Abstract:
Magnetic flow meters having a streamlined body within a flow tube provide an extended voltage path through a flowing fluid at the expense of flow passage restriction. The voltage path may extend along a circumference of an annular flow region. The use of an extended voltage sensing path increases useful signal levels, which allows for lower cost construction and lower power operation.

Description:
This application is a continuation-in-part of the inventor&#39;s application U.S. Ser. No. 09/704,913, filed Nov. 2, 2000, and now issued as U.S. Pat. No. 6,463,807. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to apparatus and method for determining the rate of flow of a fluid by measuring the electrical potential difference developed in the fluid as the fluid moves through a magnetic field. 
     BACKGROUND INFORMATION 
     In prior art in-line magnetic flow meters, the electrical potential difference developed in the fluid is generally sensed by a pair of electrodes contacting the liquid and spaced apart from each other by the diameter of a round flow sensing passage. A magnetic field generated orthogonal to both the axis between the electrodes and the direction of flow through the sensing passage is provided by two coils of wire located on opposite sides of and outside of the passage. Sophisticated electronics are used to energize the magnetic field, amplify the tiny flow-related signals generated, and reject various noise and drift signal components which would otherwise degrade measurement precision. These meters are characterized by an unobstructed flow passage offering very low pressure drop and high tolerance to solids in the fluid, high measurement precision, high power consumption, and high cost. 
     In a water metering application for irrigation, where only moderate flow rates are experienced, an unobstructed flow passage is relatively unimportant, but low cost and low power consumption for stand alone battery operation are very important. It is therefore an object of the invention to provide the basis for magnetic flow sensors which, at the expense of flow passage restriction, offer advantages of improved measurement precision, reduced operating power and lower costs. 
     BRIEF SUMMARY OF THE INVENTION 
     Various of the above and other objects are attained by magnetic flow sensors made or operated in accordance with various preferred embodiments of the present invention. In one preferred embodiment a magnetic flux generated by two electromagnets having magnetic cores is redirected by magnetic pole pieces so as to be orthogonal to both the axis between the electrodes and to a fluid flow direction. As is known in the magnetic flow metering art, the flux will generate, in the moving fluid, voltage differences proportional to the flow rate of the fluid, the magnitude of the flux and the length of the conductive path between the electrodes. These voltage differences are sensed by at least one, and preferably two pairs of electrodes arranged so that one pair is associated with each location of the pole pieces. In this embodiment, one of the electromagnets is located in a streamlined housing centered within the flow passage so as to confine the flow to a quasi-annular ring, and the other electromagnet is on the outside of the passage. The pole pieces from the two magnets are located a selected distance apart and are aligned to reinforce their radial flux through the annular flow passage at two locations along the flow axis, thereby forming a complete magnetic circuit. At each location of these paired poles, a pair of electrodes is located to sense the corresponding flow generated voltages. An electrically insulating barrier may be used both to provide mechanical support for the housing and electrodes, and to electrically isolate the paired electrodes so that the quasi-annular flow passage provides substantially the only electrical path between those electrodes. That is, flow signals are generated along a circumferential path lying between the streamlined body and the tube. In preferred embodiments, one pair of electrodes is located far enough from the other pair so that their signals have low mutual interaction. This provides a combination of an increased fluid flow velocity, a longer path between electrodes and a highly efficient magnetic circuit. These features enable a magnetic flow sensor to be produced having substantially greater flow-generated signals than is found in the prior art. 
     The presence of a streamlined housing within a flow passage reduces the cross sectional area of the flow passage and thereby increases the fluid flow rate at the expense of an increased pressure drop. At a fixed magnetic flux in the passage, the increased flow rate produces correspondingly higher electrode voltages than would be measured if the body were not there. Moreover, use of magnetic cores with pole pieces to provide a complete, shielded magnetic circuit concentrates the magnetic flux in the desired area. This arrangement enables a higher magnetic circuit efficiency to be achieved than is the case with commonly used air core magnets. Additionally, the magnetic field is generally confined to the annular flow passage in order to reduce problems of magnetic and electromagnetic compatibility. The use of magnetic cores and pole pieces with prior art magnetic flow meters is generally not practical in larger pipe sizes because the orientation of the field would require a relatively large mass of core material that would increase the size and weight of the meters considerably. 
     In a preferred flow sensing embodiment, each electrode pair may be used with its own signal amplifying and processing circuitry to provide a flow rate signal. Alternately, signals from multiple pairs may be combined in various ways to provide redundancy and improved measurement precision. Each electrode pair may also be stabilized by short-circuiting the two electrodes of the pair together, or otherwise connecting both of the two electrodes to a common potential during the period when the magnetic field is not present, thereby further helping to reduce measurement errors, 
     Although it is believed that the foregoing recital of features and advantages may be of use to one who is skilled in the art and wishes to learn how to practice the invention, it will be recognized that the foregoing is not intended to list all of the features and advantages. Moreover, it may be noted that various embodiments of the invention may provide various combinations of the hereinbefore recited features and advantages of the invention, and that less than all of the recited features and advantages of the invention may be provided by some embodiments. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     FIG. 1 is a side cross sectional view of a magnetic flow sensing configuration in accordance with a preferred embodiment of the present invention. 
