the inductor coil 86. The generation of a low frequency amplitude-modulated signal component on the sensed signal in the inductor coil 86 insures that frequency instabilities of the RF oscillator 96 do not produce any measuring inaccuracies in the flowmeter output. This significantly increases the accuracy of the sensed flow rate measurement, thereby providing improved operation over prior art devices.

The sensed signal in the inductor coil 86 is applied to the RF filter 98 for the purpose of extracting the low frequency amplitude-modulated signal component from the high frequency (i.e., RF) signal. Accordingly, the RF filter 98 operates to effectively extract the envelope of the signal applied thereto. The output of the RF filter 98 is supplied as one input to the comparator 102, with the other input thereto being a reference signal supplied by the reference signal generator 100. The comparator 102 compares the reference signal and the amplitude-modulated signal component, and in response thereto generates a shaped and amplified pulse signal at the output 103 thereof. As will be described in more detail below, the pulse signal includes a plurality of pulses each having a pulse width proportional to instantaneous flow rate of the fluid. The analog transducer circuit 92 also generates a control signal proportional to the ambient temperature in the longitudinal bore 56. In particular, the temperature sensing circuit 104 includes the temperature sensor 80 which generates an output current proportional to temperature variations. This output current is converted, using a current-to-frequency circuit in the temperature sensing circuit 104, to a pulse signal having a pulse rate proportional to temperature variations. The pulse signals output from the comparator 102 and the temperature sensing circuit 104 are then supplied to the digital processing circuit 94 for subsequent processing.

According to a feature of the invention, the programmable read only memory (PROM) 108b of the digital processing circuit 94 includes a look-up table having a plurality of addressable storage locations therein. Each of the plurality of addressable storage locations in the look-up table stores a predetermined flow rate value equal to a predetermined number of pulses per unit volume at a given flow rate of the fluid. These flow rate values are preset during calibration of the flowmeter for the specific fluid system being monitored. In particular, a known volume of fluid is passed through the flowmeter at a plurality (e.g., twenty (20)) of known flow rates during the calibration process. For each such flow rate, the analog transducer circuit 92 generates a pulse signal at the comparator output 103 having a predetermined number of pulses per unit volume. These pulse values are stored in the look-up table in the PROM 108b at predetermined locations. The address of each location is preferably equal to the number of clock cycles during the pulse width (of the pulse signal) corresponding to the known flow rate. Intermediate flow rate (and address) values are generated by a control program in the ROM 108a which linearly-extrapolates between the measured pulse values for the plurality of known flow rates to generate such intermediate values. Therefore, the look-up table thus stores a plurality of linearized values of flow rate for the fluid delivery system.

To measure an unknown flow rate, the microprocessor 106 of the digital processing circuit 94 receives the pulse signal generated from the sensed signal which, as described above, includes a plurality of pulses each having a pulse width proportional to instantaneous flow rate. The control program in the ROM 108a controls the microprocessor 106 to determine the pulse width of each pulse input to the microprocessor 106 by establishing a pulse width count equal to a number of counts (clock cycles) for each pulse. In response thereto, the control program selects one of the plurality of the addressable storage locations in the look-up table. Therefore, as noted above the pulse width of each pulse in the pulse signal output from the comparator 102 is used as a pointer to one of the plurality of addressable storage locations in the look-up table stored in PROM 108b. The control program then retrieves the predetermined flow rate value from the selected addressable storage location to form an estimate of the flow rate for the given pulse used as the pointer to the look-up table.

According to the invention, successive flow rate estimates for each of the pulses in the pulse signal output from the comparator 102 are then averaged by the control program over a predetermined time period (e.g., 1 second) to generate a flow rate signal. The flow rate signal is then normalized by the control program to a predetermined temperature (e.g., 60.degree. F.) using the pulse signal supplied from the temperature sensing circuit 104. The adjusted flow rate signal is output from the microprocessor 106 to the I/O circuit 112 for use by the flow monitor device 34 as described above with respect to FIG. 1.

According to the present invention, an improved flowmeter includes an RF oscillator for establishing oscillations in an inductor coil adjacent to the fluid path. These oscillations create a magnetic field in the path. Conductive blades of a turbine assembly are rotated by the fluid in the path with little or no resistance. These blades interrupt the magnetic field and thus change the resonant frequency of the oscillations in the inductor coil. The passage of the blades through the field also creates a low frequency amplitude-modulation on the signal in the inductor coil. This signal is filtered and then processed using a microprocessor-based look-up table encoding technique to generate instantaneous estimates of the flow rate based on linearized values of flow rate generated during calibration of the device. The flow rate estimates are then averaged over a predetermined time period and normalized to a given temperature to generate an indication of flow rate.

The flowmeter of the present invention is advantageous because the use of ferro-magnetic conductive blades 70 in the turbine assembly 60 in conjunction with the RF oscillator insures that the inductor coil current remains thermally and inductively stable and that the flow rate information is encoded in a low frequency amplitude-modulated component thereof. Accordingly, any frequency drift in the oscillator due to temperature or other environmental variations does not create measuring inaccuracies in the resulting flow rate calculation.

Referring now to FIGS. 4A-4B, detailed schematic diagrams of the electronic control circuit 90 are shown. In particular, FIG. 4A is a schematic diagram of the analog transducer circuit 92, and FIG. 4B is a schematic diagram of the digital processing circuit 94. Additionally, set forth below is a code listing of a control program for storage in the ROM 108a of the digital processing circuit 94 for use in controlling the microprocessor 106 to carry out the operations described hereinabove. ##SPC1##

Although the present invention has been described in detail, the same is by illustration and example only and is not to be taken by way of limitation, the spirit and scope of the invention being limited only by the terms of the appended claims.