Abstract:
The magnetic circuit arrangement, which is preferably used in a fluid-measuring transducer, comprises at least one coil which is traversed in operation by a current. It further comprises two armatures that are fixed to two flow tubes vibrating in phase opposition. The coil is float-mounted by means of a holder to a double flow tube configuration formed by the flow tubes. The armatures are shaped and aligned relative to each other in such a manner that magnetic fields produced by means of the magnetic circuit arrangement are essentially concentrated within the magnetic circuit arrangement, whereby the latter is also largely insensitive to extraneous magnetic fields. The magnetic circuit arrangement is marked by a long service life and, particularly if the transducer is used for fluids with high and/or varying temperatures, by constantly high accuracy in operation.

Description:
This application is based on Provisional Application, filed Apr. 27, 2001, as application No. 60/286,546. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to a magnetic circuit arrangement for use in a vibratory transducer and particularly in a Coriolis mass flow sensor. 
     BACKGROUND OF THE INVENTION 
     To determine the mass flow rate a fluid flowing in a pipe and particularly of a liquid, use is frequently made of Coriolis mass flowmeters, which, as is well known, induce Coriolis forces in the fluid and derive therefrom a measurement signal representative of mass flow rate by means of a vibratory transducer and control and evaluation electronics connected thereto. 
     Such Coriolis mass flowmeters have been known and in industrial use for a long time. For example, U.S. Pat. Nos. 4,756,198, 4,801,897, 5,048,350, 5,301,557, 5,349,872, 5,394,758, 5,796,011, and 6,138,517 as well as EP-A 803 713 disclose Coriolis mass flowmeters incorporating a transducer which comprises:
         a double flow tube configuration communicating with the pipe and comprising   a first flow tube, which vibrates in operation, and   a second flow tube, which vibrates in operation,
           the first and second flow tubes vibrating in phase opposition;   
           a vibration exciter for driving the flow tubes; and   vibration sensors for detecting inlet-side and outlet-side vibrations of the flow tubes and for producing at least one electric sensor signal influenced by the mass flow rate,   the vibration exciter and/or the vibration sensors having at least one magnetic circuit arrangement for converting electric into mechanical energy and/or vice versa which comprises:   at least one coil which is traversed at least temporarily by a current and penetrated at least temporarily by a magnetic field;   a first armature, fixed to the first vibrating flow tube of the transducer;   a second armature, fixed to the second vibrating flow tube of the transducer; and   a holder for the coil.       

     As is well known, bent or straight flow tubes of such transducers, if excited in the so-called useful mode into flexural vibrations according to a first natural vibration mode shape, can cause Coriolis forces in the fluid passing therethrough. These, in turn, result in coplanar flexural vibrations being superimposed on the excited flexural vibrations of the useful mode in the so-called Coriolis mode, so that the vibrations detected by the vibration sensors at the inlet and outlet ends have a measurable phase difference, which is also dependent on the mass flow rate of the fluid. 
     In operation, the flow tubes of the transducer are usually excited at an instantaneous resonance frequency of the first natural vibration mode, particularly with the vibration amplitude maintained constant. As this resonance frequency is also dependent on the instantaneous density of the fluid in particular, commercially available Coriolis mass flowmeters can also be used to measure the density of moving fluids. 
     In magnetic circuit arrangements as disclosed in U.S. Pat. No. 5,048,350, both the armature and the associated coil are fixed directly to the double flow tube configuration, so that in operation, both, following the motions of the associated flow tubes, are practically permanently accelerated. The resulting inertial forces, which affect particularly the coil, may lie in ranges far above 10 G (=weight). Even inertial forces up to 30 G are nothing unusual. Because of these high mechanical stresses, the coils in such magnetic circuit arrangements, and particularly their windings, must be highly loadable to ensure a long life of the vibration exciters, particularly a high number of vibration cycles, with unchanged accuracy in operation. 