     FIG. 1 a  is a sectional end view of the configuration of FIG. 1 along lines  1   a — 1   a  illustrating the quasi-annular flow ring. 
     FIG. 2 is a schematic block diagram of an electronics circuit in accordance with a preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A magnetic flow meter  10  made according to a preferred embodiment of the invention is shown in an axial cross sectional view in FIG.  1 . An end cross sectional view of the same device, taken as indicated by the double-headed arrow  1   a — 1   a  in FIG. 1, is depicted FIG. 1 a . In this embodiment a tube  22  provides an electrically insulating cylinder confining the fluid  18  that is flowing as indicated by arrows  16 . Inside, and generally centered within a selected portion of the tube  22 , is an electrically insulating streamlined housing  28  containing inner ring pole pieces  38 ,  44 , a center core  40 , and an inner magnet winding  42 . A vane  20  provides a preferred mechanical connection between the tube  22  and a housing  28 . The vane  20  is also used for mounting paired electrodes  34 ,  36  at selected axial position within the selected portion of the tubes so that they are centered with respect to the first ring pole piece  38 . The electrodes will make contact with whatever fluid  18  flows through the tube but will otherwise be electrically insulated from each other. The electrodes  34 ,  36  are thus arranged to measure a voltage difference occurring along an elongated circumferential path  39  within the quasi-annular space defined between the tube  22  and the combination of the vane  20  and streamlined body  28  inserted thereinto. Another similarly configured pair of electrodes  46 ,  48  is symmetrically located with respect to the second ring pole piece  44 . In addition, two outer ring pole pieces  52 ,  54  are disposed between the tube  22  and an outer cylindrical core  24  which has an outer magnet winding  26  wrapped around it. Slots and holes (not shown) are provided in the various components to enable electrical connections to be made to the electrodes and to the inner magnet winding  42 . 
     It will be recognized that if the vane were not present (e.g., if the streamlined body were supported in the middle of the tube by means of a support member downstream of the measurement region), the fluid would flow in an annular region defined by the streamlined body and the inner wall of the tube. In the preferred embodiments, the presence of the vane converts the annular region into a quasi-annular, or substantially annular region. Moreover, it will be recognized that a variety of diameters and lengths can be chosen for the streamlined body, which can extend beyond the selected portion of the tube in which the flow measurement is made. 
     In operation of a preferred flow meter the two coils  42 ,  26  are energized at the same time to produce magnetic flux of opposite polarity at the ends of their cores  40 ,  24 . A first pair of inner and outer ring pole pieces  38 ,  52  provide a low reluctance path and radially concentrate the magnetic flux in a quasi-annular ring of the fluid axially aligned with a first pair  34 ,  36  of electrodes. The second pair of pole pieces  44 ,  54  is correspondingly associated with a second pair  46 ,  48  of electrodes. In comparison to prior art magnetic flow meters, the average distance between electrode pairs can be easily made much greater, the magnetic flux can be easily concentrated in the region used for generating flow-related electrode voltages and the flow velocities can be made relatively high. As a result, the generated electrode voltages are relatively high, thereby enabling advantageous tradeoffs to be made. For example, electrode signal amplification and processing may be simplified because larger signals with greater immunity to noise and interference are available. Moreover, the power supplied to the electromagnets may be reduced in order to lower the overall power requirements of the meter, thereby making it more practical for self-powered and loop power applications. 
     The magnetic flow sensor  10  illustrates a configuration for achieving large flow-related signals at the electrodes and overall good performance. This configuration may be modified to gain certain advantages at the expense of others. For example, to simplify mechanical construction an embodiment of the invention could be made with only an external coil  26 . In a version such as this, the streamlined housing  28  might contain only the annular ring pole pieces  38 ,  44 , and the core  40  for concentrating the flux in the region used for generating flow related electrode voltages, but not include the winding  42 . In another approach aimed at reducing size and weight of the meter, the external winding  26  may be eliminated so that all of the flux is produced from the internal winding  42 . In addition to the winding  26 , the outer ring pole pieces  52 ,  54 , and the cylindrical core  24  could also be eliminated to enable a particularly small, low cost and lightweight magnetic flow meter for a selected pipe size. 
