Abstract:
An electromagnetic flowmeter includes at least one electromagnet coil arranged outside a liquid flow passage for generating an electromotive force in the liquid flow, and at least one pair of electrodes adjustably extending within the liquid flow for generating a velocity component signal. Preferably, two collinearly arranged coils are provided externally on opposite sides of the liquid flow passage, and a first set of two orthogonally-arranged pairs of coplanar electrodes are mounted on an adjustable probe strut that extends into the liquid flow. For greater accuracy, a Hall-effect device is provided on the probe. Additional sets of electrode pairs may be secured to the strut in planes parallel to the first electrode set. A second pair of collinear coils may be mounted externally of the liquid passage and orthogonally relative to the first coil pair, the two pairs of coils being alternately energized to produce three orthogonally arranged velocity components.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    An electromagnetic flowmeter includes at least one electromagnetic coil arranged outside a liquid flow passage for generating an electromotive force in the liquid flow, and at least one pair of electrodes adjustably extending into the liquid flow for generating a velocity component signal. Preferably, two collinearly arranged coils are provided externally on opposite sides of the liquid passage, and a first set of two orthogonally-arranged pairs of coplanar electrodes are mounted on an adjustable probe strut that extends into the liquid flow. For greater accuracy, a Hall-effect device may be provided on the probe. Additional sets of electrode pairs may be secured to the strut in planes parallel to the first electrode set. Three orthogonally-arranged velocity components are achieved by using a second pair of collinear coils mounted externally of the liquid passage in orthogonally displaced relation relative to the first pair of coils. 
         [0003]    2. Description of Related Art 
         [0004]    The Faraday Law of electromagnetic induction has been applied to water flow measuring devices for nearly 100 years. As early as 1910, a device for measuring the speed of a moving vessel was patented. Since then, a multitude of other applications and devices utilizing the Faraday Law have followed. Devices were made for measuring the volumetric flow in both open channels and full pipes where the magnetic field was applied to a large portion of the flow cross section and the induced electromotive force (emf) was averaged over an equally large portion of the cross section. Additionally, probe type velocity sensors were invented where the magnetic field was localized to a small area (generally the size of the probe) and the sensing electrodes were attached to the surface of the probe allowing for the measurement of water velocities in the vicinity of the probe. 
         [0005]    As described in the following patents, the construction of probe type velocity sensors were focused on trying to make the best velocity measurement while keeping the magnet and electrode assembly from adversely affecting the flow around the sensor thereby affecting the velocity measurement. The Olson U.S. Pat. No. 3,693,440 describes ‘an “open” cage-like housing for the magnetic field coils and electrodes that practically eliminates any physical interference with the water flow. Additional patents, such as Cushing U.S. Pat. No. 4,089,218, Marsh U.S. Pat. Nos. 4,459,858 and 4,688,432, all describe various probe type electromagnetic sensors. In all of the prior art, the electrodes and the magnetic coils were rigidly fixed in relationship to each other. Even in the Marsh U.S. Pat. Nos. 5,398,552 and 6,598,487, where the magnet assembly and the electrode assembly were separable, the magnet and electrodes were secured at a known relationship to each other when placed together for the flow measurement. 
         [0006]    The present invention described herein differs from the prior art in that the electrodes are separate from the magnetic coil assembly, and the magnetic coil assembly is placed at some distance from the electrodes, preferably out of the path of the flowing water. 
       SUMMARY OF THE INVENTION 
       [0007]    Accordingly, a primary object of the present invention is to provide an electromagnetic flowmeter including at least one electromagnet coil arranged outside a liquid flow passage for generating an electromotive force in the liquid flow, and at least one pair of electrodes adjustably extending into the liquid flow for generating a velocity component signal. The liquid flow passage may comprise either an open-topped channel or a closed conduit. 
         [0008]    According to a more specific object of the invention, two collinearly arranged coils are provided externally on opposite sides of the liquid passage, and a first set of two orthogonally-arranged pairs of coplanar electrodes are mounted on an adjustable probe strut that extends into the liquid flow. For greater accuracy, a Hall-effect device may be provided on the probe. Additional sets of electrode pairs may be secured to the strut in planes parallel to the first electrode set. 
