Abstract:
A variable area flow meter has a magnet carried by a piston which is moved to a position representing fluid flow. An array of magnetoresistive bridge sensors adjacent the path of magnet movement senses the magnet position. The bridges are unbalanced by the magnet field. The analog unbalance signals are read and processed in digital form to develop a flow signal. The flow meter circuits are powered from a 4–20 MA current loop or a remote power supply. Each bridge sensor is powered only when the bridge unbalance signal is being read, to minimize the power requirement.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   Not applicable. 
   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
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   REFERENCE TO SEQUENCE LISTING 
   Not applicable. 
   BACKGROUND OF THE INVENTION 
   A variable area flow meter as shown in Hedland U.S. Pat. No. 3,805,611 typically has a cylindrical body with an inlet at one end and an outlet at the other. A piston inside the body is spring biased toward the inlet and is moved toward the outlet by fluid flow through the body. A conical plug extends through a hole in the end of the piston and is tapered toward the outlet. As the piston is moved against the spring by increased fluid flow, the opening for flow in the end of the piston grows larger. The piston reaches a stable position in the cylindrical body which is a measure of flow. If the cylindrical body is transparent, a scale on the body indicates the flow. 
   Where the cylindrical body is not transparent, the Hedland &#39;611 patent incorporates a magnet in the piston with a follower of magnetic material outside the cylindrical body. The follower and a scale indicate the flow. Electrical output signals have been provided by switches with contacts actuated by the follower at selected flow rates. A remote electrical read out is shown in Lake U.S. Pat. No. 4,487,077. A resistor mounted on the cylindrical body is contacted by a wiper carried on the magnet follower and connected with a remote read-out. The meter of the &#39;077 patent is subject to decoupling of the follower from the magnet as a result of rapid change in flow rate and exacerbated by friction between the wiper and the resistor. 
   BRIEF SUMMARY OF THE INVENTION 
   The flow meter disclosed herein utilizes an array of anisotropic magnetoresistive (AMR or MR) sensing elements adjacent to the path of travel of the magnet on the flow meter piston. An external follower and wiper are not required. This eliminates the potential for decoupling. 
   Each of the sensing elements comprises a bridge of four MR elements. The bridge has an unbalance output related to the angle of the field of the magnet with respect to the sensitive axis of the bridge. The unbalance output signals of the bridges are processed to derive a signal representing fluid flow. The output signals are independent of magnet field strength so long as the MR elements are saturated. 
   A feature of the invention is that the flow sensor comprises a linear array of bridge circuits parallel to the path of flow meter magnet movement, each bridge circuit having four magnetoresistive elements, balanced in the absence of magnetic flux and unbalanced by flux from the magnet, the unbalance being a function of the position of the magnet along the path of movement, a voltage source connected with the bridge circuits, each bridge having an output voltage representing the unbalance of the bridge and a signal circuit responsive to the output voltages of bridges to develop a fluid flow signal. More particularly, the signal circuit includes an analog to digital converter and a processor responsive to the digital signals to develop the fluid flow signal. 
   Another feature is that a condition measuring meter with a magnet movable along a linear path in accordance with a condition to be measured includes a linear array of bridge circuits of magnetoresistive elements which are unbalanced by flux from the magnet with a current based two-wire remote indicator circuit including a remote power source. A local power supply is connected through the two-wire circuit with the remote power source and provides local power for the condition sensor with a switch circuit connecting the bridge circuits with the local power supply and a signal circuit powered by the local power supply and responsive to the output voltages of the bridges to develop a signal representing the position of the magnet and having a current output section connecting the measured condition signal with the two wire remote indicator system. More particularly, the switch circuit connects only one bridge circuit of the array with the local power supply at a time. 
   A further feature of the invention is the method of measuring fluid flow which comprises reading the analog unbalance signals of the bridge sensors, identifying the pair of adjacent sensors on each side of the magnet, selecting the sensor closest to the magnet, calculating the position of the magnet with respect of the selected sensor from the unbalance signal of the selected sensor and adding the calculated magnet position to the position in the array of the selected sensor to determine the fluid flow. 
