Abstract:
A target-type flow meter uses a target in a flowing fluid and selectively changes the orientation of the target with respect to the direction of flow of the fluid between two or mote orientations, where the target provides a different flow impedance in each of the orientations. This change in flow impedance gives rise to a corresponding difference in drag forces exerted on the target by the flowing fluid. Those forces, or displacements associated with them, are measured to determine the rate of fluid flow. In some cases the target may be a vane attached to a shaft rotated by a motor. In others, the target may be a vane structure attached to a post in a flexible fashion so that it can be oscillated transverse to the flow direction by fixed electromagnets acting on a permanent magnet portion of the vane structure.

Description:
FIELD OF THE INVENTION 
     This invention relates to apparatus for determining the rate of flow of a fluid by sensing the force exerted by the fluid on a moveable element in the flow stream. 
     BACKGROUND INFORMATION 
     Target meters are commercially available to measure the flow rates of fluids. They are generally relatively simple, low cost devices consisting basically of a target, such as a disc, suspended in the flow stream by a rod that is attached to a force transducer. The force or torque arising from flowing fluid impacting the disc is sensed by the transducer, which provides an electrical output signal responsive to the flow rate of the fluid. These meters provide typical accuracies specified as several percent of span over a 10 to 1 flow range. The relatively poor accuracy of these meters at the lower flow rates limits their applications, and it is an object of this invention to improve their accuracy. 
     BRIEF SUMMARY OF THE INVENTION 
     The above and other objects are attained by target meters in accordance with various preferred embodiments of the present invention. Preferred embodiments of the present invention use a target in a flowing fluid where the orientation of the target to the direction of flow of the fluid can be selectively changed between two or more orientations, and where the target provides a different flow impedance in each of the orientations. This change in flow impedance gives rise to a corresponding difference in drag forces exerted on the target by the flowing fluid. Those forces, or the displacement associated with them are measured to determine the rate of fluid flow. 
     In some embodiments, the target orientation is changed by continuous rotation and electrical signals responsive to the variable drag on that target are processed to provide a measurement of fluid flow rate. In one preferred embodiment, the target is in the form of a rigid vane, extending radially outwards from a shaft which is attached to a transducer and rotated in the fluid flow stream. The drag forces imposed on the vane by the stream cyclically vary from a maximum to a minimum and are converted by the transducer into corresponding electrical signals. The magnitude of only the variational component of the electrical signal is used to determine flow rate. The transducer signals, which may form part of its output signal as may be necessitated by its power requirements, either AC or DC, or friction related loads, for example, are not used. By this means, the flow sensing error and in particular transducer zero drift relating to the conversion of target drag to a flow rate signal at low flow rates, is reduced thereby enabling the meter to be effectively used at lower flow rates. 
     In another preferred embodiment the angular orientation of a vane in the fluid flow stream is cyclically oscillated in alternate directions whereby the corresponding variation in drag forces are used by the transducer to provide flow rate responsive signals as in the first embodiment. 
     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 or the invention, my be provided by some embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic presentation of the principle components of a flow meter in accordance the present invention. 
     FIG. 2 is a side cross sectional view of meter of FIG. 1 showing greater component detail pertaining to a preferred embodiment of the present invention. 
     FIG. 3 is a side view of the vane and transducer assembly of a preferred embodiment of the present invention. 
     FIG. 4 is a bottom view section of the configuration of FIG. 3 along lines  4 — 4 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 schematically depicts some the components of a flow meter  10 , comprising a transducer  12 , a shaft  14 , and a target or vane  18  that the transducer can move angularly as indicated by the arrow  28 . A fluid  20  is in contact with and flows past the target  18  in a flow direction indicated by arrows  22 . The transducer  12  is powered by an appropriate power supply  24  and provides a signal representative of its mechanical loading to a signal processor  16 . The power supply  24  and the signal processor  16  may be integral to or remotely located from meter  10  and incorporate functions and techniques well established in the flow measurement art. 
     During operation of some embodiments of the invention, the transducer  12  provides a rotary motion through the shaft  14  to the vane  18 . As the vane  18  rotates, its angular orientation with respect to the direction of fluid flow  22  changes. For half of a complete rotation a fluid flow acts to aid the rotation of the vane  18 . For the other half of the rotation the fluid flow acts to impede the vane&#39;s motion. In this embodiment the transducer  12  provides both the rotary motion to the vane  18 , and an electrical signal representing the torque load it is accommodating. When the fluid  20  flows, the transducer  12  supplies an electrical signal varying in magnitude according to the variable loading experienced by vane  18  to the signal processor  16 . In the signal processor  16 , this varying component of the signal, being an AC signal, is extracted from any non-varying portion of the transducer signal with a high pass filter (not shown) and is then magnitude detected to provide a signal responsive to the fluid flow rate. This signal is then further processed for linearization, temperature and other factors as required for the application. 