     In magnetic circuit arrangements as disclosed in U.S. Pat. Nos. 4,756,198, 5,349,872, or 6,138,517, for example, such mechanical stress on coils is avoided by holding each of the latter in a holding structure that is at rest relative to the vibrating flow tubes, such as a support plate, a meter housing, or a support frame flexibly attached directly to the flow tubes, at a nearly constant distance from a centroidal axis, here a vertical axis, of the double flow tube arrangement. 
     It turned out, however, that although the above-described mechanical stresses can thus be virtually completely eliminated, the accuracy of such a magnetic circuit arrangement may be seriously affected by, particularly temperature-induced, shifts between the holding structure and the double flow tube configuration as occur, for example, in applications for fluids with widely varying temperatures. Because of the resulting different expansion of the holding structure and the double flow t tube configuration, which are neutralizable only limitedly, the rest positions of armature and coil change relative to each other. 
     While in the magnetic circuit arrangement according to U.S. Pat. No. 6,138,517, mainly a very great temperature difference, and thus a very great expansion difference, may occur between holding structure and double flow tube configuration, in the magnetic circuit arrangements described in U.S. Pat. No. 5,349,872, whose magnetic fields, particularly in the areas of the armatures, are highly inhomogeneous, even slight disturbances may result in considerable inaccuracy. As a result, e.g., if the arrangement is used as a vibration sensor, the sensor signals may have a very poor signal-to-noise ratio and/or exhibit very high harmonic distortion. Furthermore, the magnetic field of a magnetic circuit arrangement as disclosed in U.S. Pat. No. 5,349,872 may act through a very large region, i.e., it may also penetrate adjacent components of the transducer and particularly other such magnetic circuit arrangements and/or the flow tubes with the fluid passing therethrough, thus inducing interference voltages, for example. Further disadvantages of such a magnetic circuit arrangement are discussed in detail in U.S. Pat. No. 6,138,517, for example. 
     To ensure high accuracy in operation despite those temperature-induced interfering effects on the aforementioned magnetic circuit arrangements, the large amount of technical complexity required in such mass flowmeters to compensate for temperature-dependent interferences has to be increased even further. 
     It is therefore an object of the invention to provide a magnetic circuit arrangement, particularly an arrangement for use in a fluid-measuring vibratory transducer, which has a long service life and particularly a high number of-vibration cycles, and which, particularly if the transducer is used for fluids with high and/or varying temperatures, has constantly high accuracy in operation. In addition, the magnetic circuit arrangement according to the invention is to be insensitive to extraneous magnetic fields. 
     SUMMARY OF THE INVENTION 
     To attain the object, the invention provides a magnetic circuit arrangement for converting electric into mechanical energy and/or vice versa which comprises:
         at least a first coil, traversed in operation by a current;   a first armature, fixed to a first vibrating flow tube of a transducer;   a second armature, fixed to a second vibrating flow tube of the transducer; and   a holder for the first coil, fixed to the first and second flow tubes,   the two armatures being shaped and aligned relative to each other in such a way that magnetic fields produced by means of the magnetic circuit arrangement are essentially concentrated within the magnetic circuit arrangement, and   the first coil and at least the first armature interacting via a first magnetic field.       

     In a first preferred embodiment of the invention, the magnetic circuit arrangement comprises a second coil traversed in operation by a current, the second coil and the second armature interacting via a second magnetic field. 
     In a second preferred embodiment of the invention, at least the first armature is shaped and aligned relative to the first coil in such a way that the first magnetic field spreads homogeneously at least at the side of the coil and essentially in alignment with a central axis of the coil. 
     In a third preferred embodiment of the invention, each of the two armatures is cup-shaped. 
     In a fourth preferred embodiment of the invention, the first coil is wound on a first core, and the first core and the first armature are shaped and aligned relative to each other in such a way that the magnetic flux passes through an air gap formed between the two. 
     In a fifth preferred embodiment of the invention, each of the two cores is cup-shaped. 