   Yet another feature of the method is that the unbalance signals are normalized before the pair of adjacent sensors is identified. 
   A further feature is that the unbalance signals are converted to digital form before they are normalized. 
   Still another feature is that the validity of the unbalance signal readings is confirmed after identification of the pair of adjacent sensors on each side of the magnet. 
   Further features and advantages of the invention will be apparent from the following description. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
       FIG. 1  is a longitudinal section through a variable area flow meter with an array of MR bridge sensors; 
       FIG. 2  is a fragmentary diagrammatic illustration of the flow meter piston, magnet and two MR bridge sensors; 
       FIG. 3  is a plot of the unbalance voltage ΔV as a function of the angle θ between the magnetic field and the reference axis of the sensor or the distance of the magnet from the reference axis; 
       FIG. 4  is a diagram of the circuit of the MR sensor array with signal processing and multiple analog outputs; and 
       FIG. 5  is a flow chart of the program of the microprocessor; and 
       FIG. 6  is a more detailed flow chart of the program for calculating the magnet position and fluid flow. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   A variable area of flow meter  10 , of the type shown in Hedland U.S. Pat. No. 3,805,611, is illustrated in  FIG. 1 . Cylindrical body  12 , of a non-magnetic material, has an inlet  14  and an outlet  16 . A piston  18  is biased toward the inlet by spring  20 . The piston has a central opening  22  through which a conical plug  24  extends. Fluid flow through the body  12  creates a differential pressure across piston  18  which lifts the piston to a position at which the differential pressure balances the spring force as shown in broken lines at  18 ′. A magnet  26  is carried on piston  18  and has a field which extends outside cylindrical body  12 . The position of magnet  26  and thus of the piston is sensed by an array  28  of MR sensors  30 , providing signals from which flow rate of a fluid through the meter may be determined. 
   A preferred sensor  30  is a bridge circuit of four MR elements, as Honeywell type HMC 1501. Magnet  26  and two sensors  30 - 1 ,  30 - 2  are illustrated in  FIG. 2 . The four MR elements  30   a ,  30   b ,  30   c  and  30   d  of a sensor are connected in a bridge circuit and with a DC power source, not shown in  FIG. 2 . In the absence of a magnetic field, the four MR elements have equal resistance and the bridge is balanced. In the presence of a magnetic field, the resistance of each element varies as a function of sin 2θ, where θ is the angle between the magnetic flux and the current flow through the element. With a magnetic field which saturates the elements, the effect of the field on element resistance is independent of the spacing between the magnet and the sensor. The angle between the magnetic field and the current flow is the same for elements a, d and elements b, c; but differs between elements a, d on one hand and elements b, c on the other hand. The bridge is unbalanced and has an output signal ΔV. The angle of the magnetic field and thus the position of the magnet  26  may be determined from the bridge output signal. 
   The sensor array  28  adjacent to the path of movement of the flow meter magnet  26  provides output signals from each sensor  30 , from which the magnet and piston position and thus flow rate is calculated. The Honeywell HMC 1501 sensor has a linear measuring range of about 0.5″. For a flow meter with piston travel of 3″, an array of seven sensors spaced apart at 0.5″ increments is used. The cylindrical magnet has a length of approximately 13/16″ and is magnetized axially so that the axis of the field is parallel with the linear movement of the piston and the magnet and at right angles to the axes of the bridge sensors. 
   A plot of the bridge unbalance output voltage, sometimes referred to herein as ΔV, as a function of the angle θ of the magnetic field with respect to the reference axis of the bridge and thus of the magnet position with respect to the bridge, is given in  FIG. 3 . The relationship of the voltage to field angle is substantially linear between magnet positions from −0.25″ to +0.25″. The geometry of the magnet and the sensor array is such that only one pair of adjacent sensors will have outputs of both sensors in the linear zone at the same time. This relationship is used in determining the magnet position as described below. 
