Abstract:
A device having a first flow meter ( 2 ) and a second flow meter ( 3 ). The first and the second flow meters work according to a Coriolis principle. The first flow meter includes a first measuring tube ( 7 ). The second flow meter includes a second measuring tube ( 10 ). The first flow meter and the second flow meter are disposed in a common housing ( 4,5,6 ). The first flow meter and the second flow meters have different eigenfrequencies because of a first vibration-influencing device ( 18 ) attached to the first flow meter and a second vibration-influencing device ( 19 ) attached to the second flow meter.

Description:
[0001]     This is a Continuation of International Application PCT/DK2003/000534, with an international filing date of Aug. 12, 2003, which was published under PCT Article 21(2) in German, and the disclosure of which is incorporated into this application by reference. 
     
    
     FIELD AND BACKGROUND OF THE INVENTION  
       [0002]     The invention relates to a flow meter device having a first flow meter, which works according to the Coriolis principles and is provided with a first measuring tube, and a second flow meter, which works according to the Coriolis principle and is provided with a second measuring tube.  
         [0003]     A flow meter device of this type is known from W. Kiehl, “Difference measurement using Coriolis mass flowmeters,” Flow Meas. Instrum., Vol. 2, April 1991, pp. 135 to 138. The use of two Coriolis flow meters is advantageous, for example, whenever the difference between two mass flows is to be determined. Determining this difference is useful if one wants to get information about a leak, for example.  
         [0004]     To measure the difference, the first flow meter is used to measure a first mass flow and a second flow meter to measure the second mass flow, and the difference between the two mass flows is then calculated.  
         [0005]     To ensure that the conditions created for the two measurements are as similar as possible, the two separate flow meters are usually accommodated in the same housing. To simplify production, the designs of the two flow meters are practically identical. This also facilitates the subsequent analysis of the individual signals.  
         [0006]     However, when two identical flow meters are used in the same housing, problems may arise by the two flow meters influencing each other. In flow meters that work according to the Coriolis principle vibrations are produced. These vibrations also act on the measuring tubes. The phase difference between the vibrations along different sections of the measuring tube is a measure of the mass flow. Under unfavorable circumstances, however, these vibrations are also transmitted through the common housing from one flow meter to the other. If both flow meters vibrate at the same frequency, this transmission causes significant interference and can distort the flow measurement results.  
       OBJECTS OF THE INVENTION  
       [0007]     One object of the invention is to avoid crosstalk between the flow meters.  
       SUMMARY OF THE INVENTION  
       [0008]     This and other objects are attained by flow meter devices, such as the ones disclosed, in which the two flow meters are disposed in a common housing and have different eigenfrequencies.  
         [0009]     Because of the different eigenfrequencies, the effects of crosstalk are reduced. The different eigenfrequencies make it possible to keep the mutual influence of the two flow meters small enough such that there is no, or only a tolerable level of, interference with the measurement result.  
         [0010]     In an embodiment, vibration-influencing device is mounted in a different location on the first measuring tube than on the second measuring tube. Using the vibration-influencing devices on the two measuring tubes, the eigenfrequencies of the flow meters are adjusted differently in relation to each other. The difference between the eigenfrequencies need not be large. It has been shown that an eigenfrequency difference of approximately 10 Hz is sufficient for the effects of crosstalk, i.e., the mutual influence, to become small enough to be negligible.  
         [0011]     The use of a vibration-influencing device on each measuring tube is a relatively simple measure. The design of the two flow meters can be practically identical, that is, there is no need to select a different thickness of material for the measuring tube in one flow meter than for the measuring tube in the other flow meter. Nor is it necessary to make any fundamental modifications in the flow meter. Both flow meters can be configured identically with respect to the positioning of the sensors and the exciter. If the two flow meters are identical, or at least practically identical, they also produce readily comparable results.  
         [0012]     Preferably, the vibration-influencing device is designed as a passive device. In other words, no additional energy is required to generate different eigenfrequencies in the two flow meters. The structural complexity is also reduced. Passive devices are much simpler to produce than active devices. The active devices require an electromagnet, for example, or some other excitation means.  
         [0013]     The two flow meters are preferably identical, with the exception of the vibration-influencing device. This facilitates not only their production, as described above. Only a single type of flow meter needs to be produced. It also facilitates the analysis of the measurement signals because the measurement signals are in principle based on the same conditions.  