         [0009]    In a further embodiment of the invention, three orthogonally-arranged velocity components are achieved by using a second pair of collinear coils mounted externally of the liquid passage in orthogonally displaced relation relative to the first pair of coils. These three components are combined to provide an accurate velocity measurement of the liquid flowing in the passage. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    Other object and advantages of the invention will become apparent from a study of the following specification, when viewed in the light of the accompanying drawings, in which: 
           [0011]      FIG. 1  is schematic representation of the use of a single coil for generating an electromotive force in a liquid flowing in an open-topped channel, and  FIGS. 2-4  illustrated various arrangements of a pair of electrodes relative to the liquid flow of  FIG. 1 ; 
           [0012]      FIG. 5  is a schematic representation of the flux pattern produced in the liquid flow in an open-topped channel by a single coil arranged above the liquid flow, 
           [0013]      FIG. 6  shows the flux pattern when a pair of coils are arranged collinearly above and below the channel; 
           [0014]      FIG. 7  illustrates the flux pattern produced by a pair of coils arranged collinearly on opposite sides of the channel 
           [0015]      FIG. 8  illustrates the flux paths produced by an alternately energized combination of the coil arrangements of  FIGS. 6 and 7 ; 
           [0016]      FIG. 9  illustrates a possible electrode arrangement for the single coil arrangement of  FIG. 5 , and  FIGS. 10 and 11 , illustrate electrode arrangements on a probe introduced into the liquid flows of the arrangements of  FIGS. 6 and 7 , respectively; 
           [0017]      FIG. 12  is a perspective diagrammatic illustration of a first probe arrangement in accordance with the present invention, and 
           [0018]      FIG. 13  illustrates a first modification of the probe arrangement of  FIG. 12  including a Hall effect sensor; 
           [0019]      FIG. 14  is a second embodiment of the probe arrangement of  FIG. 12 ; 
           [0020]      FIG. 15  is a schematic representation of the orthogonal arrangement of two pair coils relative to an open topped channel; 
           [0021]      FIGS. 16 and 17  are timing curves illustrating the alternate energization of the coil pairs of  FIG. 15 ; 
           [0022]      FIGS. 18 and 19  are sampling curves illustrating the sampling signals taken from the timing curves  16  and  17 , respectively; 
           [0023]      FIGS. 20-22  are schematic diagrams of the processing of the signals of  FIGS. 16-19  to produce the three orthogonally-arranged velocity output signals; 
           [0024]      FIG. 23  is a perspective view illustrating one laboratory environment in which the invention has utility; and 
           [0025]      FIGS. 24-26  are schematic illustrations of the operation of the invention in a laboratory environment. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0026]    Referring first more particularly to  FIG. 1 , it will be seen that a magnetic field β is generated by an electromagnetic coil C 1  placed above an open-topped channel  2  having a transversely curved bottom wall, which coil is supplied with electrical energy from voltage source  4 . The fluid volume that is energized by this coil will have an electromotive force e throughout this volume that is a product of the magnetic field strength and the velocity of the flowing water. In  FIG. 2 , a pair of detection electrodes  6  and  8  are provided that are separated by the distance d 1 . The voltage detected by this separation is determined largely by the electromotive forces contained in the shaded area.  FIG. 3  shows a similar configuration, but the distance d 2  between the electrodes is less, and the volume of measurement is less.  FIG. 4  shows a similar configuration, but with a greater separation distance d 3 . Here, the measured volume is greater. In addition to the measured volumes being defined by the distance between the detection electrodes, the magnitude of the signal typically increases with electrode separation since the individual electromotive forces add together when all of the flow is in the same direction. 
         [0027]    Referring now to  FIG. 5 , the first coil C 1  is placed above the liquid stream S in an open-topped channel  4 , and electrical current passing through this coil generates a magnetic field β that extends both upwardly and downwardly. The downward portion of the field extends into the liquid stream S flowing in the open-topped channel  4 . The interaction of the magnetic field and the flowing stream causes an electromotive force (emf) e 1  to be established throughout the energized portion of the flow channel. A shortcoming of using only a single coil is that the magnetic flux lines β do not remain normal to the direction of the flow, but are more curved. 
         [0028]    In the embodiment of  FIG. 6 , in addition to the first coil C 1  placed above the flow, a second coil C 2  is placed below the flow. The use of a second coil significantly corrects the shortcoming of a single coil in that the field direction between the two coils is substantially perpendicular to the flow channel. Electrical current passing through the two coils generates an additive vertical magnetic field β x,y  that extends both upwardly and downwardly. The downward portion of the field extends into the flowing stream to produce a transverse electromotive force e 2  throughout the energized portion of the flow channel. 