   The analog outputs of each of the sensors in array  28  are connected with an analog to digital (A/D) converter  34 . The A/D converter acquires each of the analog unbalance output signals and converts them sequentially to a digital format. Preferably, a delta-sigma converter is used which provides significant data resolution and rejection of 50/60 Hz noise. The digital output signals of A/D converter  34  with an identification of the sensor from which each signal is derived are in turn connected with microprocessor  36  which calculates the piston position. The flow rate, or if desired, total flow, is indicated on a display  38  which may be physically mounted on the flow meter  10 . 
   Keyboard  40  connected with microprocessor  36  provides for input as in selection of the mode of operation of the flow meter, calibration of the meter and the input of scaling factors, as discussed below. 
   In many situations, a flow meter is installed in a location remote from a control center where the flow information is needed. Output circuit  42  provides analog signals for a current output section with 4–20 mA current loop at terminals  44 ,  46  or voltage output section with a 5 or 10 volt output at terminal  48  with respect to ground terminal  50 . A fluid flow signal in pulse width modulated (PWM) format from microprocessor  36  is connected with integrator  52  which develops an analog output signal. For a 4–20 mA output, the loop current at output terminal  46  is controlled by transistor  54 . Drive amplifier  56  has an input from integrator  52  with feedback from loop current sensing resistor  58 . Amplifier  60  provides a 5 or 10 volt analog output at terminal  48 . 
   With operation in the 4–20 mA current loop output configuration, the signal circuit including the sensor array  28 , A/D converter  34 , microprocessor  36  and output circuit  42 , is powered from the current loop. The voltage drop from the signal current across isolation diode  62  and 5 volt voltage regulator  64  is approximately 5.5 volts DC. The voltage regulator  66  provides 3.3 volts DC for sensor array  28 , A/D converter  34  and microprocessor  36 . 
   When operating in the 5/10 volt output configuration, a third wire is required connecting terminal  44  with a remote voltage source of at least 13 volts. Voltage regulator  68  provides 12 volts voltage for amplifier  60 . 
   The 4–20 mA current loop can provide about 2 mA for sensor circuit operation. The analog/digital converter  34 , microprocessor  36 , display  38 , and output circuit  42 , including the voltage regulators  64  and  66 , require approximately 1.2 mA. Each MR sensor  30  draws 0.66 mA at 3.3 volts. If all seven sensors  30  in array  28  are powered simultaneously, the current requirement is too great. In accordance with the invention, each sensor bridge MR 1 –MR N  is connected with ground through a normally open Mosfet transistor switch  70   1 – 70   N . The switches are turned on and off sequentially by sensor select signals from microprocessor  36 . When a switch, as  70   1  is turned on, the circuit for sensor bridge MR 1  is completed. The output signal from the activated sensor is captured by A/D converter  34  and a digital value corresponding to the output signal is connected to microprocessor  36 . The sensor select signals identify the sensor which provides each output signal. After all sensors have been activated and the unbalance voltages react, the process is repeated. Microprocessor  36  calculates the piston/magnet position and the corresponding flow rate from a set of the sensor output signals. 
   The flow chart of  FIG. 4  illustrates the operation of microprocessor  36 . Upon start up at block  80 , the hardware and software are initialized at block  82 . Assuming that no key of keyboard  40  is pressed at decision block  84 , the sensors  30  of sensor array  28  are read, as discussed above. The digital unbalance signal from each sensor is then normalized to a value between −1 and 1. Under normal operating conditions, only one adjacent pair of sensors, the pair closest to the magnet, has unbalance voltages in the linear zone of the curve in  FIG. 3 . The unbalance voltage of one sensor is positive and that of the other is negative; and the sensor with the smaller unbalance voltage is the closer to the magnet. This voltage is used in determining the magnet position. 
   In the event that two adjacent pairs or no adjacent pair of sensors have one positive and one negative unbalance voltage, there is likely an erroneous reading. All unbalance signals for that scan of the sensors are discarded. This situation might occur in several circumstances:
         a) A surge in the fluid flow which causes a sudden movement of the magnet. During a scan of the sensors, the magnet is in two different positions.   b) Interference from an external magnetic field.   c) The magnet is beyond the end of the sensor array.