     On the other hand, when the fluid  20  is stationary its loading on the vane  18  is constant and therefore the transducer  12  output signal will not vary with angle during the course of a complete rotation. Transducer-related signal drifts affected by factors such as friction, temperature and time tend to change relatively slowly with respect to the rotational rate of a vane  18  and will be eliminated by the high pass filter in the signal processor  16 . As a result, the output signal from the signal processor  16  at zero flow rate is more stable than that of prior art target meters which utilize transducer signals from an essentially fixed target. With well defined and stable zero and span calibration points, the meter  10  is therefore useful over a broader operating range of flow rates and may be specified for providing flow measurement accuracies as a percentage of flow rate rather than of span, as is typical of the prior art meters. 
     The transducer  12  is conveniently configured as a single device providing both the rotary motion and sensing the drag effects of the fluid load on the vane  18 . Alternately, the transducer  12  may have separate components for each function. Furthermore, a complete three hundred sixty degree rotation of the vane  18  is not necessary for it to experience a variable loading responsive to fluid flow. For example, the vane  18  could be oscillated by some acute angle—e.g., thirty degrees—about a central position in which it was in alignment with the flow direction  22  in order to sense fluid loading effects. A relatively large vane could also be used in an arrangement in which its rotational angle with respect to the fluid flow direction  22  is controlled to provide a relatively constant flow induced drag. For example, a vane  18  could be angled at thirty degrees with respect to the flow direction  22  at low fluid flow rates and at a lesser angle of, say, five degrees at a high fluid flow rate in order to produce the same drag as was experienced at the low flow rate. This has the effect of increasing the measurement range of the meter and improving its linearity. The oscillating vane  18  may, in any case, be driven in both directions from the central position. 
     FIG. 2 depicts one embodiment of the flow meter  10 . In this case the output from a motor  30  drives a gearbox  32  to rotate a drive magnet  34  housed within a protective barrier  36  used to seal the drive components off from wetted portions of the apparatus. On the other side of barrier  36 , a slave magnet  38  is attached by means of a shaft  14  supported by bearings  40 , to a vane  18 . A suitable housing  48  encloses the meter components and mechanically couples the apparatus to a pipe or other flow passage  46  in which fluid  20  flows. Although the examples described herein are conveniently arranged to address the flow of fluid in a stationary pipe to which a sensing apparatus is fixedly attached, those skilled in the art will recognize that the same discussion applies to a variety of measurements of flow relative to a measurement location at which a flow target support is disposed. Such measurements embrace, but are not limited to, measurement of flow in open channels; measurement of water currents flowing past an inertially fixed structure, such as a dock; and flow of water past the hull of moving ship, in which case the flow measured is that of the ship relative to the water. 
     In the embodiment depicted in FIG. 2, a preferred motor  30  has operating power requirements that vary over a wide range in response to load changes. An example of such a motor is the type 1616E018ST manufactured by MicroMo Electronics, Inc. When used with a 10 volt power supply this motor requires only 4 milliamperes when unloaded but over 40 milliamperes fully loaded. The motor  30  drives a gear train  32  which increases its output torque and reduces its rotational speed. This causes parasitic motor generated signals, such as commutation ripple, to be much higher in frequency than the rotational speed of the vane  18  and thus allows them to be easily filtered out. A MicroMo gear train type 16AK with an 11.8:1 ratio is for example, attached to the above motor rotates the drive magnet  34  which is magnetically coupled through protective barrier  36  to the slave magnet  38 . The slave magnet  38  is coupled by the shaft  14  to rotate the vane  18 . In this embodiment the current supplied to the motor is representative of the shaft load and therefore of the flow-induced drag. Those skilled in the arts will recognized that one could alternately select a motor operated from a constant current source that produced a voltage variation responsive to load changes. Constant current operation is also advantageously used if the vane  18  is made to oscillate so that the magnitude of its deflection is responsive to loading effect of the fluid flow. 
     The rotational rate of vane  18  is typically slow and may range from a fraction of a turn per second to several turns per second. The off-center-of-gravity load represented by the vane  18  may be balanced by a compensating weight either on the shaft  14  or on the slave magnet  38 . A small amount of mechanical vibration is normally present in any flow meter application so that the slave magnet  38  will tend to, on the average, be only slightly affected by mechanical friction and will smoothly follow the movement of the drive magnet  34 , thus resulting in a precise transfer of torque between the gear train  32  and vane  18  at both high and low fluid flow rates. Because the variable torque loading on the motor  30  is transformed by the motor into a flow rate responsive signal, the variation in angular displacement between the drive and slave magnets  34  and  38 , which occurs at different angular locations of the vane  18  does not seriously affect the precision of the flow rate measurement. As a result of the above, the meter  10  may be rugged and incorporate relatively large bearings  40  for long life and the ability to withstand the impacts which may be present, for example, during startup conditions when a liquid line is not full, or a steam line when solids or slugs of condensate are circulated. 