         [0029]    In the modification of  FIG. 7 , a pair of collinear coils C 3  and C 4  are arranged horizontally externally of the passage  4  to measure the vertical component of flow. In this configuration, the additive magnetic field β z  created by coils C 3  and C 4  is generally horizontal and parallel to the bottom of the flow channel, and normal to vertical axis. The interaction of the magnetic field and any vertical flow causes a horizontal emf e 3 , to be established throughout the energized portion of the flow channel. The horizontal direction of this emf is parallel to the channel bottom and aligned longitudinally in an “upstream/downstream” direction of fluid flow. 
         [0030]    Referring now to  FIG. 8 , a combination of the coil arrangements of  FIGS. 6 and 7  is shown. By alternately first energizing the coil pair C 1  and C 2  to produce a vertical field β x,y  and then subsequently energizing coils C 3  and C 4  to produce the horizontal field β z , the energized volume is capable of measuring all three components of the flow velocity vector. The timing means of achieving is described in greater detail below with reference to  FIGS. 15-22 . 
         [0031]    Referring now to the embodiment of  FIG. 9 , a single magnet coil C 1  is placed above the flow in an open channel  18  with the electrodes  20  and  22  either just touching the top surface of the flowing fluid or lowered to other locations within the flow. In  FIG. 10 , two coils are provided, a first one C 1  being arranged above the water surface, and a second coil C 2  being arranged below the channel bottom wall. Two pairs of orthogonally arranged electrodes X 1 , X 2  and Y 1 , Y 2  contained in a common horizontal plane are placed at the lower end of a small probe P 1  that is adjustably suspended by position adjusting means  24  from above into the flow. In this configuration both the “X” and “Y” horizontal components of the flow velocity can be obtained. The velocity at any point throughout the hydraulic model can be measured by simply placing the sensor in the desired location. Note that the obstruction to flow caused by the electrode structure is very minimal as compared to what it would be if the magnet were part of the probe itself. Additionally, different probe tips could be used to vary the volume size that the electrodes detect. The greater the electrodes are separated from each other, the greater the spatial volume where the velocity is being measured. 
         [0032]      FIG. 11  shows an arrangement where all three components of the velocity vector can be measured. In this arrangement, the magnetic field is periodically alternately switched from being in a vertical orientation to one of being in the horizontal orientation, use being made of two pairs of coils C 1 , C 2 , and C 3 , C 4  having axes that are orthogonally arranged relative to each other. The probe P is suspended as before by the position adjusting means  26 . 
         [0033]    To detect the electromotive forces (emfs) generated by the local velocities in the presence of the magnetic field, various configurations of electrodes can provide the user with a means of measuring one, two and three axes of local velocities throughout the energized volume as well as being able to sum these velocities over a larger volume. 
         [0034]    In  FIG. 12 , a sensor probe structure is illustrated that provides for the measurement of the X and Y horizontal components of a velocity vector. Two orthogonally arranged electrode pairs, X 1 , X 2 , and Y 1 , Y 2 , contained in a common horizontal plane are placed at the ends of four tubular arms  30  that extend radially-outwardly from the lower end of a vertical tubular mounting strut  32 . Preferably, the Y axis of the electrode arrangement extends longitudinally parallel with the direction of fluid flow. The tubular arms and the mounting strut are formed from a suitable electrically insulating synthetic plastic material. Attached to each electrode is a conductor that extends upwardly within the probe body so as to emerge at the top of the mounting strut. These signal wires are attached to the electronic detection means  36 . The probe is placed within the magnetically energized volume in such a position that the four electrodes are contained adjacent the horizontal plane of the velocity components. This plane will also be normal to the direction of the additive magnetic field produced by the coils C 1  and C 2 . 
         [0035]    The magnitude of the signal present at the electrodes is directly proportional to the speed of the water and the strength of the magnetic field. Although the magnetic field strength is relatively uniform throughout the volume between any two coils, the variations may be such that desired accuracy of measurement cannot be achieved. To achieve higher accuracy, a small magnetic field detector  33  can be incorporated within the sensor as shown in  FIG. 13 . Typically such a sensor would be a “Hall Effect” device. The strength of the magnetic field is then used to better calibrate the instrument. 