         [0014]     Preferably, the first and the second measuring tube each have at least two loops that are interconnected by at least one coupling element, such that the coupling element is the vibration-influencing device. The configuration of a flow meter with a single measuring tube having at least two loops is known from WO 92/19940 A1. Between the loops coupling elements are provided whose function is to prevent the two loops from vibrating away from each other during operation. In principle, the coupling elements act as a fixed vibration point or a vibration node. The vibration of the measuring tube is limited to one side of the coupling elements while the other side is largely free from vibrations. This has the advantage that the measuring tube can be connected to a line without the vibrations being transmitted to that line. If the coupling elements can simultaneously be used to generate different eigenfrequencies in the two flow meters, the resulting structure is relatively simple.  
         [0015]     Preferably, two coupling elements are provided for each measuring tube. This results in a largely symmetrical configuration relative to the flow through the measuring tube.  
         [0016]     The coupling elements on each measuring tube are preferably arranged equidistantly from the ends of the respective measuring tube. This has the particular advantage that the coupling elements on the one measuring tube can be arranged farther away from the ends than those on the other measuring tube. This changes the eigenfrequency in a simple manner. However, the measurement result in the measuring tube itself remains largely unaffected by the coupling elements.  
         [0017]     The coupling elements are preferably plates, and the measuring tubes are aligned perpendicularly to the plates in the area of the coupling elements. The rigidity of the plates perpendicular to the direction of movement of the measuring tubes is sufficient to effectively absorb the vibrations. At the same time, due to the relatively high rigidity of the plates, the eigenfrequencies of the measuring tubes are effectively influenced.  
         [0018]     The plates of a flow meter are preferably fixed relative to a chassis of this flow meter. The two chassis each have at least two locating positions. The plates of the first flow meter are fixed at a first locating position and the plates of the second flow meter at a second locating position. The same chassis or housing parts can in turn be used. Basically, the difference between the eigenfrequencies of the two flow meters results simply from the fact that the plates are fixed at different positions, i.e., the so-called locating positions in each chassis. This is a relatively simple measure, which does not require any major structural changes in the chassis. This facilitates production since only a single type of chassis needs to be produced.  
         [0019]     The locating positions are preferably created by means of structures on the chassis. These structures primarily determine the site of the locating positions. At the same time, structures can also be used to mechanically fix the individual plates or auxiliary elements.  
         [0020]     The structures preferably enable an insertion of the plates. This facilitates the assembly. The plates merely need to be inserted into the structure, which defines the respectively desired locating position.  
         [0021]     A locating position is preferably defined by at least three projecting parts of the chassis, which protrude toward the measuring tube. The three projecting parts are arranged in a triangle. The plates can then be inserted into the chassis, such that two projecting parts are arranged on one side of the plate and one or more projecting parts on the other side. This configuration adequately secures the plates within the chassis.  
         [0022]     The locating positions are preferably spaced apart by a distance on the order of one centimeter. Hence, the distance between the locating positions, that is, the distance between the individual coupling elements can be relatively small. It has been found that even small differences are enough to sufficiently change the eigenfrequencies.  
         [0023]     Strain gauges are preferably arranged on each of the coupling elements. These strain gauges are inexpensive. They can register the relative longitudinal change of the coupling elements. This change can be used as a measure of the mass flow. In addition, the measurement by means of strain gauges includes a differential measurement of the curvature of the measuring tubes, such that the dependence on the flow direction can be clearly reduced.  
         [0024]     Each flow meter preferably has an electronic control unit, such that the electronic control unit of the one flow meter monitors the electronic control unit of the other flow meter. The two control units can very well be structurally combined, e.g., on a common printed circuit board. However, the printed circuit board is divided into two sections, which are functionally completely separate, such that each section controls one flow meter. Furthermore, the two sections monitor each other by means of a monitoring circuit. If one section fails, the other section will take over the control and measurement. This has the advantage of ensuring a reliable operation even in differential flow measurements, which have to meet high standards. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]     The invention will now be described in greater detail, by way of example, with reference to a preferred embodiment depicted in the drawing, in which:  
         [0026]      FIG. 1  shows a flow meter device with two flow meters,  
         [0027]      FIG. 2  shows a single flow meter,  
         [0028]      FIG. 3  depicts a housing section of a flow meter,  
         [0029]      FIG. 4  illustrates a flow meter during assembly,  
         [0030]      FIG. 5  shows a coupling element,  
         [0031]      FIG. 6  depicts a mounting plate containing a coupling element, and  
         [0032]      FIG. 7  shows the rest of the mounting plate. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0033]     A flow meter device  1  depicted in  FIG. 1  has two individual flow meters  2 ,  3 , the chassis  4 ,  5  of which are mounted to a common front plate  6 .  
