Abstract:
A flowmeter and a method of determining the mass flow rate. The flowmeter has a first measuring tube and a second measuring tube which are arranged in a common housing and are connected to one another mechanically. Also provided are an excitation device for exciting the measuring tubes to oscillation, and a detector device for detecting oscillation parameters which is connected to an evaluating device, which determines a mass flow rate signal for each measuring tube from output signals from the detector device. The object is to improve the measuring accuracy in cases in which identical conditions do not prevail in the measuring tubes. For that purpose, an amplitude detecting device is provided which detects the amplitude of the oscillation of each measuring tube, and a correcting device is connected to the evaluating device, which correcting device has a flow input and an amplitude input for each measuring tube.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a flowmeter having a first measuring tube and a second measuring tube which are arranged in a common housing and are connected to one another mechanically, an excitation device for exciting the measuring tubes to oscillation, a detector device for detecting oscillation parameters and an evaluating device, which determines a mass flow rate signal for each measuring tube from output signals from the detector device. 
     Such a flowmeter is known from WO 97/26508 A1. Using such a flowmeter, it is possible to determine the mass flow rate through each measuring tube and also the difference between or the sum of the two mass flows. Cases in which it is desirable to determine both the absolute value of the mass flow through each measuring tube and the difference between the mass flows are found, for example, in the field of medicine. In the case of purifying the blood of dialysis patients, the amount of fluid removed from the body must be monitored precisely. That amount appears in the process as a differential flow between the dialysate flow supplied and the dialysate flow removed, the latter being small in relation to the dialysate flow supplied. At the same time, it would of course also be desirable to discover the absolute amount of dialysate supplied. 
     Other applications are found in surface-coating technology, in which a certain amount must be held in store in a colour medium reservoir. It is therefore necessary to re-supply exactly the same amount of colour medium components as the amount of colour medium removed. For that reason, the difference in mass must be known. It would on the other hand of course also be desirable to know the absolute amount of colour medium used. 
     In the known flowmeter, basically two independent measuring systems operating according to the Coriolis principle are therefore used. Each measuring tube is excited to oscillation. The oscillation of the associated measuring tube is detected at a location other than at the point of excitation. The mass flow rate can then be determined from the phasing of the oscillation between the point of excitation and the measuring point or between two measuring points spaced from one another. 
     The oscillation is produced with respect to a housing and is also detected with respect to a housing. At the same time, the housing serves to secure the measuring tubes. Certain problems arise, however, as a result of the housing. The oscillations produced at the measuring tubes are transferred also to the housing or to a mechanical coupling between the tubes, which is provided to prevent oscillation loading at the point at which the tubes are fixed in the housing. The mechanical coupling between the tubes also forms an artificial node for the tubes when they are oscillating in opposite phase. 
     The mechanical coupling between the two measuring tubes is not critical provided the conditions in the two measuring tubes are identical, that is to say, when the through-flowing mass is approximately the same. That is the case in normal flowmeters, which detect only the difference between mass flows, because the two measuring tubes are then connected either in series or in parallel, see, for example, EP 0 244 692 A1. In that case, virtually no disturbances can be observed in the measurement result. 
     Problems arise, however, when the media flowing through the measuring tubes have different densities or different flow speeds or are subject to other different conditions. It has been shown that in such circumstances the measurement results do not reflect the true conditions with the necessary reliability. 
     SUMMARY OF THE INVENTION 
     The problem underlying the invention is to improve the measuring accuracy in cases in which identical conditions do not prevail in the measuring tubes. 
     The problem is solved in a flowmeter of the type mentioned at the beginning in that an amplitude detecting device is provided, which detects the amplitude of the oscillation of each measuring tube, and a correcting device is connected to the evaluating device, which correcting device has a flow input and an amplitude input for each measuring tube. 
     The flowmeter thus operates initially like a normal flowmeter according to the Coriolis principle. Each measuring tube is made to oscillate. A phase difference in the oscillations at various positions on each measuring tube is dependent upon the mass flow through the measuring tube. That phase difference (or other known values of measuring tubes operating according to the Coriolis principle) can be used to determine the mass flow, that is to say, the mass flowing through per unit time. The mass flow rate signal of each measuring tube is, however, subject to error. The “composition” of that error is now known. The amplitude of the oscillation of the other measuring tube and also the mass flow rate through the other measuring tube enter into this error. It is accordingly sufficient to supply those two values to the correcting device in order to form an error correction value and to correct the mass flow rate signal accordingly. Since the influence of one measuring tube on the other measuring tube and of the other measuring tube on the first measuring tube can be observed, only two further signals need be supplied to the correcting device in addition to the (error-affected) mass flow rate signals, namely the amplitudes of the two measuring tubes. 