 
Assuming a valid read at decision block  88 , as discussed below, magnet position is calculated from the normalized output of the sensor closest to the magnet and the position of the sensor in array  28  at block  90 . The position signal is passed through an infinite impulse response digital filter at block  92 . The filter has the following characteristic:
       

   new magnet position=K/100×last magnet position+(1−K)/100×current magnet position, 
   where K is a user programmable filter coefficient. 
   The resulting position signal is corrected for non-linearity of the flow meter. The linearized position signal is multiplied by the rate coefficient corresponding to the selected display units and the scale factor to calculate flow rate, at block  94 . The flow rate multiplied by the time since the last previous measurement is added to the totalizer figure at block  96 . At block  98 , either the flow rate or the total flow is displayed as selected by the operator. The PWM signal to analog output circuit  42  is updated at block  100 . The program then returns to block  84  and repeats. 
   If more than two pairs of sensors have a positive slope output at decision block  88 , an error is flagged to decision block  102 . If two consecutive reads of sensor array  28  produce erroneous readings, an error is displayed at block  104 . 
   Keyboard  40  is utilized by an operator to interact with the flow meter. Actuation of Menu or/and Reset keys is detected at decision block  84 . Actuation of menu key alone detected at decision block  106  directs the operator to a set-up and calibration sub-routine at block  108 . Actuation of reset key alone is detected at decision block  110  and stores the total flow information at block  112 . Concurrent actuation of the menu and reset keys is detected at decision block  114  and resets the totalizer to zero at block  116 . 
   The calculation of the position of the magnet  26  and of fluid flow, principally by microprocessor  76 , are illustrated in more detail in the flow chart of  FIG. 6 . 
   Starting at block  86 , the unbalance output voltages ΔV of each of the MR sensors  30  are read under control of microprocessor  76 . The analog signals are connected with analog to digital converter  34  and the digital unbalance signals are in turn connected with microprocessor  36 . 
   The digital unbalance signals are normalized at block  124 . During manufacture, the array  28  of MR sensors mounted on a circuit board is placed in a test fixture and calibrated. A magnet is sequentially positioned at the mid points between adjacent sensors. Analog readings are taken at each position of the minimum and maximum output levels for each sensor. This information is stored in the memory of microprocessor  36  and used to correct for the bridge zero offset and full scale unbalance voltage tolerance deviations of each sensor. The minimum and maximum signal levels for the sensing range of each sensor determined during calibration provide the basis for normalizing the signals read from each sensor. The signals are normalized to a value from −1.0 to +1.0. A normalized value of −1.0 represents the maximum valid negative unbalance signal, corresponding with a magnet position 0.25″ from the center of the sensor. A normalized value of +1.0 represents the maximum valid positive unbalance signal, corresponding with the magnet position 0.25″ from the center of the sensor on the other side. 
   At block  126 , the pair of adjacent sensors on each side of the magnet are identified by looking for a positive normalized signal from one sensor and a negative normalized signal from the other. 
   The validity of the reading is checked at block  88  by determining whether more than one pair of adjacent sensors are identified at block  126 . If the reading is valid, the program continues. If the reading is not valid; and this is the first invalid reading, the program returns to block  86  and the sensors  30  are read again. In the event of two or more consecutive errors at block  102 , an error display is given and the program returns to block  84  as shown in  FIG. 5 . 
   With a valid read, the program continues from block  88  to calculate the magnet position at block  90 . The first step is to select the closest sensor of the pair of adjacent sensors on each side of the magnet. This is the smaller of the signals from the pair of adjacent sensors on each side of the magnet. The signal is the smaller of the two, regardless of its sign. The normalized signal is multiplied by 0.25 at block  130  to obtain the distance of the magnet from the sensor of the closest sensor. This distance, which may be positive or negative, depending on the sign of the closest sensor signal is added to the sensor position in the array at block  132 . 
   The position signal is filtered at block  134  and linearized at block  136 , as described above. 
   The flow rate is calculated from the magnet position at block  94  by multiplying the magnet position signal by a rate coefficient for the desired flow dimension, e.g., milliliters per minute, barrels per hour, etc. 
   The program is completed as illustrated in  FIG. 5  and then repeats.