     The motor  30  will tend to have a higher shaft rotational rate when torque due to the flowing fluid is aiding the rotation and a lower shaft rotational rate when the flow is opposing the rotation. The resulting speed change, which reduces the dynamic range of the motor and may add further nonlinearities to the meter response, may be minimized by operating the motor  30  at a regulated constant speed. However, if the motor  30  speed is allowed to vary with vane loading the time differential between the rotational half cycle in which the vane torque aids the rotation and the rotational half cycle when that torque is in opposition can also be used to produce a signal responsive to flow rate. 
     Although the transducer  12  is depicted in FIGS. 1 and 2 as being connected to the vane  18  by means of a shaft  14 , the transducer may also be located close to the vane  18  and may even directly engage it in some embodiments of the invention. An example of such an arrangement is illustrated in the vane and transducer assembly of FIGS. 3 and 4, which comprise a preferred embodiment of the present invention. FIG. 3 is a side view of such an assembly while FIG. 4 shows a central cross section taken as shown by the double-headed arrow  4 — 4 . In this embodiment both a streamlined wetted transducer enclosure  72  and a vane  18  are extend radially outward from a non-rotating post  52 . The vane  18 , in this embodiment, is attached to the post  52  in a compliant fashion in order to permit angular oscillations of the vane as indicated by the double headed arrow  74  in FIG.  4 . The compliant attachment can take many forms, including the depicted combination of ell-shaped axle members  54  and bearings  56 , by springs, or by directly attaching a vane made of a flexible sheet of material to the shaft  52 . Moreover, the post  52  may readily be configured as a bluff body so as to efficiently produce a street of Karman vortices that would affect the vane  18 . 
     In the preferred embodiment depicted in FIGS. 3 and 4 the transducer  12  comprises a magnet  62  attached to the vane  18  that cooperates with two electromagnets  64 ,  66  supported by the wetted enclosure  72  and energized to produce magnetic flux of opposing polarities in the space between them. These electromagnets may lie on the same axis, as depicted, or may be tilted away from that axis so as to be approximately tangent to the arc along which the permanent magnet  62  moves when the vane oscillates. 
     When the electromagnets  64  and  66  are energized to provide magnetic fields with the polarities indicated in FIG. 4, the permanent magnet  62  is attracted to move up further into the magnet  64  at the same time it is being pushed out from magnet  66  below. When the electrical current through electromagnets  64  and  66  is reversed, the polarities of their magnetic fields also reverse and the magnetic forces act on the permanent magnet  62  to move it in the opposite direction. The vane  18 , being fixedly attached to the permanent magnet  62 , will oscillate as indicated by the double headed arrow  74 . These movements provide the angular displacement of vane  18  required for implementing the present invention. Those skilled in the art will recognize that other electromagnetic transducers could be used in place of the preferred embodiment depicted in the drawing. Alternate versions include, but are not limited to, one comprising a ferromagnetic portion of the vane, which may comprise a permanent magnet or other piece of metal attached thereto, arranged to cooperate with one or more windings fixedly attached to the enclosure in order to oscillate the vane. 
     If the permanent magnet  62  is of the alnico, samarium-cobalt or neodymium-iron-boron types, its presence within the magnetic fields of the electromagnets  64  and  66  substantially increases their electrical losses at high frequencies, for example several hundred kilohertz. Ferrite magnets, depending on their composition, may have a similar characteristic or may reduce the high frequency losses. The electrical signals used to produce the magnetic fields that operate to oscillate the vane are of much lower frequency, so that the magnetic generating and loss sensing signals can be easily separated by filtering. These losses are responsive to the penetration depth of the permanent magnet  62  into the electromagnets  64  and  66 . By detecting these losses, a means is provided for determining the mechanical displacement of permanent magnet  62  and therefore the angular displacement of the vane  18 . This is useful in a meter operational mode in which the forces producing the vane&#39;s angular deflection are constant, and the vane&#39;s deflection angle is responsive to the fluid induced drag forces used for determining fluid flow rate, or where a specific vane angular displacement is maintained and the forces required to provide that displacement are used to determine fluid flow rate. A regulated constant current through the electromagnets is a convenient way of providing precision control of the forces needed to produce the angular displacement and, just as in the case with the rotated vane, a transducer having a directly driven vane may be operated in several modes. 
     Alternately, the apparatus depicted in FIGS. 3 and 4 may be used in sensing arrangements in which fluid flow variations, rather than electromagnetic forces, drive the vane  18 . These flow variations may be due, for example, to fluidic oscillators or vortex shedding, as may be the case if the post  52  is configured as a bluff body. Electrical signals responsive to angular movements of the vane  18  are then generated by motion of the permanent magnet  62  within the electromagnets  64  and  66  and can be to be electrically processed to become output signals representative of fluid flow rate. High frequency signal processing may also be used to determine the angular translation of the vane  18 , if desired.