     The amplitude detecting device is preferably combined with the detector device. For error correction, it is no longer even necessary to have separate sensors. All that is required is a type of signal generation, supplemented where appropriate. One is no longer obliged to determine only the phase displacement of the oscillation at various positions on a measuring tube, but it is possible to use one or more detectors to detect the amplitude as well. 
     Preferably the detector device has a separate detector arrangement for each measuring tube. The risk of further couplings&#39; becoming involved via the detector device is thus reduced. In corresponding manner, the excitation device can have a separate excitation arrangement for each measuring tube, for example, an electromagnet. The risk of reciprocal couplings is, however, somewhat smaller in the case of excitation. 
     Advantageously the correcting device produces for each measuring tube a product of the flow, amplitude and a coupling coefficient of the respective measuring tube and feeds that product back to the mass flow rate signal from the other measuring tube. A certain transient process is of course necessary until an error-free mass flow rate signal has been obtained. By means of the backwards coupling, however, error correction can be obtained with relatively little outlay. 
     In an alternative construction, for each measuring tube the correcting device adds a product of the mass flow rate signal, amplitude and a coupling coefficient of one measuring tube to the mass flow rate signal of the other measuring tube and divides the sum by a factor which comprises the amplitudes of the measuring tubes. This is a case of forwards coupling or regenerative coupling. In that construction, a mass flow rate signal that is free of coupling errors is obtained in every operating state. 
     The correcting device is preferably in the form of an electronic circuit. By means of the electronic circuit, the individual coupling factors can be readily reproduced and coupled with the respective amplitudes. 
     It is, in that case, advantageous for the circuit to have a memory for the coupling coefficients. The coupling coefficients can then be determined in advance for each flowmeter and stored. They are then permanently available for further operation. 
     The invention relates also to a method of determining the mass flow rate through two measuring tubes which are coupled mechanically and are excited to oscillation, a mass flow rate signal being determined from oscillation parameters of each measuring tube. 
     The above-mentioned problem is solved in that method in that the mass flow rate signal for each measuring tube is corrected by means of a correction value which contains a coupling coefficient and the amplitude of the oscillation of the other measuring tube. 
     As explained above in connection with the flowmeter, in that manner “disturbances” which are exerted by the two measuring tubes on each other as a result of the mechanical coupling of the two measuring tubes, for example, by way of the housing or. by way. of fastening elements on the housing, can be eliminated. Those disturbances are not critical only as long as identical conditions prevail in the two measuring tubes, that is to say, identical mass flows, identical densities or identical temperatures. 
     When conditions are different, those disturbances amplify a measuring error. Since the composition of the measuring error has now been determined, it can be eliminated again. The measuring error is dependent, firstly, upon the amplitude of the other measuring tube and, secondly, upon the mass flow rate through that measuring tube. The equipment-related disturbances can be combined in a constant coupling coefficient. 
     It is, in that case, preferable for the correction value to be formed by the product of the amplitude, coupling coefficient and corrected mass flow rate signal, the correction value being added with the opposite sign to the uncorrected mass flow rate signal such as a disturbance caused by the mechanical coupling. That correction method is a case of backwards coupling. The issue as to what effect the disturbance has upon the mass flow rate signal can be readily determined in advance. The backwards coupling must then operate with the opposite sign. When, for example, the disturbance has the effect of reducing the mass flow rate signal, the backwards coupling must carry out an addition. 
     In an alternative construction, the correction value is formed by the product of the uncorrected mass flow rate signal, amplitude and coupling coefficient of one measuring tube, which is added to the uncorrected mass flow rate signal of the other measuring tube, the sum being normalized to a value dependent upon both amplitudes. That is a case of forwards coupling or regenerative coupling. This has the advantage of delivering an error-free signal in every operating state. 
     Advantageously the coupling coefficients and, where appropriate, their temperature dependency are determined in advance by calibration. The coupling coefficients can be determined even during manufacture or in a subsequent step and can then be imparted permanently to the corresponding flowmeter. The coupling coefficients are basically dependent upon only mechanical influences, which do not alter during operation if the temperature remains constant. If changing temperatures are to be expected, the temperature dependency of the coupling coefficients can also be determined during calibration and that dependency can then be described mathematically, for example, by a polynomial. 