     In a sixth preferred embodiment of the invention, the holder comprises a support plate for holding the at least first coil, the support plate being float-mounted by means of a resilient first leg, fixed to the first flow tube, and a resilient second leg, fixed to the second flow tube, to a double flow tube configuration formed by the two flow tubes. 
     In a seventh preferred embodiment of the invention, the support plate, extending along the double flow tube configuration, is fixed to the first and second flow tubes at the inlet and outlet ends thereof. 
     In an eighth preferred embodiment of the invention, the transducer is a Coriolis mass flow sensor. 
     In a ninth preferred embodiment of the invention, the magnetic circuit arrangement serves as a vibration exciter for driving the flow tube. 
     In a tenth preferred embodiment of the invention, the magnetic circuit arrangement serves as a vibration sensor for detecting vibrations of the flow tubes. 
     A fundamental idea of the invention is, on the one hand, to design at least one of the magnetic circuit arrangements commonly used in such transducers and particularly in Coriolis mass flow sensors or Coriolis mass flow/density sensors, i.e., the vibration exciter and/or the vibration sensors, in such a way that in operation, their coils remain in a rest position at least relative to a centroidal axis of the double flow tube configuration, particularly to the vertical axis of the latter. On the other hand, the invention is aimed at providing a magnetic circuit arrangement that is largely insensitive to temperature influences and whose magnetic field can be prevented from acting on other components while the arrangement itself is effectively shielded from other magnetic fields. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention and further advantages will become more apparent by reference to the following description of embodiments taken in conjunction with the accompanying drawings. Like reference characters have been used to designate like parts throughout the various figures; reference characters that were already used in preceding figures are not repeated in subsequent figures if this contributes to clarity. In the drawings: 
         FIG. 1  is a perspective view of a first variant of a magnetic circuit arrangement particularly suited for Coriolis-type transducers; 
         FIG. 2  is a part-sectional front view of the magnetic circuit arrangement of  FIG. 1 ; 
         FIG. 3  is a part-sectional front view of a second variant of a magnetic circuit arrangement particularly suited for Coriolis-type transducers; 
         FIG. 4  is a perspective view of the magnetic circuit arrangement of  FIG. 1 , used in a transducer with a double flow tube configuration; and 
         FIGS. 5 and 6  are perspective views of further developments of the magnetic circuit arrangement according to the invention, used in a transducer. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIGS. 1  to  3  show embodiments of a magnetic circuit arrangement for converting electrical energy into mechanical energy and/or, based on the law of electromagnetic induction, for converting mechanical into electrical energy. The magnetic circuit arrangement is particularly suited for use in a Coriolis mass flowmeter or a Coriolis mass flowmeter-densimeter. A corresponding embodiment of a vibratory transducer, wirich responds to the mass flow rate m of a fluid flowing in a pipe (not shown), is shown in FIG.  4 . As is well known, such a mass flow sensor, if used as a physical-to-electrical transducer in a Coriolis mass flowmeter, serves to produce and detect Coriolis forces in the fluid passing therethrough and to convert these forces into useful input signals for subsequent evaluation electronics. 
     To conduct the fluid to be measured, the transducer comprises a double flow tube configuration  21  with a first flow tube  211  and a second flow tube  212 , which is preferably identical in shape to flow tube  211 . As is usual with such transducers, flow tubes  211 ,  212  may be curved singly, e.g., U-shaped, or in the form of a loop; if necessary, however, they may also be straight. 
     Preferably, as shown in  FIG. 4 , flow tubes  211 ,  212  are so aligned relative to each other that an imaginary middle plane between the two tubes, which are preferably parallel to each other, corresponds to a first plane of symmetry of double flow tube configuration  21 . Furthermore, double flow tube configuration  21  is advantageously shaped so as to have a second plane of symmetry that intersects the middle plane E 1 , which also contains the above-mentioned vertical axis, particularly at right angles. 