         [0034]     The flow meter  2  has a measuring tube  7 , which is run in two loops. Sensors  8  and a driver  9  are disposed between the two loops. The flow meter  3  likewise has a measuring tube  10 , which is run in two loops. Two sensors  11  and a driver  12  are located between the loops of the measuring tube  10 .  
         [0035]     The measuring tubes  7 ,  10  can be formed by parallel loops, that is to say, by two separate measuring tubes through which the fluid flows. It is also possible, however, to design the measuring tubes  7 ,  10  as continuous tubes as disclosed in WO 92/19940 A1.  
         [0036]      FIG. 2  shows an individual flow meter  2  in detail. The two measuring tube loops  7   a ,  7   b  are mounted on the chassis  4 . The measuring tube  7   a  has an inlet  13   a  and an outlet  14   a . The measuring tube  7   b  has an inlet  13   b  and an outlet  14   b . The inlets and outlets  13   a ,  13   b  and  14   a ,  14   b  are guided through a base part  15  of the chassis  4 . Straight sections  16  extend from the inlets  13   a ,  13   b  and the outlets  14   a ,  14   b  and are supported by an anchor  17  behind the base  15 . Along the further course of the measuring tubes  7   a ,  7   b  a coupling element  18  is provided for the inlet section  13   a ,  13   b  and a coupling element  19  for the outlet section  14   a ,  14   b . The coupling elements  18 ,  19  interconnect the two measuring tubes  7   a ,  7   b  on the outside of the looped section. In the embodiment shown, the fluid can flow parallel through the two measuring tubes  7   a ,  7   b . It is also possible, however, to connect the two measuring tubes  7   a ,  7   b  in series. For this purpose, the outlet  14   a  of the measuring tube  7   a  is connected to the inlet  13   b  of the measuring tube  7   b , for example. This connection is preferably effected within the chassis  4 .  
         [0037]     The position of the coupling elements  18  determines the eigenfrequency of the flow meter  2 . If the coupling elements  18 , which are designed as plates, are shifted along the measuring tubes  7   a ,  7   b , the eigenfrequency changes because the distance between a vibration node and the driver  9  is lengthened or shortened. The distance relative to the anchor  17  is likewise shortened or lengthened. The anchor  17  together with the chassis  4  forms a base plate whose mass may be assumed to be approximately infinite. When the position of the node is changed by means of the coupling element  18 ,  19 , the eigenfrequency of the measuring tube  7   a ,  7   b  changes accordingly. The differences in the eigenfrequencies of the flow meters  2 ,  3  are adjusted simply by arranging the coupling elements  18 ,  19  in the one flow meter  2  at the positions indicated in  FIG. 2  and those same coupling elements  18 ,  19  in the other flow meter  3  somewhat closer to the anchor  17 . Otherwise the same identical flow meters can be used, that is, the measuring tubes  7   a ,  7   b  and the chassis  4 ,  5  can be practically identical.  
         [0038]     The measuring tubes  7   a ,  7   b  are provided with the mounts  8 ′,  8 ″ for the sensors  8  and the mount  9 ′ for the driver  9 .  
         [0039]      FIG. 3  shows the chassis  4  with the base  15 . The base  15  has four holes  20  through which the ends  13   a ,  13   b ,  14   a ,  14   b  of the measuring tubes  7   a ,  7   b  are threaded. Two sides  21 ,  22  extend practically symmetrically from the base  15 , such that they are substantially aligned in a U-shape relative to the base  15 . Each side  21 ,  22 , in turn, has a U-shaped cross section, i.e., it has outwardly angled projecting parts  23 ,  24  to give the chassis  4  additional stability. The sections  23 ,  24  further make it possible to mount the otherwise identical chassis  4 ,  5  to the mounting plates  25 ,  26 . The mounting plates  25 ,  26  are in turn fixed to the common housing, here the front plate  6 . The anchor  17  is thus fixed to the chassis  4 . The chassis  4  is fixed to the mounting plate  26 , which is in turn fixed to the front plate  6  that forms part of the common housing.  
         [0040]     The sides  21 ,  22  are provided with recesses  27  in which the anchor  17  can be mounted.  