     The calibration is effected advantageously in that a flow passes through one measuring tube during calibration, but not through the other measuring tube. In that case it is possible to determine relatively precisely the effect of the oscillation of one measuring tube on the other. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is described hereinafter in greater detail with reference to preferred embodiments, in conjunction with the drawings, in which: 
     FIG. 1 is a section through a flowmeter along the line B—B according to FIG. 2; 
     FIG. 2 is a section A—A according to FIG. 1; 
     FIG. 3 is a diagrammatic circuit diagram; 
     FIG. 4 is a diagrammatic representation of an error correction operation; and 
     FIG. 5 is a diagrammatic representation of an alternative error correction operation. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A flowmeter  1  shown in FIGS. 1 and 2 has a housing  2  in which there are arranged a first measuring tube  3  and a second measuring tube  4 . The two measuring tubes are coupled to one another mechanically by mechanical connections  5 ,  6 . The connections  5 ,  6  are shown only diagrammatically. They are known per se and are intended to prevent mechanical oscillations from loading the fastening of the measuring tubes  3 ,  4  to the housing  2 . Even when there are no connections  5 ,  6 , mechanical coupling takes place between the two measuring tubes  3 ,  4 , in that case via the housing  2 . 
     Each measuring tube  3 ,  4  has an excitation device  7 ,  8 , for example an electromagnet, which makes the corresponding measuring tube  3 ,  4  oscillate. Each measuring tube also has two sensors  9 ,  10  and  11 ,  12 , respectively, which detect the oscillation behaviour of the measuring tubes  3 ,  4  at a certain distance, viewed in the direction of flow, from the excitation device  7 ,  8 . 
     The flowmeter  1  operates according to the Coriolis principle. When there is no flow through the measuring tube  3 ,  4 , the oscillation produced by the excitation device  7 ,  8  is then transferred away along the measuring tube  3 ,  4  uniformly to both ends. The two sensors  9 ,  10  and  11 ,  12  arranged at the same distance from the excitation device  8  will accordingly be able to detect like-phased oscillation. 
     If, however, there is a mass flow through the oscillating measuring tube  3 ,  4 , for example, a fluid or a gas, a phase shift then occurs between the two sensors  9 ,  10  and  11 ,  12  on account of the Coriolis force thereby produced. The mass flow or mass flow rate per unit time can be determined from that phase shift. It is not absolutely necessary for two sensors  9 ,  10  and  11 ,  12  to be present. The phase shift can also be detected between the excitation device  7 ,  8  and one sensor. Basically, any other values known of mass flowmeters operating according to the Coriolis principle can be used to determine the mass flow. 
     Whereas FIGS. 1 and 2 show the mechanical structure, FIG. 3 shows diagrammatically the operational context. 
     Mass flows Q 1  and Q 2  pass through the two measuring tubes  3 ,  4 , respectively. The sensors  9 ,  10  and  11 ,  12  accordingly detect the excursion of the measuring tubes  3 ,  4  at the positions in question. By means of an evaluating device  13 ,  14 , mass flow rate signals Q 1 * and Q 2 * are formed, for example, from the phase shift discussed above. 
     On account of the mechanical coupling between the two measuring tubes  3 ,  4 , the mass flow rate signals Q 1 *, Q 2 * are, however, subject to error. That error is based on the fact that the oscillations of each measuring tube  4 ,  3  feed back to the other measuring tube  3 ,  4  by way of the mechanical coupling. The feedback is in this case dependent upon the mass flow flowing through the other measuring tube  4 ,  3  and upon the amplitude with which the measuring tube  4 ,  3  is oscillating. The greater the amplitude is, the greater is the disturbance affecting the other measuring tube. The same applies also to the mass flow, as can be readily imagined. Provided the same medium is flowing through the two measuring tubes  3 ,  4  and provided the measuring tubes  3 ,  4  receive the same mass flow, the two disturbances cancel each other out and have no noticeable disturbing effect. If, however, different media flows are flowing through the two measuring tubes, that is to say having differences, for example, in density or in flow speed, there are then in some cases considerable departures in the mass flow rate signals Q 1 *, Q 2 * from the actual mass flow rates Q 1 , Q 2 . 
     As can be seen from FIGS. 4 and 5, the disturbance-affected mass flow rate signals Q 1 *, Q 2 * can be represented as follows: 
     
       
           Q   1 *= Q   1 − Q   2 · K   21 · A   2   
       
     
     
       
           Q   2 *= Q   2 − Q   1 · K   12 · A   1   
       
     
     In those equations A 1  and A 2  are the amplitudes with which the measuring tubes oscillate when the mass flows Q 1 , Q 2  flow through them. The coefficients K 12  and K 21  are coupling coefficients with which Q 1  acts upon Q 2  (Kl 2 ) and Q 2  acts upon Q 1  (K 21 ), respectively. 