     Turning now to FIG. 2, one finds a simplified block diagram of preferred electronic circuitry used with the magnetic flow sensor  10 . The electrodes  34  and  36  provide two input signals to a signal amplification circuit block  90 , which is used to perform signal amplification and filtering functions. A switch  82 , which may be an electro-mechanical relay, a solid state relay, or any one of many other electrically controllable switching elements known in the art, may be arranged to selectively connect the two electrodes during non-measurement intervals. One of the electrodes  34  is also connected to a first input amplifier  60  and the second electrode  36  is connected to a second input amplifier  62 . The outputs from the two input amplifiers  60 ,  62  are fed to a first differential amplifier  64 . The output from the first differential amplifier  64  is fed to two sample and hold circuits  68 ,  70  that have outputs to respective buffer amplifiers  72 ,  74  that, in turn, provide inputs to a second differential amplifier  84 . The output from the second differential amplifier  84  is input to a signal processor  76 . Signals from the second pair of electrodes  46 ,  48  are supplied to a signal amplification circuit block  90   a  which is a duplicate of the signal amplification circuit block  90  and which also provides its output signal to the signal processor  76 . Other circuit elements required for operation of the sensor of the invention are generally conventional and include timing circuits  78  and driver circuits  80  used to energize the electromagnet windings  26 ,  42 . In some embodiments of the invention the circuitry also comprises a battery  88  or other exhaustible electric energy source. 
     During the course of a cycle of operation, the timing circuits  78  provide a short duration drive pulse (e.g., five milliseconds), to the driver  80  which, in turn, supplies a constant current during a portion of the drive pulse&#39;s duration (e.g., two milliseconds) to the electromagnet coils  26 ,  42 . The signals corresponding to the voltages generated in the fluid  18  responsive to both its flow rate and to the magnetic flux from the coils  26 ,  42  appear at sensing electrodes  34 ,  36  and are fed to respective input amplifiers  60  and  62 . The difference between these amplified signals is extracted by the first differential amplifier  64  and output to the sample and hold circuits  68 ,  70 . The timing circuits  78  provide a first sample pulse to enable the first sample and hold circuit  68  during a portion (e.g., one millisecond) of the interval in which the driver is supplying its constant current output. This selection of a shorter sampling interval allows for circuit tolerances and drifts as well as for amplifier circuit settling. The output from the first sample and hold  68  is buffered by its associated amplifier  72  and is then provided to the positive input of the second differential amplifier  84 . 
     After the magnetic fields produced by the electromagnets  26 ,  42  have collapsed, the timing circuits  78  provide a relatively long duration pulse—e.g., one hundred milliseconds—to the switch  82  to cause it to short circuit the electrodes  34 ,  36 . The electrodes are shorted together for most of the cycle and any difference in voltage between them, which would have otherwise existed, produces a current between them, which will neutralize their voltage difference. When switch  82  opens its contacts, timing circuits  78  provide a pulse, for example one millisecond wide, to the second sample and hold  70  so that the amplified voltage difference then existing between the first pair of electrodes  34 ,  36  is sampled, buffered by the associated buffer amplifier  74 , and provided to the negative input of the second differential amplifier  84 . The output from differential amplifier  84 , now representative of the amplified voltage difference between the electrodes  34 ,  36  for the two conditions of the magnetic field being present and not being present, is an accurate representation of the fluid flow rate and becomes one input to the signal processor  76 . 
     In the foregoing discussion, those skilled in the art will recognize that instead of using a switch  82  to short the electrodes  34 ,  36  together, one could choose to connect both of those electrodes to a common potential. A flow meter using connections of this sort is specifically taught in the inventor&#39;s co-pending application Ser. No. 09/820,057, filed on Mar. 28, 2001. The disclosure of Ser. No. 09/820,057 is herein incorporated by reference. Moreover, it will be recognized that the electrodes  34 ,  36  could be continuously connected to a signal amplifier, as is common in the prior art. 
     The combination of the second pair of electrodes  46 ,  48  and their associated signal amplification block  90   a  function similarly to the combination of the first pair of electrodes  34 ,  36  and their associated signal amplification block  90  in order to provide a similar signal to the signal processor  76 . As an alternative to the above arrangement, the two electrode pairs  34 ,  36  and  46 ,  48  can also time share a single signal amplification block. 
     The signal processor  76  can be used to integrate its input signals to provide several possible outputs. For example, the processor can sum its input signals to provide a relatively high precision signal having high noise immunity. Alternately, the difference between multiple input signals may be extracted and used as an aid to maintenance by providing redundancy. 
     There are different ways known in the magnetic flow meter art for energizing the magnetic field and for amplifying and detecting the corresponding electrode signals. For the purpose of describing this invention, single polarity DC pulsing has been used. Bipolar DC pulsing and AC energization, for example, could also be used. In these latter cases the amplifying blocks would have to be modified to include the corresponding signal polarity reversal and other necessary functions. 
     Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present invention can be implemented in a variety of forms. Therefore, while this invention has been described in connection with particular examples thereof, the true scope of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, specifications and claims.