     Each of the two flow tubes  211 ,  212  ends in an inlet manifold  213  and an outlet manifold  214 . If the meter is installed in the fluid-conducting pipe, inlet manifold  213  and outlet manifold  214  are respectively connected with straight inlet-side and outlet-side sections of the pipe and are therefore preferably aligned with each other and with a longitudinal axis A 1  of double flow tube configuration  21  which joins the two, as is usual with such transducers. If the transducer is to be detachable from the pipe, a first flange  215  and a second flange  216  are preferably formed on inlet manifold  213  and outlet manifold  214 , respectively; if necessary, however, inlet manifold  213  and outlet manifold  214  may also be connected with the pipe directly, e.g., by welding or brazing. 
     In operation, flow tubes  211 ,  212 , as mentioned above, are excited in the useful mode into flexural vibrations, particularly at a natural resonance frequency of an eigenmode, such that they vibrate in phase opposition, as is usual with such transducers. As is well known, the Coriolis forces thus induced in the fluid passing through flow tubes  211 ,  212  cause an additional elastic deformation of the tubes, also referred to as the Coriolis mode, which is superimposed on the excited vibrations of the useful mode and is also dependent on the mass flow rate m to be measured. 
     If necessary, any mechanical stresses caused by the vibrating flow tubes  211 ,  212  in inlet manifold  213  and outlet manifold  214  can be minimized, for example, by mechanically joining the tubes by means of at least a first node plate  217  at the inlet end and at least a second node plate  218  at the outlet end, as is usual with such transducers. 
     To drive flow tubes  211 ,  212 , the transducer comprises at least one vibration exciter  22 . The latter serves to convert electric excitation energy E exc , supplied from control electronics of, e.g., the above-mentioned mass flowmeter, into excitation forces F exc , e.g., pulsating or harmonic excitation forces, which act on flow tubes  211 ,  212  symmetrically, i.e., simultaneously and uniformly, but in opposite directions, thus producing the antiphase vibrations of flow tubes  211 ,  212 . The excitation forces F exc  may be adjusted in amplitude, e.g., by means of a current- and/or voltage-regulator circuit, and in frequency, e.g., by means of a phase-locked loop, in the manner familiar to those skilled in the art, see also U.S. Pat. No. 4,801,897. 
     To detect vibrations of flow tubes  211  and  212 , the transducer comprises an inlet-side first vibration sensor  23  and an outlet-side second vibration sensor  24 , which respond to motions of the tubes, particularly to their lateral deflections, and deliver corresponding first and second vibration signals S 23  and S 24 , respectively. 
     In transducers of the kind described, the magnetic circuit arrangement according to the invention, if used as vibration exciter  22 , may serve to produce the excitation forces F exc  driving the flow tubes  211 ,  212 . Furthermore, the magnetic circuit arrangement, as mentioned above, may be used as vibration sensor  23  or  24  for sensing the motions of flow tubes  211 ,  212  and for generating vibration signal S 23  or S 24 , respectively. 
     To interconvert mechanical and electric energy, the magnetic circuit arrangement comprises at least a first, preferably cylindrical, coil  13 , which is traversed in operation by a current and which is attached to double flow tube configuration  21  by means of a holder  15 . Preferably, a second coil  14 , particularly a coil aligned with coil  13 , is fixed to holder  15 . 
     Furthermore, the magnetic circuit arrangement comprises a first armature  11 , which is fixed to flow tube  211  and which in operation interacts with the current-carrying coil  13  via a first magnetic field B 1 , and a second armature  12 , particularly an armature identical in shape to armature  11 , which is fixed to flow tube  212  and can interact with coil  14  via a second magnetic field B 2 . Magnetic field B 1  may be, for example, an alternating field which is produced by means of coil  13  and on which a steady field produced by means of armature  11  may be modulated; analogously, magnetic field B 2  may be produced by means of coil  14  and armature  12 , for example. 