     In order to eliminate that error, the evaluating device  13 ,  14  determines not only the mass flow rate signal Q 1 *, Q 2 * but also the amplitude A 1 , A 2  of the oscillations of the measuring tubes  3 ,  4 . The amplitude can be detected, for example, by forming a mean value from the excursions oil each measuring tube  3 ,  4  detected by the sensors  9 ,  10  and  11 ,  12 , respectively. 
     Both the mass flow rate signals Q 1 *, Q 2 * and the two amplitudes A 1 , A 2  are sent to a correcting device  15 , the manner of operating of which will be explained with reference to FIGS. 4 and 5. 
     FIG. 4 is a first diagrammatic representation. For the purpose of clarity, as in FIG. 3, a box  16  has been sketched in to bring together certain elements from an operational point of view. It is, of course, clear that this does not mean that all those elements are housed in a common housing or that those elements must be in the form of discrete components. 
     As already explained, at the output of the evaluating device  13  there is an error-affected mass flow rate signal Q 1  and at the output of the evaluating device  14  there is an error-affected mass flow rate signal Q 2 *. The error arises from the fact that the coupling factor K 12  and the amplitude A 1  and the coupling factor K 21  and the amplitude A 2  influence the “true” mass flow rates Q 1  and Q 2 , respectively. In the present embodiment, it is assumed that the error is subtracted from the true mass flow rate Q 1 , Q 2 . 
     In order to eliminate that error, two addition points  17 ,  18  are provided in the correcting device  15 . At the addition point  17  the error-affected mass flow rate signal Q 1 * and a correcting factor are added, the latter being formed by a mass flow rate signal Q 2  that has been taken after the addition point  18  and is thus “error-free”. In similar manner, at the addition point  18  the error-affected mass flow rate signal Q 2 * and a correcting factor are added, the latter being formed by the product of the “error-free” mass flow rate signal Q 1 , the coupling factor K 12  and the amplitude A 1 . 
     The mass flow rate signal Q 1 * or Q 2 * is thus corrected by backwards coupling. After a short initial phase, disturbance-free mass flow rate signals Q 1  and Q 2  are indeed available after the addition points  17 ,  18 , with the result that the correction operation can proceed accordingly. 
     The coupling coefficients K 12  and K 21  can be determined in advance by calibration. They are basically dependent only upon mechanical parameters of the flowmeter  1  and thereby upon the temperature. That temperature dependency can also be determined during calibration and can be described adequately, for example, by a polynomial. 
     For the calibration, firstly a known mass flow Q 1  is passed through the measuring tube  3 . No flow passes through the measuring tube  4 . An error-affected mass flow rate signal Q 2 *=−Q·K 12 ·A 1  is then produced, from which the coupling coefficient K 12  can be calculated, because both Q 1  and A 1  are known or can be measured. The coupling factor K 21  can also be determined in similar manner when there is a flow through the measuring tube  4  and there is no flow through the measuring tube  3 . The coupling coefficients K 12 , K 21  can then be stored in a memory that is present in the correcting device  15 , which is preferably in the form of an electronic circuit. The term “electronic circuit” is to be understood as including miniaturized circuits also, that is to say, for example, those that can be stored on a microchip. 
     Whereas the embodiment according to FIG. 4 effects error correction by backwards coupling, FIG. 5 shows an embodiment in which the error correction is effected by forwards coupling or regenerative coupling. 
     The relationships at the output of the box  16  are the same. The correcting device  15  also has two addition points  17 ,  18 . In contrast to the construction according to FIG. 4, in which the correction values are taken after the addition points  17 ,  18 , in this case a correction value is formed from the uncorrected mass flow rate signal Q 1 *, the coupling coefficient K 12  and the amplitude A 1  and from the uncorrected mass flow rate signal Q 2 *, the coupling coefficient K 21  and the amplitude A 2 . The coupling coefficients K 12  and K 21  were determined in advance by calibration in the same manner as that described in the context of FIG.  4 . 
     After the addition points  17 ,  18  a signal is then available which must still be divided by a term (1−K 21 ·A 2 ·K 12 ·A 1 ) in order to obtain the “true” mass flow rate Q 1 , Q 2 . 
     In both embodiments, all that is required is for the two error-affected mass flow rate signals Q 1 * and Q 2 * and the two amplitudes A 1  and A 2  to be sent to the correcting device  15  in order to be able to carry out a correction operation.