         [0036]    In the sensor probe of  FIG. 14 , the measurement of multiple points of the x and y components of a velocity vector is achieved through the use of a stacked multiple electrode array. The first set of coplanar electrode pairs, X 1  and X 2 , and Y 1  and Y 2 , are respectively placed at the ends of the radially-outwardly extending arms  36  of an X-shaped probe, and directly above those pairs are arranged a second set of coplanar electrode pairs, X 11  and X 12 , and Y 11  and Y 12  respectively supported by the radial arms  38 . Similarly, above those pairs are arranged a third set of coplanar electrode pairs, X 21  and X 22 , and Y 21  and Y 22  supported by radial arms  40 . Attached to each electrode is a signal wire that is placed within the probe body so as to emerge at the top of the tubular mounting strut  50 . These signal wires are attached to the electronic signal detection means  52 . The probe is placed within the magnetically energized volume of fluid flow in such a position that the X-shaped sensor electrodes are contained in the same vertically spaced horizontal planes as the velocity vector components, respectively. These planes will also be normal to the direction of the magnetic field. 
         [0037]      FIGS. 15-19  illustrate the timing circuitry for achieving 3-axis sensor operation. In  FIG. 15 , two pairs of coils C 21 , C 22  and C 23 , C 24  are alternately energized to create magnetic fields that are orthogonal to each other. The coil pair C 23 , C 24  is energized as shown in  FIG. 16 , and the coil pair C 21  and C 22  is energized as shown in  FIG. 17 . During time period t 1 , the current is passed through the coils C 22  and C 23  to cause the magnetic field to be directed from left to right in  FIG. 15 . During this same period, the coils C 21  and C 22  are not energized. During the next period, t 2 , the magnetic field of coils C 23  and C 24  is zero, and the magnetic field of coils C 21  and C 22  is directed vertically from top to bottom ( FIG. 15 ). During time period t 3 , the current is passed through the coils C 23  and C 24  to cause the magnetic field to be directed from right to left in  FIG. 25 . During this same period, the coils C 21  and C 22  are not energized. During the next period, t 4 , the magnetic field of coils C 23  and C 24  is zero, and the magnetic field of coils C 21  and C 22  is directed vertically from bottom to top. This sequence continues as shown by  FIGS. 16 and 17 . 
         [0038]    Concurrent with the illustrated magnetic drive sequence, there are additional waveforms that are synchronized to these magnetic drivers. Shown in  FIG. 18  is the data sampling pulse for obtaining flow data when coils C 23  and C 24  are energized, and  FIG. 19  illustrates the data sampling pulse for obtaining flow data when coils C 21  and C 22  are energized. These data sampling pulses are used in conjunction with the flow signal electronic circuitry shown in  FIGS. 20-22 . The electromagnetic forces present between electrodes X 1  and X 2  are presented to differential amplifier Ax, sampled by the sampling signal S 1  applied to signal modifying means  60 , and then processed by signal processing means  62  to produce the first horizontal flow velocity component X. In a similar manner, the electromotive forces present between electrodes Y 1 , Y 2  are presented to differential amplifier Ay for modification by the sampling signal S 1  applied to signal modifying means  64  and processing circuit  66  to produce the second horizontal flow velocity component Y. Finally, the electromotive forces between the electrodes Z 1 , Z 2  are sampled by the sampling signal S 2  applied to the signal modifying means  68  and processing circuit  70  to produce the vertical flow velocity component Z. 
         [0039]      FIG. 23  illustrated a typical hydraulic laboratory flume, wherein water is circulated in a closed loop and is observed through transparent windows contained in a rectangular section of conduit. In this particular application, two sets of coils C 50 , C 51  and C 52 , C 53  ( FIGS. 24-26 ) are placed in the corners of the flume so as to energize the volume contained between the windows with a switched field that will allow for a three-dimensional detection of the velocities within the fluid stream. Substantially unobstructed probes such as those shown in  FIGS. 12-14  could be used to measure the desired velocities by moving the probe(s) anywhere within the energized volume. 
         [0040]    The present invention has been described in connection with an open-topped channel passage for the liquid flow; however, it is apparent that the passage could be a closed conduit as well. 
         [0041]    While in accordance with the provisions of the patent Statutes the preferred forms and embodiments of the invention have been illustrated and described, it will be apparent to those skilled in the art that changes may be made without deviating from the invention described above.