     The two armatures  11 ,  12  also serve to homogenize magnetic fields produced by the magnetic circuit arrangement, particularly magnetic field B 1 , and magnetic field B 2 , also outside coil  13 , and to concentrate these fields within as narrow a space as possible which lies essentially within the magnetic circuit arrangement itself. Armatures  11 ,  12  also serve to shape and direct the aforementioned magnetic fields in such a manner that they have as high a flux density as possible, particularly a constant flux density, even in air. Therefore, armatures  11 ,  12  are preferably made at least in part of ferromagnetic material, which, as is well known, has a very high permeability and thus concentrates magnetic fields. 
     In a preferred embodiment of the invention, armature  11  also serves to produce a permanent steady component of magnetic field B 1 ; analogously, a permanent steady component of magnetic field B 2  is preferably produced by means of armature  12 . Particularly in that case, armatures  11 ,  12  are made at least in part of hard magnetic, i.e., premagnetizable, material, such as AlNiCo, NyFeB, SmCo, or another rare-earth alloy. It is also possible to use far less expensive free-cutting steel or structural steel as the material for this embodiment of armatures  11 ,  12 . 
     As shown in  FIGS. 1  to  3 , armature  11  is rigidly fixed to flow tube  11  by means of a mounted-on, flexurally stiff first angle piece  11 A, and armature  12  is rigidly fixed to flow tube  212  by means of a mounted-on, flexurally stiff second angle piece  12 A. Angle pieces  11 A and  12 A may be joined to flow tubes  211  and  212 , respectively, by welding or brazing, for example. 
     As shown in  FIG. 1 , for example, coil  13 , and also coil  14  if present, is fixed by means of holder  15  to both flow tubes  211 ,  212 , such that an axis of symmetry of the magnetic circuit arrangement is virtually parallel to middle plane E 1  of double flow tube configuration  21 . Preferably, holder  15  is fixed to flow tube  211  via a first leg  15 A and to flow tube  212  via an essentially identically shaped second leg  15 B. Furthermore, the two, preferably resilient, legs  15 A,  15 B are, preferably rigidly, interconnected at the respective ends remote from double flow tube configuration  21  via a support plate  15 C. Holder  15  may either be a single part, such as a bent stamping, or be of multipart construction. It may be made of the same materials as those used for flow tubes  211 ,  212 , for example. 
     If flow tubes  211 ,  212  vibrate in phase opposition in the manner described above, holder  15  will be deformed, particularly by lateral deflection of legs  15 A,  15 B attached to flow tubes  211 ,  212 , but its symmetry axis will essentially remain in its position relative to middle plane E 1 . Thus, coil  13 , held by support plate  15 C, e.g., via a ridge portion  15 D formed on the latter, is float-mounted to double flow tube configuration  21  and kept at an essentially constant distance from middle plane E 1 . 
     To prevent the vibration mode shape of the vibrating flow tubes  211 ,  212  from being influenced by holder  15 , the latter must be made pliable. To accomplish this, legs  15 A,  15 B, which also vibrate in operation, may be formed from suitably thin sheet-metal strip. 
     In another embodiment of the invention, support plate  15 C, as shown schematically in  FIG. 5  or  6 , is shaped and attached to flow tubes  211 ,  212  in such a way as to extend essentially parallel to the flow tubes and virtually along the entire length of double flow tube configuration  21 . In that case, support plate  15 C is advantageously fastened directly to node plate  217  at the inlet end and to node plate  218  at the outlet end. To the inventors&#39; surprise it turned out that, if the, e.g., thermally induced, expansions of flow tubes  211 ,  212  are parallel to the middle plane E 1 , the holder  15  so fixed can follow these expansions to the point that any relative shift between holder  15  and double flow tube configuration  21  is negligibly small. 
     A particular advantage of this embodiment of the invention is that it eliminates the need to additionally fix holder  15  to double flow tube configuration  21  via legs  15 A,  15 B, cf. FIG.  6 . 