         [0041]     In their end regions, the two sides  21 ,  22  have two groups  28 ,  29  of projecting parts  30 . These projecting parts are arranged to form the corner points of a trapezoid as seen in the top view. The plates  31  can therefore be inserted between the front and the rear projecting parts of a group ( FIG. 4 ). Thus, the position of the plate  31  within the chassis  4  is determined by the selection of a group  28 ,  29  of projecting parts  30 . If, as shown in  FIG. 4 , the projecting parts of the group  28  are selected for the positioning of the plate  31 , then the flow meter has a frequency f. If, on the other hand, the projecting parts  30  of the group  29  are selected for the plate  31 , then the eigenfrequency is f′. The distance between the two possible positions of the plate  31  is approximately 1 cm, which results in an eigenfrequency difference of 10 Hz. The frequency f is preferably 130 Hz and the frequency f′ 140 Hz. The essential factor determining the eigenfrequency is the distance between the position of the plate  31  and the tip of the loops of the tubes.  
         [0042]      FIG. 5  to  7  show the auxiliary means used to mount the coupling elements  18 ,  19  at the correspondingly selected positions on the measuring tubes  7   a ,  7   b .  FIG. 5  shows a coupling element  18 , which prior to assembly forms part of the plate  31  shown in  FIG. 4 . The plate  31  has a central element  32  and two lateral elements  33 ,  34 , which are connected with the two coupling elements  18 ,  19  via rated break points  35 .  
         [0043]     The plates  31  are pushed over the measuring tubes  7   a ,  7   b . They can then be inserted between the projecting parts  30  of the one group  28  or the other group  29 . This determines the position of the coupling elements  18 ,  19  on the measuring tubes  7   a ,  7   b .  
         [0044]     The coupling elements can then be connected, e.g., soldered or glued, to the measuring tubes  7   a ,  7   b . The rest of the plate  31 , i.e., the central section  32  and the lateral sections  33 ,  34  can then be removed.  
         [0045]     The other flow meter  3  is basically constructed in the exact same way. The only difference is that the plate  31  is inserted between the projecting parts  30  of the other group  29 . The eigenfrequency differences thus generated are sufficient so that the two chassis  4 ,  5  can be interconnected. The direct connection of the two flow meters  2 ,  3  provides an excellent mechanical coupling between the two chassis  4 ,  5 , but is unproblematic when the above-described solution is used. The eigenfrequencies of the flow meters  2 ,  3  differ sufficiently so that crosstalk interference with the measurement results can be avoided.  
         [0046]     Once the front plate  6  has been mounted to the mounting plate  26  and the base  15  of the two chassis  4 , the differential flow meter depicted in  FIG. 1  is nearly completed. It only needs to be inserted into a housing, e.g., into an aluminum housing.  
         [0047]     It is of course also possible to use other markings in the chassis  4  instead of the projecting parts  30 . In some cases it is even sufficient to simply mark or in some other way indicate the position where the coupling elements  18 ,  19  are to be mounted in the one flow meter  2  or the other flow meter  3 . However, the use of structures makes the assembly easier.  
         [0048]     As an alternative to the above-described device, in which a vibration-influencing device is disposed at different positions on the two measuring tubes, a point mass can be used as a vibration-influencing device, which is provided at the same location on both measuring tubes  7 ,  10 . The two point masses may differ with respect to their mass, however. Instead of point masses other additional masses may of course also be used.  
         [0049]     Strain gauges may be fixed to the coupling elements  18 ,  19 . The strain gauges register the relative change in length of the coupling elements  18 ,  19 . This change can be used as the measure for the mass flow. In this case, the sensors  8 ′ and  8 ″ can be eliminated. An additional advantage is that this type of measurement using strain gauges includes a differential measurement of the curvatures of the measuring tubes  7 ,  10 . This measurement is therefore insensitive to the flow direction.  
         [0050]     The flow meter device consisting of two individual Coriolis flow meters is controlled by a joint main control unit (not depicted). This main control unit is arranged within the common housing and has two electronic control units, one for each flow meter. The control units can be arranged on the same printed circuit board. In this case, however, the printed circuit board is divided into two completely separate sections, such that each section controls one Coriolis flow meter. The control units can of course also be arranged on different printed circuit boards.  
         [0051]     In addition, the two control units monitor each other by means of a monitoring device or circuit. If the one control unit fails, the other control unit takes over the control and measurement of the flow meter. This ensures a reliable operation even in differential flow measurements, which have to meet particularly high standards.  
         [0052]     The above description of the preferred embodiments has been given by way of example. From the disclosure given, those skilled in the art will not only understand the present invention and its attendant advantages, but will also find apparent various changes and modifications to the structures disclosed. It is sought, therefore, to cover all such changes and modifications as fall within the spirit and scope of the invention, as defined by the appended claims, and equivalents thereof.