     According to a first variant of the invention, the magnetic circuit arrangement is of the electrodynamic type, i.e., an arrangement in which an electric conductor formed into a loop, e.g., coil  13 , is penetrated, particularly perpendicularly, by a magnetic field produced by at least one permanent magnet, and in which the loop and the permanent magnet are moved relative to each other. To this end, coil  13  is preferably fixed to double flow tube configuration  21  by means of holder  15  in such a way that its central axis A 13  is essentially perpendicular to middle plane E 1 . 
     To homogenize the magnetic field B 1 , B 2  and fix as high a flux density as possible, particularly outside armatures  11 ,  12 , in a preferred embodiment of the first variant of the invention, each of the two armatures  11 ,  12 , as shown schematically in  FIGS. 1 and 2 , has the form of a cup whose bottom has a, preferably hard magnetic, rod formed thereon which is coaxial with the wall of the cup. 
     In another preferred embodiment of the first variant, armatures  11 ,  12 , as is usual with such magnetic circuit arrangements, are preferably made at least in part, i.e., in the region of the above-mentioned wall of the cup, of soft magnetic material, such as ferrite or Corovac. 
     According to a second variant of the invention, the magnetic circuit arrangement is of the electromagnetic type, i.e., an arrangement in which two ferromagnetic bodies movable relative to each other are so arranged relative to each other that at least one variable air gap formed between the two is penetrated by a, preferably homogeneous, magnetic field of high flux density, cf. particularly EP-A 803 713. 
     In this second variant of the invention, the magnetic circuit arrangement further comprises a ferromagnetic first core  13 A for coil  13 , the core being fixed to holder  15 . As shown in  FIG. 3 , core  13 A, extending through at least part of coil  13 , is located opposite to and spaced from armature  11 . In this second variant of the invention, core  13 A and armature  11  serve to form a variable first air gap, across which the magnetic field B 1  extends at least in part. Preferably, the magnetic circuit arrangement further comprises a ferromagnetic second core  14 A for coil  14 , this second core being also fixed to holder  15  at a distance from armature  12 . Thus, core  14 A and armature  12  form a variable air gap, particularly an air gap penetrated by magnetic field B 2 . 
     To produce permanent steady components of the magnetic fields and attenuate eddy currents in the magnetic circuit arrangement, each of cores  13 A is preferably made at least in part of hard magnetic, but poorly conducting material, such as of any one of the aforementioned rare-earth alloys AlNiCo, NyFeB, SmCo, etc. 
     To fix a reluctance for magnetic field B 1  that is as low as possible even outside core  13 A, in a preferred embodiment of the second variant of the invention, a ferromagnetic first yoke  13 B extending outside coil  13  is formed integrally with core  13 A, as shown in  FIG. 13 ; analogously, core  14 A may have a ferromagnetic second yoke  14 B for magnetic field B 2  formed thereon, which is preferably identical in shape to yoke  13 B. Advantageously, as is usual with such magnetic circuit arrangements, yoke  13 B,  14 B may be made of soft magnetic materials, such as ferrite or Corovac. 
     In a further preferred embodiment of the invention, core  13 A and yoke  13 B are shaped and aligned relative to each other in such a way that the free end faces of core  13 A and yoke  13 B which are in contact with the air gap are essentially flat and coplanar. Then, the free end face of armature  11  that is in contact with the air gap will preferably also be flat. In that case, this end face may also be parallel to the opposite free end faces of core  13 A and yoke  13 B, for example. If necessary, armature  11 , core  13 A, and yoke  13 B may be constructed on the coil-and-plunger principle. 
     In a further preferred embodiment of the second variant of the invention, yoke  13 B is designed as a coil can, particularly as a can coaxial with coil  13 , cf. EP-A 803 713. 
     Further details and embodiments concerning the operation of a magnetic circuit arrangement according to the second variant of the invention or concerning the shape and arrangement of coil  13  and yoke  13 B, and of coil  14  and yoke  14 B if present, are disclosed, for example, in applicant&#39;s EP-A 803 713, which is therefore incorporated